Excretion is the removal of metabolic waste products. Excretion can take place in both plants and animals. For example, release of oxygen during photosynthesis to the surrounding air environment is an example of excretion.
In humans, excess amino acids are deaminated to form urea. Urea is excreted as urine from the body to maintain a suitable blood pH. The basic unit of the kidney is the nephron. Urine formation consists of (i) ultrafiltration and (ii) selective reabsorption.
Ultrafiltration occurs at the glomeruli of the nephron while selective reabsorption occurs at the proximal convoluted tubule, Loop of Henle and distal convoluted tubule and collecting duct.
Formation of urine is part of the homeostatic process to maintain osmoregulation, ie maintenance of a constant blood pH. When blood water potential is higher than normal, hypothalamus releases less anti-diuretic hormone that leads to reduced water potential at the collecting duct. When blood water potential is lower than normal, hypothalamus releases more anti-diuretic hormone that leads to increased water potential at the collecting duct.
Failure of the kidneys leads to the need for the patient to undergo dialysis.