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After completing this lesson, you should be able to:
  • Identify the structure, location, and function of the kidneys.
  • Identify the structure, location, and function of the kidneys.
  • Identify the structure, location, and function of the ureters.
  • Identify the structure, location, and function of the bladder.
  • Identify the structure, location, and function of the urethra.
  • Identify terminology, physiology, and important characteristics of urine.
  • Identify internal and external components and functions of the male genitalia.
  • Identify characteristics, structures, and functions of the female genital system.
 

Contents

 

 

 

 

 

  1. Introduction
  2. Kidneys
  3. Ureters
  4. Urinary Bladder
  1. Urethra
  2. Urine
  3. Exercises

 

Introduction

Urinary refers to the system responsible for removal of nitrogenous waste products of metabolism from the bloodstream, disposal of concentrated wastes (urine), and also water conservation.

 The urinary system's primary function is to help keep the body in homeostasis (internal environment of the body remains relatively the same, within limits) by controlling the composition and volume of blood.  The urinary system does this by removing and restoring selected amounts of water and solutes. The urinary system is made up of two kidneys, two ureters, one urinary bladder, and one urethra.  Each kidney, the primary organs of this system, excretes urine through a ureter. The urine is stored in the urinary bladder and finally expelled from the body through the urethra.

See figures 1 and 2 for organs of the urinary system

 Figure 1.  Organs of the male urinary system.

 Figure 2.  Urinary system organs.

 

NOTE: Other systems also have a part in waste elimination from the body. These systems are the respiratory, integumentary, and digestive systems

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Kidneys

The two kidneys are reddish, bean-shaped organs. They are located on the posterior wall of the abdominal cavity between the level of the twelfth thoracic vertebra (T-12) and the third lumbar vertebra (L3). Think of these organs as being just above the waist. The term for their location is retroperitoneal, which means that they are external to the peritoneal lining of the abdominal cavity. These organs are about 11 to 12 centimeters long and 5 to 6 centimeters wide.  Since the liver occupies a large space on the right side of the abdominal cavity, the right kidney is slightly lower than the left kidney. The kidneys are held in place by fat. See figure 3.

Figure 1-3.  External view of the right kidney.

Functions of the Kidneys. The kidneys accomplish these functions:

Kidney Tissue.  The kidneys are composed of three types of tissue: capsule tissue, cortex tissue, and medulla tissue. Capsule tissue is tough, white fibrous connective tissue on the kidney surface. Cortex tissue covers the outer portion of the kidney.  Cortex tissue is firm and reddish-brown in color. Medulla tissue makes up the masses of collecting tubes, which are the inner portion of the kidneys. The renal pyramids and papillae are composed of medulla tissue. Cortex tissue extends between the renal pyramids and the conical masses of tubes in the inner part of the kidneys. Cortex tissue forms an outer layer on the kidneys and medulla forms inner kidney renal columns. See figure 4.

Figure 4.  Diagram of a cross section of the right kidney.

Hilium. The hilium is a notch near the center of the rounded border of each kidney.  Blood vessels and the ureter enter each kidney at this point.

Renal Pelvis. The part of the kidney that is the collecting point for urine formed in the kidneys is called the renal pelvis.  Peristalsis carries urine from the renal pelvis to the ureter.

Nephron.  The nephron is the functional unit of the kidney. It is estimated that each kidney has about a million nephrons. Each nephron consists of a renal corpuscle and a tubular system. See figure 5.

NEPHRON = renal corpuscle + tubular system

Figure 5. A "typical" nephron.

Renal corpuscle.

Structure. The renal corpuscle has a hollow, double-walled sac called the renal capsule (Bowman's capsule).  Leading into the capsule is a very small artery called the afferent arteriole.  Within the capsule, this artery becomes a mass of capillaries known as the glomerulus. An efferent arteriole drains blood away from the capsule.  The capsule and glomerulus together are known as the renal corpuscle.

Function.  An afferent arteriole supplies blood to the glomerulus. An efferent arteriole drains blood from the glomerulus. 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 renal capsule.

AFFERENT = carry to EFFERENT = carry away from

Renal tubule

Structure. Each renal capsule is drained by a renal tubule. This part of the tubular system runs a long distance in a coiled formation and is called the proximal convoluted tubule.  A long loop, the renal loop of Henle extends down into the medulla with two straight parts and a sharp bend at the bottom. As the tube returns to the cortex layer, it once again becomes coiled and is known here as the distal convoluted tubule. The distal convoluted tubule is the end of the nephron unit.

Reabsorption. The renal tubule reabsorbs the fluid or filtrate passing through the tubular system, of the nephron.  As fluid or filtrate passes through the renal tubule, the majority of the water, glucose, and other valuable substances are removed from the fluid, reabsorbed in the tubule, and returned to the cardiovascular system. Essential electrolytes such as sodium, chloride, and bicarbonate are reabsorbed in the tubules.  The hormone aldosterone controls the reabsorption of sodium salts.  Aldosterone is a hormone produced in the adrenal gland.  Water and nonelectrolytes such as glucose, amino acids, and nutrients are also absorbed by renal tubes.

Secretion.  The function of the renal tubule is tubular secretion. Tubular reabsorption removes substances from the filtrate into the blood and also adds materials to the fluid from the blood. Substances secreted by renal tubules include potassium ions, hydrogen ions, ammonia, creatinine, and the drugs penicillin and aminohippuric acid. There are two main effects from tubular secretions.  The substances in the secretions help rid the body of certain materials and also help control the blood pH.

Summary.

The major work of the urinary system is done by the nephrons. The other parts of the urinary system are mainly passageways and storage areas. Each nephron is made up of a renal corpuscle and a tubular system. Three important functions are carried out by nephrons.

  1. Nephrons control blood concentration and volume by removing selected amounts of water and solutes.
  2. Nephrons help regulate blood pH.
  3. Nephrons remove toxic wastes from the blood.

As the nephrons go about these activities, they remove many materials from the blood, return the ones that the body requires, and eliminate the remainder. The eliminated materials are collectively called urine.  The entire volume of blood in the body is filtered by the kidneys about 60 times a day.

Collecting Tubules.  The distal convoluted tubules of several nephrons empty into a collecting tubule. That is, several tubules join larger tubules of a renal pyramid to form one tubule that opens at a renal papilla and drains into a calyx in the renal pelvis.  The final reabsorption of electrolytes, water, and glucose takes place in these collecting tubules. Also, the final secretion of electrolytes and nonelectrolytes takes place here.

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Ureters

A ureter is a tubular structure that is continuous with the renal pelvis. The ureters, one for each kidney, are musculomembranous in structure, 10 to 12 inches long, and form the upper part of the renal pelvis of the kidney.  The ureters convey urine from the kidneys to the bladder by the process of peristalsis. Urine moves along the ureters drop by drop, pushed by the wave-like muscular contractions of peristalsis of the tubular wall. From the renal pelvis, the urine drains into the ureters, entering the urinary bladder at its base. See figure 6.

Figure 6.  Urinary bladder and female urethra.

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Urinary Bladder

The urinary bladder is a musculomembranous sac located in the lower portion of the abdominal cavity behind the symphysis pubis.  The organ is highly specialized to store urine until the urine is eliminated from the body.

Trigone.  The base of the urinary bladder is known as the trigone because of its triangular shape.  The trigone is fairly solid and nonstretchable. See figure 1-6.

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

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

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 causes the urine to pass out of the body.

Increments of stretching and reorganization.  Often, however, it is not convenient to void (empty the bladder). Thus, after a short period of time, 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 has been reached.  At that limit, the urine must be voided.

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Urethra

The urethra is a tube that conducts urine from the urinary bladder to the outside of the body.  This structure begins at the anterior base of the urinary bladder.

Male-Female Differences.  The female urethra is short and opens directly to the outside. The male urethra, however, is much longer and has two curvatures.  The male urethra is divided into three sections: prostatic, membranous, and penile. The prostatic portion enters the prostate gland.  The membranous portion enters the peritoneum and the penile portion forms in the shaft of the penis.

Urethral Sphincters. The urethral sphincters are two muscular structures that 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.

 

NOTE: Remember the difference between the ureter and the urethra.  The ureter is a tube draining urine from the kidneys to the urinary bladder. The urethra is a tube draining urine from the urinary bladder to the outside.

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Urine

Urine is a by-product of the kidney's activity. Volume, pH, and solute concentration vary with the needs of the body's internal environment in a healthy person.  The characteristics of urine may change greatly when a person is ill. It is possible to find out a great deal about the state of a body by analyzing the volume, physical properties, and chemical properties of urine.

Physical Characteristics.  Normal urine is yellow or amber colored and transparent.  The color, caused by pigments from the metabolism of bile, can change because of medication or diet. A person eats beets and his urine may be a reddish color.

An individual takes large amounts of vitamin C and his urine may be deep yellow for a time. The odor of urine varies. Stale urine develops an ammonia odor (think of a baby's diaper), but the urine that is expelled after the digestion of asparagus will have a completely different, but characteristic, odor.

Composition. About 95 percent of the total volume of urine is water.  The other 5 percent is made up of solutes that come from cellular metabolism and outside sources such as drugs. Included in the 5 percent are nitrogenous waste products, electrolytes, toxins, and urea. Urea, an end product formed in the liver from protein metabolism, is the chief nitrogenous waste product in the non-water portion of urine.

 

NOTE: One of the screening tests for renal function is the blood urea nitrogen (BUN) test. It is the most commonly ordered test. It measures blood levels of nitrogen in urea.  This test is not sensitive to mild degrees of renal dysfunction, but it is a good clinical indication of significant renal dysfunction.

Specific Gravity.  This is the ratio of the weight of a volume of a substance to the weight of an equal volume of distilled water. The specific gravity of water is 1.000. The specific gravity of urine depends on the amount of solid materials in the urine.  The specific gravity of normal urine ranges from 1.010 to 1.020.  The greater the concentration of solutes in the urine, the higher its specific gravity.

Amount Voided. A normal adult eliminates about 1500 cubic centimeters of urine daily.  The amount of urine voided depends on a number of factors: blood pressure, blood concentration, diet, temperature, diuretics, mental state, and general health.

Acidity/Alkalinity of Urine.  Normal urine is slightly acid, but the acidity and alkalinity of urine varies greatly with an individual's diet. A high-protein diet increases the acidity of urine while a diet consisting of mostly vegetables increases the alkalinity of urine. Other factors influencing urinary pH include high altitude, fasting, and exercise.

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Exercises

  1. What is the primary function of the urinary system? _____
  2. List the major parts of the urinary system: _____, _____., and _____.
  3. List three functions of the kidneys: _____, _____., and _____.
  4. The notch near the center of the rounded border of each kidney is called the _____.
  5. The collecting point for urine in the kidney is called the _____.
  6. List the three important functions carried out by the nephron units in the kidneys._____, _____., and _____.
  7. Where is the urinary bladder located? _____
  8. Name the reflex that causes urine to pass out of the body. _____
  9. What is the difference between the female urethra and the male urethra? _____
  10. The two muscular structures which prevent urine from leaving the urinary bladder are the _____ and _____.
  11. Approximately 95 percent of the total volume of urine is _____.
  12. The blood urea nitrogen (BUN) test is a screening test for _____.

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Solutions to Exercises

1. The primary function of the urinary system is to help keep the body in homeostasis by controlling the composition and volume of blood.

2. Two kidneys. Two ureters. One urinary bladder. One urethra.

3. You are correct if you listed any three of the following:

Filtration of metabolic wastes.

Urine formation.

Fluid and electrolyte balance.

Acid base balance.

Influence blood pressure.

4. Hilium.

5. Renal pelvis.

6.   Control blood concentration and volume by removing selected amounts of water and solutes.

Help regulate blood pH.

Remove toxic wastes from the blood.

7. The urinary bladder is located in the lower portion of the abdominal cavity behind the symphysis pubis.

8. The voiding reflex.

9. The female urethra is short and opens directly to the outside. The male urethra is much longer and has two curvatures.

10. Urethral sphincters.

11. Water.

12. Renal function.

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