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 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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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