The renal artery that enters the kidney, branches into numerous smaller vessels, each terminating in a globular network of capillaries – the glomerulus. The filtration of blood takes place at the surface between the glomerulus and the Bowmans Capsule. A high pressure system exists in the blood flowing through the glomerulus; substances that are small enough are squeezed through the capillary wall and pass through the cellular layer lining the Bowman’s capsule – where they move into the lumen. Blood cells and proteins are retained whilst large volumes of water pass through, carrying dissolved substances. Once inside the Bowmans capsule, the fluid is known as glomerular filtrate. The process of filtrations seperates substances from blood based on their size. Glomerular filtrate is not the final product, but rather it passes into the nephrons where substances that are needed are reabsorbed into the bloodstream and wastes are filtered out. These process called tubular reabsorption and tubular secretion adjust the composition of the blood in the nephron to form urine.
The chemical composition of body fluids is precisely regulated by selective reabsorption of certain solutes from the glomerular filtrate at the proximal tubule, loop of Henle and the distal tubule. The filtrate contains substances that the body still needs, such as amino acids, glucose and ions; as such these substances are actively reabsorbed from the nephron and are passed back into the renal vein – where they flow into the kidney and back into normal circulation.
Substances that are reabsorbed
Solute reabsorption: All amino acids, glucose, vitamins and ions are reabsorbed. The differing rate of reabsorption depends on the feedback from the body. All solutes are reabsorbed from the nephron through active transport and facilitated diffusion.
Water reabsorption: water is reabsorbed by the passive process of osmosis. Approximately 99% of the filtrate is reabsorbed along the Bowmans capsule with only 1% excreted as urine. The membranes of cells can alter their permeability, thus controlling the amount of water and salts reabsorbed into the bloodstream.
Parts of the nephron involved in reabsorption
In the proximal tubule all organic nutrients are reabsorbed as well as some ions. In the ascending limb of the loop of Henle a large number of ions are reabsorbed and actively pumped out into the medulla. Some urea may move out of the colleting tubule by diffusion into the interstitial fluid.
It involves the removal of toxic substances from the blood capillaries and tissue and their active secretion into the nephron. Metabolic wastes such as urea, uric acid, ammonia and hydrogen ions are secreted into the fluid within the nephron. The movement of urea and ammonia is through diffusion, where as all other tubular secretion involves active transport. Tubular secretion occurs in the proximal part of the nephron and the descending limb of the loop of Henle.