Respiration: Carbon Dioxide Transport

Question 1

how is CO2 transported (3)

Answer 1

• physically dissolved CO2 gas in plasma
• CO2 bound to Hb
• CO2 in bicarbonate for

Question 2

CO2 transport: physically dissolved in plasma (2)

Answer 2

• small amounts in this form (~5%)
• CO2 is more soluble in body fluids than O2

Question 3

CO2 transport: Hb-bound (2)

Answer 3

• some in this form (5-23%)
• eg. carbaminohemoglobin

Question 4

CO2 transport: bicarbonate form (3)

Answer 4

• most in this weak base form
• CO2 + H20 <–> carbonic acid <–> bicarbonate (HCO3-) + protons
• reaction catalyzed by carbonic anhydrase

Question 5

carbonic anhydrase

Answer 5

• reduces half time of bicarbonate reaction from 60-90s to a milliseconds

Question 6

bicarbonate reaction simplified

Answer 6

CO2 + H20 <–> HCO3- + H+

Question 7

why is carbonic acid usually excluded from the bicarbonate formation formula

Answer 7

• it basically does not exist because its is metabolized so quickly

Question 8

why is carbonic acid usually excluded from the bicarbonate formation formula

Answer 8

• it basically does not exist because its is metabolized so quickly

Question 9

why is carbonic acid usually excluded from the bicarbonate formation formula

Answer 9

• it basically does not exist because its is metabolized so quickly

Question 10

Henderson Hasselbalch equation (2)

Answer 10

• HA <–> H+ + A-
• weak acid <–> protons + anions

Question 11

Henderson Hasselbalch formula (2)

Answer 11

• pH = pK’ + log ([A-]/[HA])
• pK’ is negative log of the acid-dissociation constant

Question 12

pK’ = pH

Answer 12

• half of the weak acid is dissociated and physically dissolved, half exists as the weak acid

Question 13

Henderson Hasselblach formula: CO2 (2)

Answer 13

• CO2 is a gas, so total dissolved CO2 = PCO2 x CO2 solubility (Bunsen solubility coefficient, alpha)
• pH = pK’ + log ([HCO3-]/ alpha * PCO2)

Question 14

pK’ and pH: CO2 (2)

Answer 14

• pK’ of CO2 is ~6.1
• blood pH is 7.4-7.8

Question 15

pK’ and pH: CO2/HCO3- percentages (2)

Answer 15

• pH is a bit more than 1 pH unit greater than pK’ for reaction
• 95% of total CO2 is HCO3- and only 5% is physically dissolved CO2

Question 16

dissolved CO2 vs HCO3- (2)

Answer 16

• HCO3- is not very permeable across membranes
• CO2 is permeable across membranes

Question 17

RBC level: CO2 from tissues into the RBC (3)

Answer 17

1. physically dissolved CO2 diffuses down partial pressure gradient from tissues to blood
2. CO2 diffuses into RBCs
3. CO2 left in plasma is converted SLOWLY into bicarbonate as carbonic anhydrase is not present outside RBC

Question 18

RBC level: CO2 from tissues inside the RBC (3)

Answer 18

1. CO2 dissociates to HCO3- + H+ due to high carbonic anhydrase levels in RBC
   2 Chloride shift: as RBC HCO3- increases, HCO3- exits RBC in exchange for Cl-
2. HCO3- is stored in the plasma for transport

Question 19

RBC level: CO2 from tissues interacting with Hb inside the RBC (2)

Answer 19

1. protons released in bicarbonate reaction stabilize the “T state”, driving O2 off Hb
2. as Hb becomes deoxygenated, carbamino complexes (bound CO2) and H+ are bound by Hb (Haldane effect)

Question 20

how do RBCs contribute to loading CO2 from the tissues into the blood (2)

Answer 20

• keep RBC H+ and HCO3- levels low
• CO2 can continue to enter blood, removing it from tissues

Question 21

RBC level: CO2 from RBC to environment

Answer 21

1. physically dissolved CO2 diffuses down its partial pressure gradient from blood to the air/water

Question 22

RBC level: CO2 from RBC to environment; inside the RBC (2)

Answer 22

1. removal of CO2 causes carbonic anhydrase to convert HCO3- and H+ into H20 and CO2 in RBC
2. chloride shift: as RBC HCO3- falls, HCO3- enters RBC in exchange for Cl-

Question 23

RBC level: CO2 from RBC to environment; Hb and CO2 (2)

Answer 23

1. as Hb becomes oxygenated, carbamino complexes (bound CO2) and H+ are released from Hb (Haldane effect)
2. H+ combine with RBC HCO3- to permit continued CO2 excretion until RBC leaves respiratory epithelium

Question 24

how does RBCs contribute to processes at the respiratory surface

Answer 24

• O2 uptake facilitates CO2 removal in RBCs at the respiratory surface

Question 25

what is the rate limiting step for CO2 excretion

Answer 25

• chloride shift

Question 26

carbon dioxide equilibrium curve

Answer 26

• relationship between PCO2 and total CO2 content of the blood

Question 27

Haldane effect: CO2 levels (3)

Answer 27

• for a given PCO2, the more H+ buffered, the greater the total CO2
• deoxygenated blood binds H+ to Hb, driving rxn to further create HCO3-
• deoxygenated blood carries more CO2 than oxygenated blood

Question 28

Haldane effect: CO2 unloading perspective (2)

Answer 28

• when deoxygenated blood is oxygenated, it elevates PCO2 and enhances CO2 removal
• Hb binds O2 and frees H+

Question 29

Haldane effect: CO2 loading perspective (2)

Answer 29

• as oxygenated blood enters tissues, unloading of O2 allows blood to hold more CO2 for same given PCO2
• facilitates CO2 removal from tissues into blood

Question 30

Hb levels in RBCs
- levels
- risks

Answer 30

• Hb levels in RBC s are very high, on the verge of solubility in the RBC
• sickle cell anemia is result of Hb levels beyond solubility limit and crystallizing

Question 31

CA levels in RBCs

Answer 31

• present at high levels to rapidly and reversibly convert CO2 to HCO3- and vice versa

Question 32

HCO3-/Cl- exchanger level in RBCs

Answer 32

• high levels to allow most CO2 to be transported in the plasma as HCO3-