Kidneys

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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.

How kidneys work?

our kidneys are as important to your health as your heart or lungs. Their main job is to remove waste products from your body. Most people have two kidneys, one on either side of the spine under the lower ribs. They are bean-shaped and reddish brown in colour. Each kidney is about the size of a clenched fist. 

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.

Kidneys are a filter system

The main job of the kidneys is to remove waste from the blood and return the cleaned blood back to the body. Each minute about one litre of blood – one-fifth of all the blood pumped by the heart – enters the kidneys through the renal arteries. After the blood is cleaned, it flows back into the body through the renal veins.

Nephrons

Each kidney contains about one million tiny units called nephrons. Each nephron is made up of a very small filter, called a glomerulus, which is attached to a tubule. As blood passes through the nephron, fluid and waste products are filtered out. Much of the fluid is then returned to the blood, while the waste products are concentrated in any extra fluid as urine.

Urine flow

The urine flows through a tube called the ureter into the bladder. Urine passes from the bladder out of the body through a tube called the urethra. The kidney usually makes one to two litres of urine every day depending on your build, how much you drink, the temperature and the amount of exercise you do.

Excess capacity

A healthy kidney can greatly increase its work capacity. With two healthy kidneys, each kidney performs 50 per cent of the normal kidney function. If one kidney is lost, the other kidney can enlarge and provide up to 75 per cent of the normal kidney function (the work of one and a half normally functioning kidneys).  

Other kidney functions

As well as filtering the blood, kidneys:

  • make and regulate important hormones in the body that help to control blood pressure, red blood cell production and calcium uptake from the intestine
  • maintain body fluid at the correct levels for the body to function
  • control body chemistry by regulating the amount of salt, water and other chemicals moving around the body.

Measuring how your kidneys work

You have two kidneys that filter your blood, removing wastes and extra water to make urine.

It is difficult to calculate the exact rate at which your kidneys work. The best measure of kidney function is called the glomerular filtration rate (GFR). The GFR can be estimated (eGFR) using a mathematical formula. This formula uses the level of creatinine in your blood to estimate how well your kidneys are filtering waste from your blood. It can indicate if there is any kidney damage.

The higher the filtration rate, the better the kidneys are working. A GFR of 100 mL/min/1.73 m2 is in the normal range. This is about equal to ‘100 per cent kidney function’. Based on this measurement system, a GFR of 50 mL/min/1.73 m2 could be called ’50 per cent kidney function’ and a GFR of 30 mL/min/1.73 m2 could be called ’30 per cent kidney function’.

If your doctor orders a blood test to learn more about your kidney function, an eGFR result is provided automatically, along with your creatinine results. 

Your doctor may also test for other signs and conditions that may indicate you have chronic kidney disease. These may include tests for:

  • protein in your urine (albuminuria or proteinuria)
  • blood in your urine (haematuria)
  • high blood pressure 
  • diabetes.
  • 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.

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