Respiratory 21 to 30
Describe the difference between diffusion limited and perfusion limited gas exchange in the lung.
- Blood in pulmonary capillary has 0.75 seconds for gas exchange
- Ability to reach partial pressure equilibrium depends on reaction with substances in the blood
- No reaction with substances in blood – gas dissolves only on plasma – rapid equilibrium reached, gas uptake limited by perfusion
- Example of N2O as perfusion limited
- Describes reaction of CO with Hb, such great affinity that PCO in capillary falls rapidly – slow equilibrium, diffusion limited
Explain how oxygen exchange is limited across the pulmonary capillary?
- Perfusion limited
- Describes O2 and Hb combination, and time frame of combination (0.3 sec)
What would you expect to be the effect of heavy exercise on oxygen uptake in the pulmonary capillary?
- Describes reduced time for combination with Hb (0.25 seconds), possible reduced O2Hb saturation
- Describes possible effect of altitude
What is the initial effect on respiration with ascent to 6000 metre?
- The ambient pressure is about half the atmospheric pressure
- Shift oxygen dissociation curve to the right
- 2 out of 2
If the person remains at the same altitude for 6 months, what additional changes would occur?
- Increase in 2,3 DPG
- Increase in the number of capillaries in peripheral tissues
- Increase maximal breathing capacity
- Pulmonary vasoconstriction resulting in pulmonary hypertension and right ventricular hypertrophy
Describe the symptoms of acute mountain sickness.
- Headache, fatigue, dizzy, palpitations, nausea, loss of appetite & insomnia
Describe the relationship between ventilation and perfusion of the lung in a person while standing?
- Max ventilation 3-4x greater at apex
- PO2 40mmHg higher at lung apex
- Max perfusion basally Q nearly 20x greater at base
- Prompt: are there regional variations in either
What are the effects of V/Q inequality on gas exchange?
- V/Q inequality impairs uptake or elimination of all gases
- Majority of blood returns from lung bases where the oxygen saturation is low
- Results in blood PO2 being lower than that of mixed alveolar PO2
What effect does increasing ventilation to the lungs have on arterial PO2 and PCO2?
- PCO2 reduces much more than PO2 increases
What factors impact on resistance in airways?
- Size of airway: R highest in medium sized bronchi, low in very small airways.
- Lung volume: R decreases with expansion as airways pulled open
- Bronchial smooth muscle tone: controlled by B sympathetics
- Gas density: eg heliox -> low R
- Forced expiration: intrathoracic pressure compresses airways = ‘dynamic compression’
What factors cause turbulent flow in airways?
- Expressed by Reynold’s number
- p is the fluid density;
- D is the diameter of the tube;
- V is the velocity of flow;
- n is the viscosity of the fluid
- The higher the value of Reynold’s number the greater the probability of turbulence’ which usually occurs when Reynold’s number is between 2000-3000
- Laminar flow only in small airways, transitional most areas, turbulent in trachea (rapid breathing)
Define lung compliance?
- Change in volume / change in pressure
- (Slope of pressure-volume curve)
- (Lung “stiffness”)
What factors influence lung compliance?
- Alveolar oedema
- Elastic tissue
- Emphysema / age
- Volume / Size of lung
- Surface tension in alveoli (Surfactant)
- 3 out of 6
What else does surfactant do?
- Reduces WOB
- Prevents collapse
- Keeps alveoli dry
- 2 out of 3
What is the effect of ventilation perfusion inequality on gas exchange?
- Impedes exchange of oxygen and carbon dioxide
- Hypoxia which cannot be corrected by increased ventilation
- Hypercapnia can be corrected by increased ventilation
- 2 out of 3
Can increasing ventilation correct these problems?
- The oxygen dissociation curve is s shaped which means that increasing ventilation to units with high VQ ratios cannot compensate for the shunt caused by low VQ units
- The carbon dioxide dissociation curve is more linear so that increasing ventilation will blow off CO2 from lung units with both high and low VQ ratios
- Pass = oxygen explanation
- Others additional information
How can we determine the effect of VQ mismatch on oxygenation in clinical practice?
- Calculate the AA gradient (= PAO2-PaO2)
- Give normal values for each
What is pulmonary compliance and what are the factors that influence it?
- Volume change per unit pressure change
- Elastic recoil of the lung especially the geometry of the elastin fibres
- Surface tension in the alveoli
- Disease such as fibrosis, oedema decrease compliance
- Ageing and emphysema increase compliance
- Bold to pass
What are the physiological advantages of surfactant?
- Increases compliance and reduces work of breathing
- Prevents small alveoli collapsing
- P = 2 x Tension / Radius
- Reduces transudation
- 2 out of 3
- Law is additional information
Can you draw the pressure volume curve of a normal lung?
- Closing volume
- Lung becomes stiffer at higher volumes
- 2 out of 3
How would you calculate pulmonary vascular resistance?
- R = Change is pressure / Blood flow
- Normally very low
- Prompt for comparison with systemic vascular resistance if necessary
- Bold to pass
What are the determinants of pulmonary vascular resistance?
- Increasing pressure as in exercise causes a reduction in resistance by recruitment and distension
- Large lung volumes pull open extra-alveolar vessels but may narrow pulmonary capillaries so that resistance rises
- Small lung volumes also cause increased resistance of extra-alveolar vessels because smooth muscle tone closes them if critical opening pressure is not reached
- Hypoxic pulmonary vasoconstriction directs blood away from hypoxic lung
- 2 out of 4
Describe Hypoxic Pulmonary Vasoconstriction.
- Alveolar hypoxia constricts pulmonary blood vessels
- Direct effect of alveolar PO2 on smooth muscle
- Important at birth
- Directs blood away from hypoxic areas
- 2 out of 4
Describe the synthesis and metabolism of cAMP.
- Formed inside the membrane
- ATP is converted to cAMP via adenyl cyclase
- Metabolised by phospho-diesterase
- 2 out of 3 to pass
Discuss the function of cAMP.
- Intracellular second messenger
- Stimulate protein synthesis
- Activate an intracellular enzyme system in the neurone
What are the factors which keep fluid out of the alveoli?
a) Starling’s Law (Theoretical Concept, exact values of pressures unknown)
I. Hydrostatic pressure (of column of blood)
- In the capillaries (positive thus outwards) = Pc
- In the interstitium (probably negative and thus also outwards) = Pi
2. Colloid osmotic pressure (of proteins in blood)
- In the capillaries (inwards) = ;cc
- In the interstitium (outwards) = xi
Net pressure probably slightly outward
Net fluid out = K[(Pc-Pi)-s(7rc-.76)] K = Filtration Coefficient a = reflection coefficient (capillary wall barrier)
b) Lymphatic drainage
c) alveolar epithelial cells
Pass: Demonstrate understanding of hydrostatic pressure & colloid osmotic pressure
Plus one other