Essence of the hematocrit number
Blood is a complex fluid tissue consisting of a liquid part—plasma—and cells suspended in it (formed elements). Hematocrit (Ht) indicates the exact fraction of the total volume occupied by these cells.
Since the vast majority of cells in the vascular bed are red blood cells, the hematocrit value depends 99% on erythrocytes. The proportion of platelet mass and leukocytes in this volume is physiologically negligible.
Normal values of this parameter differ by gender:
- For women, the normal value is 0.37–0.47.
- For men, this parameter is slightly higher: 0.40–0.54.
Rheological properties and viscosity
Hematocrit is a key parameter determining the rheological, i.e., viscous properties of blood. Blood viscosity is its internal friction, the ability to resist flow.
Normally, whole blood viscosity is about 5 arbitrary units (the viscosity of distilled water is traditionally taken as the reference unit).
This parameter is not constant and depends on two main components:
- The number of formed elements (primarily, as we found out, erythrocytes).
- The presence of proteins in blood plasma.
Viscosity directly affects the speed at which blood moves through the vascular bed: the higher the viscosity, the greater the internal friction of the fluid and the harder it is for it to flow.
Hemodynamics and cardiac workload
Understanding viscosity is critical for studying hemodynamics. The movement of blood through vessels obeys the laws of hydrodynamics, in particular, it is closely related to Poiseuille's law. According to this rule, resistance to fluid flow is directly proportional to its viscosity.
In the body, this forms a clear pathophysiological chain. If a person's hematocrit increases (cellular mass increases), blood viscosity inevitably rises. Thick blood moves with difficulty through capillaries and arterioles, leading to a sharp increase in vascular resistance. As a result, the heart has to exert significantly more effort to push blood through the system, which repeatedly increases myocardial workload.
Hemodynamic cascade scheme: ↑ Hematocrit → ↑ Viscosity → ↑ Vascular resistance → ↑ Cardiac workload.
Briefly: answers to topical questions
How does hematocrit change during dehydration?
During dehydration (e.g., due to repeated vomiting, diarrhea, or other massive fluid losses), a relative increase in hematocrit occurs. This change is associated with the loss of the liquid part of the blood—plasma—which naturally leads to hemoconcentration and an increase in the proportion of formed elements in its total volume.
In what units of measurement, besides fractions, is hematocrit expressed in a complete blood count?
In addition to fractions, hematocrit (Ht) is expressed as a percentage (%). Sources provide examples: a target hematocrit of 35% under moderate hemodilution, as well as a table of laboratory Ht values with % units.
The "HVR" rule: Hematocrit determines Viscosity, and viscosity determines Resistance.
Class and exam questions
Which blood elements make the main contribution to the hematocrit value?
Hematocrit is 99% determined by erythrocytes, as they constitute the absolute majority among all formed elements.
How does blood viscosity relate to water viscosity?
Blood viscosity is about 5 arbitrary units, provided that water viscosity is taken as 1.
How does an increase in hematocrit affect heart function?
An increase in hematocrit raises blood viscosity. According to Poiseuille's law, this leads to an increase in vascular resistance, requiring the heart to exert more effort to pump blood (myocardial workload increases).
What else to study on this topic
- Methods for laboratory determination of hematocrit
- The role of blood plasma proteins in maintaining viscosity
- Mathematical expression of Poiseuille's law in hemodynamics
- Effect of shear rate on blood rheological properties
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