Blood in the Classification of Tissues
In histological classification, blood and lymph belong to fluid connective tissues of the internal environment. This broad group also includes hematopoietic tissues (myeloid, lymphoid), proper connective tissues, specialized tissues (adipose, reticular, mucous), and skeletal tissues (bone, cartilage).
All tissues in this group share two common features:
- They develop from a single embryonic source: the mesenchyme.
- They contain an abundant amount of extracellular matrix. The physical state of this extracellular matrix (liquid, gelatinous, or solid) determines the specific type and function of the tissue.
Plasma vs. Serum
The extracellular matrix of blood is called plasma. To obtain plasma in a laboratory, blood is centrifuged after adding anticoagulants to prevent clotting. As a result, plasma separates as a pale yellow fluid above the cellular sediment.
Chemical composition of plasma:
- Water — about 90% by mass.
- Proteins (6.5–8.5%) — albumins, globulins, and fibrinogen.
- Lipids — transported inside protein-coated transport micelles.
- Low-molecular-weight compounds — metabolic products circulating between tissues.
- Inorganic ions — free or bound to proteins.
Blood serum is the fluid that separates during spontaneous blood clotting. In composition, it is identical to plasma but completely devoid of fibrinogen (since it is consumed in thrombus formation).
Formed Elements
The cellular component of blood is represented by three main types of formed elements. Under normal conditions, 1 liter of blood contains the following cell counts:
- Erythrocytes (red blood cells) — $5 \cdot 10^{12}$/L.
- Leukocytes (white blood cells) — $6 \cdot 10^9$/L (1,000 times fewer than erythrocytes).
- Platelets (thrombocytes) — $2.5 \cdot 10^{11}$/L (20 times fewer than red blood cells).
Almost all formed elements (except T cells) originate in the red bone marrow. A steady state is maintained in a healthy organism: the number of destroyed cells equals the number of newly released ones. Circulation times vary: erythrocytes live for about 120 days, whereas leukocytes spend only 4–12 hours in the bloodstream before migrating into tissues (although their overall lifespan can reach several years).
Histological Examination
The primary method for studying blood cells under a light microscope is a blood smear.
A drop of blood is placed on a glass slide and spread into an ultrathin layer using the polished edge of another slide. The smear is then fixed (typically with methanol) and stained. The gold standard is Romanovsky staining, which uses a mixture of a basic dye (Azure II) and an acidic dye (Eosin) to differentiate various cellular structures.