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#1
01-18-2013
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Respiratory dynamics

Can someone briefly explain what Pao2, Po2, Pco2, Sao2 etc represent and how they change in a normal breathing.

#2
01-18-2013
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Quote:
 Originally Posted by joset2 Can someone briefly explain what Pao2, Po2, Pco2, Sao2 etc represent and how they change in a normal breathing.
They're all related. I guess the easiest way to understand this is to think of their relationship to the oxygen content in blood. Normally, the oxygen content in blood is about 20% (1g of Hb binds to 1.34ml O2, in healthy people, Hb is about 15grams so if you do the math
1g=1.34
15g=X
X=20%
This is how much oxygen is carried in our blood! Now this oxygen can either be dissolved in plasma or attached to Hb. Normally, in order for O2 to diffuse from Hb to tissue where its needed, it needs to dissolve in plasma first and THEN go to tissues. So at any given point in time, only about 0.3% of this oxygen is dissolved in plasma (this is because if you have a lot of oxygen free in plasma it can cause damage to tissue through free radical formation, not a pretty situation). So the majority of oxygen is bound to Hb (19.7%). 0.3+19.7=20%
To understand the dynamics of these values, you need to know the factors that influence oxygen content:
1) PAO2 (big A means Alveolar pressure)--You asked for a simple explanation so basically whatever pressure you have in your alveoli is going to be transmitted to your blood vessels (in other words, alveolar O2 pressure has to equilibrate with arterial O2 pressure). So whatever affects alveolar O2 pressure is going to affect arterial O2 pressure (There's about a 5-10mmHg difference in pressure btw the two compartments but lets just ignore it for the sake of simplicity). Now, the factors that influence PAO2 must be considered in order to understand the full picture. You probably know that alveolar O2 pressure depends on the pressure outside (the atmospheric pressure, since they are in direct contact)

Say for example you decide to hike up a mountain, your atmospheric pressure decreases which in turn decreases your PA02---> decreases your PO2 (arterial)---> which further decreases your O2 content.

If you go diving on the other hand, your Patm increases and the opposite happens.

2) The other factor that affects oxygen content is Hb. We said that the majority of Oxygen in blood is attached to hemoglobin. So lets consider some factors that affect Hb.
a) Hb concentration: this tends to increase when we have polyctemia for example which then increases our oxygen content. The opposite happens when we have anemia, our oxygen content decreases.
b) Saturation. normally we have a 97% saturation (i.e. all 4 Hb sites are occupied), this is going to increase/decrease with different conditions ex: CO poisoning, PDA etc and that will affect the oxygen content respectively.

About PCO2, this related to your CO2 content, if they say PACO2, they're talking about alveolar CO2, if PaCO2 they're talking about arterial. Generally speaking, when PO2 goes up, PCO2 goes down.

I hope this clears things up.
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 The above post was thanked by: joset2 (01-18-2013), sami trailer (01-18-2013)
#3
01-18-2013
 USMLE Forums Scout Steps History: Not yet Posts: 22 Threads: 10 Thanked 3 Times in 2 Posts Reputation: 13

thank you kindly

#4
01-19-2013
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Why is it 97 percent when it should be 100?

Sent from my GT-I9100 using Tapatalk 2
#5
01-19-2013
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 824 Threads: 66 Thanked 465 Times in 298 Posts Reputation: 475

Quote:
 Originally Posted by mymleprep Why is it 97 percent when it should be 100? Sent from my GT-I9100 using Tapatalk 2
There is some admixture of bronchial venous blood and pulmonary venous blood which drops PaO2 to 95mm/Hg and SO2 to 98 as far as i remember
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 The above post was thanked by: mymleprep (01-19-2013)

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