Excretion
What you must understand
Excretion is the removal of metabolic waste, not the removal of undigested food (egestion). The main excretory organs are the kidneys (urea, excess water and salts), the lungs (carbon dioxide and water vapour) and the skin (sweat: water, salts and a little urea).
Learn the structure of the urinary system: kidneys, ureters, bladder, urethra. Inside the kidney, filtration occurs in the cortex and reabsorption of useful substances in the tubules. Useful glucose and some salts are reabsorbed; urea is not. The result is urine.
Understand the role of the kidney in homeostasis: it controls water and salt balance. If blood is too concentrated, more ADH is released, more water is reabsorbed and urine is small in volume and concentrated. If blood is too dilute, less ADH is released and urine is large in volume and dilute.
Traps examiners set
- Calling faeces an excretory product. It is egested, not excreted.
- Confusing the ureter (kidney to bladder) with the urethra (bladder to outside).
- Saying the kidney removes "poison". It removes urea, formed in the liver from excess amino acids.
- Getting the ADH direction wrong. Remember: more ADH, more water reabsorbed, less urine.
Worked example
After drinking a lot of water, a person produces a large volume of dilute urine. Explain why. Blood becomes too dilute, so less ADH is secreted, the tubules reabsorb less water, and more dilute urine is produced.
Rule of thumb: think of ADH as the "save water" hormone. It always increases water reabsorption and reduces urine volume.
Link to other systems
Excretion connects to respiration (lungs) and to the skin's role in temperature control, so be ready to combine topics in longer answers.