26

26. Topography and structure of urinary system organs

 

ORGANIZATION OF THE URINARY SYSTEM

The urinary system (Figure 26-1a) includes the kidneys, ureters, urinary bladder, and urethra. The excretory functions of the urinary system are performed by the two kidneys. These organs produce urine, a fluid containing water, ions, and small soluble compounds. Urine leaving the kidneys travels along the paired ureters to the urinary bladder for temporary storage. Urine excretion, a process called urination, or micturition, occurs when the contraction of the muscular urinary bladder forces urine through the urethra and out of the body.

THE KIDNEYS

The kidneys are located on either side of the vertebral column between vertebrae T₁₂ and L₃. The left kidney lies slightly superior to the right kidney (Figure 26-1a,b).

On gross dissection, the anterior surface of the right kidney is covered by the liver, the right colic (hepatic) flexure of the colon, and the duodenum. The anterior surface of the left kidney is covered by the stomach, pancreas, jejunum, and left colic (splenic) flexure of the colon. The superior surface of each kidney is capped by an adrenal gland (Figures 26-1a,b and 26-2a, b). The kidneys and adrenal glands lie between the muscles of the dorsal body wall and the parietal peritoneum in a retroperitoneal position (Figure 26-1c).

The position of the kidneys in the abdominal cavity is maintained by (1) the overlying peritoneum, (2) contact with adjacent visceral organs, and (3) supporting connective tissues. Each kidney is protected and stabilized by three concentric layers of connective tissue (Figure 26-1c):

1. The renal capsule is a layer of collagen fibers that covers the outer surface of the entire organ. This layer is also known as the fibrous tunic of the kidney.

2. The adipose capsule, a layer of adipose tissue, surrounds the renal capsule. This layer can be quite thick, and on dissection it generally obscures the outline of the kidney.

3. The renal fascia is a dense outer layer. Collagen fibers extend outward from the renal capsule through the adipose capsule to this layer. The renal fascia anchors the kidney to surrounding structures. Posteriorly, the renal fascia fuses with the deep fascia surrounding the muscles of the body wall. Anteriorly, the renal fascia forms a thick fibrous layer that fuses with the peritoneum.

4. 

In effect, each kidney hangs suspended by collagen fibers from the renal fascia and packed in a soft cushion of adipose tissue. This arrangement prevents the jolts and shocks of day-to-day existence from disturbing normal kidney function. If the suspensory fibers break or become detached, a slight bump or jar may displace the kidney and stress the attached vessels and ureter. This condition, called a floating kidney, can be especially dangerous, because the ureters or renal blood vessels may become twisted or kinked during movement.

Superficial Anatomy of the Kidneys

Each reddish brown kidney has the shape of a kidney bean. A typical adult kidney (Figures 26-2a, b and 26-3) is about 10 cm (4 in.) in length, 5.5 cm (2.2 in.) in width, and 3 cm (1.2 in.) in thickness. Each kidney weighs about 150 g (5.25 oz). The hilus, a prominent medial indentation, is the point of entry for the renal artery and renal nerves and the point of exit for the renal vein and ureter.

Sectional Anatomy of the Kidneys

The fibrous renal capsule has inner and outer layers. In sectional view (Figure 26-3a ), the inner layer folds inward at the hilus and lines an internal cavity, the renal sinus. Renal blood vessels and the ureter draining the kidney pass through the hilus and branch within the renal sinus. A thickened, outer layer of the capsule extends across the hilus and stabilizes the position of these structures.

The renal cortex is the outer layer of the kidney in contact with the capsule. The cortex is reddish brown and granular in texture. The renal medulla consists of 6 to 18 distinct conical or triangular structures called renal pyramids. The base of each pyramid faces the cortex, and the tip of each pyramid, a region known as the renal papilla, projects into the renal sinus. Each pyramid has a series of fine grooves that converge at the papilla. Adjacent renal pyramids are separated by bands of cortical tissue called renal columns, which extend into the medulla. The columns have a distinctly granular texture, similar to that of the cortex. A renal lobe consists of a renal pyramid, the overlying area of renal cortex, and adjacent tissues of the renal columns.

Urine production occurs in the renal lobes. Ducts within each renal papilla discharge urine into a cup-shaped drain called a minor calyx. Four or five minor calyces merge to form a major calyx, and two or three major calyces combine to form the renal pelvis, a large, funnel-shaped chamber. The renal pelvis, which fills most of the renal sinus, is connected to the ureter at the hilus of the kidney.

Urine production begins in microscopic structures called nephrons in the cortex of each renal lobe. There are roughly 1.25 millioephrons in each kidney, with a combined length of about 145 km (85 miles).

The Nephron

Each nephron consists of a renal corpuscle and a renal tubule roughly 50 mm long. The renal tubule begins at the renal corpuscle, a cup-shaped chamber (Figure 26-4). The renal corpuscle is approximately 200 µm (0.2 mm) in diameter. It contains a capillary network called the glomerulus (plural, glomeruli), which consists of about 50 intertwining capillaries. Blood arrives at the glomerulus via the afferent arteriole and departs in the efferent arteriole. Filtration occurs in the renal corpuscle as blood pressure forces fluid and dissolved solutes out of the glomerular capillaries and into the capsular space. Filtration produces an essentially protein-free solution, known as a filtrate, that is otherwise very similar to blood plasma.

From the renal corpuscle, the filtrate enters a long tubular passageway. The renal tubule has two convoluted (coiled or twisted) segments–the proximal convoluted tubule (PCT) and the distal convoluted tubule (DCT)–separated by a simple U-shaped tube, the loop of Henle. The convoluted segments are in the cortex, and the loop extends partially or completely into the medulla. For clarity, the nephron diagrammed in Figure 26-4 has been shortened and straightened. The regions of the nephron vary in their structural and functional characteristics. As it travels along the tubule, the filtrate, now called tubular fluid, gradually changes in composition. The changes that occur and the characteristics of the urine that results vary with the activities under way in each segment of the nephron; Figure 26-4 provides an overview of the regional specializations.

Each nephron empties into the collecting system. A connecting tubule carries the tubular fluid from the distal convoluted tubule to a nearby collecting duct. The collecting duct, which receives tubular fluid from many different nephrons, leaves the cortex and descends into the medulla, carrying fluid to a papillary duct that drains into a minor calyx.

The urine arriving at the renal pelvis is very different from the filtrate produced at the renal corpuscle. Filtration is a passive process that permits or prevents movement across a barrier solely on the basis of solute size. A filter with pores large enough to permit the passage of organic waste products is unable to prevent the passage of water, ions, and other organic molecules, such as glucose, fatty acids, and amino acids. These useful substances must be reclaimed and the waste products excreted. The segments of the nephron distal to the renal corpuscle are responsible for:

• Reabsorbing all the useful organic substrates that enter the renal tubule.

• Reabsorbing over 90 percent of the water present in the filtrate.

• Secreting into the tubular fluid any waste products that were missed by the filtration process.

Additional water and salts will be removed from the tubular fluid in the collecting system before the fluid is released into the renal sinus as urine. Table 26-1 gives an overview of important information concerning the regions of the nephron and collecting system.

Nephrons differ slightly in structure, depending on their location. Roughly 85 percent of all nephrons are cortical nephrons; they are located in the superficial cortex of the kidney. The remaining 15 percent of nephrons, termed juxtamedullary nephrons (juxta, near), are located closer to the medulla. Because they are more numerous than juxtamedullary nephrons, cortical nephrons perform most of the reabsorptive and secretory functions of the kidneys. However, the juxtamedullary nephrons are responsible for the ability to produce a concentrated urine.

We shall now examine the structure of each segment of a representative nephron.

The Renal Corpuscle

The renal corpuscle (Figure 26-5a,b,c) has a diameter averaging 150-250 µm. It includes (1) the glomerular capillary network and (2) a region known as Bowman’s capsule. Connected to the initial segment of the renal tubule, Bowman’s capsule forms the outer wall of the renal corpuscle and covers the glomerular capillaries.

Bowman’S Capsule
The glomerulus projects into Bowman’s capsule much as the heart projects into the pericardial cavity (Figure 26-5c). The outer wall of the capsule is lined by a simple squamous parietal epithelium (capsular epithelium). This layer is continuous with the visceral epithelium (glomerular epithelium) that covers the glomerular capillaries. The visceral epithelium consists of large cells with complex processes, or “feet,” that wrap around the lamina densa, the specialized basement membrane of the glomerular capillaries (Figure 26-5c, e). These unusual cells are called podocytes (podos, foot + -cyte, cell). The podocyte feet are known as pedicels. Materials passing out of the blood at the glomerulus must be small enough to pass between the narrow gaps, or filtration slits, between adjacent pedicels. These slits are small enough to prevent the loss of all but the smallest plasma proteins.

The capsular space separates the visceral and parietal epithelia (Figures 26-4,and 26-5b,c). The connection between the two epithelial layers lies at the vascular pole of the renal corpuscle. At the vascular pole, blood flows into and out of the glomerular capillaries. Blood arrives in an afferent arteriole and departs in an efferent arteriole.

The Glomerular Capillaries
The glomerular capillaries (Figure 26-5c) are fenestrated capillaries whose endothelium contains large-diameter pores. The openings are small enough to prevent the passage of blood cells but too large to restrict the diffusion of dissolved or suspended compounds, even those the size of plasma proteins.

The endothelial cells lining the capillaries are surrounded by the lamina densa (Figure 26-5e). During filtration, the lamina densa restricts the passage of large plasma proteins but permits the movement of smaller molecules, including albumin, many organic nutrients, and ions. Unlike basement membranes elsewhere, the lamina densa may encircle two or more capillaries. When it does, mesangial cells are situated between the capillaries. Mesangial cells have several important functions:

• Providing physical support to the capillaries.

• Engulfing organic materials that might otherwise clog the filter at the lamina densa.

• Contracting or relaxing to change the diameter of the glomerular capillaries and to change the filtration rate.

Together, the fenestrated endothelium, the lamina densa, and the filtration slits form the filtration membrane. During filtration, blood pressure forces water and small solutes across this membrane and into the capsular space. The larger solutes, especially plasma proteins, are excluded. Filtration at the renal corpuscle is both effective and passive, but it has one major limitation: In addition to metabolic wastes and excess ions, compounds such as glucose, free fatty acids, amino acids, vitamins, and other solutes enter the capsular space. These potentially useful materials are recaptured before the filtrate leaves the kidneys, with much of the reabsorption occurring in the proximal convoluted tubule.

The Proximal Convoluted Tubule

The entrance to the proximal convoluted tubule (PCT) lies almost directly opposite the vascular pole, at the tubular pole of the renal corpuscle (Figure 26-5c). The lining of the PCT consists of a simple cuboidal epithelium whose exposed surfaces are blanketed with microvilli (Figure 26-4). The cuboidal tubular cells actively absorb organic nutrients, ions, and plasma proteins (if any) from the tubular fluid and release them into the peritubular fluid, the interstitial fluid surrounding the renal tubule. As these solutes are absorbed and transported, osmotic forces pull water across the wall of the PCT and into the peritubular fluid. Although reabsorption is the primary function of the PCT, the epithelial cells can also secrete substances into the lumen.

The Loop of Henle

The PCT makes an acute bend that turns the renal tubule toward the renal medulla. This turn marks the start of the loop of Henle (Figures 26-4b and 26-5a). The loop of Henle can be divided into a descending limb and an ascending limb. Fluid in the descending limb travels toward the renal pelvis, and that in the ascending limb travels toward the renal cortex. Each limb contains a thick segment and a thin segment. (The terms thick and thin refer to the height of the epithelium, not to the diameter of the lumen.)

The thick segments have a cuboidal epithelium, whereas a thin squamous epithelium lines the thin segments (Figure 26-4). The thick descending limb has functions similar to those of the PCT. The thick ascending limb pumps sodium and chloride ions out of the tubular fluid. The effect of this pumping is most noticeable in the medulla, where the long ascending limbs of juxtamedullary nephrons create unusually high solute concentrations in the peritubular fluid.

The Distal Convoluted Tubule

The thick ascending limb of the loop of Henle ends where it forms a sharp angle near the vascular pole of the renal corpuscle. The distal convoluted tubule (DCT) begins there. The initial portion of the DCT passes between the afferent and efferent arterioles (Figure 26-5c).

In sectional view (Figure 26-4), the DCT differs from the PCT in that (1) the DCT has a smaller diameter, (2) the epithelial cells of the DCT lack microvilli, and (3) the boundaries between the epithelial cells in the DCT are distinct. The DCT is an important site for:

• The active secretion of ions, acids, and other materials.

• The selective reabsorption of sodium ions and calcium ions from the tubular fluid.

• The selective reabsorption of water, which assists in concentrating the tubular fluid.

The Juxtaglomerular Apparatus
The epithelial cells of the DCT near the vascular pole of the renal corpuscle are taller than those elsewhere along the DCT, and their nuclei are clustered together. This region, detailed in Figure 26-5c, is called the macula densa. The cells of the macula densa are closely associated with unusual smooth muscle fibers in the wall of the afferent arteriole. These fibers are known as juxtaglomerular cells. Together, the macula densa and juxtaglomerular cells form the juxtaglomerular apparatus (JGA). The juxtaglomerular apparatus is an endocrine structure that secretes the erythropoietin and renin, as we described in Chapter 18.

The Collecting System

The DCT, the last segment of the nephron, opens into the collecting system. The collecting system consists of connecting tubules, collecting ducts, and papillary ducts (Figure 26-4). Individual connecting tubules connect each nephron to a nearby collecting duct (Figure 26-6). Each collecting duct receives tubular fluid from many connecting tubules. Several collecting ducts converge to empty into a larger papillary duct, which in turn empties into a minor calyx. The epithelium lining the collecting system begins with simple cuboidal cells in the connecting tubules and changes to a columnar epithelium in the collecting and papillary ducts.

In addition to transporting tubular fluid from the nephron to the renal pelvis, the collecting system adjusts its composition and determines the final osmotic concentration and volume of the urine.

The Blood Supply to the Kidneys

Your kidneys receive 20-25 percent of your total cardiac output. Iormal individuals, about 1200 ml of blood flows through the kidneys each minute. That is a phenomenal amount of blood for organs with a combined weight of less than 300 g (10.5 oz)!

Each kidney receives blood from a renal artery that originates along the lateral surface of the abdominal aorta near the level of the superior mesenteric artery (Figure 21-26). As it enters the renal sinus, the renal artery provides blood to the segmental arteries (Figure 26-7a). Segmental arteries further divide into a series of interlobar arteries that radiate outward through the renal columns between the renal pyramids. The interlobar arteries supply blood to the arcuate arteries, which arch along the boundary between the cortex and medulla of the kidney. Each arcuate artery gives rise to a number of interlobular arteries, which supply parts of the adjacent renal lobe. Branching from each interlobular artery are numerous afferent arterioles (Figure 26-7b).

Blood reaches the vascular pole of each glomerulus through an afferent arteriole and leaves in an efferent arteriole (Figures 26-5c and 26-7c). Blood travels from the efferent arteriole to form a capillary plexus, a network of peritubular capillaries that supplies the PCT and DCT. The peritubular capillaries provide a route for the pickup or delivery of substances that are reabsorbed or secreted by these portions of the nephron.

In juxtamedullary nephrons, the efferent arterioles and peritubular capillaries are connected to a series of long, slender capillaries that accompany the loops of Henle into the medulla (Figure 26-7d). These capillaries, known as the vasa recta (rectus, straight), absorb and transport solutes and water reabsorbed into the medulla from tubular fluid in the loops of Henle and collecting ducts. Under normal conditions, the removal of solutes and water by the vasa recta balances the rates of solute and water reabsorption in the medulla.

From the peritubular capillaries and vasa recta, blood enters a network of venules and small veins that converge on the interlobular veins. In a mirror image of the arterial distribution, the interlobular veins deliver blood to arcuate veins, which empty into interlobar veins. The interlobar veins drain into the segmental veins, which merge to form a renal vein. Many of the blood vessels just described are visible in the corrosion cast of the kidneys shown in Figure 26-8.

Innervation of the Kidneys

The kidneys and ureters are innervated by renal nerves. Most of the nerve fibers involved are sympathetic postganglionic fibers from the superior mesenteric ganglion. A renal nerve enters each kidney at the hilus and follows the tributaries of the renal arteries to reach individual nephrons. The sympathetic innervation targets (1) the juxtaglomerular apparatus, (2) the smooth muscles in the walls of the afferent and efferent arterioles, and (3) mesangial cells. Known functions of sympathetic innervation include the following:

• Regulation of glomerular blood flow and pressure, through control of the diameters of the afferent and efferent arterioles and the glomerular capillaries.

• Stimulation of renin release from the juxtaglomerular apparatus.

• Direct stimulation of water and sodium ion reabsorption.

CONCEPT CHECK QUESTIONS

1. What portions of the nephron are located in the renal cortex?

2. Why don’t plasma proteins pass into the capsular space under normal circumstances?

3. Damage to what part of the nephron would interfere with the control of blood pressure?

URINE TRANSPORT, STORAGE, AND ELIMINATION

Filtrate modification and urine production end when the fluid enters the renal pelvis. The remaining parts of the urinary system (the ureters, urinary bladder, and urethra) are responsible for the transport, storage, and elimination of urine. A pyelogram is an image of the urinary system, obtained by taking an X-ray of the kidneys after a radiopaque compound has been administered. Such an image provides an orientation to the relative sizes and positions of these organs. The sizes of the minor and major calyces, the renal pelvis, the ureters, the urinary bladder, and the proximal portion of the urethra are somewhat variable, because these regions are lined by a transitional epithelium that can tolerate cycles of distension and contraction without damage.

We shall now examine these components of the urinary system.

The Ureters

The ureters are a pair of muscular tubes that extend inferiorly from the kidneys for about 30 cm (12 in.) before reaching the urinary bladder. Each ureter begins at the funnel-shaped renal pelvis (Figure 26-3). The ureters extend inferiorly and medially, passing over the anterior surfaces of the psoas major muscles (Figures 26-1c and 26-2b). The ureters are retroperitoneal and are firmly attached to the posterior abdominal wall. The paths taken by the ureters in men and women are different due to variations in the nature, size, and position of the reproductive organs. As Figure 26-19a shows, in males, the base of the urinary bladder lies between the rectum and the pubic symphysis; in females, the base of the urinary bladder sits inferior to the uterus and anterior to the vagina (Figure 26-19b).

The ureters penetrate the posterior wall of the urinary bladder without entering the peritoneal cavity. They pass through the bladder wall at an oblique angle, and the ureteral openings are slit-like rather than rounded (Figure 26-19c). This shape helps prevent the backflow of urine toward the ureter and kidneys when the urinary bladder contracts.

Histology of the Ureters

The wall of each ureter consists of three layers: (1) an inner mucosa covered by a transitional epithelium, (2) a middle muscular layer made up of longitudinal and circular bands of smooth muscle, and (3) an outer connective tissue layer that is continuous with the fibrous renal capsule and peritoneum. About every 30 seconds, a peristaltic contraction begins at the renal pelvis and sweeps along the ureter, forcing urine toward the urinary bladder.

The Urinary Bladder

The urinary bladder is a hollow, muscular organ that functions as a temporary storage reservoir for urine. The dimensions of the urinary bladder vary with the state of distension, but the full urinary bladder can contain about a liter of urine.

The superior surfaces of the urinary bladder are covered by a layer of peritoneum, and several peritoneal folds assist in stabilizing its position (Figure 26-19c). The middle umbilical ligament extends from the anterior and superior border toward the umbilicus (navel). The lateral umbilical ligaments pass along the sides of the bladder and also reach the umbilicus. These fibrous cords contain the vestiges of the two umbilical arteries, which supplied blood to the placenta during embryonic and fetal development. The urinary bladder’s posterior, inferior, and anterior surfaces lie outside the peritoneal cavity. In these areas, tough ligamentous bands anchor the urinary bladder to the pelvic and pubic bones.

In sectional view, the mucosa lining the urinary bladder is usually thrown into folds, or rugae, that disappear as the bladder fills. The triangular area bounded by the ureteral openings and the entrance to the urethra constitutes the trigone of the urinary bladder. The mucosa here is smooth and very thick. The trigone acts as a funnel that channels urine into the urethra when the urinary bladder contracts.

The urethral entrance lies at the apex of the trigone, at the most inferior point in the urinary bladder. The region surrounding the urethral opening, known as the neck of the urinary bladder, contains a muscular internal urethral sphincter, or sphincter vesicae. The smooth muscle fibers of the internal urethral sphincter provide involuntary control over the discharge of urine from the urinary bladder. The urinary bladder is innervated by postganglionic fibers from ganglia in the hypogastric plexus and by parasympathetic fibers from intramural ganglia that are controlled by branches of the pelvic nerves.

Histology of the Urinary Bladder

The wall of the urinary bladder contains mucosa, submucosa, and muscularis layers (Figure 26-20).The muscularis layer consists of inner and outer longitudinal smooth muscle layers, with a circular layer sandwiched between. Collectively, these layers form the powerful detrusor muscle of the urinary bladder. Contraction of this muscle compresses the urinary bladder and expels its contents into the urethra.

The Urethra

The urethra extends from the neck of the urinary bladder (Figure 26-19c) to the exterior. The female and male urethrae differ in length and in function. In females, the urethra is very short, extending 3-5 cm (1-2 in.) from the bladder to the vestibule (Figure 26-19b). The external urethral opening, or external urethral meatus, is situated near the anterior wall of the vagina.

In males, the urethra extends from the neck of the urinary bladder to the tip of the penis, a distance that may be 18-20 cm (7-8 in.). The male urethra can be subdivided into three portions (Figure 26-19a, c): (1) the prostatic urethra, (2) the membranous urethra, and (3) the penile urethra.

The prostatic urethra passes through the center of the prostate gland (Figure 26-19c). The membranous urethra includes the short segment that penetrates the urogenital diaphragm, the muscular floor of the pelvic cavity. The penile urethra extends from the distal border of the urogenital diaphragm to the external urethral meatus at the tip of the penis (Figure 26-19a). We shall consider the functional differences among these regions in Chapter 28.

In both genders, as the urethra passes through the urogenital diaphragm, a circular band of skeletal muscle forms the external urethral sphincter. This muscular band acts as a valve. The external urethral sphincter is under voluntary control, via the perineal branch of the pudendal nerve. This sphincter has a resting muscle tone and must be voluntarily relaxed to permit micturition.

Histology of the Urethra

The urethral lining consists of a stratified epithelium that varies from transitional at the neck of the urinary bladder, through stratified columnar at the midpoint, to stratified squamous near the external urethral meatus. The lamina propria is thick and elastic, and the mucous membrane is thrown into longitudinal folds. Mucin-secreting cells are located in the epithelial pockets. In males, the epithelial mucous glands may form tubules that extend into the lamina propria. Connective tissues of the lamina propria anchor the urethra to surrounding structures. In females, the lamina propria contains an extensive network of veins, and the entire complex is surrounded by concentric layers of smooth muscle.

The Micturition Reflex and Urination

Urine reaches the urinary bladder by the peristaltic contractions of the ureters. The process of urination is coordinated by the micturition reflex. Components of this reflex are diagrammed in Figure 26-21.

Stretch receptors in the wall of the urinary bladder are stimulated as the bladder fills with urine. Afferent fibers in the pelvic nerves carry the impulses generated to the sacral spinal cord. Their increased level of activity (1) facilitates parasympathetic motor neurons in the sacral spinal cord and (2) stimulates interneurons that relay sensations to the thalamus and on to the cerebral cortex. As a result, you become consciously aware of the fluid pressure in your urinary bladder.

The urge to urinate generally appears when the bladder contains about 200 ml of urine. The micturition reflex begins to function when the stretch receptors have provided adequate stimulation to parasympathetic preganglionic motor neurons. Action potentials carried by efferent fibers within the pelvic nerves then stimulate ganglionic neurons in the wall of the urinary bladder. These neurons in turn stimulate sustained contraction of the detrusor muscle.

This contraction elevates fluid pressures in the urinary bladder, but urine ejection does not occur unless both the internal and external urethral sphincters are relaxed. Relaxation of the external urethral sphincter occurs under voluntary control. When the external urethral sphincter relaxes, so does the internal sphincter. If the external urethral sphincter does not relax, the internal sphincter remains closed and the urinary bladder gradually relaxes.

A further increase in bladder volume begins the cycle again, usually within an hour. Each increase in urinary volume leads to an increase in stretch receptor stimulation that makes the sensation more acute. Once the volume of the urinary bladder exceeds 500 ml, the micturition reflex may generate enough pressure to force open the internal urethral sphincter. This opening leads to a reflexive relaxation of the external sphincter, and urination occurs despite voluntary opposition or potential inconvenience. At the end of a normal micturition, less than 10 ml of urine remains in the bladder.

Infants lack voluntary control over urination, because the necessary corticospinal connections have yet to be established. Toilet training before age 2 typically involves training the parent to anticipate the timing of the reflex rather than training the child to exert conscious control.