Oxygen falls in partial pressure at every stage between the air and the cell, and each fall has a named cause and a calculable size. This module follows that descent: first getting oxygen into the blood, where humidification, alveolar ventilation, diffusion and ventilation-perfusion matching each take their share; then carrying it, where almost all of it is bound to haemoglobin and the dissociation curve decides how readily it is given up again.
Orientation
What this module covers, and how to work through it
The split between the two lessons is the split between tension and content. Lesson 1 is about partial pressure: what sets the alveolar oxygen tension, what the alveolar-arterial gradient is and why a healthy lung has one at all, and how regional differences in ventilation and perfusion make the lung imperfect by design. Lesson 2 is about how much oxygen is actually carried and delivered, which is a different quantity governed by haemoglobin rather than by pressure.
Keeping those two apart is most of what this module asks. A patient can have a normal arterial oxygen tension and a dangerously low oxygen content; a patient can have a normal content and inadequate delivery. Tension, saturation and content are three different things, and the equations that connect them are worth being able to write from memory.
Prerequisites for the module as a whole
These carry material this module builds on directly and does not repeat. Work through them first if they are not already secure:
- Cellular physiology — membranes and transport: diffusion and its determinants, which Fick’s law of diffusion applies to the alveolar membrane.
- Cardiovascular — the circulation: cardiac output, which is one of the two terms in the oxygen delivery equation and the reason delivery can fail with entirely normal lungs.
Syllabus
MMed syllabus mapping
| Syllabus area | Lessons | What is covered |
|---|---|---|
| Uptake and gas exchange | Lesson 1 | The oxygen cascade from dry atmosphere to mitochondria with normal values and equations; alveolar-capillary transfer, perfusion against diffusion limitation and the alveolar-arterial gradient; regional ventilation and perfusion, the V/Q = 1 point, high and low V/Q, shunt and dead space |
| Carriage and delivery | Lesson 2 | Haemoglobin structure and the T to R allosteric transition; cooperative binding and heterotropic modulation; HbA against HbF and HbS; arterial oxygen content and delivery; the oxyhaemoglobin dissociation curve, P50, shifts, and the Bohr, Haldane and double Bohr effects |
The pathway
All 2 lessons, in order
The two halves of the problem
Getting oxygen into the blood, then carrying it and giving it up. The first is about partial pressure; the second is about content.
- Uptake and ventilation-perfusion matching
The oxygen cascade with its normal values and equations, alveolar-capillary transfer and the alveolar-arterial gradient, and how regional ventilation and perfusion produce shunt at the base and dead space at the apex.
- Carriage and delivery
Haemoglobin structure and cooperative binding, the calculation of arterial oxygen content and delivery, and the dissociation curve with its P50, its shifts and the Bohr and Haldane effects.
Objectives
The full objective wording
- Uptake and V/Q matchingDraw and explain the oxygen cascade from dry atmosphere to mitochondria, including equations and normal PO₂ values; explain alveolar-capillary transfer, perfusion against diffusion limitation, the alveolar-arterial gradient, and regional ventilation-perfusion matching
- Carriage and deliveryExplain haemoglobin structure and the T to R allosteric transition; calculate arterial oxygen content and delivery; and draw and interpret the oxyhaemoglobin dissociation curve, P50, arterial and mixed venous points, shifts, Bohr, Haldane and double Bohr effects