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.
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.
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.
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.
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.