Composition of Blood 

Blood is a vital connective tissue that circulates throughout the body, delivering oxygen and nutrients to cells and removing waste products. It plays a key role in transport, regulation, and protection within the human body. Understanding the composition of blood helps explain how it supports vital functions like respiration, immunity, and temperature regulation.

1.0What Is Blood?

Blood is a fluid connective tissue that circulates continuously through the heart and blood vessels. It consists of two major components:

1. Plasma – the liquid part of blood.
2. Formed elements – the cellular components suspended in plasma.

An average adult has about 5–6 liters of blood, constituting roughly 8% of total body weight.

2.0Average Composition of Blood Percentages

3.0Components of Blood

Plasma

Plasma is the straw-colored fluid that makes up about 55% of total blood volume. It serves as the transport medium for nutrients, hormones, gases, and metabolic waste.

Composition of Plasma

Major Plasma Proteins

• Albumin: Maintains osmotic pressure and regulates fluid balance.
• Globulins: Function in immunity (antibodies) and transport.
• Fibrinogen: Essential for blood clotting.

Formed Elements

Formed elements constitute about 45% of the blood and include red blood cells (RBCs), white blood cells (WBCs), and platelets.

1. Red Blood Cells (Erythrocytes)

• Shape: Biconcave, circular, and non-nucleated (in mammals).
• Function: Transport oxygen from lungs to tissues and carbon dioxide from tissues to lungs.
• Count: About 5–5.5 million per µL of blood.
• Pigment: Contain hemoglobin (Hb), which binds oxygen.
• Lifespan: Around 120 days; destroyed in the spleen (graveyard of RBCs).
• Formation: Produced in the bone marrow (erythropoiesis).
• Significance: RBCs determine the oxygen-carrying capacity of blood.

2. White Blood Cells (Leukocytes)

• Function: Defend the body against infections and foreign substances.
• Count: About 6,000–8,000 per µL of blood.
• Types: Divided into granulocytes and agranulocytes based on the presence or absence of cytoplasmic granules.

a. Granulocytes

b. Agranulocytes

c. Platelets (Thrombocytes)

• Shape: Small, irregular cell fragments without nuclei.
• Count: About 1.5–3.5 lakh per µL of blood.
• Origin: Produced in bone marrow from megakaryocytes.
• Function: Play a critical role in blood clotting by releasing clotting factors.
• Lifespan: Around 7–10 days.

Also Read: Functions of Blood

4.0Functions of Blood

• Transport Function: Blood transports oxygen from lungs to tissues and CO₂ from tissues to lungs and nutrients, hormones, and waste materials throughout the body
• Regulation: Maintains body temperature, pH, and osmotic pressure
• Protection: WBCs defend against infections. Platelets and clotting factors prevent excessive bleeding
• Homeostasis: Keeps internal conditions stable despite external changes