Excretion in Humans
Senior 2 · Biology
Excretion in Humans
Lesson 4 of 4
22 min
Let's begin
Your urine is not always the same. After an afternoon of football in the sun with little to drink it is dark and there is not much of it; after drinking several glasses of water it is pale and plentiful. Your kidneys are constantly adjusting to keep your blood at the right concentration. Looking after them is far cheaper and easier than treating them when they fail.
Worked example
Explain the difference in urine between (a) a footballer who played for two hours in hot sun and drank little, and (b) a student who drank 2 dm³ of water at a family celebration.
Keeping the water and salt concentration of the blood constant in this way is called osmoregulation.
When the blood is too dilute, less ADH is released, the collecting ducts let less water through, and a large volume of dilute urine is produced. Because the response reverses the change that caused it, this is called negative feedback.
| Factor | Effect on urine |
|---|---|
| Drinking a lot of water | more urine, more dilute (less ADH) |
| Hot weather or hard exercise (sweating) | less urine, more concentrated (more ADH) |
| Cold weather | more urine, because less water is lost as sweat |
| Salty food | more concentrated urine as the extra salt is excreted, and you feel thirsty |
| Protein-rich diet | more urea in the urine |
| Alcohol and large amounts of caffeine | more, dilute urine: they act as diuretics (alcohol reduces ADH release), so the body can become dehydrated |
Kidney stones are hard crystals of salts that can form in the kidney, especially when a person often drinks too little water. They can block the ureter and cause severe pain.
If both kidneys fail, urea and excess salts and water build up in the blood. There are two treatments:
Worked example
Dialysis fluid contains glucose and salts at the same concentrations as normal blood plasma, but no urea. Why? There is no concentration gradient for glucose and useful salts, so they stay in the blood. Urea is high in the blood and absent from the fluid, so it diffuses out. Any excess salt in the blood also diffuses out until the levels are normal.
Tip
ADH = Anti-Diuretic Hormone. "Diuresis" means producing lots of urine, so ADH works against it: more ADH → less urine, more concentrated.
In ADH questions, write the full chain: stimulus → brain detects → ADH released → collecting ducts more permeable → more water reabsorbed → less, concentrated urine.
Why does a person produce less urine on a hot day?
Solution: More water is lost in sweat, so the blood becomes more concentrated. More ADH is released, the collecting ducts reabsorb more water, and less urine is made.
Why does drinking alcohol make a person urinate more?
Solution: Alcohol reduces the release of ADH, so the collecting ducts reabsorb less water and a large volume of dilute urine is produced.
Give three habits that help keep the kidneys healthy.
Solution: Any three of: drink enough clean water; do not hold urine for long; limit salt; do not misuse painkillers; control blood pressure and diabetes; practise good hygiene; avoid alcohol.
Why must a dialysis machine's fluid be replaced continuously?
Solution: Urea builds up in the fluid as it diffuses out of the blood. Fresh fluid keeps the urea concentration in it low, so the concentration gradient stays steep and urea keeps diffusing out.
Remember
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