The general pattern of blood circulation

ROLE OF BLOOD

Blood transports respiratory gases, products of digestion, metabolic wastes and hormones round the body. The heart has its own blood supply - the left and right coronary arteries. 

Blood always flows from a higher pressure to a lower pressure in the circulatory system. The vena cava is the final blood vessel that takes the blood back to the heart from the body, so it has the lowest pressure.

CIRCULATORY SYSTEM IN MAMMALS

Mammals have a closed, double circulatory system in which blood is confined to vessels and passes twice through the heart for each complete circuit of the body. 

STRUCTURE OF THE HEART

The pump on the left deals with oxygenated blood from the lungs, while the one on the right deals with deoxygenated blood from the body. By having two pumps the pressure increases of the blood as when it is at the lungs for gas exchange, the pressure reduces. 

Each pump has two chambers:

• The atrium is thin-walled and elastic and stretches as it collects blood.

• The ventricle has a much thicker muscular wall as it has to contract strongly to pump blood some distance, either to the lungs or to the rest of the body. Left ventricular wall is thicker because it needs to pump blood to the extremities.

VALVES

Valves prevent the backflow of blood, ensuring unidirectional flow. 

Blood flows from high pressure to low pressure, they open and close as pressure changes. 

ATRIOVENTRICULAR VALVES

Open when pressure in atria > ventricle. 

Close when pressure in ventricle > atria. 

Closure of these valves ensures that, when the ventricles contract, blood within them moves to the aorta and pulmonary artery rather than back to the atria.

SEMILUNAR VALVES

Open when pressure in ventricle > arteries. 

Close when pressure in arteries > ventricles. 

These prevent backflow of blood into the ventricles. This arises when the elastic walls of the vessels recoil increasing the pressure within them and when the ventricle walls relax reducing the pressure within the ventricles.

STRUCTURE OF CIRCULATORY SYSTEM

1. Superior and inferior vena cava bring deoxygenated blood to the heart from the upper and lower body tissues (not lungs). 

2. Blood enters the cavity of the right atrium. 

3. Through the tricuspid valve. 

4. Into the right ventricle cavity, the heart contracts. 

5. Blood forced into the pulmonary artery through the pulmonary semilunar valve. 

6. Travels to the lungs where gas exchange occurs and the blood becomes oxygenated. 

7. Blood goes back to the heart via the pulmonary vein. 

8. It enters the left atrium cavity. 

9. Into the left ventricle cavity through the bicuspid valve. 

10. The heart contracts forcing blood into the aorta through the aortic semilunar valve. 

11. The aorta carries oxygenated blood away from the heart, branching into arterioles which take blood to capillaries at tissues.