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Gas Exchange + Transport-Chapter 18

Key Concepts:

Terms in this set (35)

-Once air reaches the alveoli, O2 and CO2 diffuse from the alveolar air space into the blood

Normal alveolar PO2 at sea level is about 100 mmHg
PO2 of deoxygenated venous blood arriving at the lungs is about 40 mmHg
-O2 therefore goes down its partial pressure (concentration) gradient from the alveoli into the capillaries

PO2 of arterial blood leaving the lungs is the same as in the alveoli: 100 mmHg

When arterial blood reaches tissue capillaries, the gradient is reversed.
(cells are continuously using O2 for oxidative phosphorylation)
In the cells of a person at rest, intracellular PO2 averages 40 mm Hg. Arterial blood arriving at the cells has a PO2 of 100 mmHg
-Because PO2 is lower in the cells, oxygen diffuses down its partial pressure gradient from plasma into cells
*As a result, venous blood has the same PO2 as the cells it just passed


Conversely, PCO2 is higher in tissues than in the systemic capillary blood because of CO2 production during metabolism. Cellular PCO2 in a person at rest is about 46 mmHg, compared to an arterial plasma PCO2 of 40 mmHg. Gradient causes CO2 to diffuse out of the cells into the capillaries. Diffusion goes to equilibrium, and the systemic venous blood averages a PCO2 of 46 mmHg


At the pulmonary capillaries, the process reverses. Venous blood bringing waste CO2 from the cells has a PCO2 of 46 mmHg. Alveolar PCO2 is 40 mmHg. Because PCO2 is higher in the plasma, CO2 moves from the capillaries into the alveoli. When blood leaves the alveoli it has a PCO2 of 40 mmHg, identical to the PCO2 of the alveoli