Body Fluids and Circulation

Syllabus Unit: Human Physiology

Why it matters

Chapter overview

Blood and lymph form the body's transport network: blood carries gases, nutrients, wastes and hormones, driven by a four-chambered heart through a closed system of vessels in a strict rhythmic cycle. NEET tests blood composition, cardiac anatomy, the mechanical cardiac cycle, and how an ECG traces that cycle electrically — plus the common disorders arising when any part of this system fails.

Biology Syllabus Human Physiology 20%–22% weightage

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Plasma~55% of blood volume

ErythrocytesO2 transport via haemoglobin

LeucocytesImmune defence

PlateletsClotting

1Blood & Its Components

Blood is a connective tissue: a fluid plasma matrix carrying three distinct cell types, each with a dedicated job.

Learning Objectives

List blood's two main components and their approximate proportion by volume.
State the function of erythrocytes, leucocytes and platelets.
Name the five types of leucocytes (granulocytes and agranulocytes) and one function each.
Describe the ABO and Rh blood group systems and their basis in antigens/antibodies.

Key Terms

PlasmaThe straw-coloured fluid matrix of blood (~55% of volume), containing water, proteins (albumin, globulin, fibrinogen), salts and dissolved substances.

Erythrocyte (RBC)Biconcave, anucleate blood cell packed with haemoglobin; the primary carrier of O2, and some CO2.

Leucocyte (WBC)Nucleated blood cell responsible for immune defence; classified as granulocytes (neutrophil, eosinophil, basophil) or agranulocytes (lymphocyte, monocyte).

Platelet (Thrombocyte)Small anucleate cell fragment essential for blood clotting at a wound site.

Blood is a fluid connective tissue: roughly 55% **plasma** (the liquid matrix) and 45% formed elements (cells and cell fragments) suspended in it.

Plasma

Plasma is ~90-92% water and carries plasma proteins (albumin for osmotic pressure, globulins for immunity, fibrinogen for clotting), glucose, amino acids, hormones, and dissolved gases and wastes.

The three formed elements

Erythrocytes (RBCs) — biconcave, enucleate discs packed with haemoglobin; carry O2 and some CO2; produced in red bone marrow, destroyed in the spleen ('graveyard of RBCs').
Leucocytes (WBCs) — nucleated, fewer in number than RBCs, involved in immunity; granulocytes (neutrophils, eosinophils, basophils) have granular cytoplasm, agranulocytes (lymphocytes, monocytes) do not.
Platelets (thrombocytes) — small cell fragments from megakaryocytes; release clotting factors and plug small wounds.

Exam TipDifferential WBC count order by abundance (NEET favourite): Neutrophils > Lymphocytes > Monocytes > Eosinophils > Basophils — remember with 'Never Let Monkeys Eat Bananas'.

Blood groups

ABO system — based on antigens A and/or B on the RBC surface; a person's plasma carries antibodies against whichever antigen is absent from their own RBCs.
Rh system — Rh antigen present (Rh+) or absent (Rh-); Rh incompatibility between an Rh- mother and Rh+ foetus can cause erythroblastosis foetalis in a subsequent pregnancy.