Question regarding having high blood pressure?

Question about Partial Pressure and Arterial Blood Pressure (BP)?

• I need a bit of help understanding this: Partial pressure for example (PaO2) is the pressure a gas would exert if it were occupying that space on it's own, but it's NOT occupying the space on it's own right there's RBC's and plasma etc in the arteries (and veins) that exert pressure as well. What I'm having trouble getting my head around is if partial pressure of PaO2 is 90mmHg and your blood pressure is 120/80mmHg, does that mean that the extra components of blood only add 30mmHg of pressure to Systolic BP, and also could you explain why it is that diastolic BP is lower than partial pressure of oxygen? I'm not sure if it's a stupid question but I was never taught about this stuff so I don't understand. Thanks in advance for your help.

• Answer:

  Very good questions, it made me think and review (a little in my physiology text). When they talk about partial pressure of either the blood gases or atmosphere, you need to think in terms of what is happening at the alveoli, that empty sac whose sides are in close contact with flowing blood. If the partial pressure of the gas in the sac is higher than the partial pressure in the blood, then the net flow of the gas is into the blood. If the gas is highly soluble, it still gets into the blood but it does not raise the partial pressure as much as a gas which is less soluble (given the same concentration of the gas in the blood). When you talk about 120/80 mm Hg that is in relation to the outward atmosphere, that is it is 120/80 mm above the (average) 760mm of Hg in the atmosphere. That is why when you get a cut artery it squirts out blood, due to this pressure difference. So, in venous blood the pressure gets near 0 mm Hg which is really atmospheric pressure of 760 mm Hg, my text tells me that the maximum partial pressure of oxygen that can be attained breathing a regular atmosphere is 149 mm Hg, which is (149/760)*100 = 19.6%, which is close to the 21% O2 found in air at sea level. A more exact and detailed explanation is given in Guyton and Hall, Textbook of Medical Physiology, 9th edition pages 501-511, and in a your average general chemistry text..