Transport Over Larger Distances: Systems in the Human Body — AQA Combined Science: Synergy
This topic covers respiration, exchange surfaces and the body systems that transport substances: circulatory, digestive, nervous and endocrine.
Respiration
Respiration is an exothermic reaction in all living cells that releases energy from glucose (stored in ATP). It is not the same as breathing.
- Aerobic respiration (with oxygen) releases the most energy, in the mitochondria: glucose + oxygen → carbon dioxide + water
- Anaerobic respiration (without oxygen) releases less energy. In muscles: glucose → lactic acid; in yeast: glucose → ethanol + carbon dioxide (fermentation).
During exercise, heart rate and breathing increase to supply oxygen and glucose; insufficient oxygen leads to anaerobic respiration and an oxygen debt.
Exchange surfaces
Larger organisms need specialised exchange surfaces because their surface area to volume ratio is small. Good exchange surfaces have a large surface area, thin walls (short diffusion distance), and a good blood/air supply to maintain a steep concentration gradient (e.g. alveoli in the lungs, villi in the small intestine).
The circulatory system
Humans have a double circulatory system — one loop to the lungs, one to the body.
- Right side pumps deoxygenated blood to the lungs; left side (thicker wall) pumps oxygenated blood around the body.
- Arteries carry blood at high pressure (thick walls); veins return blood at low pressure (have valves); capillaries have walls one cell thick for exchange.
Blood cells
Blood is a tissue containing:
- Plasma — carries dissolved substances.
- Red blood cells — contain haemoglobin to carry oxygen; biconcave, no nucleus.
- White blood cells — defend against pathogens.
- Platelets — help clotting.
The human digestive system
Enzymes break down food: carbohydrases → sugars, proteases → amino acids, lipases → fatty acids and glycerol. Bile neutralises stomach acid and emulsifies fats. Small molecules are absorbed into the blood.
The human nervous system
The pathway is: stimulus → receptor → sensory neurone → CNS → motor neurone → effector → response. Reflexes are fast and automatic, passing through a reflex arc with a relay neurone and synapses (where chemicals carry the signal across a gap).
The human endocrine system
Glands secrete hormones into the blood to target organs. Responses are slower but longer-lasting than nervous ones. The pituitary is the master gland; the pancreas controls blood glucose; the adrenal glands release adrenaline; ovaries/testes release sex hormones.
Exam tips
- Compare aerobic and anaerobic respiration (oxygen, energy, products).
- List the features of an efficient exchange surface.
- Link blood vessel structure to function (artery walls, vein valves, capillary thinness).
- Get the nervous pathway and reflex arc order right, mentioning synapses.
- Contrast nervous (fast, short-lived) and hormonal (slow, long-lasting) responses.