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    61 Oxygenation velocity of RBC
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    61 Oxygenation velocity of RBC

    Red cell oxygenation rate: oxygen uptake in lung capillaries, the haemoglobin–oxygen dissociation curve and what slows oxygenation.

    The oxygenation rate of red blood cells describes how quickly red cells take up oxygen in the lungs and release it in the tissues. In the lung capillaries, oxygen diffuses from the alveoli across a very thin membrane into the plasma and red cells, where it binds to haemoglobin. At rest a red cell spends about 0.75 seconds in a lung capillary, but it is normally almost fully saturated after about a third of that time. This reserve allows oxygenation to be maintained even when blood flows faster, as during exercise. Oxygen uptake and release depend on the properties of haemoglobin, the red cell membrane, the acidity and temperature of the blood and the red cell content of 2,3-diphosphoglycerate (2,3-DPG).

    Red blood cellsGas exchangeHaemoglobin
    Capillary transit at rest≈ 0.75 s
    Time to equilibrium≈ 0.25 s
    P50 of haemoglobin≈ 26–27 mmHg
    Arterial saturation≈ 95–98%
    Haemoglobin–oxygen dissociation curve: S-shaped curve of oxygen saturation against partial pressure of oxygen, with P50 near 27 mmHg; a shift to the right releases oxygen more easily, a shift to the left holds it more tightly Haemoglobin–oxygen dissociation curve 020406080100 PO₂ (mmHg) 050100% P50 ≈ 27 right shift: acidosis, ↑CO₂, ↑temperature, ↑2,3-DPG left shift: alkalosis, cold, CO, fetal haemoglobin
    Figure 1. Oxygen saturation of haemoglobin as a function of oxygen partial pressure (schematic). A right shift (dashed blue) favours oxygen release to the tissues; a left shift (dashed green) favours oxygen binding.

    What slows oxygenation

    • Thickened or damaged alveolar–capillary membrane: pulmonary fibrosis, interstitial lung disease, pulmonary oedema, pneumonia; the problem becomes evident during exercise when transit time shortens
    • Reduced capillary bed: emphysema, pulmonary embolism
    • Haemoglobin and red cell changes: anaemia, carbon monoxide, methaemoglobin, haemoglobin variants; altered red cell membranes (for example oxidative damage by lipid peroxidation)
    • Low inspired oxygen: high altitude

    Release of oxygen in the tissues is enhanced by acidity, carbon dioxide, heat and 2,3-DPG (the Bohr effect), which shift the dissociation curve to the right.

    When to see a doctor

    Breathlessness on exertion, bluish lips, oxygen saturation below about 94% at rest or a fall in saturation during exercise need medical assessment.

    References

    1. West JB, Luks AM. West’s Respiratory Physiology: The Essentials. 10th ed. Philadelphia: Wolters Kluwer; 2016.
    2. Collins JA, Rudenski A, Gibson J, Howard L, O’Driscoll R. Relating oxygen partial pressure, saturation and content: the haemoglobin–oxygen dissociation curve. Breathe. 2015;11(3):194–201.

    This article is for general information and does not replace a consultation with a doctor. Measurements must be interpreted by a physician together with symptoms and other tests.

    Published on 29 April 2024