Chapter Review

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Human Heart and Cardiac Cycle · Blood Vessels and Lymphatic System

Heart Structure and Chambers

The heart is enclosed in a double membranous pericardium containing lubricating fluid, and its wall has three layers: epicardium, myocardium (thick cardiac muscle), and endocardium. It has four chambers — two thin-walled atria (receiving) and two thick-walled ventricles (pumping) — with complete separation of oxygenated (left) and deoxygenated (right) blood.

Key Points

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    Pericardium: Double sac with pericardial fluid that reduces friction and prevents over-extension
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    Wall layers: Epicardium (outer) → Myocardium (thick muscular middle) → Endocardium (smooth inner lining)
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    Cardiac muscle: Branched cells connected by intercalated discs with gap junctions for rapid electrical spread
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    Left ventricle: Wall ~3× thicker than right because it must generate pressure to pump blood to the entire body
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    Right side: Deoxygenated blood only; Left side: Oxygenated blood only — no mixing in a normal heart

Heart Valves and Blood Flow Pathways

Four sets of valves ensure unidirectional blood flow: tricuspid (right AV, 3 flaps), bicuspid/mitral (left AV, 2 flaps), and semilunar valves at the pulmonary trunk and aorta. AV valve flaps are anchored by chordae tendinae to papillary muscles, preventing inversion during ventricular contraction.

Key Points

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    Right pathway: Venae cavae → Right atrium → Tricuspid valve → Right ventricle → Pulmonary trunk → Pulmonary arteries → Lungs
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    Left pathway: Pulmonary veins → Left atrium → Bicuspid valve → Left ventricle → Aorta → Body
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    Pulmonary arteries: The only arteries carrying deoxygenated blood
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    Pulmonary veins: The only veins carrying oxygenated blood
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    Chordae tendinae: Fibrous cords anchored to papillary muscles that prevent AV valve prolapse during systole
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    Valves are passive: They open and close in response to pressure differences, never actively push blood

Pulmonary and Systemic Circulation

The heart functions as a double pump driving two simultaneous pathways: pulmonary circulation (right ventricle → lungs → left atrium) for gas exchange, and systemic circulation (left ventricle → body → right atrium) for tissue supply. Coronary arteries are the first branches of the aorta, supplying the heart muscle itself.

Key Points

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    Pulmonary circulation: Right ventricle → pulmonary arteries → lungs (gas exchange) → pulmonary veins → left atrium
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    Systemic circulation: Left ventricle → aorta → body tissues → venae cavae → right atrium
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    Both operate simultaneously with each heartbeat
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    Coronary arteries: First pair of arteries branching from the aortic root — blockage causes myocardial infarction
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    Hepatic portal vein: Carries nutrient-rich blood from the digestive tract to the liver (not to be confused with the hepatic vein that drains the liver)

The Cardiac Cycle

One complete cardiac cycle (~0.8 s at 75 bpm) consists of three phases: diastole (all chambers relax and fill passively), atrial systole (atria contract, pushing remaining 30% into ventricles), and ventricular systole (ventricles eject blood). Heart sounds — lubb (S1, AV valves close) and dubb (S2, semilunar valves close) — result from valve closure, not opening.

Key Points

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    Diastole: All chambers relaxed; blood fills atria passively from veins (~70% of ventricular filling)
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    Atrial systole: Both atria contract simultaneously, topping up ventricles with remaining ~30%
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    Ventricular systole: Both ventricles contract; AV valves snap shut (lubb/S1), semilunar valves open, blood ejected; semilunar valves close (dubb/S2) as ventricles relax
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    Lubb (S1): AV valves closing — marks start of ventricular systole
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    Dubb (S2): Semilunar valves closing — marks start of diastole
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    Heart sounds are caused by valve CLOSING, not opening
Formula

Heart Conduction System and ECG

The SA node (natural pacemaker) at the upper right atrium initiates each heartbeat. Impulses travel to the AV node, which introduces a 0.15 s delay before relaying via the AV bundle (bundle of His) and Purkinje fibres to the ventricular myocardium. The ECG records P wave (atrial depolarisation), QRS complex (ventricular depolarisation), and T wave (ventricular repolarisation).

Key Points

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    SA node: Heart's natural pacemaker at the upper end of the right atrium; fires automatically under autonomic influence
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    AV node delay: 0.15 seconds — ensures atria complete contraction before ventricles begin
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    AV bundle → Purkinje fibres: Conduct impulses through the interventricular septum to the ventricular myocardium
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    P wave: Atrial depolarisation (impulse spreading through atria)
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    QRS complex: Ventricular depolarisation; atrial repolarisation is hidden within QRS
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    T wave: Ventricular repolarisation (electrical recovery of ventricles)
Formula

Arteries, Capillaries, and Veins — Structure and Comparison

Arteries have thick muscular elastic walls to withstand high pulsatile pressure; capillaries have a single-cell endothelial wall for material exchange; veins have thin walls, larger bore, and semilunar valves, relying on muscle contraction and breathing for venous return. Capillaries have the largest total cross-sectional area but slowest flow velocity.

Key Points

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    Arteries: Thick 3-layer wall (connective tissue, smooth muscle, endothelium); small bore; high pressure; pulse detectable; no valves (except at aorta/pulmonary trunk base)
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    Capillaries: One-cell-thick endothelial wall; site of all material exchange; slowest flow (<1 mm/s); no valves
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    Veins: Thin 3-layer wall (reduced muscle, few elastic fibres); large bore; low pressure; no pulse; semilunar valves present
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    Vasoconstriction/vasodilation: Controlled by nervous and endocrine systems; narrows/widens arterioles respectively
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    Capillary exchange: Diffusion/active transport through cells, through intercellular spaces, and by endocytosis/exocytosis
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    Precapillary sphincters: Rings of smooth muscle regulating blood flow into individual capillary beds
Formula

Blood Pressure and Vascular Disorders

Blood pressure is highest in the aorta during systole (normal: 120 mm Hg) and lowest during diastole (normal: 75–85 mm Hg). Hypertension damages vessel lining and weakens the heart. Atherosclerosis (atheroma + arteriosclerosis) narrows arteries and promotes thrombus formation, which can lead to myocardial infarction, stroke, or haemorrhage.

Key Points

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    Systolic pressure: Peak during ventricular contraction — normal ~120 mm Hg
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    Diastolic pressure: Minimum during ventricular relaxation — normal 75–85 mm Hg
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    Hypertension: Chronic high BP damages endothelium, causes left ventricular hypertrophy, and risks brain haemorrhage
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    Atherosclerosis: Atheroma (cholesterol plaque in inner layer) + arteriosclerosis (middle layer thickening) coexist
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    Thrombus: Stationary clot in a vessel; Embolus: dislodged travelling clot
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    Myocardial infarction: Heart muscle necrosis from blocked coronary artery; Stroke: brain tissue death from blocked cerebral artery; Haemorrhage: blood vessel rupture
Formula

Lymphatic System

The lymphatic system returns ~3 litres of excess interstitial fluid per day to the bloodstream via blind-ended lymph capillaries, lymph vessels, and the thoracic lymph duct (empties into the subclavian vein). It also absorbs fats via lacteals in intestinal villi, filters lymph and blood through lymph nodes and the spleen, and provides immune defence.

Key Points

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    Fluid balance: Returns ~3 L/day of excess interstitial fluid that blood capillaries cannot reabsorb
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    Lymph capillaries: Blind-ended; larger intercellular spaces than blood capillaries allow proteins and fats to enter
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    Lacteals: Lymph capillaries in intestinal villi that absorb fat globules (~1% of lymph after a fatty meal)
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    Lymph nodes: Found in neck, axilla, groin; multiple afferent vessels enter, single efferent vessel drains; contain lymphocytes and macrophages
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    Four functions: (1) Fluid return, (2) Fat absorption via lacteals, (3) Immune defence via lymph nodes, (4) Blood filtration via spleen
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    Flow direction: Always unidirectional toward the thoracic duct → subclavian vein; maintained by muscle contraction, visceral movement, breathing, and valves

Formulas

Heart Rate from Cardiac Cycle

Heart rate in beats per minute from the duration of one cardiac cycle.

Cardiac Cycle Duration

One complete heartbeat at resting rate lasts 0.8 seconds, giving ~75 bpm.

AV Node Delay

The 0.15 second delay at the AV node ensures atrial systole completes before ventricular systole begins.

Pulse Pressure

Difference between systolic and diastolic pressure. Normal range: 30–50 mm Hg.

Normal Blood Pressure

Normal systolic ~120 mm Hg, normal diastolic 75–85 mm Hg.

Blood Flow Velocity vs Area

Velocity is inversely proportional to total cross-sectional area. Aorta: 400–500 mm/s; Capillaries: <1 mm/s.

Daily Lymph Return

Approximately 3 litres of excess interstitial fluid returned to blood per day.