Control of Ventilation

PCO2 levels in the peripheral blood that are…

• Too high: hypercapnia

• Too low: hypocapnia

Reduced levels of O2…

• In the tissues or a specific tissue: hypoxia

• In peripheral blood (PO2): hypoxemia

Blood pH that is too…

• Low (high CO2): acidosis

• High (low CO2): alkalosis

Heart rate that is…

• Slow: bradycardia

• Rapid: tachycardia

Neural control of ventilation can be…

• Involuntary and voluntary

Involuntary neural control of ventilation

• ANS influences bronchioles

  • PNS: acetylcholine causes bronchoconstriction

  • SNS: norepinephrine causes bronchodilation

• Several centers located in the medulla oblongata and pons

  • Control expired minute volume → adjust frequency and depth of ventilation

  • Receive info from lungs/respiratory tract/body

Voluntary neural control of ventilation

• Cerebral cortex can regulate both:

  • Respiratory centers in medulla and pons

  • Motor neurons that innervate the respiratory muscles

Big picture neural control of ventilation

• Respiratory control centers are located in the medulla oblongata and pons

• Efferent pathways: motor neurons to the muscles of respiration

• Afferent signals:

  • Chemoreceptors located in the brain (central) and periphery (aortic and carotid bodies)

  • Mechanoreceptors in lung and muscle

Medulla oblongata → directly control the muscles of ventilation

• Respiratory rhythmicity centers (a pair of centers)

• DRG and VRG

DRG

• Dorsal respiratory group

• Functions in every respiratory cycle

• Neurons regulating inspiration

  • Controlling motor neurons to external intercostals and diaphragm

• Predominant role in integrating afferent input

VRG

• Ventral respiratory group

• Functions only during forced breathing

  • Expiratory center: controls accessory motor neurons used during forced exhalation

  • Inspiratory center: controls accessory motor neurons used during forced inhalation

Cerebrum and pons

• Pontine respiratory centers: regulate rate and depth of breath (influenced by sensory and higher center information)

• Apneustic center: excites the DRG. Inhibited by pneumotaxic center

• Pneumotaxic center: inhibits the apneustic center. Inhibited/excited by higher centers

Pneumotaxic center control

Central chemoreceptors

• Hypercapnia will stimulate an increase in ventilation

• Diffusion of CO2 into CSF

  • Note: H+ cannot cross BBB

• Formation of H+

• Diffusion of H+ into medullary interstitial fluid, stimulation of central chemoreceptor

• Increased H+ signals respiratory center to increase ventilation

• Increased PaCO2 = hypercapnia

Peripheral chemoreceptors

• Hypercapnia and hypoxemia will stimulate an increase in ventilation

• Aortic bodies and carotid bodies:

  • Aortic body (near aortic sinus) → respiratory control centers via vagus nerve

  • Carotid bodies (near carotid sinus) → respiratory control centers via glossopharyngeal nerve

• Sensitive to changes in:

  • Arterial PO2:

    • Respond when PO2 drops

    • Decrease in PaO2 → stimulate ventilation

  • Arterial pH:

    • Directly responsive to changes in pH

    • Increased PaCO2 → decreased pH

      • Decreased pH → stimulate ventilation

• Decreases in pH and PO2 will also stimulate vasomotor and cardiac control centers in medulla oblongata: vasoconstriction and tachycardia

Feedback loop for PCO2 levels in the blood

Hering-Breuer Reflexes

• Inflation reflex

• Deflation reflex

Inflation reflex

• Goal: prevent over expansion of lungs during forced inhale

• Sensory: stretch receptors within smooth muscle of bronchioles

• Pathway: vagus to respiratory centers

• Effect: inhibit DRG, activate expiratory center of VRG

Deflation reflex

• Goal: prevent over deflation during forced exhale

• Sensor: recoil receptors within the alveolar walls

• Pathway: vagus to respiratory centers

• Effect: inhibit expiratory center of VRG, activate inspiratory centers

High altitude

• Hypoxia

• Mount Everest → death zone

• Atmospheric Air PO2 = atmospheric pressure FIO2 = 250 × 0.21 = 53 mmHg

• Inspired air: PIO2 = (250-47) * 0.21 = 43 mmHg

Exposure to altitude → immediate

• Decrease in PIAO2 below 60 mmHg → PaO2 decrease to below 60 mmHg → hypoxemia: stimulates hyperventilation → PaO2 increases but PaCo2 decreases → alkalosis: inhibits ventilation

• Net effect: → → ventilation

  • Note: if only had hypoxia and no alkalosis: → → → ventilation

• Also → tachycardia

Altitude: acclimatization (days/weeks)

• Kidney: secretes (gets rid of) bicarbonate and conserves H+

  • Decrease in pH

    • Counter-acts alkalosis (increased respiration)

• Kidney: hypoxia stimulates increased production of erythropoietin

  • Increased production of red blood cells

    • Increased oxygen carrying capacity of the blood

• RBC: increased production of *BPG

  • *2,3- bisphosphoglycerate is a metabolite in RBCs that increases with hypoxemia

  • BPG binds to deoxyhemoglobin

  • Shifts oxygen-hemoglobin dissociation curve to the right

    • Facilitates unloading of oxygen to the tissues