Control of Ventilation

Question 1

Respiratory Control

Answer 1

• Maintain homeostasis

– Oxygen

– Carbon dioxide

– Hydrogen ion concentration (pH)

• Optimize mechanical efficiency
• Complex functions

– Vocalization

– Cough

– Exercise

– Adaptation to disease

Question 2

Respiratory Control

Answer 2

• Chemoreceptors

– Carbon dioxide

– Oxygen

– Hydrogen ion (pH)

• Mechanoreceptors

– Information from lung and chest wall

• Other

– Baroreceptors

– Temperature receptors

– Higher brain centers

Question 3

Respiratory Output

Answer 3

• Inspiratory muscles

– Diaphragm

– External intercostals

– Accessory muscles

• Upper airway muscles
• Expiratory muscles

– Abdominal wall

– Internal intercostals

Question 4

The Brainstem and Respiratory Control

Answer 4

1. Pneumotaxic Center
2. Apneustic Center
3. Medullary Respiratory Center
   - DRG (INSP)
   - VRG (EXP)

Question 5

DRG

Answer 5

Dorsal respiratory group: Inspiration

• Controls basic rhythm of breathing
• Oscillations in activity are due to multiple inputs +/- pacemaker cells
• Pre-Botzinger complex in ventrolateral medulla has rhythmic insp activity
• Crescendo of activity leads to inspiration and decreases in expiration
• Input from IXth and Xth nerves that terminate in nucleus tractus solitarius
• Input from pneumotaxic center in pons ends inspiration
• Output to inspiratory muscles

Question 6

VRG

Answer 6

• Inspiration (DRG) is active, and the expiration is passive without need for VRG output to expiratory muscles
• Increases activity with

– exercise

– dyspnea (shortness of breath)

– lung disease

• Pre-Botzinger complex in ventrolateral medulla has rhythmic insp activity
• VRG has both insp and exp muscle outputs

Question 7

Apneustic Center

Answer 7

• Lower pons
• Damage above this region isolates it from pneumotaxic center leads to prolonged inspiratory gasps
• Sends signals to inspiratory center (DRG) to prolong inspiratory ramping of diaphragmatic activity

Question 8

Pneumotaxic Center

Answer 8

• Located in the upper pons
• Switches off inspiratory activity, thus ending inspiration
• Controls tidal volume and respiratory rate by cycling the breath into expiration
• Normal breathing can persist without this center

Question 9

Answer 9

Question 10

Central Chemoreceptors

Answer 10

• Change in PaCO2 alters CSF pH

– Increase PaCO2 will decrease CSF pH

– Decrease PaCO2 will increase CSF pH

• Decreased pH (Increased H+) in CSF stimulates receptors to increase ventilation
• Normal eupneic control
• Severe hypoxemia suppresses output

Question 11

Peripheral Chemoreceptors

Answer 11

• Located in carotid bodies
• Aortic bodies have more impact on cardiovascular system
• Peripheral chemoreceptors are the only receptors that respond to hypoxia or acidemia to increase ventilation

Question 12

Carotid Body Chemoreceptors

Answer 12

• Hypoxia, Hypercarbia, and Acidemia increase ventilation
• 20% of response to PaCO2
• Hypoxia potentiates other responses
• Need to respond to hypoxemia or metabolic acidemia effectively

Question 13

Integration of Chemoreceptor Response

Answer 13

• Response to chemoreceptor input is integrated to protect homeostasis

– In a patient with metabolic acidosis any increase in carbon dioxide leads to a more rapid increase in ventilation

– In a patient with metabolic acidosis any hypoxemia leads to a more rapid increase in ventilation

Question 14

Pulmonary Mechanoreceptors

Answer 14

• Pulmonary stretch receptors
• Lung irritant receptors
• J-receptors

Question 15

Stretch Receptors

Answer 15

• Airway smooth muscle
• Slow adaptation to airway stretch
• Vagal inhibition of inspiration
• Promotion of expiration
• Hering-Breuer reflex

Question 16

Irritant Receptors

Answer 16

• Extra-pulmonary airway epithelium
• Rapid adaptation with vagal response
• Respond to chemical or physical irritation
• Help maintain lung volume with sighs
• Cause bronchoconstriction, cough, inspiration, and rapid, shallow breathing

Question 17

J-Receptors

Answer 17

• Alveolar walls near capillaries
• Connect via unmyelinated fibres
• Rapidly adapting.
• Stimulated by interstitial disease and inflammatory chemicals and engorgement of the capillary bed
• Cause laryngeal closure and apnea, followed by rapid shallow breathing.

Question 18

Chest Wall Proprio-receptors

Answer 18

• Joint receptors
• Tendon receptors
• Muscle spindle receptors

Question 19

Joint Receptors

Answer 19

• Ruffini, Pacinian, and Golgi receptors
• Located in the joints of the chest wall
• Respond to rib movement

Question 20

Tendon Organs

Answer 20

• Intercostal and diaphragmatic muscle tendons
• Monitor the force of contraction
• Inhibit inspiration

Question 21

Muscle Spindles

Answer 21

• Abundant in the intercostals and rare in diaphragm
• Stabilize rib cage and posture
• Spindles located on intrafusal muscle fibres aligned with extrafusal fibres that elevate the ribs
• Extrafusal = alpha motor neuron
• Intrafusal = gamma motor neuron
• Similar to limb reflexes = Intercostals are postural

Question 22

Non-Respiratory Mechanoreceptors

Answer 22

• Proprioreceptors in the limb joints
• Thermal and cardiovascular baroreceptors

Question 23

Chronic respiratory illness […] ventilatory control:

Answer 23

Chronic respiratory illness decreases ventilatory control:

• Increased work of breathing
• Decreased efficiency of gas exchange
• Impaired performance of the ventilatory pump
• Reduced response to chemical stimuli

Question 24

Hydrogen ion is a million times […] concentrated than bicarbonate ion

Answer 24

Hydrogen ion is a million times less concentrated than bicarbonate ion

Question 25

Chronically Increased P_aCO₂

Answer 25

• Kidney compensates for respiratory acidosis (increased PaCO2) by reabsorbing more bicarbonate (HCO3- )
• Eventually the HCO3 - enters the CSF and buffers any ph change due to increased PaCO2

Question 26

Chronic CO₂ Retention

Answer 26

• Central chemoreceptors reset their control point and lose sensitivity to changes in PaCO2
• Only drive left is peripheral hypoxic drive from carotid bodies
• High inspired oxygen (FIO2) turns off hypoxic drive and ventilation decreases
• PaCO2 rises to levels where is causes narcosis and apnea