Chp. 8: Control of Ventilation

Question 1

Basic elements of respiratory control system

Answer 1

1) Central controller (pons, medulla, other parts of brain)
2) Effectors (respiratory muscles)
3) Sensors (chemoreceptors, lung, other receptors)

Question 2

Where does the normal automatic process of breathing originate from?

Answer 2

Impulses from brainstem

Question 3

What controls the periodic nature of inspiration and expiration?

Answer 3

Central pattern generator (groups of neurons in pons and medulla)

Question 4

Medullary Respiratory Center

Answer 4

• In reticular formation of medulla
• Pre-Botzinger Complex (generation of rhythm)
• Dorsal respiratory group (inspiration)
• Ventral respiratory group (expiration)
• Intrinsic periodic firing
• Expiratory area quiescent during normal breathing; becomes active as a result of activity of expiratory cells

Question 5

Apneustic Center

Answer 5

• Lower pons
• Excitatory effect on inspiratory area of medulla, prolonging ramp APs

Question 6

Pneumotaxic Center

Answer 6

• Upper pons
• “Switch off” or inhibit inspiration and regulates inspiration volume and RR
• Normal rhythm can exist in absence of this center

Question 7

Cortex

Answer 7

Breathing under voluntary control to considerable extent, cortex can override function of brainstem within limits

Duration of breath-holding is limited by several factors, including arterial PCO2 and PO2

Question 8

What are the most important receptors involved in minute-by-minute control of ventilation?

Answer 8

Those situated near ventral surface of medulla in vicinity of exit of 9th and 10th nerves (central chemoreceptors)

Question 9

What do the central chemoreceptors respond to?

Answer 9

Changes in H+ concentration of brain extracellular fluid

Increase in H+ stimulates ventilation

Question 10

What governs ECF composition around central chemoreceptors?

Answer 10

CSF, local BF, local metabolism

Question 11

Central chemoreceptor MOA

Answer 11

• Blood PCO2 rises
• CO2 diffuses into CSF from cerebral blood vessels, liberating H+ ions
• Central chemoreceptors are stimulated

Question 12

How does CO2 level in blood regulated ventilation?

Answer 12

Chiefly by its effect on CSF pH

Question 13

Where are the peripheral chemoreceptors?

Answer 13

Carotid bodies at bifurcation of common carotid arteries and in aortic bodies below aortic arch

Question 14

Carotid body cell types

Answer 14

Type I: large dopamine content, in close apposition to endings of afferent carotid sinus nerve
Type II

Question 15

What do the peripheral chemoreceptors do?

Answer 15

Respond to decreases in arterial PO2 and pH and increases in PCO2

Responsible for increase of ventilation that occurs in response to arterial hypoxemia

Question 16

When does sensitivity to changes in PO2 begin for peripheral chemoreceptors?

Answer 16

500mmHg

Question 17

When does response of peripheral chemoreceptors ramp up?

Answer 17

Around PO2 100mmHg

Question 18

Pulmonary Stretch Receptors

Answer 18

• Within airway smooth muscle
• Discharge in response to lung distension
• Little adaptation (activity is sustained with lung inflation)
• Impulses via vagus nerve
• Main effect of stimulation is SLOWING OF RESP FREQUENCY DUE TO INC. IN EXPIRATORY TIME–> Hering-Breur Reflex

Question 19

Irritant Receptors

Answer 19

• Between airway epithelial cells
• Stimulated by noxious gases, cigarette smoke, inhaled dusts, cold air
• Impulses via vagus
• Effects include bronchoconstriction and hyperpnea

Question 20

J Receptors

Answer 20

• Endings of nonmyelinated C fibers
• Alveolar walls, close to capillaries
• Impulses via vagus nerve in slowly conducting fibers
• Effect is rapid, shallow breathing
• Intense stimulation causes apnea

Question 21

Bronchial C Fibers

Answer 21

• Bronchial circulation
• Respond to chemicals injected into this circulation
• Rapid shallow breathing, bronchoconstriction, mucus secretion

Question 22

Nose and Airway Receptors

Answer 22

• Nose, nasopharynx, larynx, trachea
• Extension of irritant receptors
• Sneezing, coughing, bronchoconstriction, laryngeal spasm

Question 23

Joint and Muscle Receptors

Answer 23

• Impulses from moving limbs
• Stimulus to ventilate during exercise

Question 24

Gamma System

Answer 24

• Intercostals and diaphragm contain muscle spindles that sense elongation of muscle
• Sensation of dyspnea with unusually large respiratory efforts to move lung and chest wall

Question 25

Arterial Baroreceptors

Answer 25

- Increase in ABP can cause reflex hypoventilation or apnea through aortic/carotid baroreceptor stimulation

Question 26

What is the most important factor in control of ventilation under normal conditions?

Answer 26

PCO2 of arterial blood, held to within 3mmHg

Question 27

What is the effect of lowering PO2 on PCO2?

Answer 27

- Ventilation for given PCO2 is higher

Question 28

Ventilatory Response to Hypoxia

Answer 28

- Only peripheral chemoreceptors involved - Important at high altitude and in long-term hypoxia - Raising PCO2 increases ventilation at any PO2

Question 29

Why is hypoxic drive to ventilation important in chronic lung disease?

Answer 29

- Chronic CO2 retention and brain ECF pH has returned to near normal in spite of increased PCO2 - Have thus lost most of increase in stimulation to ventilation from CO2 - Initial depression of blood pH nearly abolished by renal compensation, so little pH stimulation of peripheral CRs - Arterial hypoxemia is thus the primary stimulus to additional ventilation beyond the basic level set by medullary respiratory center

Question 30

What happens to ventilation with reduction in arterial blood pH?

Answer 30

- Stimulates ventilation via peripheral chemoreceptors

Question 31

Ventilatory response to exercise

Answer 31

- Ventilation increases promptly - Arterial PCO2 does not increase (typically falls due to inc. vent) - Arterial PO2 increases slightly - Not well elucidated

Question 32

Cheyne-Stokes Respiration

Answer 32

- Severe hypoxemia - Periods of apnea of 10-20s separated by equal periods of hyperventilation when the tidal volume gradually waxes and then wanes - At high altitude, severe heart failure, neurologic injury - Increased ventilatory responsiveness to PCO2

Question 33

KEY CONCEPTS for control of ventilation

Answer 33

1) Respiratory centers responsible for rhythmic pattern of breathing located in pons and medulla of brainstem. Output can be overridden by cortex to some extent. 2) Central chemoreceptors are located near ventral surface of medulla and respond to changes in pH of CSF, which is caused by diffusion of CO2 from brain capillaries. Alterations in bicarbonate concentration of CSF modulate the pH. 3) Peripheral chemoreceptors, chiefly in carotid bodies, respond to a reduced PO2 and increases in PCO2 and H+. Response to O2 is small above PO2 of 50mmHg. Response to CO2 less marked than central chemoreceptors but occurs more rapidly. 4) Other receptors in walls of airway and alveoli. 5) PCO2 of blood is most important factor controlling ventilation under normal conditions, most of control via central chemoreceptors. 6) PO2 of blood does not normally affect ventilation, but important at high altitude and in some patients with lung disease. 7) Exercise causes large increase in ventilation but cause is poorly understood.