How your kidneys work

Two organs, six jobs

Your kidneys sit at the back of your abdomen, one on each side, roughly at the level of the lowest ribs. They are about the size of a closed fist. Almost everyone can describe what the heart does, or the lungs. Almost nobody can describe what the kidneys do, which is why the diagnosis so often arrives as a shock: the organ was never on your mind, and now it is the only thing on your mind.

It helps to think of them not as a filter but as a chemistry department. Blood arrives, and the kidneys decide, continuously, what should stay and what should go. They are doing at least six jobs at once.

1. They remove waste products

Everything your body does, from digesting a meal to moving a muscle, produces waste. The kidneys clear it. When they stop, that waste accumulates in the blood, a state called uraemia. Uraemia is not an abstraction: it is nausea, a metallic taste, loss of appetite, itching, restless legs, broken sleep, difficulty concentrating, and a fatigue that sleep does not touch. People often describe it as feeling poisoned, which is close to what is happening.

2. They control how much fluid is in you

Healthy kidneys balance what you drink against what you need, hour by hour, and pass the difference out as urine. When they fail, fluid stays. It gathers in the ankles and the face, and eventually in the lungs, where it causes breathlessness that is worse lying flat. This is why weight matters so much once dialysis starts: on dialysis, your weight is largely a measure of the water you are carrying, and the machine has to take off what your kidneys no longer can.

3. They keep salts in a narrow safe range, and potassium above all

Potassium is the one that deserves your respect. It is essential, it is in most healthy food, and the safe range in the blood is narrow. Healthy kidneys hold it there without you ever thinking about it. Failing kidneys cannot, and a high potassium level can disturb the rhythm of the heart. This is not a reason to fear food, and it is a very good reason not to skip a dialysis session and not to use a salt substitute, most of which are potassium chloride.

4. They help control your blood pressure

The kidneys manage blood pressure through the fluid they hold or release and through hormones they produce. The relationship runs in both directions, and this is the cruel loop at the centre of most kidney disease: high blood pressure damages kidneys, and damaged kidneys raise blood pressure. In Seychelles this loop is the main engine of the problem. The Ministry of Health's 2023 national survey found high blood pressure in 42% of women and 38% of men aged 18 to 74, rising to 77% at ages 55 to 74, and it found that among people being treated for it, only about 40% were controlled to target.

5. They tell your body to make red blood cells

The kidneys make a hormone called erythropoietin, which instructs the bone marrow to produce red blood cells. Failing kidneys make too little of it, so the marrow makes too few, and the result is anaemia: breathlessness, pallor, and an exhaustion that people frequently mistake for depression or simply for getting older. It is one of the most treatable parts of kidney failure, which is exactly why it should not be quietly endured.

6. They activate vitamin D, and manage bone and blood vessels

Vitamin D from food or sunlight is not yet usable. It has to be switched on, and the final step of that activation happens in the kidney. When kidneys fail, that step fails, and the whole system that manages calcium, phosphate and the parathyroid glands goes out of balance. Phosphate rises. The parathyroid glands work harder. Calcium is pulled out of bone. Over years, calcium and phosphate deposit where they should not, including in blood vessels, and the National Kidney Foundation is blunt that this raises the long-term risk of heart attack and stroke. It also drives the itching that so many people on dialysis describe.

What the numbers on your blood test mean

Two numbers do most of the work in kidney medicine, and it is worth understanding both, because one of them is routinely over-read and the other is routinely ignored.

eGFR: an estimate, not a verdict

eGFR stands for estimated glomerular filtration rate. It is a calculation, made from a blood test (usually creatinine) together with your age and sex, that estimates how much blood your kidneys are filtering. It is reported in mL/min/1.73 m2, and it is the number most people are given.

The word to hold on to is estimated. KDIGO, the international kidney guideline body, is clear that eGFR is less reliable at the extremes of muscle mass, in amputees, in pregnancy and during acute illness, and that a different blood test (cystatin C) should be used to confirm it when a more accurate estimate would actually change what is done. A single eGFR result on a bad week is not your future. The trend across months is far more informative than any one value, and your team will be watching the trend.

Albuminuria: the number people forget to ask about

Albumin is a protein that should stay in the blood. When the kidney's filters are damaged, it leaks into the urine, and a urine test called the ACR (albumin-to-creatinine ratio) measures how much. It is one of the earliest signs of kidney damage, and it is a powerful predictor of what happens next, independently of eGFR.

This is why KDIGO stages kidney disease by cause, by filtration andby albuminuria together, rather than by eGFR alone. Two people can both be told they have "stage 3" kidney disease and be in genuinely different situations because of it. If nobody has mentioned your ACR, it is a reasonable question to ask.

Residual kidney function: the thing worth protecting

Even when kidneys are failing badly, they are usually still doing something. Some filtering. Some urine. Clinicians call this residual kidney function, and it is quietly precious.

It matters because it is subtracted from the work the treatment has to do. People who still pass urine generally have a more generous fluid allowance and a more forgiving diet, and their potassium is easier to keep in range. Residual function usually declines over time, and it tends to be preserved for longer on peritoneal dialysis than on in-centre haemodialysis. That is one of the real, practical arguments in the treatment choice, and it is worth understanding before the conversation, not after it.

What dialysis can and cannot replace

Here is the honest limit, and we would rather you hear it from us than discover it. In the words of the US National Institute of Diabetes and Digestive and Kidney Diseases, haemodialysis "can replace part, but not all, of your kidney function".

Two healthy kidneys work continuously, every second, adjusting constantly. A standard haemodialysis schedule of three sessions a week, of about four hours each, is an intermittent approximation of that. It is a remarkable treatment and it keeps people alive for decades. But it is not the same thing, which is why:

• the diet and the fluid limits do not disappear once dialysis starts, they become more important;
• medicines are still needed for phosphate, for anaemia, for bone and for blood pressure, because the machine does not make hormones;
• the two-day gap between sessions is when potassium and fluid build up most, and it is the interval clinicians worry about;
• more frequent or longer dialysis is generally gentler, which is one reason home therapies exist.

None of that means dialysis is not worth having. It means dialysis is a partnership: the machine does part of the job, and you and your team do the rest. Understanding which part is which is the beginning of getting good at this.