Respiratory 31 to 40
What are the causes of hypoxaemia in a person breathing room air.
Hypoventilation, diffusion limitation, shunt, V/Q inequality
Pass: 3 of 4
Please draw a diagram showing static lung volumes.
How does physiological dead space differ from anatomical dead space?
- Anatomical dead space, conducting zones of lung 150 mls
- Physiological dead space- Parts of lung with ventilation but no perfusion
In the lung, what is surfactant and how does it work?
a) Surfactant is a phospholipid. Dipalmitoyl phosphatidylcholine (DPPC) is an important constituent)
b) Produced in type 2 alveolar cells. Lamellated bodies within them are extruded into the alveoli and transform into surfactant.
c) Fast synthesis with rapid turnover
d) Formed relatively late in foetal life.
e) With surfactant present, surface tension changes greatly with surface area. It falls to very low values when area is small
f) Molecules of DPPC are hydrophobic at one end and hydrophilic at the other. When aligned on the surface, their repulsive forces oppose the normal attractive forces between the liquid surface molecules.
What are the physiological advantages of surfactant ?
a) Reduction in surface tension is greatest when film is compressed and molecules of DPPC are closest together.
b) Lower surface tension in the alveoli increases lung compliance and decreases work of breathing
c) Promotes alveolar stability (reduces tendency for small alveoli to empty into large alveoli)
d) 3) Keep the alveoli dry (surface tension “sucks” fluid into alveolar spaces from capillaries, by reducing hydrostatic pressure in the tissue)
Core Knowledge in Bold
How is carbon dioxide transported in the blood?
Figure: CO2 dissociation curves for blood of different O2 saturations. Note that oxygenated blood carries less CO2 for the same PCO@. The inset shows the ‘physiological’ curve between arterial and mixed venous blood.
- As carbamino compounds with proteins, especially Hb.
- Hydrated in red cells – H buffered – HCO3 in plasma.
Pass: 2 of 3
How does venous blood carry more CO2 than arterial blood?
- Deoxygenated haemoglobin binds more H and forms more carbamino compounds than oxyhemoglobin so venous blood carries more CO2 than arterial blood
- This is known as Haidance effect
Pass: Does the curve move towards the left or the right, and why?
What is the alveolar gas equation?
PAO2 = PIO2 – PACO2/R
- PA02 is the alveolar oxygen partial pressure
- P102 is the oxygen partial pressure of inspired air
- PACO2 is the alveolar CO2 partial pressure.
- R is the respiratory quotient; CO2 production/02 consumption, typically 0.8
Note that a small correction factor F of 2mmHg has been omitted from the equation.
Pass: 4 out of 4
How do you calculate the alveolar- arterial gradient?
Difference between PAO2 (alveolar) and PaO2 (arterial).
What is the physiological significance of the A-a gradient?
V/Q mismatch (eg: shunting or dead space)
Please describe the relationship between pulmonary vascular resistance and pulmonary vascular pressure?
Prompt: What mechanisms are involved in the vascular response to rising pulmonary vascular pressure?
A low resistance system
Capacity for resistance to DECREASE with INCREASE pressure*
(both INCREASED pulmonary art & INCREASED pulm venous)
Mechanisms: vascular ‘recruitment’ (with rises in pressure from low levels); vascular `distension’ (with rises in pressure at higher levels) *
Drawing diagram not essential
How does lung volume influence pulmonary vascular resistance?
Vascular resistance initially decreases as lung volume increases, then rises (see diagram below) *
At very low lung volumes (eg. lung collapse) must reach a ‘critical opening pressure’ (several cm H2O above downstream pressure) to enable any flow *
Very high lung volumes, when alveolar pressure exceeds pulmonary capillary pressure, pulmonary vascular resistance will increase (pulm capillaries squashed).
Drawing diagram not essential
What factors influence the distribution of pulmonary arterial blood?
- Alveolar Hypoxia *
- Gravity * :3 main zones
- Z1 (apical) PA>Pa>Pv
- Z2 (middle) Pa>PA.>Pv
- Z3 (basal) Pa>Pv>PA
- vascular resistance pulmonary HT / PE
- pulmonary disease : asthma /COAD / infection/ infarction/ cancer / fibrosis / pneumothorax / chest trauma
- vasoactive substances * (NO, endothelin, prostaglandin)
- low blood pH leads to pulm vasoconstriction
- Sympathetic stimulation leads to stiff pulmonary arteries leads to vasoconstriction.
Pass: * plus 2 others
What EXTRA-PULMONARY factors influence pulmonary blood flow?
- blood volume
- cardiac output
- atmospheric pressure
- pathology eg, anaemia, cancer, infection
Pass: 4 of 7
Describe the effect of high altitude on respiration.
Prompt if required: Explain the mechanism underlying hyperventilation at altitude.
*Hyperventilation – Most important factor in acclimatisation to altitude.
*Mechanism: Hypoxic stimulation of peripheral chemoreceptors [carotid bodies, aortic bodies].
Low pCO2 and alkalosis work against this but CSF pH `normalised’ by movement of bicarbonate out of CSF [-1-2 days] and renal excretion of bicarbonate [2-3 days] ‘normalises’ arterial pH taking this brake off. Sensitivity of carotid bodies to hypoxia increases during acclimatisation.
What other processes are involved in acclimatisation to high altitude?
*Polycythaemia [hypoxia, erythropoietin]
Shifts in the 02 dissociation curve
Right at moderate altitude 2° 2,3 DPG favouring unloading in tissues; left at high altitude 2°
respiratory alkalosis favouring loading in lungs.
Changes in capillary numbers/ density
Changes to oxidative enzymes in cells
Increased maximum breathing capacity
Pass: Polycythaemia with Mechanism and one other
What factors affect the diffusion of gases across the alveolar capillary membrane?
Ficks law diffusion is proportional to tissue area and concentration gradient of gas and inversely proportional to the tissue thickness x R
R = diffusion constant and relates to gas and tissue solubility
Pass: Need to mention diffusion, concentration gradient & thickness.
How is oxygen transported in the blood?
Pass: Need to name both
Describe the oxygen dissociation curve.
Name the axis Hb saturation and p02 & name 50% saturation (p02= 27 mmHg) or p40 = 75% saturation.
In what forms are carbon dioxide transported in the blood?
CO2 dissolved 10%, Bicarbonate (60%) Carbamino compounds (30%)
deoxygenated blood is better at carrying CO2 = Haldane effect
All 3 to pass