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  #1  
Old 06-05-2014
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Default High altitude explained!

I came to know that this is an FAQ in USMLE, so i took a lot of time to understand the complex mechanism behind changes at high altitude, and i hope this flow chart makes it easier for u guys. Cheers!



High altitude

--> Atmospheric pressure decreases (Remember, as we go away from the earth, the pressure of the gases will decrease and as we go towards it i.e sea level, it will increase)

--> however, percentage of gases remains the same (78%- N ,21% - O2, 1% -CO2)

--> the pressure in the atmosphere HAS TO BE EQUAL to the pressure in the lungs (else, they will collapse)

--> A normal lung is able to compensate for this decrease in atm pr

--> decreased pressure of O2 in alveoli (PAO2)

--> leads to decreased pressure of O2 in plasma (PaO2) (More appropriately, less pressure is required to keep the O2 dissolved in plasma)

--> Literally, the definition of HYPOXEMIA

--> any decrease in normal pressures of gases dissolved in plasma stimulates peripheral chemoreceptors in carotid and aortic bodies (Central ones are in the medulla)

--> Stimulation of Respiratory centre by chemoreceptors (They think enough o2 is not being absorbed by the lung. They don't know the lung is trying to save the body!!!)

-->Increased respiratory rate

--> Washing out of CO2 (Respiratory alkalosis)

--> Wow, nice! CO2 is a bad gas! It deserved to be washed out!

--> WRONG!!!

--> CO2 is required to form H2CO3 in blood which is the buffer system

--> No more CO2 available to make H2Co3

--> some other pathway required

--> H+ + HCO3- --> H2CO3 !!

--> Consumption of all the H+ in blood

--> decreased hydrogen ions

--> Increased ph of blood (Metabolic alkalosis)

--> Not because of increased H2CO3 formation, but due to depletion of hydrogen ions

--> body needs to compensate

--> a. Kidneys reabsorb more H+

b. Intracellular H+ driven out into plasma

--> increased IC ph

--> leads to activation of Phosphofructokinase

--> Glycolysis

--> Formation of 1,3 DPG --> Formation of 2,3 DPG

--> Makes Hb stable in it's taut form

--> less affinity of Hb for O2

--> More dissociation of O2 to tissues

--> RIGHT SHIFT OF OBC(O2 binding curve) !!!!!!



Also, in 1-2 weeks, the kidneys start secreting Erythropoietin which basically occurs whenever there is a decrease in PaO2.

That is why, athletes, before the start of Olympics, go to practice on the mountains. So that, by the time they come back down, their OBS will be right shifted and their blood will be delivering more oxygen to their tissues. (Smart Doping )



That's it folks.

Phew , finally!
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The above post was thanked by:
adrenaline106 (06-07-2014), ANUBHAV (06-06-2014), drneelema (06-05-2014), DrNewB (06-05-2014), nishu (06-06-2014), thelostsoul (05-21-2016), wa87el (06-05-2014)



  #2  
Old 06-05-2014
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Quote:
Originally Posted by aniraiqbal View Post
I came to know that this is an FAQ in USMLE, so i took a lot of time to understand the complex mechanism behind changes at high altitude, and i hope this flow chart makes it easier for u guys. Cheers!



High altitude

--> Atmospheric pressure decreases (Remember, as we go away from the earth, the pressure of the gases will decrease and as we go towards it i.e sea level, it will increase)

--> however, percentage of gases remains the same (78%- N ,21% - O2, 1% -CO2)

--> the pressure in the atmosphere HAS TO BE EQUAL to the pressure in the lungs (else, they will collapse)

--> A normal lung is able to compensate for this decrease in atm pr

--> decreased pressure of O2 in alveoli (PAO2)

--> leads to decreased pressure of O2 in plasma (PaO2) (More appropriately, less pressure is required to keep the O2 dissolved in plasma)

--> Literally, the definition of HYPOXEMIA

--> any decrease in normal pressures of gases dissolved in plasma stimulates peripheral chemoreceptors in carotid and aortic bodies (Central ones are in the medulla)

--> Stimulation of Respiratory centre by chemoreceptors (They think enough o2 is not being absorbed by the lung. They don't know the lung is trying to save the body!!!)

-->Increased respiratory rate

--> Washing out of CO2 (Respiratory alkalosis)

--> Wow, nice! CO2 is a bad gas! It deserved to be washed out!

--> WRONG!!!

--> CO2 is required to form H2CO3 in blood which is the buffer system

--> No more CO2 available to make H2Co3

--> some other pathway required

--> H+ + HCO3- --> H2CO3 !!

--> Consumption of all the H+ in blood

--> decreased hydrogen ions

--> Increased ph of blood (Metabolic alkalosis)

--> Not because of increased H2CO3 formation, but due to depletion of hydrogen ions

--> body needs to compensate

--> a. Kidneys reabsorb more H+

b. Intracellular H+ driven out into plasma

--> increased IC ph

--> leads to activation of Phosphofructokinase

--> Glycolysis

--> Formation of 1,3 DPG --> Formation of 2,3 DPG

--> Makes Hb stable in it's taut form

--> less affinity of Hb for O2

--> More dissociation of O2 to tissues

--> RIGHT SHIFT OF OBC(O2 binding curve) !!!!!!



Also, in 1-2 weeks, the kidneys start secreting Erythropoietin which basically occurs whenever there is a decrease in PaO2.

That is why, athletes, before the start of Olympics, go to practice on the mountains. So that, by the time they come back down, their OBS will be right shifted and their blood will be delivering more oxygen to their tissues. (Smart Doping )



That's it folks.

Phew , finally!
Thanks!!! This is awesome. I liked the part B.
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  #3  
Old 06-05-2014
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"-->decreased pressure of O2 in alveoli (PAO2)

--> leads to decreased pressure of O2 in plasma (PaO2) (More appropriately, less pressure is required to keep the O2 dissolved in plasma)"

Does this mean A-a gradient will be normal?Thanks
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Old 06-05-2014
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Quote:
Originally Posted by DrNewB View Post
"-->decreased pressure of O2 in alveoli (PAO2)

--> leads to decreased pressure of O2 in plasma (PaO2) (More appropriately, less pressure is required to keep the O2 dissolved in plasma)"

Does this mean A-a gradient will be normal?Thanks

Yes,in a physiologically normal person, it will be. However if there is any pathology like maybe a diffusion or ventilation defect, the gradient may be deranged.
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