The structure and functions of the cardio-respiratory system — AQA GCSE Physical Education

Air follows a fixed route from the environment into the lungs and back out. It enters through the nose or mouth, passes down the trachea (windpipe), and divides into two bronchi, one to each lung. Each bronchus branches into smaller bronchioles, which end in tiny air sacs called alveoli. Gas exchange happens at the alveoli, where oxygen moves into the blood and carbon dioxide moves out. On exhalation the same structures carry air in reverse. Learn the order as a sequence and be ready to pick out the correct next structure or spot a structure placed in the wrong position. The specification limits identification to this pathway, so focus on naming and ordering these structures rather than on detailed mechanics of breathing.

The mouth and nose are the two entry points for air into the respiratory system. The nose contains hairs and mucus that filter dust and warm and moisten the incoming air, while the mouth offers a larger opening that lets more air in quickly during vigorous exercise. Both passages lead to the pharynx and then the trachea. In an MCQ you may be asked which structure filters, warms or moistens air, or which passage a performer uses when breathing heavily. Link each feature to its function: nose for filtering and warming, mouth for rapid large-volume intake.

The trachea, or windpipe, carries air from the pharynx towards the lungs. It is a tube held open by C-shaped rings of cartilage so it does not collapse when air pressure changes during breathing. Its lining contains cilia and mucus that trap dust and move it upwards away from the lungs. The trachea divides into two bronchi, one to each lung. In an MCQ you may be asked which structure keeps the airway open, which structure carries air to the bronchi, or what the cartilage rings do. Link each feature to its function.

The bronchi are the two large airways that branch from the lower end of the trachea and carry air into each lung. The trachea divides into the right bronchus and the left bronchus, one serving each lung. Each bronchus then divides repeatedly into smaller tubes called bronchioles, which end in the alveoli where gas exchange happens. The bronchi are part of the conducting passages: they move air towards the exchange surfaces but do not themselves carry out gas exchange. Their walls contain cartilage, which keeps the tubes open so air can flow freely during inhalation and exhalation. In an exam question, identify the bronchi as the paired tubes between the trachea and the bronchioles, and link their structure to the function of conducting air to the lungs.

Bronchioles are the many small airways that branch from each bronchus inside the lungs. They are narrower than the bronchi and divide repeatedly, becoming finer and finer, and their smallest branches end at the alveoli. The bronchioles are part of the conducting passages: they carry air deeper into the lungs towards the exchange surfaces. Unlike the bronchi, the bronchioles have little or no cartilage; instead their walls contain smooth muscle, which can change the width of the airway. This means the bronchioles can narrow or widen to adjust airflow. In an exam question, place the bronchioles between the bronchi and the alveoli, and link their fine branching to distributing air throughout the lung tissue.

The alveoli are the tiny air sacs at the end of the bronchioles in the lungs, and they are the site of gaseous exchange. Their structure supports this role: there are very many of them, giving a large total surface area; each wall is only one cell thick, so gases diffuse over a short distance; they are surrounded by a dense capillary network, keeping a steep concentration gradient; and their moist lining lets gases dissolve before diffusing. Oxygen moves from alveolar air into the blood, and carbon dioxide moves from the blood into the alveolar air. A multiple-choice question may ask which feature assists exchange, so link each feature to the reason it helps rather than just naming it.

Gaseous exchange at the alveoli depends on diffusion, the movement of gases from a higher to a lower concentration. Several features assist this. A very large number of alveoli creates a large total surface area. Each alveolar wall is one cell thick, so gases travel a short distance. A dense capillary network carries blood close to the air, and breathing replaces air to keep oxygen high and carbon dioxide low in the alveoli, maintaining a steep concentration gradient. The moist lining allows gases to dissolve. Oxygen diffuses from the alveoli into the blood, and carbon dioxide diffuses from the blood into the alveoli to be breathed out. A multiple-choice question may ask which feature assists exchange or why a feature helps, so connect each feature to diffusion.

The alveoli are tiny air sacs clustered at the ends of the bronchioles, and their combined surface area is very large. This large surface area allows a greater volume of gas to diffuse across the alveolar walls in a given time, so oxygen moves into the blood and carbon dioxide moves out more quickly. A useful method is to picture the alveoli as a huge number of small balloons rather than one big balloon: the same volume of air meets far more wall surface. In an MCQ, link the feature to its function, so a question asking why gas exchange is efficient should be matched to the large surface area, not to a feature such as cartilage rings that serves a different purpose.

The alveolar walls are described as moist and thin, being one cell thick. The thinness shortens the diffusion distance, so oxygen and carbon dioxide cross quickly, and the moisture allows gases to dissolve before diffusing. A useful method is to treat the wall as a single layer of cells forming a short bridge between air and blood. In an MCQ, match each feature to its effect: thin walls shorten the diffusion pathway, while moisture supports dissolving of gases. Keep the feature attached to the alveoli and avoid options that describe the airways, such as cartilage or cilia, which serve different roles.

Gas exchange happens at the alveoli, the tiny air sacs at the ends of the bronchioles. For oxygen to move from the air in an alveolus into the blood, and for carbon dioxide to move the other way, the gases must cross the alveolar wall and the capillary wall. These two walls are pressed closely together, so the distance the gas has to travel is very short. A short diffusion pathway means gas molecules cross quickly, so the blood is re-oxygenated and carbon dioxide is removed efficiently. This is one of the features that makes the alveoli an effective exchange surface, alongside a large surface area, a rich blood supply and a moist, thin lining. In an exam question, link the short distance directly to faster diffusion and therefore more efficient gas exchange during exercise as well as at rest.

Each alveolus is surrounded by a dense network of capillaries, the smallest blood vessels. A rich capillary supply means a large volume of blood flows close to the air in the alveoli at any one time. This maintains a steep concentration gradient: blood arriving at the lungs carries relatively little oxygen and more carbon dioxide, while the air in the alveolus is rich in oxygen and low in carbon dioxide. Because fresh blood keeps arriving and flowing away, oxygen continues to diffuse into the blood and carbon dioxide continues to diffuse out. The many capillaries therefore work together with the thin walls and large surface area to make gas exchange efficient, especially during exercise when demand rises. In an exam answer, link lots of capillaries to maintaining a concentration gradient and to efficient gas exchange.

The lungs receive a large blood supply so that gases can be exchanged between air in the alveoli and blood in the surrounding capillaries. The pulmonary artery carries deoxygenated blood from the heart to the lungs, where it flows through dense capillary networks wrapped around each alveolus. A rich blood supply keeps a steep concentration gradient: blood arriving is low in oxygen and relatively high in carbon dioxide, so oxygen diffuses into the blood and carbon dioxide diffuses out. Blood flow also removes oxygenated blood quickly, maintaining the gradient. A large blood supply therefore supports efficient gaseous exchange during rest and, especially, during exercise when demand rises.

Diffusion is the movement of gas from a region of high concentration to a region of low concentration. In the lungs, oxygen moves from the alveoli, where its concentration is high, across the alveolar and capillary walls into the blood, where its concentration is lower. Carbon dioxide moves the opposite way, from the blood into the alveoli. Diffusion continues as long as a concentration gradient exists, and breathing and blood flow help maintain that gradient. The process requires no energy from the body and happens across thin, moist surfaces with a large surface area and a rich blood supply.

Haemoglobin is the red pigment inside red blood cells. At the lungs, oxygen diffuses into the blood and combines with haemoglobin to form oxyhaemoglobin; this is how most oxygen is transported, with only a small amount dissolved in plasma. At the tissues the oxyhaemoglobin releases its oxygen, which diffuses into the working muscle. Haemoglobin also carries carbon dioxide, but most carbon dioxide travels dissolved in plasma or as hydrogencarbonate ions, so haemoglobin's carbon dioxide role is a smaller share. A useful method is to trace one red blood cell from lung capillary to muscle capillary and name what it picks up and puts down at each point.

The three main blood vessels differ in wall structure because each has a different job. Arteries carry blood away from the heart at high pressure, so they have thick, muscular, elastic walls and a narrow lumen. Veins carry blood back to the heart at low pressure, so they have thinner walls and a wider lumen, and valves prevent backflow. Capillaries are the smallest vessels, with walls one cell thick, so substances can diffuse quickly between blood and tissue. A reliable method is to compare each vessel on three features: wall thickness, lumen size and presence of valves, then link each feature to its function.

This point concerns the size, or diameter, of the blood vessels in the cardio-respiratory system and how that measurement relates to function. Arteries carry blood away from the heart at high pressure, so they have a relatively narrow lumen and thick walls; veins return blood at low pressure and have a wider lumen. Capillaries are the narrowest vessels, with a lumen roughly one red blood cell wide, which slows blood flow and shortens diffusion distance so gases and nutrients exchange efficiently with tissue. A learner should be able to compare vessel diameters and link each to pressure, flow and exchange, rather than simply naming a vessel. In a multiple-choice question, read each option as a claim about diameter and check it against the vessel's job.

This point concerns the thickness of blood vessel walls and how it matches each vessel's function. Arteries have thick, muscular, elastic walls because they carry blood away from the heart at high pressure; the elastic tissue stretches as blood is forced in and recoils to help maintain pressure between heartbeats. Veins have thinner walls with less muscle and elastic tissue because blood returns at low pressure, and they rely on valves and the action of surrounding muscles. Capillary walls are a single cell thick, which keeps the diffusion distance short so gases and nutrients pass quickly between blood and tissue. A learner should compare wall thickness across vessels and connect it to pressure and exchange, and in a multiple-choice question should test each option against the vessel's role.

Veins carry blood back towards the heart at low pressure, so they need a mechanism that stops blood sliding backwards. Pocket-shaped valves sit inside the vein lining; when blood flows towards the heart the flaps are pushed open, and when blood starts to fall back the flaps fill and meet, closing the vessel. This one-way action, helped by the squeezing of nearby skeletal muscles during movement, keeps venous return moving upwards from the legs. A learner should be able to pick out the correct description of valve position, structure and effect, and reject options that place valves in arteries or claim they pump blood.

Each vessel type has a wall and lumen matched to its job. Arteries carry blood away from the heart at high pressure, so they have thick muscular and elastic walls and a narrow lumen that stretch and recoil with each heartbeat. Veins return blood at low pressure, so they have thin walls, a wide lumen and valves to stop backflow. Capillaries are one cell thick with a tiny lumen, giving a short diffusion distance so gases and nutrients exchange quickly with tissues. A learner should match each structural feature to the function it serves and reject options that pair a feature with the wrong vessel.

This statement asks you to link each major blood vessel to the direction it carries blood and to the heart. Arteries carry blood away from the heart; veins carry blood back to the heart. The pulmonary artery is the exception to the oxygen rule: it carries deoxygenated blood away from the heart to the lungs, while the pulmonary vein carries oxygenated blood from the lungs back to the heart. The aorta carries oxygenated blood away from the left ventricle to the body, and the vena cava carries deoxygenated blood from the body back to the right atrium. In a multiple-choice question, read whether the option names direction, oxygen state or both, then check each part against the vessel.

Gas exchange is the movement of oxygen and carbon dioxide between the air in the alveoli and the blood in the surrounding capillaries. Oxygen moves from the alveoli, where its concentration is higher, into the blood, and carbon dioxide moves from the blood into the alveoli to be breathed out. This happens by diffusion across thin, moist walls. The alveoli are well suited to this: they give a large surface area, have thin walls for a short diffusion path, and are surrounded by many capillaries to maintain a steep concentration gradient. In a multiple-choice question, check whether an option describes the correct gas, the correct direction and the correct mechanism.

Blood pressure is the force that blood exerts on the walls of the arteries as it is pumped around the body. It is written as two numbers: the higher systolic value, recorded when the ventricles contract and push blood into the arteries, and the lower diastolic value, recorded when the heart relaxes and refills. A typical resting reading for a healthy young adult is around 120/80 mmHg, where mmHg means millimetres of mercury. During exercise, systolic pressure rises because the heart pumps more blood with each beat and cardiac output increases, while diastolic pressure changes little. Regular aerobic training can lower resting blood pressure, which reduces the risk of heart disease and stroke.

During exercise the body redirects blood away from organs that need less oxygen and towards the working muscles. This redistribution happens because the arterioles supplying different areas can widen or narrow. Vasodilation is the widening of the lumen of an arteriole, which increases blood flow to that area; it occurs in the vessels serving the working skeletal muscles and the skin, helping to deliver oxygen and remove heat. Vasoconstriction is the narrowing of the lumen, which reduces blood flow; it occurs in vessels supplying the digestive system and other non-essential organs. The combined effect raises blood supply to active muscle while maintaining blood pressure.

This target asks you to name the major vessels that carry blood into and out of the heart, and to link each name to the direction of flow. Blood returning from the body arrives in the right atrium through the vena cava (superior and inferior). Blood returning from the lungs arrives in the left atrium through the pulmonary veins. Blood leaving the right ventricle travels to the lungs in the pulmonary artery, and blood leaving the left ventricle travels to the body in the aorta. Learn each vessel with its destination and oxygen status: arteries carry blood away from the heart, veins carry blood towards it, and the pulmonary vessels are the exception to the usual oxygen pattern. A multiple-choice question may ask which vessel enters the right atrium, which leaves the left ventricle, or which vessel carries oxygenated blood to the heart.

This row is a heading for the heart-structure content rather than a full assessed statement, so treat it as a signpost to the material that follows in section 3.1.1.2. Use it to organise your reading: locate the labelled heart diagram in your course material, then work through the four chambers (right and left atria, right and left ventricles), the valves, the septum and the major vessels. Read each label aloud and trace the path of blood: vena cava to right atrium, right ventricle, pulmonary artery to lungs, pulmonary vein to left atrium, left ventricle, aorta to body. Check that you can explain why the left ventricle wall is thicker. Because the statement is only a heading, do not treat it as a question to answer; use it to plan revision and confirm which diagram and labels your specification requires.

The atria are the two upper chambers of the heart, and the specification names both the left atrium and the right atrium. They act as collecting chambers: deoxygenated blood returning from the body enters the right atrium through the vena cava, and oxygenated blood returning from the lungs enters the left atrium through the pulmonary vein. When the atria contract, they push blood down through valves into the ventricles beneath them. A useful way to remember the arrangement is that atria sit above and receive, while ventricles sit below and pump out. In a multiple-choice question, match the chamber to its job and to the side of the heart it serves, and check that the option names the correct side rather than treating the atria as a single chamber.

The ventricles are the two lower chambers of the heart, and the specification names both the left ventricle and the right ventricle. They receive blood from the atria above them and pump it out of the heart. The right ventricle pumps deoxygenated blood to the lungs through the pulmonary artery, while the left ventricle pumps oxygenated blood around the body through the aorta. The left ventricle has a thicker muscular wall because it must generate enough pressure to send blood to the whole body, whereas the right ventricle only sends blood the short distance to the lungs. In a multiple-choice question, link each ventricle to its destination and to the thickness of its wall, and check that the option keeps the correct side of the heart.

The cardiac cycle is the repeating sequence in which the heart chambers relax and fill, then contract and eject blood. Diastole is the filling phase: the atria and ventricles relax, and blood flows into them. Systole is the ejection phase: a chamber contracts and pushes blood onward. The order depends on the starting chamber named in the question. Starting at the right ventricle, deoxygenated blood is ejected through the pulmonary valve into the pulmonary artery towards the lungs; the ventricle then relaxes and refills from the right atrium, which itself fills from the vena cava. The cycle continues through the left side: the left atrium fills and contracts, then the left ventricle contracts and ejects oxygenated blood through the aorta to the body. Trace each step in the stated order and name the chamber, the phase and the vessel involved.

This row is a heading introducing the pathway of the blood within the cardio-respiratory section, so treat it as course guidance rather than a standalone assessed statement. Use it to organise your reading: follow blood from the body through the heart to the lungs and back, then out to the body again. On the right side, deoxygenated blood returns via the vena cava to the right atrium, passes to the right ventricle, and is ejected through the pulmonary artery to the lungs. On the left side, oxygenated blood returns via the pulmonary vein to the left atrium, passes to the left ventricle, and is ejected through the aorta to the body. Read each vessel with the chamber it serves, and keep the two sides distinct.

This step describes the return of deoxygenated blood to the heart. Blood that has delivered oxygen to the working muscles and other tissues travels back through the venae cavae (superior and inferior) and empties into the right atrium. The right atrium is a thin-walled collecting chamber, so it holds blood at low pressure before passing it on. In an MCQ you may be asked which vessel delivers blood to which chamber, or which chamber receives deoxygenated blood. Remember the pattern: deoxygenated blood enters the right side of the heart, oxygenated blood enters the left side via the pulmonary veins. Link this to the pathway: body tissues → venae cavae → right atrium → right ventricle → pulmonary artery → lungs.

After collecting in the right atrium, deoxygenated blood passes through the tricuspid valve into the right ventricle. The ventricle has a thicker muscular wall than the atrium because it must pump blood out of the heart, here to the lungs via the pulmonary artery. The valve prevents backflow into the atrium when the ventricle contracts. In an MCQ you may be asked what follows the right atrium, which chamber pumps blood to the lungs, or which valve separates the two right chambers. Keep the sequence clear: right atrium → tricuspid valve → right ventricle → pulmonary valve → pulmonary artery → lungs. Note that the right ventricle pumps to the lungs at lower pressure than the left ventricle pumps to the body.

This statement fixes the direction and cargo of one vessel in the pulmonary circuit. The right ventricle pumps blood into the pulmonary artery, and that artery carries deoxygenated blood away from the heart to the lungs. The key examinable idea is that the pulmonary artery is the exception to the general rule that arteries carry oxygenated blood: it is an artery, it leaves the heart, yet its blood is low in oxygen because gas exchange has not yet happened. Trace the route aloud: right atrium, right ventricle, pulmonary artery, lungs. Contrast it with the pulmonary vein, which returns oxygenated blood from the lungs to the left atrium. A useful check is to ask which side of the heart the vessel serves; the pulmonary artery serves the right side and the lungs.

This statement describes what happens at the lungs: oxygen moves from the alveoli into the blood and carbon dioxide moves from the blood into the alveoli, so the blood leaving the lungs is oxygenated. The exchange happens across the thin walls of the alveoli and their surrounding capillaries, driven by concentration differences. Blood arriving at the lungs is deoxygenated; after gas exchange it is oxygenated and returns to the heart through the pulmonary vein. Remember that the lungs do not make oxygen; they move it into the blood. A reliable way to answer is to name the two gases, state the direction each travels, and confirm that the result is oxygenated blood ready to be pumped around the body.

The pulmonary vein is the vessel that carries oxygenated blood from the lungs back to the heart, emptying into the left atrium. It is unusual because, unlike most veins, it carries oxygenated rather than deoxygenated blood: the blood has just picked up oxygen in the lungs. Trace the route: blood flows from the lungs along the pulmonary vein into the left atrium, which then passes it on towards the left ventricle. In an MCQ, read each option carefully and reject any that names the right atrium, the right ventricle or a deoxygenated description. Link the vessel to its destination and to the oxygen state of the blood it carries.

This statement continues the route of oxygenated blood: after entering the left atrium, blood passes through the mitral (bicuspid) valve into the left ventricle. The left ventricle has a thick muscular wall because it pumps oxygenated blood at high pressure into the aorta and around the whole body. In an MCQ, follow the sequence lungs, pulmonary vein, left atrium, left ventricle, aorta. Reject options that send blood from the left atrium to the right ventricle or that reverse the order of atrium and ventricle. Link the chamber to its thick wall and its role in systemic circulation.

This fragment describes the final stage of the cardiac cycle. Oxygenated blood returns from the lungs through the pulmonary veins into the left atrium, passes through the left atrioventricular valve into the left ventricle, and is then ejected from the left ventricle into the aorta. The aorta is the large artery that carries this oxygenated blood away from the heart to the body tissues. In a multiple-choice question you must place the aorta correctly in this sequence: it comes after the left ventricle contracts, not before the lungs are supplied. Trace the route left atrium → left ventricle → aorta and check that the option you choose keeps oxygenated blood leaving via the aorta to the body.

The heart and veins contain valves that keep blood moving in one direction. A valve opens when pressure on one side of it is greater than on the other: pressure from blood arriving in the atrium, or from a contracting ventricle, pushes the flaps apart. A valve closes when pressure falls on the upstream side or rises downstream, so the flaps meet and stop blood travelling backwards. This prevents backflow, keeping blood flowing from atria to ventricles and from ventricles into the arteries, and in veins keeping blood returning towards the heart. You do not need to name individual valves such as the tricuspid or bicuspid, but you do need to explain the pressure change that opens a valve and the closure that prevents backflow. In a multiple-choice question, judge each option on whether its pressure and backflow reasoning is correct, even if it also mentions a valve name.

Cardiac output (Q) is the volume of blood the heart pumps out of the left ventricle in one minute, measured in litres per minute (dm³ min⁻¹ or L/min). Stroke volume (SV) is the volume ejected by one ventricle in a single beat, measured in millilitres (cm³). Heart rate (HR) is the number of beats per minute, measured in beats per minute (bpm). The three are linked: cardiac output equals stroke volume multiplied by heart rate. For example, a performer with a stroke volume of 70 cm³ and a heart rate of 60 bpm has Q = 70 × 60 = 4200 cm³ min⁻¹, which is 4.2 dm³ min⁻¹. During exercise, both stroke volume and heart rate rise, so cardiac output increases to deliver more oxygenated blood to working muscles. A trained performer usually has a larger stroke volume and a lower resting heart rate, so resting cardiac output stays similar.

The equation cardiac output (Q) = stroke volume × heart rate links the volume of blood ejected per beat to the number of beats per minute. Cardiac output is measured in dm³ min⁻¹ (or L/min), stroke volume in cm³ (or mL) and heart rate in beats per minute. To use the equation, multiply stroke volume by heart rate, then convert the result to the required unit. For example, a stroke volume of 80 cm³ and a heart rate of 150 bpm give Q = 80 × 150 = 12 000 cm³ min⁻¹, which is 12 dm³ min⁻¹. Rearranging the equation allows stroke volume to be found by dividing cardiac output by heart rate, and heart rate by dividing cardiac output by stroke volume. During exercise, stroke volume and heart rate both increase, so cardiac output rises to supply working muscles with more oxygenated blood.

A heart rate graph plots beats per minute on the vertical axis against time on the horizontal axis. Before exercise begins, heart rate often climbs slightly above resting level; this is the anticipatory rise, caused by adrenaline release preparing the body for activity. When intensity increases, the line rises steeply as the heart delivers more oxygenated blood to working muscles. During steady sub-maximal work the line plateaus, showing a steady state where supply meets demand. When exercise stops, the line falls during recovery, fastest at first then levelling towards resting values. To interpret a graph, read the value at a stated time, describe the direction and steepness of the line, and link each change to intensity and to the body's oxygen demand.

Inhalation at rest is an active process driven by muscle contraction. The diaphragm, a dome-shaped sheet of muscle below the lungs, contracts and flattens, moving downwards. At the same time the external intercostal muscles contract, pulling the rib cage upwards and outwards. These two movements increase the volume of the thoracic cavity. Because the pressure inside the lungs then falls below atmospheric pressure, air flows in down the pressure gradient through the nose, trachea, bronchi and bronchioles to the alveoli. The statement names the intercostal muscles, ribs and diaphragm; a full answer explains how each contributes to the volume and pressure change that draws air in.

The intercostals are the muscles lying between the ribs. In this section of the cardio-respiratory system you learn how they work with the diaphragm to change the volume and pressure inside the thorax so that air moves in and out of the lungs. During inhalation the external intercostals contract, lifting the rib cage up and out; this increases thoracic volume and lowers pressure so air is drawn in. During exhalation the internal intercostals can contract to pull the rib cage down and in, helping to force air out, especially during exercise. Learn the muscle name, its position between the ribs, and its role in breathing mechanics rather than treating it as a lung structure.

The rib cage is the bony structure that surrounds and protects the heart and lungs. It is made up of the ribs, the sternum at the front and the thoracic vertebrae at the back, and it forms the chest wall that encloses the thoracic cavity. In breathing, the rib cage moves: it lifts up and out during inhalation and moves down and in during exhalation, working with the intercostals and diaphragm. These movements change the volume of the thorax, which alters pressure and drives air into and out of the lungs. Learn the rib cage as a protective structure and as a moving part of the breathing mechanism, not as a muscle.

The diaphragm is a dome-shaped sheet of muscle that separates the chest cavity from the abdomen and is the main muscle of breathing at rest. When it contracts it flattens and moves downwards, which increases the volume of the thoracic cavity and lowers the pressure inside it, so air is drawn in during inhalation. When it relaxes it returns to its dome shape, the thoracic volume decreases and pressure rises, so air is pushed out during exhalation. In a multiple-choice question you must pick the option that correctly describes the diaphragm's structure or its role in breathing, and reject options that confuse it with the intercostal muscles or reverse the pressure change.

Exhalation at rest is a passive process driven by the elastic recoil of the lungs and the relaxation of the breathing muscles. The diaphragm relaxes and returns to its dome shape, and the external intercostal muscles relax so the ribcage moves down and in. Together these changes reduce the volume of the thoracic cavity, which raises the pressure inside it above atmospheric pressure, so air is forced out of the lungs. In a multiple-choice question you must choose the option that correctly describes these muscle roles and the resulting volume and pressure changes, and reject options that describe active muscle contraction or the wrong direction of pressure change.

At rest, quiet inspiration relies mainly on the diaphragm and external intercostals. During exercise, demand for oxygen rises, so accessory muscles are recruited. On inspiration, the pectorals and sternocleidomastoid lift and expand the rib cage further, increasing thoracic volume so the lungs expand more and tidal volume rises. On expiration, the abdominal muscles contract to pull the rib cage down faster and push the diaphragm up, forcing air out more quickly. So exercise changes both the muscles used and the speed and depth of breathing. A useful check is to link each muscle to the movement it produces and to the effect on air movement.

Breathing depends on pressure differences between the air outside the body and the air inside the lungs. On inhalation, the diaphragm contracts and flattens while the external intercostals lift the rib cage, so thoracic volume increases and pressure inside the lungs falls below atmospheric pressure. Air then flows in down the pressure gradient. On exhalation, the diaphragm relaxes and moves up and the rib cage falls, so thoracic volume decreases and pressure inside the lungs rises above atmospheric pressure, pushing air out. So muscle action changes volume, volume changes pressure, and the pressure difference drives air in or out. A useful check is to follow the chain: muscles, then volume, then pressure, then airflow.

A spirometer trace plots the volume of air in the lungs against time, producing a wavy line of peaks and troughs. To interpret it, read the vertical axis for volume and the horizontal axis for time. The regular small peaks and troughs during quiet breathing show tidal volume, the air moved in or out in one normal breath. The deepest peak after a forced breath in shows inspiratory reserve volume, and the lowest trough after a forced breath out shows expiratory reserve volume. The volume left after the fullest possible breath out is residual volume, which a spirometer cannot measure directly. During exercise, tidal volume rises because each breath is deeper, and the breathing rate rises so peaks come closer together. Learners should practise labelling each volume on a trace and comparing a rest trace with an exercise trace.

Tidal volume is the volume of air breathed in or out of the lungs in one normal breath while at rest. On a spirometer trace it appears as the height of a regular peak above the trough that follows it, measured on the vertical volume axis. A typical resting value is around 0.5 litres per breath, though learners should read the figure from the trace given rather than assume it. During exercise tidal volume increases because the muscles of breathing contract more strongly, so each breath is deeper and the peaks on the trace grow taller. Learners should be able to define tidal volume, locate it on a trace and explain why it rises from rest to exercise.

Expiratory reserve volume (ERV) is the extra volume of air a person can force out of the lungs after a normal, quiet expiration, over and above the air already breathed out in that quiet breath. It is measured with a spirometer and is expressed in litres, typically around 1.0–1.2 litres in a healthy young adult, though values vary with age, sex, body size and fitness. ERV is the reserve left after tidal volume has been exhaled, so it represents air that is available for deeper or more forceful breathing, for example during vigorous exercise or when blowing hard. It is not the same as residual volume, which is the air that stays in the lungs and cannot be exhaled. ERV is one component used when calculating vital capacity and total lung capacity.

Inspiratory reserve volume (IRV) is the extra volume of air a person can breathe in beyond a normal, quiet inspiration, taken by filling the lungs as fully as possible after an ordinary breath in. It is measured with a spirometer and expressed in litres, commonly around 2.5–3.0 litres in a healthy young adult, though values vary with age, sex, body size and fitness. IRV is the reserve available for deep or forceful inhalation, for example when taking a big breath before exercise or before blowing hard. It is not the same as tidal volume, which is the air moved in one quiet breath, nor vital capacity, which combines several volumes. IRV is one component used when calculating vital capacity and total lung capacity.

Residual volume is the air that stays in the lungs after the strongest possible exhalation, so it cannot be breathed out or measured with a simple spirometer. It keeps the alveoli partly inflated, which stops them collapsing and lets gas exchange continue between breaths. Typical values are around 1.0–1.2 litres in an adult, and it is not included in the vital capacity. A spirometer trace therefore shows the volumes a person can move, but the residual volume sits below the trace and is not recorded. When a question asks which volume cannot be measured by spirometry, residual volume is the answer, because the air never leaves the lungs.

A spirometer trace plots the volume of air in the lungs against time as a wavy line. At rest the peaks and troughs are shallow and regular, showing a small tidal volume and a steady breathing rate. At the onset of exercise the trace changes: the peaks rise and the troughs fall, so each wave is taller, showing a larger tidal volume, and the waves come closer together, showing a faster breathing rate. To continue a trace on paper, keep the same rhythm and amplitude pattern and extend the line forward in time. Explain each change by linking it to increased demand for oxygen by the working muscles and the need to remove more carbon dioxide.