Physiology of Finfish and Shellfish (2+1)

2.2.13. Cardio- vascular system

There are several types of circulatory systems.

1. The open circulatory system
2. The closed circulatory system

The closed circulatory, or blood vascular, system consists of the muscular contractile organ, heart, blood vessels like the arteries, the capillaries and the veins that deliver blood to all tissues is present in fishes : it is in the capillaries that the interchange of oxygen, carbon dioxide, nutrients, and other substances such as hormones and waste products takes place. The capillaries in turn lead to the veins, which return the venous blood with its waste products to the heart, kidneys and gills. There are two kinds of capillary beds, those in the gills and those in the rest of the body. All vertebrates and some invertebrates (cephalopods) have this type of system. The heart is situated ventrally behind the gills but is more anteriorly placed in teleosts than in the chondrichthyes.

All vertebrates possess a muscular chambered heart. Amphibians and the reptiles (except crocodiles) have a 3-chambered heart with two atria and a single ventricle, whereas crocodiles, birds and mammals possess a 4-chambered heart with two atria and two ventricles. Fish heart is regarded as a 2 chambered structure owing to the presence of the true chambers viz., the ventricle and the atrium. Sinus venosus is a thin walled sac and it receives deoxygenated blood of the body by a variable number of veins. It opens anteriorly into the atrium by a sinuatrial aperture guarded by a pair of reflux preventing valve. The atrium is also thin walled chamber that opens anteriorly into the ventricle by an atrio ventricular valve. Ventricle is single and a highly muscular structure giving rise to a tubular truncus ateriosus. The base of this tube swells to form the conus ateriosus possessing a variable number of semilunar valves. Conus is contractile, as it is composed of cardiac muscle fibres and it is therefore usually regarded as chamber of the heart. It continues anteriorly into a tubular ventral aorta, which extends mid ventrally in the anterior portion of the body just below the gills. The blood then passes to the afferent (receiving) arteries of the gill arches and then to the gill capillaries. There waste gases are given off to the environment and oxygen is absorbed. From there the oxygenated blood enters efferent arteries of the gill arches and then into the dorsal aorta. From there blood is distributed to the tissues and organs of the body. Valves present in between the various chambers of the heart prevent reflux and maintain a unidirectional flow of blood. The circulation of fishes thus differs from that of the reptiles, birds, and mammals, in that oxygenated blood is not returned to the heart prior to distribution to the other parts of the body.

(The last chamber of the fish heart is called the bulbus arteriosus in the teleosts, but the conus arteriosus in the elasmobranchs).

Single Circulation: In fishes, the heart pumps out deoxygenated blood which is oxygenated by the gills and supplied to the body parts from where deoxygenated blood is returned to the heart.

Incomplete Double Circulation: In amphibians and reptiles, the left atrium receives oxygenated blood from the gills/lungs/skin and the right atrium gets the deoxygenated blood from other body parts. However, they get mixed up in the single ventricle which pumps out mixed blood.

Complete Double Circulation: In birds and mammals, oxygenated and deoxygenated blood received by the left and right atria respectively passes on to the ventricles of the same sides. The ventricles pump it out without any mixing up, i.e., two separate circulatory pathways are present in these organisms; hence, these animals have double circulation.