Gas Exchange in the Lungs Flashcards
What does the ability of a gas to dissolve in blood depend on? What Law is this?
Henrys Law
-Pressure gradient from air into blood
-Solubility of gas in the blood
-Temperature of blood
-At equilibrium the partial pressure of a gas in plasma is equal to partial pressure in adjoining air
What does PO2 and PCO2 actually measure?
Only measure the conc of these gases dissolved in blood plasma
-not measure conc of these gases contained within RBCs where majority of O2 and CO2 is transported in blood
-However these values in blood ultimately determine the quantity of these gases in RBCs and thus provide a good index of lung function
What is the normal alveolar PO2 and the PO2 of systemic venous blood arriving at pulmonary arterial capillaries and what does this cause?
Normal alveolar PO2 is 100mmHg and the PO2 of systemic venous blood arriving in pulmonary arterial capillaries is 40mmHg
-This causes partial pressure gradient - O2 diffuses into blood until blood PO2 rises to 100mmHg - arterial blood returns to heart via pulmonary veins
What is the PCO2 in cells vs PCO2 of arterial blood?
-PCO2 in cells is 46mmHg and PCO2 of arterial blood is 40mmHg
-Pressure gradient - CO2 diffuses into blood until blood PCO2 is 46mmHg - systemic venous blood returns to lungs via heart and pulmonary arteries
-In lungs, blood PCO2 is 46mmHg and alveolar PCO2 is 40mmHg
-Diffusion of CO2 from blood into alveolar air until blood PCO2 drops to 40mmHg
LOOK at gas exchange diagram slide 7
What is hypoxaemia?
Abnormally low arterial PO2
-Results in hypoxia - deficiency of O2 in tissues
-Hypoxia may be evident as cyanosis - bluish colouration of skin/mucous membranes
What are causes of Hypoxaemia?
-Reduction in alveolar PO2
-Defects in gas exchange
What can cause a reduction in alveolar PO2?
-At high altitudes, atmospheric pressure falls so PO2 of air entering lungs decreases - alveolar PO2 decreases
-Obstructive pulmonary disorders (asthma) will diminish ventilation (hypoventilation) decreasing delivery of fresh air to alveoli - alveolar PO2 decreases
What can cause inefficient gas exchange?
-Restrictive pulmonary disorders (emphysema) cause destruction of alveoli - loss of alveolar surface area causing decreased diffusion of O2
-Increased fluid in tissue fluid of alveoli (pulmonary oedema) increases the diffusion distance between blood and air causing decreased diffusion of O2
How is O2 transported in the blood?
-Dissolved in plasma
-Bound to a transporter molecule, haemoglobin, inside rbcs (98% of O2)
What is the amount of O2 bound to haemoglobin dependent on?
-PO2 of plasma - determines the binding affinity of hb for O2
-Total amount of hb in blood - dependent mainly on rbc count
