Gas Exchange and Transport: CO2 (Week 2, Mod 9)

What happens when CO2 dissolves in an aqueous solution?

Makes the solution ACIDIC

** CA = carbonic anhydrase enzyme

What is the total CO2 content of blood composed of?

Total CO2 content = [CO2 dissolved] + [H₂CO₃] + [HCO₃^-] + [CO₃^2-]

If you can get rid of these ^ components, you can get rid of CO2

• No specific carrier for CO2 in blood, so needs to be gotten rid of in other ways

What are the 3 ways that the body gets rid of CO2 in the blood? Which one is the primary way?

1) Dissolved CO2 (only 5%)

• CO2 is much more soluble in blood than O2

• 5% of CO2 is transported unchanged, simply dissolving in the plasma

2) Bound to Hemoglobin (10%)

• CO2 combines reversibly with AMINO GROUPS on Hb, NOT Fe SITE

• Binds to oxyhemoglobin, converting it to carbaminohemoglobin

  • CARBAMINOHEMOGLOBIN IS BLUE TINGED

  • This is what gives deoxy blood its blue color in the first place

3) Bicarbonate ions (PRIMARY WAY: 85%)

• Majority of CO2 is transported via the “chloride shift” in RBCs

• CO2 diffuses into RBC, and is metabolized 

  • Gets converted to bicarbonate 

  • Chloride shift happens, bicarbonate switched out for chloride 

    • This “gets rid” of CO2; is metabolized down to its components by RBCs 

    • Is PUT BACK TOGETHER via the REVERSE of this process at the lungs; done to release CO2 via exhalation

    • Carbonic anhydrase can speed up this process 

What is the Haldane Effect?

Basically the CO2 version of the Bohr effect

Affinity of CO2 for the hemoglobin amino acid

• Hemoglobin has a HIGHER affinity for CO2 when deoxygenated (makes sense; when deoxygenated, needs to take up CO2 from freshly oxygenated tissues)

• Has LOWER affinity for CO2 when it is OXYGENATED

  • To release it at the lungs

The Haldane Effect essentially says that at areas of high CO2 concentration (tissues), the hemoglobin displaces O2 in FAVOR of CO2 uptake

• Vis versa at the lungs 

What is a capnograph, and what is its purpose?

A device that measures CO2 concentrations in expired air (alveolar CO2, aka P_ACO2)

• Mostly measured in intubated animals during anesthesia

• Is measuring the RATIO between P_aCO2 (ARTERIAL) and P_ACO2 (ALVEOLAR)

  • Normally similar unless there are respiratory or circulatory disturbances

What is the ETCO2 value, and what is its significance in a capnograph reading?

ETCO2 = “End Tidal CO2”

• Basically the max CO2 partial pressure before inhalation occurs

• THIS is the value you’re reading during a capnograph - is at the end of the P_ACO2 plateau of the graph

ETCO2 should always land around 40

Summarize the 3 different gas exchange processes that are occurring here with CO2… remember, this is oxygenated blood at the tissues.

1) CO2 can be seen being dissolved in plasma, as well as being broken down metabolically for H+ ions to bind to plasma proteins…

• Very small portion

2) CO2 from the tissues also is binding to the amino group of hemoglobin, turning it into carbaminohemoglobin

3) Most importantly, the CHLORIDE SHIFT can be seen

• CO2 is taken up by the RBC and metabolized by carbonic anhydrase, which reduces the CO2 to bicarbonate (HCO₃^- ) and hydrogen

• Bicarbonate in the RBC is then switched out for Cl from the plasma, releasing bicarbonate into the plasma

  • The REVERSE of this happens at the level of the alveoli… bicarbonate taken from plasma and switched out with chloride, then CO2 is reconstructed and released into the alveoli for exhalation

What are the 11 main things you NEED to remember about gas transport for O2 and CO2?

(Just remember these facts; use this notecard as a reminder)

• O₂ uptake and CO₂ removal are both critical for normal physiological function

• PO₂ in the atmosphere is altered by several factors (inhaled CO₂ = zero)

• PO₂ in alveoli (less than atmospheric PO₂ due to presence of PCO₂  and humidity) usually around ~105 mmHg.

• PO₂ in alveoli similar to PO2 in pulmonary vein and arterial blood (equilibrium)

• If PO₂  in alveoli decreases, PO₂ will decrease in arterial blood

• O₂ carried in blood by Hb (mammals, birds) –Hb shows cooperativity, 4 binding sites for O₂ , Hb gets saturated with O_2, Affinity of Hb can be altered to enhance O₂ delivery.

• CO₂ produced by respiring tissues dissociates to HCO₃^- and H⁺  (REMEMBER the equation- always associate high CO₂ with  low/acidic pH).

• CO₂ transported in 3 ways- main way is by HCO₃^- .

• Haldane effect - Hb has higher affinity for CO₂ when it is deoxygentated (CO₂ diplaces O₂ from Hb at tissues)

• All mechanisms ensure O₂ delivery to meet metabolic demand and CO₂ removal (usually matched)

• CO₂ is important to monitor clinically (as it is a gaseous acid) can do this via capnography