Final: Respiration 5 Flashcards
how is CO2 transported (3)
- physically dissolved CO2 gas in plasma
- CO2 bound to Hb
- CO2 in bicarbonate for
CO2 transport: physically dissolved in plasma (2)
- small amounts in this form (~5%)
- CO2 is more soluble in body fluids than O2
CO2 transport: Hb-bound (2)
- some in this form (5-23%)
- eg. carbaminohemoglobin
CO2 transport: bicarbonate form (3)
- most in this weak base form
- CO2 + H20 <–> carbonic acid <–> bicarbonate (HCO3-) + protons
- reaction catalyzed by carbonic anhydrase
carbonic anhydrase
- reduces half time of bicarbonate reaction from 60-90s to a milliseconds
bicarbonate reaction simplified
CO2 + H20 <–> HCO3- + H+
why is carbonic acid usually excluded from the bicarbonate formation formula
- it basically does not exist because its is metabolized so quickly
Henderson Hasselbalch equation (2)
- HA <–> H+ + A-
- weak acid <–> protons + anions
Henderson Hasselbalch formula (2)
- pH = pK’ + log ([A-]/[HA])
- pK’ is negative log of the acid-dissociation constant
pK’ = pH
- half of the weak acid is dissociated and physically dissolved, half exists as the weak acid
Henderson Hasselblach formula: CO2 (2)
- CO2 is a gas, so total dissolved CO2 = PCO2 x CO2 solubility (Bunsen solubility coefficient, alpha)
- pH = pK’ + log ([HCO3-]/ alpha * PCO2)
pK’ and pH: CO2 (2)
- pK’ of CO2 is ~6.1
- blood pH is 7.4-7.8
pK’ and pH: CO2/HCO3- percentages (2)
- 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
dissolved CO2 vs HCO3- (2)
- HCO3- is not very permeable across membranes
- CO2 is permeable across membranes
RBC level: CO2 from tissues into the RBC (3)
- physically dissolved CO2 diffuses down partial pressure gradient from tissues to blood
- CO2 diffuses into RBCs
- CO2 left in plasma is converted SLOWLY into bicarbonate as carbonic anhydrase is not present outside RBC
RBC level: CO2 from tissues inside the RBC (3)
- 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- - HCO3- is stored in the plasma for transport
RBC level: CO2 from tissues interacting with Hb inside the RBC (2)
- protons released in bicarbonate reaction stabilize the “T state”, driving O2 off Hb
- as Hb becomes deoxygenated, carbamino complexes (bound CO2) and H+ are bound by Hb (Haldane effect)
how do RBCs contribute to loading CO2 from the tissues into the blood (2)
- keep RBC H+ and HCO3- levels low
- CO2 can continue to enter blood, removing it from tissues
RBC level: CO2 from RBC to environment
- physically dissolved CO2 diffuses down its partial pressure gradient from blood to the air/water
RBC level: CO2 from RBC to environment; inside the RBC (2)
- removal of CO2 causes carbonic anhydrase to convert HCO3- and H+ into H20 and CO2 in RBC
- chloride shift: as RBC HCO3- falls, HCO3- enters RBC in exchange for Cl-
RBC level: CO2 from RBC to environment; Hb and CO2 (2)
- as Hb becomes oxygenated, carbamino complexes (bound CO2) and H+ are released from Hb (Haldane effect)
- H+ combine with RBC HCO3- to permit continued CO2 excretion until RBC leaves respiratory epithelium
how does RBCs contribute to processes at the respiratory surface
- O2 uptake facilitates CO2 removal in RBCs at the respiratory surface
what is the rate limiting step for CO2 excretion
- chloride shift
carbon dioxide equilibrium curve
- relationship between PCO2 and total CO2 content of the blood
Haldane effect: CO2 levels (3)
- 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
Haldane effect: CO2 unloading perspective (2)
- when deoxygenated blood is oxygenated, it elevates PCO2 and enhances CO2 removal
- Hb binds O2 and frees H+
Haldane effect: CO2 loading perspective (2)
- 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
Hb levels in RBCs
- levels
- risks
- 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
CA levels in RBCs
- present at high levels to rapidly and reversibly convert CO2 to HCO3- and vice versa
HCO3-/Cl- exchanger level in RBCs
- high levels to allow most CO2 to be transported in the plasma as HCO3-
