A Simple Guide to Blood Volume And Hypovolemia

A Simple Guide to Blood Volume And Hypovolemia

What is the normal blood volume? What are the methods used for determining blood volume? Describe the various causes of hypovolemia.
Answer:

Normal blood volume:

• The total amount of blood that is present in the circulatory system, blood reservoirs, tissues and organs constitute blood volume.

Value:

• 7% (6-8%) of total body weight.
• 2.8-3.1 liters per square.

Methods for determining blood volume:

1. Direct method:

• Employed by Welcker in 1854 in animals.
• In this method, first the animal is killed and its blood is collected.
• Next, the blood vessels and tissues are washed thoroughly with known amount of water/saline. This is added to the blood collected.
• The total volume is measured.
• The amount of water/saline used is deducted from it to obtain volume of blood.

2. Indirect method:

• It is used to measure the blood volume in human beings.
• It includes 2 steps.
• Determination of plasma volume.
  1. It is determined by 2 methods.
     1. Indicator/dye dilution technique.
        • 10 ml of blood is collected from the subject and is divided into 2 equal parts.
        • A known amount of dye is added to one part and this is used as control.
        • Now, a known amount of dye is injected intravenously.
        • After 10 minutes, blood sample is collected.
        • Similary another 4 samples are collected at the interval of 10 minutes.
        • These 5 samples are then centrifuged and plasma is separated.
        • From each plasma sample, concentration of dye is measured and average concentration of dye is found.
        • Subject’s urine is also collected and the amount of dye excreted is urine is measured.
        • Now, the plasma volume is determined by the formula.
        • \(\text { Volume }=\frac{\text { Amount of dye injected }- \text { Amount of dye excreted }}{\text { Average concentration of dye in plasma }}\)
     2. Radioisotope method:
        • Radioactive iodine is injected and after sometime blood sample is collected.
        • Radioactivity is determined by using appropriate counter and from it plasma volume is determined.
  2. Determination of blood volume-hematocrit value.
     • Blood is collected and mixed with anticoagulant.
     • It is then collected in hematocrit tube and centrifuged.
     • As a result, red blood cells gets collected at the bottom of the tube.
     • This gives packed cell volume (PCV).
     • If this is deducted from 100, the percentage of plasma is known.
     • Now, blood volume is calculated by blood volume = \(\frac{100 \times \text { Amount of plasma }}{100-\mathrm{PCV}}\)

Causes of hypovolemia:

• Hemorrhage – acute or chronic.
• Fluid loss.
• Hemolysis.
• Anaemia.
• Obesity blood volume is less compared to body weight.
• Hypothyroidism – blood volume decreses due to reduction in plasma volume and RBC count.