LESSON 12
Liver, gallbladder.
Pancreas
The Liver
The liver, the largest visceral organ, is one of the most versatile organs in the body. Most of its mass lies within the right hypochondriac and epigastric regions, but it may extend into the left hypochondriac and umbilical regions as well. The liver weighs about
Anatomy of the Liver
The liver is wrapped in a tough fibrous capsule and covered by a layer of visceral peritoneum. On the anterior surface, the falciform ligament marks the division between the left lobe and the right lobe of the liver (Figure 24-19a;){kind=small}
, b;){kind=small}
). A thickening in the posterior margin of the falciform ligament is the round ligament, or ligamentum teres, a fibrous band that marks the path of the fetal umbilical vein.
On the posterior surface of the liver, the impression left by the inferior vena cava marks the division between the right lobe and the small caudate lobe (Figure 24-19c). Inferior to the caudate lobe lies the quadrate lobe, sandwiched between the left lobe and the gallbladder. Afferent blood vessels and other structures reach the liver by traveling within the connective tissue of the lesser omentum. They converge at the hilus of the liver, a region known as the porta hepatis (“doorway to the liver”).
The gallbladder is a muscular sac that stores and concentrates bile prior to its excretion into the small intestine. The gallbladder is located in a recess, or fossa, in the posterior surface of the liver’s right lobe. We shall describe the gallbladder and associated structures in a later section.
The Blood Supply to the Liver
We detailed the circulation to the liver in Chapter 21 and summarized that circulation pattern in Figures 21-27;){kind=small}
and 21-35a;){kind=small}
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.
Roughly one-third of the blood supply to the liver is arterial blood from the hepatic artery. The remainder consists of venous blood from the hepatic portal vein, which begins in the capillaries of the esophagus, stomach, small intestine, and most of the large intestine. We described the distribution and major tributaries of the hepatic portal vein in Chapter 21. Liver cells, called hepatocytes, adjust circulating levels of nutrients by selective absorption and secretion. Blood leaving the liver returns to the systemic circuit through the hepatic veins, which open into the inferior vena cava.
Histological Organization of the Liver
Each lobe of the liver is divided by connective tissue into approximately 100,000 liver lobules, the basic functional units of the liver. The histological organization and structure of a typical liver lobule are shown in Figure 24-20a;){kind=small}
.
The Liver Lobule ;){kind=small}
Adjacent lobules are separated from each other by an interlobular septum. The hepatocytes in a liver lobule form a series of irregular plates arranged like the spokes of a wheel (Figure24-20a,b). The plates are only one cell thick, and exposed hepatocyte surfaces are covered with short microvilli. Within a lobule, sinusoids between adjacent plates empty into the central vein. (We introduced sinusoids in Chapter 21. ) The liver sinusoids lack a basement membrane, so large openings between the endothelial cells allow solutes—even those as large as plasma proteins—to pass out of the circulation and into the spaces surrounding the hepatocytes.
In addition to typical endothelial cells, the sinusoidal lining includes a large number of Kupffer cells, also known as stellate reticuloendothelial cells. These phagocytic cells, part of the monocyte-macrophage system, engulf pathogens, cell debris, and damaged blood cells. Kupffer cells are also responsible for storing (1) iron, (2) some lipids, and (3) heavy metals, such as tin or mercury, that are absorbed by the digestive tract.
Blood enters the liver sinusoids from small branches of the portal vein and hepatic artery. A typical lobule has a hexagonal shape in cross section (Figure 24-20a,b). There are six portal areas, or hepatic triads, one at each corner of the lobule. A portal area contains three structures: (1) a branch of the hepatic portal vein, (2) a branch of the hepatic artery, and (3) a small branch of the bile duct.
Branches from the arteries and veins deliver blood to the sinusoids of adjacent liver lobules (Figure24-20a,b). As blood flows through the sinusoids, hepatocytes absorb solutes from the plasma and secrete materials such as plasma proteins. Blood then leaves the sinusoids and enters the central vein of the lobule. The central veins ultimately merge to form the hepatic veins, which then empty into the inferior vena cava. Liver diseases, such as the various forms of hepatitis, and conditions such as alcoholism can lead to degenerative changes in the liver tissue and constriction of the circulatory supply.
Bile Secretion and Transport. Bile is secreted into a network of narrow channels between the opposing membranes of adjacent liver cells. These passageways, called bile canaliculi, extend outward, away from the central vein. Eventually they connect with fine bile ductules, which carry bile to bile ducts in the nearest portal area. The right and left hepatic ducts collect bile from all the bile ducts of the liver lobes. These ducts unite to form the common hepatic duct, which leaves the liver (Figure 24-21;){kind=small}
). The bile within the common hepatic duct may either (1) flow into the common bile duct, which empties into the duodenal ampulla, or (2) enter the cystic duct, which leads to the gallbladder.
The common bile duct is formed by the union of the cystic duct and the common hepatic duct. The common bile duct passes within the lesser omentum toward the stomach, turns, and penetrates the wall of the duodenum to meet the pancreatic duct at the duodenal ampulla.
The Physiology of the Liver
The liver is responsible for metabolic regulation, hematological regulation, and bile production. The liver has more than 200 different functions; in this discussion, we shall provide only a general overview.
Metabolic Regulation
The liver is the primary organ involved in regulating the composition of your circulating blood. All blood leaving the absorptive surfaces of the digestive tract enters the hepatic portal system and flows into the liver. Liver cells can thus extract absorbed nutrients or toxins from the blood before it reaches the systemic circulation through the hepatic veins. Excess nutrients are removed and stored, and deficiencies are corrected by mobilizing stored reserves or performing synthetic activities. For example:
- Carbohydrate metabolism. The liver stabilizes blood glucose levels at about 90 mg/dl. If blood glucose levels drop, the hepatocytes break down glycogen reserves and release glucose into the circulation. They also synthesize glucose from other carbohydrates or from available amino acids. The synthesis of glucose from other compounds is a process called gluconeogenesis. If blood glucose levels climb, liver cells remove glucose from the circulation and either store it as glycogen or use it to synthesize lipids that can be stored in the liver or other tissues. These metabolic activities are regulated by circulating hormones, such as insulin and glucagon, as we noted in Chapter 18.
- Lipid metabolism. The liver regulates circulating levels of triglycerides, fatty acids, and cholesterol. When those levels decline, the liver breaks down its lipid reserves and releases them into the circulation. When the levels are high, the lipids are removed for storage. However, because most lipids absorbed by the digestive tract bypass the hepatic portal circulation, this regulation occurs only after lipid levels have risen within the general circulation.
- Amino acid metabolism. The liver removes excess amino acids from the circulation. These amino acids may be used to synthesize proteins, or they may be converted to lipids or glucose for storage.
- Removal of waste products. When converting amino acids to lipids or carbohydrates, or when breaking down amino acids to get energy, the liver strips off the amino groups, a process called deamination. This process produces ammonia, a toxic waste product the liver neutralizes by conversion to urea, a relatively harmless compound excreted at the kidneys. Other waste products, circulating toxins, and drugs are also removed from the blood for inactivation, storage, or excretion.
- Vitamin storage. Fat-soluble vitamins (A, D, E, and K) and vitamin B12 are absorbed from the blood and stored in the liver. These reserves are called on when your diet contains inadequate amounts of those vitamins.
- Mineral storage. The liver converts the body’s iron reserves to ferritin and stores this protein-iron complex, as we learned in Chapter 19.
- Drug inactivation. The liver removes and breaks down circulating drugs, thereby limiting the duration of their effects. When they prescribe drugs, physicians must take into account the rate at which the liver removes a particular drug. For example, a drug that is absorbed relatively quickly must be administered every few hours to keep the plasma concentrations at therapeutic levels.
Hematological Regulation
The liver, the largest blood reservoir in your body, receives about 25 percent of the cardiac output. As blood passes by, the liver performs the following functions:
- Phagocytosis and antigen presentation. Kupffer cells in the liver sinusoids engulf old or damaged RBCs, cellular debris, and pathogens from the circulation. Kupffer cells are antigen-presenting cells that can stimulate an immune response.
- Plasma protein synthesis. The hepatocytes synthesize and release most of the plasma proteins. These include the albumins, which contribute to the osmotic concentration of the blood; the various types of transport proteins; clotting proteins; and complement proteins.
- Removal of circulating hormones. The liver is the primary site for the absorption and recycling of epinephrine, norepinephrine, insulin, thyroid hormones, and steroid hormones such as the sex hormones (estrogens and androgens) and corticosteroids. The liver also absorbs cholecalciferol (vitamin D3) from the blood. Liver cells then convert cholecalciferol, which may be synthesized in the skin or absorbed in the diet, into an intermediary product, 25-hydroxy-D3, that is released back into the circulation. The intermediary is absorbed by the kidneys and used to generate calcitriol, a hormone important to Ca2+ metabolism.
- Removal of antibodies. The liver absorbs and breaks down antibodies, releasing amino acids to be recycled.
- Removal or storage of toxins. Lipid-soluble toxins in the diet, such as DDT, are absorbed by the liver and stored in lipid deposits, where they do not disrupt cellular functions. Other toxins are removed from the circulation and are either broken down or excreted in the bile.
- Synthesis and secretion of bile. Bile is synthesized in the liver and excreted into the lumen of the duodenum. Bile consists mostly of water, with minor amounts of ions, bilirubin (a pigment derived from hemoglobin), cholesterol, and an assortment of lipids collectively known as the bile salts. The water and ions assist in the dilution and buffering of acids in chyme as it enters the small intestine.
Bile salts are synthesized from cholesterol in the liver. Several related compounds are involved; the most abundant are derivatives of the steroids cholate and chenodeoxycholate.
The Functions of Bile
Most dietary lipids are not water-soluble. Mechanical processing in the stomach creates large drops containing a variety of lipids. Pancreatic lipase is not lipid-soluble, so the enzymes can interact with lipids only at the surface of a lipid drop. The larger the droplet, the more lipids are inside, isolated and protected from these enzymes. Bile salts break the droplets apart, a process called emulsification.
Emulsification creates tiny emulsion droplets with a superficial coating of bile salts. The formation of tiny droplets increases the surface area available for enzymatic attack. In addition, the layer of bile salts facilitates interaction between the lipids and lipid-digesting enzymes supplied by the pancreas. After lipid digestion has been completed, bile salts promote absorption of lipids by the intestinal epithelium. More than 90 percent of the bile salts are themselves reabsorbed, primarily in the ileum, as lipid digestion is completed. The reabsorbed bile salts enter the hepatic portal circulation and are collected and recycled by the liver. The cycling of bile salts from the liver to the small intestine and back is called the enterohepatic circulation of bile.
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The gallbladder is a hollow, pear-shaped, muscular organ. It is divided into three regions: (1) the fundus, (2) the body, and (3) the neck (Figure 24-21a). The cystic duct leads from the gallbladder toward its union with the common hepatic duct to form the common bile duct. At the duodenum, the common bile duct meets the pancreatic duct before emptying into the duodenal ampulla (Figure 24-21b). The duodenal ampulla receives buffers and enzymes from the pancreas and bile from the liver and gallbladder. It opens into the duodenum at a small mound, the duodenal papilla.
The pancreaticohepatic sphincter (sphincter of Oddi), a muscular sphincter, encircles the lumen of the common bile duct and generally the pancreatic duct and ampulla as well. This sphincter remains contracted unless stimulated by the intestinal hormone cholecystokinin.
Structure, topography and functіon of the lіver and pancreas
The liver, the largest gland in the body, also performs important exocrine and metabolic functions:
· The secretion of bile.
· The protective role by detoxifying substances.
· The storehouse for various substances.
· Metabolising the products of digestion.
· The synthesis of proteins.
· The metabolism of carbohydrates and the regulation of blood glucose.
· The metabolism of fats and the regulation of blood lipids.
· The conjugation of substances.
· The transformation of substances.
· The production of carbohydrates from proteins.
· The haemopoietic function – especially during foetal life the liver is a centre for haemopoiesis and new-born.
· The production of thrombolitic agents.
· The synthesis of procoagulants.
Topography of the liver.
Holotopy: Liver occupies right hypochondriac region, proper epigastric region and small part of left hypochondriac region. Skeletotopy: The upper edge of the liver projects in right 10th intercostal space (middle axillar line). Than it lifts to level of 4th rib (middle clavicular line) and passes across the sternum a bit upper from xiphoid process, terminates in left 5th intercostal space (between middle clavicular line and parasternal lines). The lower edge of the liver passes along the costal arch from right 10th intercostal space (middle axillar line). Than it crosses cartilage of right 9th rib and runs in epigastrium 1,5 cm lower from xiphoid process to cartilage of left 8th rib and meets the upper margin.
Inferior surface of the liver
We distinguish the convex diaphragmatic surface of the liver and lower visceral surface. Visceral surface adjoins to the organs, which form on surface of the liver suitable ‘tracks’: renal, adrenal, gastric, duodenal, oesophageal and colic impressions. Diaphragmatic surface carries cardiac impression.
Liver is almost entirely covered with peritoneum except posteriorly positioned ‘area nuda’. The superior surface is attached to the diaphragm and anterior abdominal wall by a fold of peritoneum, the falciform ligament, in the free margin of which is a rounded cord, the ligamentum teres (obliterated umbilical vein). The liver is connected to the lower surface of the diaphragm by the coronal ligament and the right and left triangular ligaments. The falciform ligament conventionally separates greater right lobe of liver and lesser left lobe of liver.
The porta hepatis, the entrance into the liver forms a cross-connection between the sagittal grooves which together are shaped like an H. Visceral surface carries furrows: right sagittal sulcus and left sagittal sulcus, which communicate by transversal sulcus (is called ‘porta hepatis’). Left sagittal sulcus anteriorly contain fissure of teres ligament, where umbilical vein in foetus passes. It obliterates in adult and forms teres liver ligament. Posterior portion of left sagittal sulcus is formed by fissura of venous ligament (obliterated venous duct of Arantii). Right sagittal sulcus anteriorly contains fossa of gall bladder, and behind – sulcus of inferior vena cava. Vena portae, proper hepatic artery and nerves enter through the porta hepatis into liver, common hepatic duct and lymphatic vessels leave the parenchyma in this place. Sagittal and transversal sulcuses limit the quadrate lobe, positioned ventrally and caudate lobe, disposed dorsally. Caudate lobe carries papillary and caudate processes.
The liver is held together by a tense connective tissue capsule Glisson ‘s capsule. At the porta it separates the lobules of liver. The lobules form the chief mass of the hepatic substance. Branches of portal vein, hepatic artery and biliary duct form a hepatic triad are situated in stratums between liver lobules.
Unlike all other organs a liver obtains arterial blood from proper hepatic artery and venous – from portal vein. Entering into liver porta, a portal vein and hepatic artery disintegrate into right and left lobar, segmental and lobular veins and arteries, which pass along interlobular bile duct. Capillaries from these vessels joining together form sinusoid capillaries that receive mixed blood and empty into a central vein, which occupies the centre of the lobule. Central vein drains into hepatic veins, which leave the liver to end in the inferior vena cava. This system is called as wonderful venous liver net.
Hepatic cells ‘hepatocytes’ excrete the bile, which get into bile canaliculi. Last pass to periphery emtpy into interlobular ductuli that form right hepatic duct and left hepatic duct (from right and left hepatic lobes). Common hepatic duct, which originated in porta, passes in hepatoduodenal ligament, meets the cystic duct and forms ductus choledochus. It flows together with pancreatic duct and forms common hepalopancreatic ampulla, which opens on major duodenal papilla. The ampulla may itself be closed by its own sphincter muscle, the sphincter ampullae (Oddi).
The liver, the largest gland in the body, has both external and internal secretions, which are formed in the hepatic cells. Its external secretion, the bile, is collected after passing through the bile capillaries by the bile ducts, which join like the twigs and branches of a tree to form two large ducts that unite to form the hepatic duct. The bile is either carried to the gall-bladder by the cystic duct or poured directly into the duodenum by the common bile duct where it aids in digestion. The internal secretions are concerned with the metabolism of both nitrogenous and carbohydrate materials absorbed from the intestine and carried to the liver by the portal vein. The carbohydrates are stored in the hepatic cells in the form of glycogen which is secreted in the form of sugar directly into the blood stream. Some of the cells lining the blood capillaries of the liver are concerned in the destruction of red blood corpuscles. It is situated in the upper and right parts of the abdominal cavity, occupying almost the whole of the right hypochondrium, the greater part of the epigastrium, and not uncommonly extending into the left hypochondrium as far as the mammillary line. In the male it weighs from 1.4 to 1.6 kilogm., in the female from 1.2 to 1.4 kilogm. It is relatively much larger in the fetus than in the adult, constituting, in the former, about one-eighteenth, and in the latter about one thirty-sixth of the entire body weight. Its greatest transverse measurement is from 20 to
To obtain a correct idea of its shape it must be hardened in situ, and it will then be seen to present the appearance of a wedge, the base of which is directed to the right and the thin edge toward the left. Symington describes its shape as that “of a right-angled triangular prism with the right angle rounded off.”
Surfaces.—The liver possesses three surfaces, viz., superior, inferior and posterior. A sharp, well-defined margin divides the inferior from the superior in front; the other margins are rounded. The superior surface is attached to the diaphragm and anterior abdominal wall by a triangular or falciform fold of peritoneum, the falciform ligament, in the free margin of which is a rounded cord, the ligamentum teres (obliterated umbilical vein). The line of attachment of the falciform ligament divides the liver into two parts, termed the right and left lobes, the right being much the larger. The inferior and posterior surfaces are divided into four lobes by five fossæ, which are arranged in the form of the letter H. The left limb of the H marks on these surfaces the division of the liver into right and left lobes; it is known as the left sagittal fossa, and consists of two parts, viz., the fossa for the umbilical vein in front and the fossa for the ductus venosus behind. The right limb of the H is formed in front by the fossa for the gall-bladder, and behind by the fossa for the inferior vena cava; these two fossæ are separated from one another by a band of liver substance, termed the caudate process. The bar connecting the two limbs of the H is the porta (transverse fissure); in front of it is the quadrate lobe, behind it the caudate lobe.
The superior surface (facies superior) comprises a part of both lobes, and, as a whole, is convex, and fits under the vault of the diaphragm which in front separates it on the right from the sixth to the tenth ribs and their cartilages, and on the left from the seventh and eighth costal cartilages. Its middle part lies behind the xiphoid process, and, in the angle between the diverging rib cartilage of opposite sides, is in contact with the abdominal wall. Behind this the diaphragm separates the liver from the lower part of the lungs and pleuræ, the heart and pericardium and the right costal arches from the seventh to the eleventh inclusive. It is completely covered by peritoneum except along the line of attachment of the falciform ligament.
The superior surface of the
The inferior surface (facies inferior; visceral surface)is uneven, concave, directed downward, backward, and to the left, and is in relation with the stomach and duodenum, the right colic flexure, and the right kidney and suprarenal gland. The surface is almost completely invested by peritoneum; the only parts devoid of this covering are where the gall-bladder is attached to the liver, and at the porta hepatis where the two layers of the lesser omentum are separated from each other by the bloodvessels and ducts of the liver. The inferior surface of the left lobe presents behind and to the left the gastric impression, moulded over the antero-superior surface of the stomach, and to the right of this a rounded eminence, the tuber omentale, which fits into the concavity of the lesser curvature of the stomach and lies in front of the anterior layer of the lesser omentum. The under surface of the right lobe is divided into two unequal portions by the fossa for the gall-bladder; the portion to the left, the smaller of the two, is the quadrate lobe, and is in relation with the pyloric end of the stomach, the superior portion of the duodenum, and the transverse colon. The portion of the under surface of the right lobe to the right of the fossa for the gall-bladder presents two impressions, one situated behind the other, and separated by a ridge. The anterior of these two impressions, the colic impression, is shallow and is produced by the right colic flexure; the posterior, the renal impression, is deeper and is occupied by the upper part of the right kidney and lower part of the right suprarenal gland. Medial to the renal impression is a third and slightly marked impression, lying between it and the neck of the gall-bladder. This is caused by the descending portion of the duodenum, and is known as the duodenal impression. Just in front of the inferior vena cava is a narrow strip of liver tissue, the caudate process, which connects the right inferior angle of the caudate lobe to the under surface of the right lobe. It forms the upper boundary of the epiploic foramen of the peritoneum.
Posterior and inferior surfaces of the liver.
The posterior surface (facies posterior) is rounded and broad behind the right lobe, but narrow on the left. Over a large part of its extent it is not covered by peritoneum; this uncovered portion is about
The anterior border (margo anterior) is thin and sharp, and marked opposite the attachment of the falciform ligament by a deep notch, the umbilical notch, and opposite the cartilage of the ninth rib by a second notch for the fundus of the gall-bladder. In adult males this border generally corresponds with the lower margin of the thorax in the right mammillary line; but in women and children it usually projects below the ribs.
The left extremity of the liver is thin and flattened from above downward.
Fossæ.—The left sagittal fossa (fossa sagittalis sinistra; longitudinal fissure) is a deep groove, which extends from the notch on the anterior margin of the liver to the upper border of the posterior surface of the organ; it separates the right and left lobes. The porta joins it, at right angles, and divides it into two parts. The anterior part, or fossa for the umbilical vein, lodges the umbilical vein in the fetus, and its remains (the ligamentum teres) in the adult; it lies between the quadrate lobe and the left lobe of the liver, and is often partially bridged over by a prolongation of the hepatic substance, the pons hepatis. The posterior part, or fossa for the ductus venosus, lies between the left lobe and the caudate lobe; it lodges in the fetus, the ductus venosus, and in the adult a slender fibrous cord, the ligamentum venosum, the obliterated remains of that vessel.
The porta or transverse fissure (porta hepatis) is a short but deep fissure, about
The fossa for the gall-bladder (fossa vesicæ felleæ) is a shallow, oblong fossa, placed on the under surface of the right lobe, parallel with the left sagittal fossa. It extends from the anterior free margin of the liver, which is notched by it, to the right extremity of the porta.
The fossa for the inferior vena cava (fossa venæ cavæ) is a short deep depression, occasionally a complete canal in consequence of the substance of the liver surrounding the vena cava. It extends obliquely upward on the posterior surface between the caudate lobe and the bare area of the liver, and is separated from the porta by the caudate process. On slitting open the inferior vena cava the orifices of the hepatic veins will be seen opening into this vessel at its upper part, after perforating the floor of this fossa.
Lobes.—The right lobe (lobus hepatis dexter) is much larger than the left; the proportion between them being as six to one. It occupies the right hypochondrium, and is separated from the left lobe on its upper surface by the falciform ligament; on its under and posterior surfaces by the left sagittal fossa; and in front by the umbilical notch. It is of a somewhat quadrilateral form, its under and posterior surfaces being marked by three fossæ: the porta and the fossæ for the gall-bladder and inferior vena cava, which separate its left part into two smaller lobes; the quadrate and caudate lobes. The impressions on the right lobe have already been described.
The quadrate lobe (lobus quadratus) is situated on the under surface of the right lobe, bounded in front by the anterior margin of the liver; behind by the porta; on the right, by the fossa for the gall-bladder; and on the left, by the fossa for the umbilical vein. It is oblong in shape, its antero-posterior diameter being greater than its transverse.
The caudate lobe (lobus caudatus; Spigelian lobe) is situated upon the posterior surface of the right lobe of the liver, opposite the tenth and eleventh thoracic vertebræ. It is bounded, below, by the porta; on the right, by the fossa for the inferior vena cava; and, on the left, by the fossa for the ductus venosus. It looks backward, being nearly vertical in position; it is longer from above downward than from side to side, and is somewhat concave in the transverse direction. The caudate process is a small elevation of the hepatic substance extending obliquely lateralward, from the lower extremity of the caudate lobe to the under surface of the right lobe. It is situated behind the porta, and separates the fossa for the gall-bladder from the commencement of the fossa for the inferior vena cava.
The left lobe (lobus hepatis sinister) is smaller and more flattened than the right. It is situated in the epigastric and left hypochondriac regions. Its upper surface is slightly convex and is moulded on to the diaphragm; its under surface presents the gastric impression and omental tuberosity, already referred to page 1189.
Ligaments.—The liver is connected to the under surface of the diaphragm and to the anterior wall of the abdomen by five ligaments; four of these—the falciform, the coronary, and the two lateral—are peritoneal folds; the fifth, the round ligament, is a fibrous cord, the obliterated umbilical vein. The liver is also attached to the lesser curvature of the stomach by the hepatogastric and to the duodenum by the hepatoduodenal ligament (see page 1157).
The falciform ligament (ligamentum falciforme hepatis) is a broad and thin antero-posterior peritoneal fold, falciform in shape, its base being directed downward and backward, its apex upward and backward. It is situated in an antero-posterior plane, but lies obliquely so that one surface faces forward and is in contact with the peritoneum behind the right Rectus and the diaphragm, while the other is directed backward and is in contact with the left lobe of the liver. It is attached by its left margin to the under surface of the diaphragm, and the posterior surface of the sheath of the right Rectus as low down as the umbilicus; by its right margin it extends from the notch on the anterior margin of the liver, as far back as the posterior surface. It is composed of two layers of peritoneum closely united together. Its base or free edge contains between its layers the round ligament and the parumbilical veins.
The coronary ligament (ligamentum coronarium hepatis) consists of an upper and a lower layer. The upper layer is formed by the reflection of the peritoneum from the upper margin of the bare area of the liver to the under surface of the diaphragm, and is continuous with the right layer of the falciform ligament. The lower layer is reflected from the lower margin of the bare area on to the right kidney and suprarenal gland, and is termed the hepatorenal ligament.
The triangular ligaments (lateral ligaments) are two iumber, right and left. The right triangular ligament (ligamentum triangulare dextrum) is situated at the right extremity of the bare area, and is a small fold which passes to the diaphragm, being formed by the apposition of the upper and lower layers of the coronary ligament. The left triangular ligament (ligamentum triangulare sinistrum) is a fold of some considerable size, which connects the posterior part of the upper surface of the left lobe to the diaphragm; its anterior layer is continuous with the left layer of the falciform ligament.
The round ligament (ligamentum teres hepatis) is a fibrous cord resulting from the obliteration of the umbilical vein. It ascends from the umbilicus, in the free margin of the falciform ligament, to the umbilical notch of the liver, from which it may be traced in its proper fossa on the inferior surface of the liver to the porta, where it becomes continuous with the ligamentum venosum.
Fixation of the Liver.—Several factors contribute to maintain the liver in place. The attachments of the liver to the diaphragm by the coronary and triangular ligaments and the intervening connective tissue of the uncovered area, together with the intimate connection of the inferior vena cava by the connective tissue and hepatic veins would hold up the posterior part of the liver. Some support is derived from the pressure of the abdominal viscera which completely fill the abdomen whose muscular walls are always in a state of tonic contraction. The superior surface of the liver is perfectly fitted to the under surface of the diaphragm so that atmospheric pressure alone would be enough to hold it against the diaphragm. The latter in turn is held up by the negative pressure in the thorax. The lax falciform ligament certainly gives no support though it probably limits lateral displacement.
Liver with the septum transversum. Human embryo
Biliary flow
The biliary tree
The term biliary tree is derived from the arboreal branches of the bile ducts. The bile produced in the liver is collected in bile canaliculi, which merge to form bile ducts. Within the liver, these ducts are called intrahepatic (within the liver) bile ducts, and once they exit the liver they are considered extrahepatic (outside the liver). The intrahepatic ducts eventually drain into the right and left hepatic ducts, which merge to form the common hepatic duct. The cystic duct from the gallbladder joins with the common hepatic duct to form the common bile duct.
Bile either drains directly into the duodenum via the common bile duct, or be temporarily stored in the gallbladder via the cystic duct. The common bile duct and the pancreatic duct enter the second part of the duodenum together at the ampulla of Vater.
Functional anatomy
Correspondence between anatomic lobes and Couinaud segmentsSegment*
The central area where the common bile duct, hepatic portal vein, and hepatic artery proper enter is the hilum or “porta hepatis“. The duct, vein, and artery divide into left and right branches, and the portions of the liver supplied by these branches constitute the functional left and right lobes.
The functional lobes are separated by an imaginary plane (historically called Cantlie’s line) joining the gallbladder fossa to the inferior vena cava. The plane separates the liver into the true right and left lobes. The middle hepatic vein also demarcates the true right and left lobes. The right lobe is further divided into an anterior and posterior segment by the right hepatic vein. The left lobe is divided into the medial and lateral segments by the left hepatic vein. The fissure for the ligamentum teres also separates the medial and lateral segments. The medial segment is also called the quadrate lobe. In the widely used Couinaud (or “French”) system, the functional lobes are further divided into a total of eight subsegments based on a transverse plane through the bifurcation of the main portal vein. The caudate lobe is a separate structure which receives blood flow from both the right- and left-sided vascular branches.
Synthesis
Further information: Proteins produced and secreted by the liver
A large part of amino acid synthesis
The liver performs several roles in carbohydrate metabolism:
Gluconeogenesis (the synthesis of glucose from certain amino acids, lactate or glycerol)
Glycogenolysis (the breakdown of glycogen into glucose)
Glycogenesis (the formation of glycogen from glucose)(muscle tissues can also do this)
The liver is responsible for the mainstay of protein metabolism, synthesis as well as degradation.
The liver also performs several roles in lipid metabolism:
Cholesterol synthesis
Lipogenesis, the production of triglycerides (fats).
A bulk of the lipoproteins are synthesized in the liver.
The liver produces coagulation factors I (fibrinogen), II (prothrombin), V, VII, IX, X and XI, as well as protein C, protein S and antithrombin.
In the first trimester fetus, the liver is the main site of red blood cell production. By the 32nd week of gestation, the bone marrow has almost completely taken over that task.
The liver produces and excretes bile (a yellowish liquid) required for emulsifying fats and help the absorption of vitamin K from the diet. Some of the bile drains directly into the duodenum, and some is stored in the gallbladder.
The liver also produces insulin-like growth factor 1 (IGF-1), a polypeptide protein hormone that plays an important role in childhood growth and continues to have anabolic effects in adults.
The liver is a major site of thrombopoietin production. Thrombopoietin is a glycoprotein hormone that regulates the production of platelets by the bone marrow.
Breakdown
The breakdown of insulin and other hormones
The liver glucoronidates bilirubin, facilitating its excretion into bile.
The liver breaks down or modifies toxic substances (e.g., methylation) and most medicinal products in a process called drug metabolism. This sometimes results in toxication, when the metabolite is more toxic than its precursor. Preferably, the toxins are conjugated to avail excretion in bile or urine.
The liver converts ammonia to urea (urea cycle).
Other functions
The liver stores a multitude of substances, including glucose (in the form of glycogen), vitamin A (1–2 years’ supply), vitamin D (1–4 months’ supply)[citatioeeded], vitamin B12 (1–3 years’ supply), vitamin K, iron, and copper.
The liver is responsible for immunological effects—the reticuloendothelial system of the liver contains many immunologically active cells, acting as a ‘sieve’ for antigens carried to it via the portal system.
The liver produces albumin, the major osmolar component of blood serum.
The liver synthesizes angiotensinogen, a hormone that is responsible for raising the blood pressure when activated by renin, an enzyme that is
Development.—The liver arises in the form of a diverticulum or hollow outgrowth from the ventral surface of that portion of the gut which afterward becomes the descending part of the duodenum. This diverticulum is lined by entoderm, and grows upward and forward into the septum transversum, a mass of mesoderm between the vitelline duct and the pericardial cavity, and there gives off two solid buds of cells which represent the right and the left lobes of the liver. The solid buds of cells grow into columns or cylinders, termed the hepatic cylinders, which branch and anastomose to form a close meshwork. This network invades the vitelline and umbilical veins, and breaks up these vessels into a series of capillary-like vessels termed sinusoids (
As the liver undergoes enlargement, both it and the ventral mesogastrium of the fore-gut are gradually differentiated from the septum transversum; and from the under surface of the latter the liver projects downward into the abdominal cavity. By the growth of the liver the ventral mesogastrium is divided into two parts, of which the anterior forms the falciform and coronary ligaments, and the posterior the lesser omentum. About the third month the liver almost fills the abdominal cavity, and its left lobe is nearly as large as its right. From this period the relative development of the liver is less active, more especially that of the left lobe, which actually undergoes some degeneration and becomes smaller than the right; but up to the end of fetal life the liver remains relatively larger than in the adult.
Longitudinal section of a hepatic vein.
Longitudinal section of a small portal vein and canal.
Vessels and Nerves.—The vessels connected with the liver are: the hepatic artery, the portal vein, and the hepatic veins.
The hepatic artery and portal vein, accompanied by numerous nerves, ascend to the porta, between the layers of the lesser omentum. The bile duct and the lymphatic vessels descend from the porta between the layers of the same omentum. The relative positions of the three structures are as follows: the bile duct lies to the right, the hepatic artery to the left, and the portal vein behind and between the other two. They are enveloped in a loose areolar tissue, the fibrous capsule of Glisson, which accompanies the vessels in their course through the portal canals in the interior of the organ.
The hepatic veins convey the blood from the liver, and are described on page 680. They have very little cellular investment, and what there is binds their parietes closely to the walls of the canals through which they run; so that, on section of the organ, they remain widely open and are solitary, and may be easily distinguished from the branches of the portal vein, which are more or less collapsed, and always accompanied by an artery and duct.
The lymphatic vessels of the liver are described on page 711.
The nerves of the liver, derived from the left vagus and sympathetic, enter at the porta and accompany the vessels and ducts to the interlobular spaces. Here, according to Korolkow, the medullated fibers are distributed almost exclusively to the coats of the bloodvessels; while the non-medullated enter the lobules and ramify between the cells and even within them.
Section of injected liver
Structure of the Liver.—The substance of the liver is composed of lobules, held together by an extremely fine areolar tissue, in which ramify the portal vein, hepatic ducts, hepatic artery, hepatic veins, lymphatics, and nerves; the whole being invested by a serous and a fibrous coat.
The serous coat (tunica serosa) is derived from the peritoneum, and invests the greater part of the surface of the organ. It is intimately adherent to the fibrous coat.
The fibrous coat (capsula fibrosa [Glissoni]; areolar coat) lies beneath the serous investment, and covers the entire surface of the organ. It is difficult of demonstration, excepting where the serous coat is deficient. At the porta it is continuous with the fibrous capsule of Glisson, and on the surface of the organ with the areolar tissue separating the lobules.
The lobules (lobuli hepatis) form the chief mass of the hepatic substance; they may be seen either on the surface of the organ, or by making a section through the gland, as small granular bodies, about the size of a millet-seed, measuring from 1 to
A single lobule of the liver
If one of the sublobular veins be laid open, the bases of the lobules may be seen through the thin wall of the vein on which they rest, arranged in a form resembling a tesselated pavement, the center of each polygonal space presenting a minute aperture, the mouth of an intralobular vein .
Microscopic Appearance—Each lobule consists of a mass of cells, hepatic cells, arranged in irregular radiating columns between which are the blood channels (sinusoids). These convey the blood from the circumference to the center of the lobule, and end in the intralobular vein, which runs through its center, to open at its base into one of the sublobular veins. Between the cells are also the minute bile capillaries. Therefore, in the lobule there are all the essentials of a secreting gland; that is to say: (1) cells, by which the secretion is formed; (2) bloodvessels, in close relation with the cells, containing the blood from which the secretion is derived; (3) ducts, by which the secretion, when formed, is carried away.
1. The hepatic cells are polyhedral in form. They vary in size from 12 to 25μ in diameter. They contain one or sometimes two distinct nuclei. The nucleus exhibits an intranuclear network and one or two refractile nucleoli. The cells usually contain granules; some of which are protoplasmic, while others consist of glycogen, fat, or an iron compound. In the lower vertebrates, e.g., frog, the cells are arranged in tubes with the bile duct forming the lumen and bloodvessels externally. According to Delépine, evidences of this arrangement can be found in the human liver.
2. The Bloodvessels.—The blood in the capillary plexus around the liver cells is brought to the liver principally by the portal vein, but also to a certain extent by the hepatic artery.
The hepatic artery, entering the liver at the porta with the portal vein and hepatic duct, ramifies with these vessels through the portal canals. It gives off vaginal branches, which ramify in the fibrous capsule of Glisson, and appear to be destined chiefly for the nutrition of the coats of the vessels and ducts. It also gives off capsular branches, which reach the surface of the organ, ending in its fibrous coat in stellate plexuses. Finally, it gives off interlobular branches, which form a plexus outside each lobule, to supply the walls of the interlobular veins and the accompanying bile ducts. From this plexus lobular branches enter the lobule and end in the net-work of sinusoids between the cells.
The portal vein also enters at the porta, and runs through the portal canals, enclosed in Glisson’s capsule, dividing in its course into branches, which finally break up into a plexus, the interlobular plexus, in the interlobular spaces. These branches receive the vaginal and capsular veins, corresponding to the vaginal and capsular branches of the hepatic artery. Thus it will be seen that all the blood carried to the liver by the portal vein and hepatic artery finds its way into the interlobular plexus. From this plexus the blood is carried into the lobule by fine branches which converge from the circumference to the center of the lobule, and are connected by transverse branches. The walls of these small vessels are incomplete so that the blood is brought into direct relationship with the liver cells. The lining endothelium consists of irregularly branched, disconnected cells (stellate cells of Kupffer). Moreover, according to Herring and Simpson, minute channels penetrate the liver cells themselves, conveying the constituents of the blood into their substance. It will be seen that the blood capillaries of the liver lobule differ structurally from capillaries elsewhere. Developmentally they are formed by the growth of the columns of liver cells into large blood spaces or sinuses, and hence they have received the name of “sinusoids.” Arrived at the center of the lobule, the sinusoids empty themselves into one vein, of considerable size, which runs down the center of the lobule from apex to base, and is called the intralobular vein. At the base of the lobule this vein opens directly into the sublobular vein, with which the lobule is connected. The sublobular veins unite to form larger and larger trunks, and end at last in the hepatic veins, these converge to form three large trunks which open into the inferior vena cava while that vessel is situated in its fossa on the posterior surface of the liver.
3. The bile ducts commence by little passages in the liver cells which communicate with canaliculi termed intercellular biliary passages (bile capillaries). These passages are merely little channels or spaces left between the contiguous surfaces of two cells, or in the angle where three or more liver cells meet, and they are always separated from the blood capillaries by at least half the width of a liver cell. The channels thus formed radiate to the circumference of the lobule, and open into the interlobular bile ducts which run in Glisson’s capsule, accompanying the portal vein and hepatic artery. These join with other ducts to form two main trunks, which leave the liver at the transverse fissure, and by their union form the hepatic duct.
Structure of the Ducts.—The walls of the biliary ducts consist of a connective-tissue coat, in which are muscle cells, arranged both circularly and longitudinally, and an epithelial layer, consisting of short columnar cells resting on a distinct basement membrane.
Excretory Apparatus of the Liver.—The excretory apparatus of the liver consists of (1) the hepatic duct, formed by the junction of the two main ducts, which pass out of the liver at the porta; (2) the gall-bladder, which serves as a reservoir for the bile; (3) the cystic duct, or the duct of the gall-bladder; and (4) the common bile duct, formed by the junction of the hepatic and cystic ducts.
The Hepatic Duct (ductus hepaticus).—Two main trunks of nearly equal size issue from the liver at the porta, one from the right, the other from the left lobe; these unite to form the hepatic duct, which passes downward and to the right for about
The Gall-bladder (vesica fellea) is a conical or pear-shaped musculomembranous sac, lodged in a fossa on the under surface of the right lobe of the liver, and extending from near the right extremity of the porta to the anterior border of the organ. It is from 7 to
The gall-bladder and bile ducts laid open.
Relations.—The body is in relation, by its upper surface, with the liver; by its under surface, with the commencement of the transverse colon; and farther back usually with the upper end of the descending portion of the duodenum, but sometimes with the superior portion of the duodenum or pyloric end of the stomach. The fundus is completely invested by peritoneum; it is in relation, in front, with the abdominal parietes, immediately below the ninth costal cartilage; behind with the transverse colon. The neck is narrow, and curves upon itself like the letter S; at its point of connection with the cystic duct it presents a well-marked constriction.
Structure—The gall-bladder consists of three coats: serous, fibromuscular, and mucous.
The external or serous coat (tunica serosa vesicæ felleæ) is derived from the peritoneum; it completely invests the fundus, but covers the body and neck only on their under surfaces.
The fibromuscular coat (tunica muscularis vesicæ felleæ), a thin but strong layer forming the frame-work of the sac, consists of dense fibrous tissue, which interlaces in all directions, and is mixed with plain muscular fibers, disposed chiefly in a longitudinal direction, a few running transversely.
The internal or mucous coat (tunica mucosa vesicæ felleæ) is loosely connected with the fibrous layer. It is generally of a yellowish-brown color, and is elevated into minute rugæ. Opposite the neck of the gall-bladder the mucous membrane projects inward in the form of oblique ridges or folds, forming a sort of spiral valve.
The mucous membrane is continuous through the hepatic duct with the mucous membrane lining the ducts of the liver, and through the common bile duct with the mucous membrane of the duodenum. It is covered with columnar epithelium, and secretes mucin; in some animals it secretes a nucleoprotein instead of mucin.
The Cystic Duct (ductus cysticus).—The cystic duct about
The Common Bile Duct (ductus choledochus).—The common bile duct is formed by the junction of the cystic and hepatic ducts; it is about
It descends along the right border of the lesser omentum behind the superior portion of the duodenum, in front of the portal vein, and to the right of the hepatic artery; it then runs in a groove near the right border of the posterior surface of the head of the pancreas; here it is situated in front of the inferior vena cava, and is occasionally completely imbedded in the pancreatic substance. At its termination it lies for a short distance along the right side of the terminal part of the pancreatic duct and passes with it obliquely between the mucous and muscular coats. The two ducts unite and open by a common orifice upon the summit of the duodenal papilla, situated at the medial side of the descending portion of the duodenum, a little below its middle and about 7 to
Structure.—The coats of the large biliary ducts are an external or fibrous, and an internal or mucous. The fibrous coat is composed of strong fibroareolar tissue, with a certain amount of muscular tissue, arranged, for the most part, in a circular manner around the duct. The mucous coat is continuous with the lining membrane of the hepatic ducts and gall-bladder, and also with that of the duodenum; and, like the mucous membrane of these structures, its epithelium is of the columnar variety. It is provided with numerous mucous glands, which are lobulated and open by minute orifices scattered irregularly in the larger ducts.
A CT scan in which the liver and portal vein are shown.
Anterior MIP image of anomalous hepatic veins
The Gallbladder
is a pear-shaped, thin-walled bag, which collects up to 30–50 ml bile. We distinguish fundus, body and neck of gallbladder, which continues into cystic duct. The gallbladder lies in a fossa in the liver to which it is attached by connective tissue and covered by peritoneum from below (mesoperitoneal position). The lumen of the neck of the gallbladder and of its connections with the cystic duct is incompletely subdivided by spiral fold of mucosa, known as the spiral fold (Heisler’s valve).
The Pancreas
The pancreas is the most important intestinal gland. The pancreas is shaped like a horizontal wedge with its thin end on the left. The head is the thickest part, fills into the duodenal loop to the right of the spine. The horizontal body continues into tail. The pancreatic duct runs right through the length of the gland. It receives short, vertical tributaries from the lobules and has owns sphincter muscle of pancreatic duct. The pancreatic duct ends together with the common bile duct on the major duodenal papilla. If present, the accessory pancreatic duct ends above the bile duct on the minor duodenal papilla.
Topography of the pancreas. Pancreas lies in upper abdominal region behind the peritoneum (retroperitoneal position) at the level of the from 1st to 3d lumbar vertebrae. Along the upper margin of the pancreas runs the splenic artery. The right kidney and adrenal gland adjoin to body of pancreas. Anterior surface of gland touches the stomach, posterior surface – inferior vena cava and aorta. Tail adjoins to splenic hilus.
The pancreas and duodenum from behind
Endocrine part of pancreas is represented by islets of Langerhans. They produce insulin and glucagon that regulate metabolism of carbohydrates, regulative a sugar contents in organism. Attached to insufficient production of these hormonal disease sugar diabetes arises.
Your pancreas lies posterior to your stomach, extending laterally from the duodenum toward the spleen (Figure 24-18a;){kind=small}
). The pancreas is an elongate, pinkish gray organ with a length of approximately
The surface of the pancreas has a lumpy, lobular texture. A thin, transparent connective tissue capsule wraps the entire organ. You can see the pancreatic lobules, associated blood vessels, and excretory ducts through the anterior capsule and the overlying layer of peritoneum. Arterial blood reaches the pancreas by way of branches of the splenic, superior mesenteric, and common hepatic arteries. The pancreatic arteries and pancreaticoduodenal arteries are the major branches from these vessels. The splenic vein and its branches drain the pancreas.
The pancreas is primarily an exocrine organ, producing digestive enzymes and buffers. The large pancreatic duct (duct of Wirsung) delivers these secretions to the duodenum. A small accessory duct, or duct of Santorini, may branch from the pancreatic duct. The pancreatic duct extends within the attached mesentery to reach the duodenum, where it meets the common bile duct from the liver and gallbladder. The two ducts then empty into the duodenal ampulla, a chamber located roughly halfway along the length of the duodenum (Figure 24-21b). When present, the accessory duct generally empties into the duodenum independently, outside the duodenal ampulla.
Histological Organization
Partitions of connective tissue divide the pancreatic tissue into distinct lobules. The blood vessels and tributaries of the pancreatic ducts are situated within these connective tissue septa (Figure 24-18b). The pancreas is an example of a compound tubuloacinar gland, a gland structure that we described in Chapter 4. Within each lobule, the ducts branch repeatedly before ending in blind pockets called the pancreatic acini. Each pancreatic acinus is lined by a simple cuboidal epithelium. Pancreatic islets, the endocrine tissues of thepancreas, are scattered among the acini (Figure24-18b, c;){kind=small}
). The islets account for only about 1 percent of the cellular population of the pancreas.
The pancreas has two distinct functions, one endocrine and the other exocrine. The endocrine cells of the pancreatic islets secrete insulin and glucagon into the bloodstream. We described those hormones and their actions in Chapter 18. The exocrine cells include the acinar cells and the epithelial cells that line the duct system. Together they secrete an alkaline pancreatic juice into the small intestine. Pancreatic juice is a mixture of digestive enzymes, water, and ions. Pancreatic enzymes are secreted by the acinar cells. These enzymes do most of the digestive work in the small intestine, breaking down ingested materials into small molecules suitable for absorption. The water and ions, secreted primarily by the cells lining the pancreatic ducts, assist in diluting and buffering the acids in the chyme.
Each day your pancreas secretes about 1000 ml (
Proteolytic enzymes account for about 70 percent of the total pancreatic enzyme production. The enzymes are secreted as inactive proenzymes that are activated only after they reach the small intestine. Proenzymes discussed earlier in the text include pepsinogen, angiotensinogen, plasminogen, fibrinogen, and many of the clotting factors and enzymes of the complement system. As in the stomach, release of a proenzyme rather than an active enzyme in the pancreas protects the secretory cells from the destructive effects of their own products. Among the proenzymes secreted by the pancreas are trypsinogen
