Lecture 2 The work of breathing Surface tension (ST) Role of surfactant Lung volumes and capacities Anatomical and physiological VD Alveolar space and.
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Lecture 2 The work of breathing Surface tension (ST) Role of surfactant Lung volumes and capacities Anatomical and physiological VD Alveolar space and VE VD and uneven VE
The work of breathing Work (force X distance or pres X vol) is a measure of energy expenditure. The work that is done by resp muscles is the sum of 2 components; 1) elastic work done against the elasticity of the lung-chest wall, 2) flow-resistive work done against airway resistance. The alveolar pressure changes reflect flow-resistive work; the intrapleural pressure changes reflect both elastic and flow-resistive work. The resp muscles normally perform “work” to cause inspiration only. As VE ↑, the V T drops and the chest excursion that accompanies each breath is smaller, so the elastic work ↓. At the same time, air flows more and more rapidly through the airway, the work needed to overcome airway resistance becomes progressively greater. When the elastic and flow-resistive work components are summed to show how the energy cost of breathing varies with resp rate, the work is seen to be greatest at very low and very high resp rates.
- For typical resting rates of V E, the energy cost of V E is minimal at a value very close to the actual resting fr of about 12 b/min. - Several components of breathing that required energy includes; a) energy is required by the contracting muscles of breathing (e.g. diaphragm and external intercostal muscles). b) energy is required to overcome the viscosity of the expanding lung tissue and of the tissues of the thoracic cage. c) energy is required to stretch the thoracic and lung elastic tissues. d) energy is required to overcome airway resistance to airflow.
Surface tension and role of surfactant ST is defined as the collapsing pres exerted upon the alv. ST in the alveolus is created by interacting H 2 O molecules which direct a force inward and could caused the alv to collaps. An important factor affecting the compliance of the lungs is the ST of the film of the fluid that lines the alv. One way to think of a ST is to imagine that the surface consists of a thin rubber membrane under stretch. If an incision 1 cm long were made in this membrane it would gape, and if sutures were put in it to bring the two cut sides together the total force of the sutures would be equal to the ST.
According to Laplace's Law: P = 2T / r. where P is the pres within an alv, T the tension in the alveolar wall and r the radius of the alv. This formula shows that the pres inside a small alveolus is larger than that inside a large alveolus. Thus, if 2 alve of diff sizes were connected, we would expect the smaller one to collapse and empty its gas contents into the larger one. This formula also shows that the tension in the alveolar wall is directly proportional to the pres within the alv. The tension in the alveolar wall has 2 components: 1-The tension generated in the wall of the spherical alv. 2- the ST created by the watery liquid that lines the walls of the alv.
As seen above, surfactant lowers the ST of the lining fluid and thus ↓ the overall tension in the alveolar walls. By ↓ the tension, the pres ↓ prop. Thus the pres diff between 2 alv that are connected is ↓, and the smaller alv will not collapse. Surfactant has two very important functions: 1) It lowers ST of the lining fluid so we can breath without too much effort. 2) It prevents the alv from collapsing.
Lung volumes and capacities Capacity is the sum of 2 or more volumes. Lung vol and capacity can be measured by a spirometer. It also can be measured by vitalograph, gas dilution and body plethysmography. Lung vol includes; 1) Tidal volume (V T ): 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 vol 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). Lung capacities include; 1) Inspiratory capacity (IC): V T + IRV ( 3600 ml). 2) Functional residual capacity (FRC): ERV + RV (2400 ml). 3) Vital capacity (VC): IC + ERV (4800 ml). 4) Total lung capacity (TLC): IC + FRC (6000 ml).
Normal values of lung vol and capacities in both male & female Parameter Volume (liter) Volume (liter) MaleFemale TV0.50.5 IRV3.31.9 ERV1.00.7 RV1.21.1 IC3.82.4 FRC2.21.8 VC4.83.1 TLC6.04.2
LUNG CAPACITIES AND RESP DISEASES A) Restrictive Disease. Resp 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. Resp 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.
Anatomical and physiological V D V D is defined as the vol 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 two types of V D anatomical and physiological. (1) Anatomic V D 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 CO 2 arrives at the lips. (1) Anatomic V D 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 CO 2 arrives at the lips. (2) Physiological V D 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 V D plus the vol from the alv. (2) Physiological V D 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 V D plus the vol from the alv.
In a normal person, the anatomic and physiologic V D 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 V D may be as much as 10 times the vol of anatomic V D.
Alveolar ventilation V A is the total vol of new air entering the alv and adjacent GE area each minute. It is equal to the resp frequency times the amount of new air that enters these area with each breath; V A = fr X (V T - V D ) What is the V A in a normal person? V A = …. X (…. - ….) = 4200 ml/min Because of the V D, rapid, shallow resp produces much less V A than slow, deep resp at the same minute vol (see table).
Table: Effects of variations in respiratory rate & depth on V A. Respiratory rate 30 b/min 10 b/min VTVTVTVT 200 ml 600 ml Minute vol 6000 ml VAVAVAVA ….(…. – ….) = 1500 ml ….(…. – ….) = 4500ml
V D & uneven ventilation 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 vent is due to the effect of gravity. Similarly, a subject in the supine position will have better vent of the posterior parts of the lungs than the anterior parts. Uneven vent can significantly affect GE in the lungs. Vent 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 negative). A clinical correlate of the effect of gravity on vent is that arterial oxygenation is improved in unilateral lung diseases when patients lie on their sides so that the good lung is in the dependent position.