Each kidney has about a million nephrons, where urine formation takes place. At any given time, about 20 percent of the blood is going through the kidneys to be filtered so that the body can eliminate waste and maintain hydration, blood pH and proper levels of blood substances.
The first part of the process of urine formation occurs in the glomeruli, which are small clumps of blood vessels. The glomeruli act as filters, allowing water, glucose, salt and waste materials to pass through to the Bowman's capsule, which surrounds each glomerulus, but preventing the red blood cells from passing. The fluid in the Bowman's capsule is referred to as the nephric filtrate and resembles blood plasma. It also includes urea, produced from the ammonia which accumulates when the liver processes amino acids and is filtered out by the glomeruli.
About 43 gallons of fluid goes through the filtration process, but most is subsequently reabsorbed rather than being eliminated. Reabsorption occurs in the proximal tubules of the nephron, which is the portion beyond the capsule, in the loop of Henle, and in the distal and collecting tubules, which are further along the nephron beyond the loop of Henle.
Water, glucose, amino acids, sodium and other nutrients are reabsorbed into the bloodstream in the capillaries surrounding the tubules. Water moves via the process of osmosis: movement of water from an area of higher concentration to one of lower concentration.
Usually all the glucose is reabsorbed, but in diabetic individuals, excess glucose remains in the filtrate. Sodium and other ions are reabsorbed incompletely, with a greater proportion remaining in the filtrate when more is consumed in the diet, resulting in higher blood concentrations. Hormones regulate the process of active transport by which ions like sodium and phosphorus are reabsorbed.
Secretion is the final step in the process of urine formation. Some substances move directly from the blood in capillaries around the distal and collecting tubules into those tubules. Secretion of hydrogen ions via this process is part of the body's mechanism for maintaining proper pH, or acid-base balance. More ions are secreted when the blood is acidic, less when it is alkaline.
Potassium ions, calcium ions and ammonia also are secreted at this stage, as are some medications. The kidney is considered a homeostatic organ, one that helps maintain the chemical composition of the blood within strict limits. It does this partly by stepping up secretion of substances such as potassium and calcium when concentrations are high and by increasing reabsorption and reducing secretion when levels are low.
The urine created by this process then passes to the central part of the kidney called the pelvis, where it flows into the ureters and then the bladder.