Lecture 7
Transport of oxygen in the arterial bloodAfter the alveolar capillaries, the shunted blood from bronchial circulation (2%) and its PO2=40 combines in the pulmonary veins with the oxygenated blood from the alveolar capillaries (98%) and its PO2=104, this combination of blood is called venous admixture of blood causes the PO2 of the blood entering the left heart and pumped into the aorta to fall to about 95 mm Hg (fig 14).
Diffusion of oxygen from the peripheral capillaries into the tissue fluid
When the arterial blood reaches the peripheral tissues the PO2=95 mm Hg, the PO2 in the interstitial fluid that surrounds the tissue cells averages only 40 mm Hg. So there is a great initial pressure difference that causes oxygen to diffuse rapidly from the capillary blood into the tissues. Therefore, the PO2 of the blood leaving the tissue capillaries and entering the systemic veins is also about 40 mm Hg.
●The interstitial fluid and tissue PO2 is determined by a balance between:
1-blood flow through a tissue: when blood flow(, the rate of oxygen transport to the tissues ( and the tissue PO2 becomes correspondingly higher and vice versa.2- Rate of tissue metabolism: If the cells use more oxygen for metabolism than normally, this reduces the tissue and interstitial fluid PO2 and vice versa.
● Oxygen is always being used by the cells. Therefore, the intracellular PO2 in the peripheral tissue cells remains lower than the PO2 in the peripheral capillaries=23 mm Hg. only 1 to 3 mm Hg of oxygen pressure is normally required for full support of the chemical processes that use oxygen in the cell, so that even this low intracellular PO2 of 23 mm Hg is more than adequate and provides a large safety factor.
Diffusion of carbon dioxide from the peripheral tissue cells into the capillaries and from the pulmonary capillaries into the alveoli (fig 15)
●When oxygen is used by the cells, virtually all of it becomes carbon dioxide, and this increases the intracellular PCO2; because of this high tissue cell PCO2, carbon dioxide diffuses from the cells into the tissue capillaries and is then carried by the blood to the lungs. In the lungs, it diffuses from the pulmonary capillaries into the alveoli and is expired.
●Carbon dioxide can diffuse about 20 times as rapidly as oxygen, so the pressure differences required diffusing CO2 less than O2.
●The CO2 pressures are approximately the following:
Intracellular PCO2=46 mm Hg (interstitial PCO2=45 mm Hg ( PCO2 of the venous blood leaving the tissues= 45 mm Hg(PCO2 of the blood entering the pulmonary capillaries at the arterial end= 45 mm Hg; PCO2 of the alveolar air= 40 mm Hg( the PCO2 of the pulmonary capillary blood falls to almost exactly equal the alveolar PCO2 of 40 mm Hg before it has passed more than about one third the distance through the capillaries( PCO2 of the arterial blood entering the tissues=40 mm Hg and so.
Effect of rate of tissue metabolism and tissue blood flow on interstitial PCO2
Tissue capillary blood flow and tissue metabolism affect the PCO2 in ways exactly opposite to their effect on tissue PO2.
Role of hemoglobin in oxygen transport
Normally, about 97 % of the oxygen transported from the lungs to the tissues is carried in chemical combination with hemoglobin in the red blood cells. The remaining 3 % is transported in the dissolved state in the water of the plasma and blood cells.●The oxygen molecule combines loosely and reversibly with the heme portion of hemoglobin. When PO2 is high, as in the pulmonary capillaries, oxygen binds with the hemoglobin, but when PO2 is low, as in the tissue capillaries, oxygen is released from the hemoglobin.
Oxygen-hemoglobin dissociation curve (fig16)
This sigmoid shape curve demonstrates a progressive increase in the % of hemoglobin bound with oxygen as blood PO2 increases, which is called the % saturation of hemoglobin. Because the blood leaving the lungs and entering the systemic arteries usually has a PO2 of about 95 mm Hg and the % saturation of hemoglobin averages 97 %. Conversely, in normal venous blood returning from the peripheral tissues, the PO2 is about 40 mm Hg, and the % saturation of hemoglobin averages 75 %.Maximum amount of oxygen that can combine with the hemoglobin of the blood
The blood of a normal person contains about 15 grams of hemoglobin in each 100 ml of blood, and each gram of hemoglobin can bind with a maximum of 1.34 ml of oxygen (15 * 1.34 =20.1) which means that, on average, the 15 grams of hemoglobin in 100 ml of blood can combine with a total of almost exactly 20 ml of oxygen if the hemoglobin is 100 % saturated. This is usually expressed as 20 volumes % in oxygen-hemoglobin dissociation curve. But the per cent saturation of hemoglobin in normal systemic arterial blood is 97 % which is about 19.4 ml per 100 ml of blood (PO2 of 95mmHg (97%) blood carries 19.4ml O2).On passing through the tissue capillaries, this amount is reduced, on average, to 14.4 ml (PO2 of 40 mm Hg, 75 % saturated hemoglobin).
Thus, under normal conditions, about 5 ml of oxygen are transported from the lungs to the tissues by each 100 ml of blood flow.
Transport of oxygen during strenuous exercise
During heavy exercise, the muscle cells use oxygen at a rapid rate, which, in extreme cases, can cause the muscle interstitial fluid PO2 to fall from the normal 40 mm Hg to as low as 15 mm Hg leading to 15 ml of oxygen is delivered to the tissues by each 100 ml of blood flow which is three times normal. However the cardiac output can increase to six to seven times normal in well-trained marathon runners, so (3*7) gives a 20-fold increase in oxygen transport to the tissues.Utilization Coefficient: It is the percentage of the blood that gives up its oxygen as it passes through the tissue capillaries and normally it is 25 %. That is 25 % of the oxygenated hemoglobin gives its oxygen to the tissues. During strenuous exercise, the utilization coefficient in the entire body can increase to 75 to 85 per cent. And in local tissue areas where blood flow is extremely slow or the metabolic rate is very high, utilization coefficients approaching 100 % have been recordedthat is, essentially all the oxygen is given to the tissues.
Effect of hemoglobin to buffer the tissue PO2
Hemoglobin performs another function essential to life which is a tissue oxygen buffer system to stabilize the oxygen pressure in the tissues. The hemoglobin normally sets an upper limit on the oxygen pressure in the tissues at about 40 mm Hg because if the tissue PO2 rise above 40 mm Hg level, the amount of oxygen needed by the tissues would not be released from the hemoglobin. Conversely, during heavy exercise, the hemoglobin in the blood delivers an extra amount of oxygen (20 fold) to the tissues at a pressure that is held between about 15 and 40 mm Hg which is a little further decrease than in normally condition 40 mmHg. This is because of (1) the steep or sharp slope of the dissociation curve and (2) the increase in tissue blood flow caused by the decreased PO2.
When atmospheric oxygen concentration changes markedly, the buffer effect of hemoglobin still maintains almost constant tissue PO2
It can be seen from the oxygen-hemoglobin dissociation curve when the alveolar Po2 is decreased to as low as 60 mm Hg (as one ascends a mountain or ascends in an airplane), the arterial hemoglobin is still 89 per cent saturated with oxygen. Further, the tissues still remove about 5 ml of oxygen from each 100 ml of blood. Then the venous blood PO2 falls to 35 mm Hg-only 5 mm Hg below the normal value of 40 mm Hg, so the tissue PO2 hardly changes.
Conversely, when the alveolar PO2 rises as high as 500 mm Hg (such as deep in the sea), the maximum oxygen saturation of hemoglobin can never rise above 100 per cent, which is only 3 % above the normal level of 97 %. Only a small amount of additional oxygen dissolves in the fluid of the blood. Then, when the blood passes through the tissue capillaries and loses several ml of oxygen to the tissues, this reduces the PO2 of the capillary blood to a value only a few ml greater than the normal 40 mm Hg.
Factors that shift the oxygen- hemoglobin dissociation curve (fig 17)
A shift in the curve to the right indicates that the affinity of Hb for O2 is reduced and releasing O2.
A shift in the curve to the left means more O2 will be attached to Hb (increased affinity) and less O2 releasing.
Shift to right:
(1) Increased hydrogen ions (acidic blood)
(2) Increased CO2
(3) Increased temperature
(4) Increased 2, 3-biphosphoglycerate (BPG), (occur in hypoxic conditions that last longer than a few hours).
Bohr Effect: It is a shift of the oxygen hemoglobin dissociation curve to the right and downward in response to increases in blood carbon dioxide (formation of carbonic acid) and hydrogen ions. This shift increases the release of oxygen from the blood in the tissues and increases oxygenation of the blood in the lungs. Exactly the opposite effects occur in the lungs, where carbon dioxide diffuses from the blood into the alveoli. This reduces the blood PCO2 and decreases the hydrogen ion concentration, shifting the oxygen-hemoglobin dissociation curve to the left and upward.
Shift of the dissociation curve during exercise
During exercise, several factors shift the dissociation curve considerably to the right, thus delivering extra amounts of oxygen to the active, exercising muscle fibers.(1)The exercising muscles release large quantities of carbon dioxide and (2) several other acids released by the muscles increase the hydrogen ion concentration in the muscle capillary blood.(3) In addition, the temperature of the muscle often rises 2 to 3C.
Then, in the lungs, the shift occurs in the opposite direction, allowing the pickup of extra amounts of oxygen from the alveoli.
Fig 14 Changes in PO2 in the pulmonary capillary blood, systemic arterial blood, and systemic capillary blood, demonstrating the effect of venous admixture.
Fig 15 systemic and pulmonary circulation
Fig 16 Oxygen-hemoglobin dissociation curve
Fig 17 Shift of the oxygen-hemoglobin dissociation curve to the right caused by an increase in hydrogen ion concentration (decrease in pH) & BPG, 2, 3-biphosphoglycerate