Source: Textbook Ch. 44, Pre-Session Assignment 11
Tags: heart anatomy, cardiac cycle, ECG, electrocardiogram, conduction system, SA node, AV node, bundle of His, Purkinje fibres, systole, diastole, stroke volume, cardiac output, vertebrate hearts, foetal circulation
Difficulty: Intermediate | Prerequisites: Part 1 study notes (blood composition and circulatory systems)
Big Picture
This section covers the heart itself: where it sits, what it looks like inside and out, how its electrical conduction system coordinates each beat, and how the nervous and endocrine systems adjust heart rate on the fly. You also need to compare heart structure across vertebrate classes and understand how foetal circulation differs from the adult pattern. Mastering the cardiac cycle and ECG interpretation ties all of these pieces together.
TL;DR
The human heart is a four-chambered pump in the mediastinum. Its own conduction system (SA node, AV node, bundle of His, Purkinje fibres) generates and spreads electrical impulses that produce the P wave, QRS complex, and T wave on an ECG. Cardiac output equals stroke volume times heart rate, and both the autonomic nervous system and hormones regulate it.
Mediastinum
The central compartment of the thoracic cavity where the heart is located, between the two lungs. Think of it as the middle shelf of the chest.
Pericardium
The double-walled sac surrounding the heart. The outer fibrous pericardium anchors the heart in place; the inner serous pericardium secretes lubricating fluid to reduce friction during beating.
Atrium (plural: atria)
An upper receiving chamber of the heart. The right atrium receives deoxygenated blood from the body; the left atrium receives oxygenated blood from the lungs.
Ventricle
A lower pumping chamber of the heart. The right ventricle pumps blood to the lungs; the left ventricle pumps blood to the entire body and has the thickest wall.
Septum (interventricular / interatrial)
The muscular wall dividing the left and right sides of the heart. It prevents mixing of oxygenated and deoxygenated blood.
Atrioventricular (AV) valves
Valves between the atria and ventricles. The tricuspid valve is on the right side; the bicuspid (mitral) valve is on the left. They prevent backflow into the atria during ventricular contraction.
Sinoatrial (SA) node
The heart's natural pacemaker, located in the right atrium. It generates electrical impulses that set the rate and rhythm of the heartbeat.
Atrioventricular (AV) node
Located at the junction of the atria and ventricles. It briefly delays the electrical signal so the atria finish contracting before the ventricles begin.
Bundle of His
A bundle of specialised cardiac muscle fibres that carry the electrical impulse from the AV node down the interventricular septum. It splits into left and right bundle branches.
Purkinje fibres
Fast-conducting fibres that spread the electrical impulse throughout the ventricular walls, triggering coordinated contraction from the apex upward.
Electrocardiogram (ECG/EKG)
A recording of the electrical activity of the heart over time, captured through electrodes on the skin. It shows P waves, QRS complexes, and T waves.
Myocardium
The thick muscular middle layer of the heart wall, composed of cardiac muscle tissue. This is the layer that contracts.
Location
The heart sits in the mediastinum, slightly left of the midline, between the lungs. It rests on the diaphragm, posterior to the sternum and anterior to the vertebral column.
External surfaces
Anterior (sternocostal) surface: mostly the right ventricle, facing the front of the chest
Posterior (base) surface: mostly the left atrium, where the pulmonary veins enter
The apex (inferior tip) points down and to the left, formed by the left ventricle
Coronary sulcus (atrioventricular groove): marks the boundary between the atria and ventricles externally
Anterior and posterior interventricular sulci: mark the boundary between the left and right ventricles
External structures
Coronary arteries: branch from the aorta and supply oxygenated blood to the myocardium itself
Coronary sinus: a large vein on the posterior surface that collects deoxygenated blood from the heart muscle and empties into the right atrium
Auricles: ear-shaped flaps on each atrium that increase atrial volume
Layers of the heart wall
Epicardium (visceral pericardium): outermost layer, thin serous membrane
Myocardium: thick middle layer of cardiac muscle, responsible for contraction
Endocardium: innermost layer, smooth endothelium lining the chambers and covering the valves
Internal structures and their functions
Right atrium: receives deoxygenated blood from the superior vena cava, inferior vena cava, and coronary sinus
Tricuspid valve: AV valve between right atrium and right ventricle
Right ventricle: pumps deoxygenated blood through the pulmonary semilunar valve into the pulmonary trunk and then to the lungs
Left atrium: receives oxygenated blood from four pulmonary veins
Bicuspid (mitral) valve: AV valve between left atrium and left ventricle
Left ventricle: pumps oxygenated blood through the aortic semilunar valve into the aorta and then to the body; has the thickest myocardium because it generates the highest pressure
Chordae tendineae and papillary muscles: anchor the AV valve flaps and prevent them from inverting during ventricular contraction
Interventricular septum: separates the two ventricles
Fish (two-chambered heart)
One atrium, one ventricle
Single circulation: heart pumps deoxygenated blood to the gills, where it picks up oxygen, then flows directly to the body before returning to the heart
Blood passes through capillaries twice per circuit (gills, then body), so pressure drops significantly by the time it reaches body tissues
Amphibians (three-chambered heart)
Two atria, one ventricle
Double circulation: separate pulmonary (to lungs/skin) and systemic (to body) circuits, but some mixing of oxygenated and deoxygenated blood occurs in the single ventricle
The ventricle has internal ridges (trabeculae) that help reduce mixing somewhat
Most reptiles (three-chambered heart with partial septum)
Two atria, one ventricle with an incomplete septum
Less mixing than amphibians because the partial septum partially separates oxygenated and deoxygenated blood
Exception: crocodilians have a fully four-chambered heart, though they can shunt blood between circuits
Birds and mammals (four-chambered heart)
Two atria, two ventricles, complete separation
Double circulation with no mixing: the right side handles pulmonary circulation, the left side handles systemic circulation
Supports high metabolic rates and endothermy (warm-bloodedness)
The left ventricle wall is thicker because it must pump blood at higher pressure to the entire body
Foetal circulation includes three shunts that bypass the non-functional lungs, since the foetus receives oxygen from the placenta rather than its own lungs.
Ductus venosus: shunts a portion of oxygenated blood from the umbilical vein directly to the inferior vena cava, bypassing the liver
Foramen ovale: an opening in the interatrial septum that allows blood to flow from the right atrium directly into the left atrium, bypassing the pulmonary circuit
Ductus arteriosus: connects the pulmonary trunk to the aorta, diverting blood away from the lungs and into systemic circulation
At birth, when the newborn takes its first breath:
Lungs expand and pulmonary vascular resistance drops sharply
Increased blood flow to the lungs raises left atrial pressure
The foramen ovale closes (becomes the fossa ovalis) because left atrial pressure now exceeds right atrial pressure
The ductus arteriosus constricts and closes (becomes the ligamentum arteriosum), triggered by rising oxygen levels and falling prostaglandin levels
The ductus venosus closes (becomes the ligamentum venosum) as umbilical blood flow stops
The umbilical vein degenerates into the round ligament (ligamentum teres) of the liver
Cardiac muscle (myocardium) features
Striated like skeletal muscle, but cells are shorter, branched, and typically have one or two nuclei
Cells are joined by intercalated discs, which contain gap junctions (allowing rapid ion flow between cells) and desmosomes (providing mechanical strength)
Intercalated discs allow the myocardium to function as a functional syncytium: when one cell is stimulated, the impulse spreads rapidly to neighbouring cells so the entire chamber contracts as a coordinated unit
Cardiac muscle is autorhythmic, meaning it can generate its own electrical impulses without nervous system input
The conduction system (in order of impulse flow)
SA node (sinoatrial node): located in the upper wall of the right atrium. The pacemaker of the heart, firing at roughly 60 to 100 impulses per minute. Sets the sinus rhythm.
The impulse spreads across both atria, causing atrial depolarisation and contraction (P wave on ECG).
AV node (atrioventricular node): located at the floor of the right atrium near the interatrial septum. Delays the impulse for roughly 0.1 seconds so the atria finish emptying before the ventricles contract.
Bundle of His (AV bundle): carries the impulse from the AV node into the interventricular septum.
Left and right bundle branches: conduct the impulse down either side of the septum.
Purkinje fibres: spread the impulse rapidly throughout the ventricular myocardium, triggering contraction from the apex upward so blood is squeezed toward the outflow tracts (aorta and pulmonary trunk).
Autorhythmic cells vs contractile cells
Autorhythmic cells (pacemaker cells): have an unstable resting membrane potential that gradually depolarises (pacemaker potential) until it reaches threshold and fires. They do not contribute to contraction.
Contractile cells: make up the bulk of the myocardium. They have a stable resting potential but respond to impulses from the conduction system. Their action potential has a characteristic plateau phase due to slow calcium influx, which prolongs contraction and prevents tetanus.
An electrocardiogram records the sum of electrical activity in the heart as detected by surface electrodes. A normal ECG tracing for one heartbeat shows:
P wave
Represents atrial depolarisation (the electrical impulse spreading across both atria)
Followed by atrial contraction
A small, rounded, upward deflection
PR interval
The time from the start of the P wave to the start of the QRS complex
Represents the delay at the AV node (roughly 0.12 to 0.20 seconds)
A prolonged PR interval suggests AV block
QRS complex
Represents ventricular depolarisation (the impulse spreading through the ventricles via the bundle of His, bundle branches, and Purkinje fibres)
The atria repolarise during this time, but the signal is hidden by the larger ventricular depolarisation
A sharp, tall, complex waveform (duration roughly 0.06 to 0.10 seconds)
ST segment
The interval between the end of the QRS complex and the start of the T wave
Represents the period when the ventricles are fully depolarised and contracting (plateau phase)
ST elevation or depression can indicate myocardial ischaemia or infarction
T wave
Represents ventricular repolarisation (the ventricles recovering to their resting state)
A broader, rounded, upward deflection
After the T wave, the ventricles are relaxed and ready for the next cycle
Clinical note
The sequence on an ECG always runs: P wave, QRS complex, T wave, then a flat baseline before the next P wave. Changes in the shape, timing, or amplitude of these waves help diagnose arrhythmias, conduction blocks, and ischaemic events.
The heart's intrinsic rate (set by the SA node) is roughly 100 bpm, but resting heart rate is typically 60 to 80 bpm because the autonomic nervous system tonically slows it.
Autonomic nervous system
Parasympathetic (vagus nerve): releases acetylcholine at the SA and AV nodes, slowing heart rate (negative chronotropic effect) and reducing conduction speed. This is the dominant influence at rest.
Sympathetic: releases noradrenaline (norepinephrine) at the SA node, AV node, and myocardium, increasing heart rate (positive chronotropic effect) and increasing the force of contraction (positive inotropic effect). Activated during exercise, stress, or the fight-or-flight response.
Hormonal regulators
Adrenaline (epinephrine): released by the adrenal medulla, mimics sympathetic stimulation, increasing heart rate and contraction force
Thyroid hormones (T3/T4): elevated levels increase heart rate and cardiac output over a longer time frame
Atrial natriuretic peptide (ANP): released by atrial cells when atrial walls are stretched (high blood volume), promotes natriuresis (sodium excretion) and reduces blood volume, indirectly lowering cardiac output
Other factors affecting heart rate
Body temperature: fever increases heart rate (roughly 10 bpm per 1 degree Celsius rise)
Blood ion concentrations: elevated potassium (hyperkalaemia) slows heart rate and can cause cardiac arrest; elevated calcium increases contraction strength
Baroreceptor reflexes: pressure sensors in the carotid sinus and aortic arch detect changes in blood pressure and trigger autonomic adjustments to heart rate
Stroke volume (SV)
The volume of blood ejected by one ventricle per contraction. Typical resting value: roughly 70 mL.
Three factors regulate stroke volume:
Preload (Frank-Starling mechanism): the degree of ventricular stretch at the end of diastole. Greater venous return stretches the ventricle more, producing a more forceful contraction and a larger stroke volume.
Contractility: the inherent strength of contraction independent of preload. Increased by sympathetic stimulation and adrenaline (positive inotropic agents). Decreased by acidosis, hyperkalaemia, or certain drugs (negative inotropic agents).
Afterload: the pressure the ventricle must overcome to eject blood (essentially aortic pressure for the left ventricle). Higher afterload (e.g. hypertension) makes ejection harder and can reduce stroke volume.
The cardiac cycle
One complete heartbeat, lasting roughly 0.8 seconds at a resting rate of 75 bpm.
Atrial systole (~0.1 s): atria contract, topping off ventricular filling (the "atrial kick" contributes roughly 20% of ventricular volume)
Ventricular systole (~0.3 s): ventricles contract; AV valves close (first heart sound, S1, "lub"); isovolumetric contraction occurs until ventricular pressure exceeds arterial pressure, then semilunar valves open and blood is ejected
Ventricular diastole (~0.4 s): ventricles relax; semilunar valves close (second heart sound, S2, "dub"); isovolumetric relaxation occurs until ventricular pressure drops below atrial pressure, then AV valves open and passive ventricular filling begins
Cardiac output (CO)
The total volume of blood pumped by one ventricle per minute.
CO = SV x HR
Typical resting value: 70 mL x 75 bpm = 5,250 mL/min (roughly 5 L/min). During heavy exercise, cardiac output can increase to 20 to 25 L/min through increases in both stroke volume and heart rate.
Real-world application
Understanding cardiac output is central to clinical medicine. Heart failure is defined by the heart's inability to maintain sufficient cardiac output for the body's metabolic needs. Treatments aim to optimise preload, reduce afterload, or increase contractility.
Students often think the SA node requires a nerve signal to fire. It does not. Cardiac muscle is autorhythmic; the nervous system modulates the rate but does not initiate it.
The QRS complex represents ventricular depolarisation, not ventricular contraction. Contraction follows depolarisation, so the mechanical event lags slightly behind the electrical event.
"Lub-dub" heart sounds are caused by valve closures, not by the heart muscle contracting. S1 ("lub") = AV valves closing; S2 ("dub") = semilunar valves closing.
Students confuse the foramen ovale (foetal shunt between atria) with the fossa ovalis (the remnant of that shunt in the adult heart). The foramen ovale is open; the fossa ovalis is the closed scar.
⚠️ Be able to trace the path of blood through all four chambers, naming every valve and vessel in order.
⚠️ Know the conduction pathway in sequence: SA node, atria, AV node, bundle of His, bundle branches, Purkinje fibres.
⚠️ Match each ECG wave to its cardiac event: P = atrial depolarisation, QRS = ventricular depolarisation, T = ventricular repolarisation.
⚠️ Cardiac output = stroke volume x heart rate. Know the formula and the three factors that affect stroke volume (preload, contractility, afterload).
⚠️ Name the three foetal shunts, where each is located, what each becomes after birth, and what triggers closure.
True or false: The AV node is the heart's primary pacemaker.
False. The SA node is the primary pacemaker. The AV node serves as a relay and delay station.
Fill in the blank: The QRS complex on an ECG represents ________.
Ventricular depolarisation.
True or false: Fish have double circulation.
False. Fish have single circulation: blood passes through the heart once per circuit (heart to gills to body to heart).
Fill in the blank: Cardiac output = ________ x ________.
Stroke volume x heart rate.
True or false: The foramen ovale allows blood to bypass the lungs in the foetus.
True. It shunts blood from the right atrium to the left atrium.
Q: Trace the path of a drop of blood from the superior vena cava back to the aorta, naming each chamber, valve, and major vessel.
A: Superior vena cava, right atrium, tricuspid valve, right ventricle, pulmonary semilunar valve, pulmonary trunk, pulmonary arteries, lungs (gas exchange), pulmonary veins, left atrium, bicuspid (mitral) valve, left ventricle, aortic semilunar valve, aorta.
Q: What is the function of the AV node delay, and what would happen without it?
A: The AV node delays the impulse for roughly 0.1 seconds so the atria can finish contracting and completely empty into the ventricles before ventricular contraction begins. Without this delay, atrial and ventricular contraction would overlap, reducing filling efficiency and lowering stroke volume.
Q: How does the Frank-Starling mechanism regulate stroke volume?
A: Greater venous return stretches the ventricular walls at the end of diastole (increased preload). This stretch optimises the overlap of actin and myosin filaments in cardiac muscle, producing a more forceful contraction and a larger stroke volume. The heart automatically matches its output to the volume of blood returning to it.
Q: Compare the hearts of a fish, a frog, and a human in terms of chambers and degree of blood mixing.
A: A fish has two chambers (one atrium, one ventricle) with single circulation and no mixing issue. A frog has three chambers (two atria, one ventricle) with double circulation but some mixing of oxygenated and deoxygenated blood in the single ventricle. A human has four chambers (two atria, two ventricles) with double circulation and complete separation, so no mixing occurs.
Q: Name the three foetal cardiovascular shunts and state what each bypasses.
A: Foramen ovale bypasses the pulmonary circuit by shunting blood from the right atrium to the left atrium. Ductus arteriosus bypasses the lungs by connecting the pulmonary trunk to the aorta. Ductus venosus bypasses the liver by shunting umbilical vein blood directly to the inferior vena cava.
The conduction system links directly to the nervous system chapter, since the autonomic nervous system modulates heart rate through the vagus nerve and sympathetic fibres. The Frank-Starling mechanism ties into muscle physiology (sarcomere length-tension relationships). Foetal circulation connects to reproductive biology and embryology.
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