Blood Vessels and Haemoglobin
Arteries: thick muscle and elastic tissue, narrow lumen, high pressure. Veins: wide lumen, thin walls, valves, low pressure. Capillaries: one-cell-thick endothelium, short diffusion distance. Tissue fluid forms at the arterial end of capillaries where hydrostatic pressure exceeds the water potential effect of plasma proteins; it returns at the venous end and via lymph. Haemoglobin binds O₂ cooperatively, giving an S-shaped (sigmoid) dissociation curve: high saturation in lungs, unloading in respiring tissues. Bohr shift: more CO₂ lowers affinity (curve shifts right), so more O₂ is released where it's needed. Fetal haemoglobin has higher affinity (curve left). CO₂ is carried mostly as HCO₃⁻ (carbonic anhydrase in red cells), with the chloride shift balancing charge.
Learning objectives
- Relate vessel structure to function.
- Explain tissue fluid formation.
- Interpret oxygen dissociation curves and the Bohr shift.
- Describe CO₂ transport.
Why does haemoglobin release more oxygen to a muscle during exercise?
Core idea
Haemoglobin grabs oxygen in the lungs and lets it go in your muscles, more so when you exercise.
The sigmoid dissociation curve lets haemoglobin load fully in the lungs and unload in tissues; CO₂ shifts it right to release more.
Balance of hydrostatic pressure and water potential.
98% saturated in lungs, 30% in muscle; 20 cm³ O₂ per 100 cm³ when saturated. O₂ released?
- Goal
- cm³.
- Strategy
- Difference × capacity.
- Fall in saturation.
- Per 100 cm³ blood.
Summary
- Vessel structure.
- Tissue fluid.
- Bohr shift right.
- Bohr shift
- Lower O₂ affinity of haemoglobin at higher CO₂.
- Tissue fluid
- Fluid bathing cells, formed from plasma.
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