Block 4: 11 Carbon Dioxide Transport and Content

What is the most abundant form of CO2 transport in the blood?

Bicarbonate (HCO₃⁻): 70-90% of all CO2

Does CO2 move out of the blood into alveoli actively or passively?

Passively

Describe the path of CO2 in the body

Produced in the tissues, moved into the blood, carried to lungs, then transfered into alveoli for removal

True or False: CO2 and O2 binding are inversely related.

True, CO2 and O2 binding are inversely related. (increase one decreases other)

PRACTICE: True or false; HCO3- is formed in the tissues.

True; HCO3- is formed in the tissues (and CO2 is formed in the lungs).

With a low delta P driving CO2 movement from blood to alveoli, why is CO2 never diffusion impaired?

CO2 is so highly soluble that it is never diffusion impaired even at low PCO2

Describe the release of the three forms of CO2 in the blood at the lungs

Carbamino: H+ splits from Hb and causes CO2 release from carbamino compounds Bicarb: H+ from HHb-CO2 split reacts with HCO₃⁻ that reenters the cell to form dissolved CO2 and H2O Dissolved: Dissolved CO2 in plasma and dissolved CO2 in RBCs moves to alveoli

What is the Haldane effect?

Deoxygenation of Hb in the tissues increases the affinify of Hb for CO2 This shifts CO2-blood equilibrium curve up for venous blood, meaning venous blood can hold a higher CO2 contnt at a given PCO2 than arterial In tissues: increases CO2 loading In lungs: increases CO2 unloading

Describe the storage of the three forms of CO2 in the blood at the tissues

Dissolved: Dissolved CO2 is produced by the tissues and moved into the plasma Bicarb: Some dissolved CO2 enters the RBCs and reacts wtih H2O to form HCO₃⁻ (into plasma) and H+ Carbamino: H+ from bicarb formation binds with Hb, allowing CO2 binding and formation of carboamino compounds

PRACTICE: True or false; reduced Hb carries less CO2 when compared to HbO2.

False; reduced Hb carries more CO2 when compared to HbO2 (not less).

QUIZ 2: True or false; reduced Hb carries less CO2 when compared to HbO2.

False; reduced Hb carries more CO2 when compared to HbO2 (not less).

Is the formation of carbamino and bicarbonate reversible or not?

Formation (done in the tissues) is reversible, allowing release in the lungs

Do CO2 and O2 bind to the same sites on Hb?

No, CO2 and O2 bind to different sites on Hb

In which forms is CO2 carried in the blood?

1. Dissolved: 5% of all CO2 2. Carbamino: 5% of all CO2 (bound to terminal amine groups of blood proteins such as Hb) 3. Bicarbonate (HCO₃⁻): 90% of all CO2

What is the delta P between alveolar PCO2 and venous PCO2?

Alveolar PCO2 = 40 mmHg Venous PCO2 = 46 mmHg Delta P = 6 mmHg

What is the carrying capacity of CO2 in the arterial blood and venous blood?

Arterial: 48 ml CO2/100 ml blood Venous: 52 ml CO2/100 ml blood

QUIZ 2: True or false; capnography measures inspired CO2.

False; capnography measures expired CO2, not inspired CO2.

What enzyme catalyzes the binding of CO2 with H2O to form HCO₃⁻?

Carbonic anhydrase (CA)

PRACTICE: True or false; dissolved CO2 gas is the major form of CO2 in arterial blood.

False; HCO3- is the major form of CO2 in arterial blood (not dissolved CO2 gas).

PRACTICE: True or false; carbamino Hb is the major form of CO2 in venous blood.

False; HCO3- is the major form of CO2 in venous blood (not carbamino Hb).

True or False: CO2 content and PaCO2 are inversely related.

False, CO2 content and PaCO2 are POSITIVELY related. (increase one increases other)

What is the Bohr effect?

Increased CO2 and H+ binding to Hb (acidosis) decreases affinity of Hb for O2 This causes right shift in O2 equilibrium curve that allows easier O2 release at tissues) Occurs in the metabolic tissues that generate CO2 In tissues: increases O2 unloading In lungs: increases O2 loading

PRACTICE: Which direction will increased CO2 shift the HbO2 curve?

Increased CO2 will shift the HbO2 curve to the right.

MOCK EXAM: A post operative patient is still unconscious, but is receiving supplemental oxygen via a mask device. An ABG indicates that pH = 7.31, PaO2 = 155 mm Hg, PaCO2 = 82 mm Hg, and HCO3- = 26 mEq/L. Which of the following statements is FALSE? a) the patient is acidotic b) the HCO3- is normal c) the PaCO2 is elevated d) ventilation is sufficient given that PaO2 = 155 mm Hg

d) ventilation is sufficient given that PaO2 = 155 mm Hg Ventilation is assessed via PaCO2, not PaO2. The PaO2 is elevated because of the supplemental oxygen therapy. However, the high PaCO2 indicates that ventilation is insufficient.