Why animals need circulation, and blood vessels

Limits of diffusion

Diffusion is fast only over very short distances. As an organism gets bigger, its surface area to volume ratio falls and the distance from the surface to the inner cells grows. Large, active animals also have a high demand for oxygen and glucose.

So they need a mass transport system: a heart pumps blood in bulk through vessels close to every cell, and diffusion only has to cover the last tiny distance.

Double circulation

Mammals have a double circulation: blood passes through the heart twice per circuit, once to the lungs (pulmonary circulation) and once to the rest of the body (systemic circulation). Returning to the heart after the lungs lets blood be pumped out to the body at high pressure. Fish have a single circulation.

Arteries, veins and capillaries

• Arteries carry blood away from the heart at high pressure: thick walls with elastic fibres (stretch and recoil to smooth flow) and smooth muscle, narrow lumen, collagen for strength.
• Veins return blood at low pressure: thin walls, little elastic tissue, wide lumen, and valves to stop backflow. Skeletal muscles squeeze veins to push blood back.
• Capillaries are where exchange happens: walls one cell thick (endothelium), narrow lumen so red cells pass slowly and close to the wall, huge total surface area.

Worked example

Explain why an artery wall contains elastic fibres.

1. Blood leaves the heart in surges at high pressure.
2. Elastic fibres stretch as pressure rises, then recoil as it falls.
3. This smooths the flow and maintains pressure.

Answer: Elastic recoil evens out the pressure surges, keeping blood flowing smoothly.

Check yourself

1. Why do large animals need a circulatory system?
   • Their cells do not respire
   • Diffusion is faster in large animals
   • Their surface area to volume ratio is small and diffusion distances are too long
   • They have no lungs
   Show answer

   Their surface area to volume ratio is small and diffusion distances are too long. Diffusion alone is too slow over large distances.

2. In a double circulation, blood passes through the heart
   • twice for each complete circuit of the body
   • once for each complete circuit
   • only on its way to the lungs
   • four times per circuit
   Show answer

   twice for each complete circuit of the body. Once in the pulmonary circuit and once in the systemic circuit.

3. Which feature is found in veins but not in arteries?
   • Thick layers of elastic fibres
   • A narrow lumen
   • Valves along their length
   • Blood at high pressure
   Show answer

   Valves along their length. Low pressure in veins means valves are needed to prevent backflow.

4. How are capillaries adapted for exchange?
   • Their walls contain thick smooth muscle
   • Their walls are one cell thick, giving a short diffusion distance
   • They have valves
   • They carry blood at the highest pressure
   Show answer

   Their walls are one cell thick, giving a short diffusion distance. A thin endothelium means a short diffusion path.

5. What is the role of collagen in an artery wall?
   • It contracts to pump blood
   • It allows exchange of gases
   • It forms valves
   • It gives strength so the wall does not burst under high pressure
   Show answer

   It gives strength so the wall does not burst under high pressure. Collagen is a tough fibrous protein that resists high pressure.

6. How is blood helped back to the heart in the veins of the legs?
   • The veins have thick elastic walls that recoil
   • Contracting skeletal muscles squeeze the veins, and valves stop backflow
   • Capillaries pump the blood
   • Gravity pulls it upwards
   Show answer

   Contracting skeletal muscles squeeze the veins, and valves stop backflow. Muscle contraction plus valves keeps blood moving towards the heart.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.