11. CARDIOVASCULAR SYSTEM – ANATOMY AND PHYSIOLOGY OF HEART

CARDIOVASCULAR SYSTEM

The cardiovascular system comprises the heart, blood vessels, and blood. Blood acts as a carrier for oxygen and nutrients to maintain homeostasis.

Main Functions of the Cardiovascular System

1. Distribution of O₂ and nutrients to all body cells and tissues
2. Transportation of CO₂ and metabolic waste products from tissues to lungs and excretory organs
3. Distribution of water, electrolytes, and hormones throughout the body
4. Thermoregulation

ANATOMY OF HEART

The heart is a muscular organ present in all vertebrates that continuously pumps blood to the entire body. In humans, its size is equal to a clenched fist. Average weight: females 250-300g, males 300-350g. Average heart rate: 70-72 beats per minute. The heart is situated in the thoracic cavity, obliquely between the lungs in the mediastinum space, just above the diaphragm.

Layers of the Heart Wall

1. Epicardium (Visceral Pericardium): Outermost layer; transparent thin layer of mesothelium.
2. Pericardial Cavity: Space between epicardium and serous pericardium; contains pericardial fluid (prevents friction).
3. Myocardium: Thickest layer; cardiac muscle tissue (striated, involuntary, branched).
4. Endocardium: Innermost layer; endothelium lining inner cavities, covering valves, and lining blood vessels.

Chambers of the Heart

• Right Atrium: Thinnest chamber; receives blood from superior vena cava, inferior vena cava, and coronary sinus; releases into right ventricle via tricuspid valve.
• Right Ventricle: Contains inlet and outlet; pumps blood into pulmonary artery.
• Left Atrium: Forms base (posterior surface) of heart; receives blood from pulmonary veins; discharges into left ventricle via mitral valve.
• Left Ventricle: Muscular and thicker (3 times than RV); pumps blood into aorta.

Valves of the Heart

• Atrioventricular (AV) Valves:
  • Tricuspid valve (right side) – 3 cusps
  • Mitral valve (left side) – 2 cusps
  • Connected to chordae tendineae (prevent backward folding)
• Semilunar Valves:
  • Pulmonary valve (between right ventricle and pulmonary trunk)
  • Aortic valve (between left ventricle and aorta)
  • Cup-shaped cusps prevent backflow

Conduction System of the Heart

1. Sinoatrial (SA) Node: Pacemaker of the heart; located in right atrium; generates impulses (0.05 m/s).
2. Internodal Pathways: Anterior (Bachman), middle (Wenckebach), posterior (Thorel) – connect SA node to AV node.
3. Atrioventricular (AV) Node: Located in lower right atrium; pace-setter; controls impulse rate.
4. Bundle of His: Fibre bundle from AV node to Purkinje system; divides into left and right bundle branches.
5. Bundle Branches: Right bundle branch (supplies right ventricle); Left bundle branch (supplies left ventricle).
6. Purkinje Fibres: Network of conducting fibres in sub-endocardial regions of both ventricles.

BLOOD VESSELS

Types of Blood Vessels

1. Arteries: Elastic vessels carrying blood from heart to tissues (e.g., aorta, carotid artery).
2. Arterioles: Smallest diameter arteries; lined by muscles innervated by sympathetic nervous system.
3. Capillaries: Smallest diameter; connect arterioles to venules.
4. Venules: Smallest diameter veins; connect capillaries to larger veins.
5. Veins: Carry blood from organs back to heart (e.g., vena cava, subclavian vein).

Anatomy of Blood Vessels (Three Layers)

1. Tunica Intima: Thinnest layer; endothelial cells; internal elastic lamina.
2. Tunica Media: Thickest layer; elastic fibres, connective tissue, smooth muscles; external elastic lamina.
3. Tunica Adventitia: Collagen fibres; anchors blood vessel to nearby organs; contains vasa vasorum.

PHYSIOLOGY OF HEART

Properties of Cardiac Cells

• Conductivity: Transmission of electrical impulses between cells
• Excitability: Ability to respond to electrical impulses
• Automaticity: Ability to generate and discharge electrical impulses spontaneously

Cardiac Action Potential (Phases)

• Phase 0 (Upstroke): Sudden increase in Na⁺ conductance
• Phase 1: Brief outward increase in K⁺ conduction (primary repolarisation)
• Phase 2 (Plateau): Temporary increase in Ca²⁺ conduction and outward K⁺ conduction
• Phase 3: Repolarisation (decreasing Ca²⁺, increasing K⁺ conductance)
• Phase 4: Membrane pressed toward K⁺ equilibrium; inward Na⁺ current

BLOOD CIRCULATION THROUGH THE HEART

Systemic Circulation

Left side of the heart receives oxygenated blood from lungs and pumps it into the aorta. Aorta divides into systemic arteries → arterioles → capillaries (O₂ delivered, CO₂ picked up) → venules → systemic veins → superior/inferior vena cava and coronary sinus → right atrium.

Pulmonary Circulation

Deoxygenated blood returns to right atrium → right ventricle → pulmonary artery (only artery carrying deoxygenated blood) → lungs (alveoli: O₂ in, CO₂ out) → pulmonary veins (oxygenated blood) → left atrium → left ventricle → aorta.

Portal Circulation

The hepatic portal vein carries deoxygenated blood from digestive organs to the liver before returning to the heart. Blood enters the liver from two sources: hepatic artery (oxygenated) and hepatic portal vein (deoxygenated from digestive organs).

Coronary Circulation

The heart has its own circulation system. Left and right coronary arteries originate at the base of the aorta, supplying oxygenated blood to cardiac cells. Cardiac veins carry deoxygenated blood to the coronary sinus, which empties into the right atrium.

CARDIAC CYCLE AND HEART SOUNDS

Stages of Cardiac Cycle

1. Atrial Systole (0.18 sec): SA node stimulation; atria contract; blood pumped into ventricles; AV valves open.
2. Ventricular Systole (0.30 sec): AV node stimulation; ventricles contract; AV valves close (lub sound); blood flows into aorta and pulmonary artery.
3. Ventricular Diastole (0.32 sec): Ventricles relax; semilunar valves close (dub sound); AV valves open; blood flows into ventricles.
4. Joint Diastole (0.08 sec): Both atria and ventricles relaxed; blood flows from vena cava into atria and ventricles.

Heartbeat Regulation

• Nervous Regulation: Sympathetic (adrenaline → increases heartbeat); Parasympathetic (acetylcholine → decreases heartbeat).
• Hormonal Regulation: Thyroxine (increases BMR and heartbeat); Epinephrine/Norepinephrine (increase heartbeat).

Cardiac Output

Cardiac Output = Stroke Volume × Heart Rate = 70 ml × 72/min = 5040 ml/min ≈ 5 litres/min.

Heart Sounds

“Lub”: First sound – closing of tricuspid and mitral valves.
“Dub”: Second sound – closing of aortic and pulmonary valves.

BASICS OF ECG (ELECTROCARDIOGRAM)

ECG Waves

• P Wave: Atrial depolarisation (small upward deflection)
• QRS Complex: Rapid ventricular depolarisation (large, upright, triangular)
• T Wave: Ventricular repolarisation (dome-shaped upward deflection)

ECG Intervals

• P-Q Interval: Time from atrial contraction to ventricular contraction
• S-T Segment: Time when ventricular contracting fibres are depolarised (plateau phase)
• Q-T Interval: Time from ventricular depolarisation to end of repolarisation

BLOOD PRESSURE AND ITS REGULATION

Types of Arterial Blood Pressure

• Systolic Pressure: Maximum pressure (120 mmHg in healthy adult)
• Diastolic Pressure: Minimum pressure (80 mmHg in healthy adult)
• Pulse Pressure: Systolic – Diastolic (≈ 40 mmHg)
• Mean Arterial Pressure: Average pressure on arteries

Measurement of Blood Pressure

• Direct Method: Cannula inserted into artery connected to manometer
• Indirect Methods:
  • Palpatory method (feeling pulse)
  • Auscultatory method (Korotkoff sounds using stethoscope) – 5 phases
  • Oscillometric method (NIBP – automated electronic monitors)

Disorders Related to Blood Pressure

• Hypertension (High Blood Pressure):
  • Primary (Essential): 90-95% cases; complex multifactorial disorder
  • Secondary: Caused by other diseases (kidney disease, diabetes, tumours)
  • Symptoms: chest pain, confusion, tinnitus, irregular heartbeat, nosebleed
• Hypotension (Low Blood Pressure):
  • Symptoms: dizziness, weakness, fatigue, unsteadiness, light-headedness
  • Causes: fear, dehydration, heat reaction, blood donation, internal bleeding