A simple heart The salmon's heart is located where the gill covers converge, high up in the throat. Triangular in shape, it lacks the complexity of a human heart. The fish heart pumps blood in only one direction. The blood enters the heart through a vein and exits through a vein on its way to the gills. In the gills, the blood picks up oxygen from the surrounding water and leaves the gills in arteries, which go to the body. The oxygen is used in the body and goes back to the heart a very simple closed- circle circulatory system.

A short digestive system As cold-blooded animals that do not heat their bodies by their metabolism, fish do not require a lot of energy to be extracted from their digestive systems. The sac-like pyloric caeca act like a small intestine, producing the digestive juices needed to bread down food and absorb nutrients into the blood stream. Just as in humans, the liver is the largest organ in the fish. The alimentary canal leads to the vent, which excretes not only urine and feces, but reproductive eggs and milt. Mature fish caught in a stream or river are unlikely to have any food in their digestive system. Spawning fish stop eating when they begin their upstream run, and may go up to sixteen weeks without food before spawning and dying.

Roe and milt Produced in the ovaries of the female salmon, and the testes of the male, salmon eggs and sperm are released simultaneously from the vent during spawning for fertilization in the nest. In a mature female, a large portion of the body cavity will be filled with roe (eggs). They will be contained within the membrane of the roe sacs, unless they have loosened into the cavity just prior to spawning. Male milt, or sperm, is produced in the testis of the male.

Buoyant bladder The salmon's swim bladder is located just below the spine, under the centre line, or centre of balance of the fish. The amount of air in this membranous sac determines the depth at which the fish will float, and stabilizes its movement within the pressures of the water. When the fish wants to go deeper into the water, it releases some of the air in its bladder. The bladder helps the salmon to conserve energy from having to use excess energy for buoyancy control.