Pulmonary Ventilation  A.K.A. “Breathing”  Consists of two phases:  Inspiration: period of time when air flows into the lungs  Expiration: period of time when gases exit the lungs  Multiple mechanic factors work together to promote these gas flows

Inspiration  Diaphragm and intercostals muscles contract  Diaphragm moves inferiorly and flattens during contraction, causing height of thoracic cavity to increase  Intercostals contraction lifts the ribcage and thrusts sternum forward, increasing anterioposterior and lateral dimensions (circumference)  Lungs adhere tightly to the thorax walls (due to surface tension of fluid between pleural membranes), they are stretched to the new, larger size of the thorax.  As intrapulmonary volume increases, gases with in the lungs spread out to fill the larger space.  Resulting decrease in the gas pressure in the lungs produces a partial vacuum (pressure less than atmospheric pressure), which sucks the air into the lungs.

Expiration  Passive process that depends mostly on natural elasticity of the lungs than on muscle contraction.  As inspiratory muscles relax and resume normal resting length, rib cage descends and lungs recoil.  As the thoracic and pulmonary volume to decrease, gases inside the lungs are forced closer together and intrapulmonary pressure rises to above atmospheric pressure.  This causes gases to flow out to equalize pressure inside and outside of the lungs.  Normally this is a passive process, but if passageways are narrowed due to spasms of bronchioles (asthma) or clogged with mucus/fluid (bronchitis/pneumonia), it becomes an active process, using intercostal muscles to help depress rib cage and abdominal muscles to help squeeze air out of lungs.

Pulmonary Ventilation  Mechanical process that depends on volume changes in the thoracic cavity  Important physics rule to remember for breathing mechanics:  Volume changes lead to pressure changes  Pressure changes lead to flow of gases to equalize pressure  Boyle’s Law: (when temp constant) P 1 V 1 = P 2 V 2  P = pressure in mm Hg  V = volume in cubic mm

Events of Inspiration

Steps of Expiration

Respiratory Adjustments: Exercise  Ventilation can increase 10-20x during exercise  Breathing becomes deeper and more vigorous (“Hyperpnea”), but respiratory rate may not be significantly changed.  Any respiratory changes meet metablolic demands so O2/CO2 levels in the blood are not altered.  Change in breathing is prompted by rising CO2 and declining O2, which causes drop in blood pH.

Respiratory Adjustments: High Altitude  At elevation above 8000ft, air density and oxygen pressure are lower, which may cause symptoms of “acute mountain sickness” (AMS)  Respiratory and hematopoietic adjustments occur called “acclimiatization”.  Chemoreceptors become more responsive to increases in CO2/decreases in O2, resulting in increased ventilation.  Within a few days, respiratory volume stabilizes at a level 2-3 L/min higher than at sea level.