Internal organs and systems of organs of fish

Fishes have a closed circulatory system that pumps blood in a single loop—from the heart to the gills, from the gills to the body, and back to the heart. The heart is made up of four parts: the sinus venosus, atrium, ventricle, and bulbus arteriosus. The ventricle is the actual pumping portion of the heart. The atrium is a one-way compartment for blood that is going to enter the ventricle.

 

Fishes have well-developed nervous systems. The brain has several parts. The olfactory bulbs and cerebrum are involved with the sense of smell. The optic lobes process information from the eyes. The cerebellum coordinates body movements. 

Most fishes have a lateral line system that senses currents and vibrations in the water. Most fishes move by contracting muscles on either side of the backbone. Fins propel the fish forward and help it steer. Many fishes have a gas-filled swim bladder that keeps them from sinking.