How the Kidneys Filter Blood: Nephrons, Reabsorption and Urine

Every minute, roughly a fifth of the blood pumped by the heart passes through the kidneys. Two bean-shaped organs, each about the size of a fist, clean that blood continuously. They remove waste, balance water and salts, and help set blood pressure, all without rest.

The kidneys do this work with a surprising method. They first filter far more fluid than the body needs, then carefully reclaim almost everything useful. This article follows the process through the basic filtering unit, the nephron.

Two kidneys and about a million filters

Each kidney contains roughly one million nephrons. A nephron is a microscopic tube with a filter at one end and a long, twisting channel that runs from the outer part of the kidney into its inner core. Blood enters each nephron through small vessels and leaves through another set. Between these two vessel sets, the nephron decides what stays in the blood and what leaves it.

Because there are so many nephrons, the total filtering surface is very large. This is one reason a person can lose most of one kidney and still keep reasonable kidney function.

The glomerulus: filtering under pressure

The first stage happens in the glomerulus, a tight ball of capillaries that sits at the closed end of each nephron. Blood pressure inside these capillaries is higher than in most other capillary beds. The pressure pushes water and small dissolved molecules through a thin filter made of three layers: the capillary wall, a basement membrane, and specialised cells called podocytes.

The filter lets through water, glucose, amino acids, salts, urea and other small molecules. It holds back blood cells and most large proteins. The fluid that passes through is called filtrate, and it is almost identical to plasma except for the proteins.

The volume is enormous. Across the whole body, the kidneys filter roughly 180 litres of filtrate per day. The body holds only about 3 to 5 litres of blood, so the entire blood supply would pass through the filters dozens of times in a day. Almost all of that filtrate must be returned to the blood, because a typical urine output is only about 1 to 2 litres a day.

The tubules: reclaiming what the body needs

After the glomerulus, the filtrate enters the renal tubule. Here most of the water and useful solutes are pulled back into the surrounding capillaries. This process is called reabsorption. The tubule is divided into regions, and each region has a different job.

The proximal tubule does the bulk of the work. It reclaims about two-thirds of the filtered water and sodium, along with nearly all of the glucose and amino acids. Glucose is normally reabsorbed completely, so healthy urine contains none. Active transport pumps sodium out of the tubule cells, and water, glucose and other solutes follow.

The loop of Henle creates a salt gradient in the surrounding tissue. The descending limb lets water leave, while the ascending limb pumps salt out without letting water follow. This gradient lets the kidney later concentrate urine by pulling water out of the collecting duct.

Secretion: removing extra waste

Filtration catches some waste, but the tubules also add substances from the blood into the filtrate. This second process is called secretion. It removes potassium, hydrogen ions and certain drugs that the glomerulus did not catch. Secretion gives the kidney a second chance to clear substances that would otherwise build up in the blood.

By the time filtrate reaches the collecting duct, most useful substances have been returned and the unwanted ones have been added. The duct carries the remaining fluid toward the renal pelvis, where it collects as urine and drains to the bladder through the ureters.

Hormones that fine-tune the output

The kidney does not run on fixed settings. Hormones adjust reabsorption to match the body's current needs.

Antidiuretic hormone (ADH), released from the posterior pituitary when blood becomes too concentrated, makes the collecting duct more permeable to water. More water is reabsorbed, and the urine becomes more concentrated. Dehydration raises ADH, and drinking plenty of water lowers it.

Aldosterone, produced by the adrenal cortex, increases sodium reabsorption and potassium secretion in the distal tubule. Because water follows sodium, aldosterone also helps raise blood volume and blood pressure.

The kidney also produces hormones that act elsewhere. It releases erythropoietin, which signals the bone marrow to make red blood cells, and it activates vitamin D so the body can absorb calcium from food.

The table: what each region does

Region	Main job	Typical result
Glomerulus	Filters blood under pressure	About 180 L of filtrate per day
Proximal tubule	Reclaims most water and sodium	Glucose and amino acids fully recovered
Loop of Henle	Builds the salt gradient	Water recovery continues
Distal tubule	Fine-tunes sodium and potassium	Controlled by aldosterone
Collecting duct	Adjusts final water balance	Controlled by ADH

The figures are approximate because they vary with hydration, diet and health.

Blood pressure and acid-base balance

The kidneys help control blood pressure through several routes. They adjust blood volume by changing how much sodium and water they keep. They also release renin when blood pressure falls, which starts a chain of hormone steps that eventually raises aldosterone and narrows blood vessels.

The kidneys also manage the body's acid-base balance. They excrete excess hydrogen ions and regenerate bicarbonate, which buffers the blood. This matters because blood pH must stay in a narrow range, and even small shifts disturb enzymes and nerve function.

What changes the output

Everyday habits and substances shift the kidney's settings. Alcohol suppresses the release of ADH, which is why drinking increases urine output. Many diuretic medicines block sodium reabsorption at specific points along the tubule, so more water and salt leave in the urine. Doctors use these drugs to lower blood volume and blood pressure, but they must be dosed carefully, because too much fluid loss can drop pressure dangerously.

Urine is also a useful diagnostic window. A urine test can reveal glucose when blood sugar is high enough to exceed the reabsorption capacity of the proximal tubule, protein when the glomerular filter leaks, and blood cells when the filter or tubule is damaged. Because the kidney filters so much fluid, even small changes in the filter can appear in a routine urine sample long before symptoms develop.

When the kidneys fail

If the nephrons are damaged, the glomerular filter loses its ability to remove waste. Waste products such as urea and creatinine build up in the blood. Chronic kidney disease is often slow and silent, which is why routine checks of blood and urine matter.

Dialysis replaces some of the kidney's filtering job. In hemodialysis, blood flows through an external machine that removes waste and extra fluid. In peritoneal dialysis, the lining of the abdomen acts as the filter. Transplantation is another option, but dialysis remains the common treatment while patients wait.

Key takeaways

The kidneys filter far more fluid than the body keeps, then recover nearly all of it. Filtration, reabsorption and secretion work together.

Each nephron has a filter, a reabsorbing tubule and a set of hormonal controls. Water and sodium balance follows the body's needs.

ADH and aldosterone fine-tune the final result, while the kidney also shapes blood pressure, acid-base balance and red blood cell production.

Because so many nephrons work together, the kidneys can lose function slowly without obvious symptoms. Dialysis and transplantation step in when the filters can no longer keep up.