Excretion and Homeostasis
Excretion and the kidney
Excretion is removal of waste products of metabolism, not simply removal of undigested food (that is egestion). Carbon dioxide leaves via the lungs, urea via the kidneys, and excess salts and water in urine. The kidney filters blood, reabsorbing useful glucose, some salts and water, and excreting urea. A frequent trap is listing faeces as excreted; it is egested. Another trap is forgetting that reabsorption of water happens in the kidney tubules and is controlled by hormones.
Homeostasis
Homeostasis keeps internal conditions steady: temperature, blood glucose, water balance. Negative feedback is the key idea: a change is detected, a response reverses it. For temperature, sweating, vasodilation, shivering and vasoconstriction are the responses to learn. For blood glucose, insulin lowers glucose and glucagon raises it.
Water balance
If blood is too concentrated, more water is reabsorbed, producing small concentrated urine. If blood is too dilute, less water is reabsorbed and urine is large and dilute.
Worked example
After drinking a lot of water, blood becomes dilute; less water is reabsorbed in the kidney, so urine volume rises and concentration falls. Rule of thumb: more reabsorption means less, more concentrated urine.
Practical trap
When testing urine with Benedict's for glucose, remember heat is needed and a brick-red colour indicates a positive result; always compare with a control.