Blood Composition, Hematology, and Blood Typing

The centrifugal force causes the blood to separate into three distinct layers based on density:

1. Plasma: The top, liquid layer.
2. Buffy Coat: A thin middle layer containing leukocytes (WBCs) and platelets.
3. Packed RBCs: The bottom layer consisting of erythrocytes.

Reading and Normal Ranges

To find the hematocrit value, the tube is placed against a reader card. The bottom of the red cell column is aligned with the 0% baseline, and the value is read at the top of the RBC column.

Exam Tip: Normal Hematocrit Ranges Normal values differ by biological sex.

• Males: $47\% \pm 5\%$ ($42\text{--}52\%$)
• Females: $42\% \pm 5\%$ ($37\text{--}47\%$)

Hemoglobin Concentration

While hematocrit measures the volume of RBCs, hemoglobin concentration measures the actual amount of hemoglobin—the oxygen-binding protein—present in the blood. This is typically expressed in grams per 100 milliliters ($g/dL$).

The normal range for hemoglobin is approximately $14.1\text{--}15.6\text{ g/dL}$. It is important to note that males typically exhibit higher hemoglobin levels than females, a physiological adaptation supported by higher levels of testosterone to assist with muscle mass.

The Tallquist Scale

A traditional method for estimating hemoglobin is the Tallquist scale. In this procedure, a drop of blood is placed on chemically treated paper and allowed to change color. The color of the spot is then compared to a standardized color scale under natural light.

The scale allows clinicians to categorize the patient's status:

• Normal: High color intensity (e.g., Men $> 85\%$, Women $> 80\%$).
• Suggestive Anemia: Moderate color intensity.
• Actual Anemia: Low color intensity (Men and Women below $70%$).

Clinical Note: Anemia Anemia is a condition characterized by insufficient hemoglobin or red blood cells to meet the body's oxygen demands. A low reading on the Tallquist scale or a low hemoglobin concentration is a primary indicator of this condition.

Understanding Blood Types and Agglutination

Blood type is determined by the presence or absence of specific antigens (surface markers) on the surface of red blood cells. The two most significant systems are the ABO system and the Rh system.

The ABO and Rh Systems

The immune system produces antibodies in the plasma that act as "security guards," recognizing and attacking foreign antigens.

The Rh factor determines if a blood type is "positive" (Rh present) or "negative" (Rh absent). Unlike ABO antibodies, Anti-Rh antibodies are not naturally present; they only develop after an Rh-negative person is exposed to Rh-positive blood, such as through a transfusion or pregnancy.

Agglutination

When incompatible blood is introduced into a system (e.g., giving Type A blood to a Type B person), the recipient's antibodies will bind to the foreign antigens. This causes agglutination, which is the visible clumping of red blood cells.

In a laboratory setting, blood typing is performed by mixing an unknown sample with synthetic antibodies (Anti-A, Anti-B, and Anti-Rh). If clumping occurs in a specific well, it indicates the presence of that corresponding antigen.