Blood vessels and the components of blood
Three blood vessel types
| Vessel | Direction | Wall | Lumen | Valves | Pressure |
|---|---|---|---|---|---|
| Artery | Away from heart | Thick, muscular, elastic | Narrow | None | High |
| Vein | Back to the heart | Thin | Wide | Yes | Low |
| Capillary | Between arteries and veins | One cell thick | Just wide enough for one RBC | None | Low |
- Arteries — carry oxygenated blood at high pressure (except pulmonary artery). Thick walls cope with high pressure; elastic recoil smooths flow.
- Veins — carry deoxygenated blood (except pulmonary vein). Wide lumen reduces resistance. Valves stop blood flowing backwards.
- Capillaries — site of exchange. Walls are one cell thick so substances diffuse over a short distance.
Components of blood
Blood is a tissue. ~55% plasma, ~45% red blood cells, plus white blood cells and platelets.
- Plasma — straw-coloured liquid that carries dissolved substances: glucose, amino acids, CO_2, urea, hormones, heat.
- Red blood cells (erythrocytes) — biconcave disc, no nucleus (more space for haemoglobin), carry oxygen as oxyhaemoglobin.
- White blood cells (leukocytes) — defend against pathogens. Phagocytes engulf microbes; lymphocytes produce antibodies.
- Platelets — cell fragments that trigger clotting at wounds, sealing the cut and preventing pathogen entry.
WJEC exam tip
When asked to explain an adaptation, link the structure to the job. "Capillary walls are one cell thick B1 so the diffusion distance is short, allowing oxygen and glucose to pass quickly into the tissues B1." Two-mark answers always need two halves.
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