Breathing Mechanism I The Pathway of Air Breathing

Breathing Mechanism: I. The Pathway of Air • Breathing is also known as ventilation • Air outside body mouth/nose pharynx (throat) trachea bronchi and bronchial tree lungs alveoli (air sacs) reverse • One inhalation + one exhalation = a respiratory cycle

Breathing Mechanism: II. Inspiration (Inhalation) A. Principles of Pressure • Air moves into the lungs via atmospheric pressure (760 mm Hg) • Boyle’s Law: pressure and volume are inversely related

Breathing Mechanism: II. Inspiration (Inhalation)

Breathing Mechanism: II. Inspiration (Inhalation) B. Resting Inspiration • During resting inspiration, the pressure inside the lungs and alveoli decreases and atmospheric pressure pushes outside air into the airways, using the muscles in the dome -shaped diaphragm Prior to inspiration, the intraalveolar pressure is 760 mm Hg

Breathing Mechanism: II. Inspiration (Inhalation) B. Resting Inspiration Atmospheric pressure of 760 mm Hg on outside Atmospheric pressure of 760 mm Hg n the inside When the lungs are at rest, the pressure on the inside of the lungs is equal to the pressure on the outside of the thorax

Breathing Mechanism: II. Inspiration (Inhalation) C. Normal Inspiration • Muscle fibers of the diaphragm are stimulated to contract by impulses on the phrenic nerves • Diaphragm moves downward thoracic cavity enlarges intra-alveolar pressure decreases atmospheric pressure forces air into the alveoli • External intercostal muscles and thoracic muscles stimulated to contract, elevating the ribs and the sternum and increasing the size of the thoracic cavity • Intra-alveolar pressure falls further, and atmospheric pressure forces more air into the alveoli The intra-alveolar pressure decreases to about 758 mm Hg as the thoracic cavity enlarges, and atmospheric pressure forces air into the airways

Breathing Mechanism: II. Inspiration (Inhalation) C. Normal Inspiration • Pleural membranes dictate the expansion of lungs during movement of the diaphragm • Separation of the pleural membranes decreases pressure between the membranes, and only a thin film of serous fluid separates the pleura lining the thoracic cavity and the pleura attached to the lungs • Water molecules attract these membranes and each other, holding the two membranes together • When the intercostal muscles move thoracic wall upward or downward, the pleura of both the thoracic cavity and the lungs move too, expanding the lungs

Breathing Mechanism: II. Inspiration (Inhalation) C. Normal Inspiration

Breathing Mechanism: II. Inspiration (Inhalation) C. Normal Inspiration • Moist inner membranes of the alveoli have the opposite effect • Attraction of water molecules surface tension difficult to inflate alveoli • Alveolar cells synthesize a mixture of lipoproteins called surfactant, which is secreted into alveolar air spaces, preventing collapse and helping to expand the alveoli for inspiration Rx – Respiratory Distress Syndrome: occurs when premature infants do not produce sufficient surfactant • treated by injection of synthetic surfactant through endotracheal tube and a customized ventilator

Breathing Mechanism: II. Inspiration (Inhalation) D. Abnormal Inspiration • Deeper than normal breaths = diaphragm and external intercostal muscles contract more forcefully • Pectoralis minor, sternocleidomastoids, and scalenes also pull thoracic cage further upward and outward, enlarging the thoracic cavity and decreasing the intra-alveolar pressure Normal inspiration vs. maximal inspiration

Breathing Mechanism: II. Inspiration (Inhalation) D. Abnormal Inspiration • Compliance (distensibility): ease with which the lungs expand from pressure changes during breathing • Compliance decreases as lung volume increases • Compliance can also be reduced by obstruction of air passages or destroyed lung tissue

Breathing Mechanism: III. Expiration (Exhalation) A. Normal Expiration • The forces that stimulate normal resting expiration are derived from the elastic recoil of lung tissues and abdominal organs and from surface tension • When the diaphragm and external intercostal muscles relax after inspiration, the lungs retract from their expanded state during inspiration to their original shapes • The elastic tissues also cause abdominal organs to revert back to their preinspiration shapes, pushing the diaphragm upwards

Breathing Mechanism: III. Expiration (Exhalation) A. Normal Expiration • Simultaneously, surface tension between the moist surfaces of the alveolar linings shrinks alveoli • These events eventually increase the alveolar pressure above atmospheric pressure, forcing air inside the lungs out through the respiratory passages • Normal resting inspiration occurs passively without contraction of any muscles • The recoil of elastic fibers in lung tissues decreases the pressure on the pleural cavity, and the pressure between the pleural membranes decreases substantially (4 mm Hg less than atmospheric pressure)

Breathing Mechanism: III. Expiration (Exhalation) B. Abnormal Expiration • Exhaling more air than normal is accomplished by contracting the posterior internal (expiratory) intercostal muscles, which pull the ribs and sternum downward and inward, increasing the air pressure in the lungs, pushing more air out • The abdominal muscles, such as the internal and external obliques, transversus abdominus, and rectus abdominus also squeeze the abdominal organs inward • The abdominal wall muscles can increase pressure in the abdominal cavity and force the diaphragm against the lungs, pushing additional air out of the lungs

Breathing Mechanism: III. Expiration (Exhalation) B. Abnormal Expiration

Breathing Mechanism: III. Expiration (Exhalation) B. Abnormal Expiration Rx – Pneumothorax: Occurs when atmospheric air enters the pleural cavity after thoracic wall is punctured • May collapse lung(s) and create unnatural space between pleural membranes • Treated by covering the chest wound and impermeable bandage and passing a tube through the thoracic wall into the pleural cavity and applying suction to the tube • The suction reestablishes negative pressure in the cavity and the collapsed lung expands

Breathing Mechanism: IV. Maladies of the Respiratory System (Irritants) Rx – Asbestos: a lung irritant derived from a naturally occurring mineral used in buildings and many commercial products • Asbestos fibers, when airborne, can cause scars in the lungs when inhaled • May cause shortness of breath, lung cancer, mesothelioma

Breathing Mechanism: IV. Maladies of the Respiratory System (Irritants) Rx – Beryliosis: condition caused from beryllium, an element used in fluorescent powders and metal alloys • Exposure to beryllium may produce an immune response that damages the lungs • Symptoms (latent) include cough, shortness of breath, fatigue, fevers, night sweats, and weight loss • Steroid drug, prednisone, is used to control symptoms Rx – Extrinsic Allergic Alveolitis: caused by repeatedly inhaling organic dusts • Impairs breathing and causes fevers • Changes shapes of lungs chronically • “Bathtub refinisher's lung, ” “maple bark stripper disease, ” “popcorn worker’s lung, ” “wheat weevil’s disease”

Breathing Mechanism: IV. Maladies of the Respiratory System Rx – Air Pollution: a threat to health when particles are small enough to penetrate alveoli • Kills more than seven million people per year • WHO recommends limiting or avoiding outdoors when microparticulates exceed 25 micrograms per cubic meter or when air quality index exceeds 200