Excretion and Homeostasis
What you must understand
Excretion is removal of metabolic waste - carbon dioxide from lungs, urea from kidneys, water and salts. The kidneys filter blood, reabsorb useful substances and produce urine. Learn the nephron in outline: filtration at the glomerulus, selective reabsorption of glucose, some salts and water, then urine formation. ADH controls water reabsorption and so urine concentration.
Homeostasis means keeping internal conditions steady. Know examples: temperature control (vasodilation, vasoconstriction, sweating, shivering), blood glucose control (insulin lowers, glucagon raises), water balance (negative feedback involving ADH), and the role of the skin and liver.
Traps examiners set
- Calling defecation excretion - it is egestion of undigested food.
- Saying kidneys "remove all water" - they reabsorb most of it.
- Confusing insulin and glucagon direction of effect.
- Describing negative feedback without the "detect, correct, return to normal" sequence.
Worked rule of thumb
Every homeostasis answer should follow the loop: stimulus → receptor → coordination centre → effector → response → return to set point. For example, high blood glucose → pancreas detects → insulin released → liver converts glucose to glycogen → glucose falls.