Anatomy and Physiology of the Heart

The heart is a muscular organ that serves as the central pump of the cardiovascular system. Located in the chest between the lungs and slightly to the left of the breastbone, it continuously circulates blood throughout the body. This circulation delivers oxygen and nutrients to tissues, removes carbon dioxide and metabolic waste, and helps regulate body temperature, fluid balance, and blood pressure.

Anatomy of the Heart

The adult human heart is about the size of a clenched fist and is enclosed in a protective, double-layered sac called the pericardium. A small amount of fluid between the layers reduces friction as the heart beats. The heart wall has three main layers: the epicardium, the outer layer; the myocardium, the thick muscular middle layer; and the endocardium, a smooth inner lining that covers the chambers and valves.

The Chambers

The heart has four chambers. The right and left sides are separated by a muscular wall called the septum, which helps prevent oxygen-rich and oxygen-poor blood from mixing.

• Right atrium: Receives oxygen-poor blood returning from the body through the superior and inferior vena cava.

• Right ventricle: Pumps oxygen-poor blood to the lungs through the pulmonary artery.

• Left atrium: Receives oxygen-rich blood from the lungs through the pulmonary veins.

• Left ventricle: Pumps oxygen-rich blood through the aorta to the rest of the body. Its muscular wall is the thickest because it must generate the highest pressure.

The Heart Valves

Four valves ensure that blood flows in one direction. The tricuspid valve lies between the right atrium and right ventricle, while the pulmonary valve controls blood flow from the right ventricle into the pulmonary artery. The mitral valve is located between the left atrium and left ventricle, and the aortic valve controls blood flow from the left ventricle into the aorta. The valves open and close in response to pressure changes, producing the sounds commonly described as “lub-dub.”

Blood Supply and Electrical System

The heart muscle receives its own oxygen and nutrients through the coronary arteries, which branch from the aorta. Cardiac veins collect deoxygenated blood from the heart muscle and return it to the right atrium through the coronary sinus.

The heartbeat is coordinated by the heart’s electrical conduction system. The sinoatrial, or SA, node acts as the natural pacemaker and initiates each heartbeat. The signal travels through the atria to the atrioventricular, or AV, node, then passes through the bundle of His, the right and left bundle branches, and the Purkinje fibers. This sequence causes the atria to contract first, followed by the ventricles.

Physiology of the Heart

The heart functions through repeated cycles of contraction and relaxation. Systole is the phase in which the ventricles contract and eject blood. Diastole is the phase in which the chambers relax and fill with blood. The atria contract near the end of diastole to help complete ventricular filling.

Blood moves through two connected circuits. In the pulmonary circulation, the right side of the heart sends oxygen-poor blood to the lungs, where it releases carbon dioxide and gains oxygen. In the systemic circulation, the left side sends oxygen-rich blood to body tissues and receives oxygen-poor blood in return.

Two important measures describe the heart’s pumping function. Stroke volume is the amount of blood ejected by a ventricle with each beat. Cardiac output is the amount of blood pumped by the heart per minute and is calculated by multiplying heart rate by stroke volume. The autonomic nervous system adjusts these factors to meet the body’s changing demands.

How Heart Function Changes During Exercise

During exercise, working muscles require more oxygen and nutrients and produce more carbon dioxide and heat. To meet these demands, the cardiovascular system increases blood flow. The sympathetic nervous system stimulates the heart to beat faster and more forcefully, while blood vessels supplying active muscles generally widen.

Changes During Exercise

• Heart rate increases: The heart beats more rapidly to circulate blood more quickly. The size of this increase depends on exercise intensity, fitness level, age, temperature, and other factors.

• Stroke volume increases: More blood returns to the heart, and the ventricles contract more strongly. As a result, more blood is ejected with each beat, particularly during moderate-intensity exercise.

• Cardiac output rises: Because both heart rate and stroke volume usually increase, the heart pumps substantially more blood each minute.

• Blood flow is redistributed: More blood is directed to active skeletal muscles and the heart. Blood flow to some less immediately essential organs may temporarily decrease, while blood flow to the skin increases during prolonged exercise to help release heat.

• Blood pressure responds: Systolic blood pressure generally rises as the heart pumps more forcefully. During dynamic aerobic exercise, diastolic pressure often changes less because blood vessels in active muscles dilate.

• Venous return increases: The muscular contractions of the limbs help push blood back toward the heart. Deeper breathing also helps draw venous blood toward the chest.

During very intense exercise, heart rate may approach an individual’s maximum, and stroke volume may level off. At this point, further increases in cardiac output depend primarily on heart rate. The heart must also coordinate its contractions efficiently so that filling time remains adequate despite the faster rhythm.

Long-Term Adaptations to Regular Exercise

Regular aerobic exercise can produce beneficial adaptations in the cardiovascular system. The heart muscle may become stronger, allowing it to pump a greater volume of blood with each beat. As a result, a well-trained person may have a lower resting heart rate while maintaining an adequate resting cardiac output. Regular exercise can also improve blood-vessel function, increase the muscles’ ability to use oxygen, and improve recovery after physical activity.

Strength training also places demands on the cardiovascular system, although the pattern differs from continuous aerobic exercise. Brief, intense efforts can cause temporary increases in heart rate and blood pressure. Proper breathing and appropriate progression are important, especially for people with known cardiovascular conditions.

Recovery After Exercise

When exercise stops, the heart does not immediately return to its resting state. Heart rate and cardiac output gradually decline as the body’s oxygen needs decrease and temperature regulation continues. Heart-rate recovery is influenced by fitness, exercise intensity, hydration, environmental conditions, and overall health. A gradual cool-down helps maintain circulation while the cardiovascular system returns toward its resting level.

The heart’s anatomy and physiology work together to maintain continuous circulation at rest and to adjust rapidly during exercise. Its chambers, valves, blood vessels, muscle tissue, and electrical conduction system form an integrated pump capable of meeting the changing demands of the body.

Kota Shimada