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These are complete exercise solutions for Samacheer Kalvi 9th Science — Organ Systems in Animals. Solve each question yourself first, then verify your method and answer below. Focus on understanding how organs work together within each system.
Key Concepts (Quick Recap)
- Organ system: A group of organs that work together to perform a specific life function.
- Digestive system: Mouth → Oesophagus → Stomach → Small intestine → Large intestine → Rectum → Anus. Glands: salivary, liver, pancreas.
- Respiratory system: Nose → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli. Gas exchange: O₂ enters blood, CO₂ leaves blood at alveoli.
- Circulatory system: Heart (4 chambers), blood vessels (arteries, veins, capillaries), blood (RBCs, WBCs, platelets, plasma). Double circulation.
- Excretory system: Kidneys (nephrons) → Ureters → Urinary bladder → Urethra. Nephron is the functional unit.
- Nervous system: CNS (brain + spinal cord) + PNS (cranial + spinal nerves). Neuron: cyton, dendrites, axon.
- Reproductive system: Male (testes, vas deferens, urethra, penis) and Female (ovaries, fallopian tubes, uterus, vagina).
I. Choose the Correct Answer
Q1. The organ that produces bile is: (a) Stomach (b) Pancreas (c) Liver ✓ (d) Small intestine
Answer(c) ✓ — The liver produces bile, which is stored in the gall bladder. Bile emulsifies fats (breaks large fat globules into smaller droplets) in the small intestine.
Q2. The functional unit of the kidney is: (a) Neuron (b) Nephron ✓ (c) Alveolus (d) Villus
Answer(b) ✓ — The nephron is the structural and functional unit of the kidney. Each kidney contains about one million nephrons that filter blood and form urine.
Q3. The largest organ system in the human body is: (a) Skeletal system (b) Muscular system (c) Integumentary system (skin) ✓ (d) Digestive system
Answer(c) ✓ — The skin is the largest organ of the body. It covers the entire body surface, protects internal organs, regulates temperature, and provides sensory perception.
Q4. The process of removal of undigested food from the body is called: (a) Ingestion (b) Digestion (c) Absorption (d) Egestion ✓
Answer(d) ✓ — Egestion (defecation) is the removal of undigested waste (faeces) through the anus. Ingestion = taking in food, Digestion = breaking down food, Absorption = taking nutrients into blood.
Q5. Which blood cells are responsible for clotting? (a) RBCs (b) WBCs (c) Platelets ✓ (d) Plasma
Answer(c) ✓ — Platelets (thrombocytes) are responsible for blood clotting. They release thromboplastin which converts prothrombin to thrombin, leading to fibrin formation and clot formation.
Q6. The number of chambers in the human heart is: (a) 2 (b) 3 (c) 4 ✓ (d) 1
Answer(c) ✓ — The human heart has four chambers: two atria (right and left) and two ventricles (right and left). This allows complete separation of oxygenated and deoxygenated blood (double circulation).
Q7. The part of the brain that controls body balance and posture is: (a) Cerebrum (b) Cerebellum ✓ (c) Medulla oblongata (d) Pons
Answer(b) ✓ — The cerebellum (located below the cerebrum) coordinates voluntary movements, maintains balance and posture, and helps in muscle coordination.
Q8. The hormone insulin is secreted by: (a) Liver (b) Thyroid (c) Pancreas ✓ (d) Adrenal gland
Answer(c) ✓ — The pancreas secretes insulin from the Islets of Langerhans (beta cells). Insulin regulates blood glucose levels by promoting glucose uptake by cells.
Q9. Gas exchange in the lungs occurs at the: (a) Trachea (b) Bronchi (c) Bronchioles (d) Alveoli ✓
Answer(d) ✓ — Alveoli are tiny air sacs at the end of bronchioles. Their thin walls and rich blood supply allow diffusion of O₂ into blood and CO₂ out of blood.
Q10. The male gamete in humans is produced in the: (a) Seminal vesicle (b) Prostate gland (c) Testes ✓ (d) Urethra
Answer(c) ✓ — Testes produce sperm (male gametes) through spermatogenesis. Testes also produce the male hormone testosterone.
II. Fill in the Blanks
1. The process of taking food into the body is called ingestion. 2. Bile is produced in the liver and stored in the gall bladder. 3. The enzyme that digests proteins in the stomach is pepsin. 4. The windpipe is also called the trachea. 5. The rhythmic contraction and relaxation of the heart is called the heartbeat. 6. The blood vessels that carry blood from the heart to the body are called arteries. 7. The functional unit of the nervous system is the neuron (nerve cell). 8. The part of the brain that controls breathing and heartbeat is the medulla oblongata. 9. The process of filtration of blood in the nephron occurs in the glomerulus (within Bowman's capsule). 10. The female reproductive organ that produces eggs is the ovary.
III. True or False
1. Digestion of food begins in the stomach.
AnswerFalse — Correction: Digestion begins in the mouth, where salivary amylase starts breaking down starch into maltose (chemical digestion) and teeth mechanically break food.
2. The small intestine is the longest part of the alimentary canal.
AnswerTrue — The small intestine is about 6 metres long and is the longest part, where most digestion and absorption occur.
3. Arteries carry blood away from the heart.
AnswerTrue — By definition, arteries carry blood away from the heart. Most arteries carry oxygenated blood, but the pulmonary artery carries deoxygenated blood.
4. The right side of the heart contains oxygenated blood.
AnswerFalse — Correction: The right side of the heart (right atrium and right ventricle) contains deoxygenated blood. The left side contains oxygenated blood.
5. Alveoli have thin walls for efficient gas exchange.
AnswerTrue — Alveolar walls are extremely thin (one cell thick) and surrounded by a dense network of capillaries, allowing rapid diffusion of gases.
6. The cerebrum controls breathing and heart rate.
AnswerFalse — Correction: The medulla oblongata controls involuntary functions like breathing, heart rate, and blood pressure. The cerebrum controls voluntary actions, thinking, memory, and intelligence.
7. Nephrons filter the blood and form urine.
AnswerTrue — Each nephron filters blood in the glomerulus, reabsorbs useful substances in the tubules, and secretes wastes. The resulting fluid (urine) passes to the collecting duct.
8. The human heart is a four-chambered organ.
AnswerTrue — The human heart has two atria and two ventricles, allowing complete separation of oxygenated and deoxygenated blood.
9. Veins have thick, elastic walls.
AnswerFalse — Correction: Arteries have thick, elastic walls to withstand high blood pressure. Veins have thin walls and valves to prevent backflow of blood.
10. The nervous system consists of the brain and spinal cord only.
AnswerFalse — Correction: The nervous system consists of the central nervous system (brain + spinal cord) and the peripheral nervous system (cranial nerves + spinal nerves).
IV. Match the Following
Set 1: Digestive System Organs and Functions
| Column I (Organ) | Column II (Function) |
|---|
| 1. Salivary glands | (e) Secrete saliva with salivary amylase (digests starch) |
| 2. Stomach | (a) Secretes HCl and pepsin (digests proteins) |
| 3. Liver | (c) Produces bile (emulsifies fat) |
| 4. Pancreas | (f) Secretes pancreatic juice (amylase, trypsin, lipase) |
| 5. Small intestine | (b) Complete digestion and absorption of nutrients through villi |
| 6. Large intestine | (d) Absorbs water and minerals, forms faeces |
Set 2: Organ Systems and Their Functions
| Column I (System) | Column II (Primary Function) |
|---|
| 1. Digestive system | (c) Breaks down food, absorbs nutrients |
| 2. Respiratory system | (f) Gas exchange (O₂ in, CO₂ out) |
| 3. Circulatory system | (a) Transports O₂, nutrients, hormones, wastes |
| 4. Excretory system | (e) Removes metabolic wastes, maintains water balance |
| 5. Nervous system | (b) Receives stimuli, transmits impulses, coordinates body activities |
| 6. Reproductive system | (d) Produces gametes, supports fertilisation and development |
V. Short Answers
Q1. Define organ system. Give two examples.
Answer
An organ system is a group of organs that work together to perform a specific life function.
Examples: 1. Digestive system: Mouth, oesophagus, stomach, small intestine, large intestine, liver, pancreas — together they digest food and absorb nutrients. 2. Circulatory system: Heart, blood vessels (arteries, veins, capillaries), blood — together they transport substances throughout the body.
Q2. What is peristalsis? Where does it occur?
Answer
Peristalsis is the rhythmic, wave-like contraction and relaxation of the muscular walls of the alimentary canal that pushes food forward.
It occurs in the oesophagus (to push food from mouth to stomach), stomach (to mix food with gastric juice), and intestines (to move digested material along the gut).
Q3. Differentiate between arteries and veins.
Answer
| Feature | Arteries | Veins |
|---|
| Direction of flow | Away from heart | Towards heart |
| Wall thickness | Thick, elastic, muscular | Thin, less muscular |
| Lumen (inner space) | Narrow | Wide |
| Valves | Absent | Present (prevent backflow) |
| Blood pressure | High | Low |
| Oxygen content | Mostly oxygenated (except pulmonary artery) | Mostly deoxygenated (except pulmonary veins) |
| Pulse | Present (pulsatile flow) | Absent (steady flow) |
Q4. What are villi? What is their function?
Answer
Villi (singular: villus) are tiny, finger-like projections lining the inner wall of the small intestine.
Structure: Each villus has a network of blood capillaries and a lacteal (lymph vessel) surrounded by a single layer of epithelial cells.
Functions: 1. Increase surface area for absorption of digested food. 2. Absorb nutrients — glucose and amino acids into blood capillaries, fatty acids and glycerol into lacteals. 3. Rich blood supply ensures rapid transport of absorbed nutrients.
Q5. Describe the path of urine from kidney to outside the body.
Answer
Kidney (filtration and urine formation in nephrons) → Renal pelvis → Ureter → Urinary bladder (storage) → Urethra (exit from body)
Q6. What is double circulation? Explain.
Answer
Double circulation means blood passes through the heart twice in one complete cycle:
1. Pulmonary circulation (lesser circuit): Right ventricle → Pulmonary artery → Lungs (gas exchange) → Pulmonary veins → Left atrium. Deoxygenated blood is oxygenated in the lungs.
2. Systemic circulation (greater circuit): Left ventricle → Aorta → Body (O₂ delivery, CO₂ collection) → Venae cavae → Right atrium. Oxygenated blood supplies the body.
This separation of oxygenated and deoxygenated blood (due to four-chambered heart) ensures efficient oxygen supply to tissues.
Q7. Name the parts of the nervous system and their functions.
Answer
The nervous system has two main divisions:
1. Central Nervous System (CNS):
- Brain: Centre of thinking, memory, voluntary actions, and coordination of body activities.
- Spinal cord: Carries impulses between brain and body; controls reflex actions.
2. Peripheral Nervous System (PNS):
- Cranial nerves (12 pairs): Emerge from the brain; control head and neck functions.
- Spinal nerves (31 pairs): Emerge from the spinal cord; control body functions below the neck.
Q8. Write the differences between aerobic and anaerobic respiration.
Answer
| Feature | Aerobic Respiration | Anaerobic Respiration |
|---|
| Oxygen | Required | Not required |
| Location | Mitochondria | Cytoplasm |
| Products | CO₂ + H₂O + 38 ATP | Lactic acid + 2 ATP (in muscles) or Ethanol + CO₂ + 2 ATP (in yeast) |
| Energy released | Large (38 ATP) | Small (2 ATP) |
| Efficiency | High (complete oxidation) | Low (incomplete oxidation) |
| Examples | Normal breathing | Sprinting, fermentation by yeast |
VI. Detailed Answers
Q1. Describe the human digestive system in detail.
Answer
The human digestive system consists of the alimentary canal and digestive glands.
Alimentary Canal (Path of food):
1. Mouth (Buccal cavity):
- Teeth chew food (mechanical digestion).
- Saliva from salivary glands contains salivary amylase which digests starch into maltose.
- Tongue mixes food with saliva, forms a bolus, and pushes it to the pharynx.
2. Oesophagus (Food pipe):
- Carries food from mouth to stomach by peristalsis (rhythmic muscular contractions).
- No digestion occurs here.
3. Stomach:
- J-shaped muscular bag.
- Gastric glands secrete gastric juice containing:
- HCl: Kills germs, creates acidic medium (pH ~2), activates pepsin.
- Pepsin: Digests proteins into peptides.
- Mucus: Protects stomach lining from acid.
- Food is churned into a semi-solid paste called chyme.
4. Small intestine (longest part, ~6 m):
- Duodenum (first part): Receives bile (from liver via gall bladder) and pancreatic juice (from pancreas).
- Bile: Emulsifies fats (breaks large fat globules into small droplets).
- Pancreatic juice: Contains amylase (starch → maltose), trypsin (proteins → peptides), lipase (fats → fatty acids + glycerol).
- Jejunum and Ileum: Intestinal juice completes digestion. Villi absorb digested nutrients into blood and lymph.
- Final products of digestion: Glucose (from carbohydrates), amino acids (from proteins), fatty acids and glycerol (from fats).
5. Large intestine:
- Absorbs water and minerals from undigested material.
- Forms semi-solid faeces.
6. Rectum and Anus:
- Rectum stores faeces temporarily.
- Anus expels faeces (egestion/defecation).
Digestive Glands:
- Salivary glands: Secrete saliva (salivary amylase).
- Liver: Largest gland; secretes bile; stores glycogen, vitamins, iron; detoxifies blood.
- Pancreas: Dual gland — exocrine (pancreatic juice for digestion) and endocrine (insulin and glucagon for blood sugar regulation).
Q2. Explain the process of respiration in humans.
Answer
Respiration is the process of releasing energy from food (glucose). It involves both breathing (external respiration) and cellular respiration.
Breathing (External Respiration):
1. Inhalation (Inspiration): Diaphragm contracts and flattens; ribs move upward and outward; chest cavity expands; air pressure in lungs decreases; air rushes in through nose → pharynx → larynx → trachea → bronchi → bronchioles → alveoli.
2. Exhalation (Expiration): Diaphragm relaxes and becomes dome-shaped; ribs move downward and inward; chest cavity contracts; air pressure in lungs increases; air is pushed out.
Gas Exchange at Alveoli:
- Alveoli are surrounded by a dense network of blood capillaries.
- Their walls are extremely thin (one cell thick) and moist.
- O₂ diffuses from alveolar air into blood (binds to haemoglobin in RBCs).
- CO₂ diffuses from blood into alveolar air (to be exhaled).
- This is called external respiration (gas exchange between air and blood).
Cellular Respiration (Internal Respiration):
Aerobic respiration (in presence of O₂): Glucose + O₂ → CO₂ + H₂O + Energy (38 ATP)
Anaerobic respiration (in absence of O₂):
- In muscles during intense exercise: Glucose → Lactic acid + Energy (2 ATP)
- In yeast (fermentation): Glucose → Ethanol + CO₂ + Energy (2 ATP)
Transport of gases:
- O₂ is carried by haemoglobin in RBCs as oxyhaemoglobin.
- CO₂ is carried dissolved in blood plasma and as bicarbonate ions.
Q3. Describe the structure and working of the human heart.
Answer
Structure: The human heart is a muscular organ about the size of a fist, located in the chest cavity between the lungs, slightly to the left.
- Four chambers: Right atrium, Right ventricle, Left atrium, Left ventricle.
- Valves: Prevent backflow of blood.
- Tricuspid valve (between right atrium and right ventricle).
- Bicuspid/mitral valve (between left atrium and left ventricle).
- Semilunar valves (at the origin of aorta and pulmonary artery).
- Blood vessels:
- Venae cavae (superior and inferior) bring deoxygenated blood to the right atrium.
- Pulmonary artery carries deoxygenated blood from right ventricle to lungs.
- Pulmonary veins bring oxygenated blood from lungs to the left atrium.
- Aorta carries oxygenated blood from left ventricle to the body.
Working (Cardiac Cycle):
1. Diastole (Relaxation): Both atria relax and fill with blood — right atrium receives deoxygenated blood from the body (via venae cavae), left atrium receives oxygenated blood from lungs (via pulmonary veins).
2. Atrial Systole (Atrial contraction): Both atria contract simultaneously, pushing blood into the ventricles.
3. Ventricular Systole (Ventricular contraction): Both ventricles contract — right ventricle pumps deoxygenated blood to lungs via pulmonary artery; left ventricle pumps oxygenated blood to the body via aorta.
This completes one heartbeat (~0.8 seconds). Average heart rate is 72 beats per minute.
Blood pressure: The force exerted by blood on the walls of blood vessels. Measured as systolic/diastolic (normal: 120/80 mm Hg).
Q4. Explain the structure and function of the nephron in the excretory system.
Answer
The nephron is the structural and functional unit of the kidney. Each kidney contains about one million nephrons.
Structure of a Nephron:
1. Bowman's capsule (Glomerular capsule): A cup-shaped structure at the beginning of the nephron that encloses the glomerulus.
2. Glomerulus: A knot of capillaries inside Bowman's capsule, supplied by the afferent arteriole and drained by the efferent arteriole.
3. Proximal Convoluted Tubule (PCT): Coiled tube after Bowman's capsule; lined with microvilli for reabsorption.
4. Loop of Henle: A U-shaped loop with descending limb (permeable to water) and ascending limb (impermeable to water, active salt transport).
5. Distal Convoluted Tubule (DCT): Another coiled segment; site of selective secretion and reabsorption.
6. Collecting duct: Receives urine from multiple nephrons; passes into the renal pelvis.
Function (Urine Formation — 3 steps):
1. Ultrafiltration: Blood enters the glomerulus under high pressure. Water, glucose, amino acids, salts, urea, and uric acid are filtered from blood into Bowman's capsule, forming the glomerular filtrate. Blood cells and large proteins remain in blood.
2. Reabsorption (Selective): As filtrate passes through PCT, loop of Henle, and DCT, useful substances are reabsorbed back into blood:
- PCT: All glucose, amino acids, most Na⁺, most water reabsorbed.
- Loop of Henle: Water reabsorbed in descending limb; NaCl reabsorbed in ascending limb.
- DCT: Secretion of H⁺, K⁺, NH₄⁺ and reabsorption of Na⁺ and water (regulated by ADH hormone).
3. Tubular secretion: Additional wastes (H⁺, K⁺, NH₄⁺, drugs) are secreted from blood into the tubule to maintain acid-base balance.
The resulting fluid is urine — containing water, urea, uric acid, and excess salts. Urine flows from collecting duct → renal pelvis → ureter → urinary bladder → urethra (out of body).
Common Mistakes
1. Confusing ingestion, digestion, absorption, and egestion — Ingestion = taking food in; Digestion = breaking food down chemically/mechanically; Absorption = nutrients entering blood; Egestion = removing undigested waste.
2. Saying bile digests fat — Bile does NOT digest fat; it emulsifies fat (breaks large globules into smaller droplets). Actual fat digestion is by lipase (from pancreas).
3. Confusing arteries and veins — Arteries carry blood AWAY from the heart (usually oxygenated, except pulmonary artery). Veins carry blood TOWARDS the heart (usually deoxygenated, except pulmonary veins).
4. Forgetting that pulmonary artery carries deoxygenated blood — The pulmonary artery is the only artery carrying deoxygenated blood (from heart to lungs). The pulmonary vein is the only vein carrying oxygenated blood (from lungs to heart).
5. Mixing up the brain parts — Cerebrum = thinking, memory, voluntary action; Cerebellum = balance, posture, coordination; Medulla oblongata = breathing, heartbeat, blood pressure (involuntary); Pons = regulation of breathing rate.
6. Confusing reabsorption and secretion in the nephron — Reabsorption = useful substances taken BACK from filtrate into blood (glucose, amino acids, water, Na⁺). Secretion = additional wastes ADDED from blood into the tubule (H⁺, K⁺, drugs).
7. Writing "respiration" when you mean "breathing" — Breathing is the physical process of inhaling/exhaling. Respiration is the chemical process of releasing energy from glucose (aerobic or anaerobic).
Exam Checklist
- [ ] Definition of organ and organ system with two examples each
- [ ] Digestive system — complete path of food, enzymes at each stage, glandular secretions
- [ ] Digestive enzymes: salivary amylase (mouth), pepsin (stomach), bile (liver), pancreatic amylase/trypsin/lipase, intestinal enzymes
- [ ] Respiratory system — path of air, gas exchange at alveoli, mechanism of breathing
- [ ] Aerobic vs anaerobic respiration comparison (word equations, ATP yield)
- [ ] Circulatory system — heart structure (4 chambers, 4 valves, major blood vessels), double circulation
- [ ] Arteries vs veins comparison table
- [ ] Blood composition: plasma, RBCs, WBCs, platelets — functions
- [ ] Excretory system — nephron structure, 3 steps of urine formation
- [ ] Nervous system — CNS and PNS, brain parts and functions, neuron structure
- [ ] Common mistakes: bile emulsifies (not digests), pulmonary artery/vein exception, breathing vs respiration
Internal Links
- [Organ Systems in Animals Guide](/school/tamil-nadu-state-board/class/9/science/organ-systems-in-animals)
- [All Class 9 Science Solutions](/school/tamil-nadu-state-board/class/9/science/solutions)
- [Class 9 Science Subject Guide](/school/tamil-nadu-state-board/class/9/science)
