The volume and composition of body fluids are maintained at relatively constant values by physiological control mechanisms. The maintenance of the internal environment in a stable condition istermed homeostasis. Obviously, the kidney is a vary important homeostatic organ.
It is a very well known fact that when individuals consume large amounts of liquid, they eliminate much of the liquid through urine. On the other hand, when individuals are severly dehydrated, very little urine is formed. This certainly illustrates that control mechanisms in the organism can regulate the amount of urine that is formed.
When tap water is ingested, there is a relatively rapid elimination of the water in the urine. However, when salt is ingested, there is a considerable delay in the excretion of water because salt is ingested. The ingestion of coffee is very quickly followed by the rapid elimination of urine as a result of the effect of caffeine on the kidney. This effect would be observed with any beverage containing caffeine.
The measurement of pH, color, and specific gravity of urine is an indication of whenther urine is dilute of concentrated. If urine is concentrated, then the pH would be low, the color would be dark yellow and the specific gravity would be high. A dilute urine, however, would have a high pH, be almost colorless and have a low specific gravity.
Under normal conditions, the pH of urine is slightly acidic, because metabolic reactions in the cells generate acid materials. The normal pH of urine is about 6; although, under certain conditions, the pH could be as low as 4.5 or as high as 8. The color of urine is due to the presence of bile pigments which are end products of hemoglobin metabolism. If these pigments are concentrated in urine, then urine would have a darker color. The specific gravity of urine is due to the presence of solutes, most of which is sodium chloride. A high specific gravity results from the presence of large amounts of solutes as observed in concentrated urine.