Main groups and indicators
Blood acts as a key integrating system of the body, where any deviations are reflected in hematological parameters. In pathology, there are three main groups of disorders: normovolemia (normal volume), hypovolemia (decreased volume), and hypervolemia (increased volume). Each of these variants is subdivided according to the hematocrit level (Ht) into forms with a normal cell count (normocytic), decreased cell count (oligocytic), or increased cell count (polycythemic).
Normovolemia and its variants
This condition is characterized by a normal total blood volume with altered hematocrit:
- Oligocytic normovolemia: total volume is normal, but blood cells are decreased. Causes: massive erythrocyte hemolysis, suppressed hematopoiesis, or the post-acute hemorrhage state when volume is replenished by tissue fluid. Manifests as anemia and reduced viscosity.
- Polycythemic normovolemia: blood volume is preserved, but the number of cells increases. Causes: cellular mass infusion, chronic hypoxia, or polycythemia vera (Vaquez disease), leading to increased blood viscosity.
Hypovolemic states
A decrease in total blood volume develops in hemorrhage, shock states, or vasodilatory collapse due to blood pooling in capacitance vessels.
- Normocytic hypovolemia: volume drops while hematocrit remains normal (e.g., in acute hemorrhage).
- Oligocytic hypovolemia: volume and cell count are decreased, hematocrit drops (the post-hemorrhage period against the background of plasma replenishment).
- Polycythemic hypovolemia: the decrease in blood volume occurs mainly due to plasma loss ( hemoconcentration), so hematocrit exceeds the upper normal limit.
Hypervolemia and its causes
An increase in total blood volume is accompanied by changes in plasma and cellular composition:
- Normocytic hypervolemia: an equivalent increase in the volume of blood cells and plasma (during transfusions of large blood volumes or blood release from blood depots under exertion).
- Oligocytic hypervolemia (hydremia, hemodilution): volume increases due to the fluid part. Caused by excessive drinking, administration of plasma substitutes, impaired renal water excretion, and excess ADH.
- Polycythemic hypervolemia: volume increases due to an excess of cells (secondary erythrocytosis in hypoxia or polycythemia vera).
Briefly: answers to topical questions
What compensatory mechanisms are triggered in normocytic hypovolemia (acute hemorrhage)?
In response to normocytic hypovolemia during acute hemorrhage, a complex of vascular, fluid, and hematopoietic compensatory reactions is activated.
- Centralization of circulation — an adaptive response implemented through marked peripheral vasoconstriction to maintain blood pressure and blood flow in vital organs.
- Hemodilution — a compensatory influx of tissue fluid into the vascular bed, causing blood thinning and a decrease in hematocrit.
- Adaptation of venous vessels — with a deficit of up to 10%, venous vessels adapt to the reduced volume.
- Stimulation of hematopoiesis — mobilization of cells from depots in the first hours followed by reticulocytosis as a sign of bone marrow activation.
- Activation of neurohumoral systems — enhanced synthesis of antidiuretic hormone, stimulation of thirst, and activation of the renin-angiotensin-aldosterone system to retain water and maintain volume.
How does microcirculation change in polycythemic hypovolemia (hemoconcentration)?
In polycythemic hypovolemia, total blood volume is reduced mainly due to a decrease in plasma volume; hematocrit is above normal, and blood becomes concentrated. For microcirculation, this means a deterioration in blood rheological properties and a decrease in its fluidity.
- Aggregates of blood cells form; adhesion, aggregation, and agglutination of blood cells are enhanced.
- Sludge phenomenon and stasis develop in the microvasculature.
- Hemoconcentration exacerbates microcirculatory disorders and promotes thrombosis.
- With prolonged stasis, dystrophic tissue changes and necrosis of individual areas are possible; local foci of ischemia may form.
Mnemonic formula: three volume levels (normal, hypo, hyper) multiplied by three cell states (oligo-, normo-, polycythemia), forming a matrix of nine typical variants.
Class and exam questions
What is hematocrit and what is its normal range?
Hematocrit (Ht) is the fraction of formed elements in the total blood volume. Normally, this indicator ranges from 36 to 48%.
What is the difference between oligocytic and polycythemic normovolemia?
In both forms, the total blood volume remains normal. However, in the oligocytic form, the number of cells (erythrocytes) is decreased (e.g., after hemolysis), while in the polycythemic form, it is increased (e.g., in chronic hypoxia).
What happens to the blood in oligocytic hypervolemia?
Hydremia (hemodilution) develops, in which the total blood volume increases due to an excess of the fluid part, and the hematocrit drops below normal.
What else to study on this topic
- Mechanisms of compensation for acute hemorrhage
- Pathogenesis of shock and blood pooling
- The role of antidiuretic hormone in the regulation of fluid compartments
- Primary true polycythemia (Vaquez disease)
- The effect of blood viscosity on hemodynamics and microcirculation
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