Anatomy and Physiology of the Heart

• Endocardium: The innermost layer consisting of a thin layer of squamous epithelial cells that lines the chambers and covers the valves.
• Myocardium: The thick, middle layer composed of cardiac muscle. This is the layer responsible for the actual work of contraction.
• Epicardium: The outermost epithelial layer, which is synonymous with the visceral layer of the serous pericardium.

To provide structural integrity, the heart contains a heart skeleton made of semi-rigid, fibrous connective tissue. This skeleton provides support, prevents the valves from stretching, and acts as insulation between the chambers.

The Chambers and Septa

The heart is a dual-pump system divided into four chambers. The right and left sides are separated by muscular walls called septa:

• Atrial Chambers: The right atrium and left atrium act as reservoirs. The right atrium receives deoxygenated blood from the body, while the left atrium receives oxygenated blood from the lungs. The interatrial septum is the wall that separates these two chambers.
• Ventricle Chambers: The right ventricle and left ventricle act as the primary pumps. The right ventricle sends blood to the lungs, while the left ventricle sends blood to the rest of the body. The interventricular septum separates the two ventricles.

Blood Flow and Valvular Function

Blood flow through the heart is a continuous, highly regulated loop. To ensure blood moves in only one direction, four primary valves open and close in response to pressure changes:

1. Tricuspid valve
2. Pulmonary valve
3. Mitral (bicuspid) valve
4. Aortic valve

The Circulatory Pathway

The movement of blood follows this specific sequence:

1. Deoxygenated blood enters the right atrium via the superior vena cava and inferior vena cava.
2. Blood moves into the right ventricle, which contracts to push blood through the pulmonary valve into the pulmonary arteries.
3. The blood travels to the lungs to release $\text{CO}_2$ and pick up $\text{O}_2$.
4. Oxygenated blood returns to the heart via the pulmonary veins and enters the left atrium.
5. Blood moves into the left ventricle, which pumps it through the aortic valve into the aorta to be distributed to the organs and limbs.

Exam Tip: Remember that the right side of the heart deals exclusively with deoxygenated blood (pulmonary circulation), while the left side deals with oxygenated blood (systemic circulation).

Coronary Circulation

While the heart pumps blood to the entire body, it also requires its own dedicated blood supply to sustain the constant work of the myocardium. This is provided by coronary circulation.

The coronary arteries deliver oxygenated blood directly to the cardiac muscle cells. If these vessels become obstructed, the lack of oxygen to the muscle can lead to a myocardial infarction, commonly known as a heart attack.

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Summary Table: Heart Chambers and Functions