Organisation — section overview
Section B2 covers how cells are organised into tissues, organs and organ systems, and focuses specifically on two organ systems: the digestive system and the circulatory system (including heart disease).
The hierarchy of organisation
Cell → Tissue → Organ → Organ system → Organism
- Cell: smallest unit of life (e.g. muscle cell, red blood cell)
- Tissue: group of similar cells performing the same function (e.g. muscle tissue, epithelial tissue)
- Organ: group of tissues working together (e.g. heart, stomach)
- Organ system: group of organs with a common function (e.g. digestive system, circulatory system)
The digestive system
Key organs and their functions:
| Organ | Function |
|---|---|
| Salivary glands | Secrete amylase to digest starch → maltose |
| Stomach | Churns food; secretes protease (pepsin) and HCl |
| Small intestine | Site of digestion and absorption; villi increase surface area |
| Large intestine | Absorbs water; compact faeces |
| Liver | Produces bile; detoxifies alcohol; makes plasma proteins |
| Pancreas | Secretes amylase, protease and lipase into small intestine |
Enzymes:
- Amylase: starch → maltose (salivary glands, pancreas)
- Protease: proteins → amino acids (stomach, pancreas)
- Lipase: fats → fatty acids + glycerol (pancreas)
- Bile: emulsifies fats (produced by liver, stored in gall bladder)
The circulatory system
Double circulation: blood passes through the heart twice per circuit.
- Pulmonary circuit: right side of heart → lungs → left side
- Systemic circuit: left side of heart → body → right side
Blood vessels:
- Arteries: carry blood away from heart; thick muscular walls; high pressure
- Veins: carry blood to heart; thin walls; valves prevent backflow
- Capillaries: one cell thick; exchange of materials with tissues
Blood components:
- Red blood cells: no nucleus; haemoglobin; carry O₂
- White blood cells: immune defence
- Platelets: clotting
- Plasma: carries dissolved substances (glucose, CO₂, urea, hormones)
Health and disease (B2 link)
Coronary heart disease (CHD): fatty plaque narrows coronary arteries → reduced blood flow → angina or heart attack.
Treatments: stents, statins, heart bypass surgery, lifestyle changes.
Cancer: uncontrolled cell division forming tumours. Benign = localised. Malignant = spreads (metastasis).
Common exam mistakes in B2
- Arteries vs veins — oxygenation is not the rule — the pulmonary artery carries deoxygenated blood
- Bile digests fats — NO: bile emulsifies fats (breaks into droplets); lipase digests fats
- Confusing amylase and protease substrates — amylase = starch; protease = protein
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