Chapter 18: The Heart – Structure, Function, and Physiology

Heart Anatomy and Physiology

Overview of the Heart

The heart is a muscular organ responsible for pumping blood throughout the body, supplying oxygen and nutrients to tissues and removing waste products. It is divided into four chambers: two atria and two ventricles, separated by valves that regulate blood flow.

• Location: The heart is located in the thoracic cavity, between the lungs, in the mediastinum.

• Function: Maintains circulation of blood through pulmonary and systemic circuits.

• Chambers: Right atrium, right ventricle, left atrium, left ventricle.

Heart Valves

Types and Functions of Heart Valves

Heart valves ensure unidirectional blood flow through the heart chambers. There are two main types: atrioventricular (AV) valves and semilunar (SL) valves.

• Atrioventricular (AV) Valves: Separate atria from ventricles. Includes the tricuspid valve (right AV) and mitral valve (left AV).

• Semilunar (SL) Valves: Located at the exits of the ventricles. Includes the pulmonary SL valve (right ventricle to pulmonary artery) and aortic SL valve (left ventricle to aorta).

• Chordae tendineae: Tendinous cords that anchor AV valve flaps to papillary muscles, preventing prolapse.

Valve Position During Cardiac Cycle

The opening and closing of heart valves are synchronized with the cardiac cycle phases.

• Ventricular Relaxation (Diastole):

  • AV valves (tricuspid and mitral) are open to allow blood flow from atria to ventricles.

  • SL valves (aortic and pulmonary) are closed to prevent backflow from arteries into ventricles.

• Ventricular Contraction (Systole):

  • AV valves are closed to prevent backflow into atria.

  • SL valves are open to allow blood to exit ventricles into arteries.

Structure of Semilunar Valves

Semilunar valves consist of three cup-like cusps, which are strong and do not require chordae tendineae. This structure prevents prolapse and ensures efficient closure.

• Semilunar valves: Aortic and pulmonary valves.

• AV valves: Flap-like cusps, require chordae tendineae.

Cardiac Skeleton

Role and Composition

The cardiac skeleton is a framework of dense connective tissue that supports the heart valves and electrically isolates the atria from the ventricles.

• Function: Provides structural support and electrical insulation.

• Composition: Flexible, dense connective tissue.

Heart Valve Disease and Replacement

Types of Valve Disease

Valve diseases can be congenital or acquired, often resulting from coronary artery disease or atherosclerosis.

• Healthy valve: Normal function and structure.

• Damaged valve: May be stiff, irregularly shaped, or unable to open/close fully.

Valve Replacement Options

• Bioprosthetic valves: Made from animal tissue (e.g., pig heart cusps). Lasts ~10 years, less likely to stimulate clotting.

• Mechanical valves: Artificial, longer-lasting but may require anticoagulant therapy.

Arteriosclerosis and Atherosclerosis

Definitions and Pathology

Arteriosclerosis refers to the thickening and stiffening of arterial walls. Atherosclerosis is a specific type characterized by lipid deposits within the artery walls.

• Arteriosclerosis: General term for artery hardening.

• Atherosclerosis: Most common form; involves fatty deposits (plaques) beneath the endothelium, restricting blood flow.

• Coronary artery disease (CAD): Atherosclerosis in coronary arteries, can lead to heart attack.

Risk Factors for Atherosclerosis

• Age

• High cholesterol

• High blood pressure

• Smoking

Treatment Options for Coronary Artery Disease

• Bypass surgery: Grafting a vein (often the great saphenous vein) to bypass blocked artery segments.

• Angioplasty: Inserting a balloon catheter to widen the vessel; may be followed by stent placement to keep the artery open.

Cardiac Cycle

Phases of the Cardiac Cycle

The cardiac cycle consists of alternating periods of contraction (systole) and relaxation (diastole) in the atria and ventricles.

• Atrial systole: Atria contract, filling ventricles (~70% full before next cycle).

• Ventricular systole: Ventricles contract, pumping blood into arteries.

• Atrial diastole: Atria relax and refill with blood.

• Ventricular diastole: Ventricles relax and refill.

Isovolumetric Phases

• Isovolumetric contraction: Ventricles contract with all valves closed; pressure rises but volume remains constant.

• Isovolumetric relaxation: Ventricles relax with all valves closed; pressure falls but volume remains constant.

Cardiac Muscle vs Skeletal Muscle

Comparison of Muscle Types

Cardiac muscle differs from skeletal muscle in its contraction properties and electrical behavior.

Feature	Cardiac Muscle	Skeletal Muscle
Contraction Duration	Longer	Shorter
Tetanus	Cannot occur	Can occur
Electrical Isolation	Yes (via cardiac skeleton)	No

Action Potential in Cardiac Muscle

• Plateau phase: Ca2+ channels open, balancing Na+ loss, keeping membrane potential near 0.

• Repolarization: Ca2+ channels close, K+ channels open, restoring resting potential.

• Absolute refractory period: Cardiac muscle cannot be restimulated, preventing tetanus.

Cardiac Output

Definition and Formula

Cardiac output is the volume of blood pumped by the heart per minute. It is determined by heart rate and stroke volume.

• Formula:

$\text{Cardiac Output (CO)} = \text{Heart Rate (HR)} \times \text{Stroke Volume (SV)}$

• Example: HR = 75 bpm, SV = 80 mL/beat, CO = 6000 mL/min

• Both HR and SV can increase to meet tissue demands.

Electrical Conducting System of the Heart

Components and Function

The heart's electrical system coordinates contraction through specialized cells.

• Pacemaker cells: Initiate contraction signals (located in the sinoatrial (SA) node).

• Conducting cells: Distribute signals to contractile cells.

Pathway of Electrical Impulse

• Sinoatrial (SA) node: Primary pacemaker, located in right atrium wall; sets heart rate (60–100 bpm).

• Internodal pathways: Conduct impulses from SA node to atrial muscle cells and toward ventricles.

• Atrioventricular (AV) node: Located between atria and ventricles; backup pacemaker (40–60 bpm).

• AV bundle (Bundle of His): Conducts impulses through interventricular septum; splits into left and right bundle branches.

• Purkinje fibers: Large conducting cells; rapidly depolarize ventricular myocardium, triggering ventricular systole.

Example: If the SA node is damaged, the AV node can maintain heart rhythm at a slower rate.

Additional info: The cardiac cycle and electrical conduction system are essential for coordinated heart function and efficient blood circulation.