Carbon Dioxide

Question 1

Why is there a difference between the amount of CO2 and amount of O2 in blood

Answer 1

CO2 dissolves more easily

CO2 reacts with water

CO2 reacts with Hb

Question 2

What reaction does CO2 undergo once it dissolves in water

Answer 2

CO2 + H20 -> H+ + HCO3^-

Reversible reaction

Catalysed by carbonic anhydrase in RBCs

Question 3

How does the partial pressure of CO2 affect the pH

Answer 3

Rise CO2 causes pH to fall -> increases acidity

Fall in CO2 causes pH to rise -> increases alkalinity

This is because changes in CO2 pushes the reversible reaction either left or right, which in turn changes the pH

Question 4

What is the function of the high levels of HCO3-

Answer 4

Prevents most dissolved CO2 from reacting with water to form carbonic acid

Prevents the plasma becoming acidic

Question 5

Where is bicarbonate produced and what happens to the H+ ion produced in the reaction

Answer 5

Most bicarbonate is produced in RBCs, catalysed by carbonic anhydrase

Bicarbonate ions are then transported out the RBC via chloride-bicarbonate exchanger -> creates high plasma [HCO3-]

H+ ions bind with Hb within RBCs - bind to -vely charge part

Question 6

Describe the binding capacity of Hb for H+

Answer 6

If more O2 bound to Hb then less H+ ions bind to Hb

If less O2 bound to Hb then more H+ ions bind to Hb

Question 7

How does bicarbonate buffer acid produced by the body

Answer 7

Bicarbonate ions react with the acid to produce CO2 which is then removed by breathing

Question 8

How is [HCO3-] controlled

Answer 8

Kidneys control amount of HCO3- in the blood by varying excretion

Question 9

How does the binding of H+ ions to Hb effect the amount of CO2 present in the plasma

Answer 9

Increased binding of H+ ions to Hb means more HCO3- can be produced

This means that more CO2 can be present in the plasma as it can be converted to HCO3- in RBCs

Question 10

How is CO2 released when RBCs get to the lungs

Answer 10

Once venous blood arrives in the lungs, Hb picks up O2 and gives up the H+ ions

H+ ions react with HCO3- ions to form CO2

CO2 is then breathed out

Question 11

How are carbamino compounds formed

Answer 11

Carbamino compounds are formed when CO2 binds directly to amine groups on globin portion of Hb

More carbamino compounds are formed at tissues due to high pCO2

Question 12

How is CO2 transported

Answer 12

Dissolved in plasma

As hydrogen bicarbonate

As carbamino compounds

Question 13

What equation is used to calculate the pH

Answer 13

Henderson-Hasselback equation

Question 14

Why can pO2 fall considerably without markedly affecting saturation

Answer 14

Because the Hb is still well saturated

It takes a large drop in pO2 to decrease tissue perfusion

Question 15

What determines the plasma pH

Answer 15

The ratio of [HCO3-] to pCO2

Not the absolute values, the ratio!

Question 16

How does the kidney control the [HCO3-] and how does this compensate for changes in pCO2

Answer 16

Kidneys control [HCO3-] by changing excretion rate

If pCO2 increases and acidosis occurs, the kidneys compensate by increasing [HCO3-] by decreasing excretion

If pCO2 decreases and alkalosis occurs, kidneys increase secretion to decrease [HCO3-]

Question 17

What sensors control breathing - where are they found

Answer 17

Peripheral chemoreceptors - found in carotid and aortic bodies

Central chemoreceptors - found in medulla of the brain

Question 18

What do peripheral chemoreceptors detect and what do they stimulate

Answer 18

Peripheral chemoreceptors detect changes in pO2 due to large flow through them

Stimulate - increased breathing, changes in HR, changes in blood flow distribution

Question 19

What do central chemoreceptors detect and where abouts in the medulla are they found

Answer 19

Detect changes in pCO2 in the CSF

Are found within the choroid plexus

Question 20

Describe how central chemoreceptors respond to an increase in pCO2

Answer 20

Increased pCO2 drives more CO2 into the CSF causing a decreased pH as there is nothing in the CSF to buffer the increased CO2 - HCO3- is constant in CSF

Fall in CSF pH is detected by receptors which then increase ventilation

Increased ventilation lowers the pCO2 and restores the CSF pH