Chapter 8: The Heart

Introduction to the Heart

The heart is a central organ of the cardiovascular system, responsible for pumping blood throughout the body. Its structure and function are essential topics in Anatomy & Physiology.

• Shape and Location: The heart is not truly heart-shaped; it is roughly the size of a fist and located in the thoracic cavity between the lungs.

• Function: It circulates blood, delivering oxygen and nutrients while removing waste products.

Gross Anatomy of the Heart

Layers of the Heart Wall

The heart wall consists of three main layers, each with distinct functions:

• Pericardium: A double-walled sac that surrounds the heart, providing protection and lubrication. It consists of the fibrous pericardium (outer layer) and serous pericardium (inner layer, which includes the parietal and visceral layers).

• Myocardium: The thick, muscular middle layer responsible for the heart's contractile force.

• Endocardium: A thin inner lining that covers the heart chambers and valves, and is continuous with the lining of blood vessels.

Chambers of the Heart

The heart is divided into four chambers that facilitate the separation of oxygenated and deoxygenated blood:

• Atria: The two upper chambers (right and left atrium) receive blood returning to the heart.

• Ventricles: The two lower chambers (right and left ventricle) pump blood out of the heart.

• Septum: The wall separating the right and left sides of the heart.

Heart Valves

Types and Functions of Heart Valves

Valves ensure unidirectional blood flow through the heart and prevent backflow:

• Atrioventricular (AV) Valves: Located between atria and ventricles.

  • Right AV Valve (Tricuspid): Has three cusps.

  • Left AV Valve (Bicuspid/Mitral): Has two cusps.

• Semilunar Valves: Located at the exits of the ventricles.

  • Pulmonary Semilunar Valve: Between right ventricle and pulmonary artery.

  • Aortic Semilunar Valve: Between left ventricle and aorta.

Chordae tendineae and papillary muscles anchor the AV valves and prevent them from inverting during ventricular contraction.

Blood Flow Through the Heart

Pathway of Circulation

Blood flows through the heart in a specific sequence, ensuring separation of oxygenated and deoxygenated blood:

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

2. Passes through the tricuspid valve into the right ventricle.

3. Pumped through the pulmonary semilunar valve into the pulmonary artery, then to the lungs.

4. Oxygenated blood returns to the left atrium via the pulmonary veins (the only veins carrying oxygen-rich blood).

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

6. Pumped through the aortic semilunar valve into the aorta and distributed to the body.

Additional info: The left ventricle has a thicker wall than the right because it must generate higher pressure to pump blood throughout the systemic circulation.

Cardiac Cycle

Phases of the Cardiac Cycle

The cardiac cycle describes the sequence of events in one heartbeat:

• Diastole: Relaxation phase; atria and ventricles fill with blood.

• Systole: Contraction phase; blood is pumped out of the chambers.

Average heart rate is 70-80 beats per minute, with about 4-6 liters of blood pumped per minute.

Electrical Conduction System

Intrinsic and Extrinsic Regulation

The heart's rhythm is controlled by specialized cells and influenced by the nervous system:

• Sinoatrial (SA) Node: Located in the right atrium; initiates electrical impulses and acts as the natural pacemaker.

• Atrioventricular (AV) Node: Relays impulses from atria to ventricles.

• Bundle of His and Purkinje Fibers: Distribute the impulse throughout the ventricles.

• Autonomic Nervous System: The medulla oblongata can override the intrinsic rate, increasing or decreasing heart rate via sympathetic and parasympathetic input.

Abnormal Heart Rhythms

Common Arrhythmias

• Bradycardia: Abnormally slow heart rate.

• Tachycardia: Abnormally fast heart rate (over 100 beats/minute).

• Atrial Fibrillation: Irregular, rapid beating of the atria.

• Ventricular Fibrillation: Uncontrolled contraction of the ventricles; can be fatal if not treated immediately.

Cardiovascular Diseases

Risk Factors

Several factors increase the risk of developing cardiovascular diseases:

• Smoking

• Male sex

• Genetic predisposition

• Old age

• High cholesterol

• Obesity

• Lack of exercise

Blood Pressure Regulation

Blood pressure is determined by cardiac output and peripheral resistance:

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

• Peripheral Resistance: Resistance to blood flow in arterioles.

Normal blood pressure: Less than 140/90 mmHg. Hypertension (high blood pressure) is a major risk factor for heart disease and stroke.

Equation:

Atherosclerosis and Heart Attack

Atherosclerosis is the buildup of cholesterol and other substances in the arterial walls, leading to reduced elasticity and narrowed arteries. This can result in:

• Myocardial Infarction (MI): Commonly known as a heart attack, occurs when blood flow to part of the heart is blocked, causing cell death.

• Stroke: Occurs when blood flow to the brain is interrupted; can be ischemic (due to blockage) or hemorrhagic (due to bleeding).

Prevention and Treatment

• Cardiovascular disease is treatable with lifestyle changes, medications, and sometimes surgical interventions.

• Daily medications may be prescribed to manage cholesterol, blood pressure, and other risk factors.

Summary Table: Heart Valves

Key Definitions

• Myocardium: Muscular layer of the heart wall responsible for contraction.

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

• Pericardium: Protective sac surrounding the heart.

• Cardiac Cycle: Sequence of events in one heartbeat, including systole and diastole.

• Arrhythmia: Abnormal heart rhythm.

• Myocardial Infarction: Heart attack due to blocked blood flow.

Additional info: The heart's structure and function are foundational for understanding human physiology and the mechanisms of cardiovascular diseases.