Respiratory 11 to 20
Describe the distribution of blood flow in the lungs.
- Linear increase from top to bottom
- 3 zones explained by hydrostatic pressures
Explain how V/Q matching varies from apex to base in the normal lung.
- Slow increase in ventilation from top to bottom but not as much as perfusion.
- Highest V/Q at apex
What factors effect pulmonary vascular resistance?
- Hypoxia – arteriolar smooth muscle to contract
- One of low pH, autonomic or passive factors
What are the acute respiratory adaptations to altitude?
- Hyperventilation. (Oxygen-haemoglobin curve shifts right or left.)
What are the longer term physiologic effects of altitude exposure?
- one of increased 02 carriage and viscosity;
- more capillaries;
- increased oxidative enzymes.
Describe how blood flow is regulated at the level of the endothelium.
- Vasodilators: prostacyclins, NO, kinins
- Vasoconstrictors: endothelin, thromboxane, serotonin
What other general effects do endothelins have on the cardiovascular system?
- Positive inotrope and chronotrope
- Rise in ANP/renin/aldosterone
- Decreased GFR and renal blood flow
Describe how the renin-angiotensin system regulates blood pressure and flow.
- Describes pathway
- Fall in renal blood flow leads to renin
- Renin, angiotensin I > II
What factors regulate renin secretion?
- sympathetic nervous system, catechols, prostaglandins
- Na and Cl reabsorp, inc BP, angio II, vasopressin
What changes in arterial blood pressure do baroreceptors respond to?
- Carotid sinus (rise or fall)
- Aortic arch (rise)
What happens when the baroreceptors detect a fall in arterial pressure?
- Decreased firing rate of Hering’s nerve
- CN IX transmits to vasomotor centre
- Decreased parasympathetic outflow to heart
- Increased sympathetic outflow to heart
- Increased sympathetic outflow to vessels
- Increased heart rate, contractility
- Arteriolar and venous constriction
What is the Set Point?
- Neutral MAP for vasomotor centre Around 100 mm Hg
Please explain the concept of compliance as it relates to the lung.
- Volume change per unit pressure change (slope of pressure volume curve) =~200ml/cm H2O
- Depends on lung volumes and demonstrates hysteresis , may draw compliance curve, point out that inc compliance at low volumes
- Depends on structural proteins and surface tension
What factors affect compliance?
- Fibrosis, pulmonary oedema, not ventilated, increased pulmonary venous pressure
- Emphysema, asthma surfactant
What are the metabolic functions of the lung?
Prompt: What substances are metabolised in the lung?
Metabolism of vasoactive amines
- Activation of Angiotensin 1 à AT 2 (ACE in capillary endothelium)
- Inactivation of bradykinin (ACE); PGs E/F
- Uptake & storage of Serotonin
- Arachadonic acid metabolites à leukotrienes / SRS-A & Prostaglandins
- Proteases (collagen/elastin breakdown)
- 2 of each
What factors influence the rate of transfer of oxygen from the alveolus into a pulmonary capillary?
- Process is passive diffusion ( ficks law of diffusion)
- Affected by – surface area, membrane thickness, gradient of p O2 ( o2 in alveolus and O2 binding capacity of Hb)
- Also – constant –solubility and MW
- (V = A/T x D x (P1-P2)
- D = Sol/MW1/2
Could you give some clinical examples of when these may be affected?
- Exercise alveolar hypoxia and thickening of blood gas barrier
What factors determine the work of breathing?
- Elastic forces of the lungs and chest wall
- Viscous resistance of the airways and tissues
- Bold to pass
What variables affect elastic workload?
- Larger tidal volumes
- Reduced compliance due to:
- lung volume – a person with only one lung has halved compliance;
- slightly less during inflation than during deflation;
- increased by increased tissue mass – fibrosis or pulmonary congestion or chest wall restriction;
- loss of surfactant
What variables affect viscous resistance?
- Higher respiratory rates increasing flow rates
- Decreased airway radius due to:
- Lower lung volumes;
- Increased air density (eg SCUBA diving)
- Increased air viscosity
- Bold to pass
What is the relationship between intrapleural pressure and lung volume?
- Sigmoid curve of IP pressure vs volume, does not reach 0% lung volume
- Shows lung volume is higher during deflation than inflation for any given pressure = Hysteresis
- Shows that lung contains residual air, without any expanding pressure (due to airway closure)
- Shows that compliance decreases at higher lung volumes – lung becomes stiffer due to reaching limits of elasticity
- 3 out of 4 to pass
What variables affect pulmonary compliance?
- Slightly greater during deflation than during inflation as noted above;
- Lung volume – at very low and very high volumes compliance is reduced;
- Increased when tissue elasticity is reduced, as in emphysema;
- Decreased by increased tissue mass – fibrosis or pulmonary congestion;
- Decreased by loss of surfactant
Describe how regional differences in intrapleural pressure affect the ventilation.
- States that the intrapleural pressure is higher at the apex than at the base of the lung – to keep the lung expanded against its own weight
- Increased compliance at base, hence better ability to ventilate base compared with apex
- Bonus if gives values 10cm H2O at apex, 2.5cm at base