How does hematocrit change during dehydration?

During dehydration (e.g., due to persistent vomiting, diarrhea, or other massive fluid losses), a relative increase in hematocrit occurs. This change is caused by the loss of the liquid component of blood (plasma), which naturally leads to hemoconcentration and an increased proportion of formed elements in the total volume.

In what units, besides fractions, is hematocrit expressed in a complete blood count?

Besides fractions, hematocrit (Ht) is expressed as a percentage (%). Clinical examples include a target hematocrit of 35% during moderate hemodilution protocols, as well as standard laboratory tables listing Ht in %.

Which blood elements make the primary contribution to the hematocrit value?

99% of the hematocrit is determined by erythrocytes, as they constitute the absolute majority among all formed elements.

How does blood viscosity compare to water?

Blood viscosity is about 5 arbitrary units, assuming the viscosity of water is 1.

How does an increased hematocrit affect cardiac function?

An increased hematocrit raises blood viscosity. According to Poiseuille's law, this leads to increased vascular resistance, requiring the heart to generate more effort to pump blood (increased myocardial workload).

Home › Physiology › Hematocrit and Blood Viscosity

Hematocrit and Blood Viscosity

Haematocritus

For medical students2 min readUpdated 2026-10-10

Hematocrit is a physiological parameter representing the fraction of total blood volume composed of formed elements (cells). Together with plasma protein concentration, it is the primary determinant of blood viscosity and its rheological properties.

Normal range in femalesRanges from 0.37 to 0.47

Normal range in malesRanges from 0.40 to 0.54

Primary contributor99% of the hematocrit value is determined by the erythrocyte count

Blood viscosityApproximately 5 arbitrary units (compared to water as 1)

Concept of the Hematocrit

Blood is a complex fluid tissue consisting of a liquid component (plasma) and suspended cells (formed elements). Hematocrit (Ht) indicates the exact fraction of the total volume occupied by these cells.

Because the vast majority of cells in the vascular bed are red blood cells, the hematocrit value depends 99% on erythrocytes. The contribution of platelets and leukocytes to this volume is physiologically negligible.

Normal values show gender differences:

For females, the normal range is 0.37–0.47.
For males, the value is slightly higher: 0.40–0.54.

Rheological Properties and Viscosity

Hematocrit is a key parameter determining the rheological, or viscous, properties of blood. Blood viscosity is its internal friction, representing the ability to resist flow.

Normally, the viscosity of whole blood is about 5 arbitrary units (with the viscosity of distilled water traditionally taken as a reference unit of 1).

This parameter is not constant and depends on two main components:

The number of formed elements (primarily erythrocytes, as established).
The presence of proteins in blood plasma.

Viscosity directly affects the velocity of blood flow through the vascular bed: the higher the viscosity, the greater the internal fluid friction and the more difficult it is for blood to flow.

Hemodynamics and Cardiac Workload

Understanding viscosity is critical for studying hemodynamics. Blood flow through vessels obeys hydrodynamic laws, being closely related to Poiseuille's law. According to this principle, 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. Consequently, the heart must exert significantly more effort to push blood through the system, substantially increasing myocardial workload.

Hemodynamic cascade pathway: ↑ Hematocrit → ↑ Viscosity → ↑ Vascular resistance → ↑ Cardiac workload.

Mnemonic

The "HVS" rule: Hematocrit determines Viscosity, and viscosity determines Resistance (Poiseuille's law).

Frequently asked questions

How does hematocrit change during dehydration?: During dehydration (e.g., due to persistent vomiting, diarrhea, or other massive fluid losses), a relative increase in hematocrit occurs. This change is caused by the loss of the liquid component of blood (plasma), which naturally leads to hemoconcentration and an increased proportion of formed elements in the total volume.
In what units, besides fractions, is hematocrit expressed in a complete blood count?: Besides fractions, hematocrit (Ht) is expressed as a percentage (%). Clinical examples include a target hematocrit of 35% during moderate hemodilution protocols, as well as standard laboratory tables listing Ht in %.
Which blood elements make the primary contribution to the hematocrit value?: 99% of the hematocrit is determined by erythrocytes, as they constitute the absolute majority among all formed elements.
How does blood viscosity compare to water?: Blood viscosity is about 5 arbitrary units, assuming the viscosity of water is 1.
How does an increased hematocrit affect cardiac function?: An increased hematocrit raises blood viscosity. According to Poiseuille's law, this leads to increased vascular resistance, requiring the heart to generate more effort to pump blood (increased myocardial workload).

Go deeper

Laboratory methods for determining hematocrit
The role of plasma proteins in maintaining blood viscosity
Mathematical expression of Poiseuille's law in hemodynamics
Effect of shear rate on blood rheological properties