The kidneys filter the blood continuously, removing waste products and excess substances while keeping the water, salts, and other chemicals the body needs. This process is more sophisticated than simply straining blood through a sieve. The kidneys first filter a large amount of fluid from the blood, then selectively return useful substances to the bloodstream and add certain unwanted substances to the forming urine.
Most people have two kidneys, located toward the back of the abdomen on either side of the spine. Each kidney contains about a million tiny filtering units called nephrons. Within each nephron, a microscopic structure called the glomerulus performs the initial filtration, while a long tubular system adjusts the filtered fluid until it becomes urine.
What happens when blood reaches the kidneys?
Blood enters each kidney through a renal artery, which branches repeatedly into smaller blood vessels. Eventually, blood reaches tiny vessels that supply individual nephrons.
The filtration process begins in a cluster of specialized capillaries called the glomerulus. Blood enters the glomerulus through a small artery and flows through a network of very thin-walled capillaries.
The blood is under pressure inside these capillaries. That pressure pushes water and many small dissolved substances out of the blood and into a surrounding cup-shaped structure called Bowman’s capsule.
This produces a fluid called the filtrate.
The filtration barrier is selective. Water and small molecules such as glucose, amino acids, sodium, potassium, and urea can normally pass through. Larger components, particularly blood cells and most blood proteins, are retained in the bloodstream.
The result is not yet urine. It is a starting fluid that contains both waste products and substances the body may want to keep.
The nephron does most of the work
A nephron can be thought of as a filtration-and-adjustment system. It has two major functional parts: the glomerulus, where filtration begins, and a long series of tubules, where the filtered fluid is extensively modified.
After leaving Bowman’s capsule, the filtrate flows through the renal tubule. Along its path, the kidney retrieves substances the body needs and leaves behind substances that should be eliminated.
This is called tubular reabsorption.
For example, under normal conditions, essentially all of the glucose filtered into the nephron is reabsorbed. Large amounts of filtered water and sodium are also returned to the blood. Amino acids and other useful substances are similarly reclaimed.
At the same time, the tubules can move selected substances from the blood into the tubular fluid. This process, called tubular secretion, helps the kidneys eliminate substances that were not removed efficiently by glomerular filtration.
By the time the fluid has passed through the nephron, its composition is dramatically different from the original filtrate.
Why don’t the kidneys simply remove everything?
The kidneys are not designed to eliminate everything that enters them. Their job is to maintain the composition of the body’s internal environment.
A useful substance such as glucose illustrates the point. Glucose is small enough to pass through the glomerular filter, but under normal circumstances the kidney rapidly reabsorbs it. Losing large amounts of glucose in urine would waste an important energy source.
The same principle applies to water and electrolytes. The kidneys constantly adjust how much sodium, potassium, calcium, and water remain in the body. They can conserve these substances when the body needs them or allow more of them to leave in urine when there is an excess.
This selective handling is what makes kidney function much more than simple filtration.
How do the kidneys concentrate urine?
The kidneys must balance two competing needs: eliminating dissolved waste and preventing unnecessary water loss.
Part of the nephron called the loop of Henle helps establish conditions in the kidney that allow water to be conserved. The collecting ducts then adjust how much water leaves the tubular fluid and returns to the bloodstream.
A hormone called antidiuretic hormone (ADH) plays an important role in this process. When the body needs to conserve water, ADH increases the collecting ducts’ permeability to water. More water can then move back into the bloodstream, producing a smaller volume of more concentrated urine.
When the body has excess water, less ADH is released, the collecting ducts become less permeable to water, and more water remains in the tubular fluid and is excreted.
The kidneys can therefore produce urine that varies considerably in both volume and concentration depending on the body’s needs.
What exactly is removed from the blood?
One of the kidneys’ best-known tasks is removing urea, a waste product produced when the body breaks down proteins. Urea travels in the blood to the kidneys and is ultimately excreted in urine.
The kidneys also help eliminate other metabolic waste products, excess electrolytes, and various substances that the body does not need. They can also excrete some medications and their breakdown products.
But waste removal is only part of the kidneys’ role. Their filtering and regulatory activity helps control:
- Water balance, by adjusting how much water is excreted or conserved.
- Electrolytes, including sodium and potassium.
- Acid-base balance, by controlling hydrogen ions and bicarbonate.
- Blood pressure, partly through regulation of sodium and water and through hormonal mechanisms.
- Mineral balance, including calcium and phosphate.
The kidneys therefore function as both filters and highly precise chemical regulators.
What happens to the blood after filtration?
The blood that leaves the glomerulus still contains blood cells, most proteins, and the substances that were not filtered out. It then passes through another network of small blood vessels surrounding the renal tubules.
These vessels are important because substances can move in both directions between the tubular fluid and the blood.
Useful substances removed from the filtrate are reabsorbed into these blood vessels. Other substances can be secreted from the blood into the tubule for eventual elimination.
The blood ultimately leaves the kidney through the renal vein, carrying back the substances that have been retained or reclaimed.
Meanwhile, the remaining tubular fluid becomes urine. It flows from the nephrons into collecting ducts, then through the kidney’s drainage system into the ureters, which carry urine to the bladder.
Why is blood pressure important for filtration?
Glomerular filtration depends partly on pressure within the glomerular capillaries. That pressure must be high enough to drive fluid across the filtration barrier, but the kidney also has mechanisms that help keep filtration relatively stable despite normal changes in blood pressure.
The small arteries entering and leaving the glomerulus can change their diameter, altering blood flow and pressure within the glomerular capillaries. Hormonal and nervous-system signals also influence kidney blood flow and function.
Severe or prolonged disturbances in blood pressure can damage the delicate blood vessels and filtering structures of the kidneys. High blood pressure is therefore both a factor that can contribute to kidney damage and a condition that the kidneys themselves help regulate.
What prevents blood cells and large proteins from being filtered?
The glomerular filtration barrier has several layers that work together to restrict the passage of large particles.
The innermost layer is the wall of the glomerular capillaries, which contains tiny openings. Outside it is a specialized layer of proteins called the glomerular basement membrane. The outer layer consists of specialized cells called podocytes, whose extensions form narrow filtration gaps.
The barrier is selective not only because of molecular size but also because of electrical properties within the filtration barrier. Under normal conditions, this arrangement allows water and small dissolved molecules to cross while keeping most blood cells and large plasma proteins in the bloodstream.
Damage to the filtration barrier can allow proteins such as albumin to leak into urine. Significant protein in the urine can therefore be an important sign that the kidney’s filtering structures are not functioning normally.
How much blood do the kidneys handle?
Although the kidneys make up only a small fraction of the body’s total mass, they receive a large share of the heart’s output. This substantial blood supply reflects their constant role in monitoring and regulating the chemical composition of the blood.
The amount of fluid filtered through the glomeruli is also large compared with the relatively small amount of urine normally produced. The difference occurs because the renal tubules reabsorb most of the filtered water and many of the filtered solutes.
In other words, the kidneys do not filter only the portion of blood that will become urine. They repeatedly process a large volume of plasma and then carefully reclaim what the body needs.
How does filtration differ from urine formation?
These terms describe related but distinct stages.
Glomerular filtration is the movement of water and small dissolved substances from the blood into the nephron.
Reabsorption is the movement of useful substances and water from the nephron back into the blood.
Secretion is the movement of selected substances from the blood into the nephron.
Excretion is the final elimination of substances from the body in urine.
A simplified way to express the overall process is:
Excretion = filtration − reabsorption + secretion
This distinction explains why a substance can be filtered without necessarily appearing in urine in significant amounts. Glucose, for example, is normally filtered and then almost completely reabsorbed.
The kidneys’ remarkable ability lies not simply in filtering blood, but in continuously adjusting what is retained, what is added to the tubular fluid, and what is ultimately excreted.
