Excretion and homeostasis (kidneys, skin, temperature and water balance)
What this domain tests
The difference between excretion and egestion, the nephron and urine formation, skin structure, and negative feedback control of temperature and water balance. Expect questions on dialysis, glucose in urine as a sign of diabetes, and data on sweat rate and urine concentration.
Core rules
- Excretion removes metabolic waste made inside cells: carbon dioxide via lungs, urea via kidneys, bile pigments via liver, excess salts and water via kidneys and skin. Egestion removes undigested food that was never absorbed.
- Urea is made in the liver by deamination of excess amino acids.
- Nephron sequence: ultrafiltration at the glomerulus forces water, glucose, salts and urea into Bowman's capsule while proteins and cells stay in the blood; selective reabsorption in the proximal tubule returns all glucose, most water and some salts; the loop of Henle and collecting duct adjust water; the distal tubule secretes ions.
- Water balance: osmoreceptors in the hypothalamus detect concentrated blood, the posterior pituitary releases more ADH, collecting ducts become more permeable, more water is reabsorbed, and a small concentrated urine is produced. ADH falls when blood is too dilute.
- Temperature control by the hypothalamus: when hot, vasodilation, sweating, hairs lie flat, lower metabolic rate; when cold, vasoconstriction, shivering, hair erection, more thyroxine and adrenaline.
- Skin: epidermis with keratin and melanin; dermis with sweat glands, capillaries, hair follicles, nerve endings and fat.
Worked example
A boy plays football on a hot day. Explain why his urine is small in volume and dark in colour. Heavy sweating loses water and salts, so the blood becomes more concentrated. Osmoreceptors in the hypothalamus detect this and stimulate ADH release from the posterior pituitary. ADH makes the collecting ducts more permeable to water, so more water is reabsorbed into the blood. Less water remains in the filtrate, so urine volume falls and the urine is more concentrated and darker. Thirst also increases.
Traps that cost marks
Calling defaecation excretion. Saying ADH increases urine volume. Writing that sweat removes mainly urea. Claiming the kidneys normally filter proteins into the capsule.