Excretion

Excretion is the removal of toxic waste products of metabolism, especially carbon dioxide and urea. The deamination of excess amino acids in the liver produces ammonia, which is converted into urea, the main nitrogenous waste product. Urea is excreted in solution in water, as urine.

The kidneys regulate the concentration of various substances in the body fluids, by excreting appropriate amounts of them. Each kidney is made up of thousands of nephrons and their associated blood vessels. The kidneys produce urine by ultrafiltration and selective reabsorption, plus some secretion of unwanted substances. Different regions of a nephron have different fuctions, and this is reflected in the structure of the cells that make up their walls.

Blood is brought to the glomerulus in an afferent arteriole. High hydrostatic pressure in the glomerulus forces substances through the capillary walls, the basement membrane and inner lining of the Bowman's capsule. The basement membrane acts as a filter, allowing only small molecules through,. This filtrate collects in the Bowman's capsule and then enters the proximal convoluted tubule, where most reabsorption occurs by diffusion and active transport; substances are also reabsorbed in the distal convoluted tubule and collecting duct.

The loop of Henle acts as a counter-current multiplier, producing high concentrations of sodium and chloride ions in the tissue fluid in the medulla. This tissue has a very low water potential. Water is reabsorbed from fluid in the collecting duct by osmosis if the body is dehydrated. The water content of the blood is controlled by changing the amount of water excreted in the urine by the kidneys. This is done by regulating the permeability of thr walls of the collecting ducts to water, and hence the volume of water reabsorbed from the collecting ducts into the blood. The permeability is increased by the hormone ADH, which is secreted by the posterior pituitary gland in response to stimulation of osmoreceptors in the hypothalamus

Deamination

• to make use of energy of excess amino acids, since they cannot be stored in the body, the liver removes the amino groups in a process known as deamination
• in liver cells, the amino group of an amino acid together with an extra hydrogen atom is removed, these combine to form ammonia
• the keto acid that remains may enter the Krebs cycle and be respired, or it may be converted to glucose, or converted to glycogen or fat for storage
• ammonia is very soluble and toxic so it is immediately converted to urea, which is less soluble and toxic
• the urea cycle is responsible for combining carbon dioxide and ammonia to form urea

Urea is the main nitrogenous excretory product of humans. We also produce small quantities of other nitrogenous excretory products, mainly creatinine and uric acid. Creatine is made in the liver from certain amino acids and much of it is used in the muscles, in form of creatine phosphate, where it acts as an energy store. However, some is converted to creatinine and excreted. Uric acid is made from…