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mortenra
02-18-2008, 02:29 PM
V/Q close to 1 is said to be normal, but can anyone explain why the air and blood volumes should match?

doctormua
02-18-2008, 11:57 PM
read kaplan physio under 'respiratory'

Water
02-19-2008, 10:42 PM
think of blood as buses and oxygen (air) as passengers. too many buses without enough people means you are wasting the bus use; on the other hand, too many passengers with a few buses, then people are left behind. The optimal way is to have both things matching with each other.

mortenra
02-21-2008, 02:15 AM
I understand that something must match, but why should one liter of air match one liter of blood??
I think the explanation with busses and passengers is particularly poor (but very commonly accepted). In expired alveolar air there is about 14% O2 (140 ml pr L), while in blood that has just passed the alveolar wall there is 100% saturation (no seats left). This means that there is nowhere close to a match between passengers (molecules of O2) and busses (Hb molecules), there is a lot more O2 molecules.
So, can anyone explain the stoichiometry behind a V/Q of one.

Water
02-21-2008, 03:28 PM
Respectfully,
v/q is explained as such.
14% expired o2 refers to the partial pressure of 0.14 in the expired air. Oxygen partial pressure in fresh air is 0.21. so for simplicity, the exchange descreases fresh air by approximately 0.7 of oxygen.

Oxygen saturation in blood refers to places on hemoglobin occupied by oxygen. The saturation is determined by the partial arterial oxygen (dissolved oxygen). The higher the pressure, the closer to 100% hemoglobin saturation can be.
this pressure rises when it is juxtaposed with "fresh air" on alveolar surface to create equilibrium. so the closer the exposures of both sides of the alveolar surface, the better the equilibrium can be. This helps to increase oxygen arterial partial pressure of the blood and subsequently oxygen saturation.

v/q does not mean that for every liter of air in the lungs, there is one liter of blood. The total lung volume is about 5 liters, and blood is about 5 liters as well. But, not all of the blood is in the lungs at one time.

if I misread your question, then ignore my reply.:D

Dr. X
02-22-2008, 03:07 PM
V/Q = 1 is an approximately value.. even the MORE correct version of V/Q = 0.8 itself is approximate provided that certain conditions are met such as cardiac output, breathing frequency etc.

To specifically answer your question:

Ventilation of the lung is 4L per minute and Perfusion of the lung is 5L per minute - both being approximate because obviously each breath you are taking as you are reading this sentence is not gonna be the same rite. So, the approximate number comes from 4/5 = 0.8

To go beyond your question:

The significance of the ratio is what we should be most concerned about and especially for step1 purposes. Even inside our lung, the ratio is different and im going to assume that you have grasped the concept why apex of the lung would have higher V/Q value and a lower value at the base. Then you have to apply this to pathological states and also be able to connect this to Pa O2 and Pa CO2 values. And of course have some understaning of A-a values connected to this. Keep working hard :)

Dr. X
02-22-2008, 03:16 PM
And dont be surprised if anyone stands on one feet and says.. "i read somewhere ventilation is 5L/min and the average cardiac output is 6L/min .. etc, etc." You tell em' "more power to ya" and ask if they really understood what they read.

As for the bus situation.. what if we have a smoker in the bus.. and if all passengers are oxygen.. oh NO...

mortenra
02-26-2008, 01:44 PM
OK, I will try myself if nobody else dare.
Humans produce a certain amount of CO2 and uses a certain amount of O2 pr minute, and we need to fill our lungs with the volume of air that contains at least the amount of O2 consumed, and removes the exact amount of CO2 produced. The volumes are therefore more critical for CO2, and I will do the calculation for CO2.

The difference between the amount of CO2 dissolved at venous blood pCO2 (46 mm Hg) and the amount dissolved at arterial pCO2 (40 mm Hg) is approximately 40 ml CO2 pr liter blood. Therefore, we need an amount of air (pr liter of blood) that can take away the excess 40 ml CO2 pr liter blood, but not take away too much, because then the pCO2 in arterial blood would fall below 40 mm Hg (which our biochemistry doesn’t like). In other words, we would like the amount of air that dilutes 40 ml 100% CO2 down to 5.3% CO2 (which corresponds to 40 mm Hg). 40 ml CO2 in 800 ml air gives 5% CO2 (which is close enough for this demonstration). Therefore, for each liter of blood through the lungs (giving off 40 ml CO2) we need 800 ml of air to dilute the CO2 down to ~5% (or 40 mm Hg).

Dr. X
02-26-2008, 09:32 PM
im sorry bud, i didnt think you wanted to break it down this much out of your original question but the explanation you gave only allows to ponder even more.


The difference between the amount of CO2 dissolved at venous blood pCO2 (46 mm Hg) and the amount dissolved at arterial pCO2 (40 mm Hg) is approximately 40 ml CO2 pr liter blood

where did 40mL come from? Out of partial pressure values? Without pulling numbers out of nowhere..
standard solubility of O2 in blood = 0.03 mL O2/L/mm Hg (BRS pg.128)
standard solubility of CO2 in blood = 0.6 ml CO2/L/mm Hg (20X greater than O2)

so with 46 mmHg (venous) = 27.6 mL CO2/ L of blood
with 40 mmHg (arteries) = 16.6 mL CO2/ L of blood

difference would be 11 mL (not 40 mL)

While for partial pressure of oxygen its safer, we have to understand this would explain only the DISSOLVED CO2 which is about 5-10%. Pay much more attention to the MAJOR FORM of CO2 transport to lungs which is HCO3- (about 80%) due to "Cholride shift" in RBC's with the help of carbonic anhyrdase. Other would be carbonic compounds about 15%.


In other words, we would like the amount of air that dilutes 40 ml 100% CO2 down to 5.3% CO2 (which corresponds to 40 mm Hg). 40 ml CO2 in 800 ml air gives 5% CO2
Huh?? down to 5.3 from what? how does that correspond to 40 mmHg? 800 ml air gives 40 ml CO2? how?

Atmosphere breakdown: 78% nitrogen, 21% oxygen, 0.03% CO2. They do teach kids its 1% because decimals havent been introduced to them yet.

.03% of 800mL (the air that we are supposed to intake) does not give 40 mL of CO2.

The percent of CO2 EXHALED is 5% and INHALED is 0.03 - 0.06%. Your calculations mixed it up maybe?


Therefore, for each liter of blood through the lungs (giving off 40 ml CO2)...
It does? please enlighten us.

Dr. X
02-26-2008, 09:52 PM
For what its worth.. i'll agree we do need 800mL of air/L of blood but this comes out of my original statement..

4L of ventilation/5L of blood = 0.8L = 800mL

mortenra
02-27-2008, 03:56 PM
The 40 ml CO2 released from each litre of blood passing the lungs may be useful to know about. It can be read from the textbook curves that gives the CO2 concentration in blood as a function of partial pressures. The difference between 40 and 46 mm Hg pCO2, the arterio-venous difference of pCO2, gives a difference in concentration of CO2 of approximately 40 ml O2 pr liter blood.

Another approach: Most physiology students will have read that arterial blood contains 200 ml O2 pr litre blood, and that we use about 25% of this (50 ml), so that mixed venous blood contains 150 ml O2 pr litre. This means that each litre of blood passing the lungs must take up 50 ml of O2. We also know that most humans produce 8 molecules of CO2 for every 10 molecules of O2 taken up (the respiratory exchange ratio of 0.8), and this means that we get rid of approximately 50 ml * 0.8 = 40 ml CO2 pr litre blood.

One can also breathe in a bag for a minute and see how much CO2 comes out of you. That is approximately 200 ml CO2 pr minute, or 40 ml pr litre blood if you have a cardiac output of 5 litre pr minute (which also means that 5 litre goes through the lungs pr minute).

This was three ways of finding out that we get rid of 40 ml CO2 for each litre of blood that flows through the lungs (typical at rest).


A last thing: I am not sure if there was a question about how percentage of gas corresponds to mm Hg, but if so the connection is as follows: 5.3% in alveolar air during expiration corresponds to 40 mm Hg because 5.3% of one atmosphere (760 mm Hg) is 40.28 mm Hg.







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