Gas Exchange And transport Flashcards
Percentages of different gases in air
. 78% Nitrogen
. 21% O2
. 1% argon
. Trace amounts CO2, H2O vapor, other gases, and pollutants
Dalton’s law
. Partial pressure of a gas (x) in a gas mixture is the pressure that gas would exert if it occupied the total volume of the mixture in absence of the other components . Px = PB x Fx . Px: partial pressure of gas x . PB: total barometric dry gas pressure . Fx: fractional component of gas x
T/F total barometric pressure will change with altitude, but the percentage of each gas in the total air mixture will stay the same
T
Henry’s law
. States that the concentration of a gas dissolved in a liquid (Cx) is equal to the partial pressure of that gas (Px) times its solubility in that liquid (alpha)
Cx = alpha x Px
Partial pressure gradient
. Gas diffuses down its partial pressure gradient
. Normally, O2 diffuses from the alveoli and dissolves in pulmonary capillary blood until blood PO2 becomes equal to th partial pressure of O2 int he alveolar gas (PAO2)
. CO2 normally diffuses from pulmonary capillary blood and evolves as CO2 gas in the alveoli (PACO2) until alveolar PCO2 is equal to blood PCO2
composition of alveolar air
. Alveolar air has different composition than atmospheric air
. At body temp the partial pressure of H2O vapor is 47 mmHg
Effect of humidification of inspired air on partial pressure of gases
. It dilutes the partial pressure of the other inspired gases by 47 mmHg because the sum of all individual gas partial pressures must equal total barometric pressure
Alveolar gas equation
. PAO2 = PIO2 - (PACO2/RQ)
. PAO2: PO2 within alveoli
. PIO2: partial pressure of inspired O2
. PAO2 and PACO2 have an inverse linear relationship to one another
Partial pressure of O2 w/in alveoli throughout respiratory cycle
. Remains fairly constant
. O2 w/in alveoli continually moves down its partial pressure gradient into the blood
. New O2 arriving in the lungs w/ each breath replaces that which has already diffused into the pulmonary capillaries
. Pulmonary blood PO2 equilibrates w/ the alveolar PO2, the PO2 w/in the arterial blood also remains fairly constant at around 100 mmHg
Partial pressure of CO2 throughout respiratory cycle
. Body tissues continuously produce CO2
. CO2 added up in blood in the systemic capillary beds, and then is transported to the lungs
. When in lungs, CO2 diffuses down its partial pressure gradient, from the blood into the alveoli
. CO2 is removed from lungs during expiration
. Because CO2 arriving from the body tissues is constantly removed from the lungs, the PACO2 remains relatively constant throughout the cycle at 40 mmHg
Gas transfer w/in the pulmonary capillaries
. Blood entering pulmonary capillaries is systemic venous blood w/ a PO2 of 40 mmHg, and PCO2 of 46 mmHg
. Blood flows through pulmonary capillaries, is exposed to a PAO2 of 100 mmHg and PACO2 of 40 mmHg
. O2 flows down gradient from alveolar air into blood
. After O2 equilibration, blood leaving pulmonary capillaries has PO2 of 100 mmHg
. CO2 flows down gradient from blood into alveolar air
. After CO2 equilibration, blood leaving pulmonary capillaries has PCO2 of 40 mmHg
. Blood now higher in O2 and lower in CO2 is returned to heart and pumped to body
PO2 w/in the systemic capillaries
. Arterial blood reaches systemic capillaries is same blood that left lungs w/ PO2 of 100 mmHg and PCO2 is 40 mmHg
. Each cell consumes O2 and produces CO2
. Cellular PO2 is 40 mmHg and PCO2 is 46 mmHg
. Values change based on cellular metabolism
. O2 moves by diffusion down gradient from systemic capillary blood (PO2 100 mmHg) into adjacent cells (PO2 40 mmHg) until equilibrium
. PO2 of blood that leaves systemic capillaries is equal to tissues PO2 (40 mmHg)
PCO2 w/in systemic capillaries
. CO2 diffuses out of tissues (PCO2 46 mmHg) into capillary blood (PCO2 40 mmHg) down gradient until blood PCO2 equilibrates w/ the tissues PCO2
. PCO2 of the blood that leaves the systemic capillaries is equal to the tissue PCO2 ($^ mmHg)
Graham’s Law
. In the gas phase, the diffusion rate of molecules is inversely proportional to sq rt of molecular weights
. Diffusion rate = 1/ sq rt (MW)
Fick’s Law
. Volume of a gas that diffuses across a tissue sheet per unit time (Vgas)
. Vgas = A sol (P1-P2)/ d sq rt MW
. A: area of sheet
.d= thickness of the sheet
. D: diffusion constant (permeability coefficient of tissue for that gas)
(P1-P2): partial pressure gradient across membrane
. Sol: solubility
. MW: molecular weight
Blood flow in lungs during exercise
. During exercise when pulmonary blood pressure is raised as a result of inc. CO, many previously closed pulmonary capillaries are opened
. This inc. the SA available fo gas exchange
Surface area of respiratory membrane in emphysema
. Many alveoli coalesce with dissolution of alveolar walls
. New alveoli are much larger than the original alveoli
. Total SA of respiratory membrane may dec. to 20% its original size
Alveolar membrane thickness and affect on diffusion
. 0.5 um in thickness
. Rate of diffusion through the membrane is inversely proportional to thickness of the membrane
. Any factor that inc. the thickness can interfere w/ normal respiratory exchange of gases
Pathological conditions that inc. thickness of respiratory membrane
. Pulmonary edema: excess accumulation of interstitial fluid btw alveoli and pulmonary capillaries
. Pulmonary fibrosis: involves replacement of delicate lung tissue w/ thick fibrous tissue in response to certain chronic irritants
. Pneumonia: characterized by inflammatory fluid accumulation w/in or around the alveoli
Rate of gas transfer across the pulmonary membrane is proportional to ____
. Solubility of that gas
. Inversely proportional to its molecular weight
.
CO2 and O2 solubility
. CO2 is more soluble in body tissues than is O2
. The rate of diffusion CO2 diffusion across respiratory membrane is 20 times more rapid than O2 for a given partial pressure gradient
. CO2 passes through alveolar membrane easier than O2
Perfusion limited gas transfer
. Limit of time available for gas transfer
. N2O moves across respiratory membrane easily and dissolves in plasma
. N2O doesn’t combine w/ any substance in blood to its partial pressure in blood inc. rapidly
. Partial pressures on both sides of respiratory membrane equilibrate rapidly preventing further gas transfer
. Continued transfer of N2O can occur only if new blood is supplies to pulmonary capillary
. Amount of N2O taken up in blood depends on the rate of perfusion
Diffusion limited gas transfer
. Gas tensions in the alveoli and the capillary blood fail to reach equilibrium during blood transit time through the capillary
carbon monoxide gas transfer
. CO diffuses across respiratory membrane into the capillary blood down its partial pressure gradient
. Hb has high CO affinity so a large amount of CO is taken up by RBCs
. Partial pressure of CO in plasma remains low and CO gas continues to cross respiratory membrane down its partial pressure
. CO equilibrium btw alveoli and plasma is never achieved and the partial pressure gradient for CO will be high the entire time the blood spends in the pulmonary capillary
. CO limited only be diffusivity of alveolar membrane
O2 uptake along pulmonary capillary
. Perfusion limited
. O2 diffuses across membrane and some O2 will bind to RBC but some stays dissolved in plasma
. Rise in PO2 in plasma will eventually cause equilibration
Under resting conditions, the capillary PO2 is nearly the same as alveolar PO2 by the time ____
RBC is 1/3 along the way of the capillary
O2 uptake along the pulmonary capillary during exercise
. CO inc. and the time th eRBC spends in capillary may be reduced
. PO2 of blood will equilibrate w/ alveolar PO2 before the blood completely passes though the capillary
. W/ disease, there may not be enough time for equilibration and PO2 will inc. More slowly in capillary blood showing diffusion impairment
Diffusing capacity
. DL = Vgas/PAgas
. Vgas: amount of gas transferred
. PAgas: partial pressure of gas in alveoli
How DLCO is measured
. Single breath of CO
. Single inspiration of dilute mixture is made and the rate fo disappearance of CO for alveolar gas calculated
. Done by measuring inspired and expired concentrations of CO w/ infrared analyzer
. Normal value 25 ml/min/mmHg
. Can inc. 2-3x during exercise
Examples of conditions have thicken the barrier
. Interstitial/alveolar edema
. Interstitial/alveolar fibrosis
Conditions that cause dec. surface area
. Emphysema
. Low CO
. Low pulmonary capillary blood volume
Conditions that cause Dec. uptake by erythrocytes
. Anemia
. Low pulmonary capillary blood volume