What you need to know
- Describe plasma and the substances it transports.
- Relate red blood cell adaptations to oxygen transport.
- Compare phagocytes and lymphocytes.
- Explain why clotting is important.
- Recognise blood components in a smear and apply oxygen-loading ideas.
Plasma: the transport medium
Plasma is the pale liquid component of blood. It carries blood cells and dissolved substances around the body.
- Digested nutrients such as glucose and amino acids travel from the small intestine.
- Carbon dioxide travels from respiring tissues towards the lungs, much of it in dissolved converted form.
- Urea travels from the liver to the kidneys.
- Hormones travel from glands to target organs.
- Antibodies, ions and plasma proteins are carried in solution.
- Heat is distributed from active organs to other parts of the body.
Red blood cells and oxygen
Red blood cells contain haemoglobin, a protein that binds oxygen reversibly. In the lungs, where oxygen concentration is high, haemoglobin loads oxygen to form oxyhaemoglobin. In respiring tissues, where oxygen concentration is lower, oxygen is released.
Biconcave shape
Provides a large surface area relative to volume and a short diffusion path.
No nucleus when mature
Leaves more internal space for haemoglobin.
Flexible membrane
Allows cells to squeeze through narrow capillaries.
Misconception AlertRed blood cells carry oxygen in their nucleus.Select to reveal the correctionSelect to hide the correction
Mature mammalian red blood cells have no nucleus. Haemoglobin in their cytoplasm carries oxygen.
Extended application: the tendency of haemoglobin to load or unload oxygen depends on oxygen availability and conditions in the tissue. This allows loading at the lungs and unloading where respiration is active.
White blood cells and defence
| Cell type | Main action |
|---|---|
| Phagocyte | Engulfs pathogens by phagocytosis and digests them. |
| Lymphocyte | Recognises foreign antigens and produces specific antibodies; some form memory cells. |
Antibodies bind specifically to antigens. This can neutralise a pathogen or mark it for destruction. Some lymphocytes also produce antitoxins that neutralise toxins. White blood cells have nuclei and are generally larger and much less numerous than red blood cells.
Platelets and blood clotting
Platelets are small cell fragments. When a blood vessel is damaged, they trigger a sequence of reactions that produces a mesh of insoluble fibrin. The mesh traps blood cells and forms a clot.
Clotting reduces blood loss and creates a barrier that makes it harder for pathogens to enter. Later, repair tissue replaces the temporary clot.
A small cell fragment involved in initiating blood clotting.
Recognising cells in a blood smear
- Red blood cells are numerous, similarly sized and appear as pale-centred discs.
- White blood cells are less common, larger and have visible stained nuclei.
- Platelets appear as much smaller fragments.
A two-dimensional smear does not show cells at their true depth and preparation can distort shape. Use a scale bar rather than judging absolute size by eye.
To estimate the proportion of a cell type, count several randomly selected fields of view and calculate a mean. A larger sample reduces random sampling variation.
Quick retrieval check
1. Name four substances transported in plasma.
2. How does the biconcave shape help a red blood cell?
3. How do phagocytes defend the body?
4. Give two benefits of clot formation.
Exam connection
A patient has a greatly reduced platelet count. Predict two consequences and explain them.
Show the mark points
- Clots form more slowly or less effectively.
- Bleeding may continue for longer, increasing blood loss.
- The wound barrier forms less effectively, increasing opportunity for pathogen entry.