How Your Kidneys Keep Water and Salts in Balance

Blood enters each kidney through a large artery and passes into tiny filtering units called nephrons. Each kidney contains many nephrons, and each one begins with a bundle of small blood vessels. Pressure pushes water and many small dissolved substances out of the blood and into a thin tube.

This first fluid contains useful materials as well as waste. Water, salts, glucose and other small molecules can enter it, while blood cells and most large proteins normally remain in the bloodstream. If the kidney simply removed everything that passed through the filter, your body would quickly lose materials it still needed.

The nephron therefore does much more than strain the blood. As the fluid travels through its tubes, nearby blood vessels take back useful substances. This return process is called reabsorption. Waste and excess materials stay in the tube and continue toward the bladder.

Your kidneys do not produce the same urine every hour. After you drink plenty of water, they can allow more water to leave, making urine more dilute. When you have not had enough water, they can return more water to the blood, making urine more concentrated.

A hormone called antidiuretic hormone, or ADH, helps control this change. When the brain detects that the blood is becoming too concentrated, more ADH is released. It makes the final parts of the nephrons more able to return water to the bloodstream. When the blood contains plenty of water, less ADH is released and more water can leave in urine.

This is an example of negative feedback. A change away from the useful range triggers responses that push conditions back toward that range. The word negative does not mean bad. It means the response works against the original change.

The kidneys also adjust levels of sodium, potassium and other ions. These charged particles help nerves send signals, muscles contract and cells control water movement. Too much or too little can disturb normal body functions, so the kidneys return or remove different amounts as conditions change.

A simple nephron story looks like this:

• •Blood is filtered.
• •Useful substances are returned.
• •Extra ions and wastes remain in the fluid.
• •Water return changes with the body's needs.
• •The remaining fluid becomes urine.

Glucose is normally filtered into the nephron and then almost completely returned to the blood. The carriers that bring it back can only work so fast, though. In uncontrolled diabetes, blood glucose can rise so high that the filtered amount exceeds this limit, and glucose spills into the urine. Because dissolved glucose holds water in the tube, more water leaves too, which is why frequent urination and strong thirst are classic warning signs. The full name diabetes mellitus refers to this sweetness.

Other substances affect the same system. Alcohol reduces the release of ADH, so the kidneys return less water and more dilute urine is produced, which can leave you dehydrated. Doctors use these connections when they test urine. Protein in urine can suggest that the filters are letting through molecules they normally hold back, while very dark or very pale urine gives a rough clue about hydration. A single test rarely tells the whole story, but urine offers a useful window into kidney function.

When the kidneys fail, waste and excess water build up in the blood. In haemodialysis, blood is pumped from the body through a filter containing a partially permeable membrane, with a liquid called dialysis fluid flowing on the other side. Small waste molecules such as urea diffuse across the membrane down their concentration gradient. The dialysis fluid contains glucose and salts at about normal blood levels, so useful substances are not lost. The fluid flows in the opposite direction to the blood, keeping a concentration difference along the whole length of the filter.

Dialysis keeps people alive, but it is demanding. Patients typically need several sessions a week, each lasting several hours, and must limit what they eat and drink in between, because the machine works only part of the time while healthy kidneys work constantly. A transplant can restore normal function. The first long-term successful kidney transplant was performed in Boston in 1954, between identical twins, which avoided rejection by the immune system. Because one healthy kidney can do the work of two, living people can donate a kidney.

ADH can only help water return to the blood if something draws it out of the tubes, and that something is salt. Each nephron has a hairpin-shaped section called the loop of Henle, which dips deep into the inner part of the kidney, the medulla, and then climbs back. Cells in the rising side pump salt out of the fluid but do not let water follow. Because fluid flows down one side of the loop and up the other, this effect builds along its length, making the tissue deep in the medulla far saltier than blood. Biologists call this arrangement a countercurrent system.

The final collecting ducts pass back down through this salty tissue on their way to the bladder. When ADH makes their walls more permeable, water moves out of the ducts by osmosis into the surrounding tissue and returns to the blood, leaving concentrated urine behind. Longer loops can build a steeper salt gradient. Desert animals show this clearly. The kangaroo rat of North American deserts has very long loops and produces urine many times more concentrated than its blood, so it can survive largely on the water released when its body breaks down the dry seeds it eats.

The scale of kidney filtering is easy to underestimate. In a healthy adult, the kidneys filter roughly 180 litres of fluid from the blood every day, yet a typical day's urine is only around one and a half litres. That means more than 99 per cent of the filtered water is returned to the blood. Since an adult has only about three litres of blood plasma, the whole volume passes through the filters dozens of times a day. Most of the filtered sodium is reclaimed too, along with nearly all of the glucose, so the tiny fraction left behind can be adjusted precisely.

Filtering so much and then taking most of it back may seem wasteful, but it has an advantage. The kidneys do not need to recognise every unwanted substance individually. Anything small enough to pass the filter and not deliberately reclaimed is removed quickly, including many drugs and waste products. Doctors use this to judge kidney health. Muscles release a waste product called creatinine at a fairly steady rate, and healthy kidneys clear it efficiently. A blood test for creatinine lets doctors estimate the filtration rate, and a falling estimate over time is one of the main ways chronic kidney disease is detected.

Your kidneys protect homeostasis by filtering blood and then carefully choosing what to return. By adjusting water, salts and waste, they keep the body's internal conditions steadier even when your food, drink and activity change.