Airflow, Pressure Gradients, and Resistance

Airflow

- Amount of air that moves into and out of the lungs with each breath - Function of two factors: 1) the pressure gradient established between atmospheric pressure and intrapulmonary pressure 2) the resistance that occurs due to conditions within the airways, lungs, and chest wall

Pressure gradient

- Can be changed by altering the volume of the thoracic cavity - small volume changes during quiet respiration allow 500 mL air to enter the lungs - if accessory muscles of forced inspiration are used, volume increases more - airflow increases due to greater pressure gradient

Collapsed alveoli increase resistance

- Can occur if alveolar type II cells are not producing surfactant - high surface tension of alveoli is not overcome - usually only an important factor for premature infants - without surfactant, alveoli in their lungs collapse with expiration - infants experience greater resistance to airflow - condition is referred to as acute respiratory distress syndrome (ARDS)

Respiratory diseases and anatomic abnormalities may produce increased resistance

- Can occur via decreases in the size of lumen of bronchioles (asthma) - Can occur via decrease in compliance (pulmonary fibrosis)

Compliance

- Ease with which lungs and chest wall expand - Determined by surface tension and elasticity of the chest wall and lung - The easier the lung expansion, the greater the compliance

Resistance

- Includes all factors that make it more difficult to move air from the atmosphere to the alveoli - May be altered in three ways: 1) decrease in elasticity of the chest wall 2) change in the bronchiole diameter or the size of the passageway through which air moves 3) collapse of alveoli

Increased resistance requires more forceful inspirations

Muscles of inspiration working harder