Gas exchange
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what factors affect alveolar gas exchange?
-pressure gradient -solubility -membrane thickness -membrane area -ventilation-perfusion matching
List the diffusion pathway
-alveolar gas -liquid lining alveoli with surfactant -alveolar epithelium -epithelial basement membrane -interstitial space -capillary basement membrane -cappilary endothelial cells -plasma -RBC membrane
What diseases cause the alveolar cappilary membrane to thicken?
-pulmonary fibrosis -emphysema -pulmonary oedema
What is ficks law?
volume of gas diffusing through the membrane per minuete (ml/min)
What is ficks equation?
A X D X (P1-P2) /T A-surface area of membrane D-diffusion coefficent P1-P2- partial pressure difference of gas T- membrane thickness
what is the equation of ventilation perfusion rate?
V/Q ration = alveolar ventilation / cardiac output
define V/Q mismatch
well ventilated but poorly perfused or poorly ventilated and well perfused
what happens when ventilation is reduced or perfusion is increased?
-reduced alveolar and arterial PO2 and increased PCO2 -Reducing ventilation -build up of CO2 and a reduction in O2 in alveoli
What happens if perfusion increases without a change in ventilation?
more CO2 brought to the lungs that can be removed-decrease in PaO2 and increase in PaCO2
What happens when ventilation is increased and perfusion reduced?
-increased alveolar and arterial PO2 and decreased PCO2 -increasing ventilation causes more CO2 to be exhaled increasing PO2 in the alveoli and blood -increased PaO2 and decreased PaCO2
What is a physiological shunt?
an area which isnt ventilated will result in reduced V/Q example- alveoli blocked by secretions or a physical block such as an inhaled peanut
What is an anatomical shunt?
when blood doesnt enter the lungs example-blood passing from the right to left ventricle without passing through the lungs
What happens to V/Q in COPD/ emphysema?
-alveoli and damaged causing large air spaces to develop and the loss of cappilaries -V/Q mismatch -hypoxia -hypercapnia
What causes an increased V/Q?
an area which isnt perfused (example-pulmonary embolism)
What are the two responses to change in V/Q?
-hypoxic vasoconstriction -bronchoconstriction
Describe what happens during hypoxic vasoconstriction
-low V/Q -local vasculature vasoconstricts to reduce blood flow to the area and direct to better ventilated regions of the lungs
Describe what happens during bronchoconstriction
-high V/Q -local airways bronchoconstrict to increase airway resistance and reduce ventilation to poorly perfused areas