Mammalian transport systems II - the heart
Joanna George-Johnson, Contributor

As was already outlined, blood is the means by which substances are transported around the body. However, a driving force is necessary to ensure that this blood reaches all extremities of the mammalian anatomy. This force is provided by a muscular organ; the heart.

Structure of the Heart

The heart is composed of a special type of muscle known as cardiac muscle. What is special about this type of muscle is that it contracts rhythmically through life, never getting tired. This is clearly an adaptation of cardiac muscle. Can you imagine what would happen if cardiac muscle was capable of fatigue?

A diagram showing the various parts of the heart.

The heart is divided first into the right and left sides, which are separated by tissue known as the septum. Each side is further divided into the upper chamber (atrium) and the lower chamber (the ventricle). The atria only receive blood, while the ventricles pump blood. Consequently, the ventricular walls are thicker than the atrial walls. Moreover, the right atrium only pumps blood to the lungs while the left atrium pumps blood around the whole body. Thus, the left ventricle has the thickest walls.

Blood Flow through the Heart

Deoxygenated blood, i.e. blood coming from the body cells enters the right atrium through the venae cavae. This blood is low in oxygen, but high in carbon dioxide content.

The blood then passes through the tricuspid valve to the right ventricle.

There, it is pumped through the semilunar valves into the pulmonary arteries, which takes the blood to the lungs. Gaseous exchange occurs in the lungs. The blood is now said to be oxygenated.

The oxygenated blood enters the left atrium, passing though the bicuspid valve into the left ventricle.

The left ventricle pumps the blood into the aorta, which takes the blood into general circulation.

As you can see the blood passes through multiple valves during its passage through the heart. These ensure unidirectional flow of blood, more simply, they prevent backflow.

The Cardiac Cycle

The beating of the heart is self-regulatory, being controlled by special muscle cells within the cardiac muscle. A special area within the heart initiates the heartbeat, this being known as the sinoatrial node, or the pacemaker. For your level of study, the cycle is divided into three main stages. Systole simply means contraction, while diastole means relaxation.

In atrial systole, the atria contract, forcing blood into the ventricles. The atrioventricular valves (collective name for bicuspid and tricuspid valves) open. The semilunars remain shut.

Next is ventricular systole. The ventricles contract, forcing blood out of the heart, opening the semilunar valves in the process. The force generated by the outflow of the blood closes the atrioventricular valves.

The final stage is diastole. The heart relaxes and blood flows back in.

N.B. The left and right atria contract and relax simultaneously. Also the left and right ventricles contract and relax simultaneously as well.

Questions

1) Explain the role of the heart in the circulatory system.

2) Compare and explain the relative thickness of the muscle tissue in the left and right ventricles and atria.

3) What is the name of the vessel which supplies the cardiac muscle with blood, and give the effects on the heart if this vessel is obstructed?

Joanna George-Johnson teaches at Ardenne High School.
Email: Masterbio@gmail.com.