Week 4 - CO2 in the blood

Question 1

Why is there so much CO2 in arterial blood?

Answer 1

-Control blood pH between 7.35-7.45

Question 2

What happens to CO2 in plasma?

Answer 2

• Dissolves and reacts with water to form carbonic acid which dissociates quickly into H+ and HCO3-
• Reversible reaction

Question 3

What determines what happens to dissolved CO2 in plasma?

Answer 3

-The concentration of the reactants and products

ie more CO2 drives forward reaction, more bicarb drives reverse reaction

Question 4

What determines how much CO2 dissolves in plasma? What effect does this have on pH?

Answer 4

• pCO2
• If pCO2 rises, more CO2 dissolves and pH will fall
• If pCO2 falls, less CO2 dissolves and pH rises

Question 5

What controls pCO2? Describe effects of increases/decreases

Answer 5

• Respiratory rate
• Decreased rate -> increased pCO2
• Increased rate -> decreased pCO2 (blowing more off)

Question 6

What is the conc of HCO3- in plasma?

Answer 6

-20-25mmol/L

Question 7

What is the significance of the high HCO3- in plasma?

Answer 7

• Prevents all dissolved CO2 from reacting

- Buffers acid from metabolically active tissues

Question 8

Where does HCO3- in plasma come from?

Answer 8

• Some from reacted CO2

- Mostly from RBCs

Question 9

What is the henderson-hasselbach equation?

Answer 9

-pH=pK+Log([HCO3]/(pCO2x0.23))

pK is a constant =6.1 at 37 degrees

Question 10

What is the ratio of HCO3 to CO2 in arterial blood?

Answer 10

-20:1

Question 11

Explain how the plasma has such a high concentration of bicarb

Answer 11

• CO2 diffuses across RBC membrane
• CO2 reacts with H20 to form H+ and HCO3-
• This reaction is speeded up by CA
• RBCs have a high capacity to sequester H+ via Hb and thus the forward rxn is promoted inside RBCs
• HCO3- is pumped out of the cell by a chloride bicarbonate exchanger causing the high plasma concentration

Question 12

Explain the kidneys role in bicarb control

Answer 12

-Controls the amount of excretion of bicarb and thus its concentration in plasma and ultimately has a control in plasma pH

Question 13

What two factors determine plasma pH?

Answer 13

-CO2 and HCO3-

Question 14

How does bicarb act as a buffer in plasma?

Answer 14

• Metabolically active tissues produce acids (lactic/keto etc)
• Acids react with HCO3 to produce CO2
• CO2 excreted by the lungs
• Minimal change in pH

Question 15

What is the effect of extra acid in the body on bicarbonte?

Answer 15

-Causes bicarbonate to decrease

Question 16

Why is there a higher pCO2 in mixed venous blood?

Answer 16

-Higher pCO2 due to metabolically active tissues means more CO2 dissolves

Question 17

Explain how buffering of H+ by Hb is affected by oxygenation? How does this contribute to CO2 transport?

Answer 17

• H+ is always bound to Hb, the amount is dependant upon the state of the Hb molecule
• In the presence of O2, the R state is promoted and less H+ ions bind, as at the lungs
• In decreased pO2, the T state is promoted and more H+ is bound
• Thus at the lungs there is a higher concentration of H+ in RBC which drives the reverse reaction and CO2 is formed, pumped across the RBC membrane and breathed out
• At the tissues there is a lower concentration of H+ in the RBC, the forward reaction is promoted and HCO3 is formed and pumped across the RBC membrane into plasma

Question 18

Why doesnt pH of venous blood decrease if it has a higher pCO2?

Answer 18

• More dissolved CO2 present in plasma due to more HCO3- being produced by RBCs
• However, the amount of HCO3- produced due to Hb capacity to sequester H+ and drive the forward rxn means that the extra CO2 is buffered and the forward reaction is promoted again in the plasma
• Therefore there is a rise in both HCO3 and CO2 so there si a very small change in plasma pH

Question 19

What 3 ways can CO2 be carried in the body?

Answer 19

• Dissolved in plasma
• Reacted as HCO3-
• Bound to proteins

Question 20

Explain how CO2 is carried bound to proteins and how this acts as a CO2 transporter

Answer 20

• CO2 binds directly to proteins
• Binds directly to the amine group of Hb
• Doesnt effect acid-base balance
• More carbamino compounds formed at the tissues due to higher pCO2 and unloading of O2 facilitates binding of CO2 to Hb
• This CO2 is given up at the lungs

Question 21

In what proportions of the 3 different forms in CO2 carried?

Answer 21

• 60% bicarb
• 30% carbaminos
• 10% dissolved in plasma