7. Chemical control of breathing

Question 1

What are 2 functions of the respiratory system?

Answer 1

1. maintain oxygen and CO2 partial pressure gradients to optimise transfer
2. regulate pH of extracellular fluid

Question 2

what do peripheral chemoreceptors detect?

Answer 2

sense PO2, PCO2 and pH levels

Question 3

what do central chemoreceptors detect?

Answer 3

pH and pCO2

Question 4

what do Peripheral and central chemoreceptors do?

Answer 4

send information to brain respiratory centres receive resulting in adjustments both in depth and frequency of ventilation as needed

Question 5

where are peripheral chemoreceptors located?

Answer 5

Carotid bodies
• Located bifurcation common carotid arteries

Aortic bodies
• Located in aortic arch (thorax

Question 6

what are the carotid bodies and aortic bodies sensitive to?

Answer 6

– Both bodies primarily sensitive to decreases in arterial PO2 , although high PCO2
(hypercapnia) and low pH (acidosis) also stimulate

Major function carotid & aortic bodies sense hypoxaemia & signal cells in the medulla to increase ventilation

Question 7

what happens if peripheral chemoreceptors sense low PO2 and/or high PCO2?

Answer 7

they will feed back to medulla respiratory centres to increase minute ventilation – this leads to increase in PO2 and decrease in PCO2

Question 8

how does increasing minute ventilation compensate for acidosis?

Answer 8

CO2 + H20 HCO3- + H+

Therefore if CO2 levels increase H+ increases; Conversely decreasing CO2 will cause H+ decrease – so pH rises

Question 9

what is the innervation to carotid and aortic bodies?

Answer 9

– Carotid body – branch of CN IX

– Aortic body branch CN X

Question 10

what are peripheral chemoreceptors most sensitive to?

Answer 10

arterial partial pressure of oxygen – low PO2

Question 11

what increases peripheral chemoreceptors’ sensitivity to acidosis and hypercapnia?

Answer 11

Hyypoxaemia

Question 12

which of the chemoreseptors are first to respond?

Answer 12

peripheral

Question 13

why can the body tolerate small changes in pO2 without significant tissue hypoxia?

Answer 13

As the oxy-haemoglobin dissociation curve shows, the pO2 may vary over quite a range around the normal value of 13.3 kPa without any alteration in the degree of oxygen saturation of Hb

Question 14

what is the pH of plasma determined by?

Answer 14

ratio of [HCO3-]: pCO2

Question 15

Define hyperventilation

Answer 15

Ventilation increase without change in metabolism (without the need for it)

Question 16

Define hypoventilation?

Answer 16

Ventilation decrease without change in metabolism

Question 17

distinguish between chemoreceptors and baroreceptors

Answer 17

Chemoreceptors are located in the carotid bodies and aortic bodies, while Baroreceptors – are located in the carotid sinus and aortic arch and detect changes in arterial BP)

Question 18

What type of change in pO2 stimulates the peripheral chemoreceptors and how do they respond?

Answer 18

– Stimulated only by large falls in pO2.
– Stimulation of these receptors results in an Increase in tidal volume and rate of respiration. They also cause changes in circulation directing more blood to brain and kidneys & increased pumping of blood by the heart

Question 19

do peripheral chemoreceptors adapt to chronic hypoxaemia?

Answer 19

no

there is a respiratory drive as long as pO2 is low

Question 20

Describe peripheral chemoreceptor sensitivity to pCO2.

Answer 20

Not particularly sensitive to pCO2, needing a large change in pCO2 to stimulate them. But do respond quickly to large changes in pCO2.

Question 21

Describe peripheral chemoreceptor sensitivity to pH.

Answer 21

Directly activated by change in the pH of blood. A

low pH results in an increased respiratory rate and tidal volume.

Question 22

Where are central chemoreceptors located and what are they exposed to?

Answer 22

On the ventral surface of the medulla, and exposed to the cerebro-spinal fluid

– Specialised neurons located on BRAIN side of Blood Brain Barrier (BBB) – i.e. located within the brain parenchyma and bathed in brain extracellular fluid (BECF), which is separated from arterial blood by the blood-brain barrier (BBB)

Question 23

What do central chemoreceptors respond to?

Answer 23

• Sense increases in arterial PCO2 and—much more slowly—decreases in arterial pH, but not arterial PO2

Question 24

What does the blood brain barrier allow free passage of and vice versa?

Answer 24

Allowed CO2 but not HCO3

Question 25

define blood brain barrier

Answer 25

endothelial cells of blood vessels in brain surrounded by pericytes and foot `processes (end feet) of astrocytes to create a highly selective permeability barrier

Question 26

What determines CSF pH?

Answer 26

Determined by its own [HCO3] / [Dissolved CO2] buffer system

Question 27

What is CSF [HCO3] determined by?

Answer 27

Activity of choroid plexus cells which

pump HCO3 into and out of the CSF

Question 28

how central chemoreceptors sense changes PCO2 &

pH?

Answer 28

• BBB separates central chemoreceptors in medulla from arterial blood
• BBB has a low permeability to ions such as H+
and HCO3-, but high permeability to small molecules like CO2
• CO2 diffuses into brain brain extracellular fluid (BECF) bathing brain cells including Central Chemoreceptor Neuron Cells
• CNS very limited HCO3- buffering capacity and
therefore acidosis develops
• Even small decreases in pH raise the firing rate
of the central chemoreceptor neurons thus increasing ventilation

Question 29

How do central chemoreceptors respond to changes in CSF pH?

Answer 29

Impulses from the chemoreceptors travel to the brain stem respiratory centres, normally producing changes in breathing which tend to restore
CSF pH. This negative feedback is the principal means by which ventilation is controlled.

Question 30

How do the central chemoreceptors respond to hypercapnia?

Answer 30

• If CO2 remains elevated, pH of CSF/BECF slowly recovers (i.e., increases) because choroid plexus increases active transport of HCO3 into
CSF and BECF –bicarb buffers protons generated by increased CO2 thus CO2 - induced acidosis gradually becomes smaller- pH rises
• This transport represents CNS metabolic compensation to respiratory acidosis

Question 31

what does the central chemoreceptor response to constant hypercapnia mean?

Answer 31

means that a higher level of CO2 is needed to cause

acidosis and thereby increase ventilation – thus CO2 drive for ventilation has been “reset” at higher level.

Question 32

Why is CSF pH corrected much more quikcly than blood pH?

Answer 32

because of its small volume

Question 33

What is the drive for increased ventilation in long standing hypercapnia and what is this called?

Answer 33

Hypoxia, (hypoxic drive)

Question 34

what is the difference in extent of change needed in pCO2 and pO2 to cause response?

Answer 34

Falling pO2(acting via the peripheral chemoreceptors) produces increased ventilation only when the fall is considerable

Small changes in arterial pCO2 (acting via the central
chemoreceptors) produce very rapid changes in
breathing

Question 35

How does hyperventilation affect alveolar and arterial partial pressures and what effects does this have on blood pH?

Answer 35

Alveolar: pO2 increases, pCO2 decreases
Arterial: pO2 increases, pCO2 decreases
pH increases

Question 36

How does hypoventilation affect alveolar and arterial partial pressures?

Answer 36

Alveolar: pCO2 increases, pO2 decreases
Arterial: pCO2 increases, pO2 decreases
pH decreases

Question 37

describe how hypoventilation results in respiratory acidosis?

Answer 37

Removal of CO2 from lungs is less rapid than its production. The alveolar pCO2 rises, so dissolved
CO2 rises more than HCO3 - producing a fall in plasma pH - Respiratory Acidosis.

Question 38

how does the body respond to respiratory acidosis?

Answer 38

If this condition persists, the kidneys respond to the low pH by reducing excretion of HCO3- , thus increasing plasma [HCO3 -]and restoring the ratio of [ HCO3-]/ [Dissolved CO2] and the pH, to near to normal - Compensated Respiratory Acidosis

Question 39

describe how hyperventilation results in respiratory alkalosis?

Answer 39

Removal of CO2 from alveoli is more rapid than its production. Alveolar CO2 falls - plasma pH rises - Respiratory Alkalosis.

Question 40

how does the body respond to respiratory alkalosis?

Answer 40

If the condition persists, the kidneys respond by excreting HCO3-, so the ratio of [ HCO3-]/ [Dissolved CO2] returns tonear normal, and therefore pH is restored, but buffer base concentration is reduced - Compensated Respiratory Alkalosis

Question 41

How long do the kidneys take to respond to change in pH?

Answer 41

2-3 days

Question 42

What does the sigmoidal oxygen dissociation curve allow?

Answer 42

pO2 can fall considerably before saturation is markedly affected

Question 43

How does metabolic acidosis occur?

Answer 43

Excess Metabolic production of acid: If excessive acid
is formed in the body (e.g. as in diabetic ketoacidosis) the [HCO3-] in the blood is used up to buffer this acid, and the [HCO3-] in the blood drops. Therefore, the pH of blood falls. This is a reduction of buffer base - Metabolic Acidosis.

Question 44

What is compensated metabolic acidosis?

Answer 44

Increased ventilation which lowers pCO2, correcting the [HCO3]/[dissolved CO2] ratio and thereby increasing pH

Question 45

What does metabolic acidosis do to levels of bicarbonate in the blood?

Answer 45

Reduced, until corrected by the kidney

Question 46

How does metabolic alkalosis occur, give an example of what could cause it?

Answer 46

Excess [HCO3-] in the plasma, vomiting

Question 47

What is compensated metabolic alkalosis?

Answer 47

decreased ventilation which increases pCO2, correcting the [HCO3]/[dissolved CO2] ratio and thereby decreasing pH