Blood volume reflects the physiological state of the body and depends on the total volume of circulating fluid and the ratio of plasma to formed elements. Pathological disorders are classified into normovolemia, hypovolemia, and hypervolemia, taking into account the hematocrit value.
Total blood volume6–8% of body weight (approximately 4–5 L in adults)
Hematocrit (Ht)Fraction of blood volume occupied by formed elements, normal range 36–48%
Main categoriesNormovolemia, hypovolemia, and hypervolemia
Main Categories and Indicators
Blood acts as a key integrating system of the body, where any deviations are reflected in hematological parameters. Pathology distinguishes three main categories of disorders: normovolemia (normal volume), hypovolemia (decreased volume), and hypervolemia (increased volume). Each of these variants is further subdivided based on the hematocrit level (Ht) into forms with a normal cell count (normocytic), decreased cell count (oligocytic), or elevated cell count (polycythemic).
Normovolemia and Its Variants
This condition is characterized by a normal total blood volume with an altered hematocrit:
Oligocytic normovolemia: Total volume is normal, but formed elements are reduced. Causes: massive erythrocyte hemolysis, suppressed hematopoiesis, or the state following acute blood loss where the volume has been replenished by interstitial fluid. It manifests as anemia and decreased blood viscosity.
Polycythemic normovolemia: Blood volume is preserved, but the number of cells is increased. Causes: infusion of packed cell masses, chronic hypoxia, or polycythemia vera (Vaquez disease), which leads to increased blood viscosity.
Hypovolemic States
A decrease in total blood volume develops during hemorrhage, shock states, or vasodilatory collapse due to blood pooling in capacitance vessels.
Normocytic hypovolemia: Blood volume drops while the hematocrit remains within the normal range (e.g., in acute hemorrhage).
Oligocytic hypovolemia: Both blood volume and cell count are decreased, and the hematocrit drops (the period following blood loss during plasma replacement).
Polycythemic hypovolemia: The decrease in blood volume occurs mainly due to plasma loss (hemoconcentration), so the hematocrit exceeds the upper limit of normal.
Hypervolemia and Its Causes
An increase in total blood volume is accompanied by changes in plasma and cellular composition:
Normocytic hypervolemia: Equivalent increase in both formed elements and plasma volume (upon transfusion of large blood volumes or blood mobilization from vascular reservoirs during exercise).
Oligocytic hypervolemia (hydremia, hemodilution): Volume increases due to the fluid component. Caused by excessive fluid intake, administration of plasma volume expanders, impaired renal water excretion, or excess ADH.
Polycythemic hypervolemia: Volume increases due to an excess of cells (secondary erythrocytosis due to hypoxia or polycythemia vera).
Mnemonic
Memory formula: three volume levels (normo, hypo, hyper) multiplied by three cellular states (oligo-, normo-, polycythemia), forming a matrix of nine typical variants.
Frequently asked questions
What compensatory mechanisms are activated during 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 reaction mediated by pronounced peripheral vasoconstriction to maintain blood pressure and perfusion in vital organs.
Hemodilution — a compensatory shift of interstitial fluid into the vascular bed, causing blood thinning and a decrease in hematocrit.
Venous vessel adaptation — with a deficit of up to 10%, venous capacitance vessels adapt to the reduced volume.
Stimulation of erythropoiesis — mobilization of cells from reservoirs in the first few hours followed by reticulocytosis as a sign of bone marrow activation.
Activation of neurohumoral systems — increased 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 during polycythemic hypovolemia (hemoconcentration)?
In polycythemic hypovolemia, total blood volume is reduced mainly due to a decrease in plasma volume; the hematocrit is above normal, and the blood is concentrated. For microcirculation, this means a deterioration of blood rheological properties and a decrease in fluidity.
Aggregates of formed elements form; cell adhesion, aggregation, and agglutination are enhanced.
Sludge phenomenon and stasis develop in the microvasculature.
Hemoconcentration worsens microcirculatory disorders and promotes thrombosis.
Prolonged stasis can lead to tissue dystrophic changes and necrosis of individual areas; local foci of ischemia may form.
What is hematocrit and what is its normal range?
Hematocrit (Ht) is the fraction of the total blood volume composed of formed elements (primarily erythrocytes). Normally, this parameter ranges from 36 to 48%.
How do oligocytic and polycythemic normovolemia differ?
In both forms, the total blood volume remains normal. However, in oligocytic normovolemia, the erythrocyte count is decreased (e.g., after hemolysis), whereas in polycythemic normovolemia, 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 liquid component, and the hematocrit drops below normal.
Go deeper
Mechanisms of compensation for acute blood loss
Pathogenesis of shock and blood pooling
Role of antidiuretic hormone in water compartment regulation
Primary polycythemia vera (Vaquez disease)
Effect of blood viscosity on hemodynamics and microcirculation