About the urinary system and kidneys
If your child has a health condition that affects his or her kidney or another part of the urinary system, you may wish to find out more about these parts of the body. The urinary system has a very important job to get rid of waste from the body so that we can grow and stay healthy. It has two kidneys, two ureters, a bladder and an urethra.
Kidneys
The two kidneys remove substances our bodies no longer need. They filter blood and make urine (wee). Most of us have two kidneys. The kidneys are on either side of our spine (backbone), near the bottom edge of our ribs.
Other parts of the urinary system
Ureters
The two ureters are long tubes. They carry urine from the kidneys to the bladder.
Bladder
The bladder is a round, muscular bag that sits low down in the pelvis. The bladder fills up with urine like a balloon. When full, it sends signals to the brain which ‘tell’ the bladder muscles to push urine out through the urethra. This is how we pass urine.
Urethra
The urethra is a tube that carries urine from the bladder to the outside of the body. We urinate through the opening.
In boys and men, the urethra passes through the penis. It is much longer than in girls and women.
What the kidneys do
The kidneys remove waste from the body – things that we no longer need or that are harmful. They also control the amount of water in the blood, and make sure it has the right balance of chemicals, including salt and acid.
They also:
- control blood pressure
- help keep bones and teeth strong and healthy
- control the production of red blood cells, living parts of the blood that carry oxygen round the body.
Medical terms
You may hear or read lots of words related to the kidneys and other parts of the urinary system.
Renal
- Renal means anything related to the kidney.
- A paediatric renal unit of a hospital is where children with kidney problems are treated.
Nephrology
- Nephrology is the medical specialty of kidney diseases.
- Nephrologists are specialist doctors who treat people with kidney problems.
- If they work especially with babies, children and young people, they are called paediatric nephrologists.
Urology
- Urology is the surgical specialty of the urinary system.
- Urologists are specialist surgeons who treat people with problems in their urinary system.
- If they work especially with babies, children and young people, they are called paediatric urologists.
Urologists and nephrologists often work together.
Other doctors
- Other doctors, including general practitioners (GPs – family doctors), and general paediatricians (children’s doctors), will be able to treat some kidney conditions in children.
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This section includes information about:
- the adrenal gland, which sits on top of the kidney
- blood vessels going in, around and out of the kidney
- areas of the kidney
- the nephron (which filters blood and makes urine) is made up of the glomerulus and renal tubule
What the kidney looks like
A kidney is reddish and shaped like a kidney bean. One side bulges out (called the convex side). The other side curves in, or is indented (called the concave side).
Our kidneys grow as we get older. Each kidney is about the size of our fist.
Most newborn babies have kidneys that are about 4 centimetres long. When they reach eight years old, their kidney will have grown to about 8 centimetres. Adults’ kidneys are about 13 centimetres long.
Adrenal glands
We have two adrenal glands, one on top of each kidney. (“Ad” means on top of” and “renal” means to do with the kidney.) They are shaped like pyramids.
The adrenal glands help to control how the kidneys work. They make and release chemicals called hormones, which are carried in the blood to send messages to other parts of the body. These hormones are very important with many of the body functions, especially at times of stress such as when we are ill or have an operation.
Blood vessels
Like other parts of the body, the kidney needs blood for everything it does. Blood is pumped around the body by the heart, through tubes called blood vessels.
- The renal artery carries blood into the kidney.
- The renal vein carries blood out of the kidney. These large blood vessels go into the kidney at a dip on its side.
Other smaller blood vessels carry blood around the kidney. These have different names, depending on their size. The smallest of these blood vessels are capillaries.
Areas of the kidney
The kidney has a few different areas. Starting from the outside of the kidney, these are:
renal capsule – a membrane (like a skin) that covers and protects the kidney
renal cortex – the outer area
renal medulla – the inner area, which has about eight sections shaped like triangles called renal pyramids
renal calyces – these are small areas that collect urine made by the kidney
renal pelvis – the urine flows from the calyces to this larger area before moving down into the ureter, the tube that leads to the bladder.
Nephrons
Inside each kidney there are about one million nephrons. Nephrons are tiny but very important units that filter blood and make urine. Each nephron has two main parts:
- glomerulus
- renal tubule.
The glomerulus
Blood that comes into the kidney flows into each glomerulus, a tight bunch of capillaries (tiny blood vessels). The glomerulus acts like a sieve – some substances from the bloodstream pass through it. When we talk about more than one glomerulus, we say glomeruli.
The glomerulus acts like a sieve – waste and water from the blood pass through the glomerulus.
The space that surrounds the blood vessels of the glomerulus is called the Bowman's capsule, or glomerular capsule.
This part of the nephron is in the renal cortex, the outer part of the kidney (above the dotted line in the image).
The renal tubule
The renal tubule is a long, narrow tube, starting from the Bowman’s capsule. Everything that is filtered through the glomerulus goes into the renal tubule. This liquid eventually becomes urine.
Part of the renal tubule is in the renal cortex, and part of it is in the renal medulla.
The tubule has four sections: proximal tubule, loop of Henle, distal tubule and collecting duct.
Blood vessels
There are tiny blood vessels that go into and out of the nephron, and that surround it.
Read more about the nephron
More about the glomerulus
The wall of the glomerulus works as a filter, letting some substances out and keeping some in – it is called the glomerular filtration membrane. It is quite leaky, and so water and other substances in the blood filter out of the glomerulus. However, blood cells and some proteins are too large to pass through the wall, and so stay in the blood vessels.
The glomerular filtration membrane is like a sandwich with a few layers.
- Endothelial cells are cells are on the inside surface. They have tiny openings called pores that let small substances through.
- The glomerular basement membrane (GBM) is a layer of material around the endothelial cells.
- Podocytes are cells that wrap around the glomerulus. They have long projections that look like feet – sometimes called foot processes. Liquid can filter through the slits between these “feet”.
- Mesangial cells are cells that sit around the glomerulus and support it. They can contract (tighten) to control how much is filtered through the glomerulus.
Renal tubule
The liquid that is filtered through the glomerulus flows along the renal tubule in the order shown below.
- Proximal tubule – this is a coiled-up section next to the glomerulus and Bowman’s capsule (proximal means next to).
- Loop of Henle – this is the next part of the renal tubule. It is a U-shaped section that dips down into the renal medulla. Some nephrons have longer loops of Henle.
- Distal tubule – this is a coiled-up section that connects the loop of Henle to the collecting duct. Because it is further away from the glomerulus and Bowman’s capsule, it is called distal (distant or further away).
- Collecting duct – this is the final section of the renal tubule. It is where urine collects before it flows to the renal pelvis before leaving the kidney.
The renal tubule is lined with tubule cells. These cells help carry substances between the renal tubule and the bloodstream.
Blood vessels around the nephron
There are tiny blood vessels around the nephron.
- Blood goes into the glomerulus through the afferent arteriole (afferent means carried in).
- Blood that is not filtered through the glomerulus leaves through the efferent arteriole (efferent means carried out).
- This blood is then passed into the peritubular capillaries, which surround the nephron.
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The kidneys filter blood and make urine (wee). This means that they can:
- remove waste from the body – things that we no longer need or that are harmful
- control the amount of water in the blood, and make sure it has the right balance of chemicals, including salt and acid.
The kidneys also:
- control blood pressure
- help keep bones and teeth strong and healthy
- control the production of red blood cells.
How kidneys filter blood and make urine
The kidneys filter blood and make urine in the nephron – there are about one million of these tiny filtering units inside each kidney. There are a few stages.
The glomerulus
The first stage – filtering – happens in the glomerulus, the tight bunch of capillaries (small blood vessels).
Blood flows into the kidneys through blood vessels and into each glomerulus.
The glomerulus is quite leaky, and so most of the water and some substances filter out of the blood into the Bowman’s capsule, which surrounds the glomerulus.
The blood cells and most proteins are too large to filter through the glomerulus, and so stay in the blood.
The renal tubule
The next two stages happen along the renal tubule. They help make sure the body keeps what it needs and removes what it no longer needs into urine.
Whatever has been filtered flows into the renal tubule. The rest of it flows through the tiny blood vessels that surround the nephron.
- The body needs to take back some of the water and substances that were filtered and are in the renal tubule. These are re-absorbed (taken back) into the blood vessels.
- The body also needs to get rid of some substances that were not filtered and are still in the blood vessels. These are secreted (released) into the renal tubule.
Urine
- The liquid in the renal tubule is now urine. It is mostly water, but also has some substances that the body no longer needs, such as wastes.
- The urine flows to the renal pelvis and then leaves the kidney. It flows through the ureters to the bladder. When we are ready to go to the toilet, the bladder pushes urine out through the urethra.
- If there is a problem in the glomerulus or renal tubule, this may affect how the kidneys remove waste and extra water.
Read more about how the kidney filters blood and makes urine
Blood flow into the kidney
- Blood is pumped around the body by the heart through blood vessels.
- Blood flows into the kidney through the renal artery.
- It then flows through a network of blood vessels into to the afferent arteriole, the tiny blood vessel that leads to the glomerulus.
Filtration
The first stage is filtration, which happens in the glomerulus, the tight bunch of capillaries.
- The glomerulus filters blood. This has several layers, including the glomerular basement membrane (GBM) and some cells. Because of the high pressure in the afferent arteriole and because the glomerulus has tiny pores (holes), most of the water and substances in the blood such as chemical from food (nutrients) and waste products are forced through the glomerulus and into the Bowman’s capsule.
- This liquid is called the ultrafiltrate. The amount of ultrafiltrate made per minute by both kidneys is called the glomerular filtration rate (GFR). Measuring or estimating the GFR is one important way that doctors can find out how well the kidneys are working.
- The blood cells and most proteins stay in the bloodstream. This is mostly because they are too large to filter through the glomerulus.
Reabsorption in the renal tubule
The body needs to keep some of the water and substances in the ultrafiltrate, which is flowing through the renal tubule.
- The tubule cells, which line the renal tubule, have special transport pumps, which allow them to move substances in and out of the cells. These cells help move substances out of the renal tubule into the kidney tissue.
- The substances then pass into the peritubular capillaries.
This process is called reabsorption. This is because the substances that were filtered out of the blood are taken back, or reabsorbed, into the blood.
Secretion in the renal tubule
The body needs to get rid of some substances, including waste products, which is flowing through the peritubular capillaries.
- The substances pass out of the peritubular capillaries into the kidney tissue.
- The tubule cells help move substances into the renal tubule.
This process is called secretion. This is because the tubule cells secrete, or release, substances into the renal tubule.
Production of urine
- The liquid flows through the different sections of the renal tubule until it reaches the final part – the collecting duct. This is now urine. It is mostly water, but also has some substances that the body no longer needs, such as salt and wastes.
- The urine flows to the renal pelvis and then leaves the kidney. It flows through the ureters to the bladder. When we are ready to go to the toilet, the bladder pushes urine out through the urethra.
Removing waste from the body
Our bodies are constantly making waste – for example, when we use our muscles and when we eat food, which the body breaks down for energy. This waste needs to be removed so it does not build up and cause any damage in the body. The kidneys do this job.
Read more about how waste products are made
Waste is made during metabolism, a set of chemical reactions that are necessary for us to live. In metabolism, a chemical is converted (changed) into another chemical within the body’s cells. Often a waste product is made. For example:
- when we eat, the body converts the food into fuel so we have energy. The liver breaks down protein, which makes the waste product urea
- when we use our muscles, the body breaks down a chemical. This makes the waste product creatinine.
These waste products move round the body in the bloodstream. When blood enters the kidney, it goes into the glomerulus, where it is filtered. The waste is safely removed in urine.
Controlling the amount of water and minerals in the blood
The kidneys also control the amount of water and minerals in our body. They do this by controlling what gets kept in the body and what is removed into urine.
Water
It is very important to have the right amount of water in the body. We get water from drinking and eating. We lose water in lots of ways, including going to the toilet and sweating.
- If we do not have enough water in our body, the kidneys take back more water from the renal tubule into the bloodstream. This means that less water is removed in the urine.
- If we have too much water in our body, the kidneys remove more water in the urine.
Electrolytes
Electrolytes are chemicals in the body that help make sure that the muscles, including the heart muscle, bones, teeth and blood, are healthy. The kidneys control the amounts of some important electrolytes. If we have too much or too little, this can cause problems, and may mean there is a problem with the kidneys or another part of the body.
Some important electrolytes include:
- sodium (salt)
- potassium
- bicarbonate (which controls the amount of acid)
- phosphate
- calcium.
Read more about about electrolytes
The kidneys control the amounts of some important electrolytes. If we have too much or too little, this can cause problems, and may mean there is a problem with the kidneys or another part of the body.
- Sodium helps balance the amount of water in the body. Hypernatraemia is too much sodium (“hyper” means too much).
- Potassium is needed for muscles, including the heart muscle, to work properly. Hyperkalaemia is too much potassium and hypokalaemia is too little potassium (“hypo" means too little) – either of these may mean there is a problem with the kidneys.
- Bicarbonate balances the amount of acid in our body, or the pH balance (also called acid–base balance). If there is not enough bicarbonate then the blood is acidic. This is called acidosis.
- Phosphate is important for bones, teeth and muscles. Hyperphosphataemia is too much phosphate.
- Calcium is important for bones and teeth, helps blood to clot, and also helps the muscles, including the heart muscle, to work.
Controlling blood pressure
Blood pressure is the force, or pressure, caused mainly by the heart pumping. This pressure helps the blood flow round the body (circulate). Kidneys control blood pressure to help make sure it is at a healthy level. If kidneys are not working properly, they may not be able to control blood pressure well.
If blood pressure is too high, this is called hypertension. If blood pressure is not controlled, this can cause problems in the body.
Keeping bones and teeth strong and healthy
We need vitamin D to help keep our bones and teeth strong and healthy. It is important that everyone – including children – gets vitamin D from sunlight on the skin and from some foods (such as oily fish, eggs and breakfast cereal with added vitamin D). Kidneys help our bodies to use the vitamin D we get from food or sunlight.
Kidneys also help to control what happens to the nutrients from the food we eat, including vitamin D and other minerals such as calcium and phosphorus. Because of this, they play a part in helping children grow.
Controlling the production of red blood cells
Red blood cells (RBCs) are in the blood and carry oxygen round the body. They are made in the bone marrow, which is found in the long bones such as the leg bones. Kidneys help control this process by producing a hormone. Hormones are chemicals in the blood that send messages to other parts of the body.
When the body does not have enough red blood cells, the kidney releases a hormone called erythropoietin. Erythropoietin is carried into the bone and causes it to produce more red blood cells.
Other hormones
Other hormones affect how the kidneys work. These hormones control how much water and salt is removed in urine:
- antidiuretic hormone (ADH)
- aldosterone.
Read more about ADH and aldosterone
ADH
Antidiuretic hormone (ADH) controls how much urine the body makes. (A diuretic is a substance that makes us produce more urine; an antidiuretic is one that makes us produce less.) ADH is carried in the blood to the cells in the kidneys. When a lot of ADH is released, the kidneys re-absorb more water. This means there is less water in the urine, and so it is more concentrated.
The amount of ADH varies, depending on blood pressure and the blood volume (amount of blood in the body). For example, if there is a lot of bleeding, the body makes more ADH so that the kidneys keep more water in the blood. This allows the blood volume to increase.
Aldosterone
Aldosterone is made in the adrenal gland, which sits on top of the kidney. When aldosterone is released into the blood, it is carried to the kidneys. The kidneys reabsorb more salt and water. They also secrete more of a mineral called potassium.
© BAPN and Kidney Care UK 2013-2022, all rights reserved.
For details on any sources of information used in this topic, please contact us through the contact us form on our website www.infoKID.org.uk.
We take great care to make sure that the information in this leaflet is correct and up-to-date. However, it is important that you ask the advice of your child’s doctor or nurse if you are not sure about something. This information is intended for use in the United Kingdom, and may not apply to other countries. The British Association of Paediatric Nephrology (BAPN), Kidney Care UK and the contributors and editors cannot be held responsible for the accuracy of information, omissions of information, or any actions that may be taken as a consequence of reading this information.
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