Capillary Blood Flow Limits Oxygen Uptake From Alveoli

Pulmonary capillary blood flow has a significant influence on oxygen uptake. The effect of blood flow on oxygen uptake is illustrated in Figure 21.4. The time required for the red cells to move through the capillary, referred to as transit time, is approximately 0.75 sec, during which time the gas tension in the blood equilibrates with the alveolar gas tension. Transit time can change dramatically with cardiac output. For example, when cardiac output increases, blood flow through the pulmonary capillaries increases, but transit time decreases (i.e., the time blood is in capillaries is less).

Figure 21.4 illustrates the effect of blood flow on the uptake of three test gases. In the first case, a trace amount of nitrous oxide (laughing gas), a common dental anesthetic, is breathed. Nitrous oxide (N2O) is chosen because it diffuses across the alveolar-capillary membrane and dissolves in the blood, but does not combine with hemoglobin. The partial pressure in the blood rises rapidly and virtually reaches equilibrium with the partial pressure of N2O in the alveoli by the time the blood is one tenth of the time in the capillary. At this point, the diffusion gradient for N2O is zero. Once the pressure gradient becomes zero, no additional N2O is transferred. The only way the transfer of N2O can be increased is by increasing blood flow. The amount of N2O that can be taken up is entirely limited by blood flow, not by diffusion of the gas. Therefore, the net transfer or uptake of N2O is perfusion-limited.

When a trace amount of carbon monoxide (CO) is breathed, the transfer shows a different pattern (see Fig. 21.4). CO readily diffuses across the alveolar-capillary membrane but, unlike N2O, cO has a strong affinity for hemoglobin. As the red cell moves through the pulmonary capillary, CO rapidly diffuses across the alveolar-capillary membrane into the blood and binds to hemoglobin. When a trace amount of CO is breathed, most is chemically bound in the blood, resulting in low partial pressure (Pco). Consequently, equilibrium for CO across the alveolar-capillary membrane is never reached, and the transfer of CO to the blood is, therefore, diffusion-limited and not limited by the blood flow.

Alveolar Pulmonary Capillary

Time in capillary (sec)

^FGUHEHni^fcUptake of N2O, O2, and CO by pulmonary UtmmU^mmF capillary blood. Gas transfer is affected by pulmonary capillary blood flow. The horizontal axis shows time in the capillary. The average transit time it takes blood to pass through the pulmonary capillaries is 0.75 sec. The vertical axis indicates gas tension in the pulmonary capillary blood and the top of the vertical axis indicates gas tension in the alveoli. Individual curves indicates the time it takes for the partial pressure of a specific gas in the pulmonary capillaries to equal the partial pressure in the alveoli. Nitrous oxide (N2O) is used to illustrate how gas transfer is limited by blood flow, carbon monoxide (CO) illustrates how gas transfer is limited by diffusion. The profile for oxygen is more like that of N2O, which means oxygen transfer is limited primarily by blood flow. Pulmonary capillary PO2 equilibrates with the alveolar Po2 in about 0.25 second (arrow).

Time in capillary (sec)

^FGUHEHni^fcUptake of N2O, O2, and CO by pulmonary UtmmU^mmF capillary blood. Gas transfer is affected by pulmonary capillary blood flow. The horizontal axis shows time in the capillary. The average transit time it takes blood to pass through the pulmonary capillaries is 0.75 sec. The vertical axis indicates gas tension in the pulmonary capillary blood and the top of the vertical axis indicates gas tension in the alveoli. Individual curves indicates the time it takes for the partial pressure of a specific gas in the pulmonary capillaries to equal the partial pressure in the alveoli. Nitrous oxide (N2O) is used to illustrate how gas transfer is limited by blood flow, carbon monoxide (CO) illustrates how gas transfer is limited by diffusion. The profile for oxygen is more like that of N2O, which means oxygen transfer is limited primarily by blood flow. Pulmonary capillary PO2 equilibrates with the alveolar Po2 in about 0.25 second (arrow).

Figure 21.4 shows that the equilibration curve for oxygen lies between the curves for N2O and CO. Oxygen combines with hemoglobin, but not as readily as CO because it has a lower binding affinity. As blood moves along the pulmonary capillary, the rise in Po2 is much greater than the rise in Pco because of differences in binding affinity. Under resting conditions, the capillary Po2 equilibrates with alveolar Po2 when the blood is about one third of its time in the capillary. Beyond this point, there is no additional transfer of oxygen. Under normal conditions, oxygen transfer is more like that of N2O and is limited primarily by blood flow in the capillary (perfusion-limited). Hence, an increase in cardiac output will increase oxygen uptake. Not only does cardiac output increase capillary blood flow, but it also increases capillary hydrostatic pressure. The latter increases the surface area for diffusion by opening up more capillary beds by recruitment.

The transit time at rest is normally about 0.75 sec, during which capillary oxygen tension equilibrates with alveo lar oxygen tension. Ordinarily this process takes only about one third of the available time, leaving a wide safety margin to ensure that the end-capillary Po2 is equilibrated with alveolar Po2. With vigorous exercise, the transit time may be reduced to one third of a second (see Fig. 21.4). Thus, with vigorous exercise, there is still time to fully oxygenate the blood. Pulmonary end-capillary Po2 still equals alveolar Po2 and rarely falls with vigorous exercise. In abnormal situations, in which there is a thickening of the alveolar-capillary membrane so that oxygen diffusion is impaired, end-capillary Po2 may not reach equilibrium with alveolar Po2. In this case, there is measurable difference between alveolar and end-capillary Po2.

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Responses

  • battista
    How long does it take for pulmonary capillary po2 to equilibrate with alveolar po2?
    5 years ago
  • Kerstin
    Why does blood flowing out of the pulmonary capillaries during exercise is still fully oxygenated?
    5 years ago
  • HANNO
    Where is the pressure on the vertical axis in the capillaries?
    5 years ago
  • raimondo
    How does an increased alveolar capillary surface area improve oxygen uptake?
    5 years ago
  • alina
    How alveoli takes only oxygen?
    5 years ago
  • mackenzie
    Why skeletal capillaries open up?
    5 years ago
  • madoc
    What direction does o2 diffuse at the alveolarcapillary membrane?
    5 years ago
  • Ellen
    What is transit time in the alveoli?
    5 years ago
  • daryl
    How does blood flow affect oxygen diffusion?
    5 years ago
  • Aziz
    How capillaries help oxygen intake?
    5 years ago
  • Henrik
    What increases blood flow through capilaries?
    5 years ago
  • Teodros
    How does exercise influence oxygen uptake in the pulmonary capillaries?
    5 years ago
  • filmon
    What limits oxygen uptake?
    5 years ago
  • Nunzio
    Why does blood flow increase through skeletal muscle capillary beds?
    4 years ago
  • maria
    Does O2 increase at pulmonary capillary bed?
    4 years ago
  • Crispina
    Which direction does oxygen flow in cappilaries?
    4 years ago
  • arrigo
    How exactly does a thickening of the blood capillaries affect diffusion?
    4 years ago
  • markus lampinen
    Why blood flows slowly through the capillary of alveoli?
    4 years ago

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