Oxygen Transport From Capillary Blood To Individual Cells

The delivery of oxygen from capillary blood to the cell depends on:

• factors that influence diffusion (fig 2.4);

• the rate of oxygen delivery to the capillary (Do2);

• the position of the oxygen-haemoglobin dissociation relationship (P50);

• the rate of cellular oxygen utilisation and uptake (Vo2). The sigmoid oxygen-haemoglobin dissociation relationship is influenced by various physicochemical factors and its position is defined by the Pao2 at which 50% of the Hb is saturated (P50), normally 3.5 kPa. An increase in P50 or rightward shift in this relationship reduces the Hb saturation (Sao2) for any given Pao2, thereby increasing tissue oxygen availability. This is caused by pyrexia, acidosis, and an increase in intracellular phosphate, notably 2,3-diphosphoglycerate (2,3-DPG). The importance of correcting hypophosphataemia, often found in diabetic ketoacidosis and sepsis, is frequently overlooked.26

Mathematical models of tissue hypoxia show that the fall in cellular oxygen resulting from an increase in intercapillary distance is more severe if the reduction in tissue Do2 is caused by "hypoxic" hypoxia (a fall in Pao2) rather than "stagnant" (a fall in flow) or "anaemic" hypoxia (fig 2.5).27 Studies in patients with hypoxaemic respiratory failure have also shown that it is Pao2 rather than Do2—that is, diffusion rather than convection—that has the major influence on outcome.9

Thus, tissue oedema due to increased vascular permeability or excessive fluid loading may result in impaired oxygen diffusion and cellular hypoxia, particularly in clinical situations associated with arterial hypoxaemia. In these situations, avoiding tissue oedema may improve tissue oxygenation.

Slow diffusion

(Long diffusion distance \ Low pressure gradient I

Red cell

Rapid diffusion

(Short diffusion distance \ High pressure gradient I

Capillary

Slow diffusion

Red cell

Rapid diffusion

Capillary

Intracellular P02 1.3 kPa

Mitochondrial Pa02 <0.7 kPa

Mitochondrial Pa02 0.7-1.3 kPa

Figure 2.4 Diagram showing the importance of local capillary oxygen tension and diffusion distance in determining the rate of oxygen delivery and the intracellular Po2. On the left there is a low capillary Po2 and pressure gradient for oxygen diffusion with an increased diffusion distance resulting in low intracellular and mitochondrial Po2. On the right the higher Po2 pressure gradient and the shorter diffusion distance result in significantly higher intracellular Po2 values.

Intracellular P02 1.3 kPa

Mitochondrial Pa02 <0.7 kPa

Mitochondrial Pa02 0.7-1.3 kPa

Figure 2.4 Diagram showing the importance of local capillary oxygen tension and diffusion distance in determining the rate of oxygen delivery and the intracellular Po2. On the left there is a low capillary Po2 and pressure gradient for oxygen diffusion with an increased diffusion distance resulting in low intracellular and mitochondrial Po2. On the right the higher Po2 pressure gradient and the shorter diffusion distance result in significantly higher intracellular Po2 values.

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