Introduction

In very simple animals (like sponges), water itself carries materials in and out of the body. But in complex organisms, diffusion is too slow because body size is large and organs are far apart. Complex organisms use special fluids within their bodies to transport essential substances.

  • Blood: Main transport fluid in humans (most important)
  • Lymph: A supporting fluid that helps in transport and immunity

Key Point: Blood = transport + protection + regulation (Not just oxygen!)

What is Blood?

Blood is a fluid connective tissue made of:

  1. Plasma (fluid matrix)
  2. Formed elements (cells/cell fragments)

Typical composition:

  • Plasma ≈ 55%
  • Formed elements ≈ 45%

55% plasma + 45% cells

Plasma

Plasma is a straw-coloured, viscous fluid. It mainly helps in transporting dissolved substances.

Composition of Plasma

  • Water: 90–92% → acts as solvent and transport medium
  • Proteins: 6–8% → most important functional part
  • Also contains minerals/ions (Na⁺, Ca²⁺, Mg²⁺, HCO₃⁻, Cl⁻), nutrients (glucose, amino acids, lipids), hormones, wastes, etc.

Major Plasma Proteins

  1. Fibrinogen → blood clotting
  2. Globulins → defense/immunity (antibodies are globulins)
  3. Albumins → maintain osmotic balance (water balance in blood)

Key Point:

  • Fibrinogen = Clotting
  • Globulin = Immunity
  • Albumin = Osmotic pressure

Plasma vs Serum

  • Plasma = with clotting factors
  • Serum = plasma WITHOUT clotting factors

Serum = Plasma − Clotting factors

Formed Elements

Formed elements include:

  1. Erythrocytes (RBCs)
  2. Leukocytes (WBCs)
  3. Platelets (Thrombocytes)

They make up around 45% of blood.

1. Erythrocytes (RBCs)

Main job: Carry oxygen (via haemoglobin) and also help in CO₂ transport.

  • Most abundant cells: 5–5.5 million/mm³
  • Formed in adults: Red bone marrow
  • Shape: Biconcave → increases surface area and flexibility
  • Nucleus: Absent in most mammals → more space for haemoglobin
  • Haemoglobin: 12–16 g per 100 mL blood (oxygen carrier)
  • Life span: 120 days
  • Destroyed in: Spleen (called graveyard of RBCs)

Key Points to memorize:

  • RBC count = 5 million/mm³
  • Life = 120 days
  • Born = Red bone marrow
  • Dead = Spleen

2.Leukocytes (WBCs)

Main job: Body defense (immunity).

  • Colourless (no haemoglobin)
  • Nucleated
  • Count: 6000–8000/mm³
  • Life span: Generally short (varies with type)

Classification of WBCs

  1. Granulocytes (granules present)
  • Neutrophils
  • Eosinophils
  • Basophils
  1. Agranulocytes (no granules)
  • Lymphocytes
  • Monocytes

Functions (Must-know)

  • Neutrophils (60–65%) → most abundant, strong phagocytes (first line defense)
  • Eosinophils (2–3%) → fight parasites, linked to allergy
  • Basophils (0.5–1%) → least abundant; release histamine, serotonin, heparin → inflammation/allergic responses
  • Monocytes (6–8%) → large phagocytes; become macrophages in tissues
  • Lymphocytes (20–25%) → B cells + T cells → immune response (antibodies + cell-mediated immunity)

Key Point:

  • Neutrophil = most
  • Basophil = least

3. Platelets (Thrombocytes)

Main job: Help in blood clotting (coagulation).

  • Not full cells; cell fragments
  • Formed from megakaryocytes in bone marrow
  • Count: 1.5–3.5 lakh/mm³
  • Release substances that initiate and support clot formation

Low platelet count → clotting disorders and excess bleeding.

Blood: Plasma and Formed Elements

  • Blood = Plasma (55%) + Formed elements (45%)
  • Plasma proteins = A-G-F
  • Albumin → osmotic balance
  • Globulin → immunity
  • Fibrinogen → clotting
  • Serum = Plasma − clotting factors
  • RBC: 5 million/mm³, 120 days, bone marrow → spleen
  • WBC: 6000–8000/mm³
  • Neutrophil most (60–65%)
  • Basophil least (0.5–1%)
  • Platelets: 1.5–3.5 lakh/mm³, from megakaryocytes, clotting

1-line exam punch: Serum has NO clotting factors.

💡 Questions and Answers

Q1: What percentage of blood is plasma?

Answer: Plasma is about 55% of blood. Key point: Plasma = liquid part.

Q2: What is the difference between plasma and serum?

Answer: Plasma has clotting factors, but serum is plasma without clotting factors. Key point: Serum = Plasma − clotting factors.

Q3: Which WBC is most common and which is least common?

Answer: Neutrophils are the most common (60–65%) and basophils are the least (0.5–1%). Key point: Neutrophil most, Basophil least.

Q4: Where are RBCs formed and where are they destroyed?

Answer: RBCs are made in red bone marrow and mostly destroyed in the spleen (RBC graveyard). Key point: Bone marrow → Spleen.

Q5: Which plasma proteins maintain osmotic balance and help in clotting?

Answer: Albumin maintains osmotic balance, and fibrinogen helps in clotting. Key point: Albumin = water balance, Fibrinogen = clotting.