How does the structure of the mammalian heart produce a coordinated cardiac cycle, and how is each beat controlled?
3.1.2 Transport in animals: the structure of the mammalian heart and the events of the cardiac cycle (atrial systole, ventricular systole and diastole), the pressure and volume changes that open and close the valves, and the myogenic control of heart rate by the SAN, AVN, bundle of His and Purkyne tissue, including the interpretation of electrocardiograms (ECGs).
A focused answer to the OCR H420 3.1.2 dot point on the mammalian heart. Covers the heart's structure, the three stages of the cardiac cycle, how pressure changes open and close the valves, myogenic control by the SAN, AVN, bundle of His and Purkyne tissue, and how to read an ECG.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
OCR wants you to describe the structure of the mammalian heart, sequence the events of the cardiac cycle (atrial systole, ventricular systole and diastole), explain how pressure changes open and close the AV and semilunar valves, describe the myogenic control of heart rate by the SAN, AVN, bundle of His and Purkyne tissue, and interpret an electrocardiogram.
The answer
The structure of the heart
The heart is a double pump made of cardiac muscle. The right side receives deoxygenated blood from the body in the right atrium and pumps it from the right ventricle to the lungs; the left side receives oxygenated blood from the lungs in the left atrium and pumps it from the left ventricle to the body. The left ventricle wall is much thicker because it must generate enough pressure to drive blood around the whole body, whereas the right ventricle only pumps to the nearby lungs at lower pressure.
Valves keep blood flowing one way. The atrioventricular (AV) valves (tricuspid on the right, bicuspid on the left) lie between each atrium and ventricle; the semilunar valves lie at the start of the pulmonary artery and the aorta. The septum separates the oxygenated and deoxygenated sides.
The cardiac cycle
The cardiac cycle is one complete heartbeat, conventionally split into three stages and driven entirely by pressure differences:
- Atrial systole. Both atria contract. Atrial pressure rises above ventricular pressure, so the AV valves stay open and blood is pushed into the ventricles, topping them up.
- Ventricular systole. The ventricles contract. Ventricular pressure rises above atrial pressure, closing the AV valves (the first heart sound, "lub"). When ventricular pressure exceeds the pressure in the aorta and pulmonary artery, the semilunar valves open and blood is forced out.
- Diastole. The whole heart relaxes. Ventricular pressure falls below arterial pressure, so the semilunar valves close (the second heart sound, "dub"); as it falls below atrial pressure the AV valves open, and blood returning from the veins flows passively into the atria and ventricles, refilling the heart.
The golden rule is that a valve opens when the pressure behind it is higher than the pressure in front of it, and closes when the pressure in front is higher (preventing backflow).
Myogenic control of heart rate
Cardiac muscle is myogenic, so the rhythm is set inside the heart:
- The sinoatrial node (SAN) in the right atrium wall is the pacemaker. It fires a wave of depolarisation that spreads across both atria, causing atrial systole.
- A layer of non-conducting tissue between the atria and ventricles stops the wave passing straight through, so it can only reach the ventricles via the atrioventricular node.
- The atrioventricular node (AVN) delays the wave briefly, so the atria finish emptying before the ventricles contract.
- The AVN passes the wave down the bundle of His in the septum, which divides into the Purkyne tissue (fibres) running up the ventricle walls, so the ventricles contract from the apex upwards, pushing blood up towards the arteries.
Interpreting an ECG
An electrocardiogram records the electrical activity of the heart:
- the P wave is atrial depolarisation (atrial systole);
- the QRS complex is ventricular depolarisation (ventricular systole);
- the T wave is ventricular repolarisation (relaxation).
Heart rate is found from the time between successive R peaks. A fast rate (tachycardia), slow rate (bradycardia), or irregular trace (fibrillation) can all be read from the spacing and shape of the waves.
Examples in context
Example 1. Why a heart transplant still beats. A transplanted heart has its nerves cut, yet it still beats because cardiac muscle is myogenic; the SAN continues to set the rhythm, though the rate responds less quickly to exercise without its nerve supply.
Example 2. Artificial pacemakers. If the SAN or its conduction pathway fails, an artificial pacemaker delivers regular electrical impulses to restore a coordinated rhythm, a direct application of how the natural conduction system works.
Try this
Q1. State why the wall of the left ventricle is thicker than the wall of the right ventricle. [2 marks]
- Cue. The left ventricle pumps blood to the whole body, so it must generate a higher pressure; the right ventricle only pumps to the nearby lungs at lower pressure.
Q2. Explain the purpose of the delay imposed by the atrioventricular node. [2 marks]
- Cue. It allows the atria to finish contracting and empty fully into the ventricles before the ventricles contract, so the ventricles fill completely.
Q3. Name the part of the ECG that represents ventricular depolarisation. [1 mark]
- Cue. The QRS complex.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H420/01 20196 marksDescribe the events of the cardiac cycle, explaining how the pressure changes in the chambers open and close the heart valves.Show worked answer →
Work through the three stages in order, linking each valve action to a pressure comparison.
- Atrial systole
- The atria contract, raising atrial pressure above ventricular pressure, so the atrioventricular (AV) valves are pushed open and blood is forced into the ventricles.
- Ventricular systole
- The ventricles contract, raising ventricular pressure above atrial pressure, so the AV valves close (preventing backflow into the atria and producing the first heart sound). When ventricular pressure rises above the pressure in the aorta and pulmonary artery, the semilunar valves open and blood is ejected.
- Diastole
- The heart relaxes, ventricular pressure falls below arterial pressure so the semilunar valves close (second heart sound), and as it falls below atrial pressure the AV valves open and the chambers refill passively.
Markers reward each valve being opened or closed by the correct pressure comparison, not just named.
OCR H420/01 20224 marksExplain how the heartbeat is initiated and coordinated so that the atria contract before the ventricles.Show worked answer →
Trace the wave of excitation and explain the deliberate delay.
The sinoatrial node (SAN) in the right atrium wall is the pacemaker; it is myogenic and initiates a wave of depolarisation that spreads across both atria, causing atrial systole.
A band of non-conducting tissue stops the wave passing directly to the ventricles, so it reaches the atrioventricular node (AVN). The AVN imposes a short delay, ensuring the atria finish contracting and emptying before the ventricles contract.
The AVN then passes the wave down the bundle of His in the septum and out through the Purkyne tissue, so the ventricles contract from the apex upwards. Markers reward SAN as pacemaker, the AVN delay and its reason, and conduction via the bundle of His and Purkyne fibres.
Related dot points
- 3.1.2 Transport in animals: the structure and functions of arteries, arterioles, capillaries, venules and veins; the formation of tissue fluid from plasma at the arterial end of a capillary bed and its return at the venous end and via the lymphatic system, explained in terms of hydrostatic and oncotic (osmotic) pressure.
A focused answer to the OCR H420 3.1.2 dot point on blood vessels and tissue fluid. Covers the structure and function of arteries, arterioles, capillaries, venules and veins, and how hydrostatic and oncotic pressure form tissue fluid at the arterial end and return it at the venous end and via the lymph.
- 3.1.2 Transport in animals: the role of haemoglobin in transporting oxygen, the oxygen dissociation curve and cooperative binding, the Bohr effect, the higher oxygen affinity of fetal haemoglobin, and the transport of carbon dioxide including the formation of hydrogencarbonate ions and the chloride shift.
A focused answer to the OCR H420 3.1.2 dot point on oxygen transport. Covers haemoglobin and cooperative binding, the sigmoidal oxygen dissociation curve, loading and unloading, the Bohr effect, fetal haemoglobin, and carbon dioxide transport as hydrogencarbonate with the chloride shift.
- 3.1.1 Exchange surfaces: the need for specialised exchange surfaces as size and metabolic rate increase and surface-area-to-volume ratio falls; the features of an efficient exchange surface; the structure and function of the mammalian gas-exchange system, the counter-current system in fish gills, and the tracheal system of insects.
A focused answer to the OCR H420 3.1.1 dot point on exchange surfaces and gas exchange. Covers why surface-area-to-volume ratio drives the need for exchange surfaces, the features of an efficient surface, the mammalian lung, the fish counter-current system and the insect tracheal system.
- 5.1.3 Neuronal communication: the structure of a neurone; the establishment of the resting potential by the sodium-potassium pump; the generation of an action potential by voltage-gated channels (depolarisation and repolarisation); the all-or-nothing principle and the refractory period; saltatory conduction in myelinated neurones; and synaptic transmission by acetylcholine at a cholinergic synapse.
A focused answer to the OCR H420 5.1.3 dot point on neuronal communication. Covers neurone structure, the resting potential, the action potential and its ionic basis, the all-or-nothing principle and refractory period, saltatory conduction, and synaptic transmission by acetylcholine.
- 5.1.4 Hormonal communication: the principles of hormonal coordination and the contrast with nervous coordination; the structure and function of the adrenal glands and pancreas; the control of blood glucose concentration by insulin and glucagon (glycogenesis, glycogenolysis and gluconeogenesis); the second messenger model of adrenaline and glucagon; and the causes of type 1 and type 2 diabetes.
A focused answer to the OCR H420 5.1.4 dot point on hormonal communication. Covers hormonal versus nervous coordination, the adrenal glands and pancreas, the control of blood glucose by insulin and glucagon, the second messenger model, and the causes of type 1 and type 2 diabetes.
Sources & how we know this
- OCR A Level Biology A (H420) Specification — OCR (2023)