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Lung Mechanics
Lung Compliance (C)
Airway Resistance (R)
Determinants of CL
Stretchability of the lungs
Surface tension (type II alveolar cells secrete surfactant which lowers surface tension and increases CL)
Respiratory-distress syndrome of the newborn (surfactant is deficient)
Some important facts about pulmonary surfactant
1. A mixture of phospholipids and protein2. Secreted by type II alveolar cells3. Lowers surface tension of the water layer
at the alveolar surface, increases lung compliance
4. A deep breath increases its secretion (by stretching the type II cells)
5. Concentrations decreases when breaths are small
Airway Resistance (R)
Physical factors: transpulmonary pressure lateral traction (mucus accumulation)
Neuroendocrine factors:bronchodilators- Epinephrine ( receptor), VIPbronchoconstrictors- leukotrienes, histamine, ACh
Inversely proportional to the fourth power of the airway radii
Pathological conditions of airway resistance
Asthma: treated with anti-inflammatory drugs, bronchodilator drugs
Chronic obstructive pulmonary disease (COPD):
1. emphysema (destruction of alveolar walls)
2. chronic bronchitis (excessive mucus production in the bronchi and chronic inflammatory changes in the small airways)
3. a combination of the two
Lung volumes and capacities
Lung Volumes Tidal volume (VT): 500 ml Inspiratory reserve volume (IRV): 3000 ml Expiratory reserve volume (ERV): 1500 ml Residual volume (RV): 1000 mlLung Capacities Vital capacity (VC) = VT +IRV+ERV Functional residual capacity (FRC) = RV+ERV Inspiratory capacity (IC) = VT+IRV Total lung capacity (TLC) = FRC+IC
Lung volume and capacities
Ventilation
VE: minute ventilation (l/min)
VT: tidal volume (l)
f: respiratory frequency (cpm)
VE=VT x f
Alveolar Ventilation (VA)
the total volume of fresh air entering the alveoli per minute
VA =(VT – VD) x f
VE=VT x f
Dead Space (VD)
Definition: the space within them gas exchange is not permitted
Anatomical dead space: conducting airwaysAlveolar dead space: some fresh inspired air
is not used for gas exchange if the alveoli have little or no blood supply
Physiological dead space: the sum of the anatomical and physiological dead spaces
Effects of VD on VA
Effect of breathing patterns on alveolar ventilation
FEV1: forced expiratory volume in 1 secFVC: forced vital capacityObstructive lung diseases: increased airway resistanceRestrictive lung diseases: impaired respiratory movements because of abnormalities in the lung tissue, the pleura, the chest wall, or the neuromuscular machinery
Examples:
Gas Exchange
Respiratory Quotient (RQ) (in the steady state)
RQ = VCO2/VO2
1 for carbohydrate0.7 for fat0.8 for protein or mixed diet
Typical O2 and CO2 exchanges during 1 min at rest
Partial Pressure
The individual pressures in a mixture of gases; eg, PO2, PCO2
Dalton’s law
In a mixture of gases, the pressure exerted by each gas (x) is independent of the pressure exerted by the others
Px = P x Fx
Partial pressures of CO2 and O2
Alveolar gas pressure
Factors that determine PAO2
PO2 of inspired airVA
O2 consumption
Factors that determine PACO2
VA
VCO2
Gas exchange (Diffusion)