Partial Pressures and Simple Diffusion

week 3, unit 5, lesson 4

composition of atmospheric pressure

That air is composed of several different gases: oxygen (O₂), carbon dioxide (CO₂), nitrogen (N₂), and other gases like carbon monoxide, methane, water vapour etc. We will only focus on O₂ and CO₂ in this unit.

Atmospheric pressure is 760 mmHg, and we must change intrapulmonary pressure to become higher or lower than atmospheric pressure to exhale or inhale, respectively.

partial pressure of oxygen (PO₂),

refers to how much pressure oxygen contributes to the total atmospheric pressure (760 mmHg). 

how to calculate how much pressure oxygen contributes to the total atmospheric pressure (760 mmHg). 

if oxygen makes up 21% of the total atmospheric pressure of 760 mmHg, then the PO₂ of atmospheric air is 760 x 21%, which equals about 160 mmHg. This is the partial pressure of oxygen entering our lungs during inhalation.

partial pressure of carbon dioxide (PCO₂)

refers to how much pressure carbon dioxide contributes to the total atmospheric pressure. 

how to calculate  partial pressure of carbon dioxide (PCO₂)

if an atmospheric pressure of 760 mmHg is made of up 0.04% CO₂, then the PCO₂ is 760 x 0.04%, which equals about 0.3 mmHg. So, we breathe in very little carbon dioxide

what is the method of transport that oxygen and carbon dioxide use to cross the blood-gas barrier

simple diffusion, this requires no protein channels and no membrane transporters bc these gases are very small and hydrophobic (non-polar)  O₂ and CO₂simply squeeze between the phospholipids of the plasma membrane and cross to the other side of the barrier.

why the blood-gas barrier makes for an excellent site of simple diffusion:

1. large concentration (pressure) gradient

2. lots of membrane area

3. thin membrane thickness

why does large concentration gradient help with simple diffusion

for gases, we use the term pressure gradient instead. As you will see shortly, the PO₂ is very high inside the alveoli but much lower in the blood found inside capillaries that wrap around the alveoli. This maximizes diffusion. Remember, high to low, gases will go!

why does lots of membrane area help with simple diffusion

with millions of alveoli in the lungs, there is an enormous amount of membrane for simple diffusion to occur across.

why does thin membrane thickness help with simple diffusion

the blood-gas barrier is made of 2 thin (squamous) cell types--the type 1 epithelial cells of the alveolar wall and the thin endothelial cells of the capillary. Thin membranes maximize diffusion since the gases don't have to travel a large distance.

direction o2 and co2

CO₂, a waste product of metabolism, needs to be exhaled. So it must LEAVE the blood and enter the alveoli, while O₂ needs to be delivered to all our organs and tissues, and, therefore, it must leave the alveoli and ENTER the blood. 

levels of po2 in pulmonary circulation

pulmonary artery: low po2

pulmonary vein: high po2

order of blood-gas exchange

1. o2 enters the capillaries from the alveoli

2. o2 rich blood circulates (some CO2)

3. o2 leaves the capillaries into body tissues

4. co2 enters capillaries from the body tissues

5. co2 rich blood circulates (some o2)

6. co2 enters alveoli from capillaries

rules for gas exchange

1. down pressure gradient

2. happens only at capillaries

3. high to low until reaching equilibrium

what does oxygen do at the pulmonary capillaries

• must enter the blood at the pulmonary capillary so so the PO₂ will be higher in the alveoli than in the blood. 

po2 in atmospheric and in alveolar

in atmospheric: 160 mmHg air we breathe in

alveoli never emptys and its rich in co2 “stale air” will have a lower po2 100mmHg

alveolar po2 and pulmonary vein

same value, 100 mmHg bc blood is going to pass by the alveoli with stale air, pick up the oxygen

systemic artery po2

same as alveolar po2 and pulmonary vein bc no encounter with any capilllaries

po2 of body tissues at rest

will always use oxygen to create atp. tissue po2 is ~ 40mmHg or less

and active one will have even less than that, so like 30

pressure gradient in body tissues

100mmHg in systemic artery and 40 in tissue so oxygen will leave the blood and enter the body tissues

body tissue po2, systemic veins, pulmonary artery

body tissue and systemic veins will have the same valume, 40mmHg bc equilibrium and so will the pulmonary artery bc no capillaries

pulmonary artery and alveolar po2 and

we want oxygen to enter the blood from high to low

blood coming into pulmonary capillaries has a partial pressure of 40 which is lower than in the alveoli