Difficulty: Intermediate | Prerequisites: Parts 1 and 2 of these notes (blood flow dynamics, blood composition, blood vessels).
This section zooms in on the pump itself: the mammalian heart. You need to understand how pressure gradients drive flow (Part 1) before the cardiac cycle will make sense, because the cycle is fundamentally about generating and releasing pressure in the right sequence. The heart has its own electrical system that coordinates contraction without any input from the brain, and an ECG records this electrical activity from outside the body. Heart rate is then modulated by the autonomic nervous system layered on top of this intrinsic rhythm.
The mammalian heart has four chambers serving two circuits (pulmonary and systemic). It contracts automatically (myogenic) using an internal electrical conduction system: the SA node sets the pace, the signal travels through the AV node and bundle of His, and the ventricles contract in a coordinated sequence. The cardiac cycle alternates between systole (contraction) and diastole (relaxation), and all of this is visible on an ECG as the P wave, QRS complex and T wave.
Pulmonary circuit
The loop of blood flow from the right ventricle to the lungs (for gas exchange) and back to the left atrium. Think of it as the "lung loop."
Systemic circuit
The loop from the left ventricle to the rest of the body and back to the right atrium. Think of it as the "body loop."
Right atrium
Receives deoxygenated blood returning from the body via the superior and inferior vena cava.
Right ventricle
Pumps deoxygenated blood to the lungs via the pulmonary artery.
Left atrium
Receives oxygenated blood returning from the lungs via the pulmonary veins.
Left ventricle
Pumps oxygenated blood out to the body via the aorta. Has the thickest myocardium because it must generate the highest pressure.
Myocardium
The heart muscle tissue. Comes in two forms in the mammalian heart.
Spongy myocardium
The inner, trabeculated (rough-textured) layer of heart muscle. Receives its nutrients and oxygen by diffusion directly from the blood inside the chamber. Think of it as "self-feeding" muscle.
Compact myocardium
The outer, dense layer of heart muscle. Too thick for diffusion alone, so it requires its own dedicated blood supply via the coronary arteries and veins.
Coronary arteries and veins
The dedicated blood vessels supplying and draining the compact myocardium. Blockage of a coronary artery causes a heart attack (myocardial infarction).
Pericardium
The protective sac surrounding the heart. Contains fluid that protects against injury and infection, and lubricates against friction during beating.
Systole
The contraction phase of a heart chamber. Increases pressure inside the chamber.
Diastole
The relaxation phase. Pressure decreases, and the chamber fills with blood.
Isovolumetric contraction
The brief phase at the start of ventricular systole when all valves are closed. The ventricle is contracting and pressure is rising, but no blood is moving yet because both the AV and semilunar valves are shut. Once ventricular pressure exceeds arterial pressure, the semilunar valves open.
Isovolumetric relaxation
The brief phase at the start of ventricular diastole when all valves are closed. The ventricle is relaxing and pressure is falling, but no blood is entering yet. Once ventricular pressure drops below atrial pressure, the AV valves open.
AV valves (atrioventricular valves)
Valves between the atria and ventricles. Open during diastole to allow filling; close during systole to prevent backflow into the atria. No muscle or ATP is used to open them; they open passively by pressure gradients. Muscle is used to keep them closed (prevent them from flipping inside out).
Semilunar valves
Valves at the exits of the ventricles (pulmonary and aortic). Open when ventricular pressure exceeds arterial pressure during systole; close when pressure reverses.
SA node (sinoatrial node)
The heart's natural pacemaker. A cluster of specialised cells in the right atrium whose ionic conductances create a self-sustaining, rhythmic depolarisation. Sets the rate of the heartbeat.
AV node (atrioventricular node)
A relay station between the atria and ventricles. Receives the electrical signal from the SA node, delays it slightly (so the atria finish contracting before the ventricles start), and passes it on to the bundle of His.
Bundle of His
A bundle of conducting fibres that carries the depolarisation signal from the AV node down through the septum to the ventricles, ensuring the ventricles contract from the bottom up.
Myogenic
Cardiac muscle contracts automatically without neural input. The SA node generates its own rhythm; the nervous system modulates the rate but does not initiate it. In simple terms, the heart beats on its own.
Pacemaker potential
The slow, spontaneous depolarisation of SA node cells between action potentials. Caused by "funny" channels (HCN channels) that open at hyperpolarisation voltages, allowing Na⁺ influx. This is what makes the heartbeat self-sustaining.
Funny channels (HCN channels)
Ion channels in the SA node that open when the membrane is hyperpolarised (the opposite of most voltage-gated channels). Na⁺ is the primary cation of the "funny" current. They close upon depolarisation.
ECG / EKG (electrocardiogram)
A recording of the integrated electrical activity of the whole heart, detected from the body surface. Shows the P wave (atrial depolarisation), QRS complex (atrial repolarisation masked by ventricular depolarisation), and T wave (ventricular repolarisation).
P wave
The ECG deflection representing depolarisation (electrical activation) of the atria.
QRS complex
The ECG deflection representing depolarisation of the ventricles. Atrial repolarisation happens at the same time but is hidden within this larger signal.
T wave
The ECG deflection representing repolarisation (electrical recovery) of the ventricles.
PNS (parasympathetic nervous system)
The branch of the autonomic nervous system that slows the heart. Stimulating the PNS produces a larger hyperpolarisation in SA node cells, so it takes longer to reach threshold, and heart rate decreases.
The mammalian circulatory system has two circuits running in series:
Pulmonary circuit: right ventricle → lungs → left atrium
Systemic circuit: left ventricle → body → right atrium
Four chambers: right atrium, right ventricle, left atrium, left ventricle
The left ventricle has the thickest myocardium because it pumps blood at high pressure to the entire body
Spongy myocardium: the inner layer; receives O₂ and nutrients by diffusion from blood inside the chamber
Compact myocardium: the outer layer; too thick for diffusion, so it requires its own blood supply via the coronary arteries and veins
The pericardium (a protective sac) surrounds the heart, contains lubricating fluid, and protects against injury, infection and friction
Each chamber alternates between two phases:
Systole: contraction, which increases pressure
Diastole: relaxation, which decreases pressure
Passive filling accounts for about 70% of ventricular filling (AV valves open, blood flows in by gravity and returning venous pressure); atrial contraction (atrial systole) moves the remaining 30%
Both atria contract simultaneously; both ventricles contract simultaneously
Isovolumetric contraction: all valves closed; ventricle is contracting and pressure is rising, but no blood is moving. Ends when ventricular pressure exceeds arterial pressure and the semilunar valves are forced open → ventricular ejection
Isovolumetric relaxation: all valves closed again; ventricle is relaxing and pressure is falling. Ends when atrial pressure exceeds ventricular pressure and the AV valves open → filling resumes
No muscle or ATP is used to open valves; they open passively when the pressure gradient favours it
Muscle is used to keep the AV valves closed (chordae tendineae and papillary muscles prevent them from inverting under pressure)
AV valves are "flimsy enough" to open easily, but "strong enough" not to open the wrong way
Cardiac muscle is myogenic: it contracts automatically without neural stimulation
Depolarisation spreads in a stereotyped order via conducting cardiomyocytes, which trigger depolarisation of contracting cardiomyocytes
The conduction pathway: SA node → AV node → bundle of His → ventricular cardiomyocytes
SA node: ionic conductances create a self-sustaining pacemaker; sets the heart rate
Pacemaker potential: caused by funny channels (HCN channels) opening at hyperpolarisation voltages; Na⁺ influx slowly depolarises the cell; funny channels close upon depolarisation
Reaching threshold initiates the SA node action potential by activating voltage-gated Ca²⁺ channels
Detects integrated electrical activity of the whole heart from the body surface
P wave: depolarisation of the atria
QRS complex: depolarisation of the ventricles (atrial repolarisation occurs simultaneously but is masked by the much larger ventricular signal)
T wave: repolarisation of the ventricles
Heart rate is set by the SA node's intrinsic pacemaker rhythm
The autonomic nervous system modulates this rate:
Parasympathetic stimulation (PNS via the vagus nerve): increases the hyperpolarisation of SA node cells → takes longer to reach threshold → heart rate decreases
Sympathetic stimulation: decreases the time to threshold → heart rate increases
Funny channels open at repolarisation; Na⁺ influx begins the slow climb to threshold again
An ECG is one of the first tests ordered in an emergency room for chest pain. Clinicians read the P wave, QRS complex and T wave to diagnose arrhythmias, heart attacks and conduction blocks. Artificial pacemakers mimic the SA node's job when the heart's own conduction system fails. Beta-blockers (common blood pressure and anxiety medications) work by reducing sympathetic stimulation of the SA node, slowing heart rate.
Students often think the brain tells the heart to beat. Cardiac muscle is myogenic: the SA node generates its own rhythm. The nervous system modulates the rate, but the heart beats independently of neural input.
Students sometimes think muscle is used to open the heart valves. Valves open passively in response to pressure gradients. Muscle (papillary muscles via chordae tendineae) is used to keep the AV valves closed, not to open them.
Students often confuse the QRS complex with ventricular repolarisation. The QRS complex represents ventricular depolarisation (the start of contraction). Ventricular repolarisation is the T wave.
Students sometimes assume isovolumetric phases are unimportant because "nothing is moving." These phases are critical: they are when pressure builds or falls enough to open or close the valves that direct flow.
⚠️ Be able to trace blood flow through all four chambers and both circuits in the correct sequence.
⚠️ Know the conduction pathway: SA node → AV node → bundle of His → ventricles.
⚠️ Match each ECG wave to its event: P = atrial depolarisation, QRS = ventricular depolarisation, T = ventricular repolarisation.
⚠️ Understand the two isovolumetric phases and what opens/closes the valves at each transition.
⚠️ Be able to explain how the PNS slows heart rate (greater hyperpolarisation of SA node → longer time to threshold).
⚠️ Know the difference between spongy and compact myocardium and why the left ventricle is thicker than the right.
True or false: The heart requires signals from the brain to initiate each beat.
Fill in the blank: The ______ node is the heart's natural pacemaker.
True or false: The QRS complex on an ECG represents ventricular repolarisation.
Fill in the blank: During isovolumetric contraction, ______ valves are open.
True or false: The left ventricle has thicker walls than the right ventricle.
Answers: 1. False (cardiac muscle is myogenic; the SA node generates its own rhythm). 2. SA (sinoatrial). 3. False (QRS = ventricular depolarisation; the T wave = ventricular repolarisation). 4. No (all valves are closed during isovolumetric contraction). 5. True (the left ventricle pumps blood to the entire body and must generate higher pressure).
Q: Trace the path of a red blood cell from the right atrium through both circuits and back to the right atrium. Name each chamber, valve type and major vessel it passes through.
A: Right atrium → (through the right AV valve) → right ventricle → (through the pulmonary semilunar valve) → pulmonary artery → lungs (gas exchange) → pulmonary veins → left atrium → (through the left AV valve) → left ventricle → (through the aortic semilunar valve) → aorta → body (systemic circulation) → vena cava → right atrium.
Q: Why does the SA node fire rhythmically without any external stimulus?
A: SA node cells have funny channels (HCN channels) that open at hyperpolarisation voltages, allowing Na⁺ influx. This creates a slow, spontaneous depolarisation (the pacemaker potential). When threshold is reached, voltage-gated Ca²⁺ channels open and fire an action potential. The cycle then repeats, producing a self-sustaining rhythm.
Q: During isovolumetric contraction, all valves are closed. What determines the moment the semilunar valves open?
A: The semilunar valves open when ventricular pressure exceeds the pressure in the artery on the other side (pulmonary artery or aorta). Until that point, the ventricle is contracting against closed valves, building pressure without ejecting blood.
Q: A drug blocks funny channels in the SA node. What effect would you predict on heart rate, and why?
A: Heart rate would decrease. Blocking funny channels reduces the Na⁺ influx during diastole, slowing the pacemaker potential. It takes longer for the SA node cells to reach threshold, so there is a longer interval between action potentials and fewer beats per minute.
Q: Why does atrial repolarisation not appear as a separate wave on the ECG?
A: Atrial repolarisation occurs at the same time as ventricular depolarisation (the QRS complex). Because the ventricles are much larger than the atria, the ventricular signal is far stronger and masks the atrial repolarisation signal.
The cardiac cycle generates the pressure gradients discussed in Part 1 (ΔP in the bulk flow equation is ultimately produced by ventricular systole). The coronary arteries connect to the vessel types and resistance concepts from Part 1. Heart rate modulation by the PNS and sympathetic nervous system ties into autonomic nervous system topics elsewhere in the course. ECG interpretation will come up again in any clinical or applied physiology context.
Cardiac anatomy, heart chambers, right atrium, right ventricle, left atrium, left ventricle, pulmonary circuit, systemic circuit, myocardium, spongy myocardium, compact myocardium, coronary arteries, coronary circulation, pericardium, cardiac cycle, systole, diastole, isovolumetric contraction, isovolumetric relaxation, AV valves, atrioventricular valves, semilunar valves, SA node, sinoatrial node, AV node, atrioventricular node, bundle of His, pacemaker potential, funny channels, HCN channels, myogenic, ECG, EKG, electrocardiogram, P wave, QRS complex, T wave, heart rate, parasympathetic, vagus nerve, sympathetic, autonomic nervous system, depolarisation, repolarisation, ICU 34119