per  cent;  and  at  a  pressure  of  62  mm. — possibly  present  in  the  tissues —
B  stands  in  venous  blood  at  about  67  vols.  per  cent.  The  C02-containing
power  would  be  7  per  cent  lower  (i.  e.,  60  vols.  per  cent)  in  blood  saturated
with  02  at  the  latter  pressure.  The  oxygenation  of  blood  in  the  lungs,
therefore,  helps  to  drive  out  the  C02;  and  conversely,  its  deoxygenation
in  the  tissues  enhances  its  power  of  absorbing  this  gas.
Having  shown  how  the  blood  transports  its  charge  of  02  from  the
lungs  to  the  tissues,  we  may  now  proceed  to  study  the  call  for  02  by
the  tissues,  and  in  this  connection  we  have  to  consider  (1)  the  amount
of  .02  which  they  require  under  varying  conditions  of  rest  and  activity,
and  (2)  the  mechanisms  by  which  their  varying  demands  are  met.
