Topic 6 covers the major human body systems: digestive, circulatory, respiratory, immune, nervous, and endocrine. Students learn structure-function relationships and how systems maintain homeostasis.
Mechanical (teeth, peristalsis) and chemical (enzymes, acid, bile) digestion. Mouth: amylase (starch → maltose). Stomach: pepsin (proteins), HCl (pH 2). Small intestine: pancreatic enzymes (lipase, protease, amylase), bile salts emulsify fats. Absorption in small intestine: villi increase surface area. Large intestine: water absorption.
Double circulation: pulmonary (heart → lungs → heart) and systemic (heart → body → heart). Heart: 4 chambers (atria receive blood, ventricles pump). Left ventricle thicker (pumps to whole body). Cardiac cycle: atrial systole → ventricular systole → diastole. Arteries: thick walls, carry blood away from heart. Veins: valves, carry blood to heart. Capillaries: thin walls for exchange.
Lungs: alveoli provide large surface area, thin walls (one cell thick), moist surface, rich blood supply. O₂ diffuses into blood, CO₂ out. Ventilation: diaphragm and intercostal muscles change thoracic volume. Immune system: innate (phagocytes, skin barriers) and adaptive (B-cells produce antibodies, T-cells kill infected cells). Antibiotics for bacteria, not viruses.
Neurons: sensory, relay, motor. Nerve impulse: resting potential (−70 mV), depolarisation, action potential, repolarisation. Synapse: neurotransmitter (e.g., acetylcholine) released from pre-synaptic neuron, binds to receptors on post-synaptic neuron. Hormones: chemical messengers in blood. Examples: insulin (lowers blood glucose), glucagon (raises it), ADH (water retention), thyroxine (metabolism).
At SL, focus on: main structures and functions of each system, key enzymes and their substrates/products, the path of blood through the heart and body, alveolar gas exchange mechanisms, the difference between innate and adaptive immunity, and basic neuron structure and synaptic transmission. You do not need to memorise every enzyme or hormone — focus on the principles and named examples given in the syllabus.
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