The function of the circulatory system is to distribute fluids and gases throughout the body. There are two types of circulatory systems an open system found in lower animals and the closed system found in more developed animals.
In the open system there are no blood vessels just a heart that pumps blood throughout the body. The closed system is made up of the heart, capillaries, veins, arteries and blood cells.
The heart is a muscle. In mammals and crocodile species it has four chambers two ventricles and two atrium. In reptilian heart there are only three chambers two atrium and one ventricle. Other animals may have one or two chambered hearts.
The veins, arteries and capillaries are tubes made of different degrees of smooth muscles cells and several elastic lamina. The outer wall is called the adventitia, the middle wall is called the media and the inner wall is called the intima. The size of these vessels vary depending on where they are located in the body and the functions they perform.
The blood cells are produced in the bone marrow. They are made up of two main parts plasma and formed elements. The plasma is a clear fluid. The formed elements contain the blood cells, which are either erythrocytes or red blood cells and leukocytes or white blood cells and the platelets, which are not cells but tiny fragments of bone marrow.
The process starts at the heart. Blood cells that have lost all of its oxygen are carried from various parts of the body through the veins into the right ventricle. It is then pumped to the right atrium. From there the blood is pumped to the lungs were it absorbs oxygen and returns to the heart through the left atrium. The oxygenated blood is pumped into the left ventricle and flows through the arteries to the various systems of the body. Then it flows through the thin walls of the capillaries to distribute the gases to the tissue. Once depleted the blood flows back to the heart through the veins.