Lecture 8-3: Airway resistance

forces influencing airway diameter

1. lateral traction: airways are connected by elastic tissue to alveolar walls and are pulled upon when the alveoli expand on inspiration

2. transpulmonary pressure (Ptp): acts outward on airway walls and increases their diameter, particularly during inspiration

how does airway resistance affect airflow

airway resistance is low and does not limit airflow under normal conditions b/c of lateral traction and transpulmonary pressure

effect of parasympathetic activity on airway resistance

1. irritant receptor or paracrines (e.g., histamine) stimulate PS activity

2. increased mucus secretion → increased bronchoconstriction → increase airway resistance

3. locally decreased Pco2

effect of sympathetic activity on airway resistance

1. norepi and circulating Epi (binds to beta2 adrenergic receptors)

2. bronchodilation and/or decreased mucus secretion widen airways and decrease airway R

3. locally decreased Po2 and increased Pco2

is epinephrine or norepinephrine more important for bronchodilation

circulating epinephrine

is parasympathetic or sympthatetic innervation more important for determining airways diameter

parasympathetic (release of ACh onto airways smooth muscle)

is airway R greater during inspiration or expiration

• greater during expiration

  • during expiration, lung volume decreases and causes airway to narrow which increases R

  • during inspiration, lungs and airway expand and lowers airway R

asthma

• chronically increased airway resistance

• chronic inflammation →

  • excessive bronchoconstriction

  • excess mucus production → reduced airway diameter

bronchoconstriction (increased R) is caused by…

• increased parasympathetic activity (M-AChR)

• increased release of immune mediators (paracrines)

• decreased alveolar Pco2

bronchodilation (decreased R) is caused by

• increased circulating epinephrine (beta2 adrenergic receptors)

• increased alveolar Pco2

• decreased alveolar Po2

relationship between airway resistance and lung volume

inverse relationship

• low lung volume, high airway R

• high lung volume, low airway R

is it more difficult for an asthmatic to inhale or exhale

• exhaling is more difficult

• airways are pulled open during inspiration due to lateral traction and Ptp

what happens to residual volume during an asthma attack

inability to expire adequately means RV increases

what happens to arterial Po2 during asthma attack

high expiratory R → reduced expiratory V → high RV and smaller percentage of alveolar volume replaced w/ each breath → decreased alveolar Po2 → decreased arterial Po2

effect of exercise of airway R

airway R decreases due to increase symp activity (increase circulating Epi).

effect of COPD on airway R

• emphysema: airway lateral traction destroyed → increased R

• bronchitis: inflammation and mucus → increased R