Lecture 8 - Chemical control of breathing

Question 1

Functions of the respiration system

Answer 1

Maintain O2 and CO2 partial pressures to optimise transfer

Regulate pH of ECF

Question 2

Hypercapnia

Answer 2

Rise of CO2

Question 3

Hypoxia

Answer 3

Low O2

Question 4

Hypocapnia

Answer 4

Decrease in CO2

Question 5

Exercise

Answer 5

pO2 decreases due to increased metabolism
pCO2 increases

Therefore increased respiration rate to excrete more CO2 and absorb more O2

Question 6

Hyperventilation

Answer 6

Increased respiration rate without an increased metabolic demand

O2 increases
CO2 decreases - Respiratory alkalosis

Question 7

Hypoventilation

Answer 7

Decreased respiration rate without change in metabolic demand

Decrease O2
Increased CO2 - respiratory acidosis

Question 8

Oxygen- Hb dissociation curve

Answer 8

Sigmoid
100% saturation at 8kPa

pO2 can fall considerably to 8kPa from 13 kPa before saturation is affected.

Question 9

pH

Answer 9

pK+log ([HCO3-]/pCO2 x 0.23)

• H2CO3- quilibrates with CO2 so can be used interchangeably

pCO2 controlled by respiration
HCO3- controlled by kidneys

Question 10

pH below 7

Answer 10

Denatures enzymes

Question 11

pH above 7.6

Answer 11

Free calcium concentration drops leading to tetany

Question 12

Kidney compensation for respiratory acidosis

Answer 12

Produce more HCO3-

Secrete more H+

Takes 2-3 days

Question 13

Kidney compensation for respiratory alkalosis

Answer 13

Excrete more HCO3-

Reabsorb more H+

Takes 2-3 days

Question 14

Metabolic acidosis

Answer 14

Tissues produce more H+
Reacts with HCO3- to produce CO2 which is breathed off
Decreasing HCO3- concentration in blood which decreases pH

Question 15

Metabolic acidosis compensation

Answer 15

Increased respiratory rate

Breath off more CO2 which lowers [CO2] correcting ratio

Question 16

Metabolic alkalosis

Answer 16

e.g. vomiting

Increases [HCO3-]
Increasing pH

Question 17

Metabolic alkalosis compensation

Answer 17

Hypoventilation to an extent but can’t starve brain of oxygen therefore limited

Question 18

Central chemoreceptors

Answer 18

In the CSF of the brain medulla

1. Detect changes in pCO2
2. Increased CO2, increased hyperventilation
3. More CO2 breathed off to correct ratio

Question 19

Peripheral chemoreceptors

Answer 19

In carotid and aortic body

Sensitive to:
H+
O2
CO2 - relatively insensitive

1. Large fall in O2
2. Stimulates: Hyperventilation
   Increased heart rate
   Increased blood flow to brain and kidneys

Question 20

O2 peripheral chemoreceptors

Answer 20

Less sensitive to small changes in O2 as receives large blood flow

Question 21

Central chemoreceptors sensitivity

Answer 21

More sensitive to pCO2 changes as less proteins so lower buffer capacity

Question 22

Choroid plexus

Answer 22

CSF is separated from the blood by the blood brain barrier

CSF [HCO3-] and [H+] is controlled by the choroid plexus (normally impermeable).

CSF pCO2 is determined by arterial pCO2

Question 23

COPD

Answer 23

1. Increased pCO2 in CSF detected by the chemoreceptors in the medulla as pH decreases
2. Increased hyperventilation to blow off CO2 restoring CSF pH
3. Persistent high pCO2 causes the choroid plexus cells to become more permeable to [HCO3-]
4. HCO3- diffuses into the CSF correcting the acid base set point in the CSF therefore a new set point is produced with a higher CO2
5. pO2 becomes the new driving force for hyperventilation as low pO2 detected by the peripheral chemoreceptors