gaseous exchange 1112
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GASEOUS EXCHANGEFRESHMEN YEAR PROGRAM MEDICALSCHOOL OF MEDICAL FACULTY OF UNISBA
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Principles of Gas
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Partial Pressure of Gases
The rapid moving molecules exert a pressure,the magnitude is increased by anything thatincreases the rate of movement
The pressure a gas exerts is proportional to (1)the temperature (heat increases the speed ofmovement) and (2) the concentration of gas (thenumber of molecules per unit volume
In a mixture of gases, the pressure exerted byeach gas is independent of the pressure exerted
by otherstermed partial pressure(Daltons law) This because gas molecules are normally so far apart
that they do not interfere with each other
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Principles of Gas
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Partial Pressure of Gases (cont) Diffusion of gas in liquid When a liquid is exposed to air containing a particular
gas, molecules of the gas will enter the liquid anddissolve in it
Henrys lawstates that the amount of gas dissolvedwill be directly proportional to the partial pressure ofthe gas with which the liquid is in equilibrium
Why must the diffusion of gas into or within liquids bepresented in terms of partial pressure rather thanconcentration, the values used to deal with thediffusion of all other solutes?The reason is that theconcentration of a gas in a liquid is proportional notonly to the partial pressure of the gas but also to thesolubility of gas in the liquid; the more soluble the gas,the greater will its concentration at any given partialpressure
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Exchange of Gases in Alveoli dan
Tissues
Exchange of gases in lungs and tissues is bysimple diffusion, as a result of differences inpartial pressure. Gasses diffuse from a region of higher partial pressure
to one of lower partial pressure Normal alveolar gas pressure for oxygen is 105
mmHg and for carbon dioxide is 40 mmHg
At any given inspired PO2, the ratio of oxygen
consumption to alveolar ventilation determinesalveolar PO2the higher the ratio, the lower thealveolar PO2
The higher the ratio of carbon dioxide production toalveolar ventilation, the higher the alveolar PCO2
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Exchange of Gases in Alveoli dan
Tissues (cont)
The average value at rest for systemic venousPO2is 40 mmHg
As systemic venous blood flows through thepulmonary capillaries, there is net diffusion ofoxygen from alveoli to blood and carbon dioxide
from blood to alveoli. By the end of each pulmonary capillary, the blood gas
pressures have become equal to those in the alveoli
Inadequate gas exchange between alveoli andpulmonary capillaries may occur when the
alveolus-capillary surface area is decreased,when the alveolar walls thicken, or when thereare ventilationperfusion inequalities
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Alveolar Gas Pressure
Normal alveolar gas pressures are PO2 = 105mmHg and PCO2 = 40 mmHg; in atmospherethe PO2 = 160 mmHg
The alveolar PO2 is lower than atmospheric PO2
because some of the oxygen in the air enteringthe alveoli leaves them to enter the pulmonarycapillaries
The alveolar PCO2 is higher than atmospheric
PCO2 because carbon dioxide enters the alveolifrom the pulmonary capillaries
The factor that
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Alveolar Gas Pressure (cont) The factors that determine the precise value of
alveolar PO2are (1) the PO2of atmospheric air, (2)the rate of alveolar ventilation, and (3) the rate oftotal-body oxygen consumption.
A decreased in the PO2 of inspired air (at high
altitude), a decrease in alveolar ventilation, and anincrease in the cells consumption of oxygen willdecrease alveolar PO2
Hypoventilationexists when there is an increase inratio of carbon dioxide production to alveolarventilationPCO2 >40 mmHg
Hyperventilation exists when there is a decrease inratio of carbon dioxide production to alveolarventilationPCO2 < 40 mmHg.
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Alveolar Gas Pressure (cont)
Note: Hyperventilationis not synonymous with
increased ventilation.
Hyperventilation represents increased ventilation
relative to metabolism.
The increased ventilation that occurs during
moderate exerciseis nothyperventilation
The increase in production of carbon dioxide isexactly proportional to the increased ventilation
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Alveolar Gas Pressure (cont)
Effect ofincreasing or
decreasing
alveolar
ventilation onalveolar partial
pressure in a
person having a
constantmetabolic rate
(cellular O2
consumption and
CO2production)
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Alveolar Gas Exchange
The blood that enter the pulmonary capillarieshas a relatively high PCO2(45 mmHg) andrelatively low PO2(40 mmHg)
In normal person, the rates of oxygen andcarbon dioxide diffuse are so rapid and theblood flow the capillaries so slow that completeequilibrium is reached well before the end of thecapillaries
The blood that leaves the pulmonary capillariesto return to he heart and be pumped into thesystemic arteries has essentially the PO2andPCO
2as alveolar air
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Alveolar Gas Exchange (cont)
Given that diffusion between alveoli andpulmonary capillaries normally achievescomplete equilibration, the more capillariesparticipating in this process, the more totaloxygen and carbon dioxide can be exchanged.
The diffusion of gases between alveoli andcapillaries may be impairedin number of ways,resulting in inadequate oxygen diffusion into theblood, particularly during exercise when the time
for equilibration is reduced. In lung infection or pulmonary edema some of
alveoli may become filled with fluid
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Factors influence rate of pulmonary
and systemic gas exchange
Partial pressure difference of the gases
Surface area available for gas exchange
Diffusion distance
Molecular weight and solubility of the gases
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Gas Exchange in the Tissues
The capillary wall in tissues and plasmamembrane of cells are so thin and are highly
permeable to oxygen and carbon dioxide.
Metabolic reaction occurring within the cells are
constantly consuming oxygen and producing
carbon dioxideintracellular PO2 is lower and
PCO2 is higher than in blood net diffusion of
oxygen from the blood and carbon dioxide fromthe cellsas blood flows through systemic
capillaries, its PO2 decreases and its PCO2
increases
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O2CO2
Exchange
Changes In Partial
Pressure During
External andInternal Respiration
(Dalton & Henrys
laws)
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I ADVISE YOU TO LEARNMORE THAN YOU HAVE
HEARD
Thank You