Human Heart Anatomy and Blood Circulation Mechanics

The Human Heart: Structure and Function

The heart is a muscular organ approximately the size of a fist, situated in the center of the chest, between the two lungs.

Primary Function

Its main function is to pump blood through the blood vessels.

External and Internal Structure

Externally, it is formed by a muscle that contracts involuntarily. Internally, it consists of two halves separated by a partition. Each half is comprised of two chambers (atria at the top and ventricles at the bottom).

The Cardiac Cycle

The heart operates through two alternating movements: contraction (systole) and dilation (diastole). Each complete cycle consists of four distinct stages:

1. Diastole (Filling Phase)

• Blood from different parts of the body enters the atria, causing them to dilate.
• Atrioventricular valves are closed initially.
• Deoxygenated blood (AD - Right Atrium) arrives via the vena cava, and oxygenated blood (AI - Left Atrium) arrives via the pulmonary veins.

2. Atrial Systole

• The atria contract.
• The atrioventricular valves open, and blood passes into the ventricles.

3. Ventricular Diastole

• The ventricles receive blood and expand.
• The semilunar valves remain closed.

4. Ventricular Systole

• The ventricles contract.
• The semilunar valves open, and the atrioventricular valves close.
• Blood is forced into the arteries: the Left Ventricle (LV) to the aorta, and the Right Ventricle (RV) to the pulmonary artery.

Finally, blood passes into the arteries, and the semilunar valves close. The movements of systole and diastole occur in sequence, ensuring a continuous, uninterrupted pumping of blood.

Circulation Circuits

Pulmonary Circuit (Lesser Circuit)

Path: Right Atrium (AD) → Right Ventricle (VD) → Pulmonary Artery → Lungs → Left Atrium (AI).

Deoxygenated blood carrying carbon dioxide enters the Right Atrium (AD), moves to the Right Ventricle (VD), and then to the pulmonary arteries leading to the lungs. In the lungs, the blood releases $\text{CO}_2$, is loaded with oxygen, and returns to the Left Atrium (AI).

Systemic Circuit (Greater Circuit)

Path: Left Atrium (AI) → Left Ventricle (VI) → Aorta → Rest of the Body → Right Atrium (AD).

This circuit establishes circulation between the heart and the rest of the body. Oxygenated blood from the Left Atrium (AI) moves to the Left Ventricle (VI), and from there, through the aorta, it travels to all body organs. Oxygen and nutrients are delivered, while $\text{CO}_2$ and waste products are collected. The blood then returns to the Right Atrium (AD), restarting the pulmonary circuit.