Biology of Disease

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heart and then goes downwards carrying blood to the abdomen and legs.

In the capillaries of the tissues, oxygen is exchanged for carbon dioxide and

nutrients and hormones, for example, supplied to the tissues. The blood then

returns to the heart through the veins (Figure 14.2).

To work efficiently, the four chambers of the heart must beat in a coordinated

way, with the atria and then the ventricles contracting simultaneously (Figure

14.3). A given chamber of the heart contracts when an electrical impulse moves

across it. Each signal originates in a small bundle of specialized cells in the

right atrium called the sinoatrial orSA node(Figure 14.4). This is the natural

pacemaker that ensures the heart beats regularly by generating impulses

at a given rate. Although it produces a natural rate, this can be modified by

emotional and physical reactions and by hormones, especially adrenaline

and noradrenaline (Chapter 7), which speed up the heart rate, enabling it

to respond to varying demands. The electrical impulses generated by the SA

node travel throughout the right and left atria causing the heart muscle to

contract (Figure 14.4) and arrive at the atrioventricular orAV node situated

between the atria and the ventricles. This node delays the transmission of the

impulse to allow the atria to contract completely and the ventricles to fill with

as much blood as possible. The phase of relaxation of the ventricles is called

diastole. After passing through the AV node, the impulse travels through

thebundle of His. This is a group of modified cardiac muscle fibers, called

Purkinje fibers, that divides into two branches that serves the left and right

ventricles respectively (Figure 14.4). The fibers spread over the surface of the

ventricles in an orderly arrangement and thus initiate systole or ventricular

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Pulmonary

vein

Pulmonary

artery

Vena cava Aorta

Oxygenated blood

Deoxygenated blood

Figure 14.3 Diagrammatic representation of

the contractions and relaxation of the heart

chambers during one cardiac cycle. Start at the

top and observe oxygenated and deoxygenated

blood entering the heart, its passage from the

atria to the ventricles and finally its expulsion

from the ventricles as the cycle starts again.