Urine Formation

Urine is made in the kidneys, namely in the nephrons, in a multi-step process allowing part of the organism's waste matter to be eliminated. The kidney also has other functions related to various regulatory systems, notably regulation of ions, acids and bases.

Diuresis is the process by which urine is formed. It includes several steps. First, filtration of the blood forms the glomerular filtrate, a "preliminary" form of urine. The blood passes through fenestrated capillaries (tiny arteries having walls perforated by pores) in the renal tubules. Molecules that are sufficiently small can pass through the capillary walls, due to a significant difference in the pressure on either side and collect in the renal tubules.

The resulting filtrate can be called a "preliminary" form of urine because its composition will be modified before it is eliminated. At this stage, the filtrate is very similar to plasma.

Following filtration, reabsorption occurs in the convoluted portion of the tubules, particularly in the proximal tubule. It allows molecules and ions needed by the organism to return to the bloodstream. Transport of these substances requires the expenditure of energy by cells and is often accompanied by reabsorption of water, which is a passive process.

Chloride, sodium and potassium are the main ions reabsorbed. Molecules reabsorbed by the convoluted portions of the tubules include all glucose and proteins, as well as some amino acids and organic acids.

Finally, some substances occur in the urine as a result of secretion, which also occurs in the convoluted portion of the proximal tubule. This mechanism affects both substances present in the blood (such as contrast media used in medical examinations, or medications such as penicillin) and substances created in the tubule's epithelium (such as ammonia). Here, again, both active and passive mechanisms are involved, as well as exchanges.

In the last part of the nephron, the collecting tubule, urine attains its final form. Regulatory mechanisms come into play to concentrate the urine and acidify it even more.

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