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Respiratory Physiology.docx

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Physiology 3120
Tom Stavraky

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Respiratory Physiology Lung Volumes Spirometry • Method to measure lung volumes • Consists of a upside down bell filled with hair suspended in a liquid • Subject breathes air from the bell & the movement of the b ell (up with exhalation, down with inhalation) is recorded with a pen on a moving chart • Using the spirometer a number of lung volumes can be determined Definitions • Tidal volume (TV): volume of air inhaled with each breathe • Vital capacity (VC): volume of air that can be forcibly exhaled after a maximal inhalation • Residual volume (RV): volume of air remaining in the lungs after a maximal expiration • Functional Residual Capacity (FRC): volume of air remaining in the lungs at the end of a normal expiration • Inspiratory Reserve Volume (IRV): volume of air that can be forcibly inhaled following a normal inspiration • Expiratory Reserve Volume (ERV): volume of air that can be forcibly exhaled following a normal expiration • Total Lung Capacity (TLC): volume of air in the lungs at the end of maximal inspiration • Minute Volume or Pulmonary Ventilation o Is the volume of air inhaled per minute o Tidal volume x frequency of respiration o Normal: 500 mL x 12 breaths/min = 6000 mL/min • Forced Expiratory Volume – 1 second (FEV-1 sec) o Fraction of vital capacity expired in one second o In a normal healthy subject – about 80% of the vital capacity is expired in one second  i.e FEV-1sec (normal) is 80% of vital capacity o In restrictive diseases (pulmonary fibrosis) – vital capacity is reduced but FEV-1sec is normal or increased o In obstructive diseases (bronchial asthma) – FEV-1 sec is reduced much more than vital capacity • Maximal Voluntary Ventilation (MVV) (also called Maximal Breathing Capacity) o Volume of air that can be moved into and out fo the lung in one minute by voluntary effort o Normal MVV is in the range of 125-160 litres per min • TLC, FRC & RV cannot be measured directly with a simple spirometer FRC = ERV + RV RV = FRC – ERV Helium Dilution Method • Not all lung volumes can be measured with a spirometer • Spirometer does not allow one to determine: FV, FRC & TLC • Method utilizes a closed circuit – system that does not allow gas to escape • Subject, after normal expiration, is connected to a spirometer containing a known concentration of helium • Helium is almost insoluble in blood • After some breathes, helium concentration in the spirometer & lung becomes the same • Since no helium has been lost the amount of helium before equilibrium equals the amount after equilibrium C 1 1 C (2 +V1) 2 Where C 1 initial He concentration C2= final He concentration V 1 spirometer volume V2= FRC Pulmonary & Alveolar Ventilation • Pulmonary ventilation (or minute volume) = tidal volume x frequency • Since not all the volume of gas entering the lung participates in gas exchange, another important factor to consider is alveolar ventilation • Alveolar ventilation is the volume of air entering the respiratory zone per minute • Volume of air remaining in the conductive zone is called anatomical dead space • In a normal (70 kg) subject – deadspace volume is approx. 150 mL • For each tidal volume entering lung, portion goes to deadspace & remainder will go to respiratory zone • Only air that reaches respiratory zone will participate in gas exchange & continues alveolar ventilation • Alveolar ventilation = pulmonary ventilation – dead space ventilation • Physiological or total dead space: total amount of inhaled air that does not participate in gas exchange • In a normal subject anatomic dead space is similar to total dead space • • In certain diseases, inhaled air may enter nonfunctional alveoli – volume entering those areas would contribute to total dead space • Volume of total dead space can be calculated using Bohr equation • This calculation is based on the assumption hat all expired CO c2mes from alveolar gas and not from dead space Bohr Equation VD/VT = (PA(a)CO – 2ECO ) / 2ACO 2 Where VD = dead space volume VT = tidal volume PACO –2PCO of 2lveolar gas (or PaCO becau2e it is the same value as arterial CO ) 2 PECO – 2CO of e2pired gas • Importance of alveolar ventilation & the impact of deadspace can be observed when we determine the changes in alveolar ventilation that can occur with changes in respiratory rate and todal volume • in this scenario below, the amount of air available for gas exchange (alveolar ventilation) is drastically reduced despite no difference in pulmonary ventilation Respiratory Rate 30/min 10/min Tidal Volume
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