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Control of RespirationControl of Respiration
Dr Shihab Khogali
Ninewells Hospital & Medical School, University of Dundee
What makes the inspiratory muscles contract and relax rhythmically?
How could the respiratory activity be modified?
How could the expiratory muscles be called on during active expiration?
How could the arterial PO2 and PCO2 be maintained within narrow limits?
What is the role of the respiratory system in regulating blood H+ concentration?
What is This LectureAbout?
See blackboard for detailed learning objectives
The Neural
&
Chemical
Control of Respiration
To answer these questions we need to understand:
Neural control of Respiration
Fairly normal ventilation retained if section above medulla
Ventilation ceases if section below medulla
medulla is major rhythm generator
The Rhythm: inspiration followed by expiration
anterior
Neural control of RespirationUntil recently, it was thought the Dorsal respiratory group of neurons generate the basic rhythm of breathing
It is now generally believed that the breathing rhythm is generated by a network of neurons called the Pre-Brotzinger complex. These neurons display pacemaker activity. They are located near the upper end of the medullary respiratory centre
What gives rise to inspiration?
PONS
MEDULLA
SPINAL CORD
Dorsal respiratory group neurones (inspiratory)
Fire in bursts
Firing leads to contraction of inspiratory muscles - inspiration
When firing stops, passive expiration
What about “active” expiration during hyperventilation?
Increased firing of dorsal neurones excites a second group:
Ventral respiratory group neurones
Excite internal intercostals, abdominals etc Forceful expiration
In normal quiet breathing, ventral neurones do not activate expiratory muscles
The rhythm generated in the medulla can be modified by
neurones in the pons:
“pneumotaxic centre” (PC)
Stimulation terminates inspiration
PC stimulated when dorsal respiratory neurones fire
Inspiration inhibited
Without PC, breathing is prolonged inspiratory gasps with brief expiration - APNEUSIS
-
+
The “apneustic centre”
Apneustic centre
Impulses from these neurones excite inspiratory area of medulla
Prolong inspiration
Conclusion?
Rhythm generated in medulla
Rhythm can be modified by inputs from pons
Reflex modification of breathing
Pulmonary stretch receptors
Activated during inspiration, afferent discharge inhibits inspiration - Hering-Breuer reflex
Unlikely - only activated at large >>1litre tidal volumesMaybe important in new born babies
Do they switch off inspiration during normal respiratory cycle?
May prevent over-inflation lungs during hard exercise?
Joint receptors
Impulses from moving limbs reflexly increase breathing
Probably contribute to the increased ventilation during exercise
Factors That May Increase Ventilation During Exercise
Reflexes originating from body movement
Increase in body temperature
Adrenaline release
Impulses from the cerebral cortex
Later: accumulation of CO2 and H+ generated by active muscles
Chemical Control of Respiration
An example of a negative feedback control system
The controlled variables are the blood gas tensions, especially carbon dioxide
Chemoreceptors sense the values of the gas tensions
Peripheral Chemoreceptors
Carotid bodies
Aortic bodies
Sense tension of oxygen and carbon dioxide; and [H+] in the blood
Central Chemoreceptors Situated near the surface of the medulla of the brainstem
Respond to the [H+] of the cerebrospinal fluid (CSF)
CSF is separated from the blood by the blood-brain barrierRelatively impermeable to H+ and HCO3
-
CO2 diffuses readily
CSF contains less protein than blood and hence is less buffered than blood
CO2 + H2O H2CO3 H+ + HCO3-
Hypercapnia and Ventilation
10
20
30
40
Ven
tila
tion
(l/
min
)
20 40 60 80
Pco2 (kP) (mmHg)
2.7 5.3 8 10.6
The system is very responsive to PCO2
CO2 generated H+ through the central chemoreceptors
O2 c
on
cen
trat
ion
ml/
100
ml
5.3 13.3
Blood PO2 (kPa)
% H
aem
og
lob
in S
atu
rati
on
8.0Arterial Po2 (kPa)
0
Ven
tila
tion
(l/m
in)
50
40
30
20
10
0 8.0 13.3
Neuron depressed when hypoxia so severe
PeripheralChemoreceptorsStimulated
Hypoxia and Ventilation
Hypoxic Drive of Respiration
The effect is all via the peripheral chemoreceptors
Stimulated only when arterial PO2 falls to low levels (<8.0 kPa)
Is not important in normal respiration
May become important in patients with chronic CO2 retention (e.g. patients with COPD)
It is important at high altitudes
The H+ Drive of Respiration The effect is via the peripheral chemoreceptors
H+ doesn’t readily cross the blood brain barrier (CO2 does!)
The peripheral chemoreceptors play a major role in adjusting for acidosis caused by the addition of non-carbonic acid H+ to the blood (e.g. lactic acid during exercise; and diabetic ketoacidosis)
Their stimulation by H+ causes hyperventilation and increases elimination of CO2 from the body (remember CO2 can generate H+, so its increased elimination help reduce the load of H+ in the body)
This is important in acid-base balance
Influence of Chemical Factors on Respiration