Body Fluids and Circulation - Complete NEET Biology Notes

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.

Body Fluids

Types of Body Fluids

Fluid	Location	Function
Blood	Cardiovascular system	Transport, defense, regulation
Lymph	Lymphatic system	Immunity, fat absorption
Tissue fluid	Between cells	Nutrient/waste exchange

Blood

Composition of Blood

Blood = Plasma (55%) + Formed Elements (45%)

Plasma

Composition:

Water: 90-92%
Proteins: 6-8%
Inorganic salts: 0.9%
Other substances: Glucose, hormones, gases, waste

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)

Formed Elements

1. Red Blood Cells (Erythrocytes)

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:

4 polypeptide chains (2α + 2β in adults)
4 heme groups (each with Fe²⁺)
Each Hb binds 4 O₂ molecules

Erythropoiesis regulation: Hypoxia → Kidney releases Erythropoietin → ↑ RBC production

2. White Blood Cells (Leukocytes)

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)

3. Platelets (Thrombocytes)

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 Groups

ABO System

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

Rh System

Rh+ (85%): Has Rh antigen (D antigen)
Rh- (15%): Lacks Rh antigen

Erythroblastosis fetalis: Rh- mother + Rh+ fetus → Mother develops anti-Rh antibodies → Destroys fetal RBCs in subsequent Rh+ pregnancies

Blood Clotting (Coagulation)

Three Stages:

Formation of Thromboplastin:
- Damaged tissues release tissue factor
- Platelets release platelet factors
- Ca²⁺ required
Formation of Thrombin: Prothrombin → Thrombin (by Thromboplastin + Ca²⁺)
Formation of Fibrin: Fibrinogen → Fibrin (by Thrombin) Fibrin mesh traps blood cells → Clot

Anticoagulants:

Heparin (from mast cells, basophils)
EDTA, Citrate (used in blood banks - chelate Ca²⁺)

Lymph

Composition: Similar to blood but lacks RBCs and most proteins

Functions:

Returns tissue fluid to blood
Transports absorbed fats (lacteals)
Carries lymphocytes
Filters pathogens (lymph nodes)

Circulatory System

Types of Circulation

Type	Organism	Characteristics
Open	Arthropods, Molluscs	Blood bathes organs directly
Closed	Annelids, Vertebrates	Blood flows in vessels

Double Circulation

Blood passes through heart twice per complete circuit:

Pulmonary circulation: Heart → Lungs → Heart
Systemic circulation: Heart → Body → Heart

Human Heart

Location and Structure

Located in thoracic cavity, mediastinum
Size: Fist-sized (~300 g)
Covered by double-walled pericardium

Chambers and Vessels

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

Valves

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

Conducting System

Components (in order of conduction):

SA Node (Sinoatrial Node)
- Location: Right atrium, near SVC opening
- Pacemaker (generates impulses at 70-75/min)
- Rate: Highest intrinsic rate
AV Node (Atrioventricular Node)
- Location: Right atrium, near AV septum
- Delays impulse (0.1 sec) - allows atrial emptying
- Rate: 40-60/min
Bundle of His (AV Bundle)
- Passes through AV septum
- Divides into right and left bundle branches
Purkinje Fibers
- Spread throughout ventricular walls
- Rapid conduction to ventricular muscle

Autorhythmicity: Heart can generate its own impulses (no nerve required)

Cardiac Cycle

Definition: Sequence of events from one heartbeat to the next

Duration: 0.8 seconds (at 75 beats/min)

Phases

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

Pressure and Volume Changes

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

Heart Sounds

Sound	Cause	Phase
First (Lub)	AV valves closing	Start of ventricular systole
Second (Dub)	Semilunar valves closing	Start of diastole

Electrocardiogram (ECG)

Definition: Graphical record of electrical activity of heart

Waves and Intervals

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

Clinical Significance

Abnormality	Indication
Prolonged PR interval	AV block
Wide QRS	Bundle branch block
ST elevation	Myocardial infarction
Irregular rhythm	Arrhythmia

Regulation of Cardiac Activity

Neural Regulation

Nerve	Effect	Neurotransmitter
Vagus (Parasympathetic)	↓ Heart rate	Acetylcholine
Sympathetic	↑ Heart rate, ↑ Force	Norepinephrine

Hormonal Regulation

Hormone	Effect
Adrenaline	↑ Heart rate, ↑ Force
Thyroxine	↑ Heart rate

Blood Vessels

Types

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

Blood Pressure

Normal: 120/80 mm Hg (Systolic/Diastolic)

Condition	Systolic	Diastolic
Normal	<120	<80
Hypertension	>140	>90
Hypotension	<90	<60

Disorders of Circulatory System

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

Previous Year NEET Questions

Q1 (NEET 2023): The pacemaker of heart is:

(a) AV node
(b) Bundle of His
(c) SA node ✓
(d) Purkinje fibers

Q2 (NEET 2022): Which blood group is called universal donor?

(a) A
(b) B
(c) AB
(d) O ✓

Q3 (NEET 2021): The QRS complex in ECG represents:

(a) Atrial depolarization
(b) Ventricular depolarization ✓
(c) Ventricular repolarization
(d) Atrial repolarization

Q4 (NEET 2020): Cardiac output is:

(a) Heart rate × Blood pressure
(b) Stroke volume × Heart rate ✓
(c) Blood pressure × Stroke volume
(d) Heart rate / Stroke volume

Q5 (NEET 2019): "Lub" sound is produced by:

(a) Opening of AV valves
(b) Closing of AV valves ✓
(c) Opening of semilunar valves
(d) Closing of semilunar valves

Quick Revision Points

Blood plasma: 55% of blood
Formed elements: 45% of blood
Most abundant WBC: Neutrophils (60-65%)
RBC lifespan: 120 days
Platelet lifespan: 8-10 days
Graveyard of RBCs: Spleen
Universal donor: O
Universal recipient: AB
Pacemaker: SA node
Cardiac output: ~5 L/min
Normal BP: 120/80 mm Hg
P wave: Atrial depolarization
QRS complex: Ventricular depolarization
T wave: Ventricular repolarization

FAQs

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.

Key Takeaways

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