LESSON 8 The Human Urinary System.

LESSON ASSIGNMENT Paragraphs 8-1 through 8-7.

LESSON OBJECTIVES After completing this lesson, you should be able to identify the major function of the urinary system.

SUGGESTION After completing the assignment, complete the exercises at the end of this lesson. These exercises will help you to achieve the lesson objectives.

a. The urinary system is a collection of organs to rid the body of nitrogenous wastes. These nitrogenous wastes are created by the metabolism of proteins.

b. The urinary system includes the organs known as the kidney, the ureters, the urinary bladder, and the urethra (Figure 8-1). Together, these organs remove the nitrogenous wastes from the circulating blood, concentrate them into a fluid known as urine, and eliminate the urine from the body.

8-2. GENERAL ANATOMY OF THE KIDNEY

In the human, there are two kidneys, one right and one left.

a. Location.  Both kidneys are attached high up on the posterior abdominal wall. The left kidney is slightly higher than the right.

b. Shape.  In the adult, each kidney measures about 1x2x4 inches. The kidneys have a kidney-bean shape. That is, they are notched on the medial side, they have a convex lateral curvature, and their front and rear surfaces are somewhat flat.

c. Capsule.  Each kidney is surrounded by a dense FCT membrane called a capsule.

d. Internal Structure.  When a kidney is cut from side to side, the internal structure is similar to that in Figure 8-1. There is a fleshy portion surrounding a central opening. The fleshy portion is divided into an outer cortex layer and an inner medulla.

(1) The medulla consists of a series of pyramids whose apices (peaks) point into the hollow center of the kidney. The apex (peak) of each renal pyramid is known as the papilla.

Figure 8-1. The human urinary system.

(2) The central cavity of the kidney is known as the renal sinus. Its opening on the medial aspect of the kidney is known as the hilus (or hilum). The sinus contains a number of structures:

(a) The spaces among these structures are filled with loose areolar FCT (fibrous connective tissue) and fat.

(b) The renal NAVL enter the kidney directly from the abdominal aorta, through the hilus, and into the renal sinus. They then continue in a regular pattern throughout the medulla and cortex of the kidneys.

(c) A funnel-shaped, cup-like tube, called a calix (or calyx), surrounds the papilla of each pyramid. All of the calices are continuous with and empty into a hollow structure called the renal pelvis.

e. Adherence to the Posterior Abdominal Wall.  Each kidney is attached to the posterior abdominal wall on its respective side. Enclosing the kidneys and holding them in place are special perirenal fascial membranes and perirenal fats. During a "crash diet," an individual may lose some of this perirenal fat. This allows the kidney to move with the motions of the body. If the kidney should slump too far down, a kink may form in the ureter. This would prevent the normal flow of urine from the kidney to the bladder.

The actual unit of kidney function is the structure referred to as the nephron (Figure 8-2). It is estimated that each kidney has about a million nephrons. Each nephron consists of a renal corpuscle and a tubular system.

a. Renal Corpuscle.  A nephron begins with a renal corpuscle. The renal corpuscle is made up of a double-walled capsule and an arterial capillary network known as the glomerulus. An afferent arteriole supplies blood to the glomerulus, and an efferent arteriole drains blood from the glomerulus.

AFFERENT = carry to

EFFERENT = carry away from

The blood from the afferent arteriole fills the glomerulus. Because of a pressure gradient, a large percentage of fluid in this blood passes through the wall of the glomerular capillary. The fluid then passes through the inner wall of the capsule. This brings the fluid into the hollow space between the inner and outer walls of the capsule.

b. Tubular System.  The fluid, or filtrate, then passes through the tubular system of the nephron. Here, the majority of the water, glucose, and other valuable substances are reabsorbed from the fluid and returned to the cardiovascular system. Thus, at the end of the tubular system, the result is a very concentrated fluid containing the nitrogenous wastes. This concentrated fluid is called urine.

Figure 8-2. A "typical" nephron.

The urine from each nephron flows into a collecting tubule (straight renal tubule). The collecting tubules merge until they form one of the papillary ducts that open at the papilla of the renal pyramid. At the papilla, the urine empties into the calices. The urine then flows into the renal pelvis in the sinus of the kidney.

8-5. THE URETERS

The ureter is a tubular structure that is continuous with the renal pelvis. The ureter of each kidney passes down the posterior abdominal wall on its respective side. The ureter then enters the pelvic region. The urine moves along the ureters drop by drop, pushed by the wave like muscular contractions (peristalsis) of the tubular wall. In the pelvis, the two ureters enter the posterior inferior corners of the urinary bladder.

8-6. THE URINARY BLADDER

The urinary bladder is an organ that is highly specialized to store urine until it is eliminated from the body.

a. Trigone. The base of the urinary bladder is known as the trigone because of its triangular shape. The trigone is fairly solid and nonstretchable.

b. Stretchable Wall. The rest of the wall of the urinary bladder is very stretchable and forms a spherical sac when filled.

c. Transitional Epithelial Lining.  The mucosal lining of the urinary bladder is made up of a unique epithelium, called the transitional epithelium.

(1) Voiding reflex. The transitional epithelium has the capacity to stretch to a certain degree. At the limit of its stretchability, it causes a message to be sent to the spinal cord about the fullness of the urinary bladder. This initiates the voiding reflex, which would cause the urine to pass out of the body.

(2) Increments of stretching and reorganization. Often, however, it is not convenient to void (empty the bladder). Thus, after a short period, the transitional epithelium can reorganize itself and undergo another increment of stretching. Soon, however, the fullness message is somewhat more urgent. There can be several increments of stretching until the limit of the urinary bladder is finally reached. At that limit, the urine must be voided.

8-7. THE URETHRA

The urethra is the single tubular structure that connects the urinary bladder to the outside.

a. Sexual Dimorphism.  Relatively short and straight, the female urethra opens directly to the outside. However, the male urethra is incorporated into the penis. Since the male urethra has two more-or-less right-angle turns, one permanent and one flexible, the male is more difficult to catheterize than the female.

b. Urethral Sphincters.  The urethral sphincters are two muscular structures which prevent urine from leaving the urinary bladder. Each urethral sphincter is a circular mass of muscle tissue. Relaxation of the sphincters allows urine to be forced through them.