Transport of O2 in blood and interstitial tissues
Gases can move from one place to another by diffusion and the cause of this movement is the difference in pressure separation from the first position to the next.
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.
Transporting oxygen from the lungs to the tissues
Gases can move from one place to another by diffusion and the cause of this movement is the difference in pressure separation from the first position to the next. Thus, O2 diffuses from the alveoli into the pulmonary capillary blood because the O2 (PO2) pressure in the alveoli is greater than the PO2 in the pulmonary capillary blood. In other tissues of the body, PO2 is higher in capillary blood than in tissues causing O2 diffusion into the cells.
In contrast, when O2 is converted in cells to CO2, the intracellular CO2 pressure increases, causing CO2 diffusion into the tissue capillaries. After blood enters the lungs, CO2 diffuses out of the blood into the alveoli because the PCO2 in the pulmonary capillary blood is greater than in the alveoli.
Thus, the transport of O2 and CO2 by the blood depends on both their diffusion and the flow of the bloodstream.
Diffusion of oxygen from alveoli into the blood by pulmonary capillaries
The upper part of the figure, showing an alveolar adjacent to a pulmonary capillary, illustrates O2 diffusion between alveolar air and pulmonary blood. The mean PO2 of the alveolar gaseous O2 is 104 mm Hg, while the PO2 of venous blood entering the pulmonary capillary at its arterial end is an average of only 40 mm Hg, because a large amount of O2 has left the blood. as it passes through tissues in the periphery. Therefore, the initial oxygen gradient that causes O2 diffusion into the pulmonary capillaries is 104-40, or 64 mmHg.
Figure. Oxygen uptake into the pulmonary capillary blood.
In the graph in the lower part of the figure above, the curve shows a rapid increase in blood PO2 as the blood passes through the pulmonary capillaries, the blood PO2 increases roughly to the PO2 of the air in the alveolar lumen just before the blood PO2 has been reduced. increases by a factor of three in the capillaries, becoming close to 104 mm Hg.
Absorb oxygen into the blood in the lungs during work
During periods of heavy labour, a person's body may require 20 times more oxygen than is normal. In addition, due to increased cardiac output at work, the time that blood remains in the pulmonary capillaries can be reduced to less than half of normal. However, thanks to the very high factor of safety for the diffusion of O2 across the respiratory membrane, the blood still becomes nearly saturated with O2 before leaving the pulmonary capillaries. This can be explained as follows:
Firstly, the volume of dissolved O2 increases nearly three times when working; This result is mainly due to increased surface area of the capillaries involved in diffusion and also from a more near-ideal ventilation-perfusion ratio in the upper part of the lung.
Second, note in the curves that, under non-working conditions, the blood becomes almost saturated with O2 before it passes through one-third of the pulmonary capillaries, and almost no additional O2 diffuses into the blood. in the last two-thirds of the shipping process. That is to say, blood normally stays in the pulmonary capillaries about three times longer than it takes to provide the body with enough O2. Therefore, when working, even with a short exposure in the capillaries, the blood can still be almost full of oxygen.
Transport of oxygen in arterial blood
About 98 percent of the blood entering the left atrium from the lungs has just passed through the oxygenated alveolar capillaries, PO2 up to about 104 mmHg. The remaining 2 percent of the blood that has flowed through the aorta enters the pulmonary circulation which is the main blood supply to the tissues deep in the lungs and is not exposed to the air in the lungs. This blood flow is called "shunt flow", which means that blood is bypassed through the gas exchange zones. After leaving the lungs, the PO2 of the blood at the shunts is approximately equal to the normal value in the venous system, about 40 mmHg. When the blood in these pulmonary veins meets oxygen-rich blood from the alveolar capillaries, this is called venous fusion, causing the PO2 of the blood entering the left heart and pumping into the aorta to drop to about 95 mm Hg. .
Figure. PO2 changes in pulmonary capillary blood, systemic arterial blood and capillary blood, demonstrating the influence of venous mixing
Figure. Diffusion of oxygen from a tissue capillary into cells (PO2 in the interstitial space = 40 mm Hg, and in the cell = 23 mm Hg.)
The PO2 in the interstitial fluid surrounding the tissue cells averages only 40 mm Hg. Thus, there is a large initial pressure gradient that causes O2 to diffuse from the capillary blood into the tissues very rapidly, the PO2 in the capillaries drops rapidly to approximately 40 mm Hg – the intracellular pressure gradient. interstitial tissue. Therefore, the PO2 of the blood leaving the tissue capillaries and entering the systemic veins is also about 40 mm Hg.
Increased blood flow will increase PO2 in the interstitial fluid
If blood flow through a particular tissue is increased, the more O2 is transported into the tissues, the higher the PO2 is correspondingly higher. This effect is shown in Fig. Note that an increase in blood flow to 400 percent typically increases PO2 from 40 mm Hg (at point A in the figure) to 66 mm Hg (point B). However, the upper limit to which PO2 can be increased, even with maximal blood flow, is 95 mm Hg, since this is the O2 fraction in arterial blood. Conversely, if tissue blood flow is reduced, tissue PO2 also decreases, as shown in point C.
Increased tissue metabolism will reduce PO2 in the interstitial fluid
If cells use more O2 for metabolism than normal, PO2 in the interstitial fluid will decrease. Figure 41-4 also shows this effect, showing a decrease in PO2 in the interstitial fluid when cellular O2 consumption is increased and an increase in PO2 when consumption is reduced.
Figure. Effect of blood flow and oxygen utilization rate on tissue PO2
In summary, tissue PO2 is balanced by two factors: (1) the rate at which the blood transports O2 to the tissues and (2) the rate at which the tissue consumes O2.
Diffusion of oxygen from peripheral capillaries into tissue cells
Cells are always using oxygen. Therefore, at the periphery, intracellular PO2 in tissues is still lower than in capillary PO2. In addition, in many cases, there is a significant biological gap between capillaries and cells. Therefore, the normal intracellular PO2 ranges from 5 to 40 mm Hg, with an average of 23 mm Hg (directly measured in laboratory animals). Since only 1-3 mm Hg of the O2 fraction is normally used to participate in all cellular oxygen-using metabolisms, even intracellular PO2 as low as -23 mm Hg is sufficient and safe. whole body.