Table of Contents

The CO₂ dissociation curve describes the change in the total CO₂ content of blood which occurs with changing partial pressure of CO₂. This curve is more linear and steep than the oxygen-haemoglobin dissociation curve. It has no plateau, and as the result of this, shunt has little effect on CO2 (increasing the ventilation of already well-ventilated regions will improve CO2 exchange, even though it will not improve oxygenation).

The Haldane effect is a physicochemical phenomenon which describes the increased capacity of blood to carry CO₂ under conditions of decreased haemoglobin oxygen saturation Both Haldane and Bohr effects are the same features of the same phenomenon. Haldane effect is what happens to pH and CO₂ binding because of oxygen, and Bohr effect is what happens to oxygen binding because of CO₂ and lower pH. .

This chapter focuses on the transport of CO₂ in the bloodstream, which is an issue at least as important as the transport of oxygen.  CO₂ is transported by three major mechanisms: as bicarbonate, as carbamates, the conjugate bases of carbamino acids and as dissolved CO₂ gas.

The chloride shift or "Hamburger effect" describes the movement of chloride into RBCs which occurs when the buffer effects of deoxygenated haemoglobin increase the intracellular bicarbonate concentration, and the bicarbonate is exported from the RBC in exchange for chloride. This results in a difference of 2-4 mmol/L of chloride between the arterial and venous blood (and a similar difference in bicarbonate concentration).

The changes in respiratory physiology associated with age can be summarised as "everything gets worse". TLC remains the same, but RV FRC and closing capacity increase, so that by age 44 the closing capacity is greater than supine FRC and atelectasis ensues. Age-related changes to muscle fibres and connective tissue increase the work of breathing by degrading lung compliance at the same time as they disable the things which are doing all the work (respiratory muscles and diaphragm). Gas exchange is affected by increasing alveolar dead space and V/Q scatter, which decreases the efficiency of oxygenation and ventilation. The overall effect of these changes is a diminished capacity to tolerate physiological stress.

The Bohr effect describes the decrease in the oxygen affinity of haemoglobin in the presence of low pH or high CO₂. Both pH and CO₂ stabilise the deoxyhaemoglobin molecule and decrease its affinity for oxygen, which facilitates the release of oxygen in the peripheral tissues. Quantitatively, the changes in pH play a greater role in changing the shape of the oxygen-haemoglobin dissociation curve than do the changes in CO₂.

There are several major physiological factors which influence the affinity of haemoglobin for oxygen. Some of these are under our control. The p50 value as reported by the arterial blood gas analyser presents us with a short-hand way of determining whether the curve has shifted to the right or to the left. Simply put, p50 is the partial pressure of oxygen which is required to saturate 50% of haemoglobin. Thus, an increasing p50 would suggest that the affinity of haemoglobin for oxygen is decreasing - it takes more oxygen to achieve 50% saturation.

This chapter makes a half-arsed attempt to explore the fascinating behaviour of haemoglobin, and to discuss it in colourful toddler-like crayon diagrams. Of these diagrams, perhaps the most important is the sigmoidal curve itself, as it is often expected of the exam candidate to discuss its "left shift" and "right shift", a representation of changing affinity of haemoglobin for oxygen under the influence of various changing physiological parameters. The p50 value as reported by the arterial blood gas analyser represents a mid-point in this curve, and gives us useful information regarding this changing affinity.

This is the abnormal physiological state which occurs when whole-body ischaemia is followed by whole-body reperfusion.

In summary, it is a systemic inflammatory state which resembles every other form of vasodilatory shock; the degree of organ dysfunction depends on the sensitivity of those organs to ischaemia, and the duration of ischaemic time.