Physiology: why maintaining blood pressure is essential if RBC count is low

[schafer]

Hello,

 

When reading about the physiology of maintaining blood pressure I came across something that confused me a bit.

 

In the book the example was of hemorrhage:

Loss of blood results in low RBC (erythrocytes) and low blood pressure --> fluid moves from interstitial fluid into blood stream --> increased blood plasma volume --> normal blood pressure.

 

What confuses me is how this is helpful. I understood that blood pressure is important to maintain so all tissues/organs receive adequet supply of oxygen and dispose of CO2. Since this autotransfusion will not return RBC count to normal levels, how does it help the body?

 

In other words: I understood that the importance of blood pressure is its role in supplying the organs with erythrocytes. If this is so, how does normal blood pressure with low RBC is helpful?

[Marmul]

Your book made it confusing by defining hemorrhage as a loss of Erythrocytes. Hemorrhage is a broad term indicating any kind of bleeding. Blood consists of other components besides erythrocytes, including leukocytes, plasma and platelets. During a hemorrhage, you are losing all of these components. I haven't taken hematology so I don't know the specifics but in Physio, we learned that the main function of RBCs is oxygen/ co2 transport, WBCs is immune/ inflammatory properties, and plasma is the main center for Hormone transfer. A hemorrhage will interrupt all of these processes, therefore the most important thing during a hemorrhage would be to restore blood volume and pressure before the body can return to Homeostasis.

So to answer your question, once blood volume and pressure are restored, the kidneys will detect the decrease in O2 and secrete Erythropoeitin to begin the process of Erythropoeises and restore the lost Erythrocytes.

[Xalatan]

You still need normal blood pressure to pump what's left of the rbc around to the tissues. It's one thing to lose RBC and oxygen carrying capacity, and another to lose the blood volume required for preloading the heart. Without adequate stroke volume the body will go into shock irrespective of haematocrit.