🧬 Biology · Topic 6 of 10
Start with the study notes on Human Body Systems (definitions, examples and a quick revision), then work through 60 practice questions, each with the correct answer and a clear explanation of why it is right. When you are ready, take this topic as an interactive quiz to test yourself.
Topic overview
The skeleton is the framework of bones that gives the body its shape, supports it, protects organs, and helps us move. An adult has 206 bones.
Cartilage is softer and bendable; it is found in the ear, the nose and the ends of bones.
A joint is where two bones meet.
Remember: Ligaments join bone to bone; tendons join muscle to bone.
Muscles move bones by contracting (getting shorter) and pulling them. A muscle can only pull, so muscles work in pairs.
Example: To bend the arm, the biceps contracts while the triceps relaxes. To straighten it, the triceps contracts.
Breathing is taking air in (inhalation) and pushing it out (exhalation). Respiration is releasing energy from food inside our cells.
Example: Breathe out through lime water and it turns milky, because exhaled air has more carbon dioxide.
The heart, blood vessels and blood carry oxygen, food and wastes around the body.
Excretion is the removal of harmful wastes made by the body.
Example: If the kidneys fail, a dialysis machine filters the blood instead.
The nervous system (the brain, the spinal cord and nerves) controls and coordinates the whole body. Its basic unit is the neuron (nerve cell).
A reflex action is a quick, automatic response, controlled by the spinal cord, that protects the body.
Example: Pulling your hand away from a hot object, blinking, coughing.
Our five sense organs are the eyes, ears, nose, tongue and skin. They detect light, sound, smell, taste and touch.
| Misconception | Correct understanding |
|---|---|
| Breathing and respiration are the same. | Breathing moves air in and out; respiration releases energy in cells. |
| Muscles can push bones. | Muscles can only pull, so they work in pairs. |
| All arteries carry oxygen-rich blood. | Arteries carry blood away from the heart; the pulmonary artery carries oxygen-poor blood. |
| The brain controls every reflex. | Reflexes are controlled by the spinal cord, which makes them very fast. |
| Tendons and ligaments are the same. | Tendons join muscle to bone; ligaments join bone to bone. |
How many bones does an adult have?
206.
Name the joint in the elbow.
A hinge joint.
Why do muscles work in pairs?
A muscle can only pull, so another muscle is needed to pull the bone back.
Where does gas exchange happen in the lungs?
In the alveoli.
What causes muscle cramps during heavy exercise?
Lactic acid from anaerobic respiration.
Which blood cells fight germs?
White blood cells.
Which organs filter the blood to make urine?
The kidneys.
Give an example of a reflex action.
Pulling your hand away from a hot object (or blinking).
Key takeaway: The body's systems work together: bones and muscles move us, lungs and blood carry oxygen, kidneys remove wastes, and nerves control it all.
Notes written by Velnov for Class 6–8 students, following the topics of the NCERT science syllabus.
Q1. easy
The framework of bones that supports the human body is called the:
Answer: A — Skeleton
The skeleton gives the body its shape, supports it, protects delicate organs and works with muscles to produce movement.
Q2. easy
How many bones are there in an adult human body?
Answer: A — 206
An adult has 206 bones. A newborn baby has about 300, but many of them fuse together as the child grows.
Q3. medium
The place where two bones meet is called a:
Answer: A — Joint
Joints allow movement between bones. Some, like those in the skull, are fixed; others, like the elbow and shoulder, are freely movable.
Q4. medium
Which type of joint allows movement in ALL directions, as in the shoulder and hip?
Answer: A — Ball and socket joint
The rounded head of one bone fits into a cup-shaped socket of another, letting the limb rotate and swing in every direction.
Q6. medium
The joint that lets us turn our head from side to side is a:
Answer: A — Pivot joint
A pivot joint between the skull and the top of the backbone allows the head to rotate left and right, and to nod up and down.
Q8. medium
The rib cage protects which organs?
Answer: A — The heart and lungs
Twelve pairs of ribs curve from the backbone to the breastbone, forming a protective cage around the heart and lungs that also moves during breathing.
Q10. medium
A soft, flexible material found at the ends of bones and in the ear and nose is:
Answer: A — Cartilage
Cartilage is softer and more elastic than bone. It cushions joints and gives shape to the outer ear and the tip of the nose.
Q12. medium
The tissue that joins MUSCLE to BONE is called a:
Answer: A — Tendon
Tendons are tough, non-elastic cords that transmit the pull of a contracting muscle to the bone, producing movement.
Q14. medium
Muscles work in pairs because:
Answer: A — A muscle can only pull, so another muscle is needed to move the bone back
In the upper arm, the biceps contracts to bend the elbow while the triceps relaxes; to straighten it, the triceps contracts and the biceps relaxes.
Q16. medium
Muscles that we can move at will, such as those of the arms and legs, are called:
Answer: A — Voluntary muscles
Voluntary (skeletal) muscles are attached to bones and are under conscious control. Involuntary muscles, as in the stomach and intestines, work without our thinking about them.
Q18. medium
Taking air INTO the lungs is called:
Answer: A — Inhalation
During inhalation, the diaphragm moves down and the ribs move up and out, increasing the chest volume so air rushes in.
Q20. medium
The normal breathing rate of an adult at rest is about:
Answer: A — 15-18 times per minute
An adult at rest breathes 15-18 times a minute. The rate rises sharply during exercise because the muscles need more oxygen.
Q22. medium
The air we breathe out contains MORE:
Answer: A — Carbon dioxide and water vapour
Exhaled air has about 4% carbon dioxide compared with 0.04% in inhaled air, and it is warm and saturated with water vapour.
Q24. medium
The tiny air sacs in the lungs where gas exchange takes place are called:
Answer: A — Alveoli
Millions of thin-walled alveoli, each wrapped in blood capillaries, give an enormous surface area for oxygen to enter the blood and carbon dioxide to leave it.
Q26. medium
During heavy exercise, muscles may respire without enough oxygen, producing:
Answer: A — Lactic acid, which causes cramps
This is anaerobic respiration. The build-up of lactic acid causes muscle pain and cramps, which ease when rest allows oxygen to break it down.
Q28. medium
The organ that pumps blood throughout the body is the:
Answer: A — Heart
The heart is a muscular organ about the size of a fist, located in the chest between the lungs, slightly to the left.
Q30. medium
The rhythmic beating of the heart felt in an artery is called the:
Answer: A — Pulse
Each heartbeat sends a wave of pressure along the arteries. Feeling it at the wrist gives the pulse rate, which equals the heart rate.
Q32. medium
Blood vessels that carry blood AWAY from the heart are called:
Answer: A — Arteries
Arteries have thick elastic walls to withstand the high pressure of blood pumped from the heart. They carry oxygen-rich blood, except the pulmonary artery.
Q34. medium
The extremely thin blood vessels where exchange of materials with the cells occurs are:
Answer: A — Capillaries
Capillaries are only one cell thick, so oxygen and nutrients pass out to the tissues and carbon dioxide and wastes pass in. They connect arteries to veins.
Q36. medium
Which blood cells help fight infection?
Answer: A — White blood cells
White blood cells engulf germs and produce antibodies. Their number rises during an infection, which is why doctors order a blood count.
Q38. medium
The liquid part of blood in which cells float is called:
Answer: A — Plasma
Plasma is a pale yellow fluid, mostly water, that carries blood cells, digested food, hormones, waste products and dissolved gases around the body.
Q40. medium
The main excretory organs in humans are the:
Answer: A — Kidneys
The two bean-shaped kidneys filter the blood and produce urine. The lungs (carbon dioxide) and skin (sweat) also excrete wastes.
Q42. medium
Urine formed in the kidneys is stored in the:
Answer: A — Urinary bladder
Urine passes down the ureters to the muscular urinary bladder, where it is stored until it is passed out through the urethra.
Q44. medium
Sweating helps the body by:
Answer: A — Cooling it as the sweat evaporates, and removing some salts and water
Evaporating sweat draws heat from the skin, keeping body temperature steady. It also excretes a small amount of salt and urea.
Q46. medium
The system that controls and coordinates all body activities is the:
Answer: A — Nervous system
The nervous system - brain, spinal cord and nerves - receives information from the senses, processes it and sends instructions to muscles and glands.
Q48. medium
The basic unit of the nervous system is the:
Answer: A — Neuron (nerve cell)
Neurons carry messages as electrical impulses. Their long fibres allow signals to travel between the brain, spinal cord and every part of the body.
Q50. medium
Reflex actions are important because they:
Answer: A — Protect the body by producing an instant response to danger
Blinking at a sudden object, coughing and withdrawing from a hot surface all happen in a fraction of a second, preventing injury.
Q52. medium
The five sense organs of the human body are:
Answer: A — Eyes, ears, nose, tongue and skin
Each sense organ contains receptors that detect a particular stimulus - light, sound, smell, taste or touch - and send signals to the brain.
Q54. medium
Bone marrow, found inside large bones, is important because it:
Answer: A — Makes blood cells
Red bone marrow produces red blood cells, white blood cells and platelets - millions every second, replacing those that wear out.
Q56. medium
Smoking is harmful to the respiratory system because it:
Answer: A — Damages the alveoli and increases the risk of lung cancer and bronchitis
Tobacco smoke destroys the delicate alveolar walls, paralyses the cilia that clear the airways, and contains many cancer-causing chemicals.
Q58. medium
Which overall statement about human body systems is CORRECT?
Answer: A — Different organ systems work together - skeleton and muscles for movement, lungs and blood for oxygen, kidneys for wastes, nerves for control
This sums up the topic: no system works in isolation. The circulatory system, for instance, delivers the oxygen the lungs take in and carries wastes to the kidneys, all under nervous control.
Q59. medium
Why do doctors advise drinking plenty of water every day?
Answer: A — It helps the kidneys flush out wastes and keeps the body's fluids balanced
The kidneys need water to dissolve and carry away urea and salts. Too little water makes the urine concentrated and increases the risk of kidney stones and infections.
Q5. hard
The elbow and knee are examples of a:
Answer: A — Hinge joint
Like the hinge of a door, these joints allow bending and straightening in one plane only. That is why the elbow cannot be twisted sideways.
Q7. hard
The joints between the bones of the skull are:
Answer: A — Fixed (immovable) joints
The skull bones are locked together so the brain is protected inside a rigid case. Only the lower jaw can move.
Q9. hard
The backbone (vertebral column) is made up of small bones called:
Answer: A — Vertebrae
The 33 vertebrae, separated by discs of cartilage, allow the back to bend while protecting the spinal cord running through them.
Q11. hard
The strong tissue that joins BONE to BONE at a joint is called a:
Answer: A — Ligament
Ligaments hold the bones of a joint together while allowing controlled movement. Tendons, by contrast, join muscle to bone.
Q13. hard
Muscles produce movement by:
Answer: A — Contracting to become shorter and pulling the bone
A muscle can only pull, never push. That is why muscles work in pairs - one contracts to bend the joint and the other contracts to straighten it.
Q15. hard
Which muscle works continuously throughout life without tiring?
Answer: A — Cardiac (heart) muscle
Cardiac muscle is found only in the heart. It contracts rhythmically about 70 times a minute, every minute of life, without conscious control or fatigue.
Q17. hard
The process of taking in oxygen and giving out carbon dioxide is called:
Answer: A — Breathing
Breathing is the physical movement of air in and out of the lungs. Respiration is the chemical release of energy from food inside the cells.
Q19. hard
During inhalation, the diaphragm:
Answer: A — Moves down and flattens, increasing the space in the chest
The dome-shaped diaphragm below the lungs flattens on contracting, and the rib cage lifts. Together they enlarge the chest cavity and draw air in.
Q21. hard
Why does the breathing rate increase during exercise?
Answer: A — Muscles need more oxygen and produce more carbon dioxide that must be removed
Faster, deeper breathing supplies the extra oxygen required to release energy from food and clears the carbon dioxide produced more quickly.
Q23. hard
Exhaled air turns lime water milky. This shows it contains:
Answer: A — Carbon dioxide
Carbon dioxide reacts with lime water to form insoluble calcium carbonate, making it cloudy. This is the standard test for CO2.
Q25. hard
Respiration in which oxygen is used to break down food is called:
Glucose + Oxygen → Carbon dioxide + Water + EnergyAnswer: A — Aerobic respiration
Aerobic respiration breaks glucose completely into carbon dioxide and water, releasing a large amount of energy. It is the normal form in humans.
Q27. hard
Yeast respires anaerobically, producing:
Answer: A — Alcohol and carbon dioxide
This process, called fermentation, is used to make bread rise (the CO2 forms bubbles) and to produce alcoholic drinks.
Q29. hard
The human heart has how many chambers?
Answer: A — Four
Two upper chambers (atria) receive blood and two lower chambers (ventricles) pump it out. The four-chambered design keeps oxygen-rich and oxygen-poor blood completely separate.
Q31. hard
The normal pulse rate of a resting adult is about:
Answer: A — 72-80 beats per minute
A healthy adult's heart beats roughly 72 times a minute at rest. Children's hearts beat faster, and the rate rises with exercise, excitement or fever.
Q33. hard
Blood vessels that carry blood BACK to the heart are called:
Answer: A — Veins
Veins have thinner walls and valves that prevent blood flowing backwards. They carry carbon dioxide-rich blood, except the pulmonary vein.
Q35. hard
The red pigment in blood that carries oxygen is:
Answer: A — Haemoglobin
Haemoglobin, present in red blood cells, contains iron and binds oxygen in the lungs, releasing it in the tissues. It gives blood its red colour.
Q37. hard
Which component of blood helps in CLOTTING when we get a cut?
Answer: A — Platelets
Platelets gather at the wound and trigger the formation of a mesh of fibres that traps blood cells, forming a clot and stopping the bleeding.
Q39. hard
The removal of harmful metabolic wastes from the body is called:
Answer: A — Excretion
Excretion removes wastes produced by the body's own chemical processes, such as urea. Egestion removes undigested food, which never entered the body's cells.
Q41. hard
The functional filtering units of the kidney are called:
Answer: A — Nephrons
Each kidney contains about a million nephrons. They filter the blood, reabsorb useful substances such as glucose and most of the water, and leave urine behind.
Q43. hard
The main nitrogenous waste in human urine is:
Answer: A — Urea
The liver converts excess amino acids into urea, which the kidneys filter out of the blood. Urine also contains salts and water.
Q45. hard
A patient whose kidneys have failed may be treated with:
Answer: A — Dialysis, in which a machine filters the blood
In dialysis, blood is passed through an artificial filtering system that removes urea and excess salts and water, doing the kidneys' job until a transplant is possible.
Q47. hard
The control centre of the nervous system is the:
Answer: A — Brain
The brain, protected inside the skull, controls thinking, memory, senses, movement and the automatic functions such as breathing and heartbeat.
Q49. hard
Quickly pulling your hand away from a hot object is an example of:
Answer: A — A reflex action
A reflex action is an immediate, automatic response controlled by the spinal cord rather than the brain, so it happens before you consciously feel the pain.
Q51. hard
The spinal cord is protected by the:
Answer: A — Vertebral column (backbone)
The spinal cord runs through a canal formed by the stacked vertebrae, which shield it while allowing the back to bend.
Q53. hard
Which system carries oxygen, nutrients, hormones and wastes around the body?
Answer: A — The circulatory system
The heart, blood vessels and blood together form the circulatory system, the body's transport network linking every organ.
Q55. hard
Why is regular exercise good for the body systems?
Answer: A — It strengthens muscles and bones, and improves heart and lung efficiency
Exercise builds muscle, keeps bones dense, makes the heart pump more efficiently and increases lung capacity, while also improving mood and sleep.
Q57. hard
Why should we sit and stand with correct posture?
Answer: A — Poor posture strains the backbone and muscles and can cause lasting deformity and pain
The backbone is a stack of small bones held by muscles and ligaments. Slouching or carrying heavy bags on one shoulder puts uneven strain on it, causing pain and curvature over time.
Q60. hard
Why does a person breathe faster and feel out of breath at high altitudes?
Answer: A — The air is thinner there, so each breath supplies less oxygen
Air pressure falls with altitude, so the same volume of air contains fewer oxygen molecules. The body compensates by breathing faster and, over weeks, by making more red blood cells.