Gas Transport - CO2

Question 1

How is CO2 transported in the blood?

Answer 1

Dissolved CO2 (~10%)
Carbamino compounds (~21%)
Bicarbonate (HCO3-) (~69%) = MAJOR ROUTE

Question 2

Describe the exchange of O2 and CO2 from the vessels and alveoli

Answer 2

O2 will diffuse from alveolus to blood down conc gradient

CO2 will diffuse out of blood into alveolus down conc gradient

Question 3

What is the reaction between CO2 and H2O? Which enzyme catalyses this reaction?

Answer 3

CO2 + H2O H2CO3 H+ + HCO3-

Carbonic anhydrase (dehydratase) present in RBCs, salivary glands, stomach, pancreas, renal tubular epithelium etc NOT plasma!

Question 4

What is a strong acid?

Answer 4

Dissociates 100% (donates all H+ ions)

Question 5

What is a weak acid?

Answer 5

Doesn’t fully dissociate, will equilibrate with conjugate base forming a buffer pair that responds to changes in [H+] by reversibly binding it

Question 6

Where do bicarbonate ions originate from?

Answer 6

Bicarbonate ions in plasma are there from kidneys and chemical reaction with CO2

Question 7

Define Acid and Base

Answer 7

Acid = chemical that donate H+
Base = chemical that accept H+

Question 8

What is the equation for pH?

Answer 8

pH = -log10[H+]

[H+] is calculated in moles per litre (mol/L)

Question 9

What is the normal range for blood pH in human body?

Answer 9

7.36-7.44 (average 7.4)

Question 10

What are volatile acids?

Answer 10

• More easily vapourised
• Can leave solution + enter atmosphere
• Linked to respiratory acidosis (H2CO3)
• Excreted by lungs

Question 11

What are non-volatile acids?

Answer 11

• Fixed/non-respiratory acids
• Organic acids e.g. lactic/keto acids
• Metabolic acidosis
• Excreted by kidneys

Question 12

How does physiological buffering occur?

Answer 12

Respiratory:

• if body produces too much H+, H+ reacts with HCO3- to form CO2
• CO2 breathed out restoring pH

Renal:

• if pCO2 is too high, kidneys excrete less HCO3-
• Plasma [HCO3-] is raised, restoring pH

Question 13

[H+] and therefore pH is determined by [CO2] and HCO3-. Which directions would these push the reaction respectively?

Answer 13

CO2 = push to RHS
HCO3- = push to LHS

(increase pCO2 = decrease pH = more acidic)
(increase [HCO3-] = increase pH = more alkaline)

Question 14

What is the Henderson-Hasselbalch equation?

Answer 14

pH = pK + log10[HCO3-]/[CO2]

pK is constant = 6.1
Concs are for arterial blood

Question 15

What is important about the balance of [HCO3-] and pCO2?

Answer 15

Physiological buffer system - where different physiological mechanisms control these HCO3- and CO2.
Need very large amounts of HCO3- to buffer all H+ coming out of cells/tissues

Question 16

How is H+ buffered?

Answer 16

H+ buffered by histidine residues of globin in Hb - enhanced by deoxygenation (therefore venous blood can carry more H+)

Question 17

How is HCO3- transported in order to maintain pH balance?

Answer 17

Increase in cellular HCO3- leads to HCO3- exiting the cell and entering plasma triggering electrolyte imbalance (inside cell more +ve, outside -ve)

Cl- ions enters in exchange for HCO3- to aid this process carried by Cl-/HCO3- exchanger = balances electrolyte imbalance

End result = majority CO2 is transported into blood as bicarbonate

Question 18

Explain the relationship between CO2 and O2 in tissues

Answer 18

Taking up CO2 reduces Hb O2 affinity = Bohr effect

Giving up O2 increases CO2 carriage by blood = Haldane effect

Bohr + Haldane facilitate gas transfer required at tissue level