As a part of the excretory system, it’s the kidneys’ job to make sure the human body is free of unwanted substances. Our bodies are sophisticated factories that excrete about one-and-a-half liters of urine per day to maintain balance. Urine is composed of many different substances, thanks to the thousands of functions the kidneys silently perform.
The primary function of the kidneys is to filter. Each kidney is composed of about a million filtering stations called nephrons. Each nephron has a glomerulus and a tubule. The glomerulus filters the blood and passes substances like water and waste to the tubule, which does its own filtering. The body reabsorbs vitamins, minerals, and water while the unwanted waste and fluids pass out as urine.
Balancing the body's fluids and common electrolytes, such as sodium and potassium, is important. Electrolytes carry electrical impulses to the heart, telling it how to beat. An imbalance can cause dangerous heart disturbances such as arrhythmias. The kidneys regulate the amount of sodium and potassium in the body relative to water ingested. That means that when the body has too much sodium or potassium, the excess can leave as urine.
One of the hormones secreted by the kidneys is erythropoietin. Low levels of oxygen in the blood, known as hypoxia, trigger the creation of this hormone, which stimulates a process called erythropoiesis to create red blood cells in the bone marrow and bring oxygen levels back to optimal.
When the blood pressure drops below a certain level, the kidneys step in to bring it back up with the help of the RAAS system. Renin is an enzyme the kidneys create that, with the help of angiotensin and aldosterone, regulates blood pressure. The enzyme converts the liver hormone angiotensinogen to angiotensin I, which becomes angiotensin II, a peptide that increases blood pressure by narrowing blood vessels. Angiotensin II stimulates the secretion of aldosterone, a hormone secreted by the adrenal glands that sit atop each kidney. This further increases blood pressure to normal levels.
The blood’s natural pH range is between 7.35 and 7.45. Barring illness or kidney malfunction, the blood’s overall pH doesn’t deviate from this homeostatic range. The kidneys maintain proper pH through acid secretion. If urine is more acidic, that means the kidneys are ridding the body of excess acid to achieve alkalinity.
Parathyroid Hormone, PTH, is secreted when calcium levels are too low. Maintaining proper calcium levels is essential for the normal function of the nervous system, bones, heart, and kidneys. In response to PTH, the kidneys produce calcitriol, a form of vitamin D3. The combination of these two hormones increases calcium reabsorption by the intestines, preventing the mineral from being lost through the urine.
The liver is known to be the only organ that can fully regenerate. But researchers at Harvard Medical School have discovered that the kidneys can also regenerate and may use stem cells to rebuild. While this is promising, it turns out that any recovered tissue will not regenerate nephrons.
Glutamine, an amino acid, participates in many metabolic processes, including the process of metabolizing carbohydrates in the kidneys. One of the byproducts of that process is ammonia. Approximately half of the ammonia is excreted as urine while the other half is returned to circulation and passed to the liver.
Prostaglandins are fats that develop at the sites of infection or injury to help with tissue repair. Those made in the kidneys, however, have a slightly different function. They can act as vasodilators, which help blood vessels open and relax, reducing pressure and increasing blood flow. This function balances the blood pressure increase that occurs upon stimulation of the renin enzyme.
Healthy kidneys process about 200 quarts of blood daily, about 800 cups, with an average of about half a cup a minute. After blood flows through the renal artery and enters the Bowman capsule, it passes through fifty different capillaries. It then passes through about four tubules before heading for the bladder. On average, the body excretes about two quarts of urine, or eight cups.
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