The kidney is doing two different jobs at once, and students who never separate them find the whole topic confusing. It excretes toxic nitrogenous waste, and it osmoregulates — controls how concentrated the blood is. Same organ, same urine, two completely different purposes.
📚 What you need to know
Excretion is the removal of the toxic waste products of metabolism from the body.
The kidneys excrete nitrogenous waste, which comes from breaking down excess dietary amino acids and nucleic acids.
Nitrogenous waste is first converted to ammonia, which is highly toxic and cannot be stored.
Some organisms convert ammonia to the less toxic urea, which can be stored briefly at low concentration.
Urea must be diluted with water to form urine before it is excreted.
Osmoregulation is maintaining a safe balance of water and solutes — the osmotic concentration of body fluids.
Units of osmotic concentration are osmoles per litre (osmol L−1).
Cells in surroundings that are too dilute gain water and may burst; in surroundings that are too concentrated they lose water and shrink.
Excretion: getting rid of nitrogen
Excretion is often confused with egestion, so it is worth being precise. Excretion removes substances your own metabolism produced. Egestion removes undigested food that never entered your cells at all. Faeces are egested, not excreted.
Your body cannot store excess amino acids, so any you eat beyond what you need are broken down. Nucleic acids are broken down too. Both contain nitrogen, and that nitrogen ends up as ammonia.
Which of these an animal excretes depends on how much water it can spare. Freshwater fish are surrounded by water and simply release ammonia. Land mammals cannot afford that, so they pay the energy cost of making urea.
Why ammonia has to go
Ammonia is highly toxic, so it cannot be stored anywhere in the body and must be removed quickly.
Many organisms convert it into urea, which is less toxic and can remain in the body at low concentrations — but still has to be excreted before it builds up to a harmful level.
Urea must be dissolved in water to form urine before it can leave the body.
Urine is produced in the kidneys.
That last point is the hinge of the whole topic. Excreting urea costs you water. Every drop of urine you make to get rid of nitrogen is water leaving your blood. That is exactly why the same organ has to handle osmoregulation — the two jobs are physically tangled together.
Osmoregulation: getting the concentration right
Every cell in you is separated from tissue fluid by a partially permeable membrane, so water moves in and out by osmosis whenever there is a difference in concentration. Keeping that difference near zero is osmoregulation.
Definition
Osmoregulation is the control of the water and solute balance of body fluids, maintaining a constant osmotic concentration
The osmotic concentration is how much dissolved solute a fluid contains, measured in osmoles per litre (osmol L−1). The more solute dissolved, the lower the water potential.
Water always moves from the higher water potential to the lower one. The cell has no say in the matter, which is why the body has to control the fluid around it instead.
Putting it in exam language
A cell with a lower water potential than its surroundings gains water by osmosis. The internal pressure rises and the cell may burst.
A cell with a higher water potential than its surroundings loses water by osmosis. Internal pressure drops and the cell shrinks.
Plant cells are protected from bursting by their strong cell walls.
🧠
Water potential runs the other way to concentration
More solute means lower water potential. Pure water has the highest water potential of all. Water moves down the water potential gradient, which means it moves towards the more concentrated solution.
The two jobs, side by side
Excretion
Osmoregulation
The problem being solved
Nitrogenous waste is toxic and builds up
Cells burst or shrink if the blood is the wrong concentration
What is removed
Urea, and other metabolic wastes
Water and salts, in whatever amounts restore the balance
How much urine
Enough to carry the urea away
Varies hugely — small and concentrated, or large and dilute
Is it homeostasis?
It supports it by removing toxins
Yes — it is a textbook negative feedback loop
Why this matters for the next page. The kidney cannot decide to excrete urea without water, so it filters an enormous volume of blood and then takes back whatever water and useful solutes it needs. That two-stage design — filter everything small, then reclaim what you want — is the whole story of urine production.
Worked examples
WE 1
Why convert ammonia to urea?
Explain the advantage to a land mammal of converting ammonia into urea before excreting it. (3 marks)
Point 1: the problem with ammonia
Ammonia is highly toxic, so it cannot be stored and would have to be excreted immediately in a very large volume of water.
Point 2: what urea allows
Urea is less toxic, so it can be tolerated in the blood at low concentrations and excreted in a smaller volume of water.
Point 3: link to the environment
A land mammal has limited access to water, so conserving water is essential for survival.
Less toxic means less water needed to dilute ita good extra mark: making urea uses energy, so it is a trade-off, not a free upgrade.
WE 2
Predicting what happens to cells
Human blood plasma has an osmotic concentration of about 0.30 osmol L−1. Red blood cells are placed in three solutions: A at 0.05 osmol L−1, B at 0.30 osmol L−1 and C at 0.60 osmol L−1. Predict and explain what happens to the cells in each. (4 marks)
Solution A
A is more dilute than the cell contents, so it has a higher water potential. Water enters by osmosis, internal pressure rises and the cells may burst.
Solution B
B matches the cells at 0.30 osmol L−1, so there is no net movement of water and the cells stay the same.
Solution C
C is more concentrated at 0.60 osmol L−1, so it has a lower water potential. Water leaves by osmosis and the cells shrink.
The general rule
In every case, water moves from the higher water potential to the lower one across the partially permeable membrane.
Compare each solution with 0.30, then let water move down the gradientsay “no net movement”, not “no movement”. Water molecules still cross in both directions.
WE 3
Excretion or egestion?
A student writes that “the kidneys and the large intestine both excrete waste from the body”. Evaluate this statement. (3 marks)
Point 1: the kidney half is correct
The kidneys excrete urea, which is a toxic waste product of the organism’s own metabolism, produced from the breakdown of excess amino acids.
Point 2: the intestine half is not
Faeces are mainly undigested food that has never been absorbed into the body’s cells, so removing them is egestion, not excretion.
Point 3: give the deciding test
Excretion only applies to substances made by metabolism inside the body.
If your cells never made it, getting rid of it is not excretionbile pigments in faeces are excreted, which is a nice nuance to mention if you have space.
💡 Exam tips
Define excretion as removing the toxic waste products of metabolism — the full phrase is usually the mark.
Get the order right: amino acids → ammonia → urea → urine.
Say osmosis and partially permeable membrane whenever water movement is involved.
Compare water potentials explicitly: “the solution has a lower water potential than the cell“.
Remember the units: osmol L−1.
⚠ Common mistakes
Calling faeces excretion. It is egestion — the material was never part of the body’s metabolism.
Saying urea is made in the kidney. Urea is made in the liver. The kidney only removes it.
Getting water potential backwards. More solute means lower water potential, not higher.
Saying animal cells burst because of the cell wall. They burst because they lack one.
Treating excretion and osmoregulation as the same thing. They share an organ, not a purpose.
Up next: How Urine is Produced. This is the biggest page in the topic, so take it in pieces: the structure of the nephron first, then ultrafiltration, then selective reabsorption, then the loop of Henle, then ADH. Each stage only makes sense once you can picture where in the nephron it happens.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
