Circulatory System of a Fish

Fish are cold-blooded aquatic vertebrates and can be found in both saline and freshwater. Its circulatory system is responsible for transporting blood and nutrients throughout the body. It has a closed circulatory system, i.e. blood travels across the body through the network of blood vessels. Unlike humans, fish exhibit single cycle circulation, where the oxygen deprived blood comes to the heart, from where it is pumped to the gills and then circulated to the entire body. On the other hand, in mammals, the deoxygenated blood enters the heart, from where it is pumped into the lungs for oxygenation. Then, the oxygenated blood returns to the heart from the lungs, to be transported throughout the body.

Cardiovascular Mechanism of a Fish

The circulatory system of fish is quite simple. Like mammals, it consists of a heart, blood and blood vessels. The heart of a fish is a simple muscular structure that is located between the posterior gill arches. It is enclosed by the pericardial membrane or pericardium. In most fish, the heart consists of an atrium, a ventricle, a sac-like thin-walled structure known as sinus venosus and a tube, known as bulbus arteriosus. In spite of containing four parts, the heart of a fish is considered two-chambered.

The blood contains plasma (the fluid portion of blood) and the blood cells. The red blood cells or the erythrocytes contain hemoglobin, a protein that facilitates the transport of oxygen to the entire body, while the white blood cells are an indispensable part of the immune system. The thromocytes perform the functions that is equivalent to the role executed by the platelets in the human body, i.e. they help in blood clotting. Blood is circulated throughout the body with the help of blood vessels. The blood vessels are of two types, arteries and veins. The arteries are responsible for carrying oxygenated blood from the heart to the rest of the body, while the veins return deoxygenated blood from the different parts of the body to the heart.

In fish, the deoxygenated or oxygen deprived blood, is carried by the veins to the sinus venosus. Sinus venosus is an important constituent of the circulatory system of lower vertebrates. The deoxygenated blood collected by the veins accumulates in the sinus venosus, before entering the heart. Blood first enters the atrium of the heart, which is a large chamber. Then it enters the ventricle, from where it is pumped into the tube, bulbus arteriosus. Through bulbus arteriosus, the blood reaches the aorta and then the gills. The gills are the respiratory organ of a fish and they execute the activities performed by human lungs. They facilitate the exchange of gases, i.e. absorption of oxygen from water and elimination of carbon dioxide. Then the oxygenated blood is transported throughout the body with the help of blood vessels. Blood facilitates the transport of oxygen and nutrients. It also collects carbon dioxide which is again transported to the heart and then to the gills, to be removed from the body.

Though its circulatory system is quite simple compared to that of humans and other mammals, it serves an important purpose by illustrating the different stages of the evolution of the circulatory system in animals. Its heart is also studied to learn the progressive evolution of the four-chambered heart.