The kidneys are a pair of bean-shaped organs present in all vertebrates. They remove waste products from the body, maintain balanced electrolyte levels, and regulate blood pressure.

The kidneys are some of the most important organs. The Ancient Egyptians left only the brain and kidneys in position before embalming a body, inferring that the held a higher value.

In this article, we will look at the structure and function of the kidneys, diseases that affect them, and how to keep the kidneys healthy.

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The kidneys play a role in maintaining the balance of body fluids and regulating blood pressure, among other functions.

The kidneys are at the back of the abdominal cavity, with one sitting on each side of the spine.

The right kidney is generally slightly smaller and lower than the left, to make space for the liver.

Each kidney weighs 125–170 grams (g) in males and 115–155 g in females.

A tough, fibrous renal capsule surrounds each kidney. Beyond that, two layers of fat serve as protection. The adrenal glands lay 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. These are the urine-producing structures of the kidneys.

Blood enters the kidneys through the renal arteries and leaves through the renal veins. The kidneys are relatively small organs but receive 20–25 percent of the heart's output.

Each kidney excretes urine through a tube called the ureter that leads to the bladder.

The main role of the kidneys is maintaining homeostasis. This means they manage fluid levels, electrolyte balance, and other factors that keep the internal environment of the body consistent and comfortable.

They serve a wide range of functions.

Waste excretion

The kidneys remove a number of waste products and get rid of them in the urine. Two major compounds that the kidneys remove are:

  • urea, which results from the breakdown of proteins
  • uric acid from the breakdown of nucleic acids

Reabsorption of nutrients

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Functions of the kidneys include removing waste, reabsorbing nutrients, and maintaining pH balance.

The kidneys reabsorb nutrients from the blood and transport them to where they would best support health.

They also reabsorb other products to help maintain homeostasis.

Reabsorbed products include:

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

Maintaining pH

In humans, the acceptable pH level is between 7.38 and 7.42. Below this boundary, the body enters a state of acidemia, and above it, alkalemia.

Outside this range, proteins and enzymes break down and can no longer function. In extreme cases, this can be fatal.

The kidneys and lungs help keep a stable pH within the human body. The lungs achieve this by moderating the concentration of carbon dioxide.

The kidneys manage the pH through two processes:

  • Reabsorbing and regenerating bicarbonate from urine: Bicarbonate helps neutralize acids. The kidneys can either retain it if the pH is tolerable or release it if acid levels rise.
  • Excreting hydrogen ions and fixed acids: Fixed or nonvolatile acids are any acids that do not occur as a result of carbon dioxide. They result from the incomplete metabolism of carbohydrates, fats, and proteins. They include lactic acid, sulfuric acid, and phosphoric acid.

Osmolality regulation

Osmolality is a measure of the body's electrolyte-water balance, or the ratio between fluid 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, in turn, releases antidiuretic hormone (ADH).

In response to ADH, the kidney makes a number of 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 it draws in water

Regulating 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 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.

They work with other functions to increase the kidneys' absorption of sodium chloride, or salt. This effectively increases the size of the extracellular fluid compartment and raises blood pressure.

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

Secretion of active compounds

The kidneys release a number of important compounds, including:

  • Erythropoietin: This controls erythropoiesis, or the production of red blood cells. The liver also produces erythropoietin, but the kidneys are its main producers in adults.
  • Renin: This helps manage the expansion of arteries and the volume 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 number 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.

Diabetic nephropathy

In people with diabetic nephropathy, damage occurs to the capillaries of the kidney as a result of long-term diabetes.

Symptoms do not become clear until years after the damage starts to develop.

They include:

Kidney stones

Stones can form as a solid build-up of minerals in the kidney.

They can cause intense pain and might affect kidney function if they block the ureter.

Kidney infections

These tend to result from bacteria in the bladder that transfer to the kidneys.

Symptoms include lower back pain, painful urination, and sometimes fever. Changes in the urine may include the presence of blood, cloudiness, and a different odor.

Kidney infections are more common in women than in men, as well as in women who are pregnant. The infection often responds well to antibiotics.

Renal failure

In people with renal failure, the kidneys become unable to filter out waste products from the blood effectively.

If an injury causes kidney failure, such as the overuse of medication, the condition is often reversible with treatment.

If the cause is a disease, however, kidney failure often does not have a full cure.

Kidney hydronephrosis

This means "water on the kidney."

It usually occurs when an obstruction prevents urine from leaving the kidney, causing intense pain.

In time, the kidney might atrophy, or shrink.

Duplicated ureter

Two ureters might form between a kidney and the bladder, rather than one. There are few complications, but it can increase the risk of urinary tract infections and, in females, incontinence.

Duplicated ureter affects around 1 percent of people.

Interstitial nephritis

A reaction to medications or bacteria can inflame the spaces within the kidney.

Treatment usually involves removing the cause of inflammation or changing a course of medication.

Kidney tumor

These can be benign or malignant. Benign cancers do not spread or attack tissue, but malignant cancers can be aggressive.

The most common malignant kidney cancer is renal cell carcinoma.

Nephrotic syndrome

Damage to the kidney function causes protein levels in the urine to increase. This results in a protein shortage throughout the body, which draws water into the tissues.

Symptoms include:

Changes in urination and lower back pain, especially on one side, may be signs of kidney problems.

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Back pain is a symptom of kidney damage.

Some of the most common causes of kidney damage include:

  • Analgesics: Using pain medication over a long period of time might result in chronic analgesic nephritis. Examples include aspirin, acetaminophen, and non-steroidal anti-inflammatory drugs (NSAIDs).
  • IgA nephropathy: Also known as Berger disease, this occurs when immunoglobin A (IgA) antibodies build up in the kidney. IgA forms a vital part of the immune system, but a buildup can be harmful. The disease progresses slowly, sometimes over as long as 20 years. Symptoms include abdominal pain, rash, and arthritis. It can result in kidney failure.
  • Lithium: Doctors prescribe lithium to treat schizophrenia and bipolar disorder. However, lithium might cause nephropathy with long-term use. Despite the risk, a person can avoid the negative effects of lithium with close medical supervision.
  • Chemotherapy agents: The most common type of kidney issue in people with cancer is acute kidney injury. This might be due to the intense vomiting and diarrhea that are common side effects of chemotherapy.
  • Alcohol: Alcohol alters the kidneys' ability to filter the blood. It also dehydrates the body, making it harder for kidneys to redress internal balances, and increases blood pressure, which can also hinder the kidneys.

In the case of severe kidney damage, dialysis might be an option. It is only used for end-stage kidney failure where 85 to 90 percent of kidney function is lost.

Kidney dialysis aims to complete some of the functions of a healthy kidney.

These include:

  • removal of waste, excess salt, and water
  • maintaining the correct levels of chemicals in the blood, including sodium, bicarbonate, and potassium
  • maintaining blood pressure

The two most common types of kidney dialysis are:

Hemodialysis: An artificial kidney, or hemodialyzer, removes waste, additional fluids, and chemicals. The treating doctor makes an entry point in the body by connecting an artery and a vein under the skin to create a larger blood vessel.

Blood travels into the hemodialyzer, receives treatment, and then returns to the body. This is usually done 3 to 4 times a week. More regular dialysis has a more beneficial effect.

Peritoneal dialysis: The doctor inserts a sterile solution containing glucose into the abdominal cavity around the intestine. This is the peritoneum, and a protective membrane surrounds it.

The peritoneal membrane filters waste products as excess fluids enter the abdominal cavity.

In continuous peritoneal dialysis, the fluid drains through a catheter. The individual discards these fluids 4 to 5 times a day. In automated peritoneal dialysis, the process occurs over time.

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Drinking water can help keep the kidneys in good condition.

The follow are suggestions for keeping the kidneys healthy and avoiding kidney disease:

  • Eat a balanced diet: Many kidney problems result from high blood pressure and diabetes. As a result, maintaining a healthy diet can prevent several common causes of kidney disease. The National Heart, Lung, and Blood Institute (NHLBI) recommend the DASH diet for maintaining 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.
  • Drink plenty of water: Fluid intake is important, especially water. Around 6 to 8 cups per day can help improve and maintain kidney health.
  • Supplements: Be careful when taking supplements, as not all dietary supplements and vitamins are beneficial. Some can harm the kidneys if a person takes too many.
  • Salt: Limit sodium intake to a maximum of 2,300 milligrams (mg) of sodium each day.
  • Alcohol: Consuming more than one drink per day can harm the kidneys and impair renal function.
  • Smoking: Tobacco smoke restricts blood vessels. Without adequate blood supply, the kidneys will not be able to complete their normal work.
  • Over-the-counter (OTC) medications: A drug is not harmless simply because a person does not need a prescription to get it. Overusing OTC drugs, such as ibuprofen and naproxen, can damage the kidneys.
  • Screening: Anyone with high blood pressure or diabetes should consider regular kidney screening to help spot any possible health issues.
  • Diabetes and heart disease: Following the doctor's recommendations for managing these conditions can help protect the kidneys in the long term.
  • Sleep and stress control: The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) recommend getting 7 to 8 hours of sleep each night and seeking out activities to reduce stress.

Keeping the kidneys in full working order is essential for overall health.