Blood histology

Blood is composed of specialized cells that circulate in an extracellular fluid called plasma.

Plasma typically consists of about 92% water, 7% protein, and 1% is a combination of other solutes.

The three main types of blood cells are the red blood cells or erythrocytes, white blood cells or leukocytes, and platelets, which are also called thrombocytes.

The total blood volume of an average adult is about 6 L. Leukocytes and thrombocytes only comprise about 1 percent of the blood volume; whereas plasma, which is the largest portion of the blood, is about 55%.

Hematocrit or HCT for short, is the percent of packed red blood cells (or RBCs) in blood, by volume.

Normally, the hematocrit is between 39 to 50 percent in males and 35 to 45 percent in females.

The morphology and characteristics of blood cells can be analyzed histologically by using a common technique called a blood smear or blood film.

A drop of blood is placed on a glass slide, then literally smeared or spread across the slide from left to right, creating a thin layer of cells similar to this image of a blood smear stained with Wright’s stain.

This stain is a mixture of eosin, which is an acidic dye, methylene blue, which is a basic dye, and azures, which are also basic dyes.

It’s commonly used as a differentiating stain for blood smears, bone marrow, and blood parasites.

The head of the blood smear is where the drop of blood was applied to the slide.

When the blood was spread from the head to the right side of the slide, the thin layer of blood gradually became even thinner.

The tail of the smear is the last portion of the smear that visibly tapers even more.

The tail of the smear is not used for examination because the morphology of the cells can appear distorted.

The cells are often abnormally grouped together, and red blood cells will have a loss of central pallor, which could be mistaken for spherocytosis.

If we take a closer look at the blood smear on the left side, there are a lot more cells, but their morphology is also distorted.

In this image, we can see that the cells are overlapping and packed so tightly together that it’s hard to differentiate the individual cells from each other.

As a result, the best area of the smear to examine is typically within the body of the smear, but further to the right, near the tail.