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

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.

The kidneys are located 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. According to a 2022 study, across all sexes and weights, the left kidney is generally around 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 provide additional 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 span 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% 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 regulate fluid levels, electrolyte balance, and other factors that maintain the body’s internal environment and keep it stable and within a healthy range.

These organs carry out a wide range of bodily functions.

Excrete waste

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

Reabsorb nutrients

The kidneys reabsorb 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

Maintain pH

In humans, the acceptable pH range is 7.35 to 7.45. At levels below this range, the body can enter a state of acidosis. Above this range can lead to alkalosis. In extreme cases, these conditions can be fatal.

The kidneys and lungs help keep the body’s pH stable.

The lungs manage the level of carbon dioxide in the blood. The kidneys adjust the levels of bicarbonate and hydrogen ions, which helps neutralize acids.

Regulate water and electrolytes

Osmolality measures the balance of certain particles dissolved in body fluid. This includes electrolytes, glucose, and urea. Dehydration can cause imbalances of these substances in the body.

If osmolality rises (concentration of these particles increases) in the blood plasma, the hypothalamus in the brain signals the pituitary gland to release antidiuretic hormone (ADH).

In response to ADH, the kidneys:

  • increase urine concentration
  • allow more water to be reabsorbed through the collecting ducts
  • recycle more urea within the kidney medulla to help concentrate urine, as urea draws in water

Regulate blood pressure

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

They adjust long-term pressure in the arteries by altering the fluid outside cells. The medical term for this fluid is extracellular fluid. These fluid changes are part of the renin–angiotensin–aldosterone system, which includes angiotensin II, a vasoconstrictor, and other hormones that regulate blood pressure.

Vasoconstrictors are hormones that cause blood vessels to narrow.

These hormones help increase the kidneys’ absorption of sodium chloride, or salt. This absorption effectively increases the extracellular volume and raises blood pressure.

Anything that alters blood pressure, including excessive alcohol consumption, smoking, and obesity, can damage the kidneys over time.

Secrete hormones

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: An enzyme that helps regulate blood pressure, fluid balance, and sodium levels. It’s part of the renin-angiotensin-aldosterone system,
  • 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.

The following are suggestions for keeping the kidneys healthy and helping avoid kidney disease:

  • Eat a balanced diet: Many kidney problems result from high blood pressure and diabetes. As a result, maintaining a nutritious, well-balanced diet can help prevent several common causes of kidney disease. The National Heart, Lung, and Blood Institute recommends the DASH diet to help maintain healthy blood pressure.
  • Get enough exercise: Exercising for 30 minutes every day can reduce the risk of high blood pressure and obesity, both of which put pressure on kidney health.
  • Drinking plenty of water: Fluid intake, especially of water, is important. Many people aim to drink around 6 to 8 glasses of water per day. However, needs can vary from individual to individual. Also, those with chronic kidney disease, heart failure, or swelling may need to limit their water intake.
  • Limit salt: Limiting salt intake can help kidney health. The salt we eat is known as sodium chloride. There are varied guidelines for how much sodium a person should consume each day, but a general maximum of 2,300 milligrams (mg) per day.
  • Moderate alcohol: Consuming more than 1 drink per day for females and 2 per day for males can harm the kidneys and impair renal function.
  • Avoid smoking: Smoking damages blood vessels and can cause narrowing or hardening of vessels.
  • Screen for health conditions: Anyone with high blood pressure or diabetes should discuss regular kidney screening with a doctor to help spot potential health issues.
  • Manage diabetes and heart disease: Following a doctor’s recommendations for managing these conditions can help protect the kidneys in the long term.
  • Take steps to get quality sleep and manage stress: The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) recommends getting 7 to 8 hours of sleep each night and engaging in activities to reduce stress.

Keeping the kidneys functioning properly is essential for overall health.

The kidneys are a pair of bean-shaped organs that sit just below the rib cage. They play an important role in the body, helping to remove waste products, maintain electrolyte levels, and regulate blood pressure.

A person can help maintain kidney health by eating a balanced diet, exercising frequently, and managing underlying health conditions.