Physiology: Control of Breathing

Question 1

Why do we need to maintain normal levels of PO2 and PaCO2?

Answer 1

For metabolic and biochemical stability (e.g. pH)

Question 2

Name the three basic elements controlling breathing.

Answer 2

Central control in the brainstem
Sensors - central/peripheral
Effectors - resp muscles

Question 3

Describe the central control of respiration.

Answer 3

In the brainstem. Sets pattern/rhythm of breathing and coordinates sensors and effectors to maintain respiratory homeostasis.

Question 4

Describe what the sensors do.

Answer 4

Central or peripheral. Gather chemical/physical information.

Question 5

What do the effectors do?

Answer 5

Respiratory muscles that adjust ventilation

Question 6

Where else does the respiratory system receive inputs from?

Answer 6

Neural and humoral (chemical) inputs from peripheral and central receptors.

Question 7

Describe what the brainstem does.

Answer 7

Normal automatic process of breathing originates in impulses that come from the brainstem.

Question 8

What can override the autonomic process of breathing for voluntary control?

Answer 8

Cortex

Question 9

What controls the periodic nature of inspiration and expiration in the brainstem?

Answer 9

Neurons located in the pons and medulla

Question 10

Name the three main groups of neurons of the pons and medulla controlling periodic nature of inspiration and expiration.

Answer 10

Medullary respiratory centre (beneath the 4th ventricle)
Apneustic centre
Pneumotaxic centre

Question 11

What are the key points about the respiratory centre?

Answer 11

Medulla/pons
Responsible for generating rhythmicity
Input from chemoreceptors, lung and other receptors and cortex
Major output via phrenic nerves

Question 12

Where are the central chemoreceptors located (CCR)?

Answer 12

Located near the ventral surface of the medulla

Question 13

What are the CCR sensitive to?

Answer 13

Sensitive to the PCO2, but not the PO2 of blood.

Respond to the change in pH of the ECF/CSF when CO2 diffuses out of the cerebral capillaries

Question 14

CCR are bathed in the brain ECF through which CO2 easily diffuses from blood vessels to the CSF. The CO2 reduces the CSF thus stimulating the chemoreceptor.

Answer 14

xxx

Question 15

Where are the peripheral chemoreceptors located (PCR)?

Answer 15

Carotid and aortic bodies

Question 16

What do the PCR respond to?

Answer 16

Respond to decreased arterial PO2, increased PCO2 and H+
Rapidly responding
90% of the O2 response and 20-30% of the CO2 response comes from these receptors

Question 17

What is the most important stimulus to ventilation?

Answer 17

PaCO2 - normally tightly controlled +/- 3mmHg

Question 18

Where does the PaCO2 stimulus come from?

Answer 18

Most of the stimulus comes from the central chemoreceptors, but peripheral chemoreceptors also contribute and their response is much faster.

Question 19

What reduces the ventilatory response to PCO2?

Answer 19

Sleep
Increasing age
Genetic factors

Question 20

What magnifies the response to low PCO2?

Answer 20

Magnified is PO2 is lowered

Question 21

Describe the key points of the ventilatory response to hypoxia.

Answer 21

• Only the peripheral chemoreceptors are involved
• There is negligible control during normoxic conditions
• Augmented by hypercapnia
• Hypoxic control becomes important at high altitude and in long term hypercapnia caused by chronic lung disease - CO2 retainers q

Question 22

What does chronic hypoventilation result in?

Answer 22

CO2 retention (e.g. chronic bronchitis/emphysema)

Question 23

What does CO2 retention lead to?

Answer 23

High arterial PCO2 and acidosis leads to increased HCO3 retention in order to normalize the pH (blood and brain)
Thus the stimulus at the central chemoreceptor (H+) is reset to near normal levels.

Question 24

Why do we not administer oxygen to CO2 retainers/those with chronic lung diseases?

Answer 24

Hypoventilation due to increased work of breathing and V/Q mismatch often makes patients hypoxic which can become the main stimulus to drive ventilation.

Question 25

Describe the non-chemical control of breathing.

Answer 25

Receptors that are located in the airways and lungs can affect respiration through afferent connections to the resp centers from the vagus nerves.

Question 26

Name the lung stretch receptors and other receptors for non-chemical control of breathing

Answer 26

Slowly Adapting Stretch Receptors
Rapidly Adapting Stretch Receptors
Juxtacapillary Receptors
Other receptors - nose/upper airway, joint/muscle, arterial baroreceptors, pain and temperature