Summarise body fluids and circulation chapter class 11 in a concise and detailed manner.

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NCERT Class 11 biology Chapter 18 body fluids circulation summary blood lymph heart

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body fluids circulation blood lymph heart cardiac cycle double circulation disorders

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blood composition RBC WBC platelets cardiac cycle ECG heart structure ABO blood group coagulation

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Chapter 18: Body Fluids and Circulation - Class 11 Biology (NCERT)

18.1 Blood

Blood is a fluid connective tissue consisting of a liquid matrix called plasma and formed elements (cells suspended in it).

Plasma (~55% of blood volume)

Straw-colored, viscous fluid
~90-92% water; ~6-8% proteins
Key plasma proteins:
- Albumins - maintain osmotic balance
- Globulins - involved in defense (immune responses)
- Fibrinogens - needed for blood clotting/coagulation
Also contains minerals: Na⁺, Ca²⁺, Mg²⁺, HCO₃⁻, Cl⁻
Dissolved substances in transit: glucose, amino acids, lipids, hormones
Clotting factors are present in inactive form
Serum = plasma minus clotting factors

Formed Elements (~45% of blood volume)

Cell Type	Normal Count	Key Features	Function
Erythrocytes (RBCs)	5-5.5 million/mm³ (males); 4.5-5 million/mm³ (females)	Biconcave disc, no nucleus in humans, contain haemoglobin	Transport O₂ (and some CO₂)
Leucocytes (WBCs)	6,000-8,000/mm³	Nucleated, colourless, short-lived	Defense/immunity
Thrombocytes (Platelets)	1.5-3.5 lakh/mm³	Cell fragments, no nucleus	Blood coagulation

Types of Leucocytes

Granulocytes (have granules in cytoplasm):

Neutrophils - most abundant WBC (60-65%), phagocytic
Eosinophils - 2-3%, associated with allergic reactions
Basophils - least common (0.5-1%), secrete histamine, heparin

Agranulocytes (no granules):

Lymphocytes - 20-25%, involved in immune response (B and T cells)
Monocytes - 6-8%, phagocytic, largest WBC

Blood Groups

ABO System (discovered by Karl Landsteiner):

Blood Group	Antigens on RBC	Antibodies 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 only	All (Universal Recipient)
O	None	Anti-A & Anti-B	All (Universal Donor)	O only

Rh Factor:

Rh antigen (similar to that in Rhesus monkeys) present on RBCs of ~80% of humans - these are Rh⁺
Rh incompatibility in pregnancy: If an Rh⁻ mother carries an Rh⁺ fetus (second pregnancy), maternal anti-Rh antibodies can cross the placenta and destroy fetal RBCs - called Erythroblastosis foetalis

Blood Coagulation (Clotting)

Injury to blood vessel → platelets aggregate at wound site
Release of clotting factors → cascade reaction
Prothrombin (inactive) → Thrombin (active enzyme) - requires Ca²⁺
Thrombin converts Fibrinogen (soluble, in plasma) → Fibrin (insoluble threads)
Fibrin forms a mesh trapping RBCs → blood clot (thrombus)

18.2 Lymph (Tissue Fluid)

When blood plasma filters out of capillaries into tissue spaces, it forms tissue fluid (interstitial fluid)
This fluid enters lymph capillaries and becomes lymph
Lymph is colourless (no RBCs, contains WBCs, especially lymphocytes)
Composition similar to plasma but less protein

Functions of Lymph:

Returns excess tissue fluid back to blood
Transports large protein molecules that cannot re-enter blood capillaries directly
Carries fats absorbed from intestinal lacteals (lymph vessels in villi)
Lymphocytes in lymph are responsible for immune responses
Transports hormones, nutrients

18.3 Circulatory Pathways

Open vs. Closed Circulatory System

Open Circulatory System	Closed Circulatory System
Blood pumped into open spaces called sinuses	Blood always within closed network of vessels
Found in arthropods and molluscs	Found in annelids and chordates (including humans)
Less precise flow regulation	More precise, efficient flow regulation

Human Circulatory System (Blood Vascular System)

Components: Heart + Blood Vessels (arteries, veins, capillaries) + Blood

Structure of Blood Vessels

All blood vessels (except capillaries) have 3 layers:

Tunica intima (innermost) - squamous endothelium
Tunica media (middle) - smooth muscle and elastic fibres (thinner in veins)
Tunica externa (outermost) - fibrous connective tissue with collagen fibres

Arteries carry blood away from heart; thick, elastic walls. Veins carry blood toward heart; thinner walls, have valves to prevent backflow. Capillaries - one cell thick, site of actual exchange of nutrients/gases.

Structure of the Human Heart

Location: thoracic cavity, slightly tilted to left, between the two lungs
Pericardium: double-walled membranous sac enclosing the heart; fluid between layers reduces friction
4 chambers: 2 atria (upper, thin-walled) + 2 ventricles (lower, thick-walled)
Left ventricle wall is thicker than right (pumps blood to entire body)
Interatrial septum separates the two atria; Interventricular septum separates the two ventricles

Valves:

Tricuspid valve - between right atrium and right ventricle (3 cusps)
Bicuspid (Mitral) valve - between left atrium and left ventricle (2 cusps)
Semilunar valves - at openings of pulmonary artery (from RV) and aorta (from LV)
Valves ensure unidirectional blood flow

18.4 Cardiac Cycle

One complete heartbeat = 1 cardiac cycle (~0.8 seconds at 72 beats/min)

Phases:

Atrial Systole (0.1 sec) - Both atria contract; AV valves open; semilunar valves closed; blood pushed into ventricles
Ventricular Systole (0.3 sec) - Both ventricles contract; AV valves close (producing "lub" sound); semilunar valves open; blood ejected into aorta and pulmonary artery
Joint Diastole (0.4 sec) - All chambers relax; semilunar valves close (producing "dub" sound); heart fills with blood again

Heart Sounds:

"Lub" (1st sound) - closure of AV (tricuspid + bicuspid) valves
"Dub" (2nd sound) - closure of semilunar valves

Key Cardiac Output Terms:

Stroke volume - volume of blood pumped per beat (~70 mL)
Cardiac output = Stroke Volume × Heart Rate = ~70 mL × 72 beats/min ≈ 5 litres/min

Electrocardiogram (ECG)

A graphical record of the electrical activity of the heart.

P wave - depolarization of atria (atrial contraction)
QRS complex - depolarization of ventricles (ventricular contraction)
T wave - repolarization of ventricles (ventricular relaxation)
ECG shape is fairly constant in healthy individuals; deviations indicate cardiac abnormalities

18.4 Double Circulation

Humans have complete double circulation - blood passes through the heart twice per circuit.

Two Pathways:

1. Pulmonary Circulation (right side): Right ventricle → pulmonary artery → lungs (oxygenation) → pulmonary veins → left atrium

2. Systemic Circulation (left side): Left ventricle → aorta → all body tissues (O₂ delivery, CO₂ pickup) → veins → right atrium

Advantage: Oxygenated and deoxygenated blood are kept completely separate, ensuring efficient oxygen delivery - important for warm-blooded animals with high metabolic demands.

Hepatic Portal System:

Blood from the intestine (rich in absorbed nutrients) passes through the liver via the hepatic portal vein before entering general circulation - liver can process, store, or detoxify these substances.

18.5 Regulation of Cardiac Activity

The heart is autoexcitable (myogenic - it generates its own impulses) but is also regulated neurally and hormonally.

Intrinsic Regulation (Conduction System):

SA Node (Sinoatrial Node) - "Pacemaker of the heart"; located in right atrium wall; generates electrical impulse initiating each heartbeat (~72/min)
AV Node (Atrioventricular Node) - in the right atrium near the atrio-ventricular septum; receives impulse from SA node and relays it (with slight delay)
Bundle of His - from AV node, divides into right and left bundle branches in the interventricular septum
Purkinje fibres - spread through ventricular walls; cause ventricular contraction

Extrinsic Regulation:

Sympathetic nervous system - increases heart rate and force of contraction (in stress/exercise)
Parasympathetic nervous system (Vagus nerve) - decreases heart rate (during rest)
Hormones: Adrenaline increases heart rate; acetylcholine decreases it

18.6 Disorders of the Circulatory System

Disorder	Description
Hypertension	Blood pressure persistently above 120/80 mmHg. Normal BP = 120/80. Can damage heart and blood vessels; called "silent killer"
Coronary Artery Disease (CAD)	Deposition of calcium, fat, cholesterol, fibrous tissue in coronary artery walls (atherosclerosis) - narrows the lumen, reduces blood supply to heart muscle
Angina	Chest pain due to insufficient blood/O₂ supply to heart muscle. More common in middle age and elderly
Heart Failure	Heart cannot pump enough blood to meet body's needs; also called "congestive heart failure" (congestion of lungs is a main symptom). Different from cardiac arrest or heart attack

Quick Revision Summary Table

Topic	Key Points
Blood	Fluid connective tissue; plasma + RBC + WBC + platelets
Plasma proteins	Albumins (osmotic balance), Globulins (defense), Fibrinogen (clotting)
Blood groups	ABO + Rh system; O = universal donor; AB = universal recipient
Clotting	Fibrinogen → Fibrin via Thrombin; requires Ca²⁺
Lymph	Colourless; returns fluid to blood; carries fats from lacteals
Heart	4 chambers; pericardium; tricuspid & bicuspid & semilunar valves
Cardiac cycle	0.8 sec; systole + diastole; cardiac output ~5 L/min
ECG	P wave (atrial depol.), QRS (ventricular depol.), T wave (ventricular repol.)
Double circulation	Pulmonary (RV → lungs → LA) + Systemic (LV → body → RA)
Pacemaker	SA Node initiates impulse; AV Node → Bundle of His → Purkinje fibres
Disorders	Hypertension, CAD, Angina pectoris, Heart failure

This covers all six major sections of the chapter as per the NCERT Class 11 Biology syllabus. The most important concepts for exams are: blood composition and groups, the cardiac cycle with ECG waves, double circulation, the conduction pathway (SA node → AV node → Bundle of His → Purkinje fibres), and the key circulatory disorders.

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