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Blood Volume

For medical students2 min readUpdated 2026-10-10

Total blood volume normally ranges from 6% to 8% of total body mass, excluding adipose tissue from the calculations. This volume is divided into actively circulating and sequestered fractions, maintaining systemic internal environment stability.

Total Volume6–8% of body mass (excluding adipose tissue)
Circulating Volume3.5–4 L in active circulation
Blood Reservoir1.5–2 L reserve in organs and subcutaneous tissue
PlasmaAccounts for about 55% of the total volume

Total Volume and Calculation Principles

In normal physiology, blood quantity is always evaluated relative to overall body dimensions. In an adult, the total blood volume averages 6–8% of body mass. An important rule often tested on exams: when calculating the target volume, adipose tissue is excluded.

For clarity, consider basic math. Take an average male weighing 70 kg (with a normal build). His total blood volume will be approximately 5600 mL (or 5.6 liters). This volume is not uniform and is functionally divided into two critical parts that ensure adaptation to various conditions.

Blood Volume Fractions

Not all blood in the body is in a state of continuous and rapid movement. It is strictly divided into two functional fractions:

  1. Circulating blood volume (CBV). This is the fraction actively moving through the circulatory system at any given moment. CBV is located directly within the vascular bed and cardiac chambers. In an adult, this volume is 3.5–4 liters.
  2. Sequestered (reservoir) fraction. This is the physiological reserve temporarily excluded from active blood flow. The sequestered blood volume is 1.5–2 liters. It localizes in specific vascular reservoirs:
  3. vessels of the abdominal organs;
  4. pulmonary vascular network;
  5. subcutaneous tissue and certain other tissues.

Blood Composition: Volume Ratios

Blood is a complex fluid tissue consisting of a liquid medium and suspended cells. The internal environment of the body maintains a strict ratio of these components:

Maintaining this balance guarantees normal blood viscosity and adequate execution of its transport functions.

Mnemonic

The "55/45" rule: 55% is plasma (a higher percentage for fluidity, like liquid taking up more space), and 45% is formed elements.

Frequently asked questions

Which specific abdominal organs serve as the primary physiological blood reservoirs?

The primary blood reservoirs noted in sources are the spleen and liver; the skin and lungs are also listed as blood depots.

The sequestered blood fraction totals 1.5–2 L. When CBV drops, blood shifts from the reservoirs into the vascular bed.

What is the hematocrit (hematocrit value)?

The hematocrit is the ratio of the volume of formed blood elements to the plasma volume.

Normally, this value is 40–50% (36–48%), being slightly higher in males than in females. Formed elements account for about 45% of total blood volume, and plasma accounts for about 55%.

What methods are used to determine blood volume in physiology?

Blood volume is determined using indicator dilution methods, which include Evans blue dye and radioisotopes.

The principle of the method is as follows:

  • The circulating plasma volume is measured based on the concentration of the diluted indicator.
  • Using the hematocrit (Ht), the total CBV and globular volume are calculated.
How do CBV and hematocrit change during severe dehydration?

In severe dehydration, the circulating blood volume (CBV) decreases while the hematocrit increases.

  • A criterion for severe dehydration (e.g., in severe hyperemesis gravidarum) is a hematocrit level above 40% associated with dehydration and hemoconcentration.
Is adipose tissue included in the blood volume calculation?

No, adipose tissue mass is excluded when calculating the proper blood volume. The 6–8% calculation applies only to lean body mass.

What is CBV and where is this blood located?

CBV stands for circulating blood volume (about 3.5–4 L). This fraction is in constant motion inside the vascular bed and heart chambers.

Where in the body is blood sequestered?

The 1.5–2 liter blood reserve is located in the vessels of the abdominal organs, lungs, and subcutaneous tissue.

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