Definition of Blood. Blood Components and it's Functions

Subhajit Chanda


Definition (What is blood?):

Blood is a red colour viscous fluid, found in the cardiovascular system that carries oxygen, nutrition and carbon dioxide to cells throughout your body.

A fit normal human have arround 5 liters blood in their body.
Color range
Oxygen-rich blood is scarlet red
Oxygen-poor blood is dull red
pH must remain between 7.35-7.45
Slightly alkaline
Blood temperature is slightly higher than body temperature

Components Of Blood:

The main components of blood are classified below:

Blood is composed of several components that are classified on the above picture, each have their own functions:

Red Blood Cells (Erythrocytes):

1. RBCs are one of the primary cellular components of blood.
2. RBCs are biconcave, disc-shaped cells with a depressed centre, maximizing their surface area-to-volume ratio for efficient gas exchange.
3. RBCs contain haemoglobin, a complex protein composed of globin (protein) and heme (iron-containing pigment). Responsible for transporting oxygen from the lungs to body tissues and carrying carbon dioxide from tissues to the lungs for exhalation.

White Blood Cells (Leukocytes):

1. WBCs Play a crucial role in the immune system's defence against pathogens.
2. Include several types such as neutrophils, lymphocytes, monocytes, eosinophils, and basophils, each with specific functions in immunity.

Platelets (Thrombocytes):

1. Platelets are small, colourless cell fragments found in the bloodstream involved in blood clotting.
2. Form clots to prevent excessive bleeding when blood vessels are injured.


1. Plasma are a yellowish fluid component of blood, constituting about 55% of its volume. It contains water, electrolytes, proteins (albumin, globulins, fibrinogen), hormones, nutrients, and waste products.

Normal Level Of Blood Components in Human Body:

Blood Component

Normal Range

Red Blood Cells (RBCs)

Male: 4.5 - 5.9 million cells/μL

Female: 4.0 - 5.2 million cells/μL

White Blood Cells (WBCs)

4,500 - 11,000 cells/μL


150,000 - 400,000 platelets/μL


Constitutes about 55% of total blood volume

Functions of Blood:

    ★ Transport Function:

  1. RBC carries O2 from the lungs to the rest of the body.
  2. Nutrients, such as glucose, amino acids, and fatty acids are transported to cells for energy and other metabolic processes.
  3. In the human body hormones are secreted by some endocrine glands and those hormones are also transported by blood to the other cells, tissues and organs.
  4. Blood carries metabolic waste products, such as carbon dioxide and urea, away from cells to be eliminated through the lungs and kidneys, respectively.

    ★ Immunity Function:

    1. White blood cells (leukocytes) play a crucial role in the immune system by defending the body against pathogens, such as bacteria, viruses, and fungi. They multiply rapidly during infections.

        ★ Thermoregulation Function:

      1. Blood helps to maintain body temperature by re-distributing heat to the whole body.

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