The main parts of the circulatory system are:-
- Heart
- Arteries
- Capillaries
- Veins
The heart:- is two separate pumps wherein the left side pumps blood to the body(systematic circulation) and the right side pumps blood to the lungs(pulmonary circulation). Both the sides have the atrium and the ventricle. The function of the atrium is to receive blood when they are relaxed and to fill the ventricles when they contract.
Arteries: - Arteries posses thick elastic layer to allow stretching and absorb pressure. The walls of the arteries stretch and recoil in response to the pumping. The arteries maintain pressure in the circulatory system. The elastic layer surrounded by the circular muscles controls the diameter and the rate of blood flow. Arteries carry blood away from the heart.
Capillaries: - The capillaries are the smallest blood vessels and are less than 1mm long. the small diameter of the capillaries allows red blood cells to travel single file. The Sphincter muscles control the flow of blood to the capillaries.
The total cross-sectional area is bigger than that of veins or arteries and therefore the rate of blood flow is lowest in it.
The pressure of the blood is very high when it is in arteries but the pressure is reduced when it flows from the capillaries bit it is the lowest when it in veins.
Veins: - Veins function as reservoirs as they contain 50% to 60% blood volume.
The diameter of veins is more than that of arteries. The pressure in the blood is very low so it enables the valves in the veins to prevent backflow.
The smooth muscles in the walls of veins expand or contract and thus adjust the flow volume coming back to heart and make available more blood as per the requirement.