![]() This promotes the extraction of water from the ultrafiltrate, and allows mammals to live in some of the driest environments on Earth. Mammals produce urea from ammonia, and concentrate the urea in the urine to a high extent. Mammals have even more derived nephrons, which contain an extended loop, called the loop of Henle. Reptiles and birds excrete uric acid, which is a more concentrated form of ammonia. ![]() Ammonia is toxic in the bloodstream, and must be removed. Fish and other primitive vertebrates excrete ammonia as a byproduct of protein reactions. ![]() Vertebrates are the only group to have developed kidneys, which is mostly used to conserve water in terrestrial environments. The glomerulus is the specialized configuration of capillaries within the nephron that make kidneys possible. The ultrafiltrate travels through the various loops of the nephron, where water and important molecules are removed, and into a collecting duct which drains into the bladder. This substance, lacking the blood cells and large molecules in the bloodstream, is known as an ultrafiltrate. Ultrafiltration occurs when blood pressure forces water and other small molecules through tiny gaps in capillary walls. The nephron functions through ultrafiltration. ![]() A nephron is used separate to water, ions and small molecules from the blood, filter out wastes and toxins, and return needed molecules to the blood. A nephron is the basic unit of structure in the kidney. ![]()
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