Air Bladder:
In most fish, the swim bladder is whitish in color and elastic. Passage of gaseous matter through the oval organ into the bloodstream is controlled by a circular muscle (a sphincter). Gases are introduced into the air bladder at another place by means of a gas gland called a rete mirabile.

The Heart:
The heart circulates blood throughout the body. Oxygen and digested nutrients are delivered to the cells of various organs through the blood, and the blood transports waste products from the cells to the kidneys and liver for elimination. The heart may often continue to beat long after the fish dead.

The Kidneys:
In bony fish, blood is formed primarily in the kidneys. The kidney is differentiated into an anterior or head kidney and a posterior kidney. In smaller fish the posterior kidney often is only a narrow red strip under the vertebral column that thickens anteriorly behind the head, where it forms the head kidney. In many species the head and posterior kidneys are clearly separated from one another. The kidney is blood-red in health, but grayish white when diseased.

Internal Anatomy