Lecture 11: Respiratory Gas Exchange Flashcards
What are properties of Gases?
-Gas modules constantly moving (collide) = exert pressure on the container
-Total pressure: determined by the total # of gas molecules per volume unit
-Partial pressure of gases: in a mixture = total pressure x % of that gas (independent of other gases present in the mixture)
EX. O2=20.94% of atmospheric air
total pressure (sea level) = 760mm Hg
Partial pressure of O2 (PO2)= 0.21x760=159mm Hg
-Air (Dry) P = PN2 + PO2 + PCO2= 760mmHg
BUT air is not dry in the lungs
Since inspired air is 100% saturated with water when it reaches the respiratory zone how is the total pressure of a gas calculated?
-Some of the gas molecules colliding with water dissolve in water
-At equilibrium the relative amount of dissolved gas is constant
*depends on the solubility of the gas
*Partial pressure of the gas
-Presence of water “dilutes” gas content
Ex at 37C water vapour P is 47mm Hg
PO2= (0.21 x 760)-(0.21x47)= 150 mm Hg
PO2= (amount present in dry air) - (amount dissolved in water)
TRUE OR FALSE: Gas exchange is done by diffusion?
TRUE
-Passive movement of gas molcules from regions of high concentration (partial P) to regions of low concentrations (partial P)
-In alveoli the air velocity = 0 (due to large cross-sectional area)
-Movement from alveoli to alveolar-capillary membrane by diffusion ONLY
What happens with pressure in the alveoli?
At constant atm partial P(sea level)
-Diffusion of gases will depend on the alveolar and blood partial P
*depends on ventilation for alveoli (how much air replaced in alveoli)
*Depends on the tissue consumption for blood
-At higher altitude P O2 in atm decreases (significant impact on diffusion)
-P CO2 in atm is way lower than in alveoli and blood (changes in atm P doesn’t impact CO2 diffusion dramatically)
What happens with respiration at the level of the lung?
-Gas exchange occurs b/w alveoli and blood capillary network
-Blood from right ventricle flows in capillaries (note lungs also have some capillaries from the left side of the heart to bring oxygen to lung tissue)
-Blood entering alveolar capillaries has low pO2
-O2 diffuses from aveoli to the blood
-At the end of capillary. pO2 is the same in the blood and alveoli
-For CO2: pCO2 higher in blood entering capillary then diffuses out board alveoli
What are main determinants?
-Driving partial pressure gradient (high-> low p driving force) known as PalveoO2 - PcapO2
-Surface area available for diffusion (A) increased area=more exchange. capillaries can open during exercises to increase exchange
-Thickness of the air-blood barrier (X) can prevent to impact distance has to travel (Deep inspiration during excesses reduces the distance b/w alveoli and capillary epitheliums but pneumonia can create additional layers with mucus so gases have harder time to cross
-Physical properties of the gas (D) different solubility
VO2=DxAx (PAO2 -PcapO2)/X
What is the process of ventilation/ perfusion?
-Perfusion corresponds to the blood entering the lung
-Need to match ventilation with the blood flow for optimum gas exchange
-For bipeds (human) at rest: not optimum bc gravity = perfusion (blood flow) lowest at the top of the lung (capillaries can collapse) not so pronounced for ventilation
-Not so bad for quadrupeds
What does the term V/Q mean?
V/Q= term used for ventilation/perfusion (0.8 in humans) 2 legs fighting gravity at rest more blood below the lungs. should be 1 but goes closer to 1 during exercise
-V/Q becomes more uniform during exercise (more blood pump throughout the lung all alveoli properly ventilated)
What is hypoxic vasoconstriction?
-Certain diseases can affect either the ventilation or perfusion of certain alveoli = V/Q inequality
-Natural mechanism to min the impact: hypoxic vasoconstriction (poor ventilation so lungs soft recognize sufficient blood)
-Closing of the poorly ventilated alveoli and redirecting blood towards the wall ventilated
-Mechanism initiated by a reduction of pO2 and/or increase in pCO2 in the interstitial fluid of affected areas
What impairs pulmonary gas exchange?
-Thickening of alveolar-capillary membrane, increases the time for diffusion across the membrane and decreases rate of diffusion (ex sick and have mucus accumulation thickening if pulmonary adema)
-High altitude or low air O2, decrease alveolar O2 pressure, hence decrease driving pressure (lower atm reduce delta P reduces gas exchange but can be compensated for)
-Hypoventilation, inadequate ventilation of lung (not enough movement of respiratory zone but atm is fine)
-Ventilation-perfusion inequality (ventilated alveoli with no blood supply or vice versa) issue with blood diffusion, or normal diffusion but poor ventilation results in poor gas exchange
What is gas exchange in tissues?
-Follows the same principle as in the alveoli
-Diffusion driven by difference in partial pressure O2: blood –> interstitial CO2: interstitial –> blood
-Cells consume O2 and produce CO2
-Blood from the left heart loaded with O2
-Diffusion of O2 will occur from blood to interstitial fluid to the cells
-Diffusion of CO2 will occur from the cell to the interstitial fluid to the blood
