Understanding the Function of the Kidney?

The primary function of the kidney is to remove nitrogenous wastes (mainly urea) from the body. This is an extremely important kidney function, since toxic build-up of nitrogenous wastes in the body can lead to life-threatening diseases and eventually death.

While filtering the blood is an essential kidney function, the kidneys do much more than this.

Did you know that the kidneys are also responsible for regulating blood volume and blood pressure?

... And that they also produce certain hormones and regulate blood pH (Potential Hydrogen)?

This article highlights eight (8) essential functions of the kidneys, including some that may surprise you.

  1. Removing Wastes from the Blood

    Filtering the blood is the primary function of the kidney. Every day the kidneys process about 200 quarts (190 liters) of blood and filters out about 2 quarts of waste products and extra water. To effectively handle this volume of blood, the kidneys are equipped with lots of blood vessels.

    If you extracted all the blood vessels from both kidneys, stretched them out then measured them, their collective length will be approximately 160 km. That's an enormous amount of blood vessels. Many of them are very tiny and can only be properly observed under a microscope.

    Some of the kidney's blood vessels are demonstrated in the following diagram.

    Other blood vessels are located within microscopic structures of the kidneys called nephrons. Each kidney contains over one million of them.

    The main job of nephrons is to filter the blood to remove waste products.

    Here's How The Kidneys Remove Wastes from the Blood

    Waste is any substance that is not useful to the body. This could be any substance or chemical that the body cannot use at all, or even if the body can use it, if it is present in excess of what the body needs then the excess is classified as waste. If wastes are allowed to accumulate in the body, the results can be deadly. It is, therefore, crucial for wastes to be eliminated from the body, relatively quickly.

    The urinary system, of which the kidneys are the main organs, is responsible for eliminating mainly metabolic wastes from the body.

    A metabolic waste is a waste substance produced by the body. Nitrogenous wastes (such as urea and uric acid) are examples of metabolic wastes.

    Whenever we consume food, the body extracts the nutrients from the food and uses them to maintain vital bodily functions, such as energy and self-repair. Once the nutrients are extracted from the food, waste products are left behind in the blood and the bowels. These waste products must then be removed from the body through a process known as excretion.

    Blood supplied to the kidneys goes through a complex filtration process in order to remove toxic waste products from the blood. Chief among these waste products are urea and uric acid. If too many of these waste products are allowed to accumulate in the blood stream, this will result in life threatening illnesses. Fortunately, these two deadly substances are easily expelled from the body by the kidneys, through the process of excretion.

  2. Urine Formation - Another Vital Function of the Kidney

    Once the kidneys have filtered the blood and removed the waste products from it, the next step is to get rid of the wastes from the body. Urine formation is the process by which the kidneys prepare waste products, filtered from the blood, for elimination from the body.

    Once blood plasma goes through the kidneys and the waste products are filtered out, these wastes must be removed from the body. They are toxic and if allowed to accumulate, will cause illness and eventually death. A key function of the kidney is to remove nitrogenous waste from the body, which is done via the urine.

    The kidneys produce and eliminate urine by a sophisticated and somewhat complex process. During the filtering process, the kidneys determine the correct volume of water and the correct amount of chemicals (such as sodium, phosphorous and potassium) that should remain in the blood.

    Urinary System

    Collectively, the excess water, chemicals and nitrogenous wastes form what is known as urine, which flows from the kidneys through the ureters and into the urinary bladder.

    Once a sufficient volume of urine accumulates in the bladder, it is expelled from the body through the urethra.

    The urinary system is uniquely designed to enable urine to flow in the right direction, from the kidneys to the bladder. This is important to note, since any reversal of the direction in which urine flows can have serious consequences. Kidney reflux disease is a conditions characterized by urine flowing in the wrong direction.

  3. Regulate Water volume

    Regulating the body's water volume is another important function of the kidney.

    The body is made up primarily of water. In fact, our body-weight is made up of approximately 50-60% water. Water is absolutely essential for many of the body's biological functions, but having too much or too little of it can cause serious problems. Excessive water in the body can dilute the blood, negatively affecting the environment in which the cells operate. On the other hand, insufficient water can cause circulatory shock and neurological dysfunction.

    Thankfully, the Kidneys are equipped to regulate and maintain the correct volume of water in the body. Here's what the kidneys do to regulate the volume of water in the body...

    Let's suppose you drank a lot of water all at once, or within a very short period of time. The water is absorbed from your gut into your bloodstream, and it has the effect of diluting the blood. As the diluted blood enters the kidneys, water and other chemicals are filtered out, by the nephrons.

    Kidneys Regulate Water

    As blood moves along the capillaries, within the nephrons, some water and other chemicals that were previous filtered out are reabsorbed, thereby, ensuring that the volume and concentration of the blood are correct. Amazingly, the nephrons are able to detect the quality of the blood, as it enters the nephrons, and correctly adjust its volume and concentration through the filtering and reabsorption process.

    The excess water and other wastes substances form urine, which is transported from the kidneys to the bladder via the ureters. Eventually, as the bladder fills up the urine is expelled from the body through the process of urination.

    The kidneys will produce a high volume of urine to get rid of the excess water you drank, and you are likely to visit the bathroom quite frequently.

    If on the other hand you failed to drink sufficient water, the kidneys will filter out very little water from the blood in an effort to maintain the correct volume and concentration of blood. You will likely have very few trips to the bathroom and your urine will be more concentrated.

    Maintaining the correct volume of water in the body is an extremely important function of the kidney. If the kidneys are unable to perform this crucial function, the result could be serious illnesses, such as high blood pressure and pulmonary edema. (Edema is a condition where too much fluid is retained in the body's cells.)

  4. The Kidneys Regulate The Body's Salt Content

    Salt and water are regulated by the kidneys through a similar process. If you had a salty meal, for example, the salt is absorbed into your blood. When the salty blood reaches the kidneys the excess salt is removed and passed into the urine. Remarkably, the kidneys are able to determine the correct concentration of salt (or sodium) that should remain in the blood, and remove any excess. This is an extremely important function of the kidney.

    The relative amount of water and salt in the blood give the blood a particular concentration, which is necessary for good health. By regulating the amount of salt and water in the blood, the kidneys ensure that the concentration and volume of the blood stay at the correct level at all times.

  5. Regulating Blood Pressure

    Regulating blood-pressure is another crucial function of the kidney. This is done by hormonal reactions and regulating the volume of water in the body, which we previously looked at.

    Renin (REE-nin) is an enzyme secreted by the kidneys, which helps to control blood pressure and regulate electrolyte balance. If, for instance, blood-pressure begins to fall, the kidneys release the enzyme renin. This enzyme converts blood protein into a hormone called angiotensin.

    Angiotensin then induces the adrenal gland to release another hormone called aldosterone. This hormone causes sodium and water to be reabsorbed into the blood, thereby increasing blood-volume and blood-pressure. The response of the kidneys to aldosterone helps to regulate the level of salt in the blood.

  6. Regulating pH Balance

    Regulating the body's pH balance is another important function of the kidney. If the blood is too acidic, meaning that there is an excess of hydrogen ions, the kidneys remove these excess ions into the urine.

    Bacteria (which cause many serious degenerative diseases) flourish in an acidic environment. The kidneys help to maintain the blood plasma at a neutral pH 7.4. By regulating the body's pH, the kidneys help to maintain an environment that prevents diseases and promotes the efficient functioning of all bio-systems within the body.

  7. Production of Hormones is Another Function of The Kidney

    Some important hormones are produced in the kidneys. One such hormone is erythropoietin (eh-RITH-ro-POY-eh-tin). It plays a key role in the production of red blood cells in the bone marrow.

    When the kidneys detect a decline in the red blood cells in the body, it produces erythropoietin. This hormone is released into the bloodstream and goes to the bone marrow, which triggers the production and release of more red blood cells.

  8. Processing Vitamin D

    Not many people are aware that processing vitamin D is a function of the kidney. Here's what the kidneys do to help process vitamin D:

    The kidneys convert calcidiol to calcitriol, which is an active form of vitamin D. In this active form, vitamin D promotes calcium absorption by the small intestine and makes it available for bone development.


While many people may have been familiar with the primary function of the kidney (to filter the blood), many are unaware of the other essential kidney functions. Hopefully, this article has not only highlighted the Kidneys' immense contributions to overall good health, but underscored the importance of preserving optimum kidney function. There can be no question that the kidneys are very hard working organs and play a significant role in maintaining good health.