The formation of urine involves three processes: ultrafiltration, reabsorption and secretion (Figure 19.6). In these processes there is movement of water (solvent) and dissolved molecules of a small size (solutes) from malpighian body into the tubular lumen under physical pressure (ultrafiltration). This forms the primary urine which is isosmotic to blood from which it is derived. Later the composition of primary urine is altered by reabsorption of useful substances (e.g. glucose) into the blood, and by the secretion of unwanted substances (e.g. urea) from the blood into the tubular fluid. Reabsorption and secretion involve active transport, therefore are physiological processes; while ultrafiltration is a physical process (filtration under pressure).

The concentration of primary urine depends upon the length of loop of Henle. Animals with a longer loop (eg. beaver) produce dilute urine, while man, rabbit have both short and long loops in kidney and have intermediate ability to concentrate urine.

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The afferent vessel, which is of a larger diameter than the efferent vessel, brings the blood into the glomerulus. Here, the endothelial cells of blood vessels, the basement membrane and the single cell lining of the Bowman’s capsule form a semipermeable membrane. The blood entering the glomerulus is filtered through the semipermeable membrane into the lumen of the Bowman’s capsule under pressure. This filtration under pressure is called ultrafiltration. Blood pressure in glomerular capillaries is about 70 mm Hg. Opposing this is osmotic pressure of blood plasma proteins, which is about 32 mm Hg. Added to this is the pressure of fluid in the nephric capsule, about 20 mm Hg. Therefore the effective filtration pressure is 70- (32+20), i.e. about 18 mm Hg. The glomerular filtrate is nearly isosmotic to plasma and has the same composition of blood, except it does not contain blood cells and proteins. This glomerular filtrate is also called deprotenized plama or primary urine.