Human Physiology and Common Diseases
The human body is a coordinated assembly of about 37 trillion cells organised into tissues, organs and organ systems. Each system performs a specialised role, and together they maintain homeostasis — the stable internal environment that keeps us alive.
The major systems
- Circulatory — heart, blood vessels and blood. Transports oxygen, nutrients, hormones and wastes. The adult heart beats roughly 70 times a minute, pumping about 5 litres of blood.
- Respiratory — nose, trachea, lungs. Exchanges O₂ for CO₂ at the alveoli.
- Digestive — mouth, oesophagus, stomach, small and large intestines, liver, pancreas. Breaks food into absorbable nutrients.
- Nervous — brain, spinal cord, peripheral nerves. Senses, integrates and commands.
- Endocrine — glands (pituitary, thyroid, pancreas, adrenals, gonads) that release hormones into the blood.
- Excretory — kidneys, ureters, bladder. Filters blood and removes waste.
- Musculoskeletal — 206 bones and ~600 skeletal muscles, providing structure and movement.
- Immune — white blood cells, lymph nodes, spleen, thymus. Defends against pathogens.
- Reproductive — produces gametes and supports new life.
The maintenance of a relatively stable internal environment (temperature, pH, blood glucose, water balance) despite changing external conditions. Achieved through negative feedback loops involving the nervous and endocrine systems.
Kingdoms of life
Modern biology classifies organisms into five kingdoms (Whittaker, 1969) or — in newer schemes — into three domains (Bacteria, Archaea, Eukarya).
| Kingdom | Examples | Key features |
|---|---|---|
| Monera | Bacteria, cyanobacteria | Prokaryotic, single-celled |
| Protista | Amoeba, paramecium, algae | Eukaryotic, mostly single-celled |
| Fungi | Mushrooms, yeasts, moulds | Eukaryotic, cell wall of chitin, decomposers |
| Plantae | Mosses, ferns, flowering plants | Multicellular, photosynthetic, cellulose wall |
| Animalia | Invertebrates and vertebrates | Multicellular, heterotrophic, no cell wall |
- The scientific name of an organism follows the binomial nomenclature of Carolus Linnaeus: Genus species (e.g. Homo sapiens).
- Plants and animals share eukaryotic cell structure but differ in cell wall, chloroplasts and mode of nutrition (autotroph vs heterotroph).
- A virus is not a true cell — it has nucleic acid plus a protein coat and must hijack a host cell to reproduce. It is classified separately from the five kingdoms.
Common diseases of public-health importance
Polio (poliomyelitis)
Caused by the poliovirus, transmitted faecal-orally. Attacks motor neurons in the spinal cord and can cause irreversible paralysis. Two vaccines exist: Sabin's oral polio vaccine (OPV) and Salk's inactivated polio vaccine (IPV). Pakistan remains one of the last countries with endemic transmission.
Malaria
Caused by Plasmodium parasites (five species infect humans, P. falciparum being the deadliest) and transmitted by the bite of female Anopheles mosquitoes. Symptoms: cyclical fever, chills, anaemia. Prevention combines insecticide-treated bednets, indoor spraying and antimalarial drugs.
Dengue
Caused by the dengue virus, transmitted by the Aedes aegypti mosquito (which bites in daytime). Causes high fever, severe joint pain (hence "breakbone fever"), rash and, in severe cases, dengue haemorrhagic fever. No specific antiviral; control focuses on eliminating mosquito breeding sites.
Tuberculosis (TB)
Caused by the bacterium Mycobacterium tuberculosis, spread by airborne droplets. Affects the lungs primarily. Treated with a multi-drug regimen lasting at least six months; drug-resistant strains are a global concern.
Three of these four diseases (polio, malaria, dengue) are vector- or water-borne and respond best to public-health measures — sanitation, vaccination, vector control — rather than medical treatment alone. CSS questions often ask which prevention strategy fits which disease.
Biofuel — biology meets the energy crisis
Biofuels are liquid or gaseous fuels derived from recent biological matter rather than from ancient fossil deposits. The main categories are:
- Bioethanol — produced by fermenting sugars and starches (sugarcane, corn). Blended with petrol.
- Biodiesel — produced by trans-esterifying vegetable oils (palm, soybean, rapeseed) or animal fats. Blended with diesel.
- Biogas — methane-rich gas produced by anaerobic digestion of organic waste. Widely used at the village scale in South Asia.
- Algal biofuel — research-stage; algae can produce oil at far higher yields per hectare than land crops.
Biofuels are partially renewable and can be carbon-neutral in principle, since the CO₂ released on combustion was recently absorbed from the atmosphere during plant growth. Critics point to land-use competition with food crops as a significant downside.