Body Fluids

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Body Fluids — Quick Facts

Body fluids constitute approximately 60% of adult body weight. They are distributed between intracellular fluid (ICF — inside cells, ~40% of body weight) and extracellular fluid (ECF — outside cells, ~20% of body weight).

ECF is further divided into:

• Interstitial fluid (between cells, ~15%)
• Plasma (blood fluid, ~5%)
• Transcellular fluid (CSF, synovial, peritoneal, ~1–2%)

Blood Composition: Blood = Plasma (55%) + Blood cells (45%)

• Plasma: Water (90–92%), proteins (6–8%: albumin, globulins, fibrinogen), nutrients, hormones, electrolytes, waste products
• RBCs (Erythrocytes): 4.5–5.5 million/mm³; biconcave disc shape; no nucleus in mammals; contains haemoglobin (Hb)
• WBCs (Leucocytes): 6,000–10,000/mm³; include neutrophils (60–70%), lymphocytes (20–25%), monocytes (3–8%), eosinophils (1–4%), basophils (0.5–1%)
• Platelets (Thrombocytes): 1.5–4 lakh/mm³; not true cells; cell fragments from megakaryocytes

⚡ Exam tip: WBC count above 11,000/mm³ = leucocytosis (infection). Below 4,000/mm³ = leucopenia (AIDS, chemotherapy). Differential count helps diagnose specific infections (e.g., high eosinophils → parasitic infection).

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Body Fluids — NEET/JEE Study Guide

Haemoglobin (Hb):

• Each Hb molecule: 4 globin chains (2α + 2β in adults = HbA) + 4 heme groups
• Each heme group: Porphyrin ring + Ferrous iron (Fe²⁺) at centre
• O₂ binds to Fe²⁺ (not Fe³⁺ — methaemoglobin cannot carry O₂)
• Normal Hb: Adult male = 13.5–17.5 g/dL; Female = 12–16 g/dL
• HbCO (carboxyhaemoglobin): CO binds with 250× greater affinity than O₂ → carbon monoxide poisoning
• Foetal Hb (HbF): 2α + 2γ chains; has higher O₂ affinity (dissociation curve shifted left) — adaptive for extracting O₂ from maternal blood

Blood Groups (ABO and Rh):

ABO system (Karl Landsteiner, 1900):

Blood Group	Antigen on RBC	Antibodies in Plasma
A	A antigen	Anti-B
B	B antigen	Anti-A
AB	Both A and B	Neither
O	Neither	Both Anti-A and Anti-B

Rh factor: Rhesus antigen (D antigen is most immunogenic). Rh+ = has D antigen; Rh− = lacks D antigen.

• Rh− mother carrying Rh+ baby: During first delivery, baby’s RBCs enter maternal circulation → mother produces anti-Rh antibodies (sensitisation). Second Rh+ pregnancy → antibodies cross placenta → haemolysis of foetal RBCs → Erythroblastosis foetalis (hydrops fetalis).
• Prevention: RhoGAM (anti-D immunoglobulin) given to Rh− mother within 72 hours of first delivery.

Blood Clotting (Coagulation):

Extrinsic pathway (fast, initiated by tissue factor): Tissue factor (from damaged tissue) → Factor VII → Factor X → Prothrombinase → Prothrombin → Thrombin → Fibrinogen → Fibrin → Clot

Intrinsic pathway (slow, initiated by collagen exposure): Damaged endothelium → XII → XI → IX + VIII → Factor X (common pathway)

Vitamin K is essential for synthesis of clotting factors II (prothrombin), VII, IX, X.

⚡ NEET 2021 Qn: Which enzyme converts fibrinogen to fibrin? Answer: Thrombin (produced from prothrombin by prothrombinase/Complex Xa·Va).

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Body Fluids — Comprehensive Notes

Lymphatic System: Lymph = plasma + lymphocytes (no RBCs, fewer proteins). Formed from interstitial fluid that enters lymphatic capillaries. Returns to bloodstream via thoracic duct (empties into left subclavian vein) and right lymphatic duct. Key functions: Fat absorption (from lacteals in intestinal villi), immune surveillance (lymphocytes), protein return.

Heart and Circulation:

• Heart: 4 chambers (RA, RV, LA, LV); 2 pumps (right heart pumps to lungs, left heart pumps to systemic circulation)
• Cardiac muscle (myocardium): Auto-rhythmic (SA node = pacemaker at 70 bpm), intercalated discs with gap junctions
• Cardiac cycle: Systole (contraction) + Diastole (relaxation)
• Stroke volume: ~70 mL per beat; Cardiac output = SV × HR = ~5 L/min

Blood Pressure:

• Systolic (left ventricular contraction): Normal = 120 mmHg
• Diastolic (left ventricular relaxation): Normal = 80 mmHg
• Written as 120/80 mmHg
• Hypertension: Sustained BP > 140/90 mmHg; major risk factor for stroke, heart attack, kidney failure
• Hypotension: BP < 90/60 mmHg; can cause fainting (syncope)

ECG (Electrocardiogram):

Wave	Represents
P wave	Atrial depolarisation
QRS complex	Ventricular depolarisation
T wave	Ventricular repolarisation
PR interval	Time from atrial to ventricular depolarisation (normal: 0.12–0.2s)

Oxygen-haemoglobin Dissociation Curve: Sigmoidal (S-shaped) due to cooperative binding. Shifted RIGHT by: ↑ CO₂ (Bohr effect), ↑ H⁺ (↓ pH), ↑ temperature, ↑ 2,3-DPG. Shifted LEFT by: ↓ CO₂, ↑ pH, ↓ temperature, foetal Hb, CO poisoning.

Blood Disorders (NEET-Relevant):

Disorder	Cause	Key Feature
Anaemia	Iron deficiency, B12 deficiency, haemolysis, blood loss	Low Hb, pallor, fatigue
Sickle cell anaemia	Point mutation in β-globin gene (Glu→Val at position 6)	Crescent-shaped RBCs, pain crises
Thalassaemia	Mutations in α or β globin genes	Microcytic hypochromic anaemia
Haemophilia	X-linked recessive (Factor VIII or IX deficiency)	Prolonged bleeding, joint haemorrhage
Leukaemia	Uncontrolled WBC proliferation	High WBC count, bone pain, anaemia
Thrombocytopenia	Low platelet count (<1.5 lakh/mm³)	Bleeding, petechiae

NEET Pattern Analysis: Body fluids contributes 2 questions per year. Key areas: blood group genetics and transfusion, Rh incompatibility, haemoglobin structure, clotting factors, ECG interpretation, and blood disorders.

⚡ NEET 2023 Qn: A person with blood group AB needs blood transfusion. Which blood groups can this person receive from? Answer: Universal recipient — can receive from all groups (A, B, AB, O) since they have no anti-A and anti-B antibodies in plasma. However, Rh compatibility must still be checked.

⚡ Blood group genetics: IA, IB, and i alleles. IA and IB are codominant; both dominant over i. Genotypes: IAIA or IAi = Group A; IBIB or IBi = Group B; IAIB = Group AB; ii = Group O.