e. Venous blood haematocrit is more than arterial blood. Explain.
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venous blood haematocrit higher than arterial blood explanation
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The retrieved result is not directly relevant to haematocrit differences between venous and arterial blood. I'll answer from core physiological knowledge.
Venous Haematocrit > Arterial Haematocrit
This is a well-established physiological fact explained by the chloride shift (Hamburger phenomenon) and the behaviour of red blood cells (RBCs) in venous circulation.
Key Mechanism: The Chloride Shift and RBC Swelling in Venous Blood
In the capillaries and venous circulation, tissues produce CO₂ which enters RBCs. The following sequence occurs:
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CO₂ enters RBCs and is rapidly hydrated by carbonic anhydrase:CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻
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HCO₃⁻ exits the RBC into plasma in exchange for Cl⁻ (chloride shift). This is an electrically neutral exchange via the Band 3 anion exchanger.
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The entry of Cl⁻ into the RBC increases its intracellular osmolality.
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Water follows osmotically into the RBC → the RBC swells slightly.
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Swollen RBCs have a larger volume → venous haematocrit increases compared to arterial blood.
Additional Contributing Factor: Plasma Skimming & Fahraeus Effect
- In small capillaries, RBCs travel faster than plasma (axial streaming), and the capillary haematocrit is lower than large vessel haematocrit.
- When blood re-collects into venules and veins, this redistribution effectively makes venous haematocrit slightly higher than the corresponding arterial side.
Summary Table
| Parameter | Arterial Blood | Venous Blood |
|---|---|---|
| CO₂ content | Low | High |
| HCO₃⁻ in plasma | Lower | Higher |
| Cl⁻ in RBC | Lower | Higher (post chloride shift) |
| RBC water content | Less | More (osmotic water influx) |
| RBC volume | Smaller | Slightly larger |
| Haematocrit | Lower | Higher |
Magnitude
The difference is small but measurable — venous haematocrit is approximately 3% higher than arterial haematocrit, primarily due to the osmotic swelling of RBCs during CO₂ transport.
Clinical Relevance
- This is why blood samples for haematocrit measurement should ideally be standardized in terms of sampling site.
- The venous haematocrit is what is routinely measured in clinical labs (from antecubital venepuncture), and it is slightly higher than true whole-body (large-vessel arterial) haematocrit.
- The spleen can also modestly influence systemic haematocrit by sequestering or releasing RBCs, but this is not the primary explanation for the arteriovenous difference.
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