Since  the  air  in  .these  alveoli  has  been  in  contact  with  the  blood  enter-
ing the  lungs,  it  has  a  high  C02  content,  which  results,  when  compared
with  the  uniformly  low  002  content  found  in  the  tidal  air,  in  giving  a
large  figure  for  the  dead  space.  Since  the  capacity  of  the  dead  space
is  not  increased,  the  blood  in  the  normal  alveoli  is  probably  being  super-
ventilated  in  order  to  compensate  for  the  high  G02  tension  in  the  blood
entering  the  left  heart  from  the  diseased  alveoli.  However,  the  02
content  of  the  blood  leaving  the  sound  alveoli  is  practically  normal  (be-
cause superventilation  can  not  cause  it  to  take  up  more),  and  "can  not
compensate  for  the  low  02  content  in  the  blood  coming  from  the  dis-
eased alveoli,  the  net  effect  being  therefore  a  low  tension  of  02  in  the
blood  leaving  the  heart,  which  accounts  for  the  cyanosis  often  seen  in
emphysema  (Pearce).  A  somewhat  similar  explanation  can  be  given
for  the  cyanosis  present  in  pulmonary  edema,  if  we  assume  that  all  the
alveoli  in  this  condition  do  not  share  alike  in  the  edema  (Hoover).
