The Heart

The heart is a ‘muscular bag’ filled with blood that contributes to the circulatory system by pumping blood around the body. It is about the size of an adult’s fist and weighs around 300g. In humans, the heart is in the chest, enclosed by the ribs and sternum (the breastbone, the bone located in the middle of the chest). Since the heart is filled with blood and constantly pumping, it is surrounded by an inelastic pericardial membrane, which helps prevent the heart from over-distending with blood.

The heart is made of specialised muscle cells called cardiac muscles cells. These cells can continuously contract and relax without fatigue. The movement of these cells does not require stimulation by the nervous system.

What does the heart look like from the outside?

Let’s explore the heart from the outside first.

The two sides of the heart

This muscular organ is divided into two parts: the left side and the right side.

Fig. 1 - A model of the heart

You may wonder what purpose it serves for the heart to be divided into two. The left side receives oxygenated blood from the lungs and pumps it to the body, and the right side receives deoxygenated blood from the body and pumps it to the lungs. Blood has to pass through the lungs first to be oxygenated. There is a large drop in blood pressure after passing through the lungs, and the heart needs to pump this blood to increase its pressure. Hence, the heart is split into two separate pumps lying side by side instead of one pump.

Regardless, both sides of the heart contract and relax at the same time.

Fig. 2 - The heart pumping blood. Notice that both sides relax at the same time

Blood vessels of the heart

The heart is connected to four blood vessels - two veins (vena cava and pulmonary vein) and two arteries (aorta and pulmonary artery).

Fig. 3 - The heart labelled

Veins carry blood to the heart. The two veins that connect to the heart include:

• Vena cava - brings deoxygenated blood from both the lower and upper body

• Pulmonary vein - brings oxygenated blood from the left and right lungs

In contrast, arteries carry blood away from the heart. The two veins that connect to the heart include:

• Aorta - large and arching, also branches into several smaller arteries to transport oxygenated blood up to the head and the rest of the lower body

• Pulmonary artery - branches into two to transport deoxygenated blood into the left and right lungs

Blood vessels supply oxygen and nutrients to the heart muscles; they need to contract and relax throughout the organism’s lifetime. Heart muscles have coronary arteries and cardiac veins. Coronary arteries branch off from the aorta and supply the heart muscles with oxygenated blood, whereas cardiac veins bring deoxygenated blood containing metabolic waste into the vena cava. On the other hand, cardiac veins bring deoxygenated blood containing metabolic waste into the vena cava.

Four chambers of the heart

The human heart comprises four chambers – two on the right side and another two on the left. The chambers of the heart are divided by a muscular structure called the septum. The septum stops the blood from both sides of the heart from mixing together (Figure 3).

The upper chambers that are smaller in size are the atria (singular atrium). The atria are connected to the veins, with the right atrium connected to the vena cava and the left atrium connected to the pulmonary vein. The atria have thin walls as they only pump blood into the small ventricles, whose lower pumping pressure prevents the ventricles from bursting.

The larger lower chambers are the ventricles. The ventricles are connected to the arteries, with the right ventricle connected to the pulmonary artery and the left ventricle connected to the aorta. The ventricles have thicker walls than the atria as they pump blood into organs further away from the heart. Furthermore, the walls of the left ventricle are thicker than the right. This is because the blood is pumped from the left ventricles to the extremities and needs to overcome the elastic recoil of arteries. As for the thinner walls of the right ventricles, the lungs that receive blood from the right ventricles are closer to the heart and smaller than the rest of the body, but the lungs’ capillaries are extremely delicate.

Heart valves

How the heart achieves a regular direction of blood flow across the different chambers?

This is due to structures called valves, which prevent the backflow of blood. The valves located between the atria and ventricles are called atrioventricular valves. The left and right sides of the heart each have an atrioventricular valve. The atrioventricular valve on the left side is called the bicuspid (or mitral) valve, whereas the atrioventricular valve on the right side is called the tricuspid valve. The atrioventricular valves have string-like, tendinous support? Atrioventricular valves have tendinous cords to prevent pressure from the heart from turning the valves inside out when the ventricles contract.

Fig. 4 - Heart valves

Moreover, there are valves located on the arteries connected to the heart. These valves are called semilunar valves, including aortic valves and pulmonary valves. The name ‘semilunar’ is derived from the shape as the valves look like two crescents. Both atrioventricular and semilunar valves have flaps that open and close. The key difference between bicuspid and tricuspid valves is the number of flaps these valves have. Bicuspid valves have two flaps, whereas tricuspid valves have three flaps. Semilunar valves, like the tricuspid valve, have three flaps.

How does the heart function?

The heart functions by a series of contractions and relaxations of the heart muscles. These move the blood across the different chambers and send it to respective organs via blood vessels. The valves also help regulate the unidirectionality of blood flow.

As such, the components of the heart all work together to form the cardiac cycle.

In brief, the heart functions to:

• Pump oxygenated blood around the body.

• Receive and pump deoxygenated blood to the lungs for oxygenation.

• Transport hormones and other vital substances to different parts of the body.

• Maintain a constant blood pressure across the different vessels.