Single Circulation System

In a single circulation system, blood passes through the heart once for each complete circuit of the body. Fish have a single circulation.The route is:heart → gills → body tissues → heartThe fish heart has two main chambers: one atrium, which receives deoxygenated blood returning from the body, and one ventricle, which pumps it to the gills. At the gills the blood is oxygenated, and then it flows on directly to the body organs without returning to the heart.The main disadvantage is pressure. The ventricle pumps blood out at high pressure, but the blood then has to pass through the very narrow capillaries of the gills. This causes a large drop in pressure. By the time the blood reaches the capillaries of the body tissues, it is at low pressure and flows slowly, so oxygen and nutrients are delivered to the tissues relatively slowly.This is sufficient for fish because:• they are ectotherms, so they do not use energy to keep a constant body temperature and have a lower metabolic rate than mammals
• their body weight is supported by water, so less energy is needed for movement against gravityBlood passing through the heart of a fish is always deoxygenated, because it has already given up oxygen to the body before it returns. The heart muscle itself receives oxygen from some of the oxygenated blood through small coronary vessels.