Chapter 18: Heart Part 1

The Circulatory System: The Heart

Functions of the Cardiovascular System

The cardiovascular system is essential for maintaining homeostasis and supporting cellular function throughout the body.

• Distribution: Transports oxygen (O2), carbon dioxide (CO2), nutrients, and metabolic waste products.

• Acid-Base Balance: Maintains pH balance in body fluids.

• Protection: Defends against disease via immune cells and antibodies in the blood.

• Hemostasis: Prevents hemorrhage by clotting mechanisms.

• Thermoregulation: Regulates body temperature by redistributing blood flow.

Components of the Cardiovascular System

The system consists of the heart, blood vessels, and blood, each with specialized roles.

• The Heart: Muscular organ that pumps blood.

• Blood Vessels:

  • Arteries: Carry blood away from the heart (usually oxygenated).

  • Veins: Return blood to the heart (usually deoxygenated).

  • Capillaries: Microscopic vessels for exchange of substances between blood and tissues.

• The Blood: Fluid connective tissue transporting cells, nutrients, gases, and wastes.

Heart Anatomy and Structure

Heart Position, Size, and Shape

The heart is centrally located in the thoracic cavity, within the mediastinum, posterior to the sternum and between the lungs.

• Base: Broad, superior portion where major vessels attach.

• Apex: Inferior, pointed end, tilting left.

• Dimensions: ~3.5 in. wide at base, 5 in. from base to apex, 2.5 in. anterior-posterior; weighs ~10 oz.

Pericardium

The pericardium is a double-walled sac that encloses and protects the heart, allowing frictionless movement during contraction.

• Parietal Pericardium: Outer, tough, fibrous connective tissue layer.

• Pericardial Cavity: Space filled with 5–30 mL of pericardial fluid, reducing friction.

• Visceral Pericardium (Epicardium): Inner, thin, moist serous layer covering the heart surface.

Heart Wall Layers

• Epicardium: Outer layer (visceral pericardium); epithelial and connective tissue.

• Myocardium: Thick, middle muscular layer; composed of cardiac muscle with intercalated discs.

• Endocardium: Smooth, inner lining of the heart chambers.

Structure of Cardiac Muscle

• Short, branched cells with a single central nucleus.

• Intercalated discs connect myocytes end-to-end, containing gap junctions for ion flow and electrical coupling.

• Abundant myoglobin and glycogen for energy storage.

• Large mitochondria for aerobic metabolism.

• Highly fatigue-resistant.

Chambers of the Heart

The heart contains four chambers:

• Right Atrium

• Right Ventricle

• Left Atrium

• Left Ventricle

Chambers are separated by septa:

• Interatrial Septum: Divides right and left atria.

• Interventricular Septum: Divides right and left ventricles.

Heart Valves

Valves ensure unidirectional blood flow through the heart:

• Atrioventricular (AV) Valves:

  • Right AV (Tricuspid) Valve: Three cusps between right atrium and ventricle.

  • Left AV (Bicuspid/Mitral) Valve: Two cusps between left atrium and ventricle.

• Semilunar Valves:

  • Pulmonary Valve: Right ventricle to pulmonary trunk.

  • Aortic Valve: Left ventricle to aorta.

• Chordae Tendineae: Tendinous cords connecting AV valves to papillary muscles, preventing valve prolapse.

Blood Flow Through the Heart

Blood follows a specific pathway through the heart chambers and valves:

1. Blood enters right atrium from superior and inferior venae cavae.

2. Passes through right AV valve into right ventricle.

3. Right ventricle contracts, blood flows through pulmonary valve into pulmonary trunk and arteries to lungs.

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

5. Passes through left AV valve into left ventricle.

6. Left ventricle contracts, blood flows through aortic valve into aorta and systemic circulation.

Valve Operation

• AV Valves: Open when ventricles relax (pressure drops), allowing blood flow from atria to ventricles; close when ventricles contract.

• Semilunar Valves: Open when ventricles contract (pressure rises), allowing blood flow into great vessels; close when ventricles relax.

Coronary Circulation and Heart Disease

Coronary Vessels

• Coronary Arteries: Supply oxygenated blood to myocardium; right and left branches susceptible to myocardial infarction (heart attack).

• Coronary Veins: Drain deoxygenated blood from myocardium into coronary sinus, returning it to right atrium.

Blockage of Coronary Arteries

• Angina Pectoris: Partial obstruction causes chest pain due to ischemia; often activity-dependent.

• Myocardial Infarction (MI): Complete obstruction causes death of cardiac cells; pain may radiate down left arm.

Cardiac Catheterization & By-pass Surgery

• Angioplasty Stent: Device inserted to open blocked arteries.

• By-pass Surgery: Grafting vessels to bypass blocked coronary arteries.

Coronary Atherosclerosis

Fat deposition in arterial walls leads to obstruction and increased risk of heart disease.

• Risk Factors: High LDL, sedentary lifestyle, obesity, heredity, type "A" personality, hypertension, smoking, high-fat/low-fiber diet.

• Prevention: Lifestyle modification is key.

Electrical Properties and Cardiac Cycle

Myogenic Properties of the Heart

• SA Node: Pacemaker; small, modified muscle cells generate electrical signals for heart contraction.

• Conducting Tissue: Depolarizes automatically, initiating heartbeat within the heart itself.

Cardiac Conduction System

• SA Node: Initiates heartbeat, sets heart rate, signal travels through atria.

• AV Node: Electrical gateway to ventricles; slows signal, allowing atria to contract before ventricles.

• AV Bundle (Bundle of His): Pathway for signals from AV node; divides into right and left bundle branches.

• Purkinje Fibers: Spread signal upward through ventricular myocardium.

Inherent Rates of Cardiac Conduction

Structure	Intrinsic Rate (bpm)
SA Node	60–100
AV Node	40–50
AV Bundle	35–40
Purkinje Fibers	15–20

• Nervous Influence: Sympathetic nerves increase heart rate; parasympathetic (vagal) nerves decrease it. Normal adult resting rate: 70–80 bpm.

Cardiac Rhythm

• Systole: Ventricular contraction.

• Diastole: Ventricular relaxation.

• Ectopic Foci: Abnormal pacemaker sites; if SA node fails, AV node or ventricles may take over (slower rates).

Action Potential of Myocyte

Cardiac muscle cells undergo rapid voltage changes during contraction:

1. Na+ gates open

2. Rapid depolarization

3. Na+ gates close

4. Slow Ca2+ channels open

5. Ca2+ channels close, K+ channels open (repolarization)

Equation (Resting Membrane Potential):

$V_m = \frac{RT}{F} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$

Additional info: This is the Nernst equation for potassium, a major determinant of cardiac cell resting potential.

Refractory Period

• Time after a myocyte responds to a stimulus during which it cannot be re-excited.

• Skeletal muscle: 1–2 ms

• Cardiac muscle: 250 ms (prevents tetanus, allows heart to relax and refill)

Significance of the Heart's Long Refractory Period

• Prevents sustained contraction (tetanus) in the heart.

• Allows for proper filling and ejection of blood.

• Premature Ventricular Contraction (PVC): Extra systole caused by hypoxia, electrolyte imbalance, stimulants, or stress; can lead to compensatory pause.