A nephron consists of a network of tiny blood vessels, the glomerulus, surrounded by Bowman's capsule, a two-layer membrane that opens into a convoluted tubule. Pressure forces much of the blood plasma (fluid portion of the blood) through the glomerulus and into Bowman's capsule. The resulting tubular fluid, which contains water and dissolved chemicals, then passes into the convoluted tubule. The portion of the blood that remains in the glomerulus flows into small vessels called capillaries, which surround the convoluted tubule. As the tubular fluid flows through the tubule, substances needed by the body are absorbed by the cells of the tubule wall. These substances, which include amino acids, glucose, and about 99 per cent of the water, then rejoin the blood in the capillaries. The capillaries return the blood to the heart by way of the renal vein.
Substances not absorbed in the tubule are wastes that the body cannot use. Other wastes are secreted into the tubular fluid by the tubular cells of the kidney. These various substances, which include ammonia, urea, uric acid, and excess water, make up urine. The urine passes from the convoluted tubules into larger collecting tubules and then into the pelvis layer of the kidney. A tube called the ureter carries urine from each kidney into the urinary bladder. Urine collects in the bladder until it passes out of the body through another tube, the urethra. Healthy kidneys produce from 1 to 2 litres of urine daily.