Today I am gonna talk about the Anemias of blood loss, particularly the acute cases of posthemorrhagic anemia.
Lets suppose a 27- yr old healthy male met with an accident which resulted in an acute episode of loss of 500- 1000 ml of blood (upto 20% of circulating blood volume) due to hemorrhage in just a few minutes! Such large amounts of blood loss over a brief period of time may result in anemia even though the iron stores remain adequate. This condition is called "Acute Post- hemorrhagic anemia".
The sequence of events characteristic in such case may be divided into two phases:-
Phase 1 or Hypovolemic phase with no or little anemia lasts for 2-3 days.
Phase 2 or Anemic phase in which blood volume is restored to near normal levels with signs of anemia and active regeneration of new red blood cells (reticulocytes).
Why is there no sign of anemia right after an episode of acute massive blood loss?
Massive amounts of blood are lost which means all the constituent cells of blood are lost making the residual blood volume low. In spite of that the red cell indices (MCH, MCHC, MCV and RDW) after the episode are normal. The red cell count per unit volume remains the same and the red cell morphology is normal too. This is because red cells and other constituents of blood are lost along with plasma which leaves the remaining blood in normal state though the total volume decreases. Therefore, the state of Hypovolemia precedes phase of restoration when the red cell count in comparison to total blood volume is low, thus making the patient anemic.
Note here that if blood loss continues in small amounts for prolonged periods then the two phases are not distinct anymore and the manifestations of hypovolemia and regenerative anemia may occur at the same time.
Healthy subjects are able to tolerate such loss with a few or no symptoms at all. But some of them are vasovagal reactors and show symptoms of anemia such as weakness, sweating, nausea, hypotension followed by light- headedness and finally syncope (fainting). It is observed that such reactors are either hypochondriacs or psychologically depressed and hopeless.
In cases when even more blood is lost rapidly (upto 40% of circulating blood volume), signs and symptoms of anemia may appear even if the subject is recumbent (not showing any mobility). There may be a decrease in BP and CVP though the pulse may be rapid, low in volume, thin and thready.
Adrenergic stimulation in such cases may potentiate redistribution of blood flow with more blood flowing to the vital organs and less to the peripheries. Thus the skin of extremities becomes cold, clammy and pale (which explains the pallor seen in face and palms of anemic patients).
Restoration of blood volume occurs in two phases:-
In phase 1, there is decreased capillary pressure resulting in partial compensation with electrolytes and proteins.
In phase 2, there is shift of intracellular fluid to the interstitium and then to the intravascular compartment.
Phase 2 of restoration of blood volume takes hours to days depending upon the level of activity by the patient. In recumbent subjects, expansion of plasma volume occurs in 24 hours. In ambulatory cases, the plasma volume expansion takes several days (related primarily to extravascular albumin pool) due to increasing demand of nutrients by cells and tissues. But if blood loss is massive (more than 30%), the pathway takes a U- turn and more fluid moves back to cells and tissues due to less blood supply and increase in demands, thus countering the outflow.
Hope that helped :)
- Jaskunwar Singh