Chapter 18: The Heart and Cardiovascular Function

Overview of the Cardiovascular System

The cardiovascular system consists of the heart and blood vessels, functioning together to transport blood throughout the body. The heart acts as a muscular pump, circulating blood through two primary circuits: the pulmonary and systemic circuits.

• Pulmonary circuit: Carries blood between the heart and lungs for gas exchange (oxygenation and removal of carbon dioxide).

• Systemic circuit: Delivers oxygenated blood from the heart to the rest of the body and returns deoxygenated blood back to the heart.

• Cardiovascular system: Includes the heart (located directly behind the sternum in the thoracic cavity) and all blood vessels.

Heart Location and Borders

The heart is situated in the mediastinum, the central compartment of the thoracic cavity between the lungs. It is enclosed by the pericardial cavity.

• Superior border (base): Formed by the atria, especially the left atrium.

• Right border: Formed by the right atrium.

• Left border: Formed by the left ventricle and part of the left atrium.

• Inferior border: Formed by the right ventricle.

Heart Chambers

The heart is a two-sided pump with four chambers:

• Right atrium

• Right ventricle

• Left atrium

• Left ventricle

Each chamber plays a specific role in receiving and pumping blood to either the lungs or the systemic circulation.

Pericardium and Pericardial Cavity

The heart is surrounded by the pericardium, a double-walled sac that provides protection and reduces friction during heartbeats.

• Fibrous pericardium: Outer layer, tough connective tissue anchoring the heart to surrounding structures.

• Serous pericardium: Inner layer, composed of parietal and visceral layers with pericardial fluid in between to minimize friction.

• Pericarditis: Inflammation of the pericardium, which can lead to cardiac tamponade (excess fluid accumulation impairing heart function).

Layers of the Heart Wall

The heart wall consists of three main layers, each with distinct structure and function:

• Epicardium: Outer layer (visceral pericardium), provides protection.

• Myocardium: Middle, muscular layer responsible for contraction; composed of cardiac muscle tissue.

• Endocardium: Inner layer lining the heart chambers and valves.

Cardiac Muscle Structure and Function

Cardiac muscle tissue is specialized for continuous, rhythmic contraction.

• Cell characteristics: Small, typically uninucleate, striated, and highly branched.

• Intercalated discs: Specialized connections between cells containing desmosomes and gap junctions, allowing rapid electrical communication and mechanical stability.

• Metabolism: Highly dependent on aerobic metabolism; abundant mitochondria and myoglobin for oxygen storage.

Blood Flow Through the Heart

Blood follows a specific path through the heart, passing through chambers, valves, and major vessels:

1. Deoxygenated blood enters the right atrium via the superior and inferior vena cava.

2. Passes through the tricuspid (right AV) valve into the right ventricle.

3. Pumped through the pulmonary semilunar valve into the pulmonary trunk and arteries to the lungs.

4. Oxygenated blood returns via pulmonary veins to the left atrium.

5. Passes through the bicuspid (mitral, left AV) valve into the left ventricle.

6. Pumped through the aortic semilunar valve into the aorta for systemic circulation.

Heart Valves and Their Function

Valves ensure unidirectional blood flow and prevent backflow:

• Atrioventricular (AV) valves: Tricuspid (right) and bicuspid/mitral (left) valves between atria and ventricles.

• Semilunar valves: Pulmonary and aortic valves at the exits of the right and left ventricles.

• Valves open and close in response to pressure changes during the cardiac cycle.

Comparison of Heart Chambers

The atria have similar workloads and wall thickness, while the ventricles differ significantly:

• Right ventricle: Thinner wall, pumps blood to lungs (low pressure).

• Left ventricle: Thicker wall, pumps blood to entire body (high pressure).

Cardiac Skeleton

The cardiac skeleton is a framework of dense connective tissue that:

• Stabilizes valve positions

• Electrically isolates atria from ventricles

• Provides structural support for the heart

Coronary Circulation

The heart has its own blood supply via the coronary arteries:

• Left and right coronary arteries: Branch from the ascending aorta and supply oxygen/nutrients to the myocardium.

• Blood flow is greatest during ventricular relaxation (diastole).

• Coronary veins return deoxygenated blood to the right atrium.

Arteriosclerosis and Atherosclerosis

Arteriosclerosis is the thickening and hardening of arterial walls, often leading to cardiovascular disease.

• Atherosclerosis: Most common form, characterized by lipid (plaque) deposits in arterial walls, restricting blood flow.

• Major risk factors: age, male sex, high cholesterol, high blood pressure, smoking.

• Complications include coronary artery disease (CAD) and stroke.

Treatment Options

• Balloon angioplasty: Compresses plaque to open vessel.

• Stent placement: Wire-mesh tube holds vessel open.

• Bypass surgery: Replaces damaged vessel segments.

Valvular Heart Disease (VHD)

Valvular heart disease occurs when valve function deteriorates, impairing blood flow.

• Causes: Congenital malformations, inflammation (endocarditis).

• Severe cases may require prosthetic valve replacement (bioprosthetic valves from pigs or cows).

Summary Table: Heart Chambers and Valves

Key Equations

• Cardiac Output: The amount of blood pumped by the heart per minute.

Example Application

If a patient suffers damage to the semilunar valve on the right side of the heart, blood flow to the pulmonary trunk is affected. A prolapsed mitral valve can lead to regurgitation and increased effort by the left ventricle.

Additional info: Some details, such as the full path of blood flow and the specific structure of cardiac muscle, have been expanded for clarity and completeness.