The heart, blood vessels and haemoglobin — Edexcel International AS Level Biology
Why animals need a circulation, how blood vessels suit their jobs, the cardiac cycle, and how haemoglobin carries oxygen and carbon dioxide.
Revision notes
6 short notes, in the order of the specification. Each one in short:
Many animals have a heart and circulation because diffusion alone cannot meet the needs of large organisms. As size increases, the surface area to volume ratio falls and diffusion distances rise, while active animals have a high metabolic rate. Mass transport of blood, pumped under pressure by the heart, supplies cells quickly and removes waste.
Arteries have thick, elastic, muscular walls to withstand high pressure and recoil to smooth the pulse. Veins have thin walls, wide lumens and valves to return blood at low pressure. Capillaries have walls one cell thick and a narrow lumen, giving a short diffusion distance and slow flow for exchange with cells.
Structure of the mammalian heart
PracticalThe mammalian heart has four chambers: two thin-walled atria that receive blood and two thicker-walled ventricles that pump it out. The septum separates oxygenated from deoxygenated blood. Atrioventricular and semilunar valves prevent backflow, and the left ventricle has the thickest wall because it pumps blood at high pressure around the whole body.
The cardiac cycle is the sequence of events in one heartbeat: atrial systole, ventricular systole and cardiac diastole. Contraction of a chamber raises its pressure and forces blood into the next chamber or artery. Valves open and close passively as pressure differences change, so blood flows in only one direction.
Haemoglobin is a protein in red blood cells with four haem groups, each able to bind one oxygen molecule, forming oxyhaemoglobin. Most carbon dioxide is carried as hydrogencarbonate ions in the plasma, formed in red blood cells by carbonic anhydrase. Some binds to haemoglobin and some dissolves in plasma.
The oxygen dissociation curve plots percentage saturation of haemoglobin against the partial pressure of oxygen and is S-shaped. The Bohr effect is the shift of the curve to the right at high carbon dioxide levels, so more oxygen unloads in respiring tissues. Fetal haemoglobin has a higher affinity, so its curve is to the left.
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Quick check questions
Why does a large animal need a circulatory system?
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Its surface area to volume ratio is small and diffusion distances are long, so diffusion alone is too slow to supply all cells.
Why is the left ventricle wall thicker than the right?
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It pumps blood to the whole body at high pressure, whereas the right ventricle only pumps to the lungs.
What are the three stages of the cardiac cycle?
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Atrial systole, ventricular systole and cardiac diastole.
How many oxygen molecules can one haemoglobin molecule carry?
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Four, one on each haem group.
What happens to the oxygen dissociation curve when CO₂ concentration increases?
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It shifts to the right (Bohr effect), so haemoglobin releases more oxygen.
Frequently asked questions
Why do large animals need a heart and circulation?
Large animals have a small surface area to volume ratio and long diffusion distances, and active ones have high metabolic rates, so diffusion alone is too slow. A heart pumps blood in bulk (mass transport) through vessels, delivering oxygen and nutrients to all cells quickly and removing waste.
How are arteries, veins and capillaries different?
Arteries have thick, elastic, muscular walls to withstand high pressure. Veins have thin walls, wide lumens and valves to return blood at low pressure. Capillaries have walls one cell thick and a very narrow lumen, giving a short diffusion distance for exchange between blood and cells.
What happens in the cardiac cycle?
The cardiac cycle has three stages. In atrial systole the atria contract and fill the ventricles. In ventricular systole the ventricles contract, the atrioventricular valves shut and the semilunar valves open to force blood out. In cardiac diastole the heart relaxes and the atria refill.
What is the Bohr effect?
The Bohr effect is the shift of the haemoglobin oxygen dissociation curve to the right when carbon dioxide concentration rises and pH falls. Haemoglobin then has a lower affinity for oxygen, so more oxygen is unloaded at the same partial pressure, supplying respiring tissues.
Why does fetal haemoglobin have a higher affinity for oxygen?
Fetal haemoglobin has a different structure from adult haemoglobin and holds oxygen more tightly, so its dissociation curve is to the left. In the placenta, where pO₂ is low, the mother’s haemoglobin unloads oxygen and fetal haemoglobin loads it.
Written and checked against the Edexcel International AS Level Biology (XBI11) specification · Updated October 2026