5+ Year Member
Jul 24, 2014
Im tripped up on this bio question and I was hoping you could help me out. So this passage is about partial pressure of the gases (CO2, O2) as it moves from the air--> into our bloodstream. So this question:

#75 The partial pressure of O2 in systemic arterial blood is normally 5-10 mmHg less than that in alveolar gas. The difference in partial pressure exits because: _______?

A.)A fraction of the O2 is directly dissolved into the pulmonary blood flow
B.)A fraction of the O2 is replaced by the partial pressure of water
C.)The venous blood from the bronchial venues and heart vessels contaminate the pulmonary venous outflow
D.)The partial pressure of gases dissolved in liquid is not equal to the partial pressure of the gas phase at equilibrium.

-Sorry in advanced if you don't have Berkley. Ive been trying to reason through this question for quite sometime and Im just stuck lol. Idk if its the wording that tricking me up or what.

We are looking for an explanation to the decrease in O2 partial pressure. From the passage, we know that the bronchial venous flow along with vessels from the heart join with pulmonary venules. In effect, by increasing the volume of blood without changing the amount of oxygen, we are decreasing the concentration of oxygen in blood. The correct choice is C.

-I don't get when they say increased blood volume, while keeping the O2 pp the same... where is this happening? I thought the volume was the same throughout (however more area in the capillaries).

Jan 29, 2012
Well the amount of blood that mixes with O2 in the lungs is not the entire pulmonary circuit (i.e. the heart doesn't pump directly into the lungs, you have to go through the pulmonary artery first which isn't exposed to the outside O2, and come back through the pulmonary vein which also isn't exposed to the alveolar O2).