Understanding the Human Heart and Cardiac Cycle

The Heart: The Engine of Human Circulation

The heart is the motor of the circulation. If it stops, the brain stops receiving oxygen and dies.

The Phases of the Cardiac Cycle

Every beat of the heart follows a set of phenomena that constitute a cardiac cycle:

• Atrial systole: The atria contract and push blood into the ventricles.
• Ventricular systole: The ventricles contract and the blood leaves through the arteries by forcing the opening of the pulmonary artery and aortic valves. Blood cannot flow backward toward the atria, and its momentum causes the closure of the atrioventricular valves. This closure causes the first heart sound.
• Diastole: The musculature of the heart walls relaxes, but the blood in the arteries cannot flow back toward the ventricles. The closure of the pulmonary and aortic valves prevents this and causes the second heart sound. The atria fill with blood from the veins, and during diastole, blood slips into the ventricles.

Heart Activity and Blood Pathways

The heartbeat is the most visible manifestation of its activity. It often varies according to the needs of the organism.

The Two Circuits of Blood Flow

• Pulmonary circuit (Lesser circulation): The blood leaves the right ventricle through the pulmonary arteries and goes to the lungs. There, gas exchange happens between the alveoli and the capillaries into which these arteries branch. Blood collects oxygen, leaves CO2, and returns to the heart through the pulmonary veins, reaching the left atrium.
• Systemic circuit (Greater circulation): The blood leaves the left ventricle through the aorta and reaches all the organs. It circulates through the capillaries, delivering the oxygen and nutrients needed for their activity and collecting waste. Capillaries from different organs lead into veins that return blood to the right atrium of the heart.

The superior vena cava collects blood coming from the upper body, and the inferior vena cava collects blood from the lower body.

Characteristics of the Human Circulatory System

In a full cycle, blood passes twice through the heart: once during the pulmonary circuit and once through the systemic circuit. The human circulatory system, in addition to being a double track, is closed since blood passes twice through the heart at every turn, and there is a complete separation between oxygen-rich and oxygen-poor blood.