The urinary system is one of two systems that make up the urogenital system. This system helps to maintain the correct balance of water, salts and other substances in the body. This function is performed mainly by the kidneys, with special cells filtering water and wastes from the blood. Most of the water goes back to the blood, while the leftover (water not consumed by the body) and wastes constitute urine.
The volume of urine secreted is influenced by various factors. Included in these factors are the effect of the solids excreted, the link to the volume of blood, and the process of sugar diuresis.
Effect of solids excreted:
When there are more dissolved solids in the blood plasma for the kidney to excrete, the volume of urine correspondingly becomes greater. This is a common observation that can easily be proven by experiments. By clear description, a certain quantity of water must pass out of the kidney for each molecule of dissolved solid excreted. When, for instance, the concentration of glucose in the blood is above the threshold level (much like in diabetes), not only is glucose present in the voided urine but the volume of urine is greatly increased.
In like manner, urine flow is increased by an elevation of the blood's urea concentration. The urea of the blood works to accelerate excretion of the urea by stimulating urine flow. A similar example can be made out of another body waste - carbon dioxide. By its influence on the respiratory system, waste carbon dioxide acts as the stimulus for its own elimination.
Link to volume of blood:
It has been observed that the volume of urine is decreased following a hemorrhage; the relationship between the volume of blood and arterial blood pressure has a direct effect on this. A decrease in the volume of blood brings down the blood pressure. The blood pressure determines filtration through capillary walls. When the blood pressure is brought down, the glomerular filtration pressure is also reduced. Consequently, the urine's filtration into the capsule is decreased, and the volume of urine is less than sufficient. In this way, body fluids are maintained.
By analogy, any tendency for the volume of blood to become greater (an example is after ingesting great quantities of fluids) is offset partially by an increased filtration of urine into the capsule. As the kidneys excrete more water, the volume of blood is restored to normal level. There is, therefore, a direct connection between the blood pressure and the volume of urine, as there is a link between the rate of blood flow through the kidneys and the volume of urine formed.
Process of sugar diuresis:
Certain factors influence the occurrence of sugar diuresis. In the individual affected with diabetes, for example, the proximal convoluted tubule fails to reabsorb all the sugar from the tubular urine. Unable to reach the tubular epithelium, the sugar acts in much the same way as how any molecule would at a semipermeable membrane - it puts forth an osmotic effect. This effect tends to pull water into the tubular lumen from the blood and tissue fluids.
But since the reabsorption of water by the tubular cells occurs normally, the osmotic action tends to offset and reduce the rate of the absorption. This results in an increased diuresis, or the volume of urine excretion.
1. "Control of Urine Volume" - http://www.nsbri.org/HumanPhysSpace/focus4/ep-urinecontrol.html
2. "Fluid Exchange Processes in the Kidneys" - http://www.elmhurst.edu/~chm/vchembook/253fluidkidneys.html
3. "Your Urinary System and How It Works" - http://kidney.niddk.nih.gov/kudiseases/pubs/Yoururinary/