RT Book, Section
A1 Naureckas, Edward T.
A1 Solway, Julian
A2 Loscalzo, Joseph
A2 Fauci, Anthony
A2 Kasper, Dennis
A2 Hauser, Stephen
A2 Longo, Dan
A2 Jameson, J. Larry
SR Print(0)
ID 1190499026
T1 Disturbances of Respiratory Function
T2 Harrison's Principles of Internal Medicine, 21e
YR 2022
FD 2022
PB McGraw-Hill Education
PP New York, NY
SN 9781264268504
LK accessmedicine.mhmedical.com/content.aspx?aid=1190499026
RD 2024/04/23
AB The  primary  functions  of  the  respiratory  system—to  oxygenate  blood  and  eliminate  carbon  dioxide—require  virtual  contact  between  blood  and  fresh  air,  which  facilitates  diffusion  of  respiratory  gases  between  blood  and  gas.  This  process  occurs  in  the  lung  alveoli,  where  blood  flowing  through  alveolar  wall  capillaries  is  separated  from  alveolar  gas  by  an  extremely  thin  membrane  of  flattened  endothelial  and  epithelial  cells,  across  which  respiratory  gases  diffuse  and  equilibrate.  Blood  flow  through  the  lung  is  unidirectional  via  a  continuous  vascular  path  along  which  venous  blood  absorbs  oxygen  from  and  loses  CO2  to  inspired  gas.  The  path  for  airflow,  in  contrast,  reaches  a  dead  end  at  the  alveolar  walls;  thus,  the  alveolar  space  must  be  ventilated  tidally,  with  inflow  of  fresh  gas  and  outflow  of  alveolar  gas  alternating  periodically  at  the  respiratory  rate  (RR).  To  provide  an  enormous  alveolar  surface  area  (typically  70  m2)  for  blood-gas  diffusion  within  the  modest  volume  of  a  thoracic  cavity  (typically  7  L),  nature  has  distributed  both  blood  flow  and  ventilation  among  millions  of  tiny  alveoli  through  multigenerational  branching  of  both  pulmonary  arteries  and  bronchial  airways.  Ideally,  for  the  lung  to  be  most  efficient  in  exchanging  gas,  the  fresh  gas  ventilation  of  a  given  alveolus  must  be  matched  to  its  perfusion.  However,  as  a  consequence  of  variations  in  tube  lengths  and  calibers  along  these  pathways  as  well  as  the  effects  of  gravity,  tidal  pressure  fluctuations,  and  anatomic  constraints  from  the  chest  wall,  the  alveoli  vary  in  their  relative  ventilations  and  perfusions  even  in  health.