The first stage of urine formation takes place in the glomerulus. Blood under pressure enters the glomerulus, inside which water, electrolytes and waste substances undergo filtration. The pressure in the glomerular capillary forces these substances between thethat make up the capillary walls, through the basement membrane (which has pores in it), and through the slits between the projections of the podo-cytes. The larger elements of such as red , white cells, platelets, and the plasma proteins are too big to pass through the small pores in the basement membrane and the slit-like spaces between the podo-cytes. They are therefore carried away from the glomerulus in the efferent arteriole. Each day the heart delivers about 150 litres of fluid to the tubules of the kidney. This fluid contains about 1 kg of salt (sodium chloride), 500 gm of sodium bicarbonate, 250 gm of glucose and smaller quantities of other substances. As less than 2 litres of urine per day are excreted, containing only a fraction of the above chemicals, it is obvious that the renal tubules do a lot of work and modify the fluid presented to them. The process of modification takes place in both the con-vulted tubules and in the collecting ducts. The whole object of this modification, known as the reabsorp-tion/secretion mechanism, is to return valuable chemicals and water to the stream, leaving a concentrated fluid containing waste products to be excreted as urine.
Three-quarters of the water in each renal tubule is reabsorbed in the proximal convoluted tubule. Also, at this site amino acids, glucose, and phosphate are reabsorbed by the cells that line the tubule. After passing through the proximal convoluted tubule, the fluid (which is now much more concentrated resulting from the loss of much water) enters the loop of Henle. The length of the loop of Henle depends on its location in the kidney. Most loops are short, but those lying in the inner part of the kidney are long and dive down to the tip of the renal papillae. The long loops pass into an area of very concentrated tissue fluid, and the thin walls of the loops are designed to allow the passage of salt and water across the wall from within and into the surrounding. This process is an example of osmosis. After leaving the loop the fluid remaining in the tubule enters the distal convoluted tubule. This part of the tubule is responsible for the acid-base balance of the body. Special cells lining the tubule exchange sodium for potassium and hydrogen, enabling a close regulation of the acidity of the blood (blood pH). The processed and concentrated fluid finally enters the collecting tubules in which the final adjustment of the water content of the liquid takes place. This last modification to the fluid, which by now can be called urine, is governed by the hormone vasopressin, or antidiuretic hormone (ADH).