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The kidneys help remove waste products from the body, maintain balanced electrolyte levels, and regulate blood pressure.

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The kidneys are a pair of bean-shaped organs located below the rib cage, one on each side of the spine.

This article looks at the structure and function of the kidneys, the diseases that affect them, and how to keep them healthy.

Trusted Source just below the rib cage, with one on each side of the spine. The right kidney is generally slightly lower than the left kidney to make space for the liver.

Each kidney is approximately 3 centimeters (cm) thick, 6 cm wide, and 12 cm long. Across all sexes and weights, the left kidney is 10 grams (g) heavier than the right kidney.

The kidneys are two bean-shaped organs that are roughly the size of a fist. A tough, fibrous renal capsule surrounds each kidney and provides support for the soft tissue inside. Beyond that, two layers of fat serve as further protection. The adrenal glands lie on top of the kidneys.

Inside the kidneys are a number of pyramid-shaped lobes. Each consists of an outer renal cortex and an inner renal medulla. Nephrons flow between these sections. Each nephron includes a filter, called the glomerulus, and a tubule.

The glomerulus filters blood, which enters the kidneys through the renal arteries and leaves through the renal veins. The kidneys are relatively small organs, but they receive 20% to 25%Trusted Source of the heart’s output.

The tubule returns necessary substances to the blood and removes waste that then becomes urine. The kidneys excrete urine through the ureter, a tube that leads to the bladder.

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The main role of the kidneys is to maintain homeostasis. They manage fluid levels, electrolyte balance, and other factors that keep the internal environment of the body consistent and comfortable.

These organs carry out a wide range of bodily functions.

The kidneys removeTrusted Source various waste products and eliminate them in the urine. They filter around 200 quarts (qt) of blood every 24 hours, and about 2 qt leave the body in the form of urine.

Some major compounds that the kidneys remove are:

• urea, which results from the breakdown of proteins
• uric acid from the breakdown of nucleic acids
• drugs and their metabolites

The kidneys reabsorbTrusted Source nutrients from the blood using tubules and transport them to where they will best support health. They also reabsorb other products to help maintain homeostasis. Reabsorbed products include:

• glucose
• amino acids
• bicarbonate
• water
• phosphate
• chloride, sodium, magnesium, and potassium ions

In humans, the range of acceptable pH levels is 7.35 to 7.45Trusted Source. At levels below or above this range, the body enters a state of acidemia or alkalemia, respectively. In these states, proteins and enzymes break down and can no longer function. In extreme cases, this can be fatal.

The kidneys and lungs help keep the body’s pH stable. The lungs achieve this by moderating the concentration of carbon dioxide in the blood. The kidneys manage the pH by reabsorbing and producing bicarbonate from urine, which helps neutralize acids.

The kidneys can retainTrusted Source bicarbonate if the pH is tolerable and release it if acid levels rise. They can produce new bicarbonate by excreting acid.

Osmolality is a measure of the body’s electrolyte-water balance, which is the ratio between fluids and minerals in the body. Dehydration is a primary cause of electrolyte imbalance.

If osmolality rises in the blood plasma, the hypothalamus in the brain responds by passing a message to the pituitary gland. This gland releases antidiuretic hormone (ADH). In response to ADH, the kidney makes several changes, including:

• increasing urine concentration
• increasing water reabsorption
• reopening portions of the collecting duct that water cannot normally enter, allowing water back into the body
• retaining urea in the medulla of the kidney rather than excreting it, as this compound draws in water

The kidneys regulate blood pressure when necessary, but they are responsible for slower adjustments.

They adjust long-term pressure in the arteries by causing changes in the fluid outside of cells. The medical term for this fluid is extracellular fluid. These fluid changes occur after the release of a vasoconstrictor called angiotensin II. Vasoconstrictors are hormones that cause blood vessels to narrow.

These hormones play a role in increasing the kidneys’ absorption of sodium chloride, or salt. This absorption effectively increases the size of the extracellular fluid compartment and raises blood pressure. Anything that alters blood pressure, including excessive alcohol consumption, smoking, and obesity, can damage the kidneysTrusted Source over time.

The kidneys release several important compounds, including:

• Erythropoietin: This controls erythropoiesis, which is the production of red blood cells. The liver also produces erythropoietin, but the kidneys are its main producers in adults.
• Renin: This enzyme helps manage the expansion of arteries and the volumes of blood plasma, lymph, and interstitial fluid. Lymph is a fluid that contains white blood cells, which support immune activity, and interstitial fluid is the main component of extracellular fluid.
• Calcitriol: This is the hormonally active metabolite of vitamin D. It increases both the amount of calcium that the intestines can absorb and the reabsorption of phosphate in the kidney.

A range of diseases can affect the kidneys. Environmental or medical factors may lead to kidney disease, and they can cause functional and structural problems from birth in some people.

In people with diabetic nephropathy, damage occurs to the capillaries of the kidney as a result of long-term diabetes. The symptoms may not become apparent until yearsTrusted Source after the damage starts to develop. They can include:

• fluid buildup
• sleep difficulty
• poor appetite
• upset stomach
• weakness
• difficulty concentrating

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