Excretion plays an important part in the process by which the internal environment is regulated to maintain homoeostasis. It is the elimination of the unwanted products of metabolism and of substances present in excess within the organism. Animals excrete nitrogenous waste products such as ammonia, uric acid or urea. These or similar products to that produced by the deamination of any excess protein that has been eaten as nitrogenous compounds are formed in metabolism of proteins and nucleic acids. Ammonia is lost by diffusion from small aquatic organisms but more complex animals such as mammals rely on kidneys to filter wastes out of the blood and eliminate them from the body. Excretion also involves eliminating any excess or toxic substance taken in with the diet, including water and salts.
High levels of ammonia in the body can lead to coma and even death. This is because ammonium ions can substitute for potassium ions in ion-exchange mechanisms. An excessive amount of ammonia in the body raises bodily pH, which causes changes in the tertiary structure of proteins and thus cellular functions can be altered. The major nitrogenous waste excreted by mammals is urea and is synthesised from the ammonia in the liver using the ornithine cycle. This is because urea is less toxic than ammonia and requires less water for elimination, meaning urine can be produced and excreted from the body. Moreover, urea is a very small molecule and can easily diffuse across permeable membranes such as the capillaries in the glomerulus. The kidney contains numerous functional units called nephrons, which produce urine through a series of steps: ultrafiltration of the blood; selective reabsorption of the filtrate and secretion of the harmful substances. Blood flows under high pressure from the afferent arteriole into the glomerulus, which are tightly coiled capillaries enclosed in the Bowman’s capsule. Here ultrafilteration occurs and 15-25% of the water and all the small solutes and ions are filtered from the blood, through a single-cell layer of the capillary walls and into the lumen of the Bowman’s capsule. The capillaries have specialised epithelial cells called podocytes. Podocytes have small protections called pedicels packed closely together; creating slit pores though which ultrafiltration can occur, but larger molecules such as proteins and red blood cells cannot pass into the filtrate. About 180 litres of glomerulus filtrate is produced everyday and this filtrate then flows into the renal tubule to undergo reabsorption. As the glomerular filtrate moves through the nephron, most substances present are reabsorbed back onto the blood. First, the filtrate moves into the proximal convoluted tubule, where about 70% of the sodium ions are reabsorbed by active transport. Water, chloride ions, glucose, amino acids and urea are reabsorbed passively. Approximately 65% of the glomerular filtrate is reabsorbed in the proximal convoluted tubule. This is aided by the brush border of microvilli on the epithelial cells, which increase the surface area for reabsorption. After moving through the proximal convoluted tubule, the filtrate moves on to the Loop of Henle. The ascending limb is highly permeable to sodium and chloride ions and impermeable to water and urea. As the filtrate moves up the Loop of Henle, sodium and chloride ions diffuse out because there is a higher concentration in the filtrate then in the surrounding tissues. Furthermore, chloride ions are constantly pumped into the surrounding fluid and the sodium ions follow due to the electrochemical gradient created. This again further increases the solute concentration outside the Loop of Henle. At the descending limb, the cells have a low permeability to urea and salt, but are very permeable to water. As the filtrate descends the Loop of Henle, water diffuses out because of the high salt concentration in the surrounding tissue, meaning the Loop of Henle has a countercurrent mechanism. For this reason, mammals living in desert environments have longer Loops of Henle meaning there is a greater area from water reabsorption so they can conserve more water by producing more concentrated urine.
The distal tubule functions in transporting potassium ions, hydrogen ions, and ammonia into the lumen, and sodium ions, chloride ions, and ammonium ions out. Absorption of salts in this area is under endocrine control and so fine adjusts can be made according to the concentrations in the blood. The filtrate then moves into the collecting duct, which carries the fluid to the renal pelvis. The epithelium of the collecting duct is permeable to water, but not salt or urea. The permeability of the collecting duct to water is controlled by the hormone ADH from the posterior pituitary gland. The more ADH present, the greater the permeability to water and so more water is reabsorbed.
The urine produced is transferred to the bladders via the ureters. It contains excess urea, sodium, chloride and potassium ions dissolved in water. Glucose should not be present in the urine as all of it should be reabsorbed. When the bladder is full, the urine is excreted from the body via the urethra.