Respiratory Physiology 5.1- Chemoreceptors

Question 1

What does ventilatory control require?

Answer 1

requires stimulation of the skeletal muscles of inspiration

Question 2

What innervates the skeletal muscles of inspiration?

Answer 2

phrenic nerve- which goes to diaphragm intercostal nerves (to external intercostal muscles)

Question 3

Where does ventilatory control reside?

Answer 3

resides within ill defined centres located in the pons and medulla

Question 4

What are the factors that modulate respiratory centres?

Answer 4

1. emotion (via limbic system in the brain)

2.Voluntary over-ride (via higher centres in the brain)

3. Mechano-senosry input from the thorax (e.g. stretch reflex)
4. Chemical composition of the blood (PCO2, PO2 AND pH ) -detected by chemoreceptors

Question 5

Where do we find these ill defined respiratory centres?

Answer 5

between pons and the medulla

Question 6

What are the two major groups of neurons that are within the respiratory centres?

Answer 6

Dorsal Respiratory Group of neurons
- primarily involved in setting up stimulation of the inspiratory muscles (diaphragm and external intercostal muscles mainly)

Ventral Respiratory Group - stimulates the muscles of the tongue, pharynx, larynx and our muscles of expiration. Maintains basal tone which helps to maintain a patent airway.

Question 7

What are the two types of chemoreceptors?

Answer 7

Central and peripheral chemoreceptors

Question 8

Where are the central chemoreceptors found?

Answer 8

found in CNS- located in medulla

Question 9

Give characteristics of the central chemoreceptors?

Answer 9

respond directly to hydrogen ions (responding to carbon dioxide)

Question 10

Where are the peripheral chemoreceptors found?

Answer 10

carotid and aortic bodies

Question 11

Give characteristics of peripheral receptors?

Answer 11

respond primarily to changes in partial pressure of oxygen and less so to carbon dioxide
respond to changes in plasma hydrogen ion concentration
–secondary ventilatory drive

Question 12

Describe the central chemoreceptors in the medulla?

Answer 12

detect changes in hydrogen ion concentration in cerebrospinal fluid (interstitial fluid of the brain) that surrounds the brain

central chemoreceptors respond to that by increasing our rate and depth of breathing

Question 13

What is hypercapnea?

Answer 13

increase in partial pressure of carbon dioxide in systemic arterial blood

Question 14

What hydrogen ions will central chemoreceptors respond to and why?

Answer 14

can only respond to hydrogen ions that are in the cerebrospinal fluid

Question 15

What will happen if we get a reduction in hydrogen ion concentration of the cerebrospinal fluid?

Answer 15

reduces stimulation of the central chemoreceptors and if we turn off the central chemoreceptors we basically tell the respiratory centres in the brainstem to quiet down breathing, reducing inspiratory and expiratory breathing.

Question 16

What is the function of the blood brain barrier?

Answer 16

forms a much tighter barrier between plasma and interstitial fluid in the brain than it does elsewhere in the body.

Question 17

Why does the blood brain barrier not allow ions to cross?

Answer 17

because the brain is very sensitive to changes in ion composition

Question 18

What does the blood brain barrier allow to pass between the capillary and cerebrospinal fluid?

Answer 18

gas

Question 19

What will happen if we have an increase in partial pressure of carbon dioxide?

Answer 19

carbon dioxide will equilibrate between the capillary and cerebrospinal fluid

Question 20

What will happen if we have an increase in the partial pressure of carbon dioxide in our plasma?

Answer 20

increase in carbon dioxide in the cerebrospinal fluid and we end up with an increase in the hydrogen ion concentration. Increase in hydrogen ion concentration of cerebrospinal fluid is what then stimulates the central chemoreceptors. Which then feeds into the respiratory control centres and leads to increase in ventilation.

Question 21

What does increase in ventilation help with?

Answer 21

blow off that carbon dioxide

Question 22

What does a 10 percent increase in partial pressure of carbon dioxide cause?

Answer 22

100 percent increase in ventilation

Question 23

What are the complications of chronic lung disease?

Answer 23

in chronic lung disease the partial pressure of carbon dioxide can become chronically elevated

individuals become densitised to carbon dioxide levels and start relying on their peripheral chemoreceptors in order to affect their breathing pattern “hypoxic drive”

Question 24

Describe what hypoxic drive is?

Answer 24

as their tissues become hypoxic (as partial pressure of oxygen in tissues reduces) that is what stimulates ventilation in these individuals.

Question 25

What do peripheral chemoreceptors detect changes in?

Answer 25

detect changes in partial pressure of oxygen in systemic arterial blood and hydrogen ion concentration in systemic plasma

Question 26

When will the peripheral chemoreceptors kick into action and stimulate ventilation?

Answer 26

once partial pressure of oxygen falls below 60 mmHg because increasing ventilation would just put more oxygen into the plasma.

Question 27

What oxygen are the peripheral chemoreceptors responding to?

Answer 27

oxygen that is in solution in the blood, in the plasma
and not the total oxygen content

Question 28

What will happen to respiration rate in an anaemic patient with normal lung function, who has a blood oxygen content half the normal value?

Answer 28

It will stay the same.
If lungs are working normally, diffusion will take place normally and therefore the amount of oxygen in solution in plasma will be normal. And partial pressure of oxygen in blood is what peripheral receptors monitor- there will be no increase in respiratory rate.

Question 29

Describe the conflict of adding more oxygen to the plasma of those with anemia?

Answer 29

Adding more oxygen to plasma would do little to increase total blood oxygen content as the RBC are already 98% saturated at normal partial pressure oxygen in blood. The problem here is that there are not enough red blood cells or enough haemoglobin inside them (patient deficient in iron)

Question 30

Give characteristic of most gaseous anaesthetic agents?

Answer 30

increase respiratory rate but decrease tidal volume and so decrease alveolar volume.

Question 31

What is our biggest factor on alveolar ventilation?

Answer 31

tidal volume

Question 32

Describe effects of barbiturates and opioids?

Answer 32

depress the respiratory centres and overdoses of barbiturates or opioids often result in death as a result of respiratory failure. These drugs decrease sensitivity to pH and also reduce the response to PCO2.
Also turn down sensitivity of our peripheral chemoreceptor and get a decrease in response to falling partial pressure of oxygen as well.

Question 33

Describe effects of nitrous oxide?

Answer 33

commonly used as a sedative/ light anaesthetic agent
It blunts the peripheral chemoreceptor response to falling pao2.
Very safe because most individuals are not relying on the peripheral chemoreceptors.
Has no impact on central chemoreceptors and therefore has no impact on our monitoring of carbon dioxide in blood

Question 34

When is nitrous oxide problematic?

Answer 34

if give nitric oxide to someone who is already on ‘hypoxic drive’ - knock out their peripheral chemoreceptor response. Have no means of regulating their blood gas composition. O2 levels tumble and CO2 through roof.