Our kidneys are small organs with impressively complex structure and function. Kidneys not only remove extra fluid and waste from body but also help to balance chemicals to maintain stable internal environment and even perform some endocrine function.
Excretory function.
The main job of the kidneys is to remove waste products, extra minerals and drug metabolites. This removal happens through the constant process of filtration of blood in the unique kidney structures (glomeruli). As a result of this process, toxins are separated from nutrients, vitamins, minerals and proteins. The latter return to bloodstream and waste products along with water leave the body.
Our urine consists of primarily water (95%) and other components including:
urea (end product of protein metabolism), creatinine (breakdown product from muscle and protein metabolism), uric acid (product of purine metabolism);
ions such as sodium, potassium, calcium, magnesium, chloride, bicarbonate, phosphate and ammonium. Keeping these electrolytes in balance so that our body can function properly is another important kidney's job;
drug metabolites;
other products of normal metabolism.
2. Regulatory function.
Kidneys play important role in regulation of body water volume, body fluid osmolality, electrolyte balance, acid-base balance and blood pressure.
A healthy person with a normal fluid intake produces between 0.8 litres and 2 litres of urine each day. However, kidneys can control the daily urine output over the wide range, from 0.3 litres up to 23 litres a day, to keep stable body fluid volume. This function is essential in circumstances when we loose more water than we need, for example, when we sweat a lot in a hot day or when we have diarrhea. Or in circumstances, when we drink excessive amounts of water.
Serum osmolality test measures the concentration of dissolved particles in blood. For our body to function normally, this concentration must remain constant, therefore, extra particles (solutes) must leave the body with urine. The volume of urine might vary day to day but the amount of solutes dissolved in urine is fairly stable. So kidney together with some hormones concentrate or dilute urine by shifting the amounts of sodium and water they excrete.
Along with sodium, kidneys regulate other electrolytes in our body such as potassium, calcium, phosphate and magnesium. Potassium is essential for nerve and muscle cells function, including heart muscle. Calcium and phosphate are the main minerals of bones. Magnesium is important for muscle and nerve function, energy storage and production, maintaining blood sugar levels and blood pressure. We get all these minerals from food but their levels in blood have to be stable within narrow range. For example, normal potassium level in blood is 3.5 to 5.1 mmol/L and any deviation from these levels might be life threatening.
Kidney participate in maintaining a normal pH or acid-base balance. Together with other systems (chemical buffers in blood and respiratory system) kidneys balance bicarbonate and hydrogen ions. Kidneys work by excreting or retaining acids and bases as needed.
Kidneys produce the enzyme renin which is a part of the renin-angiotensin-aldosterone mechanism regulating our blood pressure. Kidneys release renin in response to decreased blood volume and blood pressure due to dehydration, lack of sodium or bleeding. Renin activates Angiotensin II, enzyme that constricts blood vessels and stimulates the adrenal gland to secrete hormone aldosterone. Aldosterone then signals kidneys to keep sodium and water in blood which also causes increase in blood pressure.
3. Metabolic function.
Kidneys function as an endocrine organ producing chemical mediators that participate in blood pressure control, calcium metabolism and red blood cell production. Renin is one of these chemical mediators and it's a part of described above mechanism that controls blood pressure.
Another chemical mediator activated in kidneys is Vitamin D. Vitamin D through blood travels to gastrointestinal tract where it increases absorption of calcium from food and also regulates calcium storage in bones.
The third important chemical produced in the kidneys is hormone erythropoietin. It stimulates bone marrow to produce red blood cells which is necessary to prevent anemia.
Kidneys, though often overlooked, work hard to maintain our body's health. Their job is equally important as the heart's and lungs', they truly deserve proper attention and care.
References:
Thomas, N. (2014). Renal Nursing. Willey Blackwell, 22-43.
https://pubmed.ncbi.nlm.nih.gov/15453232/#:~:text=The%20kidneys%2C%20in%20concert%20with,of%20sodium%20and%20water%20excreted. Assessed and Endorsed by the MedReport Medical Review Board
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