Pathology > Basic Hematology > Normal Hematopoiesis > RBC Basics

RBC Basics

It is possible to count reticulocytes by precipitating the residual RNA using new methylene blue to form a reticular network in the cell. There are numerous reticulocytes in this field. A normal reticulocyte count is often expressed as a percent of total RBCs (normal 0.5-1.5 %), but may be expressed as an absolute number (normal 21,000-94,500).

Every 24 hrs approximately 2 x10 RBCs are generated, replacing over 1% of the red cell mass. Over 95% of RBCs are found in the peripheral blood where the erythrocyte functions during its life span of 120 days.

How might you use the reticulocyte count (decreased, normal or increased) in evaluating a patient with too few RBCs? Then try the case below.

You are the Doctor:

Your patient is a 44 year -old woman complaining of "exhaustion". Her hematocrit is 0.15, her RBC count 1.0 million and her reticulocyte count 2%. You decide that her reticulocyte count is increased and determine that there is no evidence of bleeding. The increased reticulocyte count would indicate that her marrow is responding with acclerated erythropoiesis. Thus you decide to watch and wait. But that night you realize your mistake in regard to the reticulocytes! What was wrong?

You decided that the 44 year -old woman with a hematocrit of 0.15 and a reticulocyte count of 2% had an increased reticulocyte count indicating that her marrow is responding with increased RBC production thus you decide to watch and wait. What was wrong in regard to the reticulocytes?

AHHH HA! When expressed as a % of total RBCs the reticulocyte count may overstate the actual number of reticulocytes. Therefore:

In this case:

Thus the retic count is not increased (normal being 0.5-1.5%), but is in fact relatively low in an anemic patient indicating no marrow response and suggesting marrow failure.

The use of the absolute rectic count avoids this problem. In this case:

(0.02 x 1,100,000) = 22,000 retics, which is abnormally low for someone with such a low Hct.

If RBCs are destroyed as in a hemolytic anemia, or lost as in a GI bleed

the rate of erythropoiesis may increase as much as 8 fold. This is known as accelerated erythropoiesis and is secondary to the increased input of stem cells and a shortened precursor generation time. After a brief lag period increased RBC precursors appear in the bone marrow (lower left). Compare the erythroid hyperplasia with a normal marrow on the right.

 

Normally, there are three myeloid precursors for each erythroid precursor resulting in a 3:1 ratio, known as the M:E (myeloid to erythroid) ratio.

Accelerated red cell production would be expected to:

or

the M:E ratio?

The major regulator of erythrocyte production is erythropoietin. Erythropoietin stimulates the differentiation of committed erythroid stem cells (CFU-E) into red cell precursors. The production of erythropoietin is mediated by tissue oxygen levels such that RBC production exactly balances RBC loss, maintaining sufficient oxygen delivery to body tissues.

When blood is destroyed or lost:

1. the O2 receptor detects insufficient tissue oxygenation
2. erythropoietin production increases;
3. which increases committed erythroid stem cell production
4. restoring red blood cell mass to normal.

While working for NASA you take part in a short-term high altitude simulation study. Check your Hct, at right. Is it normal?

It's a busy August night in the ER; you're juggling 3 Pts.

Patient 1: A 22 yr old man is bleeding profusely from a knife wound.

Patient 2: A 24 yr old pregnant woman in her 8th month is hypertensive and is retaining fluid.

Patient 3: A 26 yr old cyclist collapsed after a gruelling race. The cyclist dies.

You spin the Hcts, but do not believe the results. Did you mix them up?

A. Pt samples 1 and 2 reversed.
B. Pt samples 2 and 3 reversed.
C. Pt samples 1 and 3 reversed.
D. Pt sample1 belongs to 2; 2 to 3, and 3 to 1.
E. Correct as is.

The Hematology Laboratory confirmed your spun hematocrits, but you noticed that the values from the laboratory were 1-2 % lower than the spun Hcts. Do you remember why?

The function of the erythrocyte is to produce, package, protect, and transport hemoglobin between the various body tissues.

The normal mature erythrocyte is a biconcave disc approximatly 8 um in diameter; 2.5 um thick at the periphery and 1.0 um thick at the center. Each contains 27-34 pg (10-12 g) of hemoglobin representing about 95% of the dry weight of a RBC.

For information regarding erythrocyte metabolism and hemoglobin click the Erythrocyte button.

Sophisticated automated partical counters determine directly the number of RBCs; RBC size or Mean Corpuscular Volume (MCV) and hemoglobin (Hgb) content.

Other RBC parameters are calculated,ie. Mean Corpuscular Hemoglobin (MCH) and Mean Corpuscular Hemoglobin Concentration (MCHC). In automated instruments the Hct is calculated:

MCV gives you the average volume of erythrocytes.
MCH gives you the average weight of Hgb per eyrthrocyte.
MCHC gives you the average Hgb concentration per erythrocyte.

Automated instruments measure cells using either electrical impedance or LASER techology.

This is a typical automated instrument report screen.

Looking only at the RBC parameters, we see a % RDW which is the coefficient of variation (CV) about the MCV. Thus the RDW is a quantitative measure of RBC size variation (anisocytosis). The RBCc, MCV, MCH and RDW are numerical correlates of RBC variations of #, size, Hgb conc. and shape, as seen on Wright's stained peripheral blood (PB) smears.

Notice the normal, minor variations in RBC size (anisocytosis) and shape (poikilocytosis). The area of central pallor is usually 1/3 the RBC diameter, reflecting normal Hgb content.

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