PROBLEM SESSION ANSWERS: Problem 3

Curves in quiet breathing:

Muscles of inspiration contract (which? Mainly diaphragm, also external intercostals). This lowers intrapleural pressure, and the pressure change is transmitted into the alveoli. Since the alveolar pressure is now negative (below atmospheric), air flows into the lungs, increasing their volume. The volume increase causes the lung to "pull back" more, causing the alveolar pressure to return toward zero (atmospheric), the volume will stop increasing, and inspiration ends. :

When the inspiratory muscles stop contracting at the beginning of expiration, the stretched lungs now "pull back" harder than the chest wall pulls "out", so the alveolar pressure becomes positive and the lungs empty until the system reaches balance again. Note that this is quiet breathing at rest, so active contraction of the expiratory muscles is not involved.

Curves in exercise:

Larger intrapleural pressure swings (more vigorous contraction of respiratory muscles); intrapleural pressure less negative in expiration (active contraction of muscle during expiration - internal intercostals and abdominal wall); larger alveolar pressure swings; larger volume changes and faster flow rates.

Restrictive lung disease (fibrosis):

Low compliance ("stiff" lungs), so "pull back" harder at rest position. Results in lower (more negative) intrapleural pressure, smaller swings in alveolar pressure (more of the work of the respiration muscles goes into stretching the lungs, less into moving air), smaller tidal volume. Usually increased respiratory rate to compensate for low tidal volume and maintain alveolar ventilation.

Emphysema:

Loss of lung tissue leads to lower static lung compliance. Result: larger lungs at rest position and less negative intrapleural pressure. Inspiration is normal but in expiration, higher intrapleural pressure leads to airway narrowing (reduced expiratory flow rate) and my cause airway collapse (gas trapping).