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Kidneys

Print Print | Email  Email article to a friend | Last updated: October 24, 2013

The kidneys are specialised organs that ensure that unwanted substances and excess water are removed from the bloodstream.
Image showing section through a kidney

Graphic showing a section through the right kidney with the main structures labelled.

Where are my kidneys?

The kidneys are small bean-shaped organs approximately 6cm wide and 12cm long and consist of two main layers - an inner layer called the medulla and an outer layer called the cortex. Most people have two kidneys that are situated at the back of the abdomen on either side of the spine.

What do the kidneys do?

The kidneys ensure that the make-up and volume of the fluids in the body is correct. They help control the chemical balance of the blood and regulate the body’s level of sodium, potassium and calcium. The kidneys remove waste products and excess water from the body and so help to regulate blood pressure. They activate vitamin D, which helps to maintain strong bones, and produce erythropoietin, a hormone that is vital for the production of red blood cells.

Each kidney contains 1.0-1.5 million small tubes called nephrons. The kidneys filter blood through a network of small blood vessels called the glomerulus. This produces a solution that then flows through the nephrons. As this fluid passes through the nephron, substances that the body wants to retain (such as sodium, potassium and most of the water) are re-absorbed back into the blood. The substances that need to be removed from the body, such as waste products including the remains of drugs and alcohol, are retained in the fluid and form urine. The kidneys filter around 200 litres of blood a day and produce between one to two litres of urine.

What hormones do the kidneys produce?

The kidneys make two main hormones, vitamin D and erythropoietin.

Vitamin D is essential for a number of different functions in the body. Most of the vitamin D that is in the blood is inactive and it is modified by the kidney and other tissues to activate it. Active vitamin D stimulates the uptake of calcium from food, is important for the maintenance of healthy bones and also helps to regulate the response of the immune system to infection.

Erythropoietin is produced by the kidney and acts on bone marrow to stimulate the production of mature red blood cells. The kidneys also produce prostaglandins.

Renin is an enzyme produced by the kidneys that plays an important role in the renin-angiotensin-aldosterone hormonal system. This helps to control blood pressure. In addition to making hormones, the kidneys also respond to a number of hormones including vitamin D, aldosterone, prostaglandins, cortisol, parathyroid hormone and calcitonin.

What could go wrong with the kidneys?

When the kidneys are not working correctly, waste products and excess fluid can build up and the levels of sodium, potassium, phosphate and calcium are not regulated correctly.  When these substances gather together, this causes the symptoms of kidney disease, which can include high blood pressure, excessive tiredness, fluid retention and possibly lower back pain.

Kidney damage can occur for a number of reasons - diabetes, high blood pressure, infections and a group of diseases that affect the glomerulus. The kidneys also need an adequate supply of blood, so if there is something wrong with the blood vessels to the kidney, such as a narrowing, this will prevent the kidneys from working efficiently.

 

Written: March 2011