TY  - CHAP
M1  - Book, Section
TI  - Anemia Due to Acute Blood Loss
A1  - Longo, Dan L.
A2  - Loscalzo, Joseph
A2  - Fauci, Anthony
A2  - Kasper, Dennis
A2  - Hauser, Stephen
A2  - Longo, Dan
A2  - Jameson, J. Larry
Y1  - 2022
N1  - 
T2  - Harrison's Principles of Internal Medicine, 21e
AB  - Blood  loss  causes  anemia  by  two  main  mechanisms:  (1)  by  the  direct  loss  of  red  cells;  and  (2)  if  the  loss  of  blood  is  protracted,  it  will  gradually  deplete  iron  stores,  eventually  resulting  in  iron  deficiency.  The  latter  type  of  anemia  is  covered  in  Chap.  97.  Here,  we  are  concerned  with  the  former  type,  that  is,  posthemorrhagic  anemia,  which  follows  acute  blood  loss.  This  can  be  external  (e.g.,  after  trauma  or  obstetric  hemorrhage)  or  internal  (e.g.,  from  bleeding  in  the  gastrointestinal  tract,  rupture  of  the  spleen,  rupture  of  an  ectopic  pregnancy,  subarachnoid  hemorrhage,  leaking  aneurysm).  In  any  of  these  cases,  after  the  sudden  loss  of  a  large  amount  of  blood,  there  are  three  clinical/pathophysiologic  stages.  (1)  At  first,  the  dominant  feature  is  hypovolemia,  which  poses  a  threat  particularly  to  organs  that  normally  have  a  high  blood  supply,  like  the  brain  and  the  kidneys;  therefore,  loss  of  consciousness  and  acute  renal  failure  are  major  threats.  It  is  important  to  note  that  at  this  stage  an  ordinary  blood  count  will  not  show  anemia  because  the  hemoglobin  concentration  is  not  affected.  On  physical  exam,  tachycardia,  tachypnea,  decreased  pulse  pressure,  cold  skin  that  appears  pale  and  mottled,  and  decreased  urine  output  may  be  noted.  (2)  Next,  as  an  emergency  response,  baroreceptors  and  stretch  receptors  will  cause  release  of  vasopressin  and  other  peptides,  and  the  body  will  shift  fluid  from  the  extravascular  to  the  intravascular  compartment,  producing  hemodilution;  thus,  the  hypovolemia  gradually  converts  to  anemia.  The  degree  of  anemia  will  reflect  the  amount  of  blood  lost.  If  after  3  days  the  hemoglobin  is,  for  example,  7  g/dL,  it  means  that  about  half  of  the  entire  blood  has  been  lost.  (3)  Provided  bleeding  does  not  continue,  the  bone  marrow  response  will  gradually  ameliorate  the  anemia.  In  this  phase  of  the  process,  the  reticulocyte  count  and  erythropoietin  levels  will  be  elevated.  The  physiologic  increase  in  marrow  red  cell  production  reflected  by  the  increase  in  reticulocytes  is  similar  to  the  marrow  response  to  hemolysis.  
SN  - 
PB  - McGraw-Hill Education
CY  - New York, NY
Y2  - 2024/04/20
UR  - accessmedicine.mhmedical.com/content.aspx?aid=1198034253
ER  -