Acid-Base in the Control of Ventilation

Question 1

What are the three components that are involved in the control of ventilation?

Answer 1

A central controller, sensors and effeects

Question 2

What three types of receptors does the respiratory control system have?

Answer 2

Central and peripheral chemoreceptors and peripheral mechanoreceptors

Question 3

What do central chemoreceptors detect?

Answer 3

CO2, specifically hypercapnia in the CSF they bathe in (within the medulla oblongata) as a result of increased acidity of the CSF

Question 4

Where are the central chemoreceptors, involved in control of ventilation, found?

Answer 4

Specialised neurones located neurones in the ventral surface of the medulla oblongata

Question 5

What is the normal pH of CSF?

Answer 5

7.32

Question 6

Why does the CSF have a far weaker buffering capacity than the blood?

Answer 6

Because no plasma proteins are filtered through the choroid plexus, the CSF contains almost no protein

Question 7

What stimulates most of the urge to breathe?

Answer 7

Drive from the central chemoreceptors in the medulla oblongata

Question 8

Describe the role of the central chemoreceptors in the control of ventilation

Answer 8

Specialised neurones in medulla oblongata, which detect hypercapnia (as a result of increased acidity in the CSF) –> activates chemoreceptors –> stimulate neurones in the respiratory centres in the medulla –> increased ventilation rate

Question 9

How can central chemoreceptors detect hypercapnia due to an increased acidity?

Answer 9

Carbon dioxide diffuses freely through the BBB into the CSF where it reacts with water to form carbonic acid which is then converted to protons and bicarbonate via carbonic anhydrase.

Question 10

Why do central chemoreceptors become desensitised in chronic pulmonary disease?

Answer 10

There are chronically high CO2 levels in the blood, and therefore the central chemoreceptors gradually become less sensitive to this stimuli and therefore the drive for ventilation from CO2 (hypercapnia) is diminished

Question 11

Where are the peripheral chemoreceptors, involved in control of ventilation, found?

Answer 11

In the carotid bodies at the bifurcation of the common carotid arteries and in the aortic bodies above and below the aortic arch.

Question 12

What does the carotid body contain?

Answer 12

Most heavily vascularised tissue in the human body

Question 13

What do peripheral chemoreceptors detect?

Answer 13

Mainly to reduced pO2 and plasma pH (acidity caused by hypercapnia) and less so to raised pCO2 directly

Question 14

Describe the afferents from the peripheral chemoreceptors

Answer 14

Afferents from the chemoreceptors travel mainly in the glossopharyngeal and vagus nerves to the brainstem

Question 15

What cells make up the carotid body?

Answer 15

Glomus cells

Question 16

What are the two types of cell that make up the carotid body?

Answer 16

Glomus type I (chief) cells, and glomus type II (sustentacular) cells

Question 17

Describe glomus type I (chief) cells of the carotid body

Answer 17

These cells are derived from the neuroectoderm like nerve cells and they release neurotransmitters that stimulate the sensory endings of the vagus and glossopharyngeal afferent nerves that project to the medullary respiratory centres

Question 18

Describe glomus type II (sustentacular) cells of the carotid body

Answer 18

Resemble ganglia and act as supporting cells

Question 19

How do type I glomus (chief) cells of the carotid body detect hypoxia?

Answer 19

Decrease in pO2 or arterial pH–> depolarisation of cell membrane –> opens voltage-gated Ca2+ channels –> rise in intracellular Ca2+ concentration –> release of variety of neurotransmitters –> act on receptors on afferent nerve fibres adjacent to glomus cell –> cause action potential which travels in vagus and glossopharyngeal fibres to the respiratory centres in the medulla

Question 20

What are the medullary respiratory centres?

Answer 20

Group of neurones in the reticular formation of the medulla that run from the top of the spinal cord to the thalamus

Question 21

What are the two parts of the medullary respiratory centre?

Answer 21

Has two parts: nucleus of the solitary tract and nucleus ambiguus.

Question 22

What are the respiratory centres of the brainstem?

Answer 22

Respiration is controlled by groups of neurones in the pons and medulla and there are three main groups; pontine respiratory centre, ventral and dorsal medullary respiratory centres

Question 23

What is the efferent action of the respiratory centres?

Answer 23

Axons arise from the cells in all parts of the reticular formation and descend down the spinal cord in the reticulospinal tracts (down spinal cord, around margin of ventral horn and activates the phrenic nerve)

Question 24

What is the function of the reticulospinal tract?

Answer 24

The reticulospinal tract arises in the pons and medulla and descends diffusely down the spinal cord around the margin of the ventral horn (it also carries the drive to the ANS) and activates the phrenic nerve

Question 25

Describe the nucleus of the solitary tract in the medullary respiratory centre (dorsal respiratory group)

Answer 25

Sensory integrating centre of the respiratory centre which receives input from pH detectors, pulmonary stretch, cough and J receptors. This centre is found on the dorsal surface of the medulla.

Question 26

Describe the nucleus ambiguous in the medullary respiratory centre (ventral respiratory group)

Answer 26

Sends axons into the more ventrally positioned motor output centre which sends impulses down to the spinal cord and to the phrenic nerve/

Question 27

What is Cheyne-Stokes breathing?

Answer 27

A gradual increase in depth and rate of breathing followed by gradual decrease that results in temporary stop of breathing (apnoea)

Question 28

What is the cause of Cheyne-Stokes breathing?

Answer 28

Heart failure or strokes that affect the blood supply to the medulla or individuals with traumatic injuries or tumours of the brainstem; indicates damage to the brainstem or to the inputs from the respiratory receptors to the brainstem

Question 29

Describe the pontine respiratory centres

Answer 29

These regulate and modulate the medullary centres during speech when you want to hold your breath voluntarily, for example. These centres are controlled from the cerebral cortex which can modulate but not overcome the actions of the medullary centres.

Question 30

What is the role of the pulmonary stretch receptors at the end of inspiration?

Answer 30

They send action potentials through afferents of the vagus nerve to the respiratory centres in the pons and medulla to inhibit further respiration (Hering-Breuer reflex)

Question 31

Describe pulmonary stretch receptors

Answer 31

Depolarise in response to distension in the lung (activity sustained with lung inflation), found in the bronchioles and small bronchi and act as cut-off switches to inhibit inspiration when the lungs are fully inflated (prevents damage due to over-inflation)

Question 32

Describe irritant receptors of the lungs

Answer 32

Mechanoreceptors that lie between airway epithelial cells and are stimulated by noxious gases, smoke, inhaled dusts and cold air and are found in the lining of the trachea and the large bronchi. Stimulates the cough reflex to clear the airway of danger.

Question 33

Describe J receptors in the lungs

Answer 33

Chemoreceptors that respond to pulmonary oedema, pulmonary embolus, pneumonia and congestive heart failure –> stimulate increase in ventilation by detecting fluid and sending afferent signals to vagus nerve to increase respiratory rate

Question 34

Describe how the higher centres of the brain can control respiration

Answer 34

Higher centres of the brain are involved in voluntary control of ventilation rate and this is affected by pain, emotion and temperature, and the brain works to modulate the impulses from the pontine respiratory centre to the medullary respiratory centre before action is sent to the lungs

Question 35

How do irritant receptors in the lung modulate ventilation when stimulated?

Answer 35

Supresses inhalation by acting at the medullary respiratory centre in order to allow the release of substance inhaled (helps to blow off irritant)

Question 36

What is the main chemical drive for ventilation?

Answer 36

Hypercapnia as hypoxia on its own doesn’t stimulate breathing until the partial pressure of oxygen is less than 60mmHg, but moderate hypoxia does increase the sensitivity of the respiratory centres for hypercapnia to increase ventilation rate.