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    Pulmonary Function Tests and Alveolar Ventilation: Introductory Human Physiology

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    Sherida Gibbs

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    © All Rights Reserved
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    Pulmonary Function Tests and Alveolar Ventilation: Introductory Human Physiology

    Uploaded by

    Sherida Gibbs
    22 views3 pages

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    LectureNotes5-4

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    Pulmonary Function Tests and Alveolar Ventilation: Introductory Human Physiology

    Uploaded by

    Sherida Gibbs

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    of 3

    Introductory Human Physiology ©copyright Jennifer Carbrey & Emma Jakoi

    4. Pulmonary function tests and alveolar ventilation

    8 FORCED EXPIRATORY VITAL CAPACITY TEST


    7
    provides an indirect assessment of airway
    X resistance. In this pulmonary function test, the
    6
    subject inhales to total lung capacity and then
    5 exhales into a spirometer as forcefully, rapidly, and
    VOLUME
    4(LITERS)
    as completely as possible. The volume expired
    Y under these conditions is called the forced vital
    3
    capacity (FVC) (Fig 3).
    2
    Z
    1
    The forced expiratory vital capacity test also
    measures the volume exhaled in 1 second, called
    0 2 4 6 8 10 the 1-second forced expiratory volume (FEV1).
    TIME (SECONDS) This value is often expressed as a % of FVC (i.e.,
    FEV1/FVC %). Normally FEV1 is at least 80% of
    FVC (curve Y). Patients with restrictive lung disease
    Figure 3. Forced expiratory vital will have a normal value of 80% (curve Z). In patients
    capacity curves generated by
    such as asthmatics, who have obstructed airways,
    individuals X, Y and Z.
    this value will be reduced (<80%) (curve X).

    Exchange of Gases in Alveoli & Tissues


    Respiration involves two processes:
    (1) Delivery of O2 to and removal of CO2 from the cells of the body.
    (2) Use of O2 in oxidative metabolism to generate ATP, water, and CO2.

    In a steady state, the amount of O2 that is consumed by the cells per unit time is equal
    to the amount of O2 added to the blood in the lungs during the same time period.
    Likewise the rate at which CO2 is generated by the cells is equal to the rate at which
    CO2 leaves the blood in the lungs and is exhaled.

    Gases move by diffusion from regions of high concentration to regions of low


    concentration. Therefore to provide adequate gradients for diffusion, the pulmonary
    system must increase the amount of oxygen in the alveoli above that found in the mixed
    venous (MV) blood of the lung. Additionally it must lower the carbon dioxide in the
    alveoli below that of mixed venous blood.

    A second set of gradients must exist at the tissue-blood interface. Here the amount of
    O2 consumed by cells and CO2 produced are not necessarily identical and depend on
    the fuel source consumed. The ratio of CO2 produced to O2 consumed is called the
    respiratory quotient (RQ). For a mixed diet, 8 molecules of CO2 are produced for
    every 10 molecules of O2 consumed (i.e., RQ = 0.8). For a diet composed of
    carbohydrates, the RQ is 1.0. For a diet of fat, the RQ is 0.7.
    Introductory Human Physiology ©copyright Jennifer Carbrey & Emma Jakoi

    Minute & Alveolar Ventilation


    Minute ventilation (VE) is the total volume of gas entering (or leaving) the lung per
    minute. It is equal to the tidal volume (TV) multiplied by the respiratory rate (f).

    Minute ventilation = VE = TV x f

    At rest, a normal person moves ~450 ml/breath x 10 breath/min = 4500 ml/min.

    However, because of the anatomical dead space (VD), not all of this entering air is
    available for exchange with the blood . Recall that the conducting airway (anatomical
    dead space) has a volume of ~150 ml. If 450 ml of fresh air is inspired, the first gas to
    reach the respiratory zone comes from this anatomical dead space (150 ml). Then 300
    ml of fresh gas reaches the respiratory zone and the last 150 ml of inspired gas remains
    in the dead space. Thus, the total amount of fresh air reaching the alveoli during each
    inspiration equals the tidal volume minus the volume of the anatomical dead space:

    TV – VD = 450 - 150 ml = 300 ml.

    Alveolar ventilation (VA) is the total volume of fresh air entering the alveoli per minute.
    It is calculated as:

    Alveolar ventilation = VA = (TV – VD) x f

    When evaluating the efficiency of ventilation, one should focus on the alveolar
    ventilation not minute ventilation.

    For example, in the table below, Subjects A and B have the same minute ventilation (VE
    = 6 L) but very different alveolar ventilations (VA). Subject A has no alveolar ventilation
    and would be become unconscious in a few minutes but Subject B is breathing
    normally.

    Subject TV f VE VD VA
    A 150ml 40 6000ml 150ml 0
    B 500ml 12 6000ml 150ml 4200ml

    One other important point shown in the table above is that the depth of breathing (TV)
    is far more effective in elevating the alveolar ventilation than an increase in ventilation
    rate (f). This is because for each tidal breath a fixed volume is dead space. As tidal
    volume decreases, the fraction going to dead space increases. The respiratory system
    will respond to O2 need (as in exercise) by reflexively increasing ventilation by
    increasing the depth of breathing.

    The anatomical dead space is not the only type of dead space in the lung. Some fresh
    air is not used for gas exchange even though it reaches the alveoli because some
    alveoli may have little or no blood supply (i.e., blood perfusion). This volume of air is
    called alveolar dead space. In normal individuals this is quite small but may be large in
    Introductory Human Physiology ©copyright Jennifer Carbrey & Emma Jakoi

    several kinds of lung disease. As we will discuss later, a mismatch in ventilation and
    blood perfusion is minimized by local mechanisms that match air and blood flow. The
    sum of the anatomical dead space and alveolar dead space is the physiologic
    dead space.

    Partial Pressure of Gases


    The amount of various gases can be measured by comparing the pressure they exert.
    Gas molecules behave like individual particles that are in a constant state of motion.
    When the particles collide with one another or the sides of the container they exert a
    pressure. The pressure exerted depends on the number of collisions. Two factors affect
    the number of collisions: the temperature of the gas and the number of gas
    molecules. Dalton's law states that in a mixture of gases, the pressure exerted by each
    gas is the same as it would be if that gas alone occupied the entire container. These
    individual pressures are called partial pressures and are denoted as P in front of the
    symbol for the gas.

    To calculate the partial pressure of gas "X":

    PX = Patm x FX

    Where, Patm is the atmospheric pressure (at sea level = 760 mm Hg), and FX is the
    fractional concentration of gas X.

    Atmospheric air contains mostly nitrogen (79%) and oxygen (21% O2) with trace
    amounts of CO2 and other gases. Air also contains water vapor. At sea level, water
    vapor is 47 mm Hg. For simplicity, respiratory physiologists and physicians generally
    assume that room air is always dry. Since 21% of dry room air is oxygen, the fraction
    of O2 in inspired air (FiO2) is:

    FiO2 x Patm = 0.21 X 760 mm Hg = 160 mm Hg.

    The concentration of carbon dioxide in room air is so low (0.04%), it is considered to be


    0.

    When inspired, the room air is warmed to 37oC and becomes humidified as it passes
    through the nasal passages. The water vaporizes into the air until the P H2O= 47 mm Hg.
    What this means is that only 760 - 47 mm Hg or 713mm Hg is available for other gases
    besides water. Therefore,

    PO2 of inspired gas = 0.21 X (760mm Hg - 47 mm Hg) = 150 mmHg.

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