ADH Control and Kidney Treatments Revision Notes | GCSE Edexcel Higher Biology

1. Why Water Balance Matters

Your body is roughly 60% water, and keeping this level steady is literally a matter of life and death. Too much water and your cells swell up like balloons. Too little and they shrivel like raisins.

Your kidneys are the master controllers here. They can save water when you're dehydrated or dump excess when you've drunk too much. But how do they know what to do?

The answer lies in a clever hormone system that acts like a thermostat for your body's water levels. Let's see how this life-saving mechanism works.

2. ADH - Your Body's Water Controller

Think of ADH as your body's water-saving switch. When your blood becomes too concentrated (not enough water), your brain detects this and releases ADH from the pituitary gland.

ADH travels through your bloodstream to the kidneys, where it makes the tubule walls more permeable to water. This means more water gets reabsorbed back into your blood instead of being lost in urine.

Trap: Many students think ADH directly controls how much you drink. Actually, ADH controls how much water your kidneys save - drinking is controlled by thirst, which is separate.

3. The Negative Feedback Loop

ADH control works through negative feedback - a system that reverses changes to keep things stable. Here's how it works step by step:

Worked example: ADH Negative Feedback Control

Trace what happens when you become dehydrated and blood concentration rises

1. Receptors in the brain detect blood is too concentrated — These receptors are in the brain
2. Pituitary gland releases more ADH into bloodstream — ADH travels to the kidneys via blood
3. ADH makes kidney tubules more permeable to water — More water channels open in tubule walls
4. More water reabsorbed, blood concentration decreases — Less water lost in urine, blood diluted
5. Brain detects normal concentration, reduces ADH release — This is the 'negative' part - reverses the original change

Answer: Blood concentration returns to normal through negative feedback

Notice how this system is self-correcting. When the problem is fixed, the response automatically switches off. This prevents overcorrection.

4. When Kidneys Fail - Treatment Options

Sometimes kidneys stop working properly due to disease, infection, or injury. When this happens, waste products build up in the blood and water balance goes haywire.

There are two main treatment options: dialysis (artificial filtering) or transplant (replacement kidney). Each has advantages and disadvantages.

Dialysis works on the principle of diffusion. Blood flows through a machine containing a partially permeable membrane. Waste products diffuse out of the blood into dialysis fluid, while useful substances stay in the blood.

The dialysis fluid contains the same concentration of useful substances as healthy blood, so these don't diffuse out. Only waste products move across the membrane.

5. Comparing Treatment Options

Both dialysis and transplant can save lives, but they work very differently. Understanding their pros and cons helps explain why treatment choice depends on individual circumstances.

Worked example: Evaluating Kidney Treatments

Compare dialysis and transplant for a 45-year-old patient with kidney failure

1. No surgery required, no rejection risk, can start immediately — Lower immediate risk, readily available
2. Time-consuming (3 sessions per week), dietary restrictions, not portable — Significantly impacts quality of life
3. Normal life possible, no regular hospital visits, better long-term health — Much better quality of life if successful
4. Major surgery, rejection risk, immunosuppressive drugs needed, donor shortage — Higher risk but potentially better outcome

Answer: Choice depends on age, health, donor availability and patient preference

6. Exam Technique and Common Mistakes

ADH questions often test your understanding of negative feedback and the ability to explain cause and effect. Here's how to tackle them successfully.

Exam depth extension: In adh control and kidney treatments, connect why water balance matters to adh - your body's water controller using clear cause-and-effect statements. When you explain biological processes, include what changes, why it changes, and the consequence for cells, organisms, or populations. If calculations appear, show each step, include units, and end with a sentence that interprets the result in biological context.

Common Mistake: Students often say ADH 'controls thirst'. Actually, ADH controls water reabsorption in kidneys. Thirst is a separate mechanism that makes you want to drink.

Mark-winning phrase: 'ADH increases the permeability of kidney tubules to water' - this precise wording often appears in mark schemes.