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Lecture 2

Lecture 2. Lung volumes and capacities Anatomical and physiological VD Alveolar space and VE VD and uneven VE Ventilation-perfusion relations. Lung volumes and capacities. Capacity is the sum of 2 or more volumes.

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Lecture 2

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  1. Lecture 2 • Lung volumes and capacities • Anatomical and physiological VD • Alveolar space and VE • VD and uneven VE • Ventilation-perfusion relations

  2. Lung volumes and capacities • Capacity is the sum of 2 or more volumes. • It can be measured by a spirometer. It also can be measured by vitalograph, gas dilution and body plethysmography. • Lung vol includes; 1) Tidal volume (TV): It is the vol of air expired and inspired in each breath (500 ml). 2) Inspiratory reserve volume (IRV): It is the max vol of additional air that can be inspired from the end of a normal insp (3100 ml). 3) Expiratory reserve volume (ERV): It is the max volume of additional air that can be expired from the end of a normal exp (1200 ml). 4) Residual volume (RV): It is the vol of air that remains in the lung after maximal exp (1200 ml).

  3. Lung capacities include; 1) Inspiratory capacity (IC) = TV + IRV 2) Functional residual capacity (FRC) = ERV + RV 3) Vital capacity (VC) = IC + ERV 4) Total lung capacity (TLC) = IC + FRC

  4. LUNG CAPACITIES AND RESPIRATORY DISEASES • Restrictive Disease. Respiratory disease which make it more difficult to get air in to the lungs. They “restrict” inspiration. Includes fibrosis, sarcoidosis, muscular diseases, and chestwall deformities. B) Obstructive Disease. Respiratory disease which make it more difficult to get air out of the lungs. Includes emphysema, chronic bronchitis, asthma. C) Lung capacity changes during disease—a summary • Restrictive Disease: ↓ VC; ↓ TLC, RV, FRC. • Obstructive Disease: ↓ VC; ↑ TLC, RV, FRC.

  5. Anatomical and physiological VD • VD is defined as the volume of inspired air that does not participate in GE. • The normal VD in a young adult man is about 150 milliliters. This ↑ slightly with age. • There are 2 types of VD, anatomical and physiological. (1) Anatomic VD is the vol of an inspired breath which has not mixed with the gas in the alv. It is anatomical because it measures the anatomical vol of the conducting airways leading up to the alv. It can be measured from the vol of expired gas leaving the mouth and nose before the 'front' of alveolar gas containing CO2 arrives at the lips. (2) Physiological VD is the vol of an inspired breath which has not taken part in GE. It is physiological because it assesses one of the functions of the lungs - GE. It can be estimated using the Bohr equation, which is derived from the fact that the vol of gas expired equals the vol from the VD plus the vol from the alv.

  6. In a normal person, the anatomic and physiologic VD are nearly equal because all alv are functional in the normal lung, but in a person with partially function or nonfunctional alv in some parts of the lungs, the physiologic VD may be as much as 10 times the vol of anatomic VD.

  7. Alveolar ventilation • VA is the total vol of new air entering the alv and adjacent GE area each minute. • It is equal to the respiratory rate times the amount of new air that enters these area with each breath; VA = fr X (VT- VD) • In a normal person VA = … X (… - …) = …… ml/min • Because of the VD, rapid, shallow respiration produces much less VA than slow, deep respiration at the same minute vol (see table).

  8. Table: Effects of variations in respiratory rate and depth on VA.

  9. VD and uneven VE • In the upright subject the bases of the lungs are found to be better ventilated than the apices. This can be demonstrated by breathing radioactive xenon. • The uneven VE is due to the effect of gravity. Similarly, a subject in the supine position will have better VE of the posterior parts of the lungs than the anterior parts. Uneven VE can significantly affect GE in the lungs. • VE is preferentially distributed to the more dependent portions of the lungs because, as a result of the weight of the lungs, the intrapleural pres is lower (i.e. less -ve).

  10. Ventilation-perfusion relations • VE is defined as the flow of air into and out of the alveoli.Perfusion is defined as blood flow to the alveoli. • VE and perfusion are normally matched in the lungs so that GE (VE) nearly matches pulmonary arterial BF (perfusion). If mismatched, impairment of O2 and CO2 transfer results. • The concentration of O2 (PO2) in any lung unit is measured by the ratio of VE to BF: VENTILATION/PERFUSION or V/Q • The VE – Perfusion relationship can be measured by calculating the Alveolar (A) – arterial (a) PO2 diff.  PAO2 can be calculated using the following equation: PAO2 = FIO2 (Patm – PH2O) – (PaCO2/R) at sea level, FIO2 = 0.21, PH20 = 47, Patm = 760, PaCO2 measured by lab analysis, R = 0.8 PAO2 = 150 – (PaCO2/0.8)

  11. Physiologic shunt • A shunt is when perfused alveoli are not ventilated. Thus, the blood passing through those parts of the lung is not receiving O2 at all. • This shunted blood (without O2) will eventually mix with the rest of the pulmonary blood. The result is a lower O2 saturation for the entire arterial blood. • There are 2 types of shunt; 1) Pulmonary Shunts:These shunts occur within the parenchyma of the lung. i.e. in the alveoli.2) Extrapulmonary Shunts:These shunts occurs outside the lung itself such as in a major bronchus.

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