Human Physiology
Domain 5 covers digestion, circulation, respiration, excretion and neural and endocrine control. Questions ask for enzyme actions, heart and nephron function, gas transport and hormone effects.
Core rules
- Digestion: salivary amylase starts starch; pepsin acts on protein in acid stomach; pancreatic lipase, amylase and trypsin act in the small intestine; bile emulsifies fats but is not an enzyme.
- Circulation: SA node sets heart rate; AV node delays impulse; cardiac output = stroke volume × heart rate. Arteries carry blood away, veins toward the heart; capillaries allow exchange.
- Respiration: haemoglobin binds O2 in lungs; CO2 is transported mainly as bicarbonate. Oxygen dissociation curve shifts right with high CO2, low pH and high temperature.
- Excretion: nephron filters blood at glomerulus; reabsorption in proximal tubule; loop of Henle creates medullary gradient; ADH increases water reabsorption.
- Neural control uses action potentials and synapses; endocrine control uses hormones such as insulin, thyroxine and adrenaline.
Worked example
Stroke volume is 70 mL and heart rate is 72 beats per minute. Cardiac output = 70 × 72 = 5040 mL/min, about 5 L/min. Reasoning: cardiac output is the volume pumped by one ventricle per minute; multiply the volume per beat by beats per minute. During exercise, both stroke volume and heart rate rise, so output increases.
Traps
- Calling bile an enzyme; it emulsifies fat.
- Saying veins always carry deoxygenated blood; pulmonary veins carry oxygenated blood.
- Confusing ADH with aldosterone; ADH increases water reabsorption, aldosterone increases Na+ reabsorption.
- Thinking CO2 is carried mainly dissolved in plasma; most is bicarbonate.