The Production of Erythrocytes: part A



A. What are RBC’s and what do they do?


These are erythrocytes that are 7-8 micron wide and 1-2 micron thick.

They are biconcave, flexible and contain haemoglobin.


There are three values that are important for the pathology:
1. Mean Cell Volume (MCV): expressed in femtoliters (fl). Normal value 80-96 fl
2. Mean Cell Haemoglobin (MCH) expressed in picograms (pg) per red cell. Normal value 27-33 pg
3. Mean Cell Haemoglobin Concentration (MCHC): percentage of the red cell mass that is made up of hemoglobin; normal value 33-35%.

(femto = 10-15; pico = 10-12).

Another value is the hematocrit. (Ht) (also called packed cell volume; PCV). This is the volume of all red cells expressed as a fraction of the total blood volume. Normal values for males (0.42-0.53) and females (0.36-0.45).


Function 1:

Oxygen transport (lungs to tissues)

Function 2:

CO2 transport with carbonic anhydrase:
CO2 + H2O <-> H2CO3 <-> H+ + HCO3-

This enzyme only occurs in the erythrocytes!

Function 3:

Because of the H+ in the previous reaction, erythrocytes also play an important role in the acid-base buffer.



B. Where are RBC's produced?


The erythrocytes are produced by the bone marrow (= tissue inside the bones). In adults, this occurs mostly in the sternum, the ribs and the vertebra but in babies and children, who need a lot of blood for growing, it is also produced in the marrow of long bones such as the femur and the humerus.


Inside the bone marrow, there are stem cells that can produce all types of blood cells (erythrocytes, leukocytes etc). Depending on the needs of the body, signals are sent to a stem cell to differentiate into either a red blood cell, a white blood cell or other types of blood cells.


In the diagram, the steps that occur in the bone marrow are shown. These steps demonstrate the maturation of proerythroblast into reticulocytes and erythrocytes.

Once these cells have become reticulocytes or erythrocytes, they are released into the blood circulation.



C. What regulates RBC production?


The signal to differentiate stem cells into erythrocytes is a hormone called erythropoietin. It is mainly the kidney that secretes this hormone.


This is a feedback system; if there is not enough oxygen in the tissues, then erythropoietin will increase which will make more erythrocytes that will transport more oxygen. If there is too much oxygen, the reverse will happen.


There are many situations when there is a lack of oxygen and production of erythropoietin. This reaction by the body is useful because more erythrocytes will transport more oxygen.


Here are four examples or situations that induce an increase in erythropoietin and an increase in RBC production:


High altitude (= high in the atmosphere, there is less air and therefore less oxygen). This is for example famous in the Indians who live in the Andes Mountains (in South America). These people look very healthy with red cheeks. In reality, their cheeks are red because the blood vessels in the skin are filled with erythrocytes.


After bleeding, when one has lost 1-2 liters of blood; in an accident for example. This is actually an anaemia (= lack of enough erythrocytes).


Many types of heart and lung diseases, that reduce the transportation of oxygen, will increase, as compensation, the number erythrocytes.


Doping: unfortunately, an increase in erythrocytes is also useful in sport activities. Some people will try to enhance their performance unfairly. One way to do this is by auto-transfusion but a more popular method nowadays is by taking artificial erythropoietin called EPO. 2014