2.7.1Inquiry Process
Blood Vessels — IB Biology HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 73 of 160
Part 1 of 4Arteries and Arterioles
Arteries carry blood away from the heart at high pressure.
Thick muscular wall
- Arteries have a wall with smooth muscle and collagen that withstands the high pressure produced by each heart beat without bursting.
Elastic fibres
- Elastic tissue stretches as the heart pumps and recoils afterwards, which smooths the flow and maintains pressure between beats.
Narrow lumen
- A small lumen relative to wall thickness helps keep blood pressure high as blood travels to the organs.
Arterioles
- Smaller branches with muscle in their walls can constrict or dilate to control how much blood reaches a particular tissue.
Part 2 of 4Capillaries
Wall one cell thick
- The wall consists of a single layer of endothelial cells, giving a very short diffusion distance between blood and tissue.
Narrow diameter
- Capillaries are only just wide enough for a red blood cell, which slows blood flow and brings cells close to the wall.
Gaps between cells
- Small spaces between endothelial cells allow plasma and dissolved substances to leak out and form tissue fluid.
Capillary beds
- Extensive networks of capillaries give a large total surface area for exchange of oxygen, nutrients, carbon dioxide and wastes.
Part 3 of 4Veins and Comparison
Wide lumen
- Veins offer little resistance to the slow, low-pressure flow of blood returning to the heart.
Valves
- Pocket valves prevent backflow and ensure blood travels in one direction towards the heart.
Thin wall
- Because pressure is low, less muscle and elastic tissue are needed, so the wall is thinner than in an artery.
Muscle assistance
- Skeletal muscle contractions squeeze veins, pushing blood forwards past the valves.
| Feature | Artery | Capillary | Vein |
|---|---|---|---|
| Pressure | High | Falling | Low |
| Wall | Thick, elastic | One cell thick | Thin |
| Lumen | Narrow | Very narrow | Wide |
| Valves | No (except at heart) | No | Yes |
Part 4 of 4Exam Checklist & Tips
2.7.1 – Key Points for the Exam
- Artery A vessel that carries blood away from the heart under high pressure.
- Capillary A very narrow vessel whose thin wall allows exchange between blood and tissues.
- Vein A vessel that returns blood to the heart at low pressure, with valves.
- Endothelium The single layer of flat cells lining all blood vessels.
- Lumen The central space inside a vessel through which blood flows.
Common Mistakes
- Stating that arteries always carry oxygenated blood; the pulmonary artery carries deoxygenated blood.
- Saying veins have thick walls because they carry blood back to the heart.
- Writing that capillary walls have no cells; they are made of one layer of endothelial cells.
- Claiming valves are found in arteries; they occur in veins and in the heart outlets.
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