6.1 Structure and function of the heart

The heart, a muscular powerhouse, pumps blood through our bodies. It's divided into four chambers: two atria on top and two ventricles below. These work together to move blood through the pulmonary and systemic circulations.

The heart's structure is key to its function. Layers like the epicardium, myocardium, and endocardium protect and power the heart. Valves ensure blood flows in the right direction, while the coronary arteries feed the heart itself.

Heart Anatomy and Function

Cardiac Chambers and Septa

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• The heart is a four-chambered muscular organ located in the thoracic cavity between the lungs, posterior to the sternum, and slightly left of the midline
• The right and left atria are the upper chambers of the heart that receive blood from the venous system (right atrium) and the lungs (left atrium)
• The right and left ventricles are the lower chambers of the heart that pump blood to the lungs (right ventricle) and the systemic circulation (left ventricle)
• The interventricular septum is a thick muscular wall that separates the left and right ventricles, preventing the mixing of oxygenated and deoxygenated blood
• The interatrial septum separates the left and right atria

Coronary Circulation and Great Vessels

• The coronary arteries, arising from the base of the aorta, supply oxygenated blood to the myocardium (heart muscle)
• The great vessels include:
  • Superior vena cava: returns deoxygenated blood from the upper body to the right atrium
  • Inferior vena cava: returns deoxygenated blood from the lower body to the right atrium
  • Pulmonary arteries: carry deoxygenated blood from the right ventricle to the lungs
  • Pulmonary veins: carry oxygenated blood from the lungs to the left atrium
  • Aorta: carries oxygenated blood from the left ventricle to the systemic circulation

Layers of the Heart Wall

Epicardium and Pericardium

• The epicardium is the outermost layer of the heart, consisting of a serous membrane that reduces friction during heart contractions and provides protection
• The pericardium is a fibrous sac that surrounds and protects the heart, anchoring it to the surrounding structures and preventing overfilling of the heart
  • Parietal pericardium: outer layer of the pericardium, attached to the great vessels and diaphragm
  • Visceral pericardium (epicardium): inner layer of the pericardium, directly attached to the heart surface

Myocardium and Endocardium

• The myocardium is the middle layer of the heart, composed of cardiac muscle tissue responsible for the heart's contractile function
  • Cardiac muscle cells (cardiomyocytes) are striated, branched, and interconnected by intercalated discs
  • Intercalated discs contain gap junctions that allow for rapid electrical impulse propagation and coordinated contraction
• The endocardium is the innermost layer of the heart, lining the chambers and valves, providing a smooth surface for blood flow and preventing blood clotting
  • Endothelial cells of the endocardium secrete nitric oxide (NO) and endothelin to regulate vascular tone and cardiac function
  • Purkinje fibers, specialized conduction cells, are found in the subendocardial layer and rapidly conduct electrical impulses

Function of Heart Valves

Atrioventricular (AV) Valves

• The atrioventricular (AV) valves, namely the tricuspid (right) and mitral (left) valves, are located between the atria and ventricles
• AV valves prevent backflow from the ventricles to the atria during ventricular contraction (systole)
• The tricuspid valve has three cusps, while the mitral valve has two cusps (bicuspid valve)
• The papillary muscles and chordae tendineae anchor the AV valves to the ventricular walls, preventing the valves from inverting during ventricular contraction

Semilunar Valves

• The semilunar valves, namely the pulmonary (right) and aortic (left) valves, are located at the base of the pulmonary artery and aorta, respectively
• Semilunar valves prevent backflow from these vessels into the ventricles during ventricular relaxation (diastole)
• Each semilunar valve has three crescent-shaped cusps that open and close passively in response to pressure changes
• The cusps of the semilunar valves are thinner and more delicate compared to the AV valves, as they do not have chordae tendineae or papillary muscle attachments

Pulmonary vs Systemic Circulation

Pulmonary Circulation

• The pulmonary circulation is the portion of the cardiovascular system that carries deoxygenated blood from the right ventricle to the lungs and returns oxygenated blood to the left atrium
• The pulmonary circulation is a low-pressure, low-resistance system compared to the systemic circulation
  • Pulmonary arteries have thinner walls and larger diameters relative to their systemic counterparts
  • Pulmonary capillaries have a larger total cross-sectional area, allowing for efficient gas exchange
• The pulmonary arteries carry deoxygenated blood from the right ventricle to the lungs, while the pulmonary veins carry oxygenated blood from the lungs to the left atrium

Systemic Circulation

• The systemic circulation is the portion of the cardiovascular system that carries oxygenated blood from the left ventricle to the body tissues and returns deoxygenated blood to the right atrium
• The aorta, the largest artery in the body, carries oxygenated blood from the left ventricle to the systemic circulation
  • Ascending aorta: initial portion of the aorta, gives rise to the coronary arteries
  • Aortic arch: curved portion of the aorta, gives rise to the brachiocephalic, left common carotid, and left subclavian arteries
  • Descending aorta: portion of the aorta that descends through the thorax (thoracic aorta) and abdomen (abdominal aorta)
• The superior and inferior vena cava are the major veins that return deoxygenated blood from the systemic circulation to the right atrium
  • Superior vena cava: formed by the union of the left and right brachiocephalic veins, draining the head, neck, and upper extremities
  • Inferior vena cava: formed by the union of the left and right common iliac veins, draining the lower extremities, pelvis, and abdomen