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Master Body Fluids and Circulation for NEET 2026 with comprehensive notes on blood, heart, cardiac cycle, ECG, and circulatory system. NCERT-aligned content with diagrams and PYQs.
5 points to remember for NEET
Body Fluids and Circulation is one of the most important chapters in Human Physiology, contributing 5-7 questions in NEET annually. This comprehensive guide covers all NCERT concepts.
| Fluid | Location | Function |
|---|---|---|
| Blood | Cardiovascular system | Transport, defense, regulation |
| Lymph | Lymphatic system | Immunity, fat absorption |
| Tissue fluid | Between cells | Nutrient/waste exchange |
Blood = Plasma (55%) + Formed Elements (45%)
Composition:
Plasma Proteins:
| Protein | Percentage | Function |
|---|---|---|
| Albumin | 60% | Osmotic pressure, transport |
| Globulins | 35% | Immunity (γ), transport (α, β) |
| Fibrinogen | 4% | Blood clotting |
| Prothrombin | <1% | Blood clotting |
Serum = Plasma - Clotting factors (Fibrinogen)
| Feature | Description |
|---|---|
| Shape | Biconcave disc |
| Nucleus | Absent (in mammals) |
| Count | 5-5.5 million/μL (males), 4.5-5 million/μL (females) |
| Lifespan | 120 days |
| Formation | Red bone marrow (Erythropoiesis) |
| Destruction | Spleen (graveyard of RBCs) |
| Contains | Hemoglobin (~270 million/RBC) |
Hemoglobin:
Erythropoiesis regulation: Hypoxia → Kidney releases Erythropoietin → ↑ RBC production
Count: 6,000-8,000/μL
Types:
| Type | Percentage | Nucleus | Granules | Function |
|---|---|---|---|---|
| Granulocytes | ||||
| Neutrophils | 60-65% | Multi-lobed | Fine, light | Phagocytosis (bacteria) |
| Eosinophils | 2-3% | Bi-lobed | Coarse, red | Parasites, allergies |
| Basophils | 0.5-1% | S-shaped | Large, blue | Histamine, heparin |
| Agranulocytes | ||||
| Lymphocytes | 20-25% | Round, large | None | Immunity (B and T cells) |
| Monocytes | 6-8% | Kidney-shaped | None | Phagocytosis (→ macrophages) |
Mnemonic: Never Let Monkeys Eat Bananas (Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils - decreasing order)
| Feature | Description |
|---|---|
| Shape | Irregular fragments |
| Nucleus | Absent |
| Count | 1.5-3.5 lakh/μL |
| Lifespan | 8-10 days |
| Origin | Megakaryocytes in bone marrow |
| Function | Blood clotting |
| Blood Group | Antigen on RBC | Antibody in Plasma | Can Donate To | Can Receive From |
|---|---|---|---|---|
| A | A | Anti-B | A, AB | A, O |
| B | B | Anti-A | B, AB | B, O |
| AB | A and B | None | AB only | All (Universal recipient) |
| O | None | Anti-A, Anti-B | All (Universal donor) | O only |
Erythroblastosis fetalis: Rh- mother + Rh+ fetus → Mother develops anti-Rh antibodies → Destroys fetal RBCs in subsequent Rh+ pregnancies
Three Stages:
Formation of Thromboplastin:
Formation of Thrombin: Prothrombin → Thrombin (by Thromboplastin + Ca²⁺)
Formation of Fibrin: Fibrinogen → Fibrin (by Thrombin) Fibrin mesh traps blood cells → Clot
Anticoagulants:
Composition: Similar to blood but lacks RBCs and most proteins
Functions:
| Type | Organism | Characteristics |
|---|---|---|
| Open | Arthropods, Molluscs | Blood bathes organs directly |
| Closed | Annelids, Vertebrates | Blood flows in vessels |
Blood passes through heart twice per complete circuit:
| Chamber | Receives Blood From | Sends Blood To |
|---|---|---|
| Right Atrium | Superior & Inferior Vena Cava | Right Ventricle |
| Right Ventricle | Right Atrium | Pulmonary Artery |
| Left Atrium | Pulmonary Veins (4) | Left Ventricle |
| Left Ventricle | Left Atrium | Aorta |
Wall Thickness: Left ventricle > Right ventricle > Atria
| Valve | Location | Function |
|---|---|---|
| Tricuspid | RA-RV junction | Prevents backflow to RA |
| Bicuspid (Mitral) | LA-LV junction | Prevents backflow to LA |
| Pulmonary semilunar | RV-Pulmonary artery | Prevents backflow to RV |
| Aortic semilunar | LV-Aorta | Prevents backflow to LV |
Components (in order of conduction):
SA Node (Sinoatrial Node)
AV Node (Atrioventricular Node)
Bundle of His (AV Bundle)
Purkinje Fibers
Autorhythmicity: Heart can generate its own impulses (no nerve required)
Definition: Sequence of events from one heartbeat to the next
Duration: 0.8 seconds (at 75 beats/min)
| Phase | Duration | Events |
|---|---|---|
| Atrial systole | 0.1 s | Atria contract, AV valves open, blood enters ventricles |
| Ventricular systole | 0.3 s | Ventricles contract, AV valves close, semilunar valves open |
| Joint diastole | 0.4 s | All chambers relax, semilunar valves close, blood fills atria |
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End Diastolic Volume (EDV): ~120 mL (blood in ventricle after filling) End Systolic Volume (ESV): ~50 mL (blood remaining after ejection) Stroke Volume (SV): EDV - ESV = 70 mL
Cardiac Output = Heart Rate × Stroke Volume = 75 × 70 = 5250 mL/min ≈ 5 L/min
| Sound | Cause | Phase |
|---|---|---|
| First (Lub) | AV valves closing | Start of ventricular systole |
| Second (Dub) | Semilunar valves closing | Start of diastole |
Definition: Graphical record of electrical activity of heart
| Component | Represents | Duration |
|---|---|---|
| P wave | Atrial depolarization | 0.08 s |
| QRS complex | Ventricular depolarization | 0.08 s |
| T wave | Ventricular repolarization | 0.16 s |
| PR interval | AV conduction time | 0.12-0.2 s |
| QT interval | Ventricular activity | 0.4 s |
Note: Atrial repolarization is hidden within QRS complex
| Abnormality | Indication |
|---|---|
| Prolonged PR interval | AV block |
| Wide QRS | Bundle branch block |
| ST elevation | Myocardial infarction |
| Irregular rhythm | Arrhythmia |
| Nerve | Effect | Neurotransmitter |
|---|---|---|
| Vagus (Parasympathetic) | ↓ Heart rate | Acetylcholine |
| Sympathetic | ↑ Heart rate, ↑ Force | Norepinephrine |
| Hormone | Effect |
|---|---|
| Adrenaline | ↑ Heart rate, ↑ Force |
| Thyroxine | ↑ Heart rate |
| Vessel | Wall Structure | Function |
|---|---|---|
| Arteries | Thick, elastic, muscular | Carry blood away from heart |
| Veins | Thin, less muscular, have valves | Carry blood to heart |
| Capillaries | Single layer endothelium | Exchange of materials |
Normal: 120/80 mm Hg (Systolic/Diastolic)
| Condition | Systolic | Diastolic |
|---|---|---|
| Normal | <120 | <80 |
| Hypertension | >140 | >90 |
| Hypotension | <90 | <60 |
| Disorder | Description |
|---|---|
| Atherosclerosis | Plaque buildup in arteries |
| Angina pectoris | Chest pain due to reduced heart blood supply |
| Heart attack (MI) | Death of heart muscle due to blocked coronary artery |
| Heart failure | Heart unable to pump adequately |
| Hypertension | High blood pressure |
Q1 (NEET 2023): The pacemaker of heart is:
Q2 (NEET 2022): Which blood group is called universal donor?
Q3 (NEET 2021): The QRS complex in ECG represents:
Q4 (NEET 2020): Cardiac output is:
Q5 (NEET 2019): "Lub" sound is produced by:
Q: Why is SA node called the pacemaker? A: SA node generates impulses at the highest rate (70-75/min) among all conducting tissues. It sets the rhythm for the entire heart, hence called the pacemaker.
Q: Why is left ventricle wall thicker than right ventricle? A: Left ventricle pumps blood to the entire body (systemic circulation) against higher resistance, requiring more force. Right ventricle only pumps to nearby lungs (pulmonary circulation) at lower pressure.
Q: What happens if the AV node fails? A: If SA node fails, AV node can take over as pacemaker (at 40-60 beats/min). If AV node also fails, Purkinje fibers can maintain a very slow rhythm (25-40/min), but this is not sustainable.
Q: Why do arteries not have valves but veins do? A: Arteries receive high-pressure blood directly from the heart, which maintains unidirectional flow. Veins carry low-pressure blood against gravity (especially from legs), so valves prevent backflow.
Q: What is the significance of the delay at AV node? A: The 0.1-second delay allows complete atrial contraction and ventricular filling before ventricles contract. Without this delay, atria and ventricles would contract simultaneously, reducing cardiac efficiency.
The numbers in this chapter — cardiac cycle duration, the AV node delay, ECG wave intervals — are where most marks are lost or won. Message us on WhatsApp at +91 88264 44334 with a cardiac cycle or ECG question you got wrong, and we'll walk through where the confusion actually is.
Related reading: Breathing and Exchange of Gases Notes · Human Physiology Complete Notes · Class 11 Biology Coaching
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About the author
Dr. Shekhar C. Singh
AIIMS New Delhi alumnus who has taught biology since 2014, from NCERT foundations to the International Biology Olympiad.
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