CB8 · Exchange and transport in animalsTopic 8 — Exchange and transport in animals
Exchange surfaces, circulation and respiration.
Revise the key ideas
Why animals need exchange and transport
Cells need oxygen and glucose for aerobic respiration, water for reactions, and nutrients for growth and repair. Dissolved food molecules and mineral ions must reach the cells that need them.
Digestion breaks proteins into amino acids and lipids into fatty acids and glycerol so these products can be absorbed. Respiration transfers energy from nutrients; it is not the same process as digestion.
Carbon dioxide is a waste product of respiration and is removed through the lungs. Urea is made in the liver during breakdown of excess amino acids and is carried to the kidneys for excretion.
Diffusion is net movement from a higher to a lower concentration. A large exchange area, a short diffusion distance and a steep concentration gradient increase the rate.
Small organisms can exchange substances across their surface because they have a high surface area relative to volume and short distances to internal cells.
Large multicellular organisms have a smaller surface area relative to volume and longer internal distances. Specialised exchange surfaces and a transport system meet the demands that diffusion alone cannot meet.
Surface area to volume calculations
For a cube of side length l, surface area = 6l² and volume = l³. Calculate both using the same length unit before finding surface area : volume.
A cube with 1 cm sides has an area of 6 cm² and a volume of 1 cm³, giving SA:V = 6:1.
A cube with 2 cm sides has an area of 24 cm² and a volume of 8 cm³, giving SA:V = 3:1. Its total area is greater, but its area relative to volume is smaller.
A cube with 3 cm sides has SA:V = 54:27 = 2:1. As size increases, volume grows faster than surface area.As size increases, volume grows faster than surface area.
Lower SA:V means less surface for exchange per unit of tissue. Larger organisms also need transport to overcome long diffusion distances.
Report a simplified ratio in the format requested by a question. Surface area and volume have different units, so the numerical ratio depends on the length unit used.
Alveoli and gas exchange
Alveoli are tiny air sacs in the lungs. Their large combined surface area allows rapid gas exchange between air and blood.
The alveolar wall and the adjacent capillary wall are each one cell thick, creating a short diffusion distance.
Oxygen dissolves in the moist surface lining and diffuses from alveolar air into the blood down its concentration gradient.
Carbon dioxide diffuses from the blood into alveolar air down its own concentration gradient, then is breathed out.
Ventilation replaces air in the alveoli, while a rich blood supply carries oxygen away and brings carbon dioxide. Together they maintain steep concentration gradients.Oxygen and carbon dioxide diffuse in opposite directions down their own gradients.
Breathing is movement of air into and out of the lungs; gas exchange is diffusion between air and blood. Cellular respiration is a chemical process in cells, rather than another name for either.
Blood components and their functions
Blood contains plasma, red blood cells, white blood cells and platelets. Each component has a different role in transport or defence.
Plasma is the liquid carrying cells and dissolved substances, including glucose, amino acids, carbon dioxide, urea and hormones. It also distributes heat.
Red blood cells, or erythrocytes, contain haemoglobin, which binds oxygen in the lungs and releases it in tissues where oxygen concentration is lower.
Red blood cells are biconcave discs: both sides dip inwards. This gives a large surface area compared with their volume and a short diffusion distance. Mature human red blood cells have no nucleus, leaving more room for haemoglobin.
White blood cells defend against pathogens. Phagocytes engulf pathogens; lymphocytes are involved in specific responses, including antibody production and immune memory.
Platelets are cell fragments involved in clotting. A clot reduces blood loss and helps stop pathogens entering through a wound.
Most oxygen is carried bound to haemoglobin in red cells, rather than simply dissolved in plasma. Plasma carries much of the carbon dioxide in dissolved forms.
Arteries, capillaries and veins
Arteries carry blood away from the heart. Thick muscular and elastic walls withstand pressure and stretch and recoil as blood is pumped.
A pulse is the pressure wave associated with heartbeats in arteries. It is not blood flowing alternately forwards and backwards.
Capillaries connect small arteries and veins. Their walls are one cell thick so dissolved substances can exchange over a short distance with surrounding tissues.
Many narrow capillaries form networks with a large combined exchange area and bring blood close to cells. Substances move according to their concentration gradients.
Veins carry blood towards the heart at lower pressure. Compared with similar-sized arteries, they generally have thinner walls and a wider central space (lumen).
Many veins, especially in the limbs, contain valves preventing backflow. Skeletal-muscle contractions help push blood towards the heart.Compare wall thickness and lumen; sketches are not to scale. Vein valve shown schematically.
Artery and vein are defined by direction of flow, not oxygen content. The pulmonary artery carries deoxygenated blood to the lungs; pulmonary veins carry oxygenated blood back.
The heart and double circulation
The heart is a muscular pump with four chambers: right and left atria above right and left ventricles. The atria receive blood; the ventricles pump it out.
Humans have a double circulatory system: the pulmonary circuit links heart and lungs, and the systemic circuit links heart and the rest of the body. Blood passes through the heart twice in one complete circuit.
Deoxygenated blood returns from the body in the venae cavae to the right atrium. It passes through a valve to the right ventricle, then through another valve into the pulmonary artery to the lungs.
Oxygenated blood returns from the lungs through pulmonary veins to the left atrium. It passes through a valve to the left ventricle, then through another valve into the aorta to the body.
Valves prevent backflow when pressure changes during contraction and relaxation. Atria contract to move blood into ventricles; ventricular contraction pumps blood out.
The left ventricle has a thicker muscular wall than the right because it must generate higher pressure to pump around the whole body. The right ventricle pumps to the nearby lungs.
The septum separates the right and left sides, preventing oxygenated and deoxygenated blood mixing within a healthy heart. Right and left are the person's anatomical sides, not necessarily the viewer's sides on a diagram.Follow the arrows through pulmonary and systemic circuits. Chamber boxes show routes rather than anatomical shape; additional outflow valves are described in the notes.
Coronary arteries supply the heart muscle itself with oxygen and glucose. A blockage can damage or kill part of that muscle in a heart attack; a heart attack is not automatically the same event as the heart stopping in cardiac arrest.
Cardiac output and exercise
Heart rate is the number of beats per minute. Stroke volume is the volume pumped by one ventricle in each beat; cardiac output is the volume pumped by one ventricle per minute.
Cardiac output = stroke volume × heart rate. If stroke volume is in cm³ per beat and heart rate in beats per minute, output is in cm³ per minute.
For a stroke volume of 70 cm³ and a heart rate of 75 beats per minute, cardiac output = 70 × 75 = 5,250 cm³ per minute, or 5.25 litres per minute.
Rearrange when needed: stroke volume = cardiac output ÷ heart rate, and heart rate = cardiac output ÷ stroke volume. Keep volume units consistent.
During exercise, muscles need more energy transferred by respiration. Heart rate and often stroke volume increase, delivering more oxygen and glucose and removing more carbon dioxide.
Regular training can increase stroke volume. A trained person may have a lower resting heart rate while maintaining a similar resting cardiac output; a lower rate alone does not prove greater fitness.
Measure pulse over a known time and convert to beats per minute. Repeated readings improve comparisons; keep exercise intensity and measurement conditions consistent.
Aerobic respiration and energy transfer
Cellular respiration consists of reactions that transfer energy from glucose and other fuels for cell processes. It occurs continuously in living cells and is exothermic overall.
Energy is transferred for movement, active transport, building larger molecules and maintaining body temperature. Energy is not a substance made from nothing or an extra atom in a balanced equation.
The word equation for aerobic respiration is glucose + oxygen → carbon dioxide + water. Energy is transferred to useful processes and the surroundings.
The balanced symbol equation is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. Oxygen is required for aerobic respiration.
Most reactions of aerobic respiration take place in mitochondria in eukaryotic cells. The first stage takes place in the cytoplasm, so not every respiration reaction happens in mitochondria.
Do not confuse respiration with breathing, or say that plants only respire at night. Plants and animals respire continuously; photosynthesis is a separate light-dependent process.
Anaerobic respiration and recovery
If muscles cannot get enough oxygen during vigorous exercise, anaerobic respiration transfers energy from glucose without using oxygen. It takes place in the cytoplasm.
In animal muscle, the word equation is glucose → lactic acid. Lactic acid is commonly called lactate in the body; energy is transferred, but less per glucose molecule than in aerobic respiration.
Anaerobic activity can support intense exercise for a short time, but cannot sustain the same energy demand indefinitely. Fatigue has several causes and should not be explained only as lactic acid poisoning.
Breathing and heart rate stay higher for a while after exercise. The extra oxygen helps break down lactate and restore energy and oxygen stores. At GCSE, this extra oxygen needed for recovery is called the oxygen debt.
Lactate travels in the blood to organs including the liver. It can be broken down using oxygen (oxidised) or converted back into glucose. Carbon dioxide produced during its breakdown leaves through the lungs; lactate itself is not breathed out.
In yeast, anaerobic respiration produces ethanol and carbon dioxide: glucose → ethanol + carbon dioxide. This differs from anaerobic respiration in human muscle.
Compare the processes: aerobic respiration requires oxygen and releases more energy per glucose; anaerobic respiration uses no oxygen and releases less, with different products in muscle and yeast.Word summaries compare products and energy transfer; aerobic respiration also consumes oxygen.
Core practical: respiration with a respirometer
A respirometer estimates respiration rate by measuring oxygen consumption. Investigate the effect of temperature on small organisms using a simple tube apparatus in water baths at different temperatures.
Place soda lime or another suitable carbon dioxide absorbent in the tube, separated from the organisms by cotton wool. It absorbs carbon dioxide released during respiration, so the gas-volume decrease reflects oxygen uptake.
Seal the apparatus and connect it to a capillary with coloured liquid. As oxygen is consumed and carbon dioxide is absorbed, pressure inside falls and the liquid moves towards the organism chamber.Carbon dioxide is absorbed, so the fall in gas volume estimates oxygen consumed.
Allow the apparatus and organisms to reach the water-bath temperature before timing. Record liquid displacement over a fixed interval; use the same capillary cross-section for comparisons.
Control organism species and mass or number, measurement time, apparatus volume and other relevant conditions. Compare with a suitable non-respiring control to account for pressure changes unrelated to respiration.
Rate can be expressed as distance moved ÷ time for the same capillary. If area is known, oxygen volume = capillary cross-sectional area × distance moved; volume rate = volume ÷ time.
For a capillary area of 0.20 mm² and movement of 15 mm in 3 minutes, oxygen uptake = 3 mm³ and rate = 1 mm³ per minute. To compare organisms of different masses, divide the volume rate by mass to get the rate per gram (a mass-normalised rate).
Repeat at each temperature and calculate a mean. Interpret increases over a safe temperature range using enzyme activity; avoid assuming that the rate increases indefinitely at damaging temperatures.
Keep apparatus airtight, use eye protection and handle the absorbent under school instructions. Protect organisms from absorbent contact, extremes of temperature and prolonged oxygen depletion; minimise stress and return them to suitable conditions.
Interpreting respiration measurements
An airtight seal matters because air leaking in can hide the decrease caused by oxygen uptake. Gas volume also changes with temperature. Allow the apparatus to reach the water-bath temperature and use a non-respiring control so these changes do not distort the result.
Use consistent units and state whether a result measures displacement, gas volume per time or volume per time per gram. Do not describe oxygen uptake and carbon dioxide production as the same measured quantity.
A yeast investigation can compare carbon dioxide production under controlled glucose concentrations and temperatures. Under anaerobic conditions it measures fermentation output, rather than oxygen consumption.
Compare means from repeat readings and identify anomalies before drawing conclusions. Change one independent variable at a time and control the others.
When comparing exercise data, distinguish an immediate response to activity from longer-term training adaptations. A higher exercise heart rate does not mean the person necessarily has a higher resting rate.
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