Circulation of Blood Through the Mammalian Heart

The mammalian heart is 4 chambered and shows double circulation. This means that the blood passes through the heart twice for the body to be supplied once.

The 2 circulations are systemic circulation and pulmonary circulation.

The aorta or the great artery arising from the left ventricle takes the oxygenated blood to the body organs through a number of arteries. From these organs, the deoxygenated blood is collected by the superior and inferior vena cava and brought back to the right auricle. The right auricle pumps the deoxygenated blood into the right ventricle. The deoxygenated blood from the right ventricle is taken by the pulmonary artery to the lungs for oxygenation. The blood after oxygenation is returned to the left auricle of the heart by the pulmonary vein. From the left auricle the blood flows to the left ventricle.

Diagram of human circulation

The cardiac muscles of the heart have the property of excitability and conductivity. When these muscles are stimulated, they get excited and initiate waves of electric potential called cardiac impulses, which are conducted along the special cardiac muscle fibres on the wall of the heart chambers.

Origin and Conduction of Heart Beat

The impulse originates from the SA node which lies on the right wall of the right auricle below the opening of the vena cava. It is also called pacemaker as it determines the rate of heart beat.

The impulse originated from the sinu-auricular node is picked up and propagated by a special system of tissues present in the heart. The conducting system includes the following components.

a) Auriculo ventricular node

b) Bundle of His (AV Bundle)

c) Purkinje fibres

The impulses arising from the sino auricular node is picked up by the auriculo ventricular node (AV node) located at the posterior right border of the inter auricular septum. It functions as a relay station and it transmits the impulses to other parts of the heart through the bundle of His. When the SA node fails to function, it acts as a resume pace maker as it can also initiate the cardiac impulse.

The bundle of His originates from the AV node as a bundle of tissue. Immediately after its origin it divides into 2 branches. These branches run along the inner border of each ventricle and reach the tip of the ventricle and then runs upwards along the outer margin of the ventricle. The bundle of His and its branches produce minute branches called Purkinje fibres on the wall of the ventricles.

During a heart beat, the auricles contract first and the ventricles contract later. This is because there is no muscular continuity between the auricles and the ventricles. The auricles receive the impulses directly from the SA node. The impulses reach the AV node about 0.03 seconds after their origin from the SA node. So the ventricles always contract after the auricles.

Neurons connecting the heart to the cardiovascular system

The heart beat is controlled by the nervous system, hormones, temperature and pH.