Total volume and calculation principles
In normal physiology, blood volume is always evaluated relative to overall body dimensions. In an adult, the total blood volume averages 6–8% of body weight. An important rule often forgotten in exams: when calculating the target volume, adipose tissue is not included.
For clarity, we can use simple math. Let's take an average male weighing 70 kg (assuming normal body build). His total blood volume will be approximately 5600 mL (or 5.6 liters). This volume is not monolithic and is functionally divided into two crucial parts that ensure the body's adaptation to various conditions.
Blood volume fractions
Not all blood in the body is in the same state of continuous and rapid movement. It is strictly distributed into two functional fractions:
- Blood volume (BV). This is the portion that is actively circulating through the cardiovascular system at any given time. BV is located directly within the vascular bed and heart chambers. In an adult, this volume is 3.5–4 liters.
- Pooled fraction. This is a physiological reserve temporarily excluded from active blood flow. The volume of pooled blood is 1.5–2 liters. It is localized in specific vascular reservoirs:
- vessels of the abdominal cavity;
- pulmonary vascular network;
- 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:
- Plasma (the liquid part) occupies the larger share of the space — approximately 55% of the total blood volume.
- Cellular elements (or formed elements, which include erythrocytes, leukocytes, and platelets) make up the remaining 45% of the total volume.
It is the maintenance of this balance that guarantees normal blood viscosity and adequate performance of its transport functions.
Briefly: answers to topical questions
Which specific abdominal organs serve as the primary physiological blood depots?
The primary blood depots indicated in sources are the spleen and liver; the skin and lungs are also listed among blood depots.
The pooled blood fraction generally totals 1.5–2 L. When CBV decreases, blood leaves the depots and enters the vascular bed.
What is hematocrit (hematocrit value)?
Hematocrit (hematocrit value) is the ratio of the volume of blood formed elements to the volume of plasma.
Normally, this indicator is 40–50% (36–48%), and it is slightly higher in men than in women. Formed elements account for about 45% of the total blood volume, and plasma accounts for about 55%.
What methods are used to determine blood volume in physiology?
Blood volume is determined using injected indicators, which include Evans blue dye and radioisotopes.
The essence of the method is as follows:
- The volume of circulating plasma is measured by the concentration of the diluted indicator.
- Taking into account the hematocrit (Ht), the total blood volume and globular volume are calculated.
How do blood volume and hematocrit change in severe dehydration?
In severe dehydration, the circulating blood volume (CBV) decreases, while the hematocrit increases.
The criterion for severe dehydration (e.g., in severe hyperemesis gravidarum) is an increase in the hematocrit level above 40% against the background of dehydration and hemoconcentration.
The "55/45" rule: 55% is plasma (a figure that needs more space, like fluid for flow), and 45% is formed elements.
Class and exam questions
Is adipose tissue included in the blood volume calculation?
No, adipose tissue mass is not taken into account when calculating the proper blood volume. The 6–8% calculation is based only on active or "lean" body mass.
What is CBV and where is this blood located?
CBV is the circulating blood volume (about 3.5–4 L). This fraction is in constant motion within the vascular bed and heart chambers.
Where is blood pooled in the body?
The reserve 1.5–2 liters of blood are located in the blood vessels of the abdominal cavity, lungs, and subcutaneous tissue.
What else to study on this topic
- Mechanisms of blood mobilization from depots
- Hematocrit and assessment of cellular elements
- Methods for determining CBV in clinical practice
- Changes in the ratio of fractions during blood loss
Master the entire topic in the "Physiology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Blood Volume" and another 4,128 pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 114 video lessons on the subject
Other topics in the "Blood" section
Free video lessons: PhysiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.