Anatomy of finfish and shellfish

Unit 5 - Circulatory System

5.1.1. Circulatory System of a Fish
The circulatory system of fish is quite simple. Like mammals, the circulatory system of fish 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 of the fishes, 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. Inspite 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 thrombocytes 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.