7.18B–7.22BMaintaining water and nitrogen balance
Biology only — not on the Combined Science specification
Water leaves the body by three routes, and only one of them is controlled.
• Via the lungs during exhalation. • From the skin in sweat, which also loses ions and urea. • Via the kidneys in the urine, which removes excess water, ions and urea.
There is no control over water, ion or urea loss by the lungs or the skin. You lose water when you breathe whether you can spare it or not, which is why the kidneys have to do all the adjusting.
Osmotic changes in body fluids act directly on cells. If body cells lose or gain too much water by osmosis they do not function efficiently: in fluid that is too dilute water enters and the cell swells, and in fluid that is too concentrated water leaves and the cell shrinks. Either way the reactions inside are disturbed, which is why water balance is regulated rather than left to chance.
The kidneys produce urine by filtration of the blood and selective reabsorption of useful substances such as glucose, some ions and water. Selective is the important word: everything small is filtered out first, and what the body still needs is taken back.
You are not required to know other parts of the urinary system, the structure of the kidney, or the structure of a nephron.
When you are given a table of glucose, ions and urea before and after filtration, the pattern is always the same: glucose disappears from the urine entirely because it is all reabsorbed, urea is concentrated because none is, and ions are somewhere in between depending on what the body needs.
Translating between the table and the bar chart is examined in both directions, and each form makes a different thing obvious. The table gives you the exact figures to calculate with. The chart shows the pattern at a glance: one bar disappearing entirely, one growing slightly, one growing enormously.
Reading the chart, say what each pair of bars means rather than describing their heights. Glucose falls to zero because all of it is reabsorbed. Urea rises steeply because none is reabsorbed while water is, so what remains is concentrated. Ions rise only somewhat, because some are reabsorbed according to what the body needs.
Worked example
A table gives concentrations in g/dm³ before filtration (blood plasma) and in the urine: glucose 1.0 → 0.0, ions 3.7 → 6.0, urea 0.3 → 20.0. Describe what has happened to each.
- Glucose: 1.0 → 0.0. All of it has been reabsorbed, because the body needs it.
- Ions: 3.7 → 6.0. Some are reabsorbed and the excess is concentrated as water is taken back.
- Urea: 0.3 → 20.0. None is reabsorbed, and it is concentrated as water returns to the blood.
Answer: Glucose is completely reabsorbed, ions are partly reabsorbed according to need, and urea is not reabsorbed at all — so it is the most concentrated substance in the urine.
Higher tier only: the digestion of proteins from the diet results in excess amino acids, which need to be excreted safely. In the liver these amino acids are deaminated to form ammonia. Ammonia is toxic, so it is immediately converted to urea for safe excretion.
Higher tier only: the water level in the body is controlled by the hormone ADH, which acts on the kidney tubules. ADH is released by the pituitary gland when the blood is too concentrated. It acts on the kidney tubules by increasing their permeability to water, so more water is reabsorbed back into the blood from the tubules and less leaves in the urine. This is controlled by negative feedback — more ADH means more water reabsorbed, which dilutes the blood, which reduces ADH release.
People who suffer from kidney failure may be treated by organ transplant or by a mechanical device — the dialysis machine. Evaluating the two against each other is a standard question, and the specification frames it exactly that way: mechanical device or transplant. In dialysis the patient's blood flows on one side of a partially permeable membrane, with dialysis fluid on the other. The fluid contains the normal concentrations of glucose and ions but no urea, so urea diffuses out of the blood down its concentration gradient, while glucose and useful ions do not — there is no gradient for them to move down. Dialysis has to be repeated regularly because it does nothing between sessions.
Common mistake
Dialysis fluid is not pure water. If it were, glucose and useful ions would diffuse out of the blood as well and the patient would lose them. It contains normal blood concentrations of what should stay, and none of what should go.
- Selective reabsorption
- the process in the kidneys by which useful substances such as glucose, some ions and water are taken back into the blood after filtration
- Urea
- the waste product removed from the body in urine, and lost in smaller amounts in sweat
Before you practise
- Filtration at the glomerulus is by size: water, glucose, ions and urea pass out; proteins and blood cells are too large.
- All the glucose is selectively reabsorbed, along with the water and ions the body needs; urea and excess water and ions leave as urine.
- Excess amino acids are deaminated in the liver, producing ammonia, which is converted to urea.
- ADH is released when the blood is too concentrated, and makes the collecting duct more permeable so more water is reabsorbed.
- Dialysis works, but has to be repeated several times a week and controls the blood composition only between sessions.
- A transplant is a long-term fix but needs a donor and immunosuppressant drugs to prevent rejection.