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Your Kidneys & How They Work
Learn how your kidneys filter blood, why kidneys are important, and how kidneys help maintain a healthy balance of water, salts, and minerals in your body.
Your Kidneys & How They WorkOn this page:
Why are the kidneys important?
How do my kidneys work?
How does blood flow through my kidneys?
The kidneys are two bean-shaped organs, each about the size of a fist. They are located just below the rib cage, one on each side of your spine.
Healthy kidneys filter about a half cup of blood every minute, removing wastes and extra water to make urine. The urine flows from the kidneys to the bladder through two thin tubes of muscle called ureters, one on each side of your bladder. Your bladder stores urine. Your kidneys, ureters, and bladder are part of your urinary tract.
You have two kidneys that filter your blood, removing wastes and extra water to make urine.
Why are the kidneys important?
Your kidneys remove wastes and extra fluid from your body. Your kidneys also remove acid that is produced by the cells of your body and maintain a healthy balance of water, salts, and minerals—such as sodium, calcium, phosphorus, and potassium—in your blood.
Without this balance, nerves, muscles, and other tissues in your body may not work normally.
Your kidneys also make hormones that help
control your blood pressure
make red blood cells
keep your bones strong and healthy
Watch a video about what the kidneys do External link.
How do my kidneys work?
Each of your kidneys is made up of about a million filtering units called nephrons. Each nephron includes a filter, called the glomerulus, and a tubule. The nephrons work through a two-step process: the glomerulus filters your blood, and the tubule returns needed substances to your blood and removes wastes.
Each nephron has a glomerulus to filter your blood and a tubule that returns needed substances to your blood and pulls out additional wastes. Wastes and extra water become urine.
The glomerulus filters your blood
As blood flows into each nephron, it enters a cluster of tiny blood vessels—the glomerulus. The thin walls of the glomerulus allow smaller molecules, wastes, and fluid—mostly water—to pass into the tubule. Larger molecules, such as proteins and blood cells, stay in the blood vessel.
The tubule returns needed substances to your blood and removes wastes
A blood vessel runs alongside the tubule. As the filtered fluid moves along the tubule, the blood vessel reabsorbs almost all of the water, along with minerals and nutrients your body needs. The tubule helps remove excess acid from the blood. The remaining fluid and wastes in the tubule become urine.
How does blood flow through my kidneys?
Blood flows into your kidney through the renal artery. This large blood vessel branches into smaller and smaller blood vessels until the blood reaches the nephrons. In the nephron, your blood is filtered by the tiny blood vessels of the glomeruli and then flows out of your kidney through the renal vein.
Your blood circulates through your kidneys many times a day. In a single day, your kidneys filter about 150 quarts of blood. Most of the water and other substances that filter through your glomeruli are returned to your blood by the tubules. Only 1 to 2 quarts become urine.
Blood flows into your kidneys through the renal artery and exits through the renal vein. Your ureter carries urine from the kidney to your bladder.
The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) and other components of the National Institutes of Health (NIH) conduct and support research into many diseases and conditions.
What are clinical trials, and are they right for you?
Clinical trials are part of clinical research and at the heart of all medical advances. Clinical trials look at new ways to prevent, detect, or treat disease. Researchers also use clinical trials to look at other aspects of care, such as improving the quality of life for people with chronic illnesses. Find out if clinical trials are right for you .
What clinical trials are open?
Clinical trials that are currently open and are recruiting can be viewed at www.ClinicalTrials.gov .
Last Reviewed June 2018
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This content is provided as a service of the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), part of the National Institutes of Health. The NIDDK translates and disseminates research findings to increase knowledge and understanding about health and disease among patients, health professionals, and the public. Content produced by the NIDDK is carefully reviewed by NIDDK scientists and other experts.
Glomerular filtration (glomerulus)
Renal physiology: Glomerular filtration
What is glomerular filtration?
Unless things go wrong, most of us don’t spend much time thinking about what it takes to urinate, but in fact, your kidneys and urinary system are quite amazing. Together they receive over a liter of blood each minute, and eliminate around 1.5 litres of urine per day, efficiently getting rid of excess water and waste products that would otherwise cause you some serious problems.
Glomerular filtration is the first step in making urine. It is the process that your kidneys use to filter excess fluid and waste products out of the blood into the urine collecting tubules of the kidney, so they may be eliminated from your body.
Why we make urine
Your body metabolizes (processes) the things you eat and drink, which produces energy, as well as the various building blocks that you need to keep your tissues and organs healthy. While doing this, a variety of other substances are produced that can’t be used or stored for later, and must be eliminated to prevent toxic build-up. In addition to waste products, our diets often contain substances in amounts that are far more than we need on a day-to-day basis (such as carbohydrates and fats). You get rid of non water-soluble waste products in feces (e.g., undigested fibre and bacteria), and water-soluble waste products in urine (e.g., urea and electrolytes - sodium and potassium). Some foods and medications can change the colour of your urine; e.g., beets, blackberries and rhubarb can turn urine red or pink. This is evidence that your kidneys are eliminating the colourful soluble pigments from your body.
The main substances excreted in urine are:
metabolic waste products - e.g., urea and creatinine
electrolytes - inorganic compounds (including sodium, potassium, calcium, chloride and bicarbonate) that your body uses to control the fluid content inside your body fluids.
You can think of your kidneys as being your body’s natural blood filter. They are able to control the amount of water and substances dissolved in your body fluids (solutes) by reabsorbing what you need and producing urine to get rid of the rest. The production of urine is obligatory, meaning that it is made regardless of what is going on with your body; for example, you still make urine even when you are severely dehydrated. This occurs because of the need to remove various solutes from the body in order to keep internal conditions stable and relatively constant (homeostasis), so that your all of your body’s physiological processes continue operating effectively. Making urine is a complicated process, and to do it, each of your kidneys contain around a million specialized structures, called nephrons.
Figure of urinary system and a nephron
How glomerular filtration works
The first step in making urine is to separate the liquid part of your blood (plasma), which contains all the dissolved solutes, from your blood cells. Each nephron in your kidneys has a microscopic filter, called a glomerulus that is constantly filtering your blood.
Blood that is about to be filtered enters a glomerulus, which is a tuft of blood capillaries (the smallest of blood vessels). The glomerulus is nestled inside a cup-like sac located at the end of each nephron, called a glomerular capsule. Glomerular capillaries have small pores in their walls, just like a very fine mesh sieve. Most capillary beds are sandwiched between arterioles and venules (the small vessels delivering blood to and collecting blood from capillary beds), and the hydrostatic pressure drops as blood travels through the capillary bed into the venules and veins.The glomerulus, on the other hand, is sandwiched between two arterioles - afferent arterioles deliver blood to the glomerulus, while efferent arterioles carry it away. Constriction of efferent arterioles as blood exits the glomerulus provides resistance to blood flow, preventing a pressure drop, which could not be achieved if blood were to flow into venules, which do not really constrict. The two arterioles change in size to increase or decrease blood pressure in the glomerulus. In addition, efferent arterioles are smaller in diameter than afferent arterioles. As a result, pressurized blood enters the glomerulus through a relatively wide tube, but is forced to exit through a narrower tube. Together, these unique features plus the fact that your heart is supplying your kidneys with over a liter of blood per minute (around 20% of its output) maintain a high glomerular capillary pressure and the filtration function of the kidney, regardless of fluctuations in blood flow. For example, the sympathetic nervous system can stimulate the efferent arteriole to constrict during exercise when blood flow to the kidney is reduced.
The physical characteristics of the glomerular capillary wall determine what is filtered and how much is filtered into the glomerular capsule. Working from the inside out, the capillary walls are made up of three layers:
this has relatively large pores (70-100 nanometers in diameter), which solutes, plasma proteins and fluid can pass through, but not blood cells.
this membrane is also made up of three layers, and is fused to the endothelial layer. Its job is to prevent plasma proteins from being filtered out of the bloodstream.
Kidneys and Urinary Tract (for Teens)
The kidneys perform several functions that are essential to health, the most important of which are to filter blood and produce urine.
Kidneys and Urinary Tract
Reviewed by: Larissa Hirsch, MD
Print en español
Los riñones y las vías urinarias
What Are the Kidneys and Urinary Tract?
The urinary tract is one of the systems that our bodies use to get rid of waste products. The kidneys are the part of the urinary tract that makes urine (pee). Urine has salts, toxins, and water that need to be filtered out of the blood. After the kidneys make urine, it leaves the body using the rest of the urinary tract as a pathway.
What Are the Parts of the Urinary Tract?
People usually have two kidneys, but can live a normal, healthy life with just one. The kidneys are under the ribcage in the back, one on each side. Each adult kidney is about the size of a fist.
Each kidney has an outer layer called the cortex, which contains filtering units. The center part of the kidney, the medulla (pronounced: meh-DUH-luh), has fan-shaped structures called pyramids. These drain urine into cup-shaped tubes called calyxes (pronounced: KAY-luh-seez).
From the calyxes, pee travels out of the kidneys through the ureters (pronounced: YUR-uh-ters) to be stored in the bladder (a muscular sac in the lower belly). When a person urinates, the pee exits the bladder and goes out of the body through the urethra (pronounced: yoo-REE-thruh), another tube-like structure. The male urethra ends at the tip of the penis; the female urethra ends just above the vaginal opening.
What Do the Kidneys Do?
Kidneys have many jobs, from filtering blood and making pee to keeping bones healthy and making a hormone that controls the production of red blood cells.
The kidneys also help regulate blood pressure, the level of salts in the blood, and the acid-base balance (the pH) of the blood. All these jobs make the kidneys essential to keeping the body working as it should.
How Do the Kidneys and Urinary Tract Work?
Blood travels to each kidney through the renal artery. The artery enters the kidney at the hilus (pronounced: HY-luss), the indentation in middle of the kidney that gives it its bean shape. The artery then branches so blood can get to the nephrons (pronounced: NEH-fronz) — 1 million tiny filtering units in each kidney that remove the harmful substances from the blood.
Each of the nephrons contain a filter called the glomerulus (pronounced: gluh-MER-yuh-lus). The fluid that is filtered out from the blood then travels down a tiny tube-like structure called a tubule (pronounced: TOO-byool). The tubule adjusts the level of salts, water, and wastes that will leave the body in the urine. Filtered blood leaves the kidney through the renal vein and flows back to the heart.
Pee leaves the kidneys and travels through the ureters to the bladder. The bladder expands as it fills. When the bladder is full, nerve endings in its wall send messages to the brain. When a person needs to pee, the bladder walls tighten and a ring-like muscle that guards the exit from the bladder to the urethra, called the sphincter (pronounced: SFINK-tur), relaxes. This lets pee go into the urethra and out of the body.
What Can Help Keep the Kidneys and Urinary Tract Healthy?
To help keep your kidneys and urinary tract healthy:
Get plenty of exercise.
Eat a nutritious diet.
For girls: Wipe from front to back after pooping so germs don't get into the urethra.
Avoid bubble baths, sitting in the tub after shampoo has been used, and scented soaps. These can irritate the urethra.
Wear cotton underwear.
Promptly change out of wet bathing suits.
Go for regular medical checkups.
Talk to your doctor before taking any supplements or herbal treatments.
Let the doctor know about any family history of kidney problems, diabetes, or high blood pressure.
Let the doctor know if you have any swelling or puffiness, have pain with peeing, need to pee often, have foamy urine or blood in the urine, or are constipated.
The kidneys do a lot, but their most important job is to take waste out of the blood and make urine (pee). The urinary tract takes this waste out of the body when a person pees.
Click through this slideshow to learn more about the kidneys and urinary tract.
© 2020 The Nemours Foundation/KidsHealth. All rights reserved.Reviewed by: Larissa Hirsch, MD
Date reviewed: September 2018
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