The 8 Functions of the Kidney

Most people assume that the main job of the kidneys is to remove waste products from the blood – and that’s true. However, the kidneys are a multi-faceted organ, involved in all sorts of chemical reactions and biological processes.

In this post, we answer the question: what is the function of the kidneys? We explore each in turn, giving you a quick overview of what the kidney does and why it is important. 

 

Remove toxins

What is the main function of the kidneys? If you ask most medical professionals, they will tell you that it is to remove toxins from the blood and transform waste into urine. Toxins that kidneys remove include creatinine, hormone metabolites, ammonia, uric acid, and urea.

The kidneys remove toxins through several mechanisms:

 

The particular mechanism the kidneys use depends on the type and concentration of the toxin in question. 

 

Control water balance

The function of kidney tissue is also to control the water balance in the body by continually filtering blood. When patients overly hydrate, the kidneys expel water by making urine more dilute. If they dehydrate, the kidneys preserve water content and make the urine more concentrated. 

Kidneys control the level of water in the body by maintaining extracellular fluid volume. Swollen extremities, such as feet and ankles, are a sign that the kidneys are losing their ability to regulate water balance.

 

Electrolyte balance

Electrolytes are particles capable of carrying electric charges when dissolved in the blood. Common electrolytes include sodium, potassium, calcium, and magnesium. 

The body needs to maintain electrolyte balance to maintain nerve function, the normal functioning of muscles, and the formation of bone and tooth tissue. Without the right electrolyte concentrations, patients can experience lethargy, irregular heartbeat, fatigue, vomiting, nausea, diarrhea, and constipation. 

Electrolyte imbalance is often a sign of renal failure. Glomerular that do not function properly may not be able to remove electrolytes from the bloodstream. 

 

Acid-base balance

Kidneys are also responsible for maintaining acid-base balance in the body. They do this in two ways:

 

By continually adjusting these secretions and reabsorptions, they can maintain acid balance in the body indefinitely, keeping the bloodstream’s pH within acceptable bounds. 

Please note, however, that acid-base balance requires the cooperation of multiple major organ systems, not just the kidneys. The liver and lungs are also involved. The liver is responsible for the metabolism of organic acid anions and ammonium, while the lungs excrete large quantities of acidic CO2 gas.

 

Control blood pressure

Healthy kidneys control blood pressure via the release of the hormone, aldosterone. This chemical adjusts the balance of sodium and potassium in the body. It signals to your kidneys and colon to put more sodium into the bloodstream or instructs the kidneys to release more potassium into the urine. 

When you consume sodium (found in table salt and most processed foods), it causes your body to hold onto more water, causing blood pressure to rise. Sensing this, the kidneys release aldosterone, leading to a reduction of sodium in the system, and water along with it. 

Patients may notice this effect when fasting after a high-salt meal. During the night, they may have to visit the bathroom several times to eliminate extra water as the body attempts to reduce sodium levels. 

By contrast, potassium (found in fresh fruits and vegetables, beans, and whole grains) reduces blood pressure by helping artery walls relax. Kidneys control potassium excretion by increasing aldosterone production from the adrenal glands. 

 

Produce erythropoietin and create blood cells

The kidneys (along with the liver) produce erythropoietin which stimulates the bone marrow to produce red blood cells. 

Small amounts of erythropoietin circulate in the bloodstream continuously to maintain the body’s natural stores. However, production can increase in response to blood loss, ischemic stress, high altitude, and anemia. 

Erythropoietin is made by the kidney’s interstitial cells. It is a type of glycoprotein. Low levels can result from kidney failure, and chronic diseases, such as HIV/AIDS and rheumatoid arthritis. 

If levels are low, patients may be able to increase them by taking echinacea supplements. Studies also show that massaging the areas around the kidneys is effective. 

 

Activate vitamin D

Vitamin D is a hormone mainly produced in the skin. People can also obtain smaller amounts via dietary sources, such as mushrooms and animal products. 

However, vitamin D from sunlight and diet is not yet in active form. It first requires conversion in the kidneys. 

People with chronic kidney disease tend to have low levels of active vitamin D. That’s because the kidneys are less able to convert it into the type that cells can use for metabolic processes. 

The rate at which kidneys convert vitamin D into active form is determined by body chemistry, including electrolyte balance. Low calcium intake, for instance, stimulates vitamin D3 precursors in the kidney, increasing gastrointestinal and bone calcium reabsorption. 

 

Regulate salt balance

Lastly, one of the main functions of the kidney is to regulate salt balance. 

Sodium chloride, also called dietary salt, is an essential electrolyte in the human body. In the past, researchers believed that the kidneys controlled salt levels via straightforward mechanisms. When salt levels are too high, the kidneys send signals to the brain encouraging us to drink more and excrete more urine, thereby expelling excess salt. 

However, new evidence suggests that aldosterone hormone rhythms regulate weekly and monthly salt expulsion patterns. Changing salt levels appears to affect levels of glucocorticoid and aldosterone in the body, the hormones that control general salt and water balance. Increasing salt intake increases sodium excretion but also causes kidneys to conserve water. Sodium becomes more concentrated in the urine, even as people drink less. 

 

Author
Uday Khosla, MD Uday M. Khosla, MD, is board certified in nephrology and hypertension. Dr. Khosla has served as a Houston kidney consultant since 2004. He currently practices at Kidney360 in Houston, Texas, and is always looking for ways to improve patient care. Dr. Khosla enjoys sharing his knowledge with colleagues and students, and his works have been published in numerous scientific journals and textbooks. Dr. Khosla is available as a kidney consultant in various settings, including in-hospital, outpatient clinics, outpatient dialysis, and home care.

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