Excretion

The major excretory products in animals are nitrogenous compounds such as urea, ammonia, and uric acid from breakdown of proteins and nucleic acids, carbon dioxide from cellular respiration and bile pigments from breaking down of worn out red blood cells in the liver.

Breakdown of nitrogen containing molecules such as amino acids results in excess nitrogen that must be removed from the body. When amino acids are broken down by the body to generate energy or converted into fats or carbohydrates, the amino ($NH_2$) group must be removed because they are not needed, and they may be toxic. This excess nitrogen may be excreted in the form of ammonia, urea, or uric acid.

Ammonia

Ammonia is quite toxic and highly soluble; hence it can be a nitrogenous excretory product if sufficient water is available to wash it from the body.

It is excreted by most fish and other aquatic animals whose gills or body surface are in direct contact with water. These animals are called ammonotelic.

Urea

Animals like sharks, adult amphibians, and mammals usually excrete urea as their nitrogenous waste. Urea is a much less toxic, and less soluble than ammonia. It can be excreted in a moderately concentrated solution. This elimination strategy allows body water to be conserved, an important advantage for terrestrial animals with limited access to water. The animals in this group are termed as ureotelic.

Uric acid

Uric acid is not toxic and it is insoluble in water. Poor solubility is an advantage if water conservation is needed.

Uric acid can be concentrated even more readily than urea can. Uric acid is usually excreted by insects, reptiles, and birds, these are collectively termed as uricotelic animals

Kidneys are paired bean-shaped organs found on each side of the back of the lower portion of the abdominal cavity. The larger left kidney is located a bit higher than the right kidney. Unlike other organs found in the abdomen, kidneys are located behind the lining (peritoneum) of the abdominal cavity. Thus, they are considered retroperitoneal organs. These bean-shaped organs are protected by the back muscles and the ribs as well as the fat (adipose tissue) that surrounds them like a protective padding. Located above each kidney is an adrenal gland which secretes adrenaline hormone. For each kidney, the renal artery supplies blood which a renal vein returns to the vena cava. The ureter removes urine produced by the kidney, transfering it to the urinary bladder for temporary storage, before being released through the urethra. The exit from the bladder is controlled by a sphincter (a ring of muscles).

The bean-shaped kidneys have an outer convex side and an inner concave side called the renal hilus. A transverse section (T.S) of the kidney reveals an outer part called cortex and an inner part called medulla. A thin connective tissue called fibrous capsule surrounds each kidney. This capsule maintains the kidneys' shape and protects the inner tissues. The cortex contains parts of the nephrons, glomeruli and capsule, while the medulla contains tubular part of the nephrons and blood vessels, forming the renal pyramids which project into the pelvis, leading to the ureter. Kidneys are well supplied by blood vessels, forming a network of blood capillaries.