Excretory System 
Nitrogenous Wastes: are by products of the breakdown of proteins. Amino acids must be deaminated if they are not going to be used in protein synthesis. The NH2 end group is removed and a Hydrogen is added to form ammonia. Animals will dilute this with water and excrete it into the environment. Land animals must change it into the substance urea. This change occurs in the liver. Birds, reptiles, and insects change ammonia into uric acid. Uric acid requires a tremendous amount of energy to produce.
The removal of nitrogenous waste from body fluids is tied to osmoregulation, the balance between salts and water.
Freshwater Bony fish:
Marine bony fish:
Marine birds:
Whales: drink water and have concentrated urine.
Function of the Excretory Organs
a). collects fluids from somewhere in the body.
b). modifies the fluids by reabsorbing substances the body needs.
c). provides a way to expel the excretory product from the body.
These organs expend metabolic energy to carry out these functions. The human kidney makes up .05% of the bodies weight and uses 7.2% of the oxygen it consumes.
The Human Kidney:

Function of the Nephrons: Let's follow the process of urine formation in a nephron.
The nephron's cup shaped Bowmans Capsule surrounds a knot of blood capillaries called the glomerulus. Much of the bloods fluid filters into the glomerulus due to pressure from the circulatory system. Left behind are large proteins and whole cells, which are too large to fit through. This filtrate is then passed through the nephron tubule, which has 4 main parts ( proximal convoluted tubule, loop of Henle, distal convoluted tubule, and the collecting duct).
In the proximal convoluted tubule, a considerable amount of resorbtion occurs. Small proteins, glucose, and ions are returned to the blood by active transport. Negatively charged ions follow passively, followed by the osmosis of water. About 75% of the filtrate is returned in this section of the nephron.
The loop of Henle, lie in the medulla of the kidney, the other parts of the nephron lie outside of the medulla in the cortex. Salt is actively transported out of the filtrate, and since this area impermeable to water, water cannot follow the ions out here.
The distal convoluted tubule, urine is then passed through here to the collecting duct where it is concentrated due to its permeability to water. It leaves the collecting duct into the pelvis of the kidney, which leads to the ureter and the bladder where it is stored.
Regulation of Kidney function
Urine composition and the rate of urine formation are largely regulated by hormones vasopressin, aldosterone, and angiotensin, and the enzyme rennin.
Vasopressin (ADH) is produced by the posterior pituitary gland and increases the bodies ability to resorbtion of water. it increase the permeability of the collecting ducts.
Diabetes insipidus is caused due to a lack of this hormone. It is characterized by thirst, and the production of large amounts of dilute urine.
Angiotensis and Aldosterone are hormones that regulate the amount of sodium that is resorbed from the filtrate in the nephrons. Aldosterone is produced by the adrenal gland. it promotes the resorbtion of sodium by the distal tube. Its action is directed by the bloods salt content. The kidney secretes an enzyme rennin, which changes a plasma protein angiotensinogen into angiotensin, which helps to increase the production of aldosterone and the blood pressure of the kidney.