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)