The circulatory system of fish is responsible for transporting blood and nutrients throughout the body. Blood travels across the body through the network of blood vessels. Unlike humans, fish have 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. 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. 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 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 help in blood clotting. Blood is circulated throughout the body with the help of arteries and veins (blood vessels). 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.