Body Fluids and Circulation — Biology Class 11 Notes (CBSE & HBSE)
Free NCERT Biology notes for Body Fluids and Circulation (Class 11) on Siksha Sarovar, aligned to CBSE and Haryana Board (HBSE). This chapter is broken into 3 topics with clear explanations, formulas, solved examples and board-pattern practice — free to read, no sign-up required.
Board exam focus — Body Fluids and Circulation (CBSE & HBSE)
Body Fluids and Circulation explains how blood and lymph circulate to deliver nutrients and gases and remove wastes. The chapter covers blood composition (plasma and formed elements), blood groups and coagulation, lymph, the structure of the human heart, the cardiac cycle and ECG, double circulation, neural and hormonal regulation of cardiac activity, and circulatory disorders. Cardiac output, ECG waves and the cardiac cycle are highly tested in NEET and board exams.
Blood, Blood Groups, Coagulation and Lymph
Body Fluids
Most higher organisms use specialised fluids — blood and lymph — for transport. Blood is a special connective tissue consisting of a fluid matrix, plasma, and formed elements.
Plasma
- Plasma is a straw-coloured, viscous fluid constituting nearly 55% of the blood.
- It is 90-92% water and 6-8% proteins.
- Major plasma proteins: fibrinogen (clotting), globulins (defence/immunity) and albumins (osmotic balance).
- Plasma without clotting factors is called serum.
Formed Elements (≈ 45%)
| Element | Other Name | Key Features |
|---|---|---|
| Erythrocytes | RBCs | Most abundant (5-5.5 million/mm³); biconcave, enucleate (in mammals); contain haemoglobin (12-16 g/100 mL); lifespan ~120 days |
| Leucocytes | WBCs | Nucleated, colourless; 6000-8000/mm³; granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes) |
| Thrombocytes | Platelets | Cell fragments from megakaryocytes; 1.5-3.5 lakh/mm³; help in clotting |
Neutrophils (60-65%) are the most abundant WBCs; lymphocytes (20-25%) are responsible for immune responses.
Blood Groups
ABO grouping is based on surface antigens A and B on RBCs:
| 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, B | none | AB | All (universal recipient) |
| O | none | anti-A, anti-B | All (universal donor) | O |
Rh grouping: ~80% people have the Rh antigen (Rh+). An Rh- mother carrying an Rh+ foetus may develop anti-Rh antibodies in a subsequent pregnancy, causing erythroblastosis foetalis.
Coagulation of Blood
Clotting is a cascade producing a network of threads. Injured tissue/platelets release thromboplastin, which (with Ca²⁺ and clotting factors) converts inactive prothrombin → thrombin. Thrombin then converts soluble fibrinogen → insoluble fibrin threads, forming the clot.
Lymph (Tissue Fluid)
- As blood passes through capillaries, water and small solutes filter out into spaces forming interstitial/tissue fluid.
- This fluid drained back by the lymphatic system is called lymph.
- Lymph is colourless, contains lymphocytes, but lacks RBCs and most plasma proteins.
- Functions: returns interstitial fluid to blood, absorbs and transports fats (via lacteals in intestinal villi), and carries immune cells.
Human Heart, Cardiac Cycle and ECG
Structure of the Human Heart
The heart is a mesodermally derived, four-chambered muscular organ enclosed by a double-walled pericardium with pericardial fluid.
- Two atria (upper, thin-walled) and two ventricles (lower, thick-walled).
- Right atrium–right ventricle: guarded by the tricuspid valve.
- Left atrium–left ventricle: guarded by the bicuspid (mitral) valve.
- Valves allow flow only towards the ventricles, preventing backflow. Semilunar valves guard the openings of the pulmonary artery and aorta.
Conducting (Nodal) System
- Sino-atrial node (SAN): in the right upper corner of the right atrium; the pacemaker that generates ~70-75 action potentials/min.
- Atrio-ventricular node (AVN): at the lower left atrial septum.
- Atrio-ventricular bundle (Bundle of His) → right and left bundle branches → Purkinje fibres, spreading impulse through ventricles.
The SAN sets the heart rate, so it is called the pacemaker; the action it generates is myogenic (originates in heart muscle itself).
Cardiac Cycle
The sequential events in one heartbeat constitute the cardiac cycle: joint atrial diastole → atrial systole → ventricular systole → joint diastole. The cycle lasts ~0.8 s (at 72 beats/min).
- Stroke volume = volume pumped out by a ventricle per beat ≈ 70 mL.
- Cardiac output = stroke volume × heart rate ≈ 70 mL × 72 = ~5000 mL/min (5 L/min).
- Heart sounds: 'lubb' (first) = closure of tricuspid and bicuspid valves at start of ventricular systole; 'dup' (second) = closure of semilunar valves at end of ventricular systole. These are clinically important.
Electrocardiograph (ECG)
An ECG is the graphical representation of the electrical activity of the heart during a cardiac cycle.
| Wave | Represents |
|---|---|
| P wave | Depolarisation of the atria (atrial contraction) |
| QRS complex | Depolarisation of the ventricles (initiates ventricular contraction) |
| T wave | Repolarisation of the ventricles (return to resting state) |
By counting the number of QRS complexes in a given time, the heart rate can be determined. Deviations in shape/size of waves help diagnose heart conditions.
Double Circulation, Regulation and Disorders
Double Circulation
In humans, blood passes twice through the heart in one complete cycle — this is double circulation, comprising two circuits:
- Pulmonary circulation: deoxygenated blood from the right ventricle → pulmonary artery → lungs (oxygenated) → pulmonary veins → left atrium.
- Systemic circulation: oxygenated blood from the left ventricle → aorta → body tissues → (deoxygenated) → vena cava → right atrium.
A special hepatic portal system carries blood from the intestine to the liver before reaching the heart; the coronary system of vessels supplies the cardiac musculature itself.
Double circulation keeps oxygenated and deoxygenated blood completely separate, allowing efficient O2 delivery — essential for endothermic mammals and birds.
Regulation of Cardiac Activity
The heartbeat is myogenic (initiated by the SAN), but it is modulated by:
- Neural control: the medulla oblongata controls cardiac function via the autonomic nervous system (ANS). Sympathetic signals increase heart rate, force of contraction and cardiac output; parasympathetic (vagus) signals decrease heart rate.
- Hormonal control: adrenal medullary hormones (adrenaline/noradrenaline) can also increase cardiac output.
Disorders of the Circulatory System
| Disorder | Description |
|---|---|
| Hypertension (high BP) | BP higher than the normal 120/80 mm Hg; readings of ≥140/90 indicate hypertension. It affects vital organs (brain, heart, kidney). |
| Coronary Artery Disease (CAD / atherosclerosis) | Deposits of calcium, fat, cholesterol and fibrous tissue narrow the coronary arteries, reducing blood flow to heart muscle. |
| Angina (angina pectoris) | Acute chest pain when not enough O2 reaches the heart muscle; can occur in men and women of any age. |
| Heart failure | State when the heart is not pumping blood effectively enough to meet the body's needs (congestion of lungs gives the name 'congestive heart failure'). Different from cardiac arrest (heart stops) and heart attack (muscle damage). |
Normal blood pressure: 120 mm Hg (systolic) / 80 mm Hg (diastolic). Repeated high readings (e.g., 140/90 or above) indicate hypertension, a 'silent killer' that strains the heart and arteries.
Frequently asked questions
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Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Body Fluids and Circulation.