MadSci Network: General Biology

Re: Why does the chemical content of urine change throughout the day?

Date: Mon Nov 27 19:39:58 2000
Posted By: Jim Bridger, Faculty, Biology, Prince Georges Community College
Area of science: General Biology
ID: 974420174.Gb

Stephanie:  That is a good question!  As you undoubtedly know, urine is 
simply a filtrate of blood.  As the blood goes through the nephrons of the 
kidney ( about 1,000,000/kidney )the blood is filtered, then some of the 
filtrate is reabsorbed and then materials are secreted into the urine by 
active transport.  So basically, anything that the blood contains that is 
small enough to go through the pores of the capsule and capillaries will 
wind up in the filtrate.  Some things, like glucose, are reabsorbed 
actively back into the blood.  Water is passively reabsorbed along with 
some electrolytes etc.  Some materials that are not found in the blood are 
manufactured (secreted) by the kidney tubules so that they can be excreted. 
These include urea and uric acid.  Since the content of your blood changes 
due to your diet or disease, so does your urine.  For example, the color 
varies due water content and because of beets or rhubarb in the diet.  The 
pH gets more basic in a vegetarian's diet and some foods such as asparagus 
and garlic change urine's odor.  In addition to these normal variations, 
diseases such as diabetes will greatly effect urine's composition and odor. 
 Also, the amount of water varies due to sweating or vomiting.  The body 
homeostatically decreases urine output in an attempt to conserve water in 
the body.  Hence, the concentrations of solutes increase.  We normally 
think of the kidneys as the eliminator of liquid wastes for the body, but 
we must realize that by doing that, they regulate our body's chemistry in a 
very precise manner.  Without this activity, we die, or will need dialysis 
and/or a transplant.  I hope this helps you.  J. Bridger

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