Why do humans retain two kidneys?

If one kidney can function just as well as two, why do humans have two kidneys? The cost of growing two kidneys must surely be quite high, especially since one kidney is all that is really needed.

One of the easiest answers to the question; "Why do we have ____?" is "Because our ancestors did". This is not trivial or flippant, as it is a significantly important answer that is so often overlooked. Humans are members of Bilataria; a deep branch of the animal tree that is characterized by bilateral symmetry. Humans are typical of other vertebrates in having two kidneys. Selective pressure to reduce to one of two organs is typical in snakes, whose body form favors reduction. But even snakes have two kidneys. Natural selection does not reduce features we don't need unless such a reduction would increase fitness.

Kidney Stone Causes

Kidney stones happen when your pee has a high concentration of minerals and other substances -- like calcium, oxalate, and uric acid -- that come together to make crystals. Crystals stick together to make one or more stones. Stones happen when your urine doesn’t have enough fluid and other substances to keep them from happening.

A kidney stone can be as tiny as a grain of sand, and you can pass it without ever knowing. But a bigger one can block your urine flow and hurt a lot. Some people say the pain can be worse than childbirth.

Different things can bring on kidney stones, including what you eat and certain medications. If you or someone in your family has had a kidney stone, you’re more likely to have them.

How Does the Urinary System Work?

How the urinary system works is relatively simple, although the supplementary roles of the kidneys can be complex.

Blood is transported to the kidneys via the renal artery. A system of filtration units within the kidney regulates levels of dilution (water), salts and other small molecules in the filtrate. Any excess or undesired products travel through each ureter and are deposited into the reservoir of the bladder, while purified blood re-enters the circulatory system by way of the renal vein. Urine is stored in the bladder until the urinary nervous system releases the contents through the urethra and out of the body. The passing of urine is known as micturition or urination.

The urinary system is split into the upper and lower urinary tract. The former consists of the kidneys and ureters, the latter of the bladder and urethra.

How much urine is expelled depends on various factors: the ratio of waste products to water, dietary habits, physical activity levels, medications, comorbidities, and the state of health of various organs and systems within the body.

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Home / Homeostasis - kidneys and water balance


The amount of a substance (solute) in a solution


The maintenance of a constant internal environment in the body


A list of often difficult or specialised words with their definitions.


Chemical needed in very small amounts as part of a balanced diet to keep the body healthy


A polymer made up of amino acids joined by peptide bonds. The amino acids present and the order in which they occur vary from one protein to another.


A type of sugar: a mono saccharide with 6 carbon atoms (a hexose sugar).


Reddish brown organs which get rid of waste urea from the body and balance the water and mineral ion concentration of the blood

The basic unit from which all living organisms are built up, consisting of a cell membrane surrounding cytoplasm and a nucleus.


The liquid which leaves your body through the urethra. It contains water, salts urea and other chemicals.

Charge particles formed when an atom loses or gains electrons during the formation of ionic bonds

The poisonous waste compound produced when excess amino acids are broken down in your liver

What do the kidneys do?

Humans have two kidneys. They are complex organs that are vital for life. The kidneys produce urine which is made up of waste products, excess mineral ions and excess water from the body.

The main job of your kidneys is to regulate the amount of water in the body and balance the concentration of mineral ions in the blood. They also get rid of waste products, especially a nitrogen-containing compound called urea. At the same time they hold on to useful substances such as glucose and protein so none is lost from the body.

Your body is made up of millions of cells. For them to work properly, the conditions inside your body need to be as constant as possible. However everything you do tends to change your internal conditions. You take millions of new molecules into your body when you eat and digest food, you release heat energy every time you move about, the amount of water you take into and lose from your body varies all the time and your cells are constantly producing poisonous waste.

Homeostasis describes the functions of your body which work to keep your internal environment constant within a very narrow range. One of your most important organs of homeostasis is the kidney.

Understanding the kidneys' role in blood glucose regulation

While not traditionally discussed, the kidneys' contributions to maintaining glucose homeostasis are significant and include such functions as release of glucose into the circulation via gluconeogenesis, uptake of glucose from the circulation to satisfy their energy needs, and reabsorption of glucose at the level of the proximal tubule. Renal release of glucose into the circulation is the result of glycogenolysis and gluconeogenesis, respectively involving the breaking down and formation of glucose-6-phosphate from precursors (eg, lactate, glycerol, amino acids). With regard to renal reabsorption of glucose, the kidneys normally retrieve as much glucose as possible, rendering the urine virtually glucose free. The glomeruli filter from plasma approximately 180 grams of D-glucose per day, all of which is reabsorbed through glucose transporter proteins that are present in cell membranes within the proximal tubules. If the capacity of these transporters is exceeded, glucose appears in the urine. The process of renal glucose reabsorption is mediated by active (sodium-coupled glucose cotransporters) and passive (glucose transporters) transporters. In hyperglycemia, the kidneys may play an exacerbating role by reabsorbing excess glucose, ultimately contributing to chronic hyperglycemia, which in turn contributes to chronic glycemic burden and the risk of microvascular consequences. This article provides an extensive review of the kidneys' role in normal human physiology, the mechanisms by which they contribute to glucose regulation, and the potential impact of glucose imbalance on the kidneys.

Regulatory functions

The kidneys regulate three essential and interrelated properties of the tissues—water content, acid-base balance, and osmotic pressure—in such a way as to maintain electrolyte and water equilibrium in other words, the kidneys are able to maintain a balance between quantities of water and the quantities of such chemicals as calcium, potassium, sodium, phosphorus, and sulfate in solution. Unless the concentrations of mineral ions such as sodium, crystalloids such as glucose, and wastes such as urea are maintained within narrow normal limits, bodily malfunction rapidly develops leading to sickness or death.

The removal of both kidneys causes urinary constituents to accumulate in the blood ( uremia), resulting in death in 14–21 days if untreated. (The term uremia does not mean that urea is itself a toxic compound responsible for illness and death.) Whenever the blood contains an abnormal constituent in solution or an excess of normal constituents including water and salts, the kidneys excrete these until normal composition is restored. The kidneys are the only means for eliminating the wastes that are the end products of protein metabolism. They do not themselves modify the waste products that they excrete, but transfer them to the urine in the form in which they are produced in other parts of the body. The only exception to this is their ability to manufacture ammonia. The kidneys also eliminate drugs and toxic agents. Thus, the kidneys eliminate the unwanted end products of metabolism, such as urea, while limiting the loss of valuable substances, such as glucose. In maintaining the acid-base equilibrium, the kidneys remove the excess of hydrogen ions produced from the normally acid-forming diet and manufacture ammonia to remove these ions in the urine as ammonium salts.

To carry on its functions the kidney is endowed with a relatively huge blood supply. The blood processed in the kidneys amounts to some 1,200 millilitres a minute, or 1,800 litres (about 475 gallons) a day, which is 400 times the total blood volume and roughly one-fourth the volume pumped each day by the heart. Every 24 hours 170 litres (45 gallons) of water are filtered from the bloodstream into the renal tubules and by far the greater part of this—some 168.5 litres of water together with salts dissolved in it—is reabsorbed by the cells lining the tubules and returned to the blood. The total glomerular filtrate in 24 hours is no less than 50–60 times the volume of blood plasma (the blood minus its cells) in the entire body. In a 24-hour period, an average man eliminates only 1.5 litres of water, containing the waste products of metabolism, but the actual volume varies with fluid intake and occupational and environmental factors. With vigorous sweating it may fall to 500 millilitres (about a pint) a day with a large water intake it may rise to three litres, or six times as much. The kidney can vary its reabsorption of water to compensate for changes in plasma volume resulting from dehydration or overhydration.

Human Organ Systems

Functionally related organs often cooperate to form whole organ systems. Figure (PageIndex<3>) and Figure (PageIndex<4>) show 11 human organ systems, including separate diagrams for the male and female reproductive systems. Some of the organs and functions of the organ systems are identified in the figure. Each system is also described in more detail in the text that follows. Most of these human organ systems are also the subject of separate chapters in this book.

Integumentary System

Organs of the integumentary system include the skin, hair, and nails. The skin is the largest organ in the body. It encloses and protects the body and is the site of many sensory receptors. The skin is the body&rsquos first defense against pathogens, and it also helps regulate body temperature and eliminate wastes in sweat.

Skeletal System

The skeletal system consists of bones, joints, teeth. The bones of the skeletal system are connected by tendons, ligaments, and cartilage. Functions of the skeletal system include supporting the body and giving it shape. Along with the muscular system, the skeletal system enables the body to move. The bones of the skeletal system also protect internal organs, store calcium, and produce red and white blood cells.

Muscular System

The muscular system consists of three different types of muscles, including skeletal muscles, which are attached to bones by tendons and allow for voluntary movements of the body. Smooth muscle tissues control the involuntary movements of internal organs, such as the organs of the digestive system, allowing food to move through the system. Smooth muscles in blood vessels allow vasoconstriction and vasodilation and thereby help regulate body temperature. Cardiac muscle tissues control the involuntary beating of the heart, allowing it to pump blood through the blood vessels of the cardiovascular system.

Nervous System

The nervous system includes the brain and spinal cord, which make up the central nervous system, and nerves that run throughout the rest of the body, which make up the peripheral nervous system. The nervous system controls both voluntary and involuntary responses of the human organism and also detects and processes sensory information.

Figure (PageIndex<3>): Organ systems: Integumentary, Skeletal, Muscular, Nervous, Endocrine, and Cardiovascular

Endocrine System

The endocrine system is made up of glands that secrete hormones into the blood, which carries the hormones throughout the body. Endocrine hormones are chemical messengers that control many body functions, including metabolism, growth, and sexual development. The master gland of the endocrine system is the pituitary gland, which produces hormones that control other endocrine glands. Some of the other endocrine glands include the pancreas, thyroid gland, and adrenal glands.

Cardiovascular System

The cardiovascular system (also called the circulatory system) includes the heart, blood, and three types of blood vessels: arteries, veins, and capillaries. The heart pumps blood, which travels through the blood vessels. The main function of the cardiovascular system is transport. Oxygen from the lungs and nutrients from the digestive system are transported to cells throughout the body. Carbon dioxide and other waste materials are picked up from the cells and transported to organs such as the lungs and kidneys for elimination from the body. The cardiovascular system also equalizes body temperature and transports endocrine hormones to cells in the body where they are needed.

Urinary System

The urinary system includes the pair of kidneys, which filter excess water and a waste product called urea from the blood and form urine. Two tubes called ureters carry the urine from the kidneys to the urinary bladder, which stores the urine until it is excreted from the body through another tube named the urethra. The kidneys also produce an enzyme called renin and a variety of hormones. These substances help regulate blood pressure, the production of red blood cells, and the balance of calcium and phosphorus in the body.

Respiratory System

Organs and other structures of the respiratory system include the nasal passages, lungs, and a long tube called the trachea, which carries air between the nasal passages and lungs. The main function of the respiratory system is to deliver oxygen to the blood and remove carbon dioxide from the body. Gases are exchanged between the lungs and blood across the walls of capillaries lining tiny air sacs (alveoli) in the lungs.

Lymphatic System

The lymphatic system is sometimes considered to be part of the immune system. It consists of a network of lymph vessels and ducts that collect excess fluid (called lymph) from extracellular spaces in tissues and transport the fluid to the bloodstream. The lymphatic system also includes many small collections of tissue, called lymph nodes, and an organ called the spleen, both of which remove pathogens and cellular debris from the lymph or blood. In addition, the thymus gland in the lymphatic system produces some types of white blood cells (lymphocytes) that fight infections.

Digestive System

The digestive system consists of several main organs &mdash including the mouth, esophagus, stomach, and small and large intestines &mdash that form a long tube called the gastrointestinal (GI) tract. Food moves through this tract where it is digested, its nutrients absorbed, and its waste products excreted. The digestive system also includes accessory organs (such as the pancreas and liver) that produce enzymes and other substances needed for digestion but through which food does not actually pass.

Male and Female Reproductive Systems

The reproductive system is the only body system that differs substantially between individuals. There is a range of Biological sex, but most books divide them into male and female. We will discuss the Biology of sex in detail in the reproductive and development chapters.

Figure (PageIndex<4>): Organ Systems: Lymphatic, Respiratory, Digestive, Urinary, male and female reproductive

Feature: Human Biology in the News

Organ transplantation has been performed by surgeons for more than six decades, and you&rsquove no doubt heard of people receiving heart, lung, and kidney transplants. However, you may have never heard of a penis transplant. The first U.S. penis transplant was performed in May of 2016 at Massachusetts General Hospital in Boston. The 15-hour procedure involved a team of more than 50 physicians, surgeons, and nurses. The patient was a 64-year-old man who had lost his penis to cancer in 2012. The surgical milestone involved grafting microscopic blood vessels and nerves of the donor organ to those of the recipient. As with most transplant patients, this patient will have to take immunosuppressing drugs for the rest of his life so his immune system will not reject the organ. The transplant team said that their success with this transplant &ldquoholds promise for patients with devastating genitourinary injuries and disease.&rdquo They also hope their experiences will be helpful for gender reassignment surgery.

Keep Your Kidneys Healthy

Your kidneys aren’t very big—each is about the size of your fist—but they do important work. They keep you healthy by maintaining just the right balance of water and other substances inside your body.

Unfortunately, if your kidneys start to malfunction, you might not realize it for a long while. Kidney disease usually doesn’t make you feel sick until the problem becomes serious and irreversible. March is National Kidney Month, a perfect time to learn more about how to keep your kidneys healthy and how to catch problems early.

Your kidneys are 2 reddish, bean-shaped organs located on either side of your spine in the middle of your back. Their main job is to filter your blood. Each kidney contains about a million tiny filters that can process around 40 gallons of fluid every day—about enough to fill a house’s hot water heater. When blood passes through the kidney, the filters sift and hold onto the substances your body might need, such as certain nutrients and much of the water. Harmful wastes and extra water and nutrients are routed to the nearby bladder and flushed away as urine.

Your kidneys also produce several hormones Molecules sent through the bloodstream to signal another part of the body to grow or react a certain way. . These hormones help to control your blood pressure, make red blood cells and activate vitamin D, which keeps your bones strong.

We all lose a little of our kidney function as we get older. People can even survive with just one kidney if they donate the other to a friend or family member.

But when kidney function drops because of an underlying kidney disease, it’s something to be concerned about. Toxins and extra water can build up in your blood. Falling hormone production can cause other problems. About 1 in 10 adults nationwide, or about 20 million people, have at least some signs of kidney damage.

There are different types of kidney disease. Most strike both kidneys at the same time, harming the tiny filters—called nephrons—and reducing their filtering ability. When damage to nephrons happens quickly, often because of injury or poisoning, it’s known as acute kidney injury. It’s more common, though, for nephrons to worsen slowly and silently for years or even decades. This is known as chronic kidney disease.

“Most people have few or no symptoms until chronic kidney disease is very advanced,” says Dr. Andrew Narva, a kidney specialist at NIH. “You can lose up to three-fourths of your kidney function and essentially have no symptoms.”

Chronic kidney disease can strike people of any race, but African Americans are especially at risk. African Americans also tend to have high rates of diabetes and high blood pressure, the 2 leading causes of kidney disease. Other risk factors for kidney disease include heart disease and a family history of kidney failure—a severe form of kidney disease.

If you have these risk factors, it’s important to be screened for kidney disease,” says Narva. “That usually involves simple laboratory tests: a urine test to look for kidney damage, and a blood test to measure how well the kidneys are working.”

The urine test checks for a protein called albumin, which isn’t routinely detected when your kidneys are healthy. The blood test checks your GFR—glomerular filtration rate. GFR is an estimate of your kidney’s filtering ability. A GFR below 60 is a sign of chronic kidney disease. A GFR below 15 is described as kidney failure.

“I tell my patients they should know their numbers,” says NIH kidney expert Dr. Jeffrey B. Kopp. “We usually cannot cure chronic kidney disease, but if we catch it early, we can slow down its progression.”

Without treatment, kidney disease often gets worse. If your GFR drops below 15, you may feel tired and weak, with nausea, vomiting and itching. By that point, you may need a kidney transplant or dialysis. It’s a good idea to talk with your doctor about the possibility of these therapies long before they’re needed. It takes time to understand your options, and it’s easier to figure things out when you’re feeling healthy.

“In general, the preferred therapy for kidney failure is to have a kidney transplant, but not everyone can have a transplant,” says Kopp. Some obstacles include long waiting lists for healthy kidneys and finding a well-matched donor.

Dialysis is a treatment that filters wastes and water from the blood, allowing patients with kidney failure to feel better and continue with everyday activities. NIH kidney specialist Dr. Paul Kimmel leads an NIH program to improve the lives of patients on dialysis. “Although dialysis is a life-saving therapy, it can be challenging for patients and families,” Kimmel says. “We’re encouraging researchers to explore innovative ways to improve the quality of life and long-term outcome for these patients.”

You can take many steps to avoid or delay reaching the point of kidney failure. The best thing you can do is control your blood pressure. A healthy lifestyle, including physical activity and a heart-healthy diet, can help to normalize blood pressure and also slow kidney disease.

“Most Americans eat more sodium and protein than the body needs. It’s your kidneys’ job to filter and get rid of the leftovers 24 hours a day, 7 days a week,” says registered dietitian Theresa Kuracina, who advises NIH on kidney health and nutrition. Healthy kidneys can generally handle the workload. “But if you have kidney damage, too much sodium and protein can have a negative effect,” Kuracina says. “We generally recommend eating less sodium and more fruits, vegetables and whole grains. To reduce fats, choose lean meats and low-fat or fat-free dairy products.”

If you have kidney disease, your health care provider may recommend additional changes to your diet. And if lifestyle changes aren’t enough to slow down kidney damage, your doctor may prescribe medications to reduce blood pressure, control blood glucose and lower your cholesterol.

Don’t wait to take the first step to keep your kidneys healthy. Talk to your health care provider about your kidneys, and ask if you should be tested for kidney disease.


Figure (PageIndex<1>): Blood glucose concentration is maintained within a range of 70 to 110 mg/dL by two negative feedback loops. The top loop uses insulin to bring higher values back within the normal range, and the bottom loop uses glucagon to bring lower values back within the normal range.

When the human body is maintained in a steady state, the condition is called homeostasis. The body consists of trillions of cells that perform many different functions, but all of them require a similar internal environment with important variables kept within narrow ranges. For example, cells require a certain range of body temperature, pH of extracellular fluids, and concentrations of mineral ions and glucose in the blood. Each of these variables must be maintained within a narrow range of values regardless of changes in the external environment, food that has been consumed, the body&rsquos activity level, or other changes in the human organism.


Keeping all of the body&rsquos internal variables within normal ranges is the function of physiological mechanisms called homeostats. Different variables are controlled by different homeostats, but all homeostats work in the same general way. A stimulus from the variable in question is sensed and compared with the normal range of values for the variable. If the actual value of the variable is outside the normal range, it elicits a response that works to move the variable back within the normal range. All homeostats use negative feedback loops to bring excessively high or low values of a variable back within the normal range.

Controlling Blood Glucose

Consider the control of the concentration of glucose in the blood as an example. The homeostat that controls this variable is illustrated and described in Figure (PageIndex<1>). The primary sensors that monitor blood glucose concentration are beta cells in the pancreas. If beta cells detect a rise in the blood glucose concentration above the normal range, they secrete the hormone insulin into the blood. Insulin, in turn, acts on cells throughout the body, stimulating them to take up glucose from the blood and use it for cellular respiration. Insulin also stimulates cells in the liver to take up glucose from the blood and turn it into the complex carbohydrate glycogen for storage. At the same time, insulin inhibits the liver from breaking down stored glycogen and releasing it as glucose. Insulin also inhibits the endoplasmic reticula (ER) of cells from converting amino acids and glycerol into glucose.

If the beta cells detect a drop in the blood glucose concentration below the normal range, they stop secreting insulin into the blood, and the alpha cells of the pancreas are stimulated to secrete the hormone glucagon into the blood. Glucagon, in turn, inhibits the uptake of glucose from the blood by the liver and by fat and muscle cells throughout the body. Instead, the liver is stimulated to make glucose by breaking down stored glycogen, and the ER in cells is stimulated to make glucose from amino acids and glycerol. Glucose from all these sources is released into the blood to bring the blood glucose concentration back to the normal range.

Excretory system

The excretory system is the system of an organism's body that performs the function of excretion, the bodily process of discharging wastes.

The Excretory system is responsible for the elimination of wastes produced by homeostasis.

There are several parts of the body that are involved in this process, such as sweat glands, the liver, the lungs and the kidney system.

Every human has two kidneys.

Each kidney is made up of three sections: the renal cortex, the renal medulla and the renal pelvis.

The blood arrives at the kidney via the renal artery, which splits into many afferent arterioles.

These arterioles go to the Bowman's Capsules of nephrons, where the wastes are taken out of the blood by pressure filtration.

Peritubular capillaries also surround the nephron so substances can be taken in and out of the blood.

The renal cortex is the outer layer of the kidney and the medulla is the inner layer of the kidney.

The renal pelvis takes urine away from the kidney via the ureter.

Both of the ureters lead the urine into the body's only urinary bladder, which expands and sends nerve impulses when full.