Your kidneys are two fist-sized organs that quietly perform several essential jobs every minute. They remove waste products from the blood, control how much water and salt remain in the body, help regulate blood pressure, maintain the right balance of acids and bases, and produce hormones involved in red blood cell production and bone health.
Most people can live normally with one healthy kidney because each kidney has considerable functional reserve. But when kidney function is seriously impaired, wastes, fluids, acids, and minerals can build up in the body and affect many organs.
Understanding how the kidneys work starts with their basic job: they continuously filter blood and then precisely adjust what the body keeps and what it eliminates.
What do the kidneys do?
The kidneys have several closely connected functions.
Their best-known role is producing urine. As blood passes through the kidneys, they filter out water and small dissolved substances. The kidney then takes back most of the substances the body needs, while leaving excess water and unwanted substances to become urine.
The kidneys also maintain the body’s internal chemical balance. They regulate levels of electrolytes such as sodium, potassium, calcium, and phosphate and help keep the blood’s acidity within a narrow range.
They also have hormonal functions. The kidneys release erythropoietin, a hormone that signals the bone marrow to make red blood cells when the body needs more of them. They also activate vitamin D into a form that helps the intestines absorb calcium and supports normal bone metabolism. In addition, the kidneys participate in the hormonal systems that regulate blood pressure and fluid balance.
These functions are not separate tasks performed at different times. They are integrated into the kidney’s continuous processing of blood.
Where are the kidneys and how are they organized?
The kidneys sit toward the back of the abdomen, one on each side of the spine, beneath the lower ribs. The right kidney is usually positioned slightly lower than the left because of the liver.
Blood enters each kidney through a renal artery. After the kidney processes the blood, the filtered blood leaves through a renal vein. Urine drains from each kidney through a ureter into the bladder, where it is stored until it leaves the body through the urethra.
Inside each kidney are roughly a million microscopic filtering units called nephrons. A nephron is the kidney’s basic functional unit. Each one contains a filtering structure called the glomerulus and a long series of tubules that modify the filtered fluid.
The large number of nephrons allows the kidneys to process a substantial amount of blood while making highly selective adjustments to the final urine.
How does a kidney filter blood?
Blood enters a nephron’s glomerulus through a tiny blood vessel called an afferent arteriole. The glomerulus is a cluster of small blood vessels enclosed by a structure called Bowman’s capsule.
Blood pressure inside these capillaries pushes water and small dissolved substances through a specialized filtration barrier. The barrier allows many small molecules to pass but normally keeps blood cells and most large proteins in the bloodstream.
The resulting fluid is called filtrate. It contains water and substances such as glucose, sodium, chloride, amino acids, urea, and other small molecules. At this point, however, it is not yet urine.
This distinction is important: the kidneys do not simply filter the blood and throw the filtered material away. Most of the filtered water and useful substances are recovered before the final urine is produced.
What happens to the filtered fluid?
The filtrate flows from Bowman’s capsule into the nephron’s tubules. Along the way, the kidney selectively moves substances between the tubular fluid and the surrounding blood.
This process has two complementary parts: reabsorption and secretion.
Reabsorption returns useful substances to the blood
Reabsorption means moving substances from the tubular fluid back into the bloodstream. The kidneys reabsorb most of the water that was initially filtered, along with virtually all filtered glucose under normal conditions and large amounts of sodium, chloride, bicarbonate, amino acids, and other substances the body needs.
The proximal tubule, the first major segment of the nephron, performs much of this recovery. Cells lining the tubule use transport proteins and energy-dependent processes to move substances across their membranes.
Water follows dissolved substances through the appropriate pathways, allowing the kidney to recover large quantities of fluid without losing essential nutrients and electrolytes.
Secretion adds selected substances to the tubular fluid
The kidneys also actively move certain substances from the blood into the tubular fluid. This is called secretion.
Secretion helps eliminate substances that were not removed sufficiently by filtration. It is particularly important for controlling potassium and acid-base balance and for removing certain drugs and other organic compounds.
Together, filtration, reabsorption, and secretion determine what ultimately leaves the body in urine.
How do the kidneys decide how much water to keep?
The kidneys constantly adjust urine concentration according to the body’s hydration and other needs.
A major part of this process occurs in the loop of Henle, a U-shaped portion of the nephron. Its descending and ascending limbs have different properties. The surrounding kidney tissue becomes increasingly concentrated deeper into the kidney, creating an environment that allows the nephron to conserve water when necessary.
The hormone antidiuretic hormone (ADH), also called vasopressin, is central to this regulation. When the body needs to conserve water, ADH makes portions of the kidney’s collecting ducts more permeable to water. Water can then move out of the tubular fluid and back into the bloodstream.
When the body has excess water, less ADH is released. The collecting ducts become less permeable to water, so more water remains in the tubular fluid and is excreted as more dilute urine.
This is why urine can range from relatively concentrated to very dilute depending on hydration and other physiological conditions.
How do the kidneys control salt and electrolytes?
Sodium is especially important because it influences both fluid volume and blood pressure. The kidneys continuously adjust how much sodium is reabsorbed and how much is excreted.
Other electrolytes are regulated through similarly selective processes. Potassium, for example, is filtered and then handled differently in various parts of the nephron. Under the influence of hormones and other signals, the kidneys can increase or decrease potassium secretion into urine.
The hormone aldosterone promotes sodium reabsorption and potassium secretion in parts of the distal nephron. This helps regulate the composition of the blood and contributes to control of blood volume and blood pressure.
The kidneys also regulate calcium and phosphate balance, working with hormones such as parathyroid hormone and the active form of vitamin D.
How do the kidneys help control blood pressure?
The kidneys influence blood pressure both by controlling fluid and sodium balance and through hormonal signaling.
Specialized cells near each nephron monitor conditions related to blood flow, sodium delivery, and pressure. When the kidneys detect circumstances suggesting that blood pressure or effective circulating volume is too low, they can release renin.
Renin initiates a hormonal cascade called the renin-angiotensin-aldosterone system. This system ultimately promotes blood-vessel constriction and increased sodium and water retention, actions that can raise blood pressure and restore circulating volume.
The kidneys therefore participate in blood-pressure regulation in both immediate and longer-term ways.
How do the kidneys control blood acidity?
Cells in the body constantly produce acids as a result of normal metabolism. The kidneys help prevent these acids from accumulating.
They do this partly by secreting hydrogen ions, which are associated with acidity, into the tubular fluid. The kidneys also conserve and generate bicarbonate, an important buffer that helps neutralize acids in the blood.
The lungs and kidneys share responsibility for maintaining acid-base balance. The lungs regulate carbon dioxide relatively quickly by changing breathing, while the kidneys provide more sustained control by adjusting acid excretion and bicarbonate handling.
When kidney function is severely reduced, the body’s ability to remove acid can become impaired, potentially leading to a condition called metabolic acidosis.
How is urine formed and eliminated?
By the time tubular fluid reaches the collecting ducts, most of its useful contents have been returned to the blood. What remains includes substances the body needs to eliminate, such as urea and excess electrolytes, along with a variable amount of water.
The fluid becomes urine as it passes through the collecting ducts and then drains into larger structures inside the kidney. It enters the renal pelvis, a funnel-shaped area that leads into the ureter.
The ureters carry urine to the bladder. The bladder stores it until nerve signals and muscle contractions coordinate urination through the urethra.
Urine therefore represents the final result of a highly selective process. It is not simply a sample of filtered blood; it is the product of filtration followed by extensive recovery and targeted secretion.
What happens to waste products in the kidneys?
One important waste product is urea, which is produced when the body breaks down proteins. Urea enters the bloodstream and is carried to the kidneys, where it is filtered and ultimately excreted in urine.
The kidneys also help eliminate creatinine, a waste product generated largely from normal muscle metabolism. Because the amount produced is relatively steady in an individual, blood creatinine is commonly used as one indicator of kidney function.
The kidneys also excrete excess electrolytes, metabolic acids, and many substances that the body does not need. Some medications and their breakdown products are eliminated partly through renal filtration and secretion.
What happens when the kidneys lose function?
When kidney function declines, the kidneys become less able to filter blood and maintain the body’s chemical balance.
Early kidney disease may cause few or no noticeable symptoms because healthy kidney tissue can compensate for lost function. As impairment becomes more advanced, waste products can accumulate, fluid and electrolyte regulation can become abnormal, and acid-base balance may be disrupted.
Reduced kidney function can also interfere with hormone-related processes. For example, inadequate production of erythropoietin can contribute to anemia, while impaired activation of vitamin D and altered mineral regulation can affect bones.
Kidney disease can also contribute to high blood pressure, while high blood pressure and diabetes are important causes of chronic kidney damage. These relationships can reinforce one another, which is why preserving kidney function involves more than simply preventing waste from accumulating.
Why can someone live with one kidney?
The kidneys have substantial functional reserve. A person with one healthy kidney can often maintain normal fluid, electrolyte, and waste regulation because the remaining kidney can handle the body’s needs.
After the loss of one kidney, the remaining kidney can undergo functional and structural adaptations that allow it to perform more work.
Having one kidney does not mean kidney function is irrelevant, however. Protecting the remaining kidney becomes particularly important because there is less reserve if additional damage occurs.
The kidney’s central job is selective control
The kidneys are often described as blood filters, but that description captures only the first step of what they do.
Their real achievement is selectivity. They filter large amounts of fluid, then reclaim the water and molecules the body needs while excreting wastes and adjusting the amounts of salts, acids, and other substances that remain in the bloodstream.
That continuous process allows the kidneys to keep the composition and volume of the body’s internal environment within narrow limits. Their role in waste removal is important, but their broader function is maintaining the chemical conditions that cells need to survive.
