The Human Heart: Structure, Function, and Blood Flow

The Heart: Structure and Function

The heart is a contractile, muscular organ, roughly the size of a fist and conical in shape. Its primary function is to pump blood to reach all parts of the body. The heart is situated above the diaphragm, nestled between the lungs.

The heart is composed of two distinct halves, separated by a septum, which ensures the right and left halves do not communicate directly. Each half of the heart has two cavities:

• An upper, thinner-walled cavity called an atrium.
• A lower, muscular-walled cavity called a ventricle, which drives the blood.

Understanding the Cardiac Cycle

The period between the end of one contraction and the end of the subsequent one is called the cardiac cycle. It includes two main phases:

• Diastole: An expansion or relaxation phase, during which the heart chambers fill with blood. In this phase, the mitral and tricuspid valves open, while the semilunar valves (aortic and pulmonary) close.

• Systole: A contraction phase, where blood is pumped out of the heart. Systole occurs in two stages:

  1. Atrial Systole: Blood passes from the atria to the ventricles through the tricuspid and mitral valves.

  2. Ventricular Systole: Blood leaves the ventricles of the heart. During this stage, the tricuspid and mitral valves close, and the semilunar valves open to allow blood passage into the aorta and pulmonary artery. These semilunar valves then close after the blood has passed.

Heart sounds, audible during monitoring, are produced by the closing of valves. The longer sound corresponds to the closing of the atrioventricular valves (tricuspid and mitral), while the shorter sound corresponds to the closing of the semilunar valves (aortic and pulmonary).

The Right Atrium

Deoxygenated blood enters the right atrium from two major veins: the superior vena cava, which collects blood from the head, neck, and upper limbs, and the inferior vena cava, which brings blood from the trunk and lower limbs. From the right atrium, blood passes through the tricuspid valve into the right ventricle. The right ventricle then pumps this blood into the pulmonary artery, which carries it to the lungs. This blood is rich in CO2 and low in O2, and is often depicted as blue.

The Left Atrium

Oxygen-rich blood, returning from the lungs, enters the left atrium through the four pulmonary veins. The left atrium communicates with the left ventricle through the mitral valve (also known as the bicuspid valve). The left ventricle has thick, muscular walls because it must pump blood throughout the entire body. From the left ventricle, blood is pumped into the aorta, which branches to supply all organs of the body.

The Circulatory Pathways

Pulmonary Circulation (Minor Circulation)

This pathway involves the movement of deoxygenated blood from the heart to the lungs and back to the heart. The blood leaves the right ventricle, travels to the lungs via the pulmonary artery, where it becomes oxygenated, and then returns to the left atrium via the pulmonary veins.

Pathway: Right Atrium → Right Ventricle → Pulmonary Artery → Lungs → Pulmonary Veins → Left Atrium

Systemic Circulation (Major Circulation)

This pathway carries oxygenated blood from the left ventricle into the aorta, distributing it to all organs of the body. Deoxygenated blood then returns from the body's tissues via the vena cava to the right atrium.

Pathway: Left Atrium → Left Ventricle → Aorta → Body Tissues → Vena Cava → Right Atrium