What do the kidneys do?

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

The kidneys have long been regarded as important organs. The Ancient Egyptians left only the brain and kidneys in position before embalming a body, inferring some higher value.

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

Structure

The kidneys are located at the back of the abdominal cavity, one on each side of the spine. Due to the asymmetry caused by the liver, the right kidney is generally slightly smaller and lower than the left.

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

Surrounding each kidney is a tough, fibrous renal capsule and, beyond that, two layers of fat that serve as protection. On top of each kidney are the adrenal glands.

Inside the kidneys are a number of pyramid-shaped lobes. Each consists of an outer renal cortex and an inner renal medulla. Flowing between these sections are nephrons, 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 they receive up to 25 percent of the heart's output.

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

Function

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

Here we will look at some of the kidneys' functions.

Waste excretion

A number of waste products are removed via the kidneys and expelled in the urine. Two major compounds that are removed are:

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

Reabsorption of nutrients

Nutrients from the blood are reabsorbed and transported to where they are needed. Other products are reabsorbed to help maintain the balance.

It is called reabsorption, not absorption, because the compounds have already been absorbed once, normally in the intestines.

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 lead to death.

The kidneys and lungs help keep a stable pH within the human body. The lungs do this by moderating carbon dioxide concentrations.

The kidneys manage it through two processes:

Reabsorbing and regenerating bicarbonate from urine: Bicarbonate is used to neutralize acids. The kidneys can either retain it if the pH is tolerable or release if acid levels rise.

Excreting hydrogen ions and fixed acids: Fixed or nonvolatile acids are any acids not produced 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. In other words, it is the ratio between fluid and minerals in the body. Dehydration is a key cause of electrolyte imbalance.

If plasma osmolality rises, 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 are normally not permeable to water, to allow water back into the body
• retaining urea in the medulla of the kidney rather than excreting it, as it attracts in water

Regulating blood pressure

The kidneys regulate blood pressure when needed, but they are responsible for slower adjustments. The renin-angiotensin system adjusts arterial pressure over the long term by impacting the extracellular fluid compartment, or the fluid outside of cells.

They do this by releasing a vasoconstrictor called angiotensin II. This hormone works with other functions to increase the kidneys' absorption of sodium chloride. 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 excess alcohol, smoking, and obesity.

Secretion of active compounds

The kidneys release a number of physiologically important products, including:

Erythropoietin: This controls erythropoiesis, or the production of red blood cells. The liver also produces erythropoietin, but the kidneys are main producers in adults. This hormone also plays an important part in wound healing and the response to neuronal injury.

Renin: This helps mediate the expansion of arteries and the volume of blood plasma, lymph, and interstitial fluid.

Calcitriol: This is the hormonally active metabolite of vitamin D. It increases the level of calcium absorbed by the intestines and ups the reabsorption of phosphate in the kidney.

Diseases

A number of problems can affect the kidneys. They can become diseased or damaged due to environmental or medical factors, and they can be malformed from birth. Here are some problems that can arise.

Diabetic nephropathy

The capillaries of the kidney become damaged due to long-term diabetes. Symptoms do not appear until years after the damage begins. They include headaches, tiredness, nausea, swollen legs, and itchy skin.

Kidney stones

Stones can form as a solid build-up of minerals in the kidney. They can cause intense pain and, if they block the ureter, they can affect kidney function.

Kidney infections

These tend to result from bacteria in the bladder traveling 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 that in men. They respond well to antibiotics.

Renal failure

The kidneys become unable to filter out waste products from the blood sufficiently. If it is caused by an injury such as a medication being overused, renal failure is often reversible with treatment. If the cause is disease, however, the condition is often irreversible.

Kidney hydronephrosis

This means "water on the kidney." It usually happens when an obstruction prevents urine from leaving the kidney. It produces intense pain. In time, the kidney can atrophy, or shrink.

Duplicated ureter

Two ureters 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. It affects around 1 percent of people.

Interstitial nephritis

A reaction to medications or bacteria can cause the spaces within the kidney to become inflamed. Treatment usually involves removing the cause.

Kidney tumor

These can be benign or malignant. The most common malignant kidney cancer is renal cell carcinoma.

Nephrotic syndrome

Damage to the kidney function causes the amount of protein in the urine to increase. This results in a loss of proteins in the body, which causes water to be drawn into the tissues. Symptoms include puffy eyes, increased cholesterol levels, fluid in the lungs, and anemia.

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

Causes

Here are some of the most common ways in which kidneys can be damaged:

Analgesics: Using pain medication over a long period of time can result in chronic analgesic nephritis. Examples include aspirin, acetaminophen, and nonsteroidal anti-inflammatory drugs (NSAIDs).

IgA nephropathy: Also known as Berger disease, it happens when IgA antibodies build up in the kidney. 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: Prescribed to treat schizophrenia and bipolar disorder, lithium can cause nephropathy with long-term use. However, if supervised carefully, the negative effects can be avoided.

Chemotherapy agents: The most common type of kidney disease in cancer patients is acute kidney injury. This is thought to be due to the intense vomiting and diarrhea that are side effects of chemotherapy.

Alcohol: Alcohol changes the kidney's 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 hinder the kidneys.

Dialysis

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. An entry point is made in the patient by joining an artery and a vein under the skin to create a larger blood vessel. Blood travels into the hemodialyzer, is treated 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: A sterile solution containing glucose is inserted into the abdominal cavity around the intestine. The peritoneal membrane acts as a filter as the osmotic gradient pulls waste products and excess fluid into the abdominal cavity. In continuous peritoneal dialysis, the fluid is drained through a catheter and thrown away 4 to 5 times a day. In automated peritoneal dialysis, the process occurs over time.

Maintaining healthy kidneys

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

Here are some suggestions for keeping your kidneys healthy:

Eat a balanced diet: Many kidney problems result from high blood pressure and diabetes, so maintaining a healthy diet can stave off many of the common causes of kidney disease. The DASH diet is recommended for maintaining a healthy blood pressure.

Get enough exercise: Exercising for 30 minutes a day can reduce the risk of high blood pressure and obesity, which put pressure on kidney health.

Drink plenty of water: Fluid intake is important, and especially water. Around 6 to 8 cups per day is recommended.

Supplements: Be cautious, as not all dietary supplements and vitamins are beneficial. Some can harm the kidneys if taken to excess.

Salt: Limit your 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 hamper their functioning.

Smoking: Tobacco smoke restricts blood vessels. Without an adequate blood supply, the kidneys will not be able to complete their normal work.

Over-the-counter medications: Just because a drug does not need a prescription, this does not mean it is harmless. Overusing 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, in order to boost overall health.