Life Processes
What this domain covers
Life Processes tests transport, gaseous exchange, excretion and homeostasis, support and movement, and reproduction - the systems that keep a multicellular body working. Questions often ask you to link structure to function and to explain a change using negative feedback.
Transport
- Heart: four chambers; the right side pumps deoxygenated blood to the lungs, the left pumps oxygenated blood to the body; valves stop backflow; the sinoatrial node (pacemaker) sets the rhythm.
- Blood: plasma transports dissolved substances; red cells carry oxygen as oxyhaemoglobin (no nucleus in mammals); white cells defend; platelets clot.
- Vessels: arteries have a thick muscular elastic wall, narrow lumen and high pressure; veins have a thin wall, wide lumen, valves and low pressure; capillaries are one cell thick for exchange.
Gaseous exchange, excretion and homeostasis
- Alveoli: large total surface area, thin walls, moist surface, dense capillary network - fast diffusion of O₂ and CO₂.
- Kidney: filtration of blood in the glomerulus; selective reabsorption of glucose, amino acids, some water and salts in the tubule; urine carries urea, excess salts and water.
- Negative feedback: insulin lowers blood glucose and glucagon raises it; ADH increases water reabsorption when blood water potential falls; sweating, vasodilation and vasoconstriction control temperature.
Support, movement and reproduction
- The skeleton supports and protects; joints (hinge, ball-and-socket) work with antagonistic muscles such as biceps and triceps.
- Fertilisation occurs in the oviduct; the zygote divides and implants in the uterus; the placenta allows exchange by diffusion between fetal and maternal blood.
- The menstrual cycle is about 28 days, with ovulation near day 14.
Worked example
After heavy sweating, a person's urine is small in volume and concentrated. Explain. Sweating removes water, so blood water potential falls. Osmoreceptors in the hypothalamus detect this and stimulate release of ADH from the pituitary. ADH makes the distal tubule and collecting duct more permeable to water, so more water is reabsorbed into the blood. The remaining urine is therefore smaller in volume and more concentrated. When water intake rises, ADH falls and urine becomes dilute.
Traps that cost marks
- Saying all arteries carry oxygenated blood - the pulmonary artery does not.
- Confusing filtration (glomerulus) with reabsorption (tubule).
- Claiming sweat removes most urea - it is mainly water and salts.
- Confusing ovulation (release of the egg) with menstruation.
- Writing that the placenta lets blood mix; exchange is by diffusion and the bloods do not normally mix.
- Saying the heart pumps blood to the lungs and body at the same pressure - the left ventricle wall is thicker.