Human Body

The human body is organized into 11 major organ systems, each with specific roles.

1. Integumentary system: skin, hair, nails. Protects, regulates temperature, vitamin D synthesis.
2. Skeletal system: 206 bones in adults (270 at birth). Support, protection, blood cell production (in marrow), mineral storage.
3. Muscular system: ~600 muscles. Three types:
   • Skeletal (voluntary, striated).
   • Smooth (involuntary, in walls of organs).
   • Cardiac (heart only).
4. Nervous system: brain, spinal cord, nerves. Receives signals, processes, responds.
5. Endocrine system: hormone-producing glands (pituitary, thyroid, adrenals, pancreas, gonads).
6. Circulatory system: heart + blood + vessels. Transports O₂, CO₂, nutrients, hormones.
7. Respiratory system: nose, trachea, lungs. Gas exchange.
8. Digestive system: mouth to anus. Breaks down food, absorbs nutrients.
9. Urinary / Excretory system: kidneys, bladder, ureters, urethra. Removes nitrogenous waste.
10. Reproductive system: ovaries/uterus in females; testes in males.
11. Lymphatic / Immune system: lymph nodes, spleen, thymus. Fights infection.

Bones — interesting facts:

• Femur (thigh bone): longest. ~25% of body height.
• Stapes (in ear): smallest, ~3 mm.
• Skull: 22 bones (cranium 8 + face 14).
• Spine: 33 vertebrae (cervical 7, thoracic 12, lumbar 5, sacrum 5, coccyx 4).
• Hand has 27 bones; foot has 26.

Heart:

• Size of a fist; weight ~300 g.
• Located slightly left of centre in the chest.
• Pumps ~5 L of blood per minute at rest.
• Has its own electrical pacemaker — SA node (sinoatrial).
• Blood pressure: normal ~120/80 mmHg (systolic/diastolic).

Blood:

• Plasma (55%) + cells (45%).
• RBC (red blood cells / erythrocytes): contain haemoglobin; no nucleus in mature form; ~5 million/mm³.
• WBC (white blood cells / leucocytes): fight infection; ~7000/mm³.
• Platelets: aid clotting; ~250,000/mm³.

Blood groups (ABO + Rh):

• A, B, AB, O.
• AB is universal recipient; O is universal donor.
• Rh+ or Rh− based on Rh antigen.

Brain (~1.4 kg in adult):

• Cerebrum: largest part; controls thinking, memory, sensory perception, voluntary movement. Divided into left and right hemispheres.
• Cerebellum: balance, posture, fine motor coordination.
• Medulla oblongata: controls involuntary actions — heartbeat, breathing, swallowing.
• Hypothalamus: regulates hunger, thirst, body temperature.
• Brain uses ~20% of body's energy at rest.

Eye:

• Cornea + lens form image on retina (inverted, smaller).
• Iris controls aperture (pupil).
• Retina has rods (B&W vision, dim light) and cones (colour, bright light).
• Three cone types: red, green, blue.

Ear (hearing + balance):

• Outer ear (pinna + canal) collects sound.
• Middle ear: 3 tiny bones (malleus, incus, stapes) amplify vibrations.
• Inner ear: cochlea converts to nerve signals; semicircular canals provide balance.

Teeth (adults: 32):

• 8 incisors (cutting), 4 canines (tearing), 8 premolars (grinding), 12 molars (grinding).
• 4 wisdom teeth (last molars) may not erupt.
• Children have 20 milk teeth.

Skin:

• Largest organ; ~16% of body weight.
• Two layers: epidermis (outer) + dermis (inner).
• Sweat glands cool body; sebaceous glands produce oil.
• Vitamin D synthesized when UV from sun hits skin.

Reproduction:

• Sperm produced in testes; ova in ovaries.
• Fertilization in fallopian tube.
• Pregnancy: 40 weeks (280 days from last menstrual period).
• Sex determination: father's Y chromosome → male; X → female.

Immune system:

• Antibodies neutralize antigens.
• Vaccination introduces weakened antigen → body makes antibodies for future protection.
• B cells (antibodies), T cells (cellular immunity).
• White blood cell types: neutrophils, lymphocytes, monocytes, eosinophils, basophils.

Common conditions:

• Anaemia: low haemoglobin → fatigue, paleness. Eat iron-rich foods.
• Diabetes: high blood sugar; insulin deficiency or resistance.
• Hypertension: high blood pressure; risk of stroke and heart attack.
• Goitre: thyroid enlargement; often from iodine deficiency.

Universal facts:

• Resting heart rate: 60–100 bpm.
• Resting respiration rate: 12–18 breaths/min.
• Normal body temperature: 36.5–37.5°C (97.7–99.5°F).
• Total length of blood vessels in body: ~100,000 km (could circle Earth 2.5 times).