8.6 - Blood and its components
Connect the structures of red and white blood cells, plasma and platelets to oxygen transport, defence, clotting and the movement of dissolved substances.
8.6 — Blood is a transport tissue
Blood is a tissue made from cells and cell fragments suspended in a liquid called plasma. Its components travel together, but their different structures give them different jobs.
| Component | Useful structural features | How structure supports function |
|---|---|---|
| Red blood cell (erythrocyte) | A biconcave disc, thin through its centre, flexible, and without a nucleus when mature; packed with haemoglobin | The biconcave shape gives a large surface area and a short route for oxygen exchange. Having no nucleus leaves more space for haemoglobin, which binds oxygen. Flexibility lets the cell deform through narrow capillaries. |
| Phagocyte | A white blood cell with a flexible cell surface and enzyme-containing compartments | Its surface can extend around a pathogen and enclose it. Enzymes then digest the engulfed material: this is phagocytosis. |
| Lymphocyte | A white blood cell with a nucleus and receptor molecules of particular shapes on its surface | The nucleus controls the cell's active response, while a matching receptor allows recognition of a particular antigen. Activated B lymphocytes can develop into cells that produce specific antibodies. |
| Plasma | A water-rich liquid in which substances can dissolve and cells can be suspended | Flowing plasma carries the blood cells and dissolved glucose, amino acids, carbon dioxide, urea, hormones and other substances. It also transfers thermal energy around the body. Most oxygen, however, is carried bound to haemoglobin inside red blood cells. |
| Platelet | A small cell fragment that becomes activated and sticky at a damaged vessel wall | Platelets collect at the break and help trigger formation of a clot. The clot seals the vessel, reducing blood loss and helping block pathogen entry. |
The word structure therefore applies at two scales: the shape and contents of an individual component, and the fact that the solid components are carried in a flowing liquid.
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