Respiratory System 4

Question 1

Describe neural control of respiration

Answer 1

Nerve impulses from medullary respiratory centres sent via motor neurons i.e.

 - phrenic nerve to diaphragm
 - intercostal nerves to intercostal muscles (increase width and depth of ribcage = increase volume)

Stimulate contraction → inhalation
When impulse ceases → Expiration (passive)

Question 2

Describe chemical control of respiration

Answer 2

Chemoreceptors = Sensors that detect PCO2, pH (i.e. H+) & PO2

Peripheral chemoreceptors - in the aorta and carotid arteries (outside the CNS)

 - Strongly detect changes in plasma pH (caused by changes in CO2)
 - Weaker response directly to PCO2
 - Weak response to PO2 – ONLY when very low (<60 mmHg; <90% saturation)

PO2 levels drop = Carbon dioxide increase = increase ventilation

Central chemoreceptors – in medulla
- Detect changes in pH in the cerebrospinal fluid caused by changes in arterial PCO2 (CO2 freely diffusible across blood-brain barrier)

Question 3

Describe the negative feedback mechanism of chemical control of breathing

Answer 3

Increased arterial Pressure of carbon dioxide → Homeostasis is disturbed

→ Stimulation of arterial chemoreceptors → Nerve impulses via sensory afferent fibres to medulla
→ Increased Pressure of carbon dioxide, decreased pH in CSF → Stimulation of CSF chemoreceptors at medulla oblongata

→ Stimulation of respiratory muscles → Increased respiratory rate with increased elimination of Carbon dioxide at alveoli (Increased ventilation where more CO2 is exhaled) → Carbon dioxide is blown off at lungs or Homeostasis restored, where there is a normal arterial pressure of carbon dioxide (Arterial PCO2 and pH return to normal)

Question 4

How ventilation affects blood gases?

Answer 4

Hyperventilate → blows off CO2 (removes the breathing stimulus)

Hyperventilation does not increase the amount of O2 in blood (Haemoglobin saturated)

CO2 rises, O2 falls, pO2 may reach critically low values BEFORE pCO2 reaches the value which stimulates breathing

Blackout and drowning may follow (especially during ascent from a deep dive)