Human Body: Systems, Organs, Diseases, and Vitamins

Human body and biology questions appear in every PSC exam — expect 4-8 questions on body systems, vitamins, diseases, and organs.

Digestive System

Path of food: Mouth → Pharynx → Oesophagus → Stomach → Small Intestine (Duodenum → Jejunum → Ileum) → Large Intestine (Caecum → Colon → Rectum) → Anus

Organ	Enzyme/Juice	Function
Mouth	Salivary amylase (ptyalin)	Starch → Maltose
Stomach	HCl + Pepsin (from pepsinogen)	Protein digestion; pH 1.5–2
Liver	Bile (stored in gallbladder)	Emulsification of fats (no enzyme)
Pancreas	Trypsin, Lipase, Amylase	Proteins, fats, carbs
Small intestine	Intestinal juice (succus entericus)	Final digestion + absorption
Large intestine	No enzymes	Water absorption; faecal formation

Key facts:

• Largest gland: Liver
• Largest organ: Skin (external); Liver (internal)
• Small intestine length: ~6 metres (longest part of alimentary canal)
• Appendix is attached to caecum
• Bile pigment: Bilirubin (gives yellow colour to urine and faeces)

Circulatory System

Fact	Detail
Heart chambers	4 — 2 atria (upper), 2 ventricles (lower)
Heartbeat rate	72 beats/min (average adult)
Pacemaker	SA Node (Sinoatrial Node) — “natural pacemaker”
Largest artery	Aorta
Largest vein	Inferior Vena Cava
Blood volume	~5–6 litres (adult)
Blood pH	7.35–7.45 (slightly alkaline)
RBC lifespan	120 days
RBC formation	Bone marrow (in adults)
RBC destruction	Spleen (“graveyard of RBCs”)
WBC types	Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils
Platelets	Thrombocytes — blood clotting; lifespan 8–10 days
Universal donor	O negative
Universal recipient	AB positive

Blood Groups

Group	Antigen	Antibody	Can donate 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	A, B, AB, O
O	None	Anti-A, Anti-B	A, B, AB, O	O

Rh factor: Discovered from Rhesus monkey. Rh+ has D antigen; Rh- lacks it.

Respiratory System

Path of air: Nose → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli

Fact	Detail
Gas exchange	Alveoli (300 million in lungs)
Breathing rate	12–20 per minute (adult)
Lung capacity	~6 litres (total); tidal volume ~500 ml
Diaphragm	Main muscle for breathing
Voice box	Larynx
Epiglottis	Prevents food entering trachea
Inhaled air	21% O₂, 0.04% CO₂
Exhaled air	16% O₂, 4% CO₂
Oxygen carrier	Haemoglobin (in RBCs)

Nervous System

Component	Function
Brain	Control centre; ~1.4 kg; ~86 billion neurons
Cerebrum	Largest part; thinking, memory, intelligence, voluntary action
Cerebellum	Balance and coordination
Medulla Oblongata	Involuntary actions (breathing, heartbeat, blood pressure)
Spinal Cord	Reflex actions; ~45 cm long
Neurons	Basic unit of nervous system; 3 types — sensory, motor, relay

Key facts:

• Largest cranial nerve: Vagus nerve (10th cranial nerve)
• Total cranial nerves: 12 pairs
• Spinal nerves: 31 pairs
• Meninges: 3 protective layers (Dura mater, Arachnoid, Pia mater)
• CSF (Cerebrospinal Fluid): protects brain and spinal cord

Endocrine System (Glands and Hormones)

Gland	Hormone	Function
Pituitary (master gland)	GH, TSH, ACTH, FSH, LH, Prolactin, ADH, Oxytocin	Growth, controls other glands
Thyroid	Thyroxine (T4), T3	Metabolism; needs iodine
Parathyroid	Parathormone (PTH)	Calcium regulation
Adrenal	Adrenaline (epinephrine), Cortisol	Fight-or-flight; stress response
Pancreas	Insulin (beta cells), Glucagon (alpha cells)	Blood sugar regulation
Pineal	Melatonin	Sleep-wake cycle
Thymus	Thymosin	T-cell maturation (immunity)

Key: Pituitary is the “master gland” but is itself controlled by Hypothalamus (“master of the master gland”).

Vitamins — Complete Table

Vitamin	Chemical Name	Source	Deficiency Disease
A	Retinol	Carrot, liver, milk	Night blindness (Nyctalopia), Xerophthalmia
B1	Thiamine	Cereals, pulses	Beriberi
B2	Riboflavin	Milk, eggs, liver	Cheilosis (cracked lips), Ariboflavinosis
B3	Niacin	Meat, fish, groundnut	Pellagra (3 Ds: Dermatitis, Diarrhoea, Dementia)
B5	Pantothenic acid	Widespread	Burning feet syndrome
B6	Pyridoxine	Cereals, meat	Anaemia, convulsions
B7	Biotin	Egg yolk, liver	Dermatitis, hair loss
B9	Folic Acid	Green leafy vegetables	Megaloblastic anaemia; neural tube defects in foetus
B12	Cyanocobalamin	Meat, fish, dairy	Pernicious anaemia (only vitamin with cobalt)
C	Ascorbic Acid	Citrus, amla, guava	Scurvy (bleeding gums)
D	Calciferol	Sunlight, fish oil	Rickets (children), Osteomalacia (adults)
E	Tocopherol	Nuts, vegetable oils	Sterility, muscular dystrophy
K	Phylloquinone	Green vegetables, liver	Delayed blood clotting

Fat-soluble vitamins: A, D, E, K (mnemonic: ADEK) Water-soluble vitamins: B-complex, C

Important Diseases

Deficiency Diseases

Disease	Deficiency
Goitre	Iodine
Anaemia	Iron / B12 / Folic acid
Kwashiorkor	Protein (children)
Marasmus	Protein + calories (children)
Osteoporosis	Calcium / Vitamin D
Scurvy	Vitamin C
Rickets	Vitamin D

Communicable Diseases

Disease	Causative Agent	Spread By
Tuberculosis	Mycobacterium tuberculosis	Airborne
Malaria	Plasmodium (parasite)	Female Anopheles mosquito
Dengue	Dengue virus	Aedes aegypti mosquito
Cholera	Vibrio cholerae	Contaminated water
Typhoid	Salmonella typhi	Contaminated food/water
COVID-19	SARS-CoV-2	Respiratory droplets
Rabies	Rhabdovirus	Animal bite (dog)
AIDS	HIV	Blood, sexual contact
Chickenpox	Varicella-zoster virus	Airborne/contact
Hepatitis B	HBV	Blood, body fluids

Non-Communicable / Lifestyle

Disease	Related Organ	Key Fact
Diabetes mellitus	Pancreas	Insulin deficiency/resistance
Hypertension	Heart/blood vessels	”Silent killer”
Alzheimer’s	Brain	Progressive memory loss
Parkinson’s	Brain	Dopamine deficiency; tremors
Cancer	Various	Uncontrolled cell growth
Arthritis	Joints	Inflammation of joints

Quick-Fire Recall

• Largest gland? Liver
• Smallest bone? Stapes (ear)
• Largest bone? Femur (thigh)
• Total bones in adult? 206
• Total muscles? ~640
• Hardest substance in body? Tooth enamel
• Blood protein for clotting? Fibrinogen
• pH of blood? 7.35–7.45
• Vitamin stored in liver? A, D, E, K, B12
• Only vitamin body produces? D (sunlight) and K (gut bacteria)
• Colour of oxygenated blood? Bright red
• Graveyard of RBCs? Spleen
• Master gland? Pituitary