|MadSci Network: General Biology|
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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