Circulation

Humans, like most animals that have a body fluid, have a system of tubes which fluid flows. It is called a circulatory system. A muscular pump (heart) propels a fluid containing special cells (blood) through tubes (vessels).
The basic function of a circulatory system is to transport materials through the organism’s body. This system provides the raw materials needed by the cells and removes wastes from the cells environment.
Human circulation occurs in a closed system. A single muscular heart with four chambers pumps blood through the system. There are two kinds of blood vessels: 1) Arteries - have rather thick muscular walls and carry blood away from the heart. In this kind of vessels the blood is rich in oxygen. 2) Veins – have relatively thin walls with less muscle. They carry blood practically out of oxygen towards the lungs and the heart. 3) Capillaries are thin walled, narrow tubes that connect arteries and veins.
Ih humans as in all mammals and birds the heart is separated into right and left sides. Each side has its own set of veins and arteries. In human the right side receives blood from all parts of the body and sends it to the lungs. The left side receives oxygenated blood from the lungs and returns it to the body.
Each side has two chambers. The first chamber (atrium) forms the upper part of the heart and receives incoming blood from the tissues of the body. The thin-walled atrium bulges with this blood and as the lower heart muscle relaxes the blood flows into second chamber -...

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