How does haemoglobin load and unload oxygen, and how does the heart move blood around the body?
Mass transport in animals: the role of haemoglobin in oxygen transport, the oxygen dissociation curve and the Bohr effect; the structure of the heart and the cardiac cycle; the structure of arteries, veins and capillaries in relation to function.
An AQA A-Level Biology answer on mass transport in animals. Explains cooperative oxygen binding by haemoglobin, the sigmoid dissociation curve and the Bohr effect, the structure of the heart, the cardiac cycle and pressure changes, and how vessels are adapted to function.
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What this dot point is asking
AQA wants you to explain how haemoglobin transports oxygen (loading, unloading, cooperative binding and the Bohr effect), interpret the oxygen dissociation curve, describe the structure of the heart and the cardiac cycle with its pressure changes, and relate the structure of arteries, veins and capillaries to their functions.
Haemoglobin and oxygen transport
Haemoglobin (Hb) is a globular protein with four polypeptide chains, each containing a haem group that binds one oxygen molecule - so one haemoglobin carries up to four oxygen molecules.
- Loading (association) occurs where the partial pressure of oxygen (pO2) is high, e.g. the lungs.
- Unloading (dissociation) occurs where pO2 is low, e.g. respiring tissues.
The binding is cooperative: when the first oxygen binds, it changes the shape of haemoglobin, making it easier for the next oxygens to bind. This is why the dissociation curve is S-shaped (sigmoid), not a straight line.
The oxygen dissociation curve
The curve plots percentage saturation of haemoglobin against partial pressure of oxygen.
- At high pO2 (lungs): haemoglobin is nearly fully saturated (high affinity), so it loads oxygen.
- At low pO2 (tissues): saturation falls steeply, so haemoglobin readily unloads oxygen.
- The steep middle section means a small fall in pO2 (in active tissue) causes a large release of oxygen - perfect for delivery.
The Bohr effect
When carbon dioxide concentration rises (in actively respiring tissue), the curve shifts to the right. This is the Bohr effect.
- A right-shifted curve means lower affinity for oxygen at any given pO2.
- So haemoglobin unloads more oxygen to the tissues that need it most.
This is adaptive: active tissue produces more carbon dioxide, which automatically triggers more oxygen release exactly where demand is highest.
Organisms in low-oxygen environments (e.g. a foetus, or a llama at altitude) have haemoglobin with a higher affinity for oxygen - a left-shifted curve - so they can still load oxygen where pO2 is low. Foetal haemoglobin must have a higher affinity than maternal haemoglobin to draw oxygen across the placenta.
Structure of the heart
The mammalian heart has four chambers: two atria (thin-walled, receive blood) and two ventricles (thick-walled, pump blood out).
- The right side pumps deoxygenated blood to the lungs (pulmonary circuit).
- The left side pumps oxygenated blood to the body (systemic circuit) - its wall is thicker because it generates higher pressure.
- Atrioventricular valves (tricuspid right, bicuspid left) prevent backflow from ventricles to atria.
- Semilunar valves (in the aorta and pulmonary artery) prevent backflow into the ventricles.
- Coronary arteries supply the heart muscle itself with oxygenated blood.
The cardiac cycle
The cardiac cycle is one full heartbeat, driven by pressure changes. Valves open and close because of pressure differences: a valve opens when pressure behind it is higher, and closes (preventing backflow) when pressure in front becomes higher.
Vessels and their functions
| Vessel | Key structural features | Why |
|---|---|---|
| Artery | Thick wall, thick layer of elastic tissue and smooth muscle, narrow lumen | Withstands and maintains high pressure; elastic recoil smooths blood flow between heartbeats |
| Capillary | One-cell-thick endothelium, very narrow lumen, large total surface area | Short diffusion path and large area for rapid exchange of substances with tissues |
| Vein | Thin wall, large lumen, valves | Low pressure; valves prevent backflow; large lumen reduces resistance and acts as a blood reservoir |
Try this
Q1. Explain why the oxygen dissociation curve for haemoglobin is S-shaped. [3 marks]
- Cue. Binding of the first oxygen is difficult (flat start); it changes haemoglobin's shape so subsequent oxygens bind more easily (steep middle - cooperative binding); the last binding site is hard to fill as the molecule saturates (flat top).
Q2. During ventricular systole, explain why the atrioventricular valves close but the semilunar valves open. [3 marks]
- Cue. Ventricular contraction raises ventricular pressure above atrial pressure, closing the atrioventricular valves to prevent backflow into the atria; when ventricular pressure exceeds arterial pressure, the semilunar valves are forced open and blood leaves the heart.
Q3. A high-altitude bird has haemoglobin with a dissociation curve to the left of a lowland bird's. Explain the advantage. [3 marks]
- Cue. A left-shifted curve means higher affinity for oxygen at a given pO2, so the high-altitude bird's haemoglobin can still load oxygen effectively where the partial pressure of oxygen in the air is low, ensuring adequate oxygen uptake.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2019 AQA Paper 24 marksExplain how the Bohr effect helps to supply oxygen to respiring muscle tissue during exercise.Show worked answer →
A 4-mark answer needs the rise in carbon dioxide, the shift of the curve, the lower affinity, and the outcome at the tissues.
- Point 1 (more CO2)
- During exercise, respiring muscle produces more carbon dioxide, lowering the local pH (more acidic).
- Point 2 (curve shifts right)
- This shifts the oxygen dissociation curve to the right - the Bohr effect.
- Point 3 (lower affinity)
- At a right-shifted curve, haemoglobin has a lower affinity for oxygen at any given partial pressure, so it releases (unloads) oxygen more readily.
- Point 4 (outcome)
- More oxygen is therefore released to the respiring muscle exactly where, and when, demand is highest, supporting aerobic respiration.
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