The kidneys are a pair of bean-shaped organs that are found in all vertebrates. They are regulatory in nature and carry out a great number of tasks, including the removal of waste products from the body, maintaining the balance of electrolyte levels and regulating blood pressure.
The kidneys have been regarded as important organs since early history. In biblical times, the kidneys were considered to be the seat of conscience and reflective thought. God is described as inspecting the heart and kidneys of men.
Tellingly, ancient Egyptian embalmers left only the brain and kidneys in position before permanently fixing bodies, inferring some higher value.
In the Jewish text, the Talmud, it is claimed that one kidney prompts man to do what is good, and the other evil.
Modern nephrologists still have huge respect for the wonders of the kidney, although the exact reasons for this respect have changed since historical times. In the following article, we will look at the structure and function of the kidneys, diseases that affect them and how to keep the kidneys healthy.
Contents of this article:
Here are some key points about kidneys. More detail and supporting information is in the main article.
- The kidneys are a pair of organs within the abdomen
- Much of the kidneys' work revolves around maintaining the body's internal equilibrium
- Despite being relatively small organs, the kidneys use a high percentage of the body's cardiac output
- Kidneys help maintain the body's pH balance
- Dialysis is used if the kidneys have lost the majority of their function
- Blood pressure is partially maintained by the kidneys
- The kidneys secrete a number of hormones
- The adrenal glands sit on top of the kidneys
- Certain analgesics can damage the kidneys.
Structure and location of the kidneys
The right kidney is often smaller than the left.
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 125-170 g in males and 115-155 g in females.
Surrounding the kidneys is the 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. Despite being relatively small organs, together the kidneys receive up to 25% of the heart's entire output.1 Each kidney excretes urine through a tube called the ureter that leads to the bladder.
Function of the kidneys
The primary role of the kidneys is that of homeostasis. In other words, they manage a whole host of variables to ensure that the internal environment of the body is kept within constant parameters.
Here, we will discuss some of the kidneys' main functions in this regard:
A number of waste products are removed via the kidneys and expelled in the urine. Two of the major compounds to be removed are urea, produced from the breakdown of proteins, and 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 equilibrium.
This process is referred to as reabsorption rather than absorption because the compounds have already been absorbed once, normally in the intestines.
Reabsorbed products include:
- Amino acids
- Chloride ions
- Sodium ions
- Potassium ions
- Magnesium ions.
Keeping pH at a tolerable level is essential. In humans, the acceptable level is between 7.38 and 7.42. Outside of the normal boundaries (states of acidemia or alkalemia), proteins and enzymes break down and can no longer function. In extreme cases, this can lead to death.
Both the kidneys and the lungs help keep a stable pH within the human body. The lungs carry out this role 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 it when acid is on the rise2
- Excreting hydrogen ions and fixed acids: fixed acids are also known as nonvolatile acids and the name refers to any acids not produced as a result of carbon dioxide. They are a result of the incomplete metabolism of carbohydrates, fats and proteins. They include lactic acid, sulfuric acid and phosphoric acid.
The kidneys maintain the correct levels of electrolytes and water in the body.
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.3 Dehydration is a key cause of electrolyte imbalance.
If a rise in plasma osmolality is detected, the hypothalamus in the brain responds by passing a message to the pituitary gland which, in turn, releases antidiuretic hormone (ADH).
In response to ADH, the kidney makes a number of changes, including:
- Increasing urine concentration
- Increasing water reabsorption
- Portions of the collecting duct that are normally not permeable to water are reopened, allowing water back into the body
- Retaining urea rather than excreting it; this compound is retained in the medulla of the kidney and attracts in water.
Regulating blood pressure
Rather than regulating blood pressure on an ad hoc basis, the kidneys are responsible for slower adjustments. The renin-angiotensin system adjusts arterial pressure over the long term by impacting the extracellular fluid compartment (fluid outside of cells).
The kidneys complete this task by releasing a vasoconstrictor called angiotensin II. This hormone, as part of an incredibly complex web, increases the kidney's absorption of sodium chloride, effectively increasing the size of the extracellular fluid compartment and raising 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: controls erythropoiesis (production of red blood cells). The liver also produces erythropoietin, but during adulthood the kidneys are the primary producers. This hormone also plays an important part in wound healing4 and the response to neuronal injury5
- Renin: helps mediate arterial vasoconstriction 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 of the kidney
There are a great number of potential ways in which the kidneys can become diseased or damaged. Additionally, kidneys can be malformed from birth.
Kidney diseases include:
- Diabetic nephropathy: caused by long-term diabetes, the capillaries of the kidney are damaged. Symptoms do not appear until years after the damage begins and include headaches, tiredness, nausea, swollen legs and itchy skin
- Kidney stones: a solid build-up of minerals in the kidney. They can cause intense pain and, if they block the ureter, kidney function can be diminished
- Kidney infections: typically caused by bacteria in the bladder traveling to the kidneys. These infections cause lower back pain, painful urination and sometimes fever. There may be observable changes in the urine, including the presence of blood, cloudiness and a different odor. Kidney infections are more common in women and respond well to antibiotics
- Renal failure: in short, this refers to the inability of the kidneys to sufficiently filter out waste products from the blood. This can be caused by an injury; in this case, renal failure is often reversible with treatment. If the cause of renal failure is disease, however, the condition is often irreversible
- Kidney hydronephrosis: meaning "water on the kidney," this condition is generally caused by an obstruction that prevents urine from exiting the kidney. It produces intense pain and leads to the progressive atrophy (shrinking) of the kidney
- Duplicated ureter: approximately 1% of the population have this condition. Two ureters form between a kidney and the bladder rather than one. There are few complications, but there can be an increase in urinary tract infections and, in females, incontinence
- Interstitial nephritis: often a reaction to specific medications or bacteria, the spaces within the kidney become inflamed. Generally, the only treatment is to remove the causal factor
- Kidney tumor: these can be benign or malignant. The most common malignant kidney cancer is renal cell carcinoma
- Nephrotic syndrome: kidney function is damaged which increases the amount of protein in the urine. The resulting loss of proteins causes water to be drawn into the tissues. Symptoms of this condition include puffy eyes, increased cholesterol levels, fluid in the lungs and anemia.
Causes of kidney disease
There are many ways in which the kidneys can become damaged.
There are a number of ways in which a kidney can become damaged; below are some of the most common:
- Analgesics: pain medication used over a long period of time can damage the kidneys. Drugs that have the potential to damage the kidneys include aspirin, acetaminophen and nonsteroidal anti-inflammatory drugs (NSAIDs). This damage is referred to as chronic analgesic nephritis
- IgA nephropathy: also known as Berger disease, 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. The end point is often 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 avoided6
- Chemotherapy agents: the most common type of kidney disease in cancer patients is acute kidney injury, thought to be caused by the intense vomiting and diarrhea brought on by 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. Additionally, alcohol increases blood pressure which can hinder the kidneys.
If the kidneys are severely damaged, dialysis might be an option. Dialysis is only used for end-stage kidney failure where 85-90% of kidney function is lost.7
Kidney dialysis aims to complete some of the functions that a healthy kidney would carry out:
- 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 following are the two most common types of kidney dialysis:
- Hemodialysis: an artificial kidney (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 returned to the body. This is generally carried out 3-4 times a week, but more regular dialysis has a more beneficial effect8
- 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. Once this has taken place, the fluid is drained through a catheter. This process is carried out 4-5 times a day.9
Maintaining healthy kidneys
Maintaining hydration is important for kidney health.
Keeping the kidneys in full working order is important for overall health. Below are some suggestions for maintaining kidney health:10
- Hydration: drinking enough water is important for many reasons, but drinking too much water will not be of benefit. Around 6-8 cups per day is a sufficient amount
- Diet: because many kidney problems are due to high blood pressure and diabetes, maintaining a healthy diet can stave off many of the common causes of kidney disease
- Exercise: exercise, as with diet, can also remove many of the risks to kidney health
- Supplements: caution should be shown with dietary supplements and vitamins. Some have the ability to harm the kidneys if taken to excess
- Salt: monitor salt intake. A maximum of 2,300 milligrams of sodium each day is preferable11
- Alcohol: any more than two drinks per day can harm the kidneys and hamper their functioning12
- Smoking: tobacco smoke restricts blood vessels. If the kidneys' blood supply is not optimal, they will struggle to complete their normal work
- Over-the-counter medications: just because a drug is available to purchase without a prescription does not mean it is harmless. Overusing drugs such as ibuprofen and naproxen can damage the kidneys
- Screening: if you have high blood pressure or diabetes, a regular kidney screening can help spot any potential health issues.
Diabetes is a major risk factor for kidney disease; around 30% of people with type 1 diabetes and 10-40% of those with type 2 diabetes will go on to experience kidney failure. But new research finds that even before a diabetes diagnosis, higher-than-normal blood sugar levels could be causing kidney damage.
Taking a class of drugs commonly used to reduce acid in the stomach is linked to a higher risk of developing chronic kidney disease, compared with not taking them.
Learn more about the causes of kidney failure.