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№ 7

June 18, 2024

№ 7. Urinary system. Kidneys. Kidney diseases.

Urinary System

Introduction

The Urinary System is a group of organs in the body concerned with filtering out excess fluid and other substances from the bloodstream. The substances are filtered out from the body in the form of urine. Urine is a liquid produced by the kidneys, collected in the bladder and excreted through the urethra. Urine is used to extract excess minerals or vitamins as well as blood corpuscles from the body. The Urinary organs include the kidneys, ureters, bladder, and urethra. The Urinary system works with the other systems of the body to help maintain homeostasis. The kidneys are the main organs of homeostasis because they maintain the acid base balance and the water salt balance of the blood.

Functions of the Urinary System

One of the major functions of the Urinary system is the process of excretion. Excretion is the process of eliminating, from an organism, waste products of metabolism and other materials that are of no use. The urinary system maintains an appropriate fluid volume by regulating the amount of water that is excreted in the urine. Other aspects of its function include regulating the concentrations of various electrolytes in the body fluids and maintaining normal pH of the blood. Several body organs carry out excretion, but the kidneys are the most important excretory organ. The primary function of the kidneys are to maintain a stable internal environment (homeostasis) for optimal cell and tissue metabolism. They do this by separating urea, mineral salts, toxins, and other waste products from the blood. They also do the job of conserving water, salts, and electrolytes. At least one kidney must function properly for life to be maintained.

Six important roles of the kidneys are:

· Regulation of plasma ionic composition. Ions such as sodium, potassium, calcium, magnesium, chloride, bicarbonate, and phosphates are regulated by the amount that the kidney excretes.

· Regulation of plasma osmolarity. The kidneys regulate osmolarity because they have direct control over how many ions and how much water a person excretes.

· Regulation of plasma volume. Your kidneys are so important they even have an effect on your blood pressure. The kidneys control plasma volume by controlling how much water a person excretes. The plasma volume has a direct effect on the total blood volume, which has a direct effect on your blood pressure. Salt(NaCl)will cause osmosis to happen; the diffusion of water into the blood.

· Regulation of plasma hydrogen ion concentration (pH). The kidneys partner up with the lungs and they together control the pH. The kidneys have a major role because they control the amount of bicarbonate excreted or held onto. The kidneys help maintain the blood Ph mainly by excreting hydrogen ions and reabsorbing bicarbonate ions as needed.

· Removal of metabolic waste products and foreign substances from the plasma. One of the most important things the kidneys excrete is nitrogenous waste. As the liver breaks down amino acids it also releases ammonia. The liver then quickly combines that ammonia with carbon dioxide, creating urea which is the primary nitrogenous end product of metabolism in humans. The liver turns the ammonia into urea because it is much less toxic. We can also excrete some ammonia, creatinine and uric acid. The creatinine comes from the metabolic breakdown of creatine phospate (a high-energy phosphate in muscles). Uric acid comes from the break down of nucleotides. Uric acid is insoluble and too much uric acid in the blood will build up and form crystals that can collect in the joints and cause gout.

Renin-angiotensin-aldosterone system.

· Secretion of Hormones. The endocrine system has assistance from the kidney’s when releasing hormones. Renin is released by the kidneys. Renin leads to the secretion of aldosterone which is released from the adrenal cortex. Aldosterone promotes the kidneys to reabsorb the sodium (Na+) ions. The kidneys also secrete erythropoietin when the blood doesn’t have the capacity to carry oxygen. Erythropoietin stimulates red blood cell production. The Vitamin D from the skin is also activated with help from the kidneys. Calcium (Ca+) absorption from the digestive tract is promoted by vitamin D.

Organs in the Urinary System

Kidneys and Their Structure

The kidney structure:

1. Renal pelvis

2. Ureter

3. Minor calyx

4. Renal capsule

5. Inferior renal capsule

6. Superior renal capsule

7. Interlobar vein

8. Nephron

9. Minor calyx

10. Major calyx

11. Renal papilla

12. Renal column

The kidneys are a pair of bean shaped, reddish brown organs about the size of your fist. It measures 10-12 cm long. They are covered by the renal capsule, which is a tough capsule of fibrous connective tissue. Adhering to the surface of each kidney is two layers of fat to help cushion them. There is a concaved side of the kidney that has a depression where a renal artery enters, and a renal vein and a ureter exit the kidney. The kidneys are located at the rear wall of the abdominal cavity just above the waistline, and are protected by the ribcage. They are considered retroperitoneal, which means they lie behind the peritoneum. There are three major regions of the kidney, renal cortex, renal medulla and the renal pelvis. The outer, granulated layer is the renal cortex. The cortex stretches down in between a radially striated inner layer. The inner radially striated layer is the renal medulla.

This contains pyramid shaped tissue called the renal pyramids, separated by renal columns. The ureters are continuous with the renal pelvis and are the very centre of the kidney.

Renal Vein

The renal veins are veins that drain the kidney. They connect the kidney to the inferior vena cava. Because the inferior vena cava is on the right half of the body, the left renal vein is generally the longer of the two.

Unlike the right renal vein, the left renal vein often receives the left gonadal vein (left testicular vein in males, left ovarian vein in females). It frequently receives the left suprarenal vein as well.

Renal Artery

The renal arteries normally arise off the abdominal aorta and supply the kidneys with blood. The arterial supply of the kidneys are variable and there may be one or more renal arteries supplying each kidney. Due to the position of the aorta, the inferior vena cava and the kidneys in the body, the right renal artery is normally longer than the left renal artery.

The right renal artery normally crosses posteriorly to the inferior vena cava. The renal arteries carry a large portion of the total blood flow to the kidneys. Up to a third of the total cardiac output can pass through the renal arteries to be filtered by the kidneys.

Ureters

The ureters are two tubes that drain urine from the kidneys to the bladder. Each ureter is a muscular tube about 10 inches (25 cm) long. Muscles in the walls of the ureters send the urine in small spurts into the bladder, (a collapsible sac found on the forward part of the cavity of the bony pelvis that allows temporary storage of urine). After the urine enters the bladder from the ureters, small folds in the bladder mucosa act like valves preventing backward flow of the urine. The outlet of the bladder is controlled by a sphincter muscle. A full bladder stimulates sensory nerves in the bladder wall that relax the sphincter and allow release of the urine.

However, relaxation of the sphincter is also in part a learned response under voluntary control. The released urine enters the urethra.

Urinary Bladder

The urinary bladder is a hollow, muscular and distendible or elastic organ that sits on the pelvic floor (superior to the prostate in males). On its anterior border lies the pubic symphysis and, on its posterior border, the vagina (in females) and rectum (in males). The urinary bladder can hold approximately 17 to 18 ounces (500 to 530 ml) of urine, however the desire to micturate is usually experienced when it contains about 150 to 200 ml. When the bladder fills with urine (about half full), stretch receptors send nerve impulses to the spinal cord, which then sends a reflex nerve impulse back to the sphincter (muscular valve) at the neck of the bladder, causing it to relax and allow the flow of urine into the urethra. The Internal urethral sphincter is involuntary. The ureters enter the bladder diagonally from its dorsolateral floor in an area called the trigone. The trigone is a triangular shaped area on the postero-inferior wall of the bladder. The urethra exits at the lowest point of the triangle of the trigone. The urine in the bladder also helps regulate body temperature. If the bladder becomes completely void of fluid, it causes the patient to chill.

Urethra

The urethra is a muscular tube that connects the bladder with the outside of the body. The function of the urethra is to remove urine from the body. It measures about 1.5 inches (3.8 cm) in a woman but up to 8 inches (20 cm) in a man. Because the urethra is so much shorter in a woman it makes it much easier for a woman to get harmful bacteria in her bladder this is commonly called a bladder infection or a UTI. The most common bacteria of a UTI is E-coli from the large intestines that have been excreted in fecal matter.

Female urethra

In the human female, the urethra is about 1-2 inches long and opens in the vulva between the clitoris and the vaginal opening.

Men have a longer urethra than women. This means that women tend to be more susceptible to infections of the bladder (cystitis) and the urinary tract.

Female urethra

Male urethra

In the human male, the urethra is about 8 inches long and opens at the end of the head of the penis.

The length of a male’s urethra, and the fact it contains a number of bends, makes catheterisation more difficult.

The urethral sphincter is a collective name for the muscles used to control the flow of urine from the urinary bladder. These muscles surround the urethra, so that when they contract, the urethra is closed.

There are two distinct areas of muscle: the internal sphincter, at the bladder neck and the external, or distal, sphincter.

Human males have much stronger sphincter muscles than females, meaning that they can retain a large amount of urine for twice as long, as much as 800mL, i.e. “hold it”.

Male urinary bladder and urethra

Nephrons

A nephron is the basic structural and functional unit of the kidney. The name nephron comes from the Greek word (nephros) meaning kidney. Its chief function is to regulate water and soluble substances by filtering the blood, reabsorbing what is needed and excreting the rest as urine. Nephrons eliminate wastes from the body, regulate blood volume and pressure, control levels of electrolytes and metabolites, and regulate blood pH. Its functions are vital to life and are regulated by the endocrine system by hormones such as antidiuretic hormone, aldosterone, and parathyroid hormone.

Each nephron has its own supply of blood from two capillary regions from the renal artery. Each nephron is composed of an initial filtering component (the renal corpuscle) and a tubule specialized for reabsorption and secretion (the renal tubule). The renal corpuscle filters out large solutes from the blood, delivering water and small solutes to the renal tubule for modification.

Nephron structure

Glomerulus

The glomerulus is a capillary tuft that receives its blood supply from an afferent arteriole of the renal circulation. The glomerular blood pressure provides the driving force for fluid and solutes to be filtered out of the blood and into the space made by Bowman’s capsule. The remainder of the blood not filtered into the glomerulus passes into the narrower efferent arteriole. It then moves into the vasa recta, which are collecting capillaries intertwined with the convoluted tubules through the interstitial space, where the reabsorbed substances will also enter. This then combines with efferent venules from other nephrons into the renal vein, and rejoins with the main bloodstream.

Afferent/Efferent Arterioles

The afferent arteriole supplies blood to the glomerulus. A group of specialized cells known as juxtaglomerular cells are located around the afferent arteriole where it enters the renal corpuscle. The efferent arteriole drains the glomerulus. Between the two arterioles lies specialized cells called the macula densa. The juxtaglomerular cells and the macula densa collectively form the juxtaglomerular apparatus. It is in the juxtaglomerular apparatus cells that the enzyme renin is formed and stored. Renin is released in response to decreased blood pressure in the afferent arterioles, decreased sodium chloride in the distal convoluted tubule and sympathetic nerve stimulation of receptors (beta-adrenic) on the juxtaglomerular cells. Renin is needed to form Angiotensin I and Angiotensin II which stimulate the secretion of aldosterone by the adrenal cortex.

Glomerular Capsule or Bowman’s Capsule

Bowman’s capsule (also called the glomerular capsule) surrounds the glomerulus and is composed of visceral (simple squamous epithelial cells) (inner) and parietal (simple squamous epithelial cells) (outer) layers. The visceral layer lies just beneath the thickened glomerular basement membrane and is made of podocytes which send foot processes over the length of the glomerulus. Foot processes interdigitate with one another forming filtration slits that, in contrast to those in the glomeruluar endothelium, are spanned by diaphragms. The size of the filtration slits restricts the passage of large molecules (eg, albumin) and cells (eg, red blood cells and platelets). In addition, foot processes have a negatively-charged coat (glycocalyx) that limits the filtration of negatively-charged molecules, such as albumin. This action is called electrostatic repulsion.

The parietal layer of Bowman’s capsule is lined by a single layer of squamous epithelium. Between the visceral and parietal layers is Bowman’s space, into which the filtrate enters after passing through the podocytes’ filtration slits. It is here that smooth muscle cells and macrophages lie between the capillaries and provide support for them. Unlike the visceral layer, the parietal layer does not function in filtration. Rather, the filtration barrier is formed by three components: the diaphragms of the filtration slits, the thick glomerular basement membrane, and the glycocalyx secreted by podocytes. 99% of glomerular filtrate will ultimately be reabsorbed.

The process of filtration of the blood in the Bowman’s capsule is ultrafiltration (or glomerular filtration), and the normal rate of filtration is 125 ml/min, equivalent to ten times the blood volume daily. Measuring the glomerular filtration rate (GFR) is a diagnostic test of kidney function. A decreased GFR may be a sign of renal failure. Conditions that can effect GFR include: arterial pressure, afferent arteriole constriction, efferent arteriole constriction, plasma protein concentration and colloid osmotic pressure.

Any proteins that are roughly 30 kilodaltons or under can pass freely through the membrane. Although, there is some extra hindrance for negatively charged molecules due to the negative charge of the basement membrane and the podocytes. Any small molecules such as water, glucose, salt (NaCl), amino acids, and urea pass freely into Bowman’s space, but cells, platelets and large proteins do not. As a result, the filtrate leaving the Bowman’s capsule is very similar to blood plasma in composition as it passes into the proximal convoluted tubule. Together, the glomerulus and Bowman’s capsule are called the renal corpuscle.

Kidney Anatomy

The kidneys are dark-red, bean-shaped organs. One side of the kidney bulges outward (convex) and the other side is indented (concave). There is a cavity attached to the indented side of the kidney, called the renal pelvis, which extends into the ureter.

Each kidney is enclosed in a transparent membrane called the renal capsule… which helps to protect them against infections and trauma. The kidney is divided into two main areas: a light outer area called the renal cortex, and a darker inner area called the renal medulla. Within the medulla there are 8 or more cone-shaped sections known as renal pyramids. The areas between the pyramids are called renal columns.

Kidney Anatomy and Excretion

The most basic structures of the kidneys, are nephrons. Inside each kidney there are about one million of these microscopic structures. They are responsible for filtering the blood. removing waste products.

The renal artery delivers blood to the kidneys each day. Over 180 liters of blood pass through the kidneys every day. When this blood enters the kidneys it is filtered and returned to the heart via the renal vein.

The process of separating wastes from the body fluids and eliminating them, is known as excretion. The body has four organ systems which are responsible for excretion. The urinary system is one of the organ systems responsible for excretion. It excretes a broad variety of metabolic wastes, toxins, drugs, hormones, salts, hydrogen irons, and water. The kidneys are the main organs of the urinary system.

Kidney Anatomy and Blood Vessels

The kidneys receive blood from the renal arteries, left and right, which branch directly from the abdominal aorta. Despite their relatively small size, the kidneys receive approximately 20% of the cardiac output.

Each renal artery branches into segmental arteries, dividing further into interlobar arteries which penetrate the renal capsule and extend through the renal columns between the renal pyramids. The interlobar arteries then supply blood to the arcuate arteries that run through the boundary of the cortex and the medulla. Each arcuate artery supplies several interlobular arteries that feed into the afferent arterioles that supply the glomeruli.

After filtration occurs the blood moves through a small network of venules that converge into interlobular veins. As with the arteriole distribution the veins follow the same pattern, the interlobular provide blood to the arcuate veins then back to the interlobar veins which come to form the renal vein exiting the kidney for transfusion for blood.

Kidney Location

The normal kidney location is towards the back of the abdominal cavity, just above the waist. If you put your hands on your hips, your kidneys are located just about where your thumbs are. One kidney is normally located just below the liver, on the right side of the abdomen and the other is just below the spleen on the left side.

In rare cases, however, one or both kidneys may be located much lower in the abdomen. This is not necessarily a problem except probably in the case of pregnancy. As the fetus begins to develop in the womb this could sometimes place pressure on the kidney which is located in the lower abdomen.

Normal Kidney size

The normal kidney size of an adult human is about 10 to 13 cm long and about 5 to 7.5 cm wide. It is approximately the size of a computer mouse.

A kidney weighs approximately 150 grams. Kidneys weigh about 0.5 percent of total body weight. The human kidney anatomy, though relatively simple, enables it to perform extremely complex but essential functions. If any area of the kidney is damaged or becomes diseased, this could significantly affect its ability to perform these functions.

Kidney Functions

The kidneys perform a wide range of vital functions in the healthy body, such as:

· Removing wastes and water from the blood

· Balancing chemicals in your body

· Releasing hormones

· Helping control blood pressure

· Helping to produce red blood cells

· Producing vitamin D, which keeps the bones strong and healthy

· Filtering the blood

· The kidneys remove wastes and excess water (fluid) collected by, and carried in, the blood as it flows through the body.

· About 190 litres of blood enter the kidneys every day via the renal arteries. Millions of tiny filters inside the kidneys separate wastes and water from the blood.

· Most of these unwanted substances come from what we eat and drink. The kidneys automatically remove the right amount of salt and other minerals from the blood to leave just the quantities the body needs.

· The cleansed blood returns to the heart and recirculates through the body.

· Excess wastes and fluid leave the kidneys in the form of urine. Urine is stored in the bladder until it is full and then leaves the body via the urethra. Most people pass about 2 litres of urine every day.

· Balancing fluid levels

· By removing just the right amount of excess fluid, healthy kidneys maintain what is called the body’s fluid balance.

· In women, fluid content stays at about 55% of total weight. In men, it stays at about 60% of total weight. The kidneys maintain these proportions by balancing the amount of fluid that leaves the body against the amount entering the body.

· Fluid comes into our bodies from what we drink, and from high-liquid foods such as soup. If we drink a lot, healthy kidneys remove the excess fluid and we pass a lot of urine. If we don’t drink much, the kidneys retain fluid and we don’t pass much urine.

· Fluid also leaves the body through sweat, breath, and feces. If the weather is hot and we lose a lot of fluid by sweating, then the kidneys will not pass so much urine.

· As your kidneys fail, maintaining this balance becomes more difficult. You may suffer symptoms of too much fluid. You may need to watch your diet and what you drink to maintain fluid balance.

· Helping control blood pressure

· One of the important functions of the kidneys is to regulate blood pressure.

· Healthy kidneys make hormones such as renin and angiotensin. These hormones regulate how much sodium (salt) and fluid the body keeps, and how well the blood vessels can expand and contract. This, in turn, helps control blood pressure.

· They do this by regulating:

· The amount of water in the body. If there is too much water in the body (fluid overload) blood pressure will go up. If there is too little water in the body (dehydration) the blood pressure will drop.

· The width of the arteries. The arteries constantly change in width as blood flows through them. The narrower the arteries, the higher the blood pressure. Renin helps control narrowing of the arteries. Failing kidneys often make too much renin. This raises blood pressure. If your blood pressure is high, your heart is working harder thaormal to pump blood through your body.

· High blood pressure (also called hypertension) caused by a breakdown in these functions is common in people with kidney failure. It is also a complication, a secondary condition caused by kidney failure.

· Helping make red blood cells

· Healthy kidneys produce a hormone known as erythropoeitin (EPO), which is carried in the blood to the bone marrow where it stimulates the production of red blood cells. These cells carry oxygen throughout the body. Without enough healthy red blood cells you develop anaemia, a condition which makes you feel weak, cold, tired, and short of breath.

· Maintaining healthy strong bones

· Healthy kidneys keep bones strong by producing the hormone calcitriol. Calcitriol maintains the right levels of calcium and phosphate in the blood and bones. Calcium and phosphate balance are important to keep bones healthy.

· When the kidneys fail they may not produce enough calcitriol. This leads to abnormal levels of phosphate, calcium, and vitamin D, causing renal bone disease.

The urinary system or renal system is the organ system that produces, stores, and eliminates urine. In humans it includes two kidneys, two ureters, the bladder and the urethra. The female and male urinary system are very similar, differing only in the length of the urethra.

1. Human urinary system: 2. Kidney, 3. Renal pelvis, 4. Ureter, 5. Urinary bladder, 6. Urethra. (Left side with frontal section)

7. Adrenal gland

Vessels: 8. Renal artery and vein, 9. Inferior vena cava, 10. Abdominal aorta, 11. Common iliac artery and vein

With transparency: 12. Liver, 13. Large intestine, 14. Pelvis

The order of impurities being excreted from the kidneys: Kidneys → Ureters → Urinary Bladder → Urethra

Physiology of urinary system

Kidney

The kidneys are bean-shaped organs that lie in the abdomen, retroperitoneal to the organs of digestion, around or just below the ribcage and close to the lumbar spine. The organ is about the size of a human fist and is surrounded by what is called perinephric fat. Situated on the superior pole of each kidney is one of the pair of adrenal glands. The kidneys receive their blood supply of 1.25 L/min (25% of the cardiac output) from the renal arteries which are fed by the abdominal aorta. This is important because the kidneys’ main role is to filter water soluble waste products from the blood. The other attachment of the kidneys are at their functional endpoints the ureters, which lie more mediallu and run down to the trigone of the urinary bladder. The kidneys perform a number of tasks, such as: concentrating urine, regulating electrolytes, and maintaining acid-base homeostasis. The kidney excretes and re-absorbs electrolytes (e.g. sodium, potassium and calcium) under the influence of local and systemic hormones. pH balance is regulated by the excretion of bound acids and ammonium ions. In addition, they remove urea, a nitrogenous waste product from the metabolism of amino acids. The end point is a hyperosmolar solution carrying waste for storage in the bladder prior to urination.

Humans produce about 2.9 liters of urine over 24 hours, although this amount may vary according to circumstances. Because the rate of filtration at the kidney is proportional to the glomerular filtration rate, which is in turn related to the blood flow through the kidney, changes in body fluid status can affect kidney function. Hormones exogenous and endogenous to the kidney alter the amount of blood flowing through the glomerulus. Some medications interfere directly or indirectly with urine production. Diuretics achieve this by altering the amount of absorbed or excreted electrolytes or osmalites, which causes a diuresis.

The kidneys are organs that serve several essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and regulation of blood pressure (via maintaining salt and water balance). They serve the body as a natural filter of the blood, and remove wastes which are diverted to the urinary bladder. In producing urine, the kidneys excrete wastes such as urea and ammonium, and they are also responsible for the reabsorption of water, glucose, and amino acids. The kidneys also produce hormones including calcitriol, erythropoietin, and the enzyme renin.

Located at the rear of the abdominal cavity in the retroperitoneum, the kidneys receive blood from the paired renal arteries, and drain into the paired renal veins. Each kidney excretes urine into a ureter, itself a paired structure that empties into the urinary bladder.

Renal physiology is the study of kidney function, while nephrology is the medical specialty concerned with kidney diseases. Diseases of the kidney are diverse, but individuals with kidney disease frequently display characteristic clinical features. Common clinical conditions involving the kidney include the nephritic and nephrotic syndromes, renal cysts, acute kidney injury, chronic kidney disease, urinary tract infection, nephrolithiasis, and urinary tract obstruction. Various cancers of the kidney exist; the most common adult renal cancer is renal cell carcinoma. Cancers, cysts, and some other renal conditions can be managed with removal of the kidney, or nephrectomy. When renal function, measured by glomerular filtration rate, is persistently poor, dialysis and kidney transplantation may be treatment options. Although they are not severely harmful, kidney stones can be painful and a nuisance. The removal of kidney stones involves ultrasound treatment to break up the stones into smaller pieces, which are then passed through the urinary tract. One common symptom of kidney stones is a sharp pain in the medial/lateral segments of the lower back..

Human kidneys viewed from behind with spine removed.

Latin Ren (Greek: nephros)

Artery – renal artery

Vein – renal vein

Nerve – renal plexus

Anatomy

Location

A CT scan in which the kidneys are shown.

In humans the kidneys are located in the abdominal cavity, more specifically in the paravertebral gutter and lie in a retroperitoneal position at a slightly oblique angle. There are two, one on each side of the spine. The asymmetry within the abdominal cavity caused by the liver typically results in the right kidney being slightly lower than the left, and left kidney being located slightly more medial than the right. The left kidney is approximately at the vertebral level T12 to L3, and the right slightly lower. The right kidney sits just below the diaphragm and posterior to the liver, the left below the diaphragm and posterior to the spleen. Resting on top of each kidney is an adrenal gland. The upper (cranial) parts of the kidneys are partially protected by the eleventh and twelfth ribs, and each whole kidney and adrenal gland are surrounded by two layers of fat (the perirenal and pararenal fat) and the renal fascia. Each adult kidney weighs between 125 and 170 grams in males and between 115 and 155 grams in females. The left kidney is typically slightly larger than the right kidney.

Surface projections of the organs of the trunk, showing kidneys at the level of T12 to L3.

Blood supply

Histology

Microscopic photograph of the renal medulla.

Microscopic photograph of the renal cortex.

Renal histology studies the structure of the kidney as viewed under a microscope. Various distinct cell types occur in the kidney, including:

· Kidney glomerulus parietal cell

· Kidney glomerulus podocyte

· Kidney proximal tubule brush border cell

· Loop of Henle thin segment cell

· Thick ascending limb cell

· Kidney distal tubule cell

· Kidney collecting duct cell

· Interstitial kidney cells

· Renal arteries and their branches

The renal artery enters into the kidney at the level of first lumbar vertebra just below the superior mesenteric artery. As it enters the kidney it divides into branches: first the segmental artery, which divides into 2 or 3 lobar arteries, then further divides into interlobar arteries, which further divide into the arcuate artery which leads into the interlobular artery, which form afferent arterioles. The afferent arterioles form the glomerulus (network of capillaries closed in Bowman’s capsule). From here, efferent arterioles leaves the glomerulus and divide into peritubular capillaries, which drain into the interlobular veins and then into arcuate vein and then into interlobar vein, which runs into lobar vein, which opens into the segmental vein and which drains into the renal vein, and then from it blood moves into the inferior vena cava.

Innervation

The kidney and nervous system communicate via the renal plexus, whose fibers course along the renal arteries to reach each kidney. Input from the sympathetic nervous system triggers vasoconstriction in the kidney, thereby reducing renal blood flow. The kidney also receives input from the parasympathetic nervous system, by way of the renal branches of the vagus nerve (cranial nerve X); the function of this is yet unclear. Sensory input from the kidney travels to the T10-11 levels of the spinal cord and is sensed in the corresponding dermatome. Thus, pain in the flank region may be referred from corresponding kidney.

The kidney participates in whole-body homeostasis, regulating acid-base balance, electrolyte concentrations, extracellular fluid volume, and regulation of blood pressure. The kidney accomplishes these homeostatic functions both independently and in concert with other organs, particularly those of the endocrine system. Various endocrine hormones coordinate these endocrine functions; these include renin, angiotensin II, aldosterone, antidiuretic hormone, and atrial natriuretic peptide, among others.

Many of the kidney’s functions are accomplished by relatively simple mechanisms of filtration, reabsorption, and secretion, which take place in the nephron. Filtration, which takes place at the renal corpuscle, is the process by which cells and large proteins are filtered from the blood to make an ultrafiltrate that eventually becomes urine. The kidney generates 180 liters of filtrate a day, while reabsorbing a large percentage, allowing for the generation of only approximately 2 liters of urine. Reabsorption is the transport of molecules from this ultrafiltrate and into the blood. Secretion is the reverse process, in which molecules are transported in the opposite direction, from the blood into the urine.

Excretion of wastes

The kidneys excrete a variety of waste products produced by metabolism. These include the nitrogenous wastes called “urea”, from protein catabolism, as well as uric acid, from nucleic acid metabolism. Formation of urine is also the function of the kidney. The concentration of nitrogenous wastes, in the urine of mammals and some birds, is dependent on an elaborate countercurrent multiplication system. This requires several independent nephron characteristics to operate: a tight hair pin configuration of the tubules, water and ion permeability in the descending limb of the loop, water impermeability in the ascending loop and active ion transport out of most of the ascending loop. In addition, countercurrent exchange by the vessels carrying the blood supply to the nephron is essential for enabling this function.

Acid-base homeostasis

Two organ systems, the kidneys and lungs, maintain acid-base homeostasis, which is the maintenance of pH around a relatively stable value. The lungs contribute to acid-base homeostasis by regulating carbon dioxide (CO2) concentration. The kidneys have two very important roles in maintaining the acid-base balance: to reabsorb bicarbonate from urine, and to excrete hydrogen ions into urine

Osmolality regulation

Any significant rise in plasma osmolality is detected by the hypothalamus, which communicates directly with the posterior pituitary gland. An increase in osmolality causes the gland to secrete antidiuretic hormone (ADH), resulting in water reabsorption by the kidney and an increase in urine concentration. The two factors work together to return the plasma osmolality to its normal levels.

ADH binds to principal cells in the collecting duct that translocate aquaporins to the membrane, allowing water to leave the normally impermeable membrane and be reabsorbed into the body by the vasa recta, thus increasing the plasma volume of the body.

There are two systems that create a hyperosmotic medulla and thus increase the body plasma volume: Urea recycling and the ‘single effect.’

Urea is usually excreted as a waste product from the kidneys. However, when plasma blood volume is low and ADH is released the aquaporins that are opened are also permeable to urea. This allows urea to leave the collecting duct into the medulla creating a hyperosmotic solution that ‘attracts’ water. Urea can then re-enter the nephron and be excreted or recycled again depending on whether ADH is still present or not.

The ‘Single effect’ describes the fact that the ascending thick limb of the loop of Henle is not permeable to water but is permeable to NaCl. This allows for a countercurrent exchange system whereby the medulla becomes increasingly concentrated, but at the same time setting up an osmotic gradient for water to follow should the aquaporins of the collecting duct be opened by ADH.

Blood pressure regulation

Although the kidney cannot directly sense blood, long-term regulation of blood pressure predominantly depends upon the kidney. This primarily occurs through maintenance of the extracellular fluid compartment, the size of which depends on the plasma sodium concentration. Renin is the first in a series of important chemical messengers that make up the renin-angiotensin system. Changes in renin ultimately alter the output of this system, principally the hormones angiotensin II and aldosterone. Each hormone acts via multiple mechanisms, but both increase the kidney’s absorption of sodium chloride, thereby expanding the extracellular fluid compartment and raising blood pressure. When renin levels are elevated, the concentrations of angiotensin II and aldosterone increase, leading to increased sodium chloride reabsorption, expansion of the extracellular fluid compartment, and an increase in blood pressure. Conversely, when renin levels are low, angiotensin II and aldosterone levels decrease, contracting the extracellular fluid compartment, and decreasing blood pressure.

Hormone secretion

The kidneys secrete a variety of hormones, including erythropoietin, and the enzyme renin. Erythropoietin is released in response to hypoxia (low levels of oxygen at tissue level) in the renal circulation. It stimulates erythropoiesis (production of red blood cells) in the bone marrow. Calcitriol, the activated form of vitamin D, promotes intestinal absorption of calcium and the renal reabsorption of phosphate. Part of the renin-angiotensin-aldosterone system, renin is an enzyme involved in the regulation of aldosterone levels.

Development

The mammalian kidney develops from intermediate mesoderm. Kidney development, also called nephrogenesis, proceeds through a series of three successive phases, each marked by the development of a more advanced pair of kidneys: the pronephros, mesonephros, and metanephros.

Evolutionary adaptation

Kidneys of various animals show evidence of evolutionary adaptation and have long been studied in ecophysiology and comparative physiology. Kidney morphology, often indexed as the relative medullary thickness, is associated with habitat aridity among species of mammals.

Related terms

Medical terms related to the kidneys commonly use terms such as renal and the prefix nephro-. The adjective renal, meaning related to the kidney, is from the Latin rēnēs, meaning kidneys; the prefix nephro- is from the Ancient Greek word for kidney, nephros (νεφρός). For example, surgical removal of the kidney is a nephrectomy, while a reduction in kidney function is called renal dysfunction.

Diseases and disorders

· Congenital

· Congenital hydronephrosis

· Congenital obstruction of urinary tract

· Duplex kidneys, or double kidneys, occur in approximately 1% of the population. This occurrence normally causes no complications, but can occasionally cause urine infections.

· Duplicated ureter occurs in approximately one in 100 live births

· Horseshoe kidney occurs in approximately one in 400 live births

· Polycystic kidney disease

· Autosomal dominant polycystic kidney disease afflicts patients later in life. Approximately one in 1000 people will develop this condition

· Autosomal recessive polycystic kidney disease is far less common, but more severe, than the dominant condition. It is apparent in utero or at birth.

· Renal agenesis. Failure of one kidney to form occurs in approximately one in 750 live births. Failure of both kidneys to form is invariably fatal.

· Renal dysplasia

· Unilateral small kidney

· Multicystic dysplastic kidney occurs in approximately one in every 2400 live births

· Ureteropelvic Junction Obstruction or UPJO; although most cases appear congenital, some appear to be an acquired condition

Acquired

Drawing of an enlarged kidney by John Hunter.

· Diabetic nephropathy

· Glomerulonephritis

· Hydronephrosis is the enlargement of one or both of the kidneys caused by obstruction of the flow of urine.

· Interstitial nephritis

· Kidney stones (nephrolithiasis) are a relatively common and particularly painful disorder.

· Kidney tumors

· Wilms tumor

· Renal cell carcinoma

· Lupus nephritis

· Minimal change disease

· Iephrotic syndrome, the glomerulus has been damaged so that a large amount of protein in the blood enters the urine. Other frequent features of the nephrotic syndrome include swelling, low serum albumin, and high cholesterol.

· Pyelonephritis is infection of the kidneys and is frequently caused by complication of a urinary tract infection.

· Renal failure

· Acute renal failure

· Stage 5 Chronic Kidney Disease

Kidney Failure

Generally, humans can live normally with just one kidney, as one has more functioning renal tissue than is needed to survive. Only when the amount of functioning kidney tissue is greatly diminished does one develop chronic kidney disease. Renal replacement therapy, in the form of dialysis or kidney transplantation, is indicated when the glomerular filtration rate has fallen very low or if the renal dysfunction leads to severe symptoms. 3D-rendered computed tomography, showing renal arteries and veins.

The interstitum (or interstitium) is the functional space in the kidney beneath the individual filters (glomeruli) which are rich in blood vessels. The interstitum absorbs fluid recovered from urine. Various conditions can lead to scarring and congestion of this area, which can cause kidney dysfunction and failure.

Kidney.

Kidneys.

Right Kidney.

Left kidney.

Kidney Posterior View.

Anterior relation of Left Kidney.

Diseases of the Kidney

Diabetic nephropathy (nephropatia diabetica), also known as Kimmelstiel-Wilson syndrome and intercapillary glomerulonephritis, is a progressive kidney disease caused by angiopathy of capillaries in the kidney glomeruli. It is characterized by nodular glomerulosclerosis. It is due to longstanding diabetes mellitus, and is a prime cause for dialysis in many Western countries.

In medicine, hematuria (or “haema-turia”) is the presence of blood in the urine. It is a sign of a large number of diseases of the kidneys and the urinary tract, ranging from trivial to lethal.

Kidney stones, also known as nephrolithiases, urolithiases or renal calculi, are solid accretions (crystals) of dissolved minerals in urine found inside the kidneys or ureters. They vary in size from as small as a grain of sand to as large as a golf ball. Kidney stones typically leave the body in the urine stream; if they grow relatively large before passing (on the order of millimeters), obstruction of a ureter and distention with urine can cause severe pain most commonly felt in the flank, lower abdomen and groin. Kidney stones are unrelated to gallstones.

Case Study. I was 34 weeks pregnant when I noticed blood in my urine. I immediately went to my OBGYN where I was told that I had a bladder infection and given an antibiotic. The next morning I experienced the most intense pain. I was rushed to the ER where I was told that I had kidney stones. The doctors explained that there was nothing they could do as long as I was pregnant. The next 3 weeks of my life were filled with intense pain and multiple painkillers. After I delivered my baby, CAT scans were done and I was informed that I had 6 kidney stones. It took three more weeks for me to pass all of the stones the largest measuring 5 mm. The stones were tested and I was informed that my body had been building up calcium due to my pregnancy and this was the cause of the kidney stones. I continued to have kidney pain for 6 months after passing the stones. I now live my life on a low calcium diet and the hope that my body will not develop more kidney stones.

Pyelonephritis.

An image of Pyelonephritis.

When an infection of the renal pelvis and calices, called pyelitis, spreads to involve the rest of the kidney as well, the result is pyelonephritis. It usually results from the spread of fecal bacterium Escherichia coli from the anal region superiorly through the urinary tract. In severe cases, the kidney swells and scars, abscesses form, and the renal pelvis fills with pus. Left untreated, the infected kidney may be severely damaged, but administration of antibiotics usually achieve a total cure.

Glomerulonephritis. Inflammation of the glomerular can be caused by immunologic abnormalities, drugs or toxins, vascular disorders, and systemic diseases. Glomerulonephritis can be acute, chronic or progressive. Two major changes in the urine are distinctive of glomerulonephritis: hematuria and proteinuria with albumin as the major protein. There is also a decrease in urine as there is a decrease in GFR (glomerular filtration rate). Renal failure is associated with oliguria (less than 400 ml of urine output per day).

Renal Failure. Uremia is a syndrome of renal failure and includes elevated blood urea and creatinine levels. Acute renal failure can be reversed if diagnosed early. Acute renal failure can be caused by severe hypotension or severe glomerular disease. Diagnostic tests include BUN and plasma creatinine level tests. It is considered to be chronic renal failure if the decline of renal function to less than 25%.

Diabetes Insipidus. This is caused by the deficiency of or decrease of ADH. The person with (DI) has the inability to concentrate their urine in water restriction, in turn they will void up 3 to 20 liters/day. There are two forms of (DI), neurogenic, and nephrogenic. Iephrogenic (DI) the kidneys do not respond to ADH. Usually the nephrogenic (DI) is characterized by the impairment of the urine concentrating capability of the kidney along with concentration of water. The cause may be a genetic trait, electrolyte disorder, or side effect of drugs such as lithium. In the neurogenic (DI), it is usually caused by head injury near the hypophysisal tract.

Urinary tract infections (UTI’s)

The second most common type of bacterial infections seen by health care providers is UTI’s. Out of all the bacterias that colonize and cause urinary tract infections the big gun is Escherichia coli. In the hospital indwelling catheters and straight catheterizing predispose the opportunity for urinary tract infections. In females there are three stages in life that predispose urinary tract infections, that is menarche, manipulation between intercourse, and menopause. However, a small percentage of men and children will get urinary tract infections. In men it is usually due to the prostate gland growth which usually occurs in older age men. In children it can occur 3% to 5% in girls and 1% in boys, uncircumcised boys it is more common than circumcised ones to have a urinary tract infection, in girls it may be the result of onset of toilet training, some predispositions for getting urinary tract infection include family history and urinary tract anomalies. Ieonates urinary tract infections is most common when bacteremia is present.

Symptoms of Urinary Tract Infection:

There are several typical symptoms that signify that you may have a Urinary Tract Infection.

· Upper back and side pain

· Fever

· Nausea and vomiting

· Persistent urge to urinate

· Burning sensation when you are urinating

· Passing urine frequently, yet with only very small amounts

· Blood in the urine

Causes of Urinary Tract Infection:

Typically a Urinary Tract Infection occurs when the bacteria enter the urinary tract through the urethra and then enter the bladder where they begin to multiply. Although we have defences to keep bacteria out, it doesn’t always work, and an infection occurs. An Infection of the bladder (Cystitis) is often caused by E.Coli bacteria that are found often in the gastrointestinal tract. Sexual intercourse can lead to an infection of the bladder. An infection of the urethra (urethritis) can be a result of the female urethra being so close to the vagina. This leads to bacteria moving from the anus to the urethra. There are also sexually transmitted diseases which can lead to urethritis, such as chlamydia and gonorrhea.

Treatment of Urinary Tract Infections:

Urinary Tract Infections can be treated easily, but if you do not treat them, they can lead to complications. In fact, they can lead to acute or chronic kidney infections which could cause permanent damage to your kidneys. This is especially important for young people, older adults, and pregnant women. When you go in for treatment, you will be able to see your general practitioner, but if you have frequent Urinary Tract Infections, you may be referred to a Urologist, who deals with urinary disorders, or a nephrologist, a doctor who deals with kidney disorders. Your doctor may ask you for a urine sample to look for red blood cells, bacteria, or pus in your urine. The urine test will tell you if you have an infection.

Typically for a simple Urinary Tract Infection, you will be prescribed antibiotics. These antiobiotics may include Ciprofloxacin, Nitrofurantoin, Levofloxacin, Sulfamethoxazole-trimethoprim, or Amoxicillin. Even though your symptoms will probably begin to clear up quickly within a few days of starting the antibiotics, do not stop taking them. You need to take the entire course of antibiotics so that you do not risk the continuation of the infection. It is also possible that your doctor may prescribe an analgesic. An analgesic will numb your bladder and urethra so that you do not feel the burning when you are urinating. Do not be alarmed if your urine is bright blue or orange. This is a possible result of the analgesic prescribed by your doctor. If your infection is a recurrent infection, however, your doctor may need to prescribe a longer set of antibiotics.

There are also other options that your doctor may choose if you have recurrent infections. One is that a doctor may recommend you taking a single dose of antibiotics after you have sexual intercourse. Another option, if you are postmenopausal, is to have vaginal estrogen therapy to lower your chance of continuing with more Urinary Tract Infections. In very rare, but very serious cases, hospitalization and treatment with intravenous antibiotics may be needed.

There are also some steps you can take to help you both prevent Urinary Tract Infections and deal with Urinary Tract Infections. It is important to drink plenty of water to help you flush out bacteria. You should also try to avoid the following drinks: alcohol, coffee, soft drinks, citrus juices, and caffeine until your Urinary Tract Infection is better. If you have bladder pressure or discomfort, using a heating pad on your abdomen may help ease your pain. Prevention includes drinking cranberry juice, which may have infection fighting ability. However, do not drink cranberry juice if you are taking Warfarin, a blood-thinning medication. Second, women should be sure to wipe from front to back to avoid moving bacteria from the anus to the urethra. Third, it is suggested that you empty your bladder immediately after intercourse and drink a glass of water. This will help to flush bacteria. Finally, women should avoid irritating feminine products, such as douches, powder, or deodorant sprays because they can irritate the urethra.

Dialysis and Kidney Transplant

Plugged into dialysis

Generally, humans can live normally with just one kidney. Only when the amount of functioning kidney tissue is greatly diminished will renal failure develop. If renal function is impaired, various forms of medications are used, while others are contraindicated. Provided that treatment is begun early, it may be possible to reverse chronic kidney failure due to diabetes or high blood pressure. If creatinine clearance (a measure of renal function) has fallen very low (“end-stage renal failure”), or if the renal dysfunction leads to severe symptoms, dialysis is commenced. Dialysis is a medical procedure, performed in various different forms, where the blood is filtered outside of the body.

Kidney Dialisis

Kidney transplantation is the only cure for end stage renal failure; dialysis, is a supportive treatment; a form of “buying time” to bridge the inevitable wait for a suitable organ.

The first successful kidney transplant was announced on March 4, 1954 at Peter Bent Brigham Hospital in Boston. The surgery was performed by Dr. Joseph E. Murray, who was awarded the Nobel Prize in Medicine in 1990 for this feat.

There are two types of kidney transplants: living donor transplant and a cadaveric (dead donor) transplant. When a kidney from a living donor, usually a blood relative, is transplanted into the patient’s body, the donor’s blood group and tissue type must be judged compatible with the patient’s, and extensive medical tests are done to determine the health of the donor. Before a cadaveric donor’s organs can be transplanted, a series of medical tests have to be done to determine if the organs are healthy. Also, in some countries, the family of the donor must give its consent for the organ donation. In both cases, the recipient of the new orgaeeds to take drugs to suppress their immune system to help prevent their body from rejecting the new kidney.

Glossary

Antidiuretic: lessening or decreasing of urine production or an agent that decreases the release of urine.

Catheterisation: a catheter is a tube that can be inserted into a body cavity, duct or vessel. Catheters thereby allow drainage or injection of fluids or access by surgical instruments. The process of inserting a catheter is catheterisation. In most uses a catheter is a thin, flexible tube: a “soft” catheter; in some uses, it is a larger, solid tube: a “hard” catheter.

Dehydration: condition resulting from excessive loss of body fluid.

Diabetes: a general term for a disease characterized by the begining stages and onset of renal failure. It is derived from the Greek word diabaнnein, that literally means “passing through,” or “siphon”, a reference to one of diabetes’ major symptoms—excessive urine production.

Diuresis: secretion and passage of large amounts of urine.

Diuretic: increasing of urine production, or an agent that increases the production of urine.

Erythropoietin: hormone that stimulates stem cells in the bone marrow to produce red blood cells

Fibrous Capsule: the kidney’s loose connective tissue

Glomerulus: capillary tuft that receives its blood supply from an afferent arteriole of the renal circulation.

Gluconeogenesis: the cycle of producing a glucose form fat or protein; preformed by the kidney in times of long fasting, initially gluconeogenesis is preformed by the liver

Juxtaglomerular (JG) cells: Renin-secreting cells that are in contact with the macula densa and the afferent arterioles of the renal nephron.

Juxtaglomerular apparatus (JGA): A site of juxtaglomerular cells connecting with the macula densa where renin is secreted and sensor for control of secretion of golmerular filtration rate.

Loop of Henle/ Nephron Loop: u-shaped tube that consists of a descending limb and ascending limb; primary role is to concentrate the salt in the interstitium, the tissue surrounding the loop

Medullary Pyramids or Renal Pyramids: the cone shaped masses in the kidney

Micturition: another name for excretions

Nephron: basic structural and functional unit of the kidney; chief function is to regulate water and soluble substances by filtering the blood, reabsorbing what is needed and excreting the rest as urine

Podocytes: filtration membrane, in the visceral layer of the bowman’s capsule

Renal Calculi: kidney stones, solid crystals of dissolved minerals in urine found inside the kidneys

Renal Cortex: outer portion of the kidney

Renal Lobe: each pyramid together with the associated overlying cortex

Renal Pelvis: a central space, or cavity that transmits urine to the urinary bladder via the ureter

Renin: hormone released by the Juxtaglomerular (JG) cells of the kidneys when blood pressure falls

TURP: transurethral resection of the prostate. During TURP, an instrument is inserted up the urethra to remove the section of the prostate that is blocking urine flow. This is most commonly caused by benign prostatic hyperplasia (BPH). A TURP usually requires hospitalization and is done using a general or spinal anesthetic. It is now the most common surgery used to remove part of an enlarged prostate.

Urethra: a muscular tube that connects the bladder with the outside of the body

Ureters: two tubes that drain urine from the kidneys to the bladder

Urine: liquid produced by the kidneys, collected in the bladder and excreted through the urethra

Urinary Bladder: a hollow, muscular and distensible or elastic organ that sits on the pelvic floor

Urinary System: a group of organs in the body concerned with filtering out excess fluid and other substances from the bloodstream

Locate the cortex of the kidney and scan over the tissue at low magnification. Note the presence of numerous glomeruli and the apparent absence of any preferred orientation of the tubules visible between the glomeruli (convoluted parts of proximal and distal tubuli). You should be able to identify the vascular pole of a good glomerulus by the attachment of the capillary tuft to the wall of the glomerulus. What would make your glomerulus VERY good would be the presence of a tubulus which contains a dense row of nuclei in the part of its wall closest to the vascular pole of the glomerulus, the macula densa. The nuclei are located side by side or may even overlap. It should be possible to find this structure in all slides. It is also very likely that it may take you a few minutes of carefully scanning the tissue at high magnification before you will find it. Proximal tubules are characterised by their eosinophilic (pink) low, columnar cells and by large amounts of fuzzy material, which may fill the entire lumen of the tubulus. This fuzzy material represents the remains of the brush border of the cells of the proximal tubules, which is difficult to preserve during the preparation of the tissue.

Tubules of the Nephron

The tubular system can be divided into proximal and distal tubules, which in turn have convoluted and straight portions. Intermediate tubules connect the proximal and distal tubules. Running from the cortex of the kidney towards the medulla (descending), then turning and running back towards the cortex (ascending), the tubules form the loop of Henle.

The proximal tubule is the longest section of the nephron (about 14 mm). The convoluted part of the proximal tubules coils close to the glomerulus in the cortex. The diameter of proximal tubules is ~65 µm. Their walls are formed by a low columnar epithelium. The eosinophilic cells of the epithelium have a wide brush border (long microvilli – What is their function?) and are active in endocytosis. They almost completely resorb substances of nutritional value from the glomerular filtrate (glucose, amino acids, protein, vitamins etc. – Which organelles would you expect to be numerous in proximal tubule cells?). In the proximal tubules the volume of the glomerular filtrate is reduced by about 75%. Sodium ions are actively resorbed from the glomerular filtrate. They are followed by passively diffusing chloride ions and the osmotic absorption of water. The straight portion of the proximal tubule descends towards the medulla.

The straight portion of the proximal tubule merges with the intermediate tubule (thin segment of the loop of Henle). A flattened, only ~1-2 µm high epithelium forms the intermediate tubule, which is only ~15 µm wide. Descending parts of the straight proximal and intermediate tubules are permeable to water but not to solutes.

The thin segment of Henle’s loop leads into the straight part of the distal tubule, which is formed by low cuboidal cells without a brush border. A few short microvilli are present, but they are difficult to see in the light microscope. The diameter of the tubule expands to ~35 µm. Epithelial cells in the ascending parts of the intermediate and straight distal tubules cells transport chloride (active) and sodium ions (passive) out of the tubular lumen into the surrounding peritubular space. The epithelium caot be penetrated by water. Consequently, the transport of ions over the epithelium sets up a gradient in osmotic pressure, which serves as driving force in the further concentration of the urine.

The straight portion of the distal tubule contacts the glomerulus forming the macula densa. Thereafter, the distal tubule forms its convoluted portion (about 5 mm long). Cells in the distal tubulus are sensitive to the hormone aldosterone, which is produced in the zona glomerulosa of the adrenal glands. Aldosterone stimulates the active resorption of sodium ions and the excretion of potassium ions.

The convoluted distal tubule merges, via connecting tubules, with the collecting ducts. In the presence of antidiuretic hormone (ADH), the epithelia of the collecting ducts are permeable to water but not to sodium ions. Osmotic forces move water out of the lumen of the tubules as they pass through the medulla, where cells of the ascending intermediate and straight distal tubules of the loop of Henle have established high concentrations of sodium in the extracellular space.

Collecting ducts merge to form papillary ducts (of Bellini), which terminate on the tips of the renal papilla and empty into a distended, funnel shaped part (minor calyx) of the ureter.

Kidney, human – H&E

Find a good spot in the medulla of the kidney. Preferably a spot in which you are able to identify a collecting ducts (cuboidal to columnar cells, well-defined boundaries between cells, cytoplasm only weakly stained or unstained, large ducts) and an intermediate (very flat epithelium, nuclei bulge into the lumen of the tubulus, diameter of the duct is small) and distal tubule (cuboidal epithelium, cells stain weakly pink). Both transversely or longitudinally cut tubules are suitable. Note that it will be difficult to identify ALL tubules that are visible.

In most of our sections only little medulla is present – try to scan along the margins of the tissue and see if you can find some medulla. If that should not be possible take a look at medullary rays instead, although they will contain few, if any, good thin tubules.

The Juxtaglomerular Apparatus

As mentioned above, the distal tubule contacts the glomerulus forming a specialized section of tubular epithelium, the macula densa. At the point of contact with the glomerulus, the distal tubule is always in close contact with the efferent and afferent arterioles of the glomerulus.

Other parts of the juxtaglomerular apparatus are extraglomerular mesangial cells and the juxtaglomerular cells surrounding the afferent arteriole (modified smooth muscle cells), which produce and secrete renin. Renin activates angiotensinogen, a precursor found in the bloodstream, leading to the formation of angiotensin I, which is converted to angiotensin II. Angiotensin II is the most potent vasoconstrictor known. It also stimulates the secretion of aldosterone.

Different theories exist that try to explain the interactions between the cells that eventually lead to the release of renin. One of them, the baroreceptor theory, assumes that the juxtaglomerular cells function as stretch receptors (high blood pressure would inhibit the release of renin). Another theory, the macula densa theory, claims that the secretion of renin is regulated by the composition of the fluid in the distal tubule and/or the afferent arteriole (low sodium would increase in the release of renin).

Excretory Passages

The minor calyces merge to form major calyces within the kidney, which in turn merge to form the renal pelvis (still within the kidney). The urine flows through these structures to the ureter and is channelled to the bladder.

The basic structure of all these components is the same. The mucosa is lined with a transitional epithelium , which occurs exclusively in the urinary system. The epithelium is virtually impenetrable to any components of the urine , which consequently does not change in composition as it passes through the excretory passages. The lamina propria consists mainly of dense connective tissue, with many bundles of coarse collagenous fibres. The muscularis usually consists of an inner longitudinal and outer circular layer of smooth muscle cells . In lower parts of the ureter and the bladder an additional outer longitudinal layer of muscles is added to the first two.

The bladder is finally emptied through the urethra. Initially, the urethra is lined by a transitional epithelium in males and females. In males, it is replaced by a pseudostratified or stratified columnar epithelium below the openings of the ejaculatory ducts into the urethra. The distal parts of the female urethra and the distal end of the male urethra are lined by a stratified squamous epithelium. The lamina propria contains loose connective tissue. Smooth muscle cells in the muscularis are mainly oriented longitudinally. They are surrounded, in the middle part of the urethra (below the prostate in males), by striated muscle cells of the sphincter urethrae.

VIDEO

Diseases of the Urinary System

REFERENCES:

Baba, T; Murabayashi, S; Tomiyama, T; Takebe, K (1990). “Uncontrolled hypertension is associated with a rapid progression of nephropathy in type 2 diabetic patients with proteinuria and preserved renal function”. The Tohoku journal of experimental medicine 161 (4): 311–8. doi:10.1620/tjem.161.311
C. Dugdale, David (16 September 2011). “Female urinary tract”. MedLine Plus Medical Encyclopedia.
Caldwell HK, Young WS III (2006). “Oxytocin and Vasopressin: Genetics and Behavioral Implications”. In Lajtha A, Lim R. Handbook of Neurochemistry and Molecular Neurobiology: Neuroactive Proteins and Peptides (3rd ed.). Berlin: Springer. pp. 573–607. ISBN 0-387-30348-0.
Maton, Anthea; Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright (1993). Human Biology and Health. Englewood Cliffs, New Jersey, USA: Prentice Hall. ISBN 0-13-981176-1.
Caldwell HK, Young WS III (2006). “Oxytocin and Vasopressin: Genetics and Behavioral Implications”. In Lajtha A, Lim R. Handbook of Neurochemistry and Molecular Neurobiology: Neuroactive Proteins and Peptides (3rd ed.). Berlin: Springer. pp. 573–607. ISBN 0-387-30348-0.
Clapp, WL. “Renal Anatomy”. In: Zhou XJ, Laszik Z, Nadasdy T, D’Agati VD, Silva FG, eds. Silva’s Diagnostic Renal Pathology. New York: Cambridge University Press; 2009.
Kidneys Location Stock Illustration. New York: McGraw-Hill Medical, 2013.
Nielsen S, Chou CL, Marples D, Christensen EI, Kishore BK, Knepper MA (February 1995). “Vasopressin increases water permeability of kidney collecting duct by inducing translocation of aquaporin-CD water channels to plasma membrane”. Proc. Natl. Acad. Sci. U.S.A. 92 (4): 1013–7. doi:10.1073/pnas.92.4.1013. PMC 42627. PMID 7532304.