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 protein 2. Secreted by type II alveolar cells 3. 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) Inversely proportional to the fourth power of the airway radii Physical factors: transpulmonary pressure lateral traction (mucus accumulation) Neuroendocrine factors: bronchodilators- Epinephrine (b receptor), VIP bronchoconstrictors- leukotrienes, histamine, ACh

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 volume 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 ml Lung Capacities Vital capacity (VC) = VT +IRV+ERV Functional residual capacity (FRC) = RV+ERV Inspiratory capacity (IC) = VT+IRV Total lung capacity (TLC) = FRC+IC

Ventilation VE=VT x f VE: minute ventilation (l/min) VT: tidal volume (l) f: respiratory frequency (cpm)

Alveolar Ventilation (VA) VE=VT x f the total volume of fresh air entering the alveoli per minute VA =(VT – VD) x f

Dead Space (VD) Definition: the space within them gas exchange is not permitted Anatomical dead space: conducting airways Alveolar 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 sec FVC: forced vital capacity Obstructive lung diseases: increased airway resistance Restrictive 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 carbohydrate 0.7 for fat 0.8 for protein or mixed diet

Typical O2 and CO2 exchanges during 1 min at rest

The individual pressures in a mixture of gases; eg, PO2, PCO2 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 air VA O2 consumption Factors that determine PACO2 VA VCO2

Gas exchange (Diffusion)