Partial pressures and transport of carbon dioxide

week 4, lesson 1, unit 5

where does carbon dioxide leave

leaves the blood in the pulmonary capillaries, enter the alveoli, and be exhaled out. If we want CO₂ to leave the pulmonary capillaries, the PCO₂ must be HIGHER inside those capillaries and LOWER inside the lungs' alveoli.

partial pressure of co2 of atmospheric air

0.3 mmHg (760 mmHg x 0.04%). So atmospheric air has low CO₂relative to oxygen

resting tissue pco2

equal or greater than (if its a metabolically active tissue) 46 mmHg. so if you were active, it would be ~48/50 bc its creating more co2 in the process of creating atp

pco2 in tissues, venous and pulmonary

the same, 46 mmHg for tissue and venous bc it reaches equilibrium but its the same for pulmonary artery because no contact with capillaries

at the pulmonary capillaries, explain the movement of co2

(blood) high to low (alveolar pco2) into alveoli

alveolar pco2, pulmonary vein, arterial pco2

40 mmhg

plasma and co2

about 7% of the CO2 we carry in our blood is dissolved in plasma. this is a higher percentage than we saw for oxygen (1.5%) simply bc CO2 is more soluble in fluids than oxygen

Red blood cells (erythrocytes.)

about 23% of the co2 we carry is attached to hemoglobin and other proteins found in the blood. we call. this carbamino form of CO2 transport. but CO2 is not attached to hemegroups like we saw with oxygen. it binds to the globin chains in hemoglobin to be carried inside red blood cells

Red blood cells/plasma:

most of the carbon dioxide we carry in our blood is not carried as co2, but as an ion called bicarbonate (HCO3) this rxn takes place inside red blood cells bc the rxn requires an enzyme that is found in the cytoplasm of red blood cells. this enzyme is called carbonic anhydrase. when bicarbonate is made thru this rxn, we also make hydrogen ion (proton)

When CO₂ inside the red blood cells reacts with water

inside the cytoplasm of the red blood cell in the presence of carbonic anhydrase, it initially creates carbonic acid (H₂CO₃). However, carbonic acid is pretty unstable, so it quickly dissociates (breaks down) into bicarbonate and hydrogen.  This entire chemical reaction is reversible. Like any chemical reaction, the direction of the reaction depends on how much of each substance is on either side of the equation. So when there is lots of CO₂ entering the blood, the reaction goes to the right and creates lots of bicarbonate. 

 Once bicarbonate (HCO3) forms, 

 it leaves the red blood cell through a transporter found in that red blood cell's plasma membrane and is carried around in plasma. Why? Well, if we didn't remove HCO₃^-, then we would eventually build up too muchHCO₃^- inside of the cell and that would shift the equation back to the left. But we want to keep picking up CO₂ at the tissue, so we want to make sure HCO₃^- levels remain low inside red blood cells.

At the lung alveoli and bicarbonate levels

At the lung alveoli, we want to get rid of CO₂. This requires the bicarbonate levels to be higher than CO₂ levels to drive the reaction in reverse and shift it to the left, changing bicarbonate back into CO₂. That same transporter must bring bicarbonate back into the red blood cell to make this happen, since carbonic anhydrase is required for that reaction.

Pulmonary capillaries: unloading CO2

If you remember the goal is to pick up CO₂ at the tissues and return it to the lungs to be exhaled out, the direction of this equation should make sense. We want to "free up" CO₂ that was carried as bicarbonate and turn it into a gas again so it can enter alveolar air.