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Physiology of the Respiratory System

Physiology of the Respiratory System. Pulmonary Ventilation. Breathing, 2 phases Inspiration: air moves into the lungs Expiration: air moves out of the lungs Gas moves down a pressure gradient Air in the atmosphere exerts pressure of 760 mm Hg. Inspiration.

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Physiology of the Respiratory System

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  1. Physiology of the Respiratory System

  2. Pulmonary Ventilation • Breathing, 2 phases • Inspiration: air moves into the lungs • Expiration: air moves out of the lungs • Gas moves down a pressure gradient • Air in the atmosphere exerts pressure of 760 mm Hg

  3. Inspiration • Diaphragm contracts, it flattens, which makes thoracic cavity longer • Intercostals muscles contract, elevated sternum & ribs, which enlarges thoracic cavity • Lungs pulled out because of cohesion of the pleura • Air pressure in alveoli & tubes decrease & air moves into lungs

  4. Elastic recoil • Tendency of the thorax & lungs to return to their preinspiration volume

  5. Expiration • Inspiratory muscles relax, decreasing size of thorax • Alveolar pressure increases thus positive pressure gradient from alveoli to atmosphere & expiration occurs

  6. Pulmonary Volumes • Tidal volume= volume of air exhaled after a typical inspiration; normal TV=500 ml • Expiratory reserve volume= largest additional volume that can be forcibly expired after expiring tidal air; normal ERV=1000-1200 ml • Inspiratory reserve volume= amount of air that can be forcibly inspired over and above normal inspiration; normal IRV=3300 ml • Residual volume= air that can not be forcibly expired but is trapped in alveoli, RV=1200 ml

  7. Vital capacity • Largest volume of air that an individual can move in and out of the lungs • VC=IRV=TV=ERV

  8. Alveolar Ventilation • Volume of inspired air that actually reaches the alveoli • Part of air inspired fills our air passageways, this is the anatomical dead space • Anatomical dead space is approximately 30% of TV, thus alveolar ventilation is 70 % of TV

  9. Pulmonary Gas Exchange • A gas diffuse “down” its pressure gradient • Concentration of O2 in air is about 21% thus the partial pressure of O2 is about 160 mmHg • 21% x 760 mm Hg = 160 mm Hg

  10. Amount of Oxygen that diffuses into blood depends on: • Oxygen pressure gradient • Total functional surface area of alveolus • Respiratory minute volume • Alveolar ventilation

  11. Hemoglobin • 4 polypeptide chains (2 alpha & 2 beta) each with an iron containing heme molecule • Oxygen can bind to iron in heme group • CO2 can bind to amino acids in chain

  12. Transport of Oxygen • Oxygen travels in two forms in blood: • Dissolved in plasma • Associated with hemoglobin as oxyhemoglobin (most) • Increasing PO2 in blood accelerates hemoglobin association with O2

  13. Transport of Carbon Dioxide • Dissolved carbon dioxide (10%) • Bound to amine (NH2) groups of amino acids to form carbaminohemoglobin (20%) • In the form of bicarbonate ions (more than 2/3) • CO2 + H20 H2CO3  H + HCO3 • Catalyzed by carbonic anhydrase

  14. Carbon Dioxide and pH • Increasing carbon dioxide content of blood increases H ion concentration thus increases the acidity and decrease the pH

  15. Respiratory Control Centers • Main integrators that control nerves that affect inspiratory & expiratory muscles are located in brainstem • Medullary rhythmicity center generates basic rhythm of respiratory cycle • Can be altered by input inputs from: • Apneustic center in pons stimulates to increase length and depth of respiration • Pneumotaxic center in pons inhibits apneustic center to prevent overinflation of the lungs

  16. Factors that influence breathing • PCO2 acts on chemoreceptors in medulla: • Increasing PCO2 increases RR • Decreasing PCO2 decreases RR • Decrease in blood pH stimulates chemoreceptors in carotid & aortic bodies • Arterial blood PO2 has little influence if it stays above a certain level • Decrease in PO2 below 70 mmHg increases RR

  17. Arterial blood pressure & breathing • Sudden rise in blood pressure results in reflex slowing of respirations

  18. Hering-Breuer reflexes • Help control respiratory depth & volume of tidal air

  19. Miscellaneous factors • Sudden painful stimulations produces reflex apnea (no respirations) but continued painful stimulus cause faster & deeper respirations • Sudden cold stimuli on skin causes reflex apnea • Stimulation of pharynx or larynx by irritating chemicals or touch causes temporary apnea-choking reflex

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