Double Circulation in Mammals
In animals such as fish, blood flows through the heart once during each circuit of the body. Fish are said to have a single circulation. However in mammals, there is a double circulation because of the presence of lungs in the body, by which blood passes though the heart twice in one complete circuit. Double circulation consists of pulmonary circulation and systemic circulation.
In pulmonary circulation, blood flows from the main circulation of the body to the heart, then to the lungs and back to the heart again before it is pumped back into the main circulation.
- From the heart, the pulmonary arteries carry the blood to the lungs.
- Oxygenated blood is returned to the heart by the pulmonary veins.
Systemic circulation is the circulation of blood around the body. Oxygenated blood leaves the left side of the heart and is distributed by arteries to all parts of the body (except the lungs). Veins carry the blood from all parts of the body back to the right side of the heart.
Blood Flow in The Heart
1) Deoxygenated blood from certain parts of the body is returned to the right atrium. Blood from the head, neck and arms is returned to the right atrium by the superior vena cava while blood from other parts of the body (excluding the lungs) is brought back by the inferior vena cava.
2) When the right atrium contracts, the blood pressure in the atrium increases causing the tricuspid valve to open and thus allowing blood to flow into the right ventricle. The tricuspid valve consists of three flaps which are attached to the walls of the right ventricle by cord-like tendons called chordae tendineae. They point downwards to permit easy flow of blood from the atrium into the ventricle.
3) When the right ventricle contracts, the blood pressure increase forces the tricuspid valve to close which prevents the backflow of blood into the atrium. The chordae tendineae prevents the flaps from being reverted into the atrium when the ventricle contracts. Blood leaves the right ventricle through the pulmonary arch which divides into two pulmonary arteries, one to each lung. Semi-lunar valves in the pulmoary arch also prevent backflow of blood into the right ventricle.
4) The blood in the pulmonary arteries is at a lower pressure than the blood in the aorta. This slows down the rate of blood flow to give more time for gaseous exchange in the lungs.
5) Oxgenated blood from the lungs is brought back to the left atrium by the pulmonary veins. When the left atrium contracts, the blood pressure in the left atrium becomes higher than that in the left ventricle, causing the bicuspid valve to open allowing blood to enter the left ventricle. The bicuspid valve (or mitral valve) is similar in structure and function to the tricuspid valve except that it has two flaps instead of three. As the left ventricle contracts, blood leaves through the aortic arch.
6) From the aortic arch, blood is distributed to all parts of the body (except the lungs). It also possesses semi-lunar valves to prevent backflow of blood into the left ventricle. Blood entering the aorta is at a very high pressure.
7) Two small coronary arteries originate from the aortic arch, bringing oxygen and nutrients to the heart muscles.
The Cardiac Cycle
1) Both the atria and ventricles are relaxed. The right atrium receives blood from the venae cavae while the left atrium receives blood from the pulmonary vein.
2) The atria contract, forcing blood into the ventricles.
3) After a short pause, the ventricles contract. This is called ventricular systole. The increase in blood pressure forces the bicuspid and tricuspid valves to close, preventing backflow of blood into the atria. This produces a loud 'lub' sound. When the pressure in the left ventricle becomes higher than the pressure in the aorta, the semi-lunar valves in the pulmonary and aortic arches are forced open. Blood flows from the left ventricle into the aortic arch. As the ventricles relax, the atria also relax.
4) When the ventricles relax, it is the period of the ventricular diastole. The drop in pressure in the ventricles causes the semi-lunar valves in the two arches to close, preventing backflow of blood into the ventricles. This produces a soft 'dub' sound. The drop in pressure in the ventricles also causes the bicuspid and tricuspid valves to open. The whole cycle then repeats itself.
A venticular systole and a ventricular diastole make up one heartbeat, which lasts for about 0.8 seconds. There is a short pause between two heartbeats. The rate of heartbeat varies with age and size of the individual. Note that the atria and the ventricles work alternately: when the atria contract, the ventricles relax and vice-versa.