Physiology 12 Flashcards
Outline the function of the peripheral chemoreceptors
Carotid bodies: respond to PaO2, PaCO2 and [H+]
Aortic bodies: respond to PaO2 and PaCO2. Less influential than carotid bodies
Peripheral chemoreceptors are the main receptors that stimulate ventilatory response to low PaO2
Outline the anatomy and structure of the carotid body
Cluster of chemoreceptor cells adjacent to carotid bifurcation
Consists of 2 types of glomus cells
- Type 1 (chief cells) - Chemoreceptors rich in dopamine. Synapse with the sinus nerve (CN IX)
- Type 2 (sheath cells) - Resemble glial cells, act as ‘supporting cells’
What is the blood flow to the carotid bodies?
How does this compare to cerebral blood flow?
Why is this important?
2000ml/min/100g
More than cerebral (50ml/min/100g)
This means that a-v O2 difference is very low and carotid bodies can derive all their O2 from dissolved O2 and respond primarily to changes in O2 tension rather than total O2 content
How does a carotid body chemoreceptor respond to changes in PaO2?
Type 1 glomus cells have a graded reduction in intracellular ATP in response to changes in PaO2, causing increased nerve activity. Response is most marked to PaO2 <7.89 kPa
Response is augmented by acidosis and hypercapnia.
Response is rapid, with a time constant of a few seconds
Outline the response of the carotid body to changes in PaCO2
Response is dependent on carbonic anhydrase and likely responds to both PaCO2 and [H+] through intracellular [H+]
Efferent nerve activity is much increased by raised intracellular [H+] in conjunction with hypoxia
Less than 20% of the response to CO2 is peripheral, but it is more rapid and may be the only response to short-lived PaCO2 changes.
How do the carotid bodies respond to hypotension?
SBP <60mmHg causes ‘stagnant hypoxia’ leading to increased ventilation
Outline the chemoreceptor function of the aortic bodies
- Blood supply lower than carotid bodies therefore greater a-v difference and less sensitivity to changes in PaO2 and PaCO2.
- Aortic bodies (unlike carotid bodies) respond to total O2 content and are thus sensitive to anaemia and CO poisoning etc
Outline the role of the pulmonary stretch receptors
- Between SM cells in large airways
- Are slowly-adapting receptors (SARs)
- Respond to distension of lungs
- Send afferents via CN X
- Stimulation causes termination of inspiration via Hering-Breuer reflex
- Strong stimulation causes activation of expiratory neurones
- Thought to be relevant at normal TV in infants but possibly only at increased TVs for adults (?via Hering-Breuer)
Outline the role of the pulmonary irritant receptors
- Rapidly adapting receptors (RARs)
- Between epithelial cells of large airways
- Respond to mechanical and chemical irritation
- Have a role in coughing, sneezing and bronchoconstriction
- Efferents travel via myelinated fibres in CN X
- Also found in nose and upper airways; implicated in laryngospasm
Outline the role of the pulmonary juxtacapillary (J) receptors
- Located in alveolar cell walls close to capillaries
- Respond to chemical stimuli and oedema
- Can cause reflex rapid, shallow breathing
- Efferents travel via unmyelinated fibres in CN X
What is a normal ventilatory response to increasing PaCO2 at a normal PO2?
How does hypoxia change this?
15-20 L/min increase in MV per kPa rise in PaCO2
Hypoxia increases this response (via integrated peripheral/central chemoreceptor mechanism)
How does the ventilatory response to metabolic acidosis differ to respiratory acidosis?
- Increased [H+] stimulates carotid chemoreceptors, increasing MV
- Resulting fall in PaCO2 reduces central ventilatory drive and thus blunts ventilatory response to metabolic acidosis
How does hypocapnia affect ventilatory response to hypoxia?
Reduced response to hypoxia (threshold reduced to <5 kPa)
Outline what is known about the ventilatory response to exercise
Extreme exercise may increase MV to >120L/min. Cause of increase largely unknown.
PaCO2 does not increase and falls during extreme exercise
PaO2 increases
Some increase in [lactate] and [H+] but only heavy exercise
Possible mechanisms for increased MV:
- Lactic acidosis
- PaCO2 oscillations
- Muscle afferents
- Catecholamines
- CO2 load
- Enhanced PO2 response
- Learned response
Define respiratory failure
Inability to maintain appropriate arterial gas tensions
T1RF / T2RF
Define ventilatory failure
Pathological reduction in alveolar ventilation below level required to maintain normal alveolar gas tensions
For a normal person at rest how much CO2 is produced per day?
288L
200mL/min
What are the possible causes of ventilatory failure?
Neurological / Muscular / Thoracic / Airway
Neurological:
- Central (drugs, severe PCO2/PO2 derangement, lesions)
- UMN (tumours, strokes, demyelination)
- Anterior horn cells (polio)
- LMN (GBS, MND, phrenic nerve injury)
Muscular:
- NMJ (toxins, autoimmunity)
- Respiratory muscles (inflammatory disease, dystrophies, atrophy, mechanical splinting)
Thoracic:
- Altered elasticity (fibrosis, contusions, plaques, kyphoscoliosis, burns, external pressure)
- Structural integrity (Trauma - flail chest, tension PTX)
Airway:
- Obstructive lung disease
- Upper airway obstruction (many causes)
- Increased dead space (emphysema, PE)
What is the normal quantity of CO2 in the body?
What effect does this have in the context of increased VCO2?
Around 120L
This acts effectively as a buffer to increased CO2 production, thus levels change slowly in response to increased VCO2
What is the normal quantity of O2 in the body?
Around 1.5L
What is the rate of PaCO2 rise during apnoea?
0.4-0.8 kPa/min
In acute alveolar hypoventilation (on room air), which gas tension gives the best warning?
Reduction in PO2 can be a useful first sign, as PaCO2 may not have had time to rise
How is distribution of alveolar ventilation affected by rate of inspiration during IPPV?
Fast inspiration: Distributed to areas with short time constants
Slow inspiration: Distributed according to regional compliances
What are the different ways of controlling the length of the respiratory cycle during IPPV?
Time cycling
Volume cycling
Pressure cycling