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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


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