What is Blood – Definition, Properties & Composition

Blood Sample Examples Medical Biology Essay Questions with Functions, Structure & Viscosity for Students & Kids


Blood is a connective tissue in fluid form. It is considered as the fluid of life because it carries oxygen from lungs to all parts of the body and carbon dioxide from all parts of the body to the lungs. It is known as fluid of growth because it carries nutritive substances from the digestive system and hormones from endocrine gland to all the tissues. The blood can also be called the fluid of health because it protects the body against the diseases and gets rid of the waste products and unwanted substances by transporting them to the excretory organs like kidneys.


Following are the properties of the blood.

1. Colour: 

Blood is red in colour. Arterial blood is scarlet red because it contains more oxygen and venous blood is purple red.

2. Volume: The volume of blood in a normal adult is 5 liters.

3. Reaction and pH: 

Blood is slightly alkaline and its pH in normal conditions is 7.4.

4. Specific gravity:

The specific gravity of total blood : 1.052 to 1.061 
The specific gravity blood cells : 1.092 to 1.101
The specific gravity of plasma : 1.022 to 1.026

5. Viscosity: Blood is five times more viscous than water. It is mainly due to red blood cells and plasma proteins.


Blood contains the blood cells which are called formed elements and the liquid portion known as plasma.


Three types of cells are present in the blood.

1. Red blood cells or Erythrocytes
2. White blood cells or Leukocytes and
3. Platelets or Thrombocytes

Hematocrit Value

If blood is collected in a hematocrit tube along with suitable anticoagulant and centrifuged for 30 minute; a speed of 3000 RPM (revolutions per minute), the blood cells settle down at the bottom having a clear plasma at the top. The plasma forms 55% and the red blood cells form 45% of the total blood. The volume of red blood cells expressed in percentage is called the hematocrit value or packed cell volume (PCV).
In between the plasma and the red blood cells there is a thin layer of white buffy coat. This white buffy coat formed by the aggregation of white blood cells and platelets.

Download Note on Blood Properties & Components

1 comment:

  1. Thanks for the usefull information you share