Composition of Blood

COMPOSITION OF BLOOD

Blood has two major components:

1. Plasma (liquid portion)
2. Formed elements (RBCs, WBCs, and platelets)

Table: Major Components of Blood

1. PLASMA

Plasma is the yellowish fluid that makes up about 55% of total blood volume. It is essential for transporting nutrients, hormones, and waste products throughout the body.

• Plasma contains 90–92% water.
• When fibrinogen is removed from plasma, the remaining fluid is called serum.
  Serum = Plasma – Fibrinogen

Contents of Plasma

Amino Acids

These are building blocks of proteins, required for growth and repair.

Nitrogenous Waste

Examples: urea, uric acid, creatinine
These are waste products of protein metabolism, carried by blood to the kidneys for excretion.

Nutrients

Glucose, amino acids, fatty acids—used for energy production, growth, and repair.

Hormones

Chemicals released by glands that travel through blood to regulate body functions.

Plasma Proteins

These contribute to osmotic pressure and transport functions.

Gases

Oxygen, carbon dioxide, and nitrogen are transported in plasma (often bound to hemoglobin in RBCs).

2. FORMED ELEMENTS

Formed elements include:

a)     Red Blood Cells (RBCs / Erythrocytes)

b)     White Blood Cells (WBCs / Leukocytes)

c)      Platelets (Thrombocytes)

Red Blood Cells (RBCs / Erythrocytes)

RBCs are biconcave, disc-shaped, non-nucleated cells about 7.5–8.7 μm in diameter. Their red color comes from hemoglobin.

Characteristics

• RBC count in men: 4.35–5.65 million/mcL
• RBC count in women: 3.92–5.13 million/mcL
• Lifespan: about 120 days
• Destroyed in spleen → called the “Graveyard of RBCs”

Functions of RBCs

1. Transport of Oxygen

• Hemoglobin combines with oxygen to form oxyhemoglobin.
  About 97% of oxygen is carried this way.

2. Transport of Carbon Dioxide

• Hemoglobin binds to CO₂ forming carbaminohemoglobin.
  About 30% of CO₂ is transported by RBCs.

DEVELOPMENT AND LIFE SPAN OF RBCs (ERYTHROPOIESIS)

RBCs are formed in bone marrow from stem cells called haemocytoblasts. The whole process takes about 7 days.

Key Steps in Erythropoiesis

Destruction of RBCs

Aged RBCs are broken down in spleen and liver.

• Iron is reused for new RBC formation.
• Heme → converted to bilirubin → excreted in bile.

White Blood Cells (WBCs / Leukocytes)

WBCs protect the body from infection and foreign substances.
Normal count: 7,000–8,000 per mm³.

• Formed in bone marrow
• Lifespan varies from hours to years depending on type
• Found in the “buffy coat” layer after centrifugation

Functions of WBCs

• Phagocytosis of pathogens
• Production of antibodies
• Inflammation and defense
• Pus formation at infected sites
• Removal of dead cells

Types of White Blood Cells

WBCs are divided into two major groups:

1. Granulocytes

These contain granules in the cytoplasm and have multilobed nuclei.

2. Agranulocytes

Monocytes also release interleukins, which:

• Stimulate globulin production
• Activate T-lymphocytes
• Raise body temperature via hypothalamus

Platelets (Thrombocytes)

Platelets are small, irregular cell fragments formed in bone marrow.

• No nucleus
• Size: 2–4 microns
• Lifespan: 8–11 days
• Normal count: 1,50,000–4,00,000 per microliter

Functions of Platelets