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The human circulatory system is a closed circulatory system. The blood flows in closed vessels without coming in direct contact with the cells of the body.
The walls of the blood vessels are made up of three layers:
- Inner endothelium - single layer of cells
- Tunica media - made of smooth involuntary muscles and elastic fibres
- Tunica externa - made of inelastic white fibres
The blood flows in them due to the contraction and relaxation of these muscles. The blood flow to and from the different tissues and cells can be regulated according to their needs because of the action of the smooth muscles. The closed vessels are narrow tubes called the arteries and the veins.
The blood vessels that take blood away from the heart are called arteries. They carry oxygenated blood (except pulmonary artery that carries deoxygenated blood). The colour of fresh blood drawn from the arteries is bright red. They have comparatively thicker walls as the blood flows through them under more pressure. This pressure is created by the pumping action of the heart. It ensures that blood reaches all the parts of the body at the earliest. The blood flows through the arteries because of the pressure generated by the heart and maintained by the action of the arterial muscles.
The two main arteries leaving the heart are:
Aorta
The branches of which supply blood to different parts of the body and
Pulmonary Artery
The artery that takes blood to the lungs.
The arteries divide again and again to form narrower arterioles. The arterioles, branch into very thin tubes called the capillaries. The capillaries are in touch with the tissues and cells. While the walls of the arteries and arterioles have layers of smooth muscles, the capillary walls only have a single layer of endothelial cells. The membranes of the endothelial cells are permeable to the nutrients and gases. They are not permeable to proteins and macromolecules. Hence, these are retained in the blood itself.
The capillaries formed by the arterioles continue as carriers of deoxygenated blood as they carry the waste products that have diffused in from the cells. These capillaries join together to form venules. The venules join together and form bigger vessels called the veins. The veins and venules are also lined by smooth muscles. However, the walls of the veins are not as thick as those of arteries. All the veins of the upper body except the pulmonary vein join together to form the superior vena cava and the veins of the lower body join together to form the inferior vena cava. These two veins pour their blood into the right auricle through separate openings. The pulmonary vein brings the blood from the lungs to the heart. Thus, the blood vessels that bring the blood to the heart are called the veins. The veins carry deoxygenated blood except for the pulmonary vein that carries oxygenated blood from the lungs. The blood drawn from the veins is dark red in colour.
The blood moves in the veins towards the heart by the action of the muscles in its walls and skeletal muscles around them. The skeletal muscles are more active during exercise. The veins all along their length have valves that prevent the backflow of blood. This is because the veins have fewer muscles to propel the blood more forcefully.
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