Human Body Systems: Diagrams, Facts, Tricks and MCQs

The human body is a remarkable machine made of trillions of cells. These cells join to form tissues, tissues form organs, and organs work together in teams called organ systems. Each system has a clear job – the digestive system breaks down food, the respiratory system brings in oxygen, the circulatory system carries blood, the nervous system controls everything, and the excretory system removes waste. No system works alone; together they keep us alive, healthy and active.

Human Body is one of the most scoring parts of General Science in SSC CGL, CHSL, MTS, Railways (RRB NTPC, Group D), WBCS, WBPSC Clerkship, Police and Banking exams. Questions are asked on bones and muscles, digestive enzymes, heart and blood circulation, blood groups, the brain, hormones, vitamins and their deficiency diseases, and the "largest and smallest" facts of the body. This guide explains every system in simple language with clear diagrams, easy tables, a list of important numbers, a quick revision box with memory tricks and 20 MCQs with answers.

➤ Levels of organisation: Cell → Tissue → Organ → Organ system → Organism
➤ Bones in an adult: 206 · Largest bone: femur · Smallest bone: stapes (ear)
➤ Heart: 4 chambers · Normal heartbeat about 72 per minute · Normal BP about 120/80 mm Hg
➤ Blood: About 5–6 litres in an adult · RBC life span about 120 days · Universal donor O, universal recipient AB
➤ Largest organ: Skin · Largest internal organ and largest gland: Liver · Master gland: Pituitary
➤ Brain: Cerebrum (thinking), Cerebellum (balance), Medulla oblongata (heartbeat and breathing)
➤ Functional units: Neuron (nervous system) · Nephron (kidney) · Alveoli (lungs, gas exchange)

Levels of Organisation in the Human Body

The cell is the basic structural and functional unit of life. A group of similar cells doing the same job is called a tissue. Different tissues join to make an organ (like the heart or stomach). Several organs that work together for one major function form an organ system. All organ systems together make a complete organism – the human being.

Four Basic Types of Animal Tissue

Tissue	Main job	Examples
Epithelial tissue	Covers and protects body surfaces, lines organs	Skin surface, lining of mouth, stomach and blood vessels
Connective tissue	Joins, supports and binds other tissues	Bone, cartilage, ligament, tendon, blood, fat (adipose)
Muscular tissue	Movement by contraction	Skeletal, smooth and cardiac muscles
Nervous tissue	Carries messages (nerve impulses)	Brain, spinal cord, nerves

Exam point: Blood is a fluid connective tissue. Ligament joins bone to bone, while tendon joins muscle to bone.

Major Organ Systems at a Glance

System	Main organs	Main function
Skeletal	Bones, cartilage, joints	Shape, support, protection, movement, makes blood cells
Muscular	Skeletal, smooth and cardiac muscles	Movement and posture
Digestive	Mouth, oesophagus, stomach, intestines, liver, pancreas	Breaks down and absorbs food
Respiratory	Nose, trachea, lungs	Takes in oxygen, removes carbon dioxide
Circulatory	Heart, blood, blood vessels	Transports oxygen, food, hormones and wastes
Lymphatic and Immune	Lymph nodes, spleen, thymus, white blood cells	Fights infection, returns fluid to blood
Excretory	Kidneys, ureters, urinary bladder, urethra	Removes nitrogen wastes like urea
Nervous	Brain, spinal cord, nerves	Fast control and coordination
Endocrine	Pituitary, thyroid, adrenal, pancreas and other glands	Slow control through hormones
Reproductive	Testes, ovaries, uterus	Produces offspring
Integumentary	Skin, hair, nails	Protection, temperature control, sensation

Skeletal System – The Body's Framework

The skeleton is the frame of the body. It gives shape, supports the body, protects soft organs (the skull protects the brain, the ribs protect the heart and lungs), helps in movement along with muscles, stores calcium and phosphorus, and makes blood cells in the red bone marrow. A newborn baby has about 300 bones, but many of them fuse as the child grows, leaving 206 bones in an adult.

Important Bone Facts

Fact	Answer
Total bones in an adult	206
Largest and longest bone	Femur (thigh bone)
Smallest bone	Stapes (stirrup) in the middle ear
Bones of the middle ear	Malleus (hammer), Incus (anvil), Stapes (stirrup)
Ribs	12 pairs (24 ribs)
Vertebrae (backbone)	33 in a child; 26 in an adult because some fuse
Only movable bone of the skull	Mandible (lower jaw)
Collar bone	Clavicle
Hardest substance in the body	Tooth enamel
Main mineral in bone	Calcium phosphate

Types of Joints

Joint	Movement	Example
Ball and socket	All directions	Shoulder, hip
Hinge	One direction, like a door	Elbow, knee
Pivot	Rotation	Neck (between the first two vertebrae – atlas and axis)
Gliding	Bones slide over each other	Wrist, ankle
Fixed (immovable)	No movement	Bones of the skull

Muscular System – Movement and Strength

Muscles move the body by contracting (becoming shorter) and relaxing. The human body has more than 600 muscles. Muscles usually work in pairs – when the biceps contracts the arm bends, and when the triceps contracts the arm straightens. Muscles that we control by will are called voluntary muscles; muscles that work on their own are called involuntary muscles.

Type	Control	Look under microscope	Found in
Skeletal muscle	Voluntary	Striped (striated)	Arms, legs, attached to bones
Smooth muscle	Involuntary	Not striped	Stomach, intestine, blood vessels, iris
Cardiac muscle	Involuntary	Striped, branched	Only in the heart; never gets tired

Record muscles: Largest muscle – gluteus maximus (buttock); longest muscle – sartorius (thigh); smallest muscle – stapedius (middle ear). Lactic acid builds up in muscles during hard exercise and causes muscle fatigue (cramps).

Digestive System – From Food to Energy

Digestion is the process of breaking big, complex food molecules into small, simple ones that the body can absorb. Carbohydrates become glucose, proteins become amino acids, and fats become fatty acids and glycerol. The food travels through a long tube called the alimentary canal, and juices from the liver and pancreas join it on the way. Figure 1 shows the path of food.

Figure 1: Path of food through the alimentary canal, with the liver, gall bladder and pancreas adding their juices to the small intestine

Digestive Juices and Enzymes

Where	Juice / Enzyme	What it does
Mouth	Saliva – salivary amylase (ptyalin)	Starch → maltose (sugar)
Stomach	Hydrochloric acid (HCl)	Makes the medium acidic, kills germs, activates pepsin
Stomach	Pepsin	Proteins → peptides
Stomach (in infants)	Rennin	Curdles milk protein
Liver	Bile (no enzyme)	Breaks fat into tiny droplets (emulsification), makes medium alkaline
Pancreas	Trypsin	Proteins → peptides
Pancreas	Pancreatic amylase	Starch → maltose
Pancreas	Lipase	Fats → fatty acids + glycerol
Small intestine	Intestinal juice (maltase, sucrase, lactase, peptidases)	Completes digestion into glucose and amino acids

Teeth: An adult has 32 permanent teeth – 8 incisors (cut), 4 canines (tear), 8 premolars and 12 molars (grind). A child has 20 milk teeth. The last molars to come out are called wisdom teeth. Tooth decay happens when bacteria convert sugar into acid that dissolves enamel.

Exam point: Most digestion and absorption take place in the small intestine. The finger-like villi increase the surface area for absorption. The liver also stores glycogen, makes urea and removes poisons from the blood.

Respiratory System – Breathing and Gas Exchange

We breathe in air that has oxygen and breathe out air that has more carbon dioxide. Air enters through the nose, where hair and mucus filter dust and the air is warmed. It then passes through a series of tubes that become smaller and smaller until it reaches millions of tiny air sacs called alveoli inside the lungs. Here, oxygen passes into the blood and carbon dioxide passes out of it. Figure 2 shows the pathway.

Figure 2: Pathway of air from the nose to the alveoli, where oxygen and carbon dioxide are exchanged

Breathing vs Respiration

Breathing	Respiration (cellular)
Physical process of taking air in and out	Chemical process of breaking glucose to release energy
Happens in the lungs	Happens inside cells, mainly in mitochondria
No energy is produced	Energy is stored as ATP

Aerobic respiration (with oxygen): Glucose + Oxygen → Carbon dioxide + Water + Energy. Anaerobic respiration in muscles (without enough oxygen) makes lactic acid. Mitochondria are called the "powerhouse of the cell". Haemoglobin in red blood cells carries oxygen. An adult at rest breathes about 12–20 times per minute. The epiglottis is a flap that closes the windpipe when we swallow, so food does not enter it.

Circulatory System – Heart, Blood and Blood Vessels

The circulatory system is the body's transport network. The heart is a muscular pump about the size of a fist. It has four chambers – two upper atria (receiving chambers) and two lower ventricles (pumping chambers). The right side of the heart handles deoxygenated blood (low in oxygen), and the left side handles oxygenated blood (rich in oxygen). A wall called the septum keeps the two kinds of blood from mixing.

Blood passes through the heart twice in one complete round of the body, so human circulation is called double circulation. The short loop from the heart to the lungs and back is pulmonary circulation; the long loop from the heart to the whole body and back is systemic circulation. Follow the arrows in Figure 3.

Figure 3: Double circulation – blue arrows show deoxygenated blood and red arrows show oxygenated blood

Remember the path: Body → Vena cava → Right atrium → (tricuspid valve) → Right ventricle → Pulmonary artery → Lungs → Pulmonary vein → Left atrium → (bicuspid valve) → Left ventricle → Aorta → Body.

Two famous exceptions: The pulmonary artery is the only artery that carries deoxygenated blood, and the pulmonary vein is the only vein that carries oxygenated blood. The heartbeat starts at the SA node (sino-atrial node) in the right atrium, which is called the natural pacemaker. William Harvey described blood circulation in 1628, and René Laennec invented the stethoscope in 1816.

Three Types of Blood Vessels

Artery	Vein	Capillary
Carries blood away from the heart	Carries blood towards the heart	Connects arteries and veins
Thick, elastic walls; high pressure	Thin walls; low pressure	Walls only one cell thick
No valves	Has valves to stop backflow	Exchange of gases, food and wastes with cells
Largest artery: aorta	Largest vein: vena cava	Finest blood vessels

Blood – The Fluid of Life

Blood is a fluid connective tissue. An adult has about 5–6 litres of blood. It is slightly alkaline, with a pH of about 7.4. Blood has two parts – a pale yellow liquid called plasma and the blood cells floating in it (Figure 4).

Figure 4: Composition of blood – plasma and the three kinds of blood cells

The spleen is known as the "graveyard of RBCs" because old red blood cells are broken down there. A shortage of haemoglobin or RBCs causes anaemia. Haemophilia is a genetic disease in which blood does not clot properly; it is also called the "royal disease" because it ran in Queen Victoria's family. Vitamin K is needed for blood clotting.

Blood Groups (ABO System)

Karl Landsteiner discovered the ABO blood groups in 1900–1901 and received the Nobel Prize in 1930. Blood group depends on the antigens present on the surface of red blood cells. The Rh factor was discovered later by Landsteiner and Wiener in 1940; people having it are Rh positive.

Blood group	Antigen on RBC	Antibody in plasma	Can give 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 groups (universal recipient)
O	None	anti-A and anti-B	All groups (universal donor)	O only

Blood pressure: Normal BP of a healthy adult is about 120/80 mm Hg – 120 is the systolic pressure (when the ventricles contract) and 80 is the diastolic pressure (when they relax). BP is measured with a sphygmomanometer. A normal adult heart beats about 72 times per minute.

Lymphatic and Immune System – The Body's Defence

The lymphatic system is a network of thin vessels that carry a colourless fluid called lymph. Lymph returns extra tissue fluid to the blood and carries fats absorbed from the intestine. Lymph nodes act like filters that trap germs. The spleen, thymus and tonsils are lymphatic organs. The thymus helps T-lymphocytes mature and is larger in children.

The immune system protects us from germs. The first line of defence is the skin, mucus and stomach acid. White blood cells eat germs or make antibodies against them. A vaccine trains the immune system by showing it a harmless form of a germ. Edward Jenner developed the first vaccine against smallpox in 1796. AIDS is caused by HIV, a virus that attacks the T-helper cells of the immune system.

Excretory System – Cleaning the Blood

Our cells produce harmful nitrogen wastes when they break down proteins. The liver turns ammonia into less harmful urea, and the kidneys filter urea out of the blood. The two bean-shaped kidneys lie on either side of the backbone. Each kidney has about one million tiny filtering units called nephrons. The urine they make flows down two tubes called ureters into the urinary bladder, and leaves the body through the urethra (Figure 5).

Figure 5: Human excretory system – kidneys, ureters, urinary bladder and urethra

How a nephron works: Blood enters a tiny ball of capillaries called the glomerulus, which sits inside a cup called Bowman's capsule. Water, salts, glucose and urea are filtered out. As the fluid passes through the long tubule, useful things like glucose and most of the water are taken back into the blood. What remains becomes urine. The hormone ADH (from the pituitary) controls how much water is taken back.

Other excretory organs: Lungs remove carbon dioxide, skin removes sweat (water and salts), and the liver removes bile pigments. When both kidneys fail, a machine cleans the blood through dialysis (artificial kidney), or a kidney transplant is done. Kidney stones are usually made of calcium oxalate.

Nervous System – The Control Centre

The nervous system controls and coordinates every activity of the body. It collects information from the sense organs, decides what to do, and sends orders to muscles and glands within a fraction of a second. It has two main parts – the Central Nervous System (brain and spinal cord) and the Peripheral Nervous System (12 pairs of cranial nerves from the brain and 31 pairs of spinal nerves from the spinal cord). The brain is protected by the skull and three membranes called meninges; the spinal cord is protected by the backbone.

Parts of the Brain

An adult human brain weighs about 1.3–1.4 kg. Figure 6 shows its main parts and their jobs.

Figure 6: Main parts of the human brain and their functions

Part	Section	Function
Cerebrum	Forebrain	Thinking, memory, reasoning, speech, voluntary actions, senses
Thalamus	Forebrain	Relay centre for sensory messages
Hypothalamus	Forebrain	Body temperature, hunger, thirst, sleep; controls the pituitary
Midbrain	Midbrain	Reflexes of eyes and ears (seeing and hearing)
Cerebellum	Hindbrain	Balance, posture and coordination of muscles
Pons	Hindbrain	Helps control breathing; bridge between parts
Medulla oblongata	Hindbrain	Involuntary actions – heartbeat, breathing, BP, swallowing, vomiting

Neuron – The Nerve Cell

The neuron is the structural and functional unit of the nervous system. It is the longest cell in the human body. Dendrites receive messages, the cell body processes them, and the long axon carries the electrical message (nerve impulse) away. The fatty myelin sheath around the axon makes the message travel faster. The tiny gap between two neurons is called a synapse, where chemicals pass the message to the next neuron (Figure 7).

Figure 7: Structure of a neuron – the message travels from dendrites to axon terminals

Reflex Action and Reflex Arc

When you touch a hot pan, your hand jerks back even before you feel the pain. Such a quick, automatic response that happens without thinking is called a reflex action. It is controlled mainly by the spinal cord, not the brain, which saves precious time. Sneezing, coughing, blinking and the knee-jerk are other examples. The path the message follows is called a reflex arc (Figure 8).

Figure 8: Reflex arc – Stimulus → Receptor → Sensory neuron → Spinal cord → Motor neuron → Effector → Response

Endocrine System – Glands and Hormones

Endocrine glands are ductless glands. They release chemical messengers called hormones directly into the blood. Hormones act slowly but their effect lasts longer than that of nerve messages. They control growth, metabolism, sugar level, reproduction and our response to stress. Too much or too little of a hormone causes disease.

Gland	Location	Main hormone(s)	Work / Disorder
Pituitary ("master gland")	Base of the brain	Growth hormone, TSH, FSH, LH, ADH, oxytocin, prolactin	Controls other glands. Excess growth hormone → gigantism; less → dwarfism. Less ADH → diabetes insipidus
Thyroid (largest endocrine gland)	Neck	Thyroxine (contains iodine)	Controls metabolism. Iodine deficiency → goitre
Parathyroid	Behind the thyroid	Parathormone	Controls calcium level in blood
Adrenal ("emergency gland")	On top of each kidney	Adrenaline, cortisol	Adrenaline prepares the body for "fight or flight"
Pancreas (both endocrine and exocrine)	Behind the stomach	Insulin (β-cells), glucagon (α-cells) from the Islets of Langerhans	Insulin lowers blood sugar; less insulin → diabetes mellitus
Pineal	Brain	Melatonin	Sleep–wake cycle (body clock)
Thymus	Chest	Thymosin	Development of immunity (T-cells)
Testes	Males	Testosterone	Male features
Ovaries	Females	Oestrogen, progesterone	Female features, menstrual cycle, pregnancy

Exam point: Insulin was discovered by Frederick Banting and Charles Best in 1921. The liver is the largest gland of the body, but the thyroid is the largest endocrine gland. Hormones are also called "chemical messengers".

Reproductive System – Continuing Life

Humans reproduce sexually. The male reproductive organs (testes) produce sperm, and the female reproductive organs (ovaries) produce eggs (ova). Fertilisation – the joining of a sperm and an egg – normally takes place in the fallopian tube (oviduct). The fertilised egg, called a zygote, divides and fixes itself in the wall of the uterus (implantation). The growing baby gets food and oxygen from the mother through the placenta. A normal pregnancy lasts about 9 months (about 280 days). The sex of a child depends on the father: an X chromosome from the father gives a girl (XX) and a Y chromosome gives a boy (XY).

Sense Organs – Eye, Ear, Nose, Tongue and Skin

Part	Job
Cornea	Transparent front window of the eye; can be donated and transplanted
Iris and pupil	Iris gives eye colour and controls the size of the pupil
Lens	Focuses light on the retina; makes a real, inverted image
Retina	Light-sensitive screen. Rods – dim light; Cones – colour and bright light
Optic nerve	Carries the message to the brain; the point where it leaves the eye is the blind spot
Ear – Cochlea	Hearing
Ear – Semicircular canals	Balance of the body
Ear – Eardrum and ear ossicles	Vibrate and pass sound inward
Skin	Largest organ; touch, pain, heat; melanin gives skin colour and protects from UV rays

Eye defect	Problem	Correction
Myopia (short-sightedness)	Cannot see far objects clearly	Concave lens
Hypermetropia (long-sightedness)	Cannot see near objects clearly	Convex lens
Presbyopia	Old-age weakening of focusing power	Bifocal lens
Astigmatism	Uneven curvature of the cornea	Cylindrical lens
Cataract	Lens becomes cloudy	Surgery – lens replacement

Vitamins, Minerals and Deficiency Diseases

Vitamins are needed in very small amounts but are essential for health. The word "vitamine" was coined by Casimir Funk in 1912. Vitamins A, D, E and K dissolve in fat (fat-soluble) and can be stored in the body; vitamins B and C dissolve in water (water-soluble) and must be taken regularly.

Vitamin / Mineral	Chemical name	Deficiency disease	Good sources
Vitamin A	Retinol	Night blindness, xerophthalmia	Carrot, milk, fish liver oil, green vegetables
Vitamin B1	Thiamine	Beriberi	Whole grains, pulses
Vitamin B2	Riboflavin	Cracking of lips and corners of mouth	Milk, eggs
Vitamin B3	Niacin	Pellagra	Meat, groundnut
Vitamin B12	Cyanocobalamin (contains cobalt)	Pernicious anaemia	Meat, milk, eggs
Vitamin C	Ascorbic acid	Scurvy (bleeding gums)	Amla, lemon, orange, guava
Vitamin D	Calciferol	Rickets (children), osteomalacia (adults)	Sunlight, milk, fish
Vitamin E	Tocopherol	Reproductive problems	Vegetable oils, nuts
Vitamin K	Phylloquinone	Blood does not clot properly	Green leafy vegetables
Iron	–	Anaemia	Spinach, jaggery, meat
Iodine	–	Goitre	Iodised salt, sea food
Calcium	–	Weak bones and teeth	Milk, cheese

Important Numbers of the Human Body

Item	Normal value (adult)
Body temperature	37 °C (98.6 °F)
Heart rate	About 72 beats per minute
Blood pressure	About 120/80 mm Hg
Blood volume	About 5–6 litres
Blood pH	About 7.4 (slightly alkaline)
RBC count	About 4.5–5.5 million per cubic millimetre
WBC count	About 4,000–11,000 per cubic millimetre
Platelet count	About 1.5–4 lakh per cubic millimetre
Life span of RBC	About 120 days
Breathing rate at rest	About 12–20 per minute
Bones / Teeth	206 bones / 32 teeth (20 milk teeth)
Chromosomes	46 (23 pairs)
Cranial / Spinal nerves	12 pairs / 31 pairs
Brain weight	About 1.3–1.4 kg

Largest, Smallest and Longest – Record Facts

Record	Answer
Largest organ	Skin
Largest internal organ / Largest gland	Liver
Largest endocrine gland	Thyroid
Largest and longest bone	Femur
Smallest bone	Stapes
Largest muscle / Longest muscle	Gluteus maximus / Sartorius
Longest cell	Neuron (nerve cell)
Largest artery / Largest vein	Aorta / Vena cava
Largest part of the brain	Cerebrum
Largest white blood cell	Monocyte
Most common WBC	Neutrophil
Hardest substance	Tooth enamel

West Bengal Connection – Kolkata and Medical Science

Kolkata has a proud place in the history of medical science in India. Questions on these facts are popular in WBCS, WBPSC and West Bengal Police exams.

Event	Details
Medical College, Calcutta (1835)	One of the first colleges in Asia to teach modern Western medicine. Pandit Madhusudan Gupta performed the first human dissection by an Indian here in 1836.
Sir Ronald Ross	Proved that mosquitoes spread malaria. He found the parasite in a mosquito in 1897 at Secunderabad and completed his work on bird malaria in Calcutta in 1898. He received the Nobel Prize in Medicine in 1902. His laboratory building at today's SSKM Hospital (IPGME&R), Kolkata, is remembered as a memorial.
Dr Upendranath Brahmachari	Discovered Urea Stibamine (1922), a drug that saved lakhs of lives from kala-azar, while working at the Campbell Medical School (now Nil Ratan Sircar Medical College), Kolkata.
Durga – India's first test-tube baby	Born in Kolkata on 3 October 1978 through the work of Dr Subhas Mukhopadhyay, only about two months after the world's first test-tube baby, Louise Brown (UK).

For more science questions from exams, practise our CSIR NAL MTS Mock Test with Solutions, and read about Indian science outreach in Darbhanga Planetarium's ISRO Space Gallery and the VAIBHAV Fellowship for Indian-origin scientists. You can find all our biology and physics notes in the Science section.

20 MCQs on the Human Body and Its Systems

Q1. How many bones are there in the body of an adult human?
(a) 300
(b) 212
(c) 206
(d) 196

Answer: (c) – An adult has 206 bones; a newborn has about 300, many of which fuse later.

Q2. The smallest bone of the human body is:
(a) Stapes
(b) Femur
(c) Malleus
(d) Incus

Answer: (a) – The stapes (stirrup) is in the middle ear; the femur is the largest bone.

Q3. Which tissue connects a muscle to a bone?
(a) Ligament
(b) Cartilage
(c) Tendon
(d) Epithelium

Answer: (c) – A tendon joins muscle to bone, while a ligament joins bone to bone.

Q4. Salivary amylase (ptyalin) acts on:
(a) Proteins
(b) Starch
(c) Fats
(d) Vitamins

Answer: (b) – Salivary amylase changes starch into maltose in the mouth.

Q5. Bile is produced by the:
(a) Gall bladder
(b) Pancreas
(c) Stomach
(d) Liver

Answer: (d) – Bile is made by the liver and only stored in the gall bladder. It emulsifies fats.

Q6. Most of the digestion and absorption of food takes place in the:
(a) Small intestine
(b) Stomach
(c) Large intestine
(d) Oesophagus

Answer: (a) – Villi in the small intestine absorb digested food; the large intestine mainly absorbs water.

Q7. Exchange of gases in the lungs takes place in the:
(a) Trachea
(b) Bronchi
(c) Alveoli
(d) Larynx

Answer: (c) – Alveoli are tiny air sacs surrounded by capillaries where O₂ enters and CO₂ leaves the blood.

Q8. Which blood vessel carries deoxygenated blood from the heart to the lungs?
(a) Pulmonary vein
(b) Pulmonary artery
(c) Aorta
(d) Vena cava

Answer: (b) – The pulmonary artery is the only artery that carries deoxygenated blood.

Q9. The valve between the left atrium and the left ventricle is the:
(a) Tricuspid valve
(b) Semilunar valve
(c) Pulmonary valve
(d) Bicuspid (mitral) valve

Answer: (d) – The tricuspid valve is on the right side; the bicuspid or mitral valve is on the left side.

Q10. The natural pacemaker of the human heart is the:
(a) SA node
(b) AV node
(c) Septum
(d) Aorta

Answer: (a) – The sino-atrial (SA) node in the right atrium starts each heartbeat.

Q11. A person with blood group O is called a universal donor because his RBCs have:
(a) Both A and B antigens
(b) Only A antigen
(c) No A or B antigen
(d) No antibodies in plasma

Answer: (c) – With no A or B antigen on its RBCs, group O blood is not attacked by the receiver's antibodies.

Q12. Who discovered the ABO blood group system?
(a) William Harvey
(b) Karl Landsteiner
(c) Edward Jenner
(d) Louis Pasteur

Answer: (b) – Karl Landsteiner discovered ABO groups (1900–01); William Harvey described blood circulation.

Q13. The "graveyard of RBCs" is the:
(a) Liver
(b) Kidney
(c) Bone marrow
(d) Spleen

Answer: (d) – Old red blood cells are broken down in the spleen; RBCs are made in the red bone marrow.

Q14. The structural and functional unit of the kidney is the:
(a) Nephron
(b) Neuron
(c) Alveolus
(d) Villus

Answer: (a) – Each kidney has about one million nephrons that filter blood and form urine.

Q15. Which part of the brain maintains balance and posture of the body?
(a) Cerebrum
(b) Cerebellum
(c) Medulla oblongata
(d) Hypothalamus

Answer: (b) – The cerebellum coordinates muscles and maintains balance; the medulla controls heartbeat and breathing.

Q16. A reflex action is mainly controlled by the:
(a) Cerebrum
(b) Heart
(c) Spinal cord
(d) Pituitary gland

Answer: (c) – In a reflex arc, the spinal cord sends a quick response before the brain is involved.

Q17. Which gland is known as the "master gland"?
(a) Thyroid
(b) Adrenal
(c) Pancreas
(d) Pituitary

Answer: (d) – The pituitary controls many other endocrine glands; the thyroid is the largest endocrine gland.

Q18. Deficiency of insulin causes:
(a) Diabetes mellitus
(b) Diabetes insipidus
(c) Goitre
(d) Dwarfism

Answer: (a) – Less insulin raises blood sugar (diabetes mellitus). Diabetes insipidus is due to shortage of ADH.

Q19. Scurvy is caused by deficiency of:
(a) Vitamin A
(b) Vitamin D
(c) Vitamin C
(d) Vitamin K

Answer: (c) – Lack of vitamin C (ascorbic acid) causes scurvy, with bleeding gums. Amla is a rich source.

Q20. India's first test-tube baby, Durga, was born in 1978 in which city?
(a) Mumbai
(b) Kolkata
(c) Chennai
(d) New Delhi

Answer: (b) – Durga was born in Kolkata on 3 October 1978 through the work of Dr Subhas Mukhopadhyay.

Want more practice? Try our Daily GK Quiz and Mock Tests.

Conclusion

The human body works like a well-run team. Bones and muscles give shape and movement, the digestive and respiratory systems supply food and oxygen, the heart and blood carry them everywhere, the kidneys clean the blood, and the nervous and endocrine systems control it all. For exams, focus on the record facts (largest, smallest, longest), digestive enzymes, the path of blood through the heart, blood groups, the parts of the brain, glands and hormones, vitamins and their deficiency diseases, and the normal body numbers. Revise the figures and memory tricks above, and practise the MCQs before your exam.

Reference: NCERT Science Class VI–X (Life Processes; Control and Coordination; How do Organisms Reproduce?); NCERT Biology Class XI–XII (Human Physiology).