The heart powers circulation by using metabolic energy to elevate the hydrostatic pressure of circulatory fluid.
Blood vessels contain a central cavity lined with endothelium,a single layer of flattened epithelial tissue. Capillaries are the smallest blood vessels. The walls of arteries and veins have more complex organizations than capillaries.
Blood is a connective tissue consisting of cells suspended in plasma. In the plasma are ions and proteins that function in osmotic regulation, transport, and defense.
A circulatory system has three components: a circulatory fluid, a set of interconnecting tubes, and a heart. There are two types of circulatory systems, open and closed. In an open system, the circulatory fluid baths the organs directly. In a closed system, blood is confined to vessels and is distict from the circulatory fluid.
The closed circulatory system of vertebrates is called the cardiovascular system. Blood circulates to and from the heart through a network of vessels.There are three main types of vessels: arteries, capillaries,and veins. Arteries carry blood away from the heart to organs throughout the body. Capillaries are vessels with very thin walls. chemicals are exchanged by diffusion across these walls. Also,veins are the vessels that carry blood back to the heart.
In vertebrates, there can be either single circulation or double circulation. Bony fishes, sharks, and rays have single circulation, meaning that blood enters in the heart and collects in the atrium before transferring to the ventricle. Amphibians, reptiles, and mammals have doubler circulation. In this type, the pumps for the circuits serve different tissues,but are combined into the heart. The following image shows double circulation in three different vertebrates: