MAVO Year 4 · Transport and immune defence · Section 13.3

The heart

Position and structure

The heart lies between the lungs, behind the sternum, with much of it slightly left of the midline. It is roughly the size of its owner's fist. A septum separates right and left sides. Each side contains an atrium, receiving blood, and a ventricle, pumping blood out.

Coronary arteries, branching from the aorta, supply the heart muscle with oxygen and nutrients. Coronary veins collect blood from the muscle, draining mainly through the coronary sinus into the right atrium.

Blood flow through the heart

• The superior and inferior venae cavae carry blood from the body into the right atrium.
• Blood enters the right ventricle, which pumps it through the pulmonary artery towards the lungs.
• Pulmonary veins return oxygen-rich blood to the left atrium.
• Blood enters the left ventricle, which pumps it into the aorta and systemic circulation.

Muscular walls

The left ventricle has a thicker muscular wall than the right because it generates higher pressure for the systemic circuit. The right pumps through the lower-pressure lung circuit. Atrial walls are thinner because they move blood over a short distance into the ventricles.

Valves

Atrioventricular valves between atria and ventricles prevent flow back into the atria during ventricular contraction. The semilunar valves at the exits to the aorta and pulmonary artery prevent blood returning into the ventricles. Valves open and close in response to pressure differences.

The cardiac cycle

The heartbeat can be described in three main stages.

• Atrial contraction: the atria push additional blood into the ventricles, which have already been filling.
• Ventricular contraction: pressure closes the atrioventricular valves and opens the outlet valves, ejecting blood towards the lungs and body.
• Relaxation: pressure falls, outlet valves close and the heart chambers refill. Blood continues returning to the atria throughout much of the cycle.