1-17 THE URINARY SYSTEM

LEARNING OBJECTIVE: Recall the parts of the urinary system and their function(s).

The urinary system is the primary filtering system of the body (fig. 1-55). This system is composed of two main organs, the kidneys and urinary bladder. The kidneys produce urine, which is drained from the kidneys by two tubes called ureters. Urine flows down both ureters to the bladder. The urinary bladder is a large reservoir where the urine is temporarily stored before excretion from the body. A tube called the urethra carries the urine from the bladder to the outside of the body. All these parts, except the length of the urethra, are the same in both sexes.

KIDNEYS

The importance of the kidney can be realized only when its structure and functions are understood. The bladder, ureters, and urethra store and pass the products of the kidneys.

The kidneys are two large, bean-shaped organs designed to filter waste materials from the blood (figs. 1-55 and 1-5 6). They also assist in controlling the rate of red blood cell formation, and in the regulation of blood pressure, the absorption of calcium ions, and the volume, composition, and pH of body fluids. The kidneys are located in the upper posterior part of the abdominal cavity, one on each side of the spinal column. The upper end of each kidney reaches above the level of the 12th rib. The suprarenal (adrenal) gland sits like a cap on top of each kidney. The kidneys are protected by a considerable amount of fat and supported by connective tissue and the peritoneum. Attached to the hollow side of each kidney is the dilated upper end of the ureter, forming the renal pelvis.

Figure 1-55.—The urinary system.

Structure

The lateral surface of the kidneys is convex in shape, and the medial side is deeply concave. The medial side of each kidney possesses a depression that leads to a hollow chamber called the renal sinus (fig. 1-55). The entrance of the renal sinus is referred to as the hilum (fig. 1-5 5). Blood vessels, nerves, lymphatic vessels, and the ureters pass through the hilum.

The superior end of the ureter forms a funnel-shaped sac called the renal pelvis (fig. 1-56). The renal pelvis is divided into two or three tubes, called major calyces. The major calyces (sing. calyx) are further subdivided into minor calyces.

There are groups of elevated projections in the walls of the renal pelvis. These projections are called renal papillae. The renal papillae connect to the minor calyces, through tiny openings in the minor calyces.

The principal portion of the kidney is divided into two distinct regions: an inner medulla and outer cortex (fig. 1-56). The renal medulla is composed of pyramid-shaped masses of tubes and tubules called renal pyramids. Renal pyramids drain the urine to the renal pelvis. The renal cortex forms a shell over the renal medulla. Renal cortex tissue dips down, like fingers, between the renal pyramids, and forms what are called renal columns. The cortex possesses very small tubes associated with nephrons. Nephrons are the functional units of the kidneys.

RENAL BLOOD VESSELS.—The renal artery supplies blood to the kidneys (fig. 1-5 6). The renal artery enters the kidneys through the hilum, and sends off branches to the renal pyramids. These arterial branches are called interlobar arteries. At the border between the medulla and cortex, the interlobar arteries branch to form the arciform arteries. The arciform arteries branch also and form the interlobular arteries.

Figure 1-56.—Principal parts of the kidney:
A. Longitudinal section of a kidney; B. A renal pyramid containing nephrons; C. A single nephron.

The venous system of the kidneys generally follow the same paths as the arteries. Venous blood passes through the interlobular, arciform, interlobar, and renal veins (fig. 1-56).

NEPHRONS.—The functional units of the kidneys are called nephrons. There are about 1 million nephrons in each kidney. Each nephron consists of a renal corpuscle and a renal tubule (fig. 1-56, view C).

The renal corpuscle (Malpighian corpuscle) is composed of a tangled cluster of blood capillaries called a glomerulus. The glomerulus is surrounded by a sac-like structure referred to as the glomeruluscapsule or Bowman's capsule (figs. 1-56, view C, and 1-57).

Leading away from the glomerulus is the renal tubule. The initial portion of the renal tubule is coiled and called the proximal convoluted (meaning coiled or twisted) tubule. The proximal convoluted tubule dips down to become the descending loop of Henle. The tubule then curves upward toward the renal corpuscle and forms the ascending loop of Henle.

Once the ascending limb reaches the region of the renal corpuscle, it called the distal convoluted tubule. Several distal convoluted tubules merge in the renal cortex to form a collecting duct. The collecting duct begins to merge within the renal medulla. The collecting ducts become increasingly larger as they are joined by other collecting ducts. The resulting tube is called the papillary duct. The papillary duct empties into the minor calyx through an opening in the renal papilla.

Figure 1-57.—The reabsorption process.

Function

The kidneys are effective blood purifiers and fluid balance regulators. In addition to maintaining a normal pH of the blood (acid-base balance), the kidneys keep the blood slightly alkaline by removing excess substances from the blood. The end product of these functions is the formation of urine, which is excreted from the body.

Urine is formed through a series ofprocesses in the nephron. These processes are filtration, reabsorption, and secretion.

FILTRATION.—Urine formation begins when water and various dissolved substances are filtered out of blood plasma from a glomerular capillary into the glomerular capsule. The filtered substance (glomerular filtrate) leaves the glomerular capsule and enters the renal tubule.

REABSORPTION.—As glomerular filtrate passes through the renal tubule, some of the filtrate is reabsorbed into the blood ofthe peritubular capillary (fig. 1-57). The filtrate entering the peritubular capillary will repeat the filtration cycle. This process of reabsorption changes the composition of urine. For instance, the filtrate entering the renal tubule is high in sugar content, but because of the reabsorption process, urine secreted from the body does not contain sugar.

SECRETION.—Secretion is the process by which the peritubular capillary transports certain substances directly into the fluid of the renal tubule (fig. 1-58). These substances are transported by similar mechanisms as used in the reabsorption process, but done in reverse. For example, certain organic compounds, such as penicillin and histamine, are secreted directly from the proximal convoluted tubule to the renal tubule. Also, large quantities of hydrogen ions are secreted in this same manner. The secretion of hydrogen ions plays an important role in regulating pH of body fluids.

Figure 1-58.—The secretion process.

The glomerulus filters gallons of blood each day. It is estimated that 2,500 gallons of blood pass through the kidneys in 24 hours, and about 80 gallons of glomerular filtrate. All the water from this filtrate is reabsorbed in the renal tubules except that containing the concentrated waste products.

The average amount of urine an adult excretes varies from 1,000 to 1,500 ml per day. However, the amount of urine excreted varies greatly with temperature, water intake, and state of health. No matter how much water one drinks, the blood will always remain at a constant concentration, and the excess water will be excreted by the kidneys. A large water intake does not put a strain on the kidneys. Instead it eases the load of concentration placed on the kidneys.

URETERS

The ureters' only function is to carry urine from each kidney to the urinary bladder. The ureters are two membranous tubes 1 mm to 1 cm in diameter and about 25 cm in length. Urine is transported through the ureters by peristaltic waves (produced by the ureter's muscular walls).

URINARY BLADDER

The urinary bladder functions as a temporary reservoir for urine. The bladder possesses features that enable urine to enter, be stored, and later be released for evacuation from the body.

Structure

The bladder is a hollow, expandable, muscular organ located in the pelvic girdle (fig. 1-59). Although the shape ofthe bladder is spherical, its shape is altered by the pressures of surrounding organs. When it is empty, the inner walls ofthe bladder form folds. But as the bladder fills with urine, the walls become smoother.

The internal floor of the bladder includes a triangular area called the trigone (fig. 1-59). The trigone has three openings at each of its angles. The ureters are attached to the two posterior openings. The anterior opening, at the apex of the trigone, contains a funnel-like continuation called the neck ofthe bladder. The neck leads to the urethra.

The wall of the bladder consists of four bundles of smooth muscle fibers. These muscle fibers, interlaced, form the detrusor muscle (which surrounds the bladder neck) and comprise what is called the internal urethral sphincter. The internal urethral sphincter prevents urine from escaping the bladder until the pressure inside the bladder reaches a certain level. Parasympathetic nerve fibers in the detrusor muscle function in the micturition (urination) process. The outer layer (serous coat) of the bladder wall consists of two types of tissue, parietal peritoneum and fibrous connective tissue.

Figure 1-59.—Urinary bladder and urethra:
A. Frontal section of the female urinary bladder and urethra;
B. Frontal section of the male urinary bladder and urethra. 

Micturition (Urination)

Micturition is the process by which urine is expelled from the bladder. It involves the contraction of the detrusor muscle, and pressure from surrounding structures. Urination also involves the relaxation ofthe external urethral sphincter. The external urethral sphincter surrounds the urethra about 3 centimeters from the bladder, and is composed of voluntary muscular tissue.

Urination is usually stimulated by the distention of the bladder as it fills with urine. When the walls of the bladder contract, nerve receptors are stimulated, and the urination reflex is triggered. The urination reflex causes the internal urethral sphincter to open and the external urethral sphincter to relax. This relaxation allows the bladder to empty. The bladder can hold up to 600 ml of urine. The desire to urinate may not occur until the bladder contains 250-300 ml.

URETHRA

The urethra is the tube that carries urine from the bladder to the outside of the body (fig. 1-59, views A and B). The urinary meatus is the external urethral orifice. In the male, the urethra is common to the urinary and reproductive systems; in the female, it belongs only to the urinary system.

Female Urethra

The female urethra is about 4 cm long, extending from the bladder to the external orifice, (fig. 1-59, view A).

Male Urethra

The male urethra is about 20 cm long and is divided into three parts: the prostatic, membranous, and penile portions. See view B of figure 1-59 for an illustration of the male urethra.

PROSTATIC URETHRA.—The prostatic urethra is surrounded by the prostate gland; it contains the orifices of the prostatic and ejaculatory ducts. This portion of the male urethra is about 2.5 cm long.

MEMBRANOUS URETHRA.—The mem­branous urethra is about 2 cm in length and is surrounded by the external urethral sphincter.

PENILE URETHRA.—The penile urethra, the longest portion, is about 15 cm long. It lies in the ventral portion of the penis. The urethra terminates with the external orifice at the tip of the penis.