Human Physiology : NEET Biology Notes

Human Physiology deals with the structure and functioning of the human body. The major systems covered in this unit are respiratory, circulatory, excretory, muscular, skeletal, nervous and endocrine systems.

1. Breathing and Respiration

1.1 Respiratory Organs in Animals — Quick Revision

Different animals possess different respiratory structures depending on their habitat and body organization.

Organ/Structure	Example
Body surface	Amoeba, earthworm
Gills	Fish
Tracheal system	Cockroach, insects
Book lungs	Spider
Skin + lungs	Frog
Lungs	Reptiles, birds, mammals

NEET Point: Fish primarily exchange gases through gills, while insects use a tracheal system.

1.2 Human Respiratory System

The human respiratory system consists of:

Nostrils → Nasal cavity → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli

Main Components

External nostrils
Nasal chamber
Pharynx
Larynx
Trachea
Primary bronchi
Secondary and tertiary bronchi
Bronchioles
Alveoli
Lungs
Respiratory muscles

Lungs

Humans have a pair of lungs.
Right lung → 3 lobes
Left lung → 2 lobes
Left lung is smaller because the heart occupies space on that side.

1.3 Conducting Part

The conducting portion includes:

Nose
Nasal cavity
Pharynx
Larynx
Trachea
Bronchi
Bronchioles

Functions

Carries air.
Filters air.
Warms air.
Moistens air.
Helps protect deeper respiratory surfaces.

1.4 Respiratory Part

The respiratory part includes:

Alveoli and their associated ducts

Alveoli

Tiny sac-like structures.
Main site of gas exchange.
Surrounded by a dense network of blood capillaries.
Have a very thin respiratory membrane.

Why are alveoli efficient?

They provide:

Huge surface area
Thin diffusion barrier
Rich blood supply
Moist surface

NEET Fact: Exchange of O₂ and CO₂ mainly occurs across the alveolar-capillary membrane.

1.5 Pleura

Each lung is surrounded by a double-layered membrane called:

Pleura

Between the two pleural layers is pleural fluid.

Functions:

Reduces friction during breathing.
Helps maintain smooth movement of lungs.

2. Mechanism of Breathing

Breathing consists of:

Inspiration
Expiration

2.1 Inspiration

Inspiration means air entering the lungs.

During normal inspiration:

Diaphragm contracts and becomes flatter.
External intercostal muscles contract.
Thoracic volume increases.
Intrapulmonary pressure decreases below atmospheric pressure.
Air enters the lungs.

Sequence

Diaphragm contracts → Thoracic volume ↑ → Lung pressure ↓ → Air enters

2.2 Expiration

Expiration means air leaving the lungs.

During normal expiration:

Diaphragm relaxes.
Thoracic volume decreases.
Intrapulmonary pressure increases.
Air moves out.

Sequence

Diaphragm relaxes → Thoracic volume ↓ → Lung pressure ↑ → Air leaves

Normal quiet expiration is mainly a passive process.

2.3 Pressure-Volume Relationship

The basic principle is:

Increase in thoracic volume → Decrease in pressure

Decrease in thoracic volume → Increase in pressure

This pressure difference drives air movement.

3. Regulation of Breathing

Breathing is controlled by respiratory centres in the brain.

Major Centres

Centre	Location/Role
Respiratory rhythm centre	Medulla
Pneumotaxic centre	Pons
Chemosensitive area	Medulla
Peripheral chemoreceptors	Carotid and aortic bodies

Medulla

The medullary respiratory centre generates the basic rhythm of breathing.

Pons

The pneumotaxic centre modifies the respiratory rhythm and helps regulate the duration of inspiration.

Chemosensitive Area

It responds mainly to changes associated with:

CO₂
H⁺ concentration

Peripheral Chemoreceptors

Located mainly in:

Carotid bodies
Aortic bodies

They respond to changes in blood gases, especially O₂, as well as CO₂/H⁺.

NEET Concept: Increased CO₂ is a major stimulus for increased ventilation.

4. Exchange of Gases

Gas exchange occurs by simple diffusion.

At Alveoli

O₂ moves from alveolar air → blood.
CO₂ moves from blood → alveoli.

At Tissues

O₂ moves from blood → tissues.
CO₂ moves from tissues → blood.

Gas movement depends on partial pressure gradients.

4.1 Respiratory Membrane

The respiratory membrane mainly consists of:

Alveolar epithelium
Basement membrane/interstitial layer
Capillary endothelium

Its thinness allows rapid diffusion.

5. Transport of Oxygen

Oxygen is transported in blood in two forms:

Dissolved in plasma
Bound to haemoglobin

Most oxygen is transported as:

Oxyhaemoglobin

Haemoglobin

Haemoglobin is present inside RBCs.

One haemoglobin molecule can bind up to:

4 molecules of O₂

5.1 Oxygen-Haemoglobin Dissociation

The oxygen dissociation curve is approximately sigmoid-shaped.

Factors Promoting O₂ Release

At actively respiring tissues:

CO₂ ↑
H⁺ ↑
Temperature ↑
pH ↓

These conditions favour oxygen unloading.

This is associated with the Bohr effect.

At Lungs

High O₂ partial pressure
Lower CO₂
Relatively higher pH

→ favours O₂ binding to haemoglobin.

6. Transport of Carbon Dioxide

CO₂ is transported in three major forms:

Form	Approximate contribution
Dissolved in plasma	~7%
As bicarbonate ions	~70%
Bound to haemoglobin as carbaminohaemoglobin	~20–23%

The largest fraction is transported as:

Bicarbonate (HCO₃⁻)

6.1 Chloride Shift

In RBCs, conversion of CO₂ to bicarbonate leads to movement of bicarbonate out of the RBC and chloride ions into the RBC.

This is called:

Chloride shift

It helps maintain electrical neutrality.

7. Respiratory Volumes

Respiratory volumes are important quantitative parameters of breathing.

Volume	Approximate value
Tidal Volume (TV)	500 mL
Inspiratory Reserve Volume (IRV)	2500–3000 mL
Expiratory Reserve Volume (ERV)	1000–1100 mL
Residual Volume (RV)	1100–1200 mL

Values can vary with age, sex, body size and physical condition.

7.1 Tidal Volume

Amount of air inhaled or exhaled during normal quiet breathing.

≈ 500 mL

7.2 Inspiratory Reserve Volume

Additional air that can be inhaled after a normal inspiration.

≈ 2500–3000 mL

7.3 Expiratory Reserve Volume

Additional air that can be forcibly exhaled after normal expiration.

≈ 1000–1100 mL

7.4 Residual Volume

Air remaining in the lungs after forceful expiration.

≈ 1100–1200 mL

NEET Fact: Residual volume prevents complete collapse of lungs.

8. Respiratory Capacities

Inspiratory Capacity

IC = TV + IRV

Approximately:

3000–3500 mL

Expiratory Capacity

EC = TV + ERV

Approximately:

1500–1600 mL

Functional Residual Capacity

FRC = ERV + RV

Approximately:

2100–2300 mL

Vital Capacity

VC = IRV + TV + ERV

Approximately:

4000–4600 mL

Total Lung Capacity

TLC = VC + RV

Approximately:

5000–5800 mL

9. Respiratory Disorders

9.1 Asthma

Asthma is characterized by:

Airway inflammation
Bronchoconstriction
Difficulty in breathing
Wheezing

Common triggers may include:

Allergens
Dust
Pollutants
Exercise in susceptible individuals

9.2 Emphysema

Emphysema involves destruction of alveolar walls.

Consequences:

Reduced respiratory surface area.
Difficulty in gas exchange.
Breathlessness.

A major risk factor is long-term cigarette smoking.

9.3 Occupational Respiratory Disorders

Long-term exposure to workplace dust can damage the lungs.

Examples:

Silicosis
Asbestosis

Commonly associated with exposure to:

Silica dust
Asbestos fibres

Prevention: Proper ventilation and appropriate respiratory protective equipment are important.

10. Body Fluids and Circulation

The circulatory system transports:

Oxygen
Nutrients
Hormones
Metabolic wastes
Heat

The major body fluids involved are:

Blood
Lymph

11. Blood

Blood is a fluid connective tissue.

It consists of:

Plasma
Formed elements

11.1 Plasma

Plasma forms about 55% of blood volume.

It mainly contains:

Water
Proteins
Electrolytes
Nutrients
Hormones
Gases
Waste products

Major Plasma Proteins

Protein	Major function
Albumin	Osmotic balance
Globulins	Immunity/transport
Fibrinogen	Blood clotting

12. Formed Elements

Formed elements account for approximately 45% of blood volume.

They include:

RBCs
WBCs
Platelets

12.1 Red Blood Cells

RBCs:

Are also called erythrocytes.
Contain haemoglobin.
Transport oxygen.
Mature human RBCs lack a nucleus.
Have a biconcave shape.
Increase surface area for gas exchange.

Normal lifespan:

≈ 120 days

RBCs are mainly removed by:

Spleen
Liver

13. White Blood Cells

WBCs protect the body against infections and foreign substances.

They are divided into:

Granulocytes

Neutrophils
Eosinophils
Basophils

Agranulocytes

Lymphocytes
Monocytes

13.1 Functions of WBCs

WBC	Important function
Neutrophils	Phagocytosis
Eosinophils	Allergy and parasitic responses
Basophils	Histamine/heparin-related inflammatory responses
Lymphocytes	Adaptive immunity
Monocytes	Become macrophages; phagocytosis

14. Platelets

Platelets are also called:

Thrombocytes

Functions:

Help stop bleeding.
Participate in blood clot formation.
Release factors involved in coagulation.

Normal platelet count is approximately:

1.5–3.5 lakh/mm³

15. Blood Clotting

Blood clotting prevents excessive blood loss following injury.

Basic Sequence

Injury → Platelet activation → Clotting factors → Thrombin formation → Fibrinogen → Fibrin → Clot

Important Proteins

Fibrinogen
Prothrombin
Thromboplastin-related factors

Fibrin

Fibrin forms a mesh that traps blood cells and forms the clot.

16. Blood Groups

The major blood group systems important for NEET are:

ABO system
Rh system

16.1 ABO Blood Group

Determined by the presence or absence of:

Antigen A
Antigen B

and corresponding antibodies in plasma.

Blood group	RBC antigen	Plasma antibodies
A	A	Anti-B
B	B	Anti-A
AB	A + B	None
O	None	Anti-A + Anti-B

Transfusion Concept

O negative is often referred to as the universal RBC donor in simplified clinical teaching.
AB positive is often referred to as the universal RBC recipient.

However, in actual clinical transfusion, proper blood typing and cross-matching are essential.

17. Rh Blood Group

Rh system is mainly associated with the:

D antigen

Rh⁺ → D antigen present.
Rh⁻ → D antigen absent.

Rh Incompatibility

An Rh⁻ mother carrying an Rh⁺ fetus may become sensitized.

In a subsequent Rh⁺ pregnancy, maternal antibodies may cross the placenta and damage fetal RBCs.

This condition is associated with:

Erythroblastosis fetalis / Hemolytic disease of the newborn

Prevention:

Anti-D immunoglobulin

18. Lymph

Lymph is a colourless fluid formed from tissue fluid.

Composition

Contains:

Water
Electrolytes
Some proteins
Lymphocytes
Nutrients
Waste products

It generally contains fewer proteins than plasma.

Functions

Returns excess tissue fluid to blood.
Transports absorbed fats from intestinal lacteals.
Participates in immune responses.

NEET Fact: Intestinal lacteals absorb dietary fats and transport them through the lymphatic system.

19. Human Circulatory System

Humans have a:

Closed circulatory system

Blood remains within:

Heart
Arteries
Veins
Capillaries

Humans also have:

Double circulation

because blood passes through the heart twice during one complete circuit.

20. Human Heart

The heart is a muscular, four-chambered organ.

Chambers

Right atrium
Right ventricle
Left atrium
Left ventricle

Septa

Separate:

Right and left atria
Right and left ventricles

This prevents mixing of oxygenated and deoxygenated blood under normal conditions.

21. Major Blood Vessels

Vessel	Direction/Function
Vena cava	Body → Right atrium
Pulmonary artery	Right ventricle → Lungs
Pulmonary veins	Lungs → Left atrium
Aorta	Left ventricle → Body

Important: Pulmonary artery carries deoxygenated blood, while pulmonary veins carry oxygenated blood.

22. Heart Valves

Valves prevent backflow of blood.

Atrioventricular Valves

Tricuspid valve → Right atrium/right ventricle
Bicuspid/Mitral valve → Left atrium/left ventricle

Semilunar Valves

Pulmonary semilunar valve
Aortic semilunar valve

23. Cardiac Cycle

One complete sequence of events in the heart is called the:

Cardiac cycle

At a heart rate of approximately 72 beats/min:

One cycle ≈ 0.8 second

Major phases:

Atrial systole
Ventricular systole
Joint diastole

23.1 Atrial Systole

Atria contract.
Blood moves into ventricles.
AV valves are open.
Semilunar valves remain closed.

Duration:

≈ 0.1 s

23.2 Ventricular Systole

Ventricles contract.
AV valves close.
Semilunar valves open.
Blood is pumped into pulmonary artery and aorta.

Duration:

≈ 0.3 s

23.3 Joint Diastole

All four chambers are relaxed.
Blood enters the atria.
Ventricles fill largely passively.

Duration:

≈ 0.4 s

24. Heart Sounds

Two major heart sounds:

First Sound — “Lub”

Due mainly to closure of AV valves.
Occurs at beginning of ventricular systole.

Second Sound — “Dub”

Due mainly to closure of semilunar valves.
Occurs at beginning of ventricular diastole.

Lub → AV valves close

Dub → Semilunar valves close

25. Cardiac Output

Cardiac output is the amount of blood pumped by each ventricle per minute.

Formula

Cardiac Output = Heart Rate × Stroke Volume

Typical values:

Heart rate ≈ 72 beats/min
Stroke volume ≈ 70 mL/beat

Therefore:

CO ≈ 5 L/min

Example

If heart rate rises during exercise and stroke volume also increases:

Cardiac output increases significantly.

26. ECG — Electrocardiogram

ECG records the electrical activity of the heart.

P Wave

Represents:

Atrial depolarization

QRS Complex

Represents:

Ventricular depolarization

T Wave

Represents:

Ventricular repolarization

ECG component	Meaning
P wave	Atrial depolarization
QRS complex	Ventricular depolarization
T wave	Ventricular repolarization

NEET Fact: Atrial repolarization is generally hidden within the QRS complex.

27. Conducting System of Heart

The normal cardiac impulse follows:

SA node → AV node → AV bundle → Bundle branches → Purkinje fibres

SA Node

Located in right atrium.
Acts as the natural pacemaker.
Initiates the heartbeat.

AV Node

Delays the impulse briefly.
Allows ventricles time to fill.

28. Double Circulation

Humans have two interconnected circuits:

Pulmonary Circulation

Right ventricle → Pulmonary artery → Lungs → Pulmonary veins → Left atrium

Function:

Gas exchange in lungs.

Systemic Circulation

Left ventricle → Aorta → Body tissues → Venae cavae → Right atrium

Function:

Supplies oxygen and nutrients to tissues.

Significance

Double circulation ensures efficient separation of:

Oxygenated blood
Deoxygenated blood

29. Regulation of Heart Activity

Heart activity is influenced by:

Autonomic nervous system
Hormones
Exercise
Emotions
Blood pressure
Chemical conditions

Sympathetic Stimulation

Generally:

Heart rate ↑

Parasympathetic Stimulation

Generally:

Heart rate ↓

30. Cardiovascular Disorders

30.1 Hypertension

Persistent elevation of arterial blood pressure is called:

Hypertension

A commonly used clinical threshold is around:

140/90 mm Hg or above

but diagnostic definitions can vary with guidelines and clinical context.

Long-term uncontrolled hypertension can damage:

Heart
Brain
Kidneys
Blood vessels

30.2 Coronary Artery Disease

Coronary arteries supply blood to the cardiac muscle.

Narrowing/blockage of these vessels can reduce blood supply to heart muscle.

Major risk factors include:

High LDL cholesterol
Smoking
Diabetes
Hypertension
Obesity
Physical inactivity

30.3 Angina Pectoris

Angina is chest pain/discomfort caused by inadequate oxygen supply to cardiac muscle, usually due to reduced coronary blood flow.

It commonly occurs during:

Exercise
Physical stress

30.4 Heart Failure

Heart failure occurs when the heart cannot pump blood sufficiently to meet the body’s requirements.

Possible symptoms:

Breathlessness
Fatigue
Fluid accumulation
Reduced exercise tolerance

Important: Heart failure is not the same as sudden cardiac arrest.

31. Excretory Products and Their Elimination

Metabolic reactions generate waste products that must be removed.

Important nitrogenous wastes:

Ammonia
Urea
Uric acid

32. Types of Nitrogenous Excretion

32.1 Ammonotelism

Main nitrogenous waste:

Ammonia

Characteristics:

Highly toxic.
Requires large amounts of water for elimination.

Examples:

Many aquatic animals
Bony fishes

32.2 Ureotelism

Main nitrogenous waste:

Urea

Advantages:

Less toxic than ammonia.
Requires less water for elimination.

Examples:

Humans
Mammals
Adult amphibians

32.3 Uricotelism

Main nitrogenous waste:

Uric acid

Advantages:

Low toxicity.
Conserves water.

Examples:

Birds
Reptiles
Many insects

Quick Table

Type	Main waste	Water requirement	Example
Ammonotelism	Ammonia	High	Many aquatic animals
Ureotelism	Urea	Moderate	Humans
Uricotelism	Uric acid	Low	Birds, reptiles

33. Human Excretory System

The human urinary system consists of:

Two kidneys
Two ureters
Urinary bladder
Urethra

Kidney

Functions:

Urine formation
Removal of nitrogenous wastes
Osmoregulation
Electrolyte balance
Acid-base balance
Blood pressure regulation
Endocrine functions

34. Nephron

The structural and functional unit of the kidney is:

Nephron

Each kidney contains a very large number of nephrons.

Two broad types:

Cortical nephrons
Juxtamedullary nephrons

35. Structure of Nephron

Main parts:

Renal Corpuscle

Glomerulus
Bowman’s capsule

Renal Tubule

Proximal convoluted tubule (PCT)
Loop of Henle
Distal convoluted tubule (DCT)
Collecting duct

36. Glomerulus

Network of capillaries.
Receives blood through the afferent arteriole.
Blood leaves through the efferent arteriole.

Important Point

The afferent arteriole is generally wider than the efferent arteriole, helping maintain high pressure within the glomerulus.

37. Urine Formation

Urine formation involves three major processes:

Glomerular filtration
Selective reabsorption
Tubular secretion

37.1 Glomerular Filtration

Blood is filtered through the glomerular capillaries into Bowman’s capsule.

The filtrate contains:

Water
Glucose
Amino acids
Ions
Urea
Other small molecules

Normally, blood cells and most plasma proteins are retained in blood.

37.2 Selective Reabsorption

Useful substances are returned from tubular fluid to blood.

Examples:

Glucose
Amino acids
Water
Na⁺
Other ions

The PCT performs extensive reabsorption.

37.3 Tubular Secretion

Certain substances are actively secreted into the tubular fluid.

Examples include:

H⁺
K⁺
NH₃/NH₄⁺
Some drugs

Importance

Acid-base balance
Ion regulation
Removal of certain wastes

38. Role of Different Nephron Parts

Part	Major function
Bowman’s capsule	Receives filtrate
PCT	Major reabsorption
Descending limb	Water reabsorption
Ascending limb	Na⁺/Cl⁻ transport; relatively impermeable to water
DCT	Selective reabsorption/secretion
Collecting duct	Water conservation and urine concentration

39. Counter-Current Mechanism

The loop of Henle and vasa recta help establish and maintain a medullary osmotic gradient.

Loop of Henle

Descending limb → permeable to water.
Ascending limb → relatively impermeable to water; transports ions.

Vasa Recta

Helps maintain the medullary gradient through counter-current exchange.

Significance

This mechanism enables the kidney to produce concentrated urine and conserve water.

40. Osmoregulation

Osmoregulation means maintaining appropriate:

Water balance
Ion concentration
Osmotic pressure

The kidneys play the major role.

41. ADH and Water Reabsorption

ADH:

Antidiuretic Hormone

Also called:

Vasopressin

It increases water reabsorption mainly by increasing water permeability of:

Late distal tubule
Collecting ducts

When body water is low

Water deficit → ADH ↑ → Water reabsorption ↑ → Urine volume ↓

42. Renin-Angiotensin System

When blood pressure or renal perfusion decreases:

Renin release → Angiotensinogen → Angiotensin I → Angiotensin II

Angiotensin II:

Promotes vasoconstriction.
Stimulates aldosterone secretion.
Promotes Na⁺ reabsorption.
Increases water retention indirectly.
Helps restore blood pressure.

Overall Effect

Blood pressure ↑

43. Aldosterone

Produced by:

Adrenal cortex

Functions:

Increases Na⁺ reabsorption.
Promotes K⁺ secretion.
Water follows Na⁺, helping increase extracellular fluid volume.

44. ANF / ANP

Atrial Natriuretic Factor/Peptide

Released by atrial cells when blood volume/atrial stretch increases.

Functions:

Promotes sodium excretion.
Promotes water excretion.
Opposes the renin-angiotensin-aldosterone system.
Helps reduce blood pressure.

Memory

RAAS → Raises pressure

ANF → Lowers pressure

45. Other Excretory Organs

Kidneys are the major excretory organs, but other organs also remove wastes.

Organ	Excretory role
Lungs	CO₂ and water vapour
Skin	Sweat: water, salts, small amounts of urea
Liver	Bile pigments, detoxification, nitrogen metabolism
Intestine	Some salts, pigments and other wastes

46. Excretory Disorders

46.1 Uremia

Accumulation of nitrogenous wastes, especially urea, in blood is called:

Uremia

It commonly occurs with severe renal dysfunction.

46.2 Renal Failure

When kidneys lose the ability to adequately perform their functions:

Renal failure

may occur.

Consequences can include:

Waste accumulation
Fluid imbalance
Electrolyte disturbances
Acid-base abnormalities

46.3 Kidney Stones

Solid crystalline deposits in the urinary tract are called:

Renal calculi / kidney stones

Common constituents may include:

Calcium oxalate
Uric acid
Other salts

Symptoms can include severe pain and difficulty in urination.

46.4 Nephritis

Inflammation of kidney tissue is called:

Nephritis

It can impair normal renal function.

47. Dialysis

Dialysis is used when kidneys cannot adequately remove metabolic wastes.

Principle

Blood is passed through a semipermeable membrane against a suitable dialysis fluid.

Waste products diffuse into the dialysis fluid.

Main Functions

Removes nitrogenous wastes.
Helps correct fluid and electrolyte imbalance.

Important: Dialysis supports kidney function but does not completely replace all endocrine and metabolic functions of healthy kidneys.

48. Artificial Kidney

An artificial kidney refers to a dialysis system used to remove wastes from blood.

It is particularly useful in:

Severe renal failure
Temporary renal support
Patients awaiting kidney transplantation

49. Locomotion and Movement

Movement is an important characteristic of living organisms.

In humans, movement occurs through coordinated action of:

Muscles
Bones
Joints
Nervous system

50. Types of Movement

50.1 Ciliary Movement

Movement produced by cilia.

Examples:

Movement of mucus in respiratory tract.
Movement of ovum through the female reproductive tract.

50.2 Flagellar Movement

Movement using flagella.

Example: Movement of human sperm.

50.3 Muscular Movement

Produced by contraction of muscles.

Examples:

Walking
Running
Blinking
Heartbeat
Breathing movements

51. Skeletal Muscles

Skeletal muscles are:

Striated
Voluntary
Attached mainly to bones through tendons

They produce body movement by contracting and pulling bones.

52. Structure of Skeletal Muscle

A skeletal muscle contains:

Muscle → Fascicles → Muscle fibres → Myofibrils → Sarcomeres

Muscle Fibre

Long and cylindrical.
Multinucleated.
Contains myofibrils.

Myofibrils

Contain:

Actin
Myosin

53. Sarcomere

The sarcomere is the basic contractile unit of skeletal muscle.

It lies between two:

Z-lines/Z-discs

Main Components

Thin filament → Actin
Thick filament → Myosin

54. Sliding Filament Theory

Muscle contraction occurs because actin filaments slide over myosin filaments.

Important point:

Filaments do not themselves become shorter; their overlap increases.

55. Mechanism of Muscle Contraction

Step 1: Nerve Signal

Motor neuron stimulates the muscle fibre.

Step 2: Calcium Release

Action potential causes release of:

Ca²⁺

from the sarcoplasmic reticulum.

Step 3: Calcium Binding

Ca²⁺ binds to:

Troponin

Step 4: Tropomyosin Movement

Tropomyosin moves away from the active sites on actin.

Step 5: Cross-Bridge Formation

Myosin heads attach to exposed actin sites.

Step 6: Power Stroke

Myosin head pulls actin toward the centre of the sarcomere.

Step 7: ATP

ATP binding and hydrolysis allow repeated cross-bridge cycling.

Overall

Ca²⁺ → Troponin → Tropomyosin shifts → Actin-myosin interaction → Contraction

56. Changes During Muscle Contraction

During contraction:

Sarcomere length ↓
I-band ↓
H-zone ↓
A-band remains approximately constant

NEET Fact: The A-band does not change in length during normal sliding-filament contraction.

57. Neuromuscular Junction

The junction between a motor neuron and skeletal muscle fibre is:

Neuromuscular junction

The neurotransmitter released is:

Acetylcholine (ACh)

Sequence:

Motor neuron → ACh release → Muscle membrane depolarization → Ca²⁺ release → Contraction

58. ATP Requirement in Muscle

ATP is required for:

Myosin head cycling
Detachment of myosin from actin
Active transport of Ca²⁺ back into the sarcoplasmic reticulum

Without ATP, cross-bridges cannot properly detach.

This contributes to rigor mortis after death.

59. Skeletal System

The adult human skeleton generally contains:

206 bones

It is divided into:

Axial skeleton
Appendicular skeleton

60. Axial Skeleton

Includes:

Skull
Vertebral column
Ribs
Sternum

Functions

Support
Protection of vital organs
Attachment of muscles

61. Appendicular Skeleton

Includes:

Pectoral girdles
Upper limbs
Pelvic girdle
Lower limbs

It is mainly involved in:

Movement
Locomotion
Attachment of limbs

62. Functions of Skeleton

The skeletal system provides:

Support
Protection
Movement
Muscle attachment
Mineral storage
Blood cell formation through bone marrow

63. Joints

A joint is the point where two or more bones meet.

Major types:

Fibrous joints
Cartilaginous joints
Synovial joints

63.1 Fibrous Joints

Little or no movement.

Example:

Skull sutures

63.2 Cartilaginous Joints

Limited movement.

Example:

Joints between vertebrae

63.3 Synovial Joints

Freely movable.
Have a synovial cavity and synovial fluid.

Examples:

Joint	Example
Ball-and-socket	Shoulder, hip
Hinge	Elbow, knee
Pivot	Atlas-axis
Gliding	Wrist/ankle
Saddle	Thumb

64. Musculoskeletal Disorders

64.1 Myasthenia Gravis

Autoimmune disorder.
Affects neuromuscular transmission.
Commonly associated with antibodies against components of the acetylcholine receptor system.
Causes muscle weakness and fatigue.

64.2 Tetany

Tetany is characterized by:

Involuntary muscle spasms

Often associated with low blood Ca²⁺ levels.

64.3 Muscular Dystrophy

A group of inherited disorders characterized by:

Progressive muscle weakness
Muscle degeneration

Example: Duchenne muscular dystrophy.

64.4 Arthritis

Inflammation of joints.

May cause:

Pain
Swelling
Stiffness
Reduced movement

64.5 Osteoporosis

Condition in which:

Bone mass decreases.
Bones become fragile.
Fracture risk increases.

Risk increases with:

Age
Hormonal changes
Calcium/vitamin D deficiency
Reduced physical activity

64.6 Gout

Gout results from accumulation of uric acid crystals in joints.

Symptoms:

Severe joint pain
Inflammation
Swelling

65. Neural Control and Coordination

The nervous system provides rapid control and coordination.

Major components:

Neurons
Neuroglia
Brain
Spinal cord
Peripheral nerves

66. Neuron

The neuron is the structural and functional unit of the nervous system.

Main parts:

Cell body
Dendrites
Axon

Dendrites

Receive signals.
Conduct impulses toward the cell body.

Axon

Conducts impulses away from the cell body.

67. Myelin Sheath

Many axons are covered by a myelin sheath.

In the peripheral nervous system, myelin is produced by:

Schwann cells

Gaps between myelin segments are:

Nodes of Ranvier

Myelination increases the speed of impulse conduction.

68. Nerves

A nerve is a bundle of nerve fibres/axons in the peripheral nervous system.

Types based on function:

Sensory nerves
Motor nerves
Mixed nerves

Sensory

Carry impulses toward the CNS.

Motor

Carry impulses away from CNS.

Mixed

Contain both sensory and motor fibres.

69. Human Nervous System

It is broadly divided into:

Central nervous system
Peripheral nervous system
Autonomic nervous system

69.1 Central Nervous System

Consists of:

Brain + Spinal cord

Brain

Main regions include:

Forebrain
Midbrain
Hindbrain

69.2 Peripheral Nervous System

Includes:

Cranial nerves
Spinal nerves

It connects the CNS with receptors and effectors.

69.3 Autonomic Nervous System

Controls largely involuntary functions.

It includes:

Sympathetic nervous system
Parasympathetic nervous system

Sympathetic

Generally prepares the body for:

Fight or flight

Effects may include:

Heart rate ↑
Pupil dilation
Digestive activity ↓

Parasympathetic

Generally associated with:

Rest and digest

Effects may include:

Heart rate ↓
Digestive activity ↑

70. Generation of Nerve Impulse

A neuron at rest has a:

Resting membrane potential

Typical value:

≈ –70 mV

This is maintained by:

Selective membrane permeability
Na⁺/K⁺ pump
Distribution of ions across the membrane

71. Resting Potential

At rest:

Outside of membrane is relatively positive.
Inside is relatively negative.

The Na⁺/K⁺ pump transports:

3 Na⁺ out + 2 K⁺ in

per ATP cycle.

72. Action Potential

When a sufficient stimulus reaches threshold:

Depolarization

Voltage-gated Na⁺ channels open.
Na⁺ enters.
Membrane potential becomes less negative and may become positive.

Repolarization

Na⁺ channels inactivate.
K⁺ channels open.
K⁺ exits.
Membrane returns toward resting potential.

Hyperpolarization

Membrane potential may briefly become more negative than the resting level before returning to normal.

73. All-or-None Principle

An action potential follows the:

All-or-none law

If threshold is reached:

Full action potential occurs.

If threshold is not reached:

No action potential occurs.

A stronger stimulus above threshold does not produce a larger action potential; it may increase the frequency of action potentials.

74. Conduction of Nerve Impulse

Unmyelinated Axon

Impulse travels continuously along the membrane.

Myelinated Axon

Impulse appears to jump from:

Node of Ranvier → Node of Ranvier

This is called:

Saltatory conduction

It is much faster.

75. Synapse

A synapse is the functional junction between:

Two neurons
A neuron and an effector

Chemical Synapse

Sequence:

Action potential → Ca²⁺ entry → Neurotransmitter release → Receptor binding → Postsynaptic response

Common neurotransmitters include:

Acetylcholine
Noradrenaline
Dopamine
Serotonin
Glutamate
GABA

76. Chemical Coordination and Integration

The endocrine system regulates body functions through:

Hormones

Hormones are chemical messengers released by endocrine cells and transported to target tissues.

They regulate:

Growth
Metabolism
Reproduction
Water balance
Stress responses
Development

77. Endocrine vs Nervous Control

Feature	Nervous system	Endocrine system
Messenger	Neurotransmitter	Hormone
Transport	Neurons	Blood
Speed	Very fast	Usually slower
Duration	Usually short	Often longer
Major role	Rapid responses	Long-term regulation

78. Hypothalamus

The hypothalamus links the nervous and endocrine systems.

Functions include regulation of:

Body temperature
Hunger
Thirst
Water balance
Sleep
Pituitary activity

It produces releasing and inhibiting hormones that regulate the anterior pituitary.

It also synthesizes:

ADH
Oxytocin

These are transported to and released from the posterior pituitary.

79. Pituitary Gland

The pituitary is located below the hypothalamus and is connected to it.

It is often called the:

Master gland

However, its activity itself is controlled largely by the hypothalamus.

Anterior Pituitary Hormones

GH
TSH
ACTH
FSH
LH
Prolactin

Posterior Pituitary Releases

ADH
Oxytocin

80. Growth Hormone

GH promotes:

Body growth
Protein synthesis
Cell growth

Hyposecretion in Childhood

→ Dwarfism

Hypersecretion in Childhood

→ Gigantism

Hypersecretion in Adults

→ Acromegaly

81. Pineal Gland

Produces:

Melatonin

Major functions:

Helps regulate sleep-wake/circadian rhythms.
Influences seasonal and reproductive rhythms in some contexts.

82. Thyroid Gland

Produces:

T₃ — Triiodothyronine
T₄ — Thyroxine

Requires:

Iodine

Functions

Thyroid hormones regulate:

Basal metabolic rate
Growth
Development
Metabolism of carbohydrates, proteins and fats

83. Thyroid Disorders

Hypothyroidism

Low thyroid hormone activity.

In children:

Cretinism — severe untreated congenital hypothyroidism causing impaired physical and mental development.

In adults, hypothyroidism can cause:

Slow metabolism
Fatigue
Weight gain
Cold intolerance

Goitre

Enlargement of thyroid gland.

Iodine deficiency can cause:

Simple goitre

Hyperthyroidism

Excess thyroid hormone activity.

A classic condition is:

Exophthalmic goitre / Graves’ disease

Features may include:

Increased metabolic rate
Nervousness
Weight loss
Increased heart rate
Protruding eyes

84. Parathyroid Glands

Usually four small glands located on the posterior surface of the thyroid.

They secrete:

Parathyroid hormone (PTH)

Functions

PTH increases blood Ca²⁺ levels by promoting:

Bone resorption
Renal Ca²⁺ reabsorption
Activation of vitamin D, indirectly increasing intestinal calcium absorption

85. Adrenal Glands

Located above the kidneys.

Two major regions:

Adrenal cortex
Adrenal medulla

85.1 Adrenal Cortex

Produces steroid hormones including:

Mineralocorticoids
Glucocorticoids
Adrenal androgens

Aldosterone

Na⁺ retention
K⁺ excretion
Helps regulate blood volume and pressure

Cortisol

Functions include:

Stress response
Glucose metabolism
Protein and lipid metabolism
Anti-inflammatory effects

85.2 Adrenal Medulla

Produces:

Adrenaline
Noradrenaline

These hormones support the:

Fight-or-flight response

Effects include:

Heart rate ↑
Blood pressure ↑
Blood glucose availability ↑
Blood flow redistributed toward skeletal muscles

86. Addison’s Disease

Hyposecretion of adrenal cortex hormones can cause:

Addison’s disease

Possible features:

Weakness
Low blood pressure
Weight loss
Electrolyte disturbances
Increased skin pigmentation in primary adrenal insufficiency

87. Pancreas

The endocrine part of pancreas consists of:

Islets of Langerhans

Major hormones:

Insulin
Glucagon

87.1 Insulin

Produced mainly by:

β-cells

Functions:

Decreases blood glucose.
Promotes glucose uptake by tissues.
Promotes glycogenesis.
Promotes storage of nutrients.

87.2 Glucagon

Produced mainly by:

α-cells

Functions:

Increases blood glucose.
Promotes glycogen breakdown.
Promotes glucose production during fasting.

Relationship

Insulin → Blood glucose ↓

Glucagon → Blood glucose ↑

88. Diabetes Mellitus

Diabetes mellitus is characterized by chronic elevation of blood glucose due to inadequate insulin secretion, impaired insulin action, or both.

Type 1 Diabetes

Usually involves autoimmune destruction of pancreatic β-cells.
Insulin deficiency is prominent.
Requires insulin therapy.

Type 2 Diabetes

Insulin resistance is prominent.
Relative insulin deficiency may develop.
Associated with genetic and lifestyle factors.

Possible long-term complications affect:

Eyes
Kidneys
Nerves
Blood vessels
Heart

89. Gonads as Endocrine Organs

The gonads produce sex hormones.

Testes

Main hormone:

Testosterone

Functions:

Male reproductive development
Secondary sexual characteristics
Spermatogenesis support

Ovaries

Major hormones:

Estrogen
Progesterone

Functions:

Female reproductive development
Menstrual cycle regulation
Pregnancy-related functions
Secondary sexual characteristics

90. Mechanism of Hormone Action

Hormones act on specific:

Target cells

because target cells possess appropriate receptors.

Two broad mechanisms:

Cell-surface receptor mechanism
Intracellular receptor mechanism

90.1 Cell-Surface Receptors

Used mainly by:

Peptide hormones
Protein hormones
Many catecholamines

These hormones generally cannot easily cross the lipid membrane.

They bind membrane receptors and activate:

Second messenger systems

Examples:

cAMP
IP₃/DAG
Ca²⁺

90.2 Intracellular Receptors

Used mainly by:

Steroid hormones
Thyroid hormones

These hormones can enter cells and bind intracellular receptors.

The hormone-receptor complex can influence:

Gene transcription → Protein synthesis → Cellular response

91. Hormones as Messengers and Regulators

Hormones act as chemical messengers between:

Endocrine glands
Target tissues

They regulate:

Metabolism
Growth
Development
Reproduction
Water balance
Blood pressure
Stress
Blood glucose

92. Hyposecretion and Hypersecretion

Hyposecretion

Hormone production is lower than normal.

Example:

Insufficient insulin → Diabetes mellitus.

Hypersecretion

Hormone production is excessive.

Example:

Excess GH in adults → Acromegaly.

93. Important Endocrine Disorders — NEET Table

Gland/Hormone	Abnormality	Disorder
GH	Low in childhood	Dwarfism
GH	High in childhood	Gigantism
GH	High in adults	Acromegaly
Thyroid hormones	Low in children	Cretinism
Thyroid	Enlargement	Goitre
Thyroid hormones	Excess	Exophthalmic goitre/Graves’ disease
Insulin	Deficiency/ineffective action	Diabetes mellitus
Adrenal cortex	Hypofunction	Addison’s disease
PTH	Low	Tetany may occur due to hypocalcaemia

94. Most Important Hormones — Quick Table

Gland	Hormone	Main function
Hypothalamus	Releasing hormones	Control pituitary
Pituitary	GH	Growth
Pituitary	TSH	Thyroid stimulation
Pituitary	ACTH	Adrenal cortex stimulation
Pituitary	FSH/LH	Gonadal functions
Pituitary	Prolactin	Lactation
Posterior pituitary	ADH	Water reabsorption
Posterior pituitary	Oxytocin	Uterine contraction/milk ejection
Pineal	Melatonin	Circadian rhythm
Thyroid	T₃/T₄	Metabolism and development
Parathyroid	PTH	Blood Ca²⁺ regulation
Adrenal cortex	Aldosterone	Na⁺/K⁺ balance
Adrenal cortex	Cortisol	Stress/metabolism
Adrenal medulla	Adrenaline	Fight-or-flight
Pancreas	Insulin	Blood glucose ↓
Pancreas	Glucagon	Blood glucose ↑
Testes	Testosterone	Male reproductive functions
Ovaries	Estrogen	Female reproductive functions
Ovaries	Progesterone	Reproductive cycle/pregnancy

95. NEET Rapid Revision — Respiratory System

Gas exchange → Alveoli
Right lung → 3 lobes
Left lung → 2 lobes
Main respiratory muscle → Diaphragm
Inspiration → Thoracic volume ↑
Inspiration → Intrapulmonary pressure ↓
Expiration → Thoracic volume ↓
O₂ transport → Mainly haemoglobin
CO₂ transport → Mainly bicarbonate
CO₂ + H₂O reaction → Mainly catalysed by carbonic anhydrase
Respiratory rhythm centre → Medulla
Pneumotaxic centre → Pons
TV → ≈500 mL
Vital capacity → IRV + TV + ERV
Residual volume → Air left after forced expiration
Asthma → Airway inflammation/bronchoconstriction
Emphysema → Alveolar wall destruction

96. NEET Rapid Revision — Circulation

Blood → Fluid connective tissue
Plasma → ~55%
Formed elements → ~45%
RBC lifespan → ≈120 days
Main O₂ carrier → Haemoglobin
Platelets → Clotting
Universal RBC donor → O negative in simplified ABO/Rh teaching
Universal RBC recipient → AB positive in simplified ABO/Rh teaching
Right ventricle → Pulmonary artery
Left ventricle → Aorta
Pulmonary artery → Deoxygenated blood
Pulmonary vein → Oxygenated blood
SA node → Pacemaker
P wave → Atrial depolarization
QRS → Ventricular depolarization
T wave → Ventricular repolarization
Lub → AV valves close
Dub → Semilunar valves close
Cardiac output → HR × SV
Humans → Double circulation

97. NEET Rapid Revision — Excretion

Ammonia → Ammonotelism
Urea → Ureotelism
Uric acid → Uricotelism
Human excretory unit → Nephron
Filtration → Glomerulus
Filtrate collection → Bowman’s capsule
Major reabsorption → PCT
Water-conserving mechanism → Counter-current mechanism
ADH → Water reabsorption
Renin → Activates RAAS
Angiotensin II → Vasoconstriction + aldosterone stimulation
ANF → Promotes natriuresis and opposes RAAS
Kidney stones → Renal calculi
Uremia → Accumulation of nitrogenous wastes
Dialysis → Removes wastes from blood

98. NEET Rapid Revision — Movement

Ciliary movement → Cilia
Flagellar movement → Flagellum
Skeletal muscle → Striated + voluntary
Contractile unit → Sarcomere
Thin filament → Actin
Thick filament → Myosin
Regulatory proteins → Troponin + Tropomyosin
Muscle contraction trigger → Ca²⁺
Neurotransmitter at NMJ → Acetylcholine
Contraction theory → Sliding filament theory
A-band → Remains constant
I-band → Shortens
H-zone → Shortens
Adult skeleton → 206 bones
Fibrous joint → Skull sutures
Ball-and-socket → Hip/shoulder
Myasthenia gravis → Autoimmune neuromuscular disorder
Tetany → Often associated with low Ca²⁺
Osteoporosis → Reduced bone mass
Gout → Uric acid crystal deposition