The Function of the Excretory System Is to Regulate Blood Composition by Removing Wastes

EXCRETION

The function of the excretory system is to regulate blood composition by removing wastes (urea, uric acid, creatinine, ammonia) resulting from normal metabolic processes and excess water, ions, etc. The kidneys are the primary organs of excretion, serving to filter the blood, reabsorbing substances vital to homeostasis and excreting wastes and excesses.

The basic unit of the kidney is the nephron. The nephron is a microscopic structure located principally in the cortex (outer part) of the kidney. Each kidney contains about 1 million nephrons. A nephron begins with a tangled mass of arterioles (small arteries), the glomerulus, through which filtrate leaves the blood. The filtrate has a composition similar to blood plasma except that normally, blood proteins (and of course, blood cells) are absent, being retained within the arterioles. In total, within a 24 hour period, the entire volume of blood is filtered about 60 times, and 150-180 liters (about 40 gallons) of filtrate enter the nephrons. Obviously, not all of this filtrate leaves the body in one day! Normally, about 1-2 liters of urine are produced and eliminated from the body daily. Most of the glomerular filtrate is reabsorbed. This is the role of the tubular system of the nephrons. The filtrate from the glomerulus is collected by a thin-walled sac called the Bowman's capsule and channelled into the tubular system composed of the proximal convoluted tubule, loop of Henle, and distal convoluted tubule (see diagram below). The tubules are wrapped by blood capillaries and serve to both passively (by diffusion and osmosis) and actively (by active transport) remove valuable substances such as water, glucose, amino acids, and vitamins and return them to the blood and concentrate wastes such as urea (a slightly toxic by-product of protein metabolism) to form urine.

Urine is collected from the nephrons by a series of collecting ducts and conducted to an internal cavity, the renal sinus. A funnel-like expansion called the renal pelvis channels the urine into a tube, the ureter, which transports the urine to the urinary bladder for storage. The urine leaves the body via the urethra, which opens to the exterior. In males the urethra also serves to convey sperm.

Refer to Figures 10.4 and 10.5 in the Mader text and examine the plastic models as you label the diagrams of the excretory structures below.

Urinalysis

Normal urine is generally clear and has a pale yellow to amber color. This color is due to the presence of a pigment called urochrome which is a metabolite arising from the breakdown of hemoglobin. The color is indicative of the concentration of solutes in the urine, i.e., pale yellow = low concentration of solutes, dark amber = high concentration of solutes. Changes in color can be caused by certain foods (carrots --> yellow urine; beets --> red urine; rhubarb --> brown urine), drugs, or the presence of blood in the urine.

Urine normally has a slightly aromatic to ammonia odor. Ammonia odor is due to bacterial action after the urine has been left standing for some time. Foods such as asparagus, drugs, or disease may also cause the odor of urine to change.

Normal urine has a pH of 6.0, but may range from 4.5 to 8.0. High protein diets, diets high in wheat products, or disease such as diabetes mellitus may cause the urine to be more acidic than normal. Bacterial infection or a vegetarian diet may increase the alkalinity of urine.

Specific gravity is a measurement of the relative weight of a specific volume of liquid compared with an equal volume of water. This is a measure of the concentration of solutes in the solution. The specific gravity of pure water is 1.000; the normal range of specific gravity of urine is 1.015 to 1.030. The higher the specific gravity, the higher the concentration of solutes present. Very dilute urine, with a specific gravity less than 1.015, may indicate a person who drinks excessive amounts of water, uses diuretics, or suffers from diabetes insipidus or chronic renal failure. Very concentrated urine may indicate a person who does not drink enough water, has a fever or kidney inflammation (pyelonephritis). Concentrated urine may allow substances such as calcium to precipitate, forming renal calculi (kidney stones).

The normal constituents of urine are, in order of decreasing amounts: water, urea (from protein metabolism), sodium, phosphate, potassium, sulfate, creatinine (from muscle metabolism), ammonia, and uric acid (from nucleotide metabolism). Lesser amounts of magnesium, calcium, and bicarbonate ions are also present.

The detection of substances not normally present in the urine may indicate disease. Normally, glucose is absent from the urine or present at a maximum concentration of 0.03 g per 100 ml (0.03% glucose). Glucose present in greater amounts is referred to as glycosuria and may indicate excessive carbohydrate intake or, more importantly, diabetes mellitus, a serious condition caused by the faulty regulation of carbohydrate metabolism. Normally, blood sugar levels range from 80 to 120 milligrams (0.080 to 0.120 grams) per 100 ml blood, and the kidneys can completely reabsorb glucose even at the level of 160 mg per 100 ml. When blood (and hence filtrate) glucose levels exceed this concentration, the excess cannot be reabsobed and is passed in the urine. Diabetics may have blood glucose concentrations well over 200 mg per 100 ml, so considerable glucose appears in the urine.

Albumin is the most abundant protein found in the blood, and it serves a key role in maintaining proper water balance in the blood, drawing water from tissue fluids by creating an osmotic pressure. Normally albumin, a large protein, does not pass out of the blood at the glomerulus and therefore is not found in the urine. When it does, the condition is called albuminuria. This may be caused by muscular exertion, prolonged exposure to cold, pregnancy, or excessive protein intake. However, albuminuria may indicate pathology such as glomerulonephritis (inflammation and damage to the glomeruli), kidney trauma due to physical injury, ingestion of heavy metal poisons, toxemia of pregnancy, bacterial toxins, or hypertension (high blood pressure).

Ketones are compounds formed during fat catabolism (breakdown). Normally, fats are broken down to carbon dioxide and water, but ketones may appear when fat catabolism becomes excessive, particularly in disease. Ketonuria (ketone in the urine) is most often associated starvation or with diabetes mellitus. In diabetes, since glucose metabolism is defective, fats are catabolized at a high rate for energy, yielding high blood and urine concentrations of ketones.

Red blood cells are never present in normal urine. The presence of red cells in urine, hematuria, is indicative of pathology such as glomerulonephritis, kidney stones, or physical trauma (e.g., jogger's kidney).

Hemoglobin in urine results from hemolysis, the destruction of red blood cells. When red blood cells lyse, their hemoglobin is released into the plasma and some passes into the glomerular filtrate and hence the urine. This condition is hemoglobinuria and may be indicative of a large range of pathological conditions such as hemolytic anemia, malaria, transfusion reactions, hepatitis (inflammation of the liver), mushroom poisoning, and renal failure. A degradation product of hemoglobin, bilirubin, may appear in small amounts in the urine. Excessive bilirubin in the urine (bilirubinuria) indicates a liver disorder, such as hepatitis or cirrhosis, since the liver is normally responsible for the elimination of bilirubin via the bile, and its failure leads to elimination via the urine.

With this brief discussion of normal and abnormal constituents of the urine as background, you should now be able to analyze urine and draw some conclusions from your results. Each group will test four urine samples (Don't worry- it's not real urine!) and determine whether each is normal or indicative of pathology. If pathology is indicated, try to determine what disorder(s) might be responsible and give reasons to support your conclusions.

Perform the following tests and record your results (last page).

Color:

Normal = clear to slightly cloudy; pale yellow to amber

Milky, cloudy ==> pus, bacteria, or fat present

Reddish-amber ==> bile products (urobilinogen and urobilin) present

Brown-yellow or green ==> bile pigments

Red to smoky brown ==> blood present

Specific gravity:

Perform this determination at the supply table. Be very careful with the urine hydrometer; it is made of thin glass and breaks easily.

Pour urine into the cylinder until 3/4 full, then place the urine hydrometer into the urine. Read the specific gravity value where the urine level matches the graduation on the hydrometer stem. (You might check the accuracy of the hydrometer by using water instead of urine. The hydrometer should yield a specific gravity of 1.000 with water.)

Hydrometer

pH:

Hand-held pH meters will be available. Remove the black protective cover from the bottom of the meter, then dip that end into a beaker of distilled water to rinse. Blot off the excess water with a paper towel. Turn on the meter, then dip it into the urine to the immersion line. Stir, then read the pH from the digital scale on the body of the meter. Finally, turn off the meter, rinse it in water, blot dry, and replace the protective cover.

pH Meter

Glucose:

Place ~ 1/2 ml of Benedict's reagent into a test tube, then add 2 drops of urine. Place in a boiling water bath for 5 minutes; a change to orange-red indicates the presence of glucose.

Protein: (Bradford test)

Place 3 drops of urine in a test tube. Add 10 drops of Bradford's

reagent. A blue color is positive for protein.

Ketones:

Place 10 drops of urine and 5 drops of NaOH (sodium hydroxide) in a test tube. Add a drop of sodium nitroferricyanide solution, mix, then add 3 drops of acetic acid. A purple or violet-red color indicates the presence of ketones.

Hemoglobin:

Your instructor will demonstrate the Multistix system for the detection of hemoglobin (blood).

URINALYSIS RESULTS

Urine #1 Urine #2 Urine #3 Urine #4

Test

Color

Specific

gravity

pH

Glucose

Proteins

Ketones

Hemoglobin

Diagnosis and

Justification

Internet Resources

To learn more about diabetes and kidney diseases, see the National Institute of Diabetes and Digestive and Kidney Diseases site at

http://www.niddk.nih.gov.

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