Mass transport and blood vesselsEdexcel International A Level Biology: Revision notes
Section 1
Why large animals need a circulatory system
In small organisms such as an amoeba, oxygen and nutrients reach every cell by diffusion across the body surface. This works because the surface area to volume ratio is large and the diffusion distance is short.
In a large animal:
- the surface area to volume ratio is small, so the surface is too small to supply all the cells
- the distance from the surface to the inner cells is long, and diffusion is too slow over long distances
- the metabolic rate is often high, so cells need a lot of oxygen and glucose and make a lot of waste
Large animals therefore have a circulatory system: a pump (the heart) moves blood around the body in vessels, an example of mass transport. Mass transport is the bulk movement of fluid, and is much faster than diffusion. It also keeps concentration gradients steep at exchange surfaces and cells.
Link all three: small surface area to volume ratio, long diffusion distance and high metabolic rate.
Section 2
The circulatory system
Mammals have a closed circulatory system: blood stays within vessels. Blood leaves the heart in arteries, flows through capillaries in the tissues, and returns to the heart in veins.
Section 3
Arteries
Arteries carry blood away from the heart at high pressure.
- Thick wall containing collagen to withstand the pressure and prevent bursting
- Elastic tissue that stretches as the ventricle contracts and recoils as it relaxes, keeping pressure and flow smooth
- Smooth muscle that contracts or relaxes to change the diameter of the lumen
- Narrow lumen which helps maintain high pressure
- Inner lining of endothelium, which is smooth to reduce friction
Section 4
Capillaries
Capillaries are the site of exchange between blood and cells.
- Wall is a single layer of endothelial cells, so a short diffusion distance
- Very narrow (about the width of a red blood cell), so red blood cells pass slowly and close to the wall
- Form extensive networks, giving a large surface area and slow flow so there is time for exchange
- Gaps (pores) between cells allow some substances to leave
Section 5
Veins
Veins carry blood back to the heart at low pressure.
- Wide lumen gives little resistance to flow
- Thin wall with little muscle and elastic tissue, because the pressure is low
- Valves prevent backflow
- Contraction of surrounding skeletal muscles squeezes the veins and moves blood towards the heart
Arteries do not have valves along their length. Valves are in veins (and the heart).
Must Know
- Large animals need a heart and circulation because of a small surface area to volume ratio, long diffusion distance and high metabolic rate
- Arteries: thick, elastic, muscular walls for high pressure
- Capillaries: one cell thick, narrow, in large networks for exchange
- Veins: thin walls, wide lumen, valves for low-pressure return
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Mass transport and blood vessels
- An amoeba is a single-celled organism less than 1 mm across and it obtains oxygen by diffusion across its cell surface membrane. A horse is a large mammal with a high metabolic rate and cannot obtain oxygen for its cells in this way.A flatworm is a thin, flat animal that has no circulatory system. Explain why it can survive without one.2 marks
- Blood flows from the heart to the leg muscles of a runner through an artery, passes through capillaries in the muscle and returns to the heart through a vein.Explain how the structure of a capillary makes it suitable for exchanging oxygen and glucose with the muscle cells.2 marks
- A student examines cross-sections of an artery and a vein from the same mammal. The artery has a much thicker wall than the vein, and the vein has a wider lumen than the artery.Explain how the structure of the vein is suited to carrying blood back to the heart.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).