Urine Composition

• Erythropoiesis—the kidneys will sense a low oxygen tension in the bloodstream and will

cause erythropoietin (EPO) secretion. People with kidney failure can lose EPO

production and will develop anemia from this.

• Blood pressure regulation—this happens mainly through the regulation of water and

sodium balance in the kidneys and in the body. A loss of kidney function can lead to a

loss of volume control in the kidneys, which can result in low blood or high blood

pressure situations.

• Osmolarity regulation— the kidneys must hold on to protein in order to maintain the

osmolarity of the blood system. If protein is lost by damaged kidneys, this can lead to a

loss of osmolarity and the potential for leakage of water into the interstitial spaces. This

leads to peripheral edema.

• pH Regulation—this involves the maintenance of the pH of the blood and body so as to

keep homeostatic mechanisms in place.

URINE COMPOSITION

So far, we have learned about the making of urine by the glomeruli and the rest of the renal

tissue. While nearly 200 liters of filtrate pass through the glomeruli per day, all but 1-2 liters are

made as urine during this time. The urine composition depends on water intake, level of

exercise, nutrient intake, environmental temperature, and others. Urine can be fairly clear or

dark amber, depending on how concentrated it is. A urinalysis is done to assess the presence or

absence of kidney diseases as well as other possible diseases (such as liver disease or diabetes).

The color of urine comes mainly from the breakdown of hemoglobin and the production of

urochrome, a yellow breakdown pigment. The urine color can also be affected by eating

berries, fava beans, or beets, which contain pigmented molecules. Blood in the urine comes

from bladder or kidney infections, urinary tract infections or cancers, and sometimes kidney

stones. Dehydration will darken the urine and will make it slightly more malodorous. Ammonia

comes from the urea in the urine but doesn’t create an odor until the urine has sat outside the

body for a period of time. Certain foods, like asparagus, fish, garlic, and onions will cause an

aromatic urine.

The volume of urine depends on the state of hydration of the body. A minimum of 500

milliliters of urine must be created in order to get rid of body waste products. Oliguria involves

severe low output of urine, while polyuria involves severe increased output of urine. Anuria

involves the absence of urine completely. The pH of the urine varies according to the diet and

the pH of the bloodstream, as urine will help regulate the homeostasis of the acid-base

situation of the body.

The specific gravity of the urine is the measure of the number of solutes in the liquid per unit of

volume. This is easier to measure than the osmolarity. Water has a specific gravity of 1.0 and

all urine will have a specific gravity greater than that.

Normally, cells won’t be present in urine but, in cases of a bladder infection, there will be

leukocytes. These cells will release leukocyte esterase, which is also a test that is done on a

urine dipstick in order to determine if there are leukocytes in the urine.

Protein is not usually found in the urine because it doesn’t pass through the glomerular

capillaries. In cases of kidney damage, however, there can be proteinuria or a positive test for

protein in the urinalysis.

The urine dipstick will also assess urine for ketones, which are byproducts of fatty acid

metabolism. This is seen in severe diabetes and in the presence of a low carbohydrate, low

protein diet.

The nitrite test on a urinalysis is a test done to evaluate the urine for bacteria. When gram-

negative bacteria are in the urine, they will metabolize nitrates into nitrites, which will be

detected in the urine.