General Science: Human Body Systems — Digestive, Circulatory, Respiratory
SSLC-level notes on the human body — digestive system, circulatory system, respiratory system. Key facts, organ functions, and common PSC questions from NCERT Class 8-10 Science.
Human body questions appear in almost every Kerala PSC exam — usually 2-4 questions per paper at SSLC level. The questions are straightforward if you know the basic facts. These notes cover the three most tested body systems based on NCERT Class 8-10 Science and PSC previous year patterns.
1. Digestive System
The digestive system breaks down food into nutrients that the body can absorb and use for energy, growth, and repair.
The digestive tract (alimentary canal)
| Organ | Function | Key fact for PSC |
|---|---|---|
| Mouth | Chewing + saliva mixes with food | Saliva contains amylase (ptyalin) which digests starch |
| Oesophagus | Passage from mouth to stomach | Moves food by peristalsis (wave-like muscle contractions) |
| Stomach | Churns food + adds gastric juice | Gastric juice contains HCl (hydrochloric acid) and pepsin (digests proteins) |
| Small intestine | Main digestion and absorption | Longest part (~6-7 metres); villi increase absorption area |
| Large intestine | Absorbs water, forms faeces | Shorter but wider than small intestine |
| Rectum/Anus | Storage and expulsion of waste |
Important digestive glands
| Gland | Secretion | Function |
|---|---|---|
| Salivary glands | Saliva (amylase) | Digests starch to sugar |
| Liver | Bile | Emulsifies (breaks down) fats; largest gland in the body |
| Pancreas | Pancreatic juice | Contains enzymes for proteins, fats, and carbohydrates |
Most asked questions:
- Largest gland in the body? Liver
- Which enzyme digests starch in the mouth? Amylase (Ptyalin)
- Which acid is found in the stomach? HCl (Hydrochloric acid)
- Longest part of the alimentary canal? Small intestine
Vitamins and deficiency diseases
| Vitamin | Also called | Source | Deficiency disease |
|---|---|---|---|
| Vitamin A | Retinol | Carrot, papaya, milk | Night blindness |
| Vitamin B1 | Thiamine | Cereals, pulses | Beriberi |
| Vitamin B12 | Cobalamin | Meat, fish, eggs | Anaemia |
| Vitamin C | Ascorbic acid | Citrus fruits, amla | Scurvy |
| Vitamin D | Calciferol | Sunlight, fish oil | Rickets (children), Osteomalacia (adults) |
| Vitamin E | Tocopherol | Nuts, seeds, oils | Sterility |
| Vitamin K | Phylloquinone | Green leafy vegetables | Delayed blood clotting |
Memory trick for fat-soluble vitamins: A, D, E, K are fat-soluble. The rest (B, C) are water-soluble. Fat-soluble vitamins can be stored in the body; water-soluble cannot.
2. Circulatory System
The circulatory system transports blood, nutrients, oxygen, and waste products throughout the body.
The heart
- Chambers: 4 — 2 atria (upper) + 2 ventricles (lower)
- Largest chamber: Left ventricle (pumps blood to the whole body)
- Wall between chambers: Septum
- Average heart rate: 72 beats per minute (normal range: 60-100 bpm)
- Blood pumped per day: ~7,000-8,000 litres
PSC trap: The heart has 4 chambers, not 2. Atria receive blood; ventricles pump blood out. Left side carries oxygenated blood; right side carries deoxygenated blood.
Blood circulation
Double circulation — blood passes through the heart twice in one complete circuit:
- Pulmonary circulation: Right ventricle → Lungs → Left atrium (blood gets oxygenated)
- Systemic circulation: Left ventricle → Whole body → Right atrium (blood delivers oxygen)
Blood components
| Component | Function | Key fact |
|---|---|---|
| Red Blood Cells (RBCs) | Carry oxygen | Contain haemoglobin; formed in bone marrow |
| White Blood Cells (WBCs) | Fight infection | Part of the immune system |
| Platelets | Blood clotting | Prevent excessive bleeding |
| Plasma | Liquid medium | 55% of blood; carries nutrients, hormones, waste |
Blood groups
| Blood group | Can donate to | Can receive from |
|---|---|---|
| A | A, AB | A, O |
| B | B, AB | B, O |
| AB | AB only | A, B, AB, O (Universal recipient) |
| O | A, B, AB, O (Universal donor) | O only |
Most asked:
- Universal donor? O group
- Universal recipient? AB group
- Who discovered blood groups? Karl Landsteiner (1901)
- Rh factor discovered by? Karl Landsteiner and Alexander Wiener (1937-40; published 1940)
3. Respiratory System
The respiratory system handles breathing — taking in oxygen and expelling carbon dioxide.
Breathing process
Inhalation (breathing in):
- Diaphragm contracts (flattens)
- Rib cage moves up and out
- Lung volume increases → air pressure decreases → air rushes in
Exhalation (breathing out):
- Diaphragm relaxes (domes up)
- Rib cage moves down and in
- Lung volume decreases → air pressure increases → air pushed out
Gas exchange
Gas exchange happens in the alveoli — tiny air sacs in the lungs.
| Inhaled air | Exhaled air |
|---|---|
| ~21% oxygen | ~16% oxygen |
| ~0.04% CO₂ | ~4% CO₂ |
| Variable moisture | Saturated with moisture |
PSC fact: The total surface area of alveoli in human lungs is approximately 70-80 square metres — about the size of a tennis court. This large area enables efficient gas exchange.
Key respiratory facts
| Fact | Answer |
|---|---|
| Normal breathing rate (adult) | 12-20 breaths per minute (avg ~15-18) |
| Total lung capacity | ~6 litres |
| Gas exchanged at | Alveoli |
| Oxygen carried by | Haemoglobin in RBCs |
| Breathing controlled by | Medulla oblongata (brain stem) |
Important Diseases (PSC Favourites)
| Disease | Cause | Affected system |
|---|---|---|
| Jaundice | Liver malfunction (excess bilirubin) | Digestive |
| Anaemia | Low haemoglobin / RBC count | Circulatory |
| Hypertension | Persistently high blood pressure | Circulatory |
| Asthma | Inflammation of airways | Respiratory |
| Pneumonia | Infection of lungs (bacteria/virus) | Respiratory |
| Tuberculosis (TB) | Mycobacterium tuberculosis bacteria | Respiratory |
| Diabetes | Insulin deficiency / resistance | Endocrine |
Quick Revision — Top 20 PSC Questions
- Largest gland in the body? — Liver
- Smallest bone? — Stapes (in the ear)
- Largest bone? — Femur (thigh bone)
- Total bones in an adult? — 206
- Total muscles? — 639
- Universal blood donor? — O group
- Universal blood recipient? — AB group
- Normal body temperature? — 37°C / 98.6°F
- Normal BP? — 120/80 mmHg
- Vitamin for night vision? — Vitamin A
- Vitamin from sunlight? — Vitamin D
- Disease from Vitamin C deficiency? — Scurvy
- Disease from Vitamin D deficiency? — Rickets
- Haemoglobin contains which metal? — Iron
- Blood is filtered by? — Kidneys
- Functional unit of kidney? — Nephron
- Functional unit of liver? — Hepatocyte (lobule)
- Which gas do we exhale more of? — CO₂
- Enzymes are made of? — Proteins
- DNA full form? — Deoxyribonucleic Acid
Notes based on NCERT Class 8 Science (Ch. 2), Class 9 Science, and Class 10 Science (Ch. 6 — Life Processes). PSC question patterns from 2020-2024 LDC and Last Grade papers. Updated April 2026.