Gas exchange in the lung Flashcards
(14 cards)
how dos oxygen get from the atmosphere to cells
-O2 inhaled from atmosphere into alveoli within lungs
-O2 diffuses from alveoli into blood within pulmonary capillaries
-O2 transported in blood, predominantly bound to haemoglobin
-O2 diffuses into cells/tissues for use in aerobic respiration
-CO2 diffuses from respiring tissues
Explain what air consists of
Air consists of a mixture of gases, which behave in accordance with their partial pressure rather than concentration
how is partial pressure and mole fraction calculated
Partial pressure is calculated by multiplying total pressure by mole fraction
-mole fraction: the % of total moles represented by dividing gases in air at sea level
what determines how much gas dissolves in a liquid
-the concentration of a gas dissolved within a liquid is determined by the partial pressure and solubility of the gas:
-the partial pressure of gas dissolved in a liquid reflects the amounts of gas that would dissolve if the liquid was placed in contact with a gas phase of equivalent partial pressure.
-double the pressure of surrounding gas= double the concentration of dissolved gas
explain how gas exchange involves diffusion of blood gases through multiple structures and mediums
1) O2 enters the alveolar airspace from the atmosphere
2) O2 dissolves in ALF
3) O2 diffuses through the alveolar epithelium, basement membrane and capillary endothelial cells
4) O2 dissolves in blood plasma
5) O2 binds to Hb moelcule
what determines the rate of diffusion
-alveolar surface area
-partial pressure gradient between alveolar air and capillary blood
-epithelial and endothelial cell thickness + basement membrane thickness + fluid layer depth
explain defects in diffusion at gas exchange surfaces
for maximum diffusion:
-increases partial pressure gradient
-increases in functional surface area
-decrease in distance (barrier thickness)
hypoventilation (type II respiratory failure)
emphysema= decrease in surface area
acute lung injury= decrease in functional surface area
fibrosis= increased basement membrane thickness
pulmonary oedema= increase in thickness of fluid layer/oedema
explain the adaptations of alveoli to maximise the rate of gas exchange
-large surface area (lungs have high SA:V ratio)
-wall= one cell layer thick + basement membrane fused with blood vessel
-richly innervated by capillaries
discuss how ventilation-perfusion coupling is maintained by hypoxic pulmonary vasoconstriction
-haemostatic mechanisms exist to reduce ventilation-perfusion mismatching. Hypoxic vasoconstriction of capillaries diverts blood flow from poor to well ventilated alveoli
-if ventilation of specific alveoli decreases, PACO2 will rise + PAO2 will fall. decrease in oxygenation of blood flowing through innervating capillaries
-under normal conditions, blood flow and ventilation are matched
-decrease in PaO2 induces vasoconstriction= decrease in blood flow
-blood flow diverted to alveoli with increased ventilation
explain ventilation-perfusion inequality
-in situations where ventilation and perfusion to indvidual alveolar units are not matched —> the overall rate of gas exchange will be reduced
-we are not referring to changes in the overall level of ventilation or perfusion to the lungs
-V-Q inequality affects both O2 and CO2 exchange, however in most cases, increase in PaCO2 will induce a reflex hyperventilation that clears the excess CO2
give examples of ventilation-perfusion inequality
-embolism occludes pulmonary artery supplying a region of the lung
-unless ventilation of these alveoli increases to match perfusion, hypoxaemia and hypercapnia will occur
-perfusion to these vessels/alveoli increases as cardiac output diverted
explain the effect of O2 therapy on hypoxaemia depends on the nature of the pathology
SDQ= a method of quantifying variation between V/Q ratios in different regions of the lung
FiO2= fraction of oxygen in inspired air
effect of increased FiO2 on shunt is much more limited (however the modest effect produced can still be critical)
explain acute lung injuy-induced shunt
-alveolar oedema and lung injury due to infection prevents ventilation in affected parts of the lung
-alveolat odema and lung injury due to infection prevents ventilation in affected parts of the lung
-increased ventilation of functional airways/alveoli supplies additional O2 to the alveoli
give a summary of key points
-gas exchange involves O2 and CO2 diffusing through multiple structures to pass between alveolar air and capillary blood
-the rate of diffusion is dependent on the partial pressure gradient, diffusion distance and surface area (diseases which negatively affect these factors will reduce gas exchange)
-ventilation and perfusion of respiratory surfaces must be matched in order to ensure an efieicent O2 exchange. V/Q defects will reduce blood oxygenation
