जीवविज्ञान – मानव शरीर

हिंदी में पढ़ें

Biology – Human Body: The Machine You Live In

◆ In 1665, an English scientist named Robert Hooke cut a thin slice of cork and looked at it under a microscope he had built himself. He saw tiny boxes, like the rooms of a honeycomb, and called them "cells". Hooke was looking at dead plant walls — but he had just found the word for the building block of every living thing, including you. Right now, as you read this line, your heart is pumping, your kidneys are filtering, your eyes are focusing and your brain is turning these letters into meaning. You never asked any of them to do it. Let us open the machine and see how it works.

What you will master today (Learning Objectives)

The cell — its parts and their jobs

The main organ systems: digestion, breathing, blood flow, excretion, nerves and hormones

Bones, muscles and sense organs — the high-yield "largest / longest / smallest" facts

Vitamins, minerals and the diseases caused by their deficiency

Communicable diseases — which germ causes what, and how it spreads

The examiner's favourite traps in this chapter and how to dodge them

1. The Cell — The Brick of Life

Every living body is built from cells, just as a house is built from bricks. Some organisms, like Amoeba, are a single cell; you are made of an enormous number of them.

Key discoveries:
Robert Hooke (1665) — saw cells in a cork slice and named them.
Robert Brown (1831) — discovered the nucleus.
Schleiden (1838) and Schwann (1839) — proposed the cell theory: all plants and animals are made of cells.
Rudolf Virchow (1855) — added that all cells arise from pre-existing cells.

Parts of a cell and their jobs:
Plasma membrane — the "gatekeeper"; selectively permeable, lets some substances in and out.
Nucleus — the "control room"; holds chromosomes made of DNA, which carry hereditary information.
Mitochondria — the "powerhouse of the cell"; release energy in the form of ATP.
Ribosomes — the "protein factories".
Endoplasmic reticulum — the "internal road network" for moving materials; rough ER carries ribosomes, smooth ER helps make fats (lipids).
Golgi apparatus — the "packing and dispatch department"; also forms lysosomes.
Lysosomes — the "suicide bags"; full of digestive enzymes that clean up waste and worn-out cell parts.
Plastids and cell wall — found in plant cells, not in animal cells. The plant cell wall is made of cellulose.
Vacuoles — large in plant cells, small in animal cells.

Mnemonic — "My Rich Granny Loves Nuts": Mitochondria (power), Ribosomes (protein), Golgi (packing), Lysosomes (cleaning), Nucleus (control).

🧠 Active Recall: Which organelle is called the powerhouse of the cell? Which one is called the suicide bag? (Answer: Mitochondria; Lysosome)

2. Tissues — Teams of Cells

Cells that look alike and do the same job form a tissue. Animal tissues are of four types:
Epithelial tissue — covers and lines (skin surface, lining of the mouth and intestine).
Connective tissue — joins and supports. Bone, cartilage, ligament, tendon and blood are connective tissues. Blood is a fluid connective tissue.
Muscular tissue — helps movement. Striated (skeletal, voluntary), smooth (involuntary, e.g., in the intestine) and cardiac (heart; involuntary and never tires).
Nervous tissue — carries messages; its cell is the neuron.

Trap alert: A ligament joins bone to bone. A tendon joins muscle to bone. Remember "LBB – TMB": Ligament Bone-Bone, Tendon Muscle-Bone.

3. Digestive System — The Food Factory

Food travels through the alimentary canal: mouth → oesophagus (food pipe) → stomach → small intestine → large intestine → anus.

Mouth: Saliva contains salivary amylase, which begins breaking down starch into sugar.
Stomach: Gastric glands release hydrochloric acid (makes the medium acidic and kills germs), pepsin (begins protein digestion) and mucus (protects the stomach lining from the acid).
Liver: The largest gland of the body. It makes bile, which is stored in the gall bladder. Bile breaks large fat globules into small droplets (emulsification) and makes the food alkaline so pancreatic enzymes can act.
Pancreas: Pancreatic juice contains trypsin (proteins), lipase (emulsified fats) and amylase (starch).
Small intestine: The longest part of the alimentary canal. Digestion is completed here, and finger-like projections called villi increase the surface area for absorption.
Large intestine: Absorbs extra water; the rest is removed as faeces.

Trap alert: Pepsin is made in the stomach; trypsin is made in the pancreas. Bile contains no digestive enzyme — it only emulsifies fats.

🧠 Active Recall: Which part of the alimentary canal has villi? Where is bile stored? (Answer: Small intestine; Gall bladder)

4. Respiratory System — Breathing and Burning Fuel

Air path: nostrils → pharynx → larynx → trachea (wind pipe) → bronchi → bronchioles → alveoli. The alveoli are tiny balloon-like sacs where oxygen enters the blood and carbon dioxide leaves it. The rings of cartilage in the trachea keep it from collapsing. The diaphragm, a muscular sheet below the lungs, helps us breathe in and out.

Breathing is not the same as respiration. Breathing is the movement of air in and out of the lungs. Respiration is the release of energy from food inside cells.
Aerobic respiration (with oxygen): glucose → pyruvate (in the cytoplasm) → carbon dioxide + water + a lot of energy (in mitochondria).
Anaerobic respiration in our muscles (shortage of oxygen during hard exercise): pyruvate → lactic acid. Lactic acid build-up causes muscle cramps.
In yeast (anaerobic): pyruvate → ethanol + carbon dioxide (fermentation).

Haemoglobin, the red pigment in red blood cells, carries oxygen. It contains iron.

5. Circulatory System — The Transport Network

The heart has four chambers: two atria above and two ventricles below. The right side handles deoxygenated blood; the left side handles oxygenated blood. The wall of the left ventricle is the thickest because it pumps blood to the whole body. Valves stop the blood from flowing backwards.

Humans have double circulation — blood passes through the heart twice in one complete cycle (once for the lungs, once for the body).

Arteries carry blood away from the heart; they have thick, elastic walls. Veins bring blood back to the heart; they have valves. Capillaries are the thinnest vessels where exchange happens.
Exception (a favourite trap): The pulmonary artery carries deoxygenated blood (heart → lungs). The pulmonary vein carries oxygenated blood (lungs → heart).

Blood components:
Plasma — the fluid part; carries food, CO2, salts and hormones.
Red blood cells (RBC) — carry oxygen using haemoglobin; mature human RBCs have no nucleus; made in the bone marrow.
White blood cells (WBC) — fight infection.
Platelets — help blood to clot at a wound.
Lymph — a colourless fluid that carries digested fat and drains excess fluid back into the blood.

Blood pressure is measured with a sphygmomanometer. Normal systolic pressure is about 120 mm of Hg and diastolic about 80 mm of Hg.

Blood groups: The ABO system was discovered by Karl Landsteiner.
Group O — universal donor (has no A or B antigen on its RBCs).
Group AB — universal recipient (has no anti-A or anti-B antibody in plasma).

Mnemonic — "O Gives, AB Grabs": O is the giver, AB takes from all.

🧠 Active Recall: Which blood vessel carries oxygenated blood but is called a vein? Which chamber of the heart has the thickest wall? (Answer: Pulmonary vein; Left ventricle)

6. Excretory System — The Cleaning Plant

The kidneys filter waste such as urea from the blood. Each kidney has many nephrons — the basic filtering units. Urine passes from the kidneys through the ureters to the urinary bladder and leaves through the urethra. Urea is made in the liver and removed by the kidneys. When the kidneys fail, an artificial kidney (dialysis) is used to filter the blood.

7. Nervous System and Hormones — Control and Coordination

The neuron (nerve cell) is the longest cell in the human body. It has dendrites (receive the message), a cell body and an axon (carries the message away). The gap between two neurons is a synapse, where a chemical passes the message on.

Brain:
Cerebrum (part of the fore-brain) — thinking, memory, intelligence and voluntary actions.
Cerebellum (part of the hind-brain) — balance and posture; precise voluntary movements like riding a bicycle or picking up a pencil.
Medulla (part of the hind-brain) — involuntary actions such as blood pressure, salivation and vomiting.
The brain is protected by the skull and a fluid-filled cushion; the spinal cord is protected by the vertebral column.

Reflex action: a quick, automatic response, such as pulling your hand away from a hot object. The pathway is called a reflex arc, and it passes through the spinal cord.

Endocrine glands (glands that pour hormones directly into the blood):
Pituitary — the "master gland"; growth hormone (too little in childhood → dwarfism).
Thyroid — thyroxin; needs iodine. Iodine shortage → goitre (swollen neck).
Adrenal — adrenaline; prepares the body for emergency ("fight or flight").
Pancreas — insulin, which controls blood sugar. Too little insulin → diabetes.
Testes — testosterone (males). Ovaries — oestrogen (females).

Mnemonic — "Pituitary is the Principal": like a school principal, it controls many other glands.

🧠 Active Recall: Which part of the brain keeps your balance? Which gland needs iodine to make its hormone? (Answer: Cerebellum; Thyroid)

8. Skeleton, Muscles and Sense Organs

An adult human has 206 bones.
Longest and strongest bone — femur (thigh bone).
Smallest bone — stapes (in the middle ear).
The middle ear has three tiny bones: malleus (hammer), incus (anvil) and stapes (stirrup).
Hardest substance in the body — tooth enamel.
Largest organ of the body — skin. Largest internal organ and largest gland — liver.

Eye: The cornea is the clear front window; the iris controls the size of the pupil; the eye lens is a convex lens that forms an image on the retina. The retina has rods (sensitive to dim light) and cones (sensitive to colour). The near point of a normal eye is about 25 cm.
Myopia (near-sightedness) — corrected with a concave lens.
Hypermetropia (far-sightedness) — corrected with a convex lens.
Presbyopia (old age) — often needs bifocal lenses.
Cataract — the eye lens becomes cloudy.

9. Nutrition — Vitamins and Minerals

Fat-soluble vitamins: A, D, E, K (mnemonic: "ADEK — they hide in fat").
Water-soluble vitamins: B group and C.

Vitamin — Chemical name — Deficiency disease
A — Retinol — Night blindness
B1 — Thiamine — Beriberi
B3 — Niacin — Pellagra
B12 — Cobalamin — Pernicious anaemia (B12 contains cobalt)
C — Ascorbic acid — Scurvy (bleeding gums)
D — Calciferol — Rickets in children (soft, bent bones); made in the skin in sunlight
K — Phylloquinone — Delay in blood clotting

Minerals: Iron → anaemia; Iodine → goitre; Calcium → weak bones and teeth.
Protein deficiency in children → kwashiorkor. Shortage of both protein and calories → marasmus.

Mnemonic — "Scurvy Citrus": C for scurvy, C for citrus fruits like lemon and amla. "D for Daylight": the skin makes Vitamin D in sunlight.

🧠 Active Recall: Deficiency of which vitamin causes rickets? Which vitamin contains cobalt? (Answer: Vitamin D; Vitamin B12)

10. Diseases — Which Germ, Which Carrier?

Bacterial: tuberculosis (TB), typhoid, cholera, tetanus, diphtheria, whooping cough, leprosy, plague.
Viral: common cold, influenza, dengue, AIDS (HIV), polio, rabies, measles, chickenpox, hepatitis, COVID-19 (SARS-CoV-2).
Protozoan: malaria (Plasmodium), kala-azar (Leishmania), amoebic dysentery (Entamoeba), sleeping sickness (Trypanosoma).
Fungal: ringworm, athlete's foot.
Worm: elephantiasis (filarial worm).

Carriers (vectors):
Female Anopheles mosquito — malaria.
Aedes mosquito — dengue (and chikungunya).
Culex mosquito — filaria (elephantiasis).
Sandfly — kala-azar.
Tsetse fly — sleeping sickness.
Rat flea — plague. Rabies spreads through the bite of an infected animal such as a dog.

Antibiotics work against bacteria, not against viruses. That is why antibiotics do not cure the common cold.

Milestones: Edward Jenner developed the smallpox vaccine (1796). Alexander Fleming discovered penicillin (1928). BCG is the vaccine against tuberculosis.

Mnemonic — "A-M, A-D, C-F": Anopheles–Malaria, Aedes–Dengue, Culex–Filaria.

🔗 Static-Dynamic Link

The WHO declared smallpox eradicated worldwide in 1980. The WHO certified India (with the South-East Asia Region) polio-free in March 2014. COVID-19, a viral disease caused by SARS-CoV-2, became a pandemic in 2020. Whenever a disease is in the news, exams ask the static fact behind it: Is it viral or bacterial? Which vector spreads it?

🔁 Linguistic Bridge — In Hindi we say this, in English we say that

कोशिका — Cell
माइटोकॉन्ड्रिया / कोशिका का बिजलीघर — Mitochondria / Powerhouse of the cell
लयनकाय — Lysosome
आहार नाल — Alimentary canal
पित्त / पित्ताशय — Bile / Gall bladder
अग्न्याशय — Pancreas
वृक्क — Kidney
फुफ्फुस (फेफड़ा) — Lung
अनुमस्तिष्क — Cerebellum
प्रमस्तिष्क — Cerebrum
मेरुरज्जु — Spinal cord
अवटु ग्रंथि — Thyroid gland
अधिवृक्क ग्रंथि — Adrenal gland
घेंघा — Goitre
रतौंधी — Night blindness
सूखा रोग — Rickets
जीवाणु / विषाणु — Bacteria / Virus

📋 High-Yield Facts at a Glance

⚠️ Caution: Examiner's Trap

Trap 1 — "Arteries always carry oxygenated blood."
Trap Type: Almost Correct Statement. Almost every artery carries oxygenated blood — except the pulmonary artery. Likewise, the pulmonary vein is the only vein that carries oxygenated blood.

Trap 2 — "Which of the following is NOT a fat-soluble vitamin?" with options A, D, K and C.
Trap Type: Negative Phrasing. The word "NOT" is easy to miss. A, D, E, K are fat-soluble; C is water-soluble, so C is the answer.

Trap 3 — Cerebrum vs Cerebellum; Pepsin vs Trypsin; Night blindness vs Colour blindness.
Trap Type: Similar Names. Cerebrum thinks, cerebellum balances. Pepsin works in the stomach, trypsin comes from the pancreas. Night blindness is caused by lack of Vitamin A; colour blindness is a genetic (inherited) disorder, not a vitamin deficiency.

Trap 4 — "Lactic acid causes muscle cramps, so cramps produce lactic acid."
Trap Type: Reverse Causation. Shortage of oxygen in muscles → lactic acid builds up → cramps. The cramp is the result, not the cause.

📊 Summary Table

📅 Revision Planner

Review after 2 days: Read only the summary table and the trap box. Cover the "Answer" column and test yourself.
Weekly review: Redraw the vitamin–disease table and the vector list from memory, then check.
Monthly blurting: On a blank page, write everything you remember about each organ system in five minutes. Compare with the chapter and mark the gaps in red — those are your next revision targets.

Review this chapter again after 2 days. Your body worked without rest while you read it — give your memory the same steady practice.