Main groups of formed blood elements
Formed blood elements are divided into three main groups, differing in count and structure:
- Erythrocytes (red blood cells) — their concentration is about $5 \cdot 10^{12}$ per 1 L.
- Leukocytes (white blood cells) — present in a smaller volume, approximately $6 \cdot 10^9$ per 1 L.
- Platelets (thrombocytes) — numbering about $2.5 \cdot 10^{11}$ per 1 L.
Their quantitative ratio is strictly constant: there are approximately 1,000 times fewer leukocytes than erythrocytes in the body, and 20 times fewer platelets than erythrocytes.
Classification and structure of leukocytes
Unlike anuclear erythrocytes, leukocytes are spherical, nucleated, and larger than red blood cells.
Based on the presence of specific granulation in the cytoplasm, white blood cells are divided into two major categories:
- Granulocytes (granular): include neutrophils, eosinophils, and basophils.
- Agranulocytes (agranular): represented by monocytes and lymphocytes.
The percentage ratio of these cell types is called the leukocyte formula (differential white blood cell count). The largest group consists of neutrophils, among which segmented forms predominate.
Origin, lifespan, and dynamics
The formation of all formed blood elements occurs in the red bone marrow, with the exception of T lymphocytes. The system maintains a principle of dynamic equilibrium: the influx of new cells strictly equals their destruction.
The lifespan and time spent in the bloodstream differ significantly among various cells:
- Erythrocytes circulate for an average of 120 days.
- Leukocytes remain in the bloodstream for only 4 to 12 hours, retaining the ability to repeatedly migrate into tissues and return. Their overall lifespan ranges from several days to years.
There are 1,000 times fewer leukocytes than erythrocytes, but they are larger, nucleated, and divided into granular (granulocytes) and agranular (agranulocytes).
Class and exam questions
What is the process of blood cell formation called?
This process is called hemocytopoiesis (or hematopoiesis).
What is the average lifespan of erythrocytes?
On average, erythrocytes circulate in the blood for 120 days.
Where are formed blood elements produced?
All formed blood elements, except for T lymphocytes, are formed in the red bone marrow.
How do leukocytes differ structurally from erythrocytes?
Leukocytes are spherical, invariably contain a nucleus, and are larger than erythrocytes, which lack a nucleus.
What else to study on this topic
- Connection between hematopoiesis and immunogenesis via lymphocytes
- Detailed composition of the adult leukocyte differential count
- Ratio of band to segmented neutrophils
- Features of leukocyte circulation and migration into tissues
- Principle of blood steady-state
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