Place of blood in the tissue system
In histological classification, blood and lymph are classified as fluid tissues of the internal environment. This large group also includes hemopoietic (myeloid, lymphoid), connective proper, special (adipose, reticular, mucous), and skeletal (bone, cartilage) tissues.
All tissues in this group share two common features:
- They develop from a single embryonic source — mesenchyme.
- They contain a large amount of extracellular matrix. It is the physical state of this intercellular matrix (liquid, gel-like, or solid) that determines the type and functions of a specific tissue.
Plasma and serum
The intercellular substance of blood is called plasma. To obtain plasma in the laboratory, blood is centrifuged after adding anticoagulants to prevent clotting. As a result, plasma separates as a light yellow fluid above the cell sediment.
Chemical composition of plasma:
- Water — about 90% by mass.
- Proteins (6.5–8.5%) — represented by albumins, globulins, and fibrinogen.
- Lipids — transported within protein-coated transport micelles.
- Low-molecular-weight compounds — metabolic products circulating between tissues.
- Inorganic ions — in a free state or complexed with proteins.
Blood serum — is the fluid that separates during natural blood clotting. In composition, it is the same as plasma, but completely devoid of the protein fibrinogen (since it was consumed in thrombus formation).
Formed elements
The cellular portion of blood is represented by three main types of formed elements. Normally, 1 liter of blood contains the following number of cells:
- Erythrocytes (red blood cells) — $5 \cdot 10^{12}$/L.
- Leukocytes (white blood cells) — $6 \cdot 10^9$/L. They are 1000 times fewer than erythrocytes.
- Platelets (thrombocytes) — $2.5 \cdot 10^{11}$/L. They are 20 times fewer than red blood cells.
Almost all formed elements (except T-lymphocytes) are formed in the red bone marrow. In a healthy organism, a steady state is maintained: the number of destroyed cells equals the number of new ones entering the bloodstream. Circulation duration varies: erythrocytes live about 120 days, while leukocytes remain in the bloodstream for only 4–12 hours before migrating into tissues (although their total lifespan can reach several years).
Histological Examination
The main method for studying blood cells under a light microscope is the preparation of a blood smear.
A drop of blood is placed on a glass slide and spread into a ultrathin layer using the polished edge of another slide. Then the smear is fixed (usually with methanol) and stained. The gold standard is Romanowsky staining. It uses a mixture of two dyes: a basic one (Azure II) and an acidic one (Eosin), which allows differentiation of various cell structures.
Briefly: answers to topical questions
What functions do blood plasma albumins and globulins perform?
Blood plasma albumins and globulins perform the following functions:
- Albumins — maintain oncotic (colloidal-osmotic) blood plasma pressure and blood suspension properties, and serve as an amino acid reserve. They transport fatty acids, calcium, bilirubin, drugs, steroid hormones, and heavy metals.
- Globulins — perform a protective function: gamma globulins are antibodies. Globulins also participate in the transport of metals (haptoglobin, transferrin, ceruloplasmin); C-reactive protein is a marker of pathology.
What are the main groups into which peripheral blood leukocytes are classified?
Peripheral blood leukocytes are classified into two large groups depending on the presence of specific granulation (granules) in the cytoplasm.
- Granulocytes (granular leukocytes) — include neutrophils, eosinophils, and basophils.
- Agranulocytes (agranular leukocytes) — include lymphocytes and monocytes. Unlike granulocytes, their nucleus is not segmented, and specific granulation is practically absent in the cytoplasm.
Class and exam questions
How does blood plasma differ from serum?
Serum is plasma from which fibrinogen has been removed during the natural blood clotting process.
From which embryonic germ layer does blood develop?
Blood, like all other internal environment tissues, develops from mesenchyme.
Which blood smear staining is considered standard?
The standard histological technique is Romanowsky staining using Azure II and eosin dyes.
What else to study on this topic
- Morphology and functions of erythrocytes
- Classification of leukocytes (leukogram)
- Structure and functions of platelets
- Hematopoiesis in the red bone marrow
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