10.3 - Mechanism of Heartbeat

Blood must be circulated continuously in the body at all times. Every time the heart contracts, it pumps the blood throughout the whole body. The heart is made up cardiac muscles which is a strong muscle. The cardiac muscle cells are interconnected. This interconnection allows the spread of electrical impulses through the heart and, at the same time, stimulates the cardiac muscle cells to contract in a coordinated movement. The cardiac muscle is myogenic , which means it contracts and relaxed without the need of stimulation by nerve impulses.

Blood Circulation in Humans

The blood can be circulated in humans thanks to the pumping action of the heart and the contraction of the skeletal muscles. The process of the pumping of the heart is initiated by a pacemaker , which is a group of specialized heart muscle cells that set the rate of contraction. The primary pacemaker is called the sinoatrial (SA) node as it keeps the heartbeats regular. From the SA node, the impulses are relayed to the atrioventricular (AV) node, located at the bottom of the right atrium.

Figure 12 - 1. The SA node generates electrical impulses. 2. The impulses are spread rapidly over the walls of both atria so that they contract simultaneously. The contraction pumps the blood into the ventricles. 3. The electrical signals reach the AV node. The bundle of His fibres, bundle branches and Purkinje fibres send the impulses to the apex of the heart. 4. The electrical impulses spread to the ventricles, which pump the blood out to the lungs and body.

The lub-dub sound produced by the heart is the result of the closing of the valves.

• ‘Lub’ sound

  • Produced when the tricuspid valve and the bicuspid valve close.

• ‘Dub’ sound

  • Produced when the semilunar valves close.

The pumping of the heart only gives enough force to supply blood through the arteries, arteriole and capillaries. In the veins, the pressure from the heart is not sufficient to force the blood back to the heart. The blood in the veins also has to work against the force of gravitational pull. Therefore, the contractions of the skeletal muscles around the veins help to push the blood back to the heart.

Figure 13 - The valves direct the flow of blood in the veins. When the lateral skeletal muscles contract, the valves open to allow the flow of blood towards the heart. The valves then close to prevent backflow.