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.

This zone will have some cells that overlap, but the majority of the cells will be evenly spaced apart, allowing the cells to be easily counted and differentiated from one another.

Now let’s take a closer look at the erythrocytes.

With Wright’s stain, the erythrocytes stain bright pink or red because they contain a large amount of hemoglobin, which is eosinophilic.

Erythrocytes normally lack organelles or nuclei and they’re typically shaped like a biconcave disc, which provides a large surface-to-volume ratio that helps facilitate gas exchange.

As a result, the center of erythrocytes are thinner, which is why they appear to have a pale center when they’re positioned parallel to the microscope slide.

Erythrocytes normally survive in the circulation for about 120 days.

The small basophilic or purple discs that are seen throughout this image are platelets.

Platelets also don’t have nuclei and are normally only about 2 to 4 μm in diameter.

Whereas the average erythrocyte is about 7 to 8 μm in diameter, which is often used by histologists as an internal standard to estimate other nearby cells or structures, such as the diameter of this small capillary that’s only slightly larger than a single erythrocyte.

Many abnormalities in erythrocytes can be seen with a blood smear, including spherocytes which lack a distinct pale center; fragmented RBCs called schistocytes; sickle cells and target cells in patients with sickle cell anemia; and many other abnormalities such as howell-jolly bodies, & bite cells.

Leukocytes in the blood can be classified into two groups, granulocytes and agranulocytes.

The two groups can be differentiated using light microscopy since the granulocytes have prominent granules that look like small dots or grains in their cytoplasm, whereas agranulocytes don’t have these granules.

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Blood histology

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