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Understanding the kidneys and urine

The two kidneys lie to the sides of the upper part of the tummy (abdomen), behind the intestines and either side of the spine. Each kidney is about the size of a large orange but bean-shaped. A large artery - the renal artery - takes blood to each kidney. The artery divides into many tiny blood vessels (capillaries) throughout the kidney. In the outer part of the kidneys, tiny blood vessels cluster together to form structures called glomeruli. Each glomerulus is like a filter. The structure of the glomerulus allows waste products and some water and salt to pass from the blood into a tiny channel called a tubule whilst keeping blood cells and protein in the bloodstream. Each glomerulus and tubule make up a nephron. There are about one million nephrons in each kidney.

As the waste products and water pass along the tubule there is a complex adjustment of the content. For example, some water and salts may be absorbed back into the bloodstream, depending on the current level of water and salt in your blood. Tiny blood vessels next to each tubule enable this 'fine tuning' of the transfer of water and salts between the tubules and the blood. The liquid that remains at the end of each tubule is called urine. This drains into larger channels (collecting ducts) which drain into the inner part of the kidney (the renal pelvis). The urine then passes down a tube called a ureter which goes from each kidney to the bladder. Urine is stored in the bladder until it is passed out when we go to the toilet. The 'cleaned' (filtered) blood from each kidney collects into a large vein - the renal vein - which takes the blood back towards the heart.

What are the main functions of the kidneys?

The main functions of the kidneys are to:

  • Filter out waste products from the bloodstream to be passed out in the urine.

  • Help control blood pressure - partly by the amount of water passed out of the body as urine and partly by making hormones which are involved in blood pressure control.

  • Make a hormone called erythropoietin ('epo') which stimulates the bone marrow to make red blood cells. This is needed to prevent anaemia.

  • Help keep various salts and chemicals in the blood at the right level. For example, sodium, potassium, calcium and phosphate. An imbalance of salts and chemicals in the bloodstream can cause problems in other parts of the body.

What is diabetic kidney disease?

Diabetic kidney disease (diabetic nephropathy) is a complication that occurs in some people with diabetes. In this condition the filters of the kidneys, the glomeruli, become damaged. Because of this the kidneys 'leak' abnormal amounts of protein from the blood into the urine. The main protein that leaks out from the damaged kidneys is called albumin. In normal healthy kidneys only a tiny amount of albumin is found in the urine. A raised level of albumin in the urine is the typical first sign that the kidneys have become damaged by diabetes.

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