THE URINARY SYSTEM: KIDNEY STRUCTURE AND FUNCTION

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THE URINARY SYSTEM:
                       KIDNEY STRUCTURE AND FUNCTION
Purpose:

Following this lab you should have an understanding of:

             Kidney gross anatomy and histology
             Normal physiological processes responsible for urinary concentrations of solutes
             Pathological processes that may produce abnormal urinary concentrations of solutes and
              their clinical significance
             Normal constituents of urine sediment and their clinical significance

Introduction:

The kidneys are responsible for the elimination of most of the waste products from the metabolism of
nitrogen-containing compounds such as urea and creatinine from protein catabolism, as well as ketone
bodies from fat catabolism. The difficulty of this kidney function is compounded by the fact that the
kidney also reabsorbs molecules necessary for normal body function such as glucose, amino acids, and
bicarbonate.

The kidneys are involved in the maintenance of a constant internal environment (homeostasis ) of
electrolyte (salt) concentrations, fluid balance, and acid-base balance. The fluid volume of the blood is
maintained by the reabsorption of 98-99% of the water that leaves the blood in the initial step of urine
formation, while the electrolyte and pH balance of the blood is maintained by the selective reabsorption
of such ions as Na+, K+, Cl- and HCO3-.

In this exercise, you will examine the gross external and internal anatomy of the sheep kidney (a
representative mammalian system), the tissues associated with the renal system, and perform some
simple clinical studies of your own urine. Participation in the last portion of the exercise is completely
voluntary.

Organs of the Urinary System
Figure 1.1 illustrates the components of the urinary system with its principal blood supply. It consists of
two kidneys, two ureters, the urinary bladder, and a urethra. The kidney is an anteroposteriorly
compressed lima bean-shaped organ embedded retroperitoneally in a fatty fibrous pouch lateral to the
vertebral column. The kidney is high in the abdominal cavity, between the levels of the twelfth thoracic
and the third lumbar vertebrae. The left kidney is usually slightly higher than the right due to the large
area occupied by the liver. Each kidney is supplied blood through a renal artery that is a branch of the
abdominal aorta. Blood leaves each kidney through a renal vein that empties into the inferior vena
cava.

    Figure 1.1. The human urinary system.

                                                   Label the designated structures.

                                                   Renal artery and vein - vessels that transport blood to
                                                     and from the kidney.
                                                   Kidney - paired organ located lateral to the vertebral
                                                     column; high in the abdominal cavity.
                                                   Ureter - tube that transports urine from the kidney to
                                                     the urinary bladder.
                                                   Urethra - tube that transports urine from the urinary
                                                     bladder out of the body.
                                                   Urinary bladder - distensible sac in the pelvic cavity,
                                                     just posterior to the symphysis pubis; stores urine
Urine passes from each kidney to the urinary bladder through a ureter. The upper end of each ureter is
enlarged to form a funnel-like renal pelvis. The lower end of each ureter enters the posterior surface of
the bladder. Leading from the urinary bladder to the exterior is a short tube, the urethra. In males the
urethra is about 20 centimeters long; in females, it is approximately 4 centimeters in length.

The exit of urine from the bladder is called micturition. Two sphincter muscles control the passage of
urine from the bladder. The sphincter viscae is a smooth muscle sphincter that is near the exit of the
bladder. When approximately 300 ml of urine has accumulated in the bladder, the muscular walls of the
bladder are stretched sufficiently to initiate a parasympathetic reflex that causes the bladder wall to
contract. These contractions force urine past the sphincter vesicae into the urethra above the sphincter
urethrae . This second sphincter, located approximately 1-3 centimeters below the sphincter vesicae,
consists of skeletal muscle fibers and is voluntarily controlled. The presence of urine in the urethra above
this sphincter creates the desire to micturate; however, since the valve is under voluntary control,
micturition can be inhibited. When both sphincters are relaxed, urine passes from the body.

Kidney Anatomy
Figure 1.2 reveals a frontal section of a mammalian kidney. Its outer surface is covered with a thin
fibrous renal capsule. In addition to this thin covering, the kidney is provided support and protection by
a fatty capsule that completely encases it.

    Figure 1.2. Longitudinal section of a kidney.




Immediately under the capsule is the cortex of the kidney. The cortex is reddish-brown due to its great
blood supply. The lighter inner portion is called the medulla. The medulla is divided into several cone-
shaped renal pyramids. Cortical tissue, in the form of renal columns , extends down between the
pyramids. Each renal pyramid terminates as a renal papilla, which projects into a calyx. The calyces are
short tubes that receive urine from the renal papillae; they empty into the large renal pelvis.

Procedure:

Sheep Kidney Dissection
1. Obtain a sheep kidney. If the kidney is still encased in fat, peel off this adipose layer carefully. As you
   lift the fat from the kidney, look carefully for the adrenal gland, which should be embedded in the fat
   near one end of the kidney. Remove the adrenal gland from the fat and cut it in half. Note that the
   gland has a distinct outer cortex and inner medulla.

2. Examine the concave hilum and identify the renal vessels and the ureter.

3. The shiny covering of the kidney is the renal capsule. Probe into the surface of the kidney with a sharp
    dissecting needle to see if you can differentiate the capsule from the underlying tissue.
4. With a scalpel, slice the kidney longitudinally to produce a frontal section (posterior and anterior
   halves) similar to Figure 1.2. Identify the renal cortex, renal medulla, renal papillae, renal pyramids,
   renal pelvis, renal artery, renal vein, and ureter.

Histological examination of renal tissue

The basic functional unit of the kidney is the nephron. Figure 1.3 illustrates a single nephron in detail. It
has been estimated that there are around one million nephrons in each kidney. Approximately 80% of
the nephrons are located in the cortex; these are called cortical nephrons. The remainder are the
juxtamedullary nephrons which are located partially in the cortex and partially in the medulla.

   Figure 1.3. Diagram of a nephron.




Each nephron is composed of two parts: 1) the glomerulus, which is a tightly woven, highly permeable
capillary bed at the end of an arteriole; and 2) the renal tubule, which is a bent and convoluted tube
composed of kidney cells. The mouth of the renal tubule (Bowman's capsule) envelops the glomerulus.
The last part of the renal tubule (the collecting duct) empties its contents into the renal pelvis, which
funnels urine to the ureters.

The formation of urine in the nephron can be divided into two stages. 1) The hydrostatic pressure of the
blood squeezes fluid out of the capillary wall of the glomerulus, producing an ultrafiltrate of blood.
Except for proteins, which are usually too large to leave the capillaries, the glomerular filtrate contains
the same solute molecules as plasma and is isotonic to plasma. 2) As the glomerular filtrate passes
through the renal tubules, the cells of the tubules selectively reabsorb solute molecules and ions. The
solution that emerges at the end of the collecting duct (urine) is thus very different in composition and
concentration from the solution that enters the tubule (glomerular filtrate).

In the adult human, 180 liters of filtrate are produced each day, but the daily urine output is only 0.6 to
2.0 liters. If we assume that the plasma volume is 3 liters, the production of 180 liters of filtrate means
that all of the blood plasma must be filtered through the glomeruli once every 40 minutes or 60 times in
24 hours. Therefore, 178 to 179 liters of fluid must be reabsorbed along with many solutes, a huge
energetically expensive process.

Procedure:
1. Examine a histological slide of a human kidney and observe it under the 10x objective. In the cortex of
    the kidney, there are groups of cells that make up the renal corpuscles. Each consists of a tuft of
    interlaced capillaries collectively called the glomerulus surrounded by the cup-like Bowman's
    capsule . You will have to search on the slide to find these structures because the plane of the slice
    does not pass directly through each nephron. In addition, nephron structure is difficult to discern in a
    prepared slide, since the plane of the section rarely coincides with the plane of the nephron.
2. Examine a histological slide of a human kidney with cancer and compare it with normal renal tissue.
   Kidney cancers are usually a malignant neoplasm of the renal parenchyma ore renal pelvis. Factors
   associated with an increased incidence of disease are exposure to aromatic hydrocarbons or tobacco
   smoke and the use of drugs containing phenactetin.




3. Examine a histological slide of a human bladder. Urine is channeled from the kidneys to the urinary
   bladder via the ureters and expelled from the body through the urethra. The mucosal layer lining the
   lumen of the ureters and urinary bladder are composed of transitional epithelium that permits
   distention of the tissue. This epithelial tissue is unique to the urinary system.




Urine Composition and Testing
Urine is a highly complex aqueous solution of organic and inorganic substances. Urea, uric acid,
creatinine, sodium chloride, ammonia, and water are its principal ingredients. Most substances are either
waste produces of cellular metabolism or products derived directly from certain foods that have been
eaten. A clinical examination of urine may provide evidence of urinary tract infection or kidney disease.
In addition, since urine is derived from plasma, an examination of the urine provides a convenient, non-
invasive means of assessing the composition of plasma and detecting a variety of systemic diseases.

NOTE: These tests are not being carried out by health care professionals in a clinical setting. Therefore,
the results are not truly indicative of health or pathology. If you have any questions about the results of
these tests, please see your private physician.
A clinical examination of the urine includes an observation of its appearance, tests of its chemical
composition, and a microscopic examination of urine sediment. The best time for collecting a urine
sample is three hours after a meal. First samples taken in the morning are least likely to reveal abnormal
substances in the urine.

Procedure:
1. Urine should be collected in a clean container and tested as soon as possible.

Visual Appearance of the Sample

2. The color and turbidity of urine can provide clues as to evidence of pathology. Evaluate your urine
   sample according the the criteria below and enter your observations in Table 1.1.

   Color: Normal urine will vary from light straw to amber color. The color of normal urine is due to a
      pigment called urochrome, which is the end product of hemoglobin breakdown:

       hemoglobin         hematin         bilirubin      urochromogen         urochrome

       The following are deviations from normal color that have pathological implications:
       a. Milky: presence of pus, bacteria, fat, or chyle.
       b. Reddish amber: presence of urobilinogen or porphyrin. Urobilinogen is produced in the
          intestine by the action of bacteria on bile pigments. Porphyrin may be evidence of liver
          cirrhosis, jaundice, Addison's disease, and other conditions.
       c. Brownish yellow or green: presence of bile pigments. Yellow foam is definite evidence of bile
          pigments.
       d. Red to smoky brown: presence of blood and blood pigments.

      Carrots may cause increased yellow color due to carotene, while beets cause reddening and
      rhubarb may cause the urine to become brown. These food items and certain drugs may color the
      urine, yet have no pathological significance.

   Transparency (cloudiness): A fresh sample of normal urine should be clear, but may become cloudy
      after standing for a while. cloudy urine maybe evidence of phosphates, urates, pus, mucus,
      bacteria, epithelial cells, fat, and chyle. Phosphates disappear with the addition of dilute acetic acid
      and urates will dissipate with heat. Other causes of turbidity can by analyzed by microscopic
      examination. Shake your sample and record the degree of cloudiness in Table 1.1.

Other Tests

Other important parameters of urine composition can be measured easily using dipsticks. We will be
using Clinistix, which measures nine different parameters at once. Although you will use one dipstick,
each parameter will be discussed individually. Pay attention to the amount of time that must pass before
reading the dipstick; this time ranges from 30 to 60 seconds.

3. Remove a strip from the bottle and replace cap tightly.

4. Completely immerse reagent areas of the strip in fresh, well-mixed urine.              Remove the strip
   immediately to avoid dissolving out the reagent areas.

5. While removing the strip, touch the side of the strip against the rim of the urine container to remove
   excess urine. Blot the lengthwise edge of the strip on a paper towel to further remove excess urine and
   avoid contamination from adjacent reagent pads.

6. Compare each reagent area to the corresponding color blocks on the color chart and read at the times
   specified. Proper read time is critical for optimal results. Changes in color after 2 minutes are of no
   diagnostic value.
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