Transport of CO2 in blood and interstitial tissues
When cells use O2, most produce PO2, and this modification increases intracellular PCO2; Because intracellular PCO2 is elevated, CO2 diffuses from the cells into the capillaries and is then transported in the blood to the lungs.
Diffusion of CO2 from cells into capillaries in peripheral tissues and from pulmonary capillaries into alveoli
After oxygen is diffused from the alveoli into the pulmonary blood, it is transported almost completely to the capillaries in the tissues in the form of bound haemoglobin. The presence of Hb in red blood cells allows the blood to transport 30 to 100 times more O2 than dissolved O2 in the blood. In the cells of the body's tissues, O2 reacts with many substances to produce CO2. This CO2 enters the capillaries in the tissues and is transported back to the lungs. CO2, like O2, combines with chemicals in the blood to increase CO2 transport by 15-20 times.
When cells use O2, most produce PO2, and this modification increases intracellular PCO2; Because intracellular PCO2 is elevated, CO2 diffuses from the cells into the capillaries and is then transported in the blood to the lungs. In the lungs, CO2 diffuses from the pulmonary capillaries into the alveoli and is excreted.
Thus, at each position in the gas transport chain, CO2 diffuses in the exact opposite direction of the diffusion of O2. However, there is a big difference between the diffusion of CO2 and O2: CO2 can diffuse about 20 times faster than O2. Thus, in each case, the pressure differential needed to cause CO2 diffusion is less than the pressure differential needed to cause O2 diffusion. The following is the CO2 pressure distribution at different positions:
Figure. Absorption of carbon dioxide into the blood in the tissue capillaries (intracellular PCO = 46 mmHg, and in the interstitial fluid 45 mmHg).
Figure. Diffusion of carbon dioxide from the pulmonary blood into the alveoli.
Intracellular PCO2: 46 mm Hg; PCO2 in the interstitial tissue: 45 mm Hg. Thus, the differential pressure is only 1 mm Hg, shown in Fig.
PCO2 of arterial blood when entering tissues: 40 mm Hg; Venous blood PCO2 out of tissue: 45 mm Hg. As shown in the figure, the tissue capillary blood has almost reached equilibrium with PCO2 at the interstitial space of 45 mm Hg.
PCO2 at the end of the capillary artery is 45 mm Hg entering the pulmonary capillaries; The PCO2 of the air in the alveolar lumen is 40 mm Hg. Thus, only a pressure gradient of 5 mm Hg is required for the diffusion of CO2 out of the pulmonary capillaries into the alveoli. Furthermore, as shown, the PCO2 of the pulmonary capillary blood drops to 40 mm Hg -nearly equal to the PCO2 in the alveoli after it has passed more than a third of the way through the capillaries. The effect is similar to that observed in the previous O2 diffusion, except that O2 diffuses in the opposite direction.
Effect of blood flow and tissue metabolism on interstitial PCO2
Capillary blood flow and tissue metabolism affect PCO2 in the opposite way to tissue PO2. The figure shows the following effects:
Figure. Effect of blood flow and metabolic rate on interstitial fluid PCO2.
A decrease in blood flow from normal (score A) to one-quarter of normal (score B) will increase peripheral tissue PCO2 from the normal value of 45 mm Hg to a high of 60 mm Hg. In contrast, in tissue capillaries, an increase in blood flow to six times normal (point C) will decrease the interstitial PCO2 from the normal value of 45 mm Hg to 41 mm Hg, down to an approximately equal with PCO2 in arterial blood (40 mm Hg).
A 10-fold increase in tissue metabolism results in a significant increase in interstitial fluid PCO2 at all blood flow levels, while a one-quarter decrease in metabolism causes interstitial fluid PCO2 to drop to about 41 mm Hg, approximately reaches its value in the arterial blood of 40 mm Hg.