Blood Composition — Plasma & Formed Elements

NCERT grounding

NCERT Class XI, Chapter 15 (§15.1 Blood, including §15.1.1 Plasma and §15.1.2 Formed Elements) defines blood as “a special connective tissue consisting of a fluid matrix, plasma, and formed elements.” The chapter fixes the proportions, names the three plasma proteins, lists the cellular components and assigns each WBC its functional niche. The NIOS Biology lesson on Circulation of Body Fluids reinforces the same composition with additional emphasis on transport functions — nutrients, gases, hormones, wastes and heat distribution. This subtopic page treats §15.1.1 and §15.1.2 in greater depth than the chapter summary does.

Plasma, formed elements and the 55/45 split

A centrifuged tube of anticoagulated whole blood resolves into two visible layers. The upper, straw-yellow supernatant is plasma, which makes up roughly 55 per cent of the total volume. The lower, densely packed red layer is the formed elements — erythrocytes, leucocytes and platelets — accounting for about 45 per cent. Between them sits a thin pale ring, the buffy coat, where the white cells and platelets concentrate. The volume fraction occupied by the packed cells alone is the haematocrit or packed cell volume (PCV); in a healthy adult it sits between 40 and 45 per cent and closely tracks the formed-elements share.

Blood qualifies as a connective tissue on three counts: it is mesodermal in origin, its cells are embedded in a non-living extracellular matrix (here a fluid rather than a fibrous one) and it links distant tissues by transport. Total blood volume in an average adult is approximately 5 litres, which equals roughly 7–8 per cent of body weight.

Plasma — the fluid matrix

Plasma is a straw-coloured, slightly viscous fluid. About 90–92 per cent of it is water, and the remaining 8–10 per cent is solute. Of that solute fraction, dissolved proteins account for 6–8 per cent of the plasma by mass — the rest is electrolytes, organic nutrients, dissolved gases, hormones and metabolic wastes in transit. The three named proteins of NCERT recall are fibrinogen, the globulins and the albumins, and each maps to a distinct physiological job.

Beyond proteins, plasma also dissolves small amounts of mineral ions — Na⁺, Ca²⁺, Mg²⁺, HCO₃⁻, Cl⁻ — together with glucose, amino acids and lipids in transit, dissolved O₂ and CO₂, and the full battery of circulating hormones. The clotting factors themselves are present, but in an inactive form; that catalogue of zymogens is what distinguishes plasma from serum. Serum is what you get when blood is allowed to clot and the fibrin mesh and trapped cells are removed: it is plasma minus the clotting factors. Serum still carries all the globulins (so antibody-based serological tests work on serum), the albumin, the electrolytes, glucose and hormones.

Formed elements — three lineages, one bone marrow origin

All three cellular components arise from haematopoietic stem cells in the red bone marrow of the adult. They differ sharply in number, nuclear status, lifespan and function. The NCERT-anchored counts and proportions are below.

Red blood cells — built for haemoglobin

RBCs are the most numerous cells in the body. An adult man carries 5.0–5.5 million per mm³; an adult woman slightly less. They are biconcave discs with no nucleus and no functioning organelles in mature mammalian form — the loss of the nucleus during the late erythroblast stage frees nearly all the cell’s internal volume for haemoglobin, and the biconcave shape maximises surface area for gas diffusion while letting the cell flex through capillaries narrower than its own resting diameter. Each cell is essentially a flexible bag of haemoglobin, the red iron-protein complex that gives blood its colour. A healthy individual has 12–16 g of haemoglobin per 100 mL of blood.

The functional consequence is direct: more haemoglobin per cell means more O₂ carried per cell. After roughly 120 days of cyclic squeezing through capillary beds, the cell membrane becomes rigid and is filtered out by splenic macrophages, which break the cell apart and recycle the iron back to the bone marrow via transferrin. Because senescent red cells systematically die there, the spleen is termed the “graveyard of erythrocytes” — a phrase NCERT introduces and NEET 2022 (Q.152) tested verbatim.

White blood cells — five types, two families

Leucocytes are nucleated and lack haemoglobin, which is why they appear colourless. They are far less numerous than RBCs (6,000–8,000 per mm³) and most live only a few days. The classical NCERT split is between cells with cytoplasmic granules visible on staining (granulocytes) and those without (agranulocytes).

The functional partition is worth memorising as a mnemonic block. Neutrophils and monocytes are the phagocytes — they engulf and destroy invading microbes. Eosinophils rise during parasitic worm infections and allergic episodes; they release histaminase and tissue-destructive enzymes. Basophils are the granule-bearing inflammatory cells, releasing histamine, serotonin and heparin — the same trio of mediators NEET 2023 (Q.189) used as a recognition cue. Lymphocytes, split into B and T forms, are the architects of acquired immunity: B-cells produce antibodies, T-cells coordinate cell-mediated responses. A neat NEET trap: monocytes — not basophils — have the characteristic kidney-shaped nucleus.

Platelets — fragments, not whole cells

Platelets (thrombocytes) are not complete cells. They are small, anucleate cytoplasmic fragments pinched off from giant precursor cells called megakaryocytes, themselves resident in the bone marrow. Normal blood contains 1,50,000–3,50,000 platelets per mm³. On contact with a damaged vessel wall, platelets release a battery of substances — most of which feed into the coagulation cascade. A drop in platelet count (thrombocytopenia) impairs clotting and leads to abnormal bleeding.

Worked examples

Common confusion & NEET traps