Facts About Humans

Brain structure

The brain is made up of water, protein, carbohydrates, salts, and fat. 

The brain is soft and gel-like, similar to tofu. 

The brain's outer layer is called the cortex, which is made up of millions of cells that send messages to other parts of the brain.

The human brain is a disproportionately energy-hungry organ. Although it makes up only about 2% of total body weight, it uses roughly 20% of the body’s energy at rest. Around 20% of the oxygen you breathe is directed to the brain to support this constant activity.

An average adult brain weighs about 1.3–1.4 kilograms (around three pounds). Despite its modest size, it contains an immense network of cells and connections that regulate thought, movement, memory, and emotion.

If the cerebral cortex were smoothed out, it would cover roughly 2,000–2,500 square centimeters (about the size of a large pillowcase). The folds, known as gyri and sulci, increase surface area and allow for greater cognitive capacity within the limited space of the skull.

The brain does produce electrical signals as neurons communicate through electrochemical impulses. However, the popular claim that it generates enough electricity to light up a room is misleading. The total electrical output is small—measurable in millivolts—and not comparable to household electrical power.

The left hemisphere of the brain primarily controls movement on the right side of the body, and the right hemisphere controls the left side. This cross-wiring occurs because many motor and sensory pathways decussate (cross over) in the brainstem.

Claims that the brain can store a specific amount of data, such as “4 terabytes,” are speculative. The brain does not function like a digital storage device, and memory capacity cannot be accurately expressed in conventional computer units.

Nerve impulse speed varies. Signals in myelinated neurons can travel up to about 100–120 meters per second, while slower fibers conduct much more slowly.

Statements comparing the number of electrical impulses in the brain to “all the phones in the world” are metaphorical and not scientifically quantifiable.

Likewise, claims that a person remembers “150 trillion bits” of information over a lifetime are not supported by measurable neuroscience data.

Modern research estimates that the human brain contains about 86 billion neurons, based on work published in 2012 in Proceedings of the National Academy of Sciences. The number of non-neuronal cells, particularly glial cells such as oligodendrocytes, is of a similar order of magnitude rather than vastly exceeding neuron counts, as was once believed.

Normal daily functioning depends heavily on gray matter in the brain and spinal cord. Gray matter contains neuronal cell bodies, dendrites, and synapses, where most information processing occurs. In contrast, white matter consists mainly of myelinated axons that transmit signals between different brain regions.

The outer surface of the brain is folded into ridges (gyri) and grooves (sulci). These folds increase the surface area of the cerebral cortex, which is composed largely of gray matter. This expanded surface allows for higher cognitive processing within the limited volume of the skull.

The brain is composed of roughly 73–75% water. Even mild dehydration can affect attention, memory, mood, and cognitive performance. Severe dehydration can disrupt normal neural function more significantly.

During the first year of life, the brain grows rapidly—nearly doubling in size. It continues to increase in volume and reorganize throughout childhood and adolescence. By around 18–25 years of age, structural maturation—especially in the prefrontal cortex—is largely complete, although subtle changes continue into early adulthood.

Headaches arise from pain-sensitive structures in the head and neck, including blood vessels, muscles, nerves, and the meninges. The brain tissue itself does not contain pain receptors. Chemical changes and inflammatory mediators can activate these pain pathways, producing different types of headaches.

Cholesterol plays a critical role in the brain. It is essential for building cell membranes, forming synapses, and maintaining myelin sheaths. However, high blood cholesterol levels can increase the risk of vascular problems, which may indirectly affect brain health. The impact varies depending on age, cardiovascular health, and other risk factors.

Dreaming occurs primarily during REM (rapid eye movement) sleep, although it can happen in other stages as well. Dreams are associated with complex neural activity involving memory consolidation, emotional processing, and imagination. Brain imaging shows that certain regions remain highly active during dreaming.

Phantom limb pain occurs when a person continues to feel sensations or pain in an amputated limb. It arises because the brain and spinal cord retain neural representations of the missing limb. The central nervous system can continue generating pain signals even in the absence of the physical structure.

Brain volume gradually decreases with age. Mild shrinkage typically begins in mid-adulthood and becomes more noticeable after age 60. The frontal lobes and hippocampus are particularly affected. Some cognitive changes are part of normal aging, while more severe decline may indicate neurodegenerative disease.

Alcohol has clear short-term effects on the brain, including slurred speech, blurred vision, slowed reaction time, and impaired coordination. These symptoms usually resolve once blood alcohol levels return to normal. However, chronic heavy drinking can cause lasting structural and functional damage, including shrinkage of brain tissue, memory impairment, and increased risk of disorders such as alcohol-related dementia and Wernicke-Korsakoff syndrome. Some damage may partially improve with prolonged abstinence, but severe cases can be irreversible.

Eyewitness testimony is not consistently reliable. Accuracy varies widely depending on stress, lighting, duration of exposure, and whether a weapon was present. High stress and traumatic events can impair detailed recall and increase false memory formation. The claim that eyewitnesses are “only 50% accurate” is an oversimplification; error rates depend heavily on context and identification procedures.

Certain video games have been shown to improve specific cognitive skills, such as visual attention, reaction time, and spatial reasoning. However, evidence for broad, long-term cognitive enhancement is mixed. Benefits are typically task-specific rather than general intelligence gains.

The brain itself does not contain pain receptors (nociceptors). It processes and interprets pain signals from other parts of the body, but the neural tissue itself cannot feel pain. This is why brain surgery can be performed while a patient is awake under local anesthesia.

“Brain freeze,” medically termed sphenopalatine ganglioneuralgia, occurs when cold substances rapidly cool the roof of the mouth and back of the throat. This triggers rapid constriction and dilation of nearby blood vessels, activating branches of the trigeminal nerve. The brain interprets the referred signal as pain in the forehead.

Cognitive processing speed and certain aspects of memory may begin to show subtle decline in the late twenties or thirties. However, many abilities—such as vocabulary, general knowledge, and emotional regulation—remain stable or even improve with age. Cognitive aging is gradual and varies significantly between individuals.

The average person blinks about 15–20 times per minute. That equals roughly 14,000–19,000 blinks per day, or around 5–7 million per year. Blinking keeps the cornea lubricated and clears debris from the eye’s surface.

An eyelash typically has a life cycle of about 3 to 5 months (around 90–150 days). After shedding, it regrows from the same follicle as part of a natural cycle.

The cornea is the only tissue in the human body without direct blood vessels. It receives oxygen primarily from the air through the tear film and nutrients from the aqueous humor inside the eye.

Claims that the human eye has “576 megapixels” are speculative. The eye does not function like a digital sensor. Visual resolution varies across the retina—sharpest in the fovea and much lower in peripheral vision—so a single megapixel number is an oversimplification.

The eye’s convex lens does project an inverted image onto the retina. The brain interprets these signals so we perceive the world as upright. However, newborns do not literally see the world upside down; visual processing develops gradually as neural pathways mature.

The statement that “at least 20 quadrillion miles are visible to us” is misleading. Under ideal dark-sky conditions, the unaided human eye can see the Andromeda Galaxy, which is about 2.5 million light-years away. One light-year equals about 5.88 trillion miles, making that distance roughly 15 quintillion (not quadrillion) miles.

There is no strong scientific consensus that blue-eyed individuals are universally more sensitive to pain. Some small studies have suggested possible variations in pain perception linked to genetics, but the evidence is limited and not definitive.

The human eye can distinguish millions of colors. A commonly cited estimate is around 10 million distinct color shades under optimal conditions, though this varies between individuals.

The 20-20-20 rule is commonly recommended to reduce digital eye strain. It suggests that every 20 minutes, you look at something about 20 feet away for at least 20 seconds. This relaxes the eye’s focusing muscles and reduces accommodative fatigue caused by prolonged near work. While it does not prevent all forms of eye strain, it is a practical strategy for people who spend long hours on screens.

Some studies have suggested that wearing a necktie very tightly may temporarily increase intraocular pressure (IOP), particularly in men. A tight collar can mildly restrict venous blood flow from the head, leading to short-term elevation in eye pressure. However, this effect is usually small and temporary. There is no strong evidence that wearing a tight tie directly causes glaucoma, though individuals already at risk may want to avoid sustained pressure around the neck.

A comprehensive eye examination can reveal systemic health problems. By examining the retina and its blood vessels, clinicians may detect signs of diabetes, hypertension, atherosclerosis, and other vascular conditions. Changes in retinal blood vessels often reflect broader circulatory health, making the retina a valuable window into systemic disease.

Having two forward-facing eyes enables binocular vision. The brain compares the slightly different images received from each eye to calculate depth and distance—a process known as stereopsis. This allows accurate spatial judgment, hand–eye coordination, and three-dimensional perception.

Most blue-eyed people likely share a distant common ancestor. Genetic research suggests that a mutation affecting the OCA2 gene occurred thousands of years ago, probably in Europe, leading to blue eye color spreading through populations. This does not mean all blue-eyed individuals are closely related, but they may trace back to a shared ancestral mutation.

Smoking can impair night vision. Chemicals in tobacco smoke reduce oxygen delivery and can damage the retina over time. Smoking is also a major risk factor for cataracts and age-related macular degeneration, both of which can affect vision in low light.

The human retina contains roughly 120 million rod cells and about 6–7 million cone cells. These photoreceptors convert light into neural signals. The claim that the eye has “two million functional elements” is inaccurate and far below the true number of light-sensitive cells.

Most people have one dominant eye, meaning the brain prefers input from that eye for precise visual tasks. However, this is not the same as anisometropia. Anisometropia refers specifically to a significant difference in refractive power between the two eyes, which may or may not be present in someone with ocular dominance.

The extraocular muscles that move the eyes are extremely efficient and capable of rapid, precise movement. While they are strong relative to their size, claims that they are “100 times stronger than necessary” are exaggerated and not based on clear physiological measurement.

Cosmetics can contribute to eye irritation or injury, especially if products are contaminated, expired, or improperly applied. Mascara wands and eyeliner pencils may cause corneal scratches or infections if used carelessly. However, they are not typically among the leading overall causes of serious eye injuries, which more commonly involve workplace accidents, sports, or foreign objects.

A contact lens cannot travel behind the eyeball. The conjunctiva forms a continuous membrane from the eyelids to the surface of the eye, preventing objects from moving into the back of the orbit. However, a lens can become displaced under the upper eyelid or fold on itself. If a lens feels “lost,” lubricating drops and gentle blinking usually help reposition it. Persistent discomfort should be evaluated by an eye care professional.

Human ears do not continuously “grow” throughout life in the way bones lengthen during childhood. However, the external ear (pinna) can appear larger with age due to cartilage changes, loss of skin elasticity, and gravity. This creates the impression of ongoing growth.

The inner ear is responsible for both hearing and balance. The cochlea processes sound, while the vestibular system (semicircular canals and otolith organs) regulates balance and spatial orientation. Because these systems are closely connected, disorders can affect both. For example, Meniere's disease can cause vertigo, fluctuating low-frequency hearing loss, tinnitus, and a feeling of ear fullness.

The cochlea is a spiral-shaped structure resembling a snail shell. If uncoiled, it measures approximately 31–35 millimeters in length. It contains specialized hair cells that convert sound vibrations into electrical signals for the brain.

The stapes is the smallest bone in the human body. It is one of three tiny middle-ear bones—the malleus, incus, and stapes—collectively known as the ossicles. These bones amplify and transmit sound vibrations from the eardrum to the inner ear. All three together are small enough to fit on a coin.

Under ideal conditions, the typical human hearing range spans roughly 20 Hz to 20,000 Hz (20 kHz). Sensitivity to higher frequencies declines with age, a process known as presbycusis.

Hearing does not completely shut off during sleep. The ears continue to detect sound, but the brain reduces responsiveness to most stimuli. However, meaningful or loud sounds—such as a baby crying or an alarm—can still trigger awakening.

Human ears can detect a wide range of sounds, but extremely loud noise does not produce a specific “frequency level” that causes ringing. Instead, ringing or buzzing after loud exposure is typically a temporary threshold shift caused by overstimulation of hair cells in the inner ear. This temporary ringing is related to the same underlying mechanisms involved in tinnitus, although tinnitus can also occur without recent loud noise exposure.

The ear has a natural self-cleaning mechanism. Earwax (cerumen) is produced by specialized glands in the outer ear canal, not by cilia themselves. Jaw movement from talking and chewing gradually helps migrate earwax outward, carrying trapped dust and debris with it. Cerumen protects the ear canal by lubricating the skin, reducing friction, and providing antimicrobial defense. Routine deep cleaning is unnecessary for most people and may even push wax deeper. Removal is only needed if there is blockage, discomfort, or hearing reduction.

The sensory receptors responsible for hearing are called hair cells, located inside the cochlea of the inner ear. These cells convert mechanical sound vibrations into electrical signals that travel to the brain via the auditory nerve. Damage to hair cells can result from aging (presbycusis), prolonged loud noise exposure, certain medications (ototoxic drugs), infections, or reduced blood supply. In humans, cochlear hair cells do not regenerate once destroyed. As a result, most sensorineural hearing loss is permanent.

Above the cochlea in the inner ear are three semicircular canals. These fluid-filled loops are oriented in different planes and detect rotational head movement. When the head turns, fluid inside the canals shifts, bending specialized hair cells. This mechanical deflection sends signals to the brain that help maintain balance and coordinate eye movements during motion.

The chorda tympani nerve, a branch of the facial nerve, passes through the middle ear and carries taste sensation from the front two-thirds of the tongue to the brain. Because of its anatomical path, middle ear infections or ear surgery can sometimes affect taste perception temporarily. However, hearing loss itself does not directly eliminate the ability to taste.

Children are more prone to ear infections than adults. Their Eustachian tubes are shorter, narrower, and more horizontally positioned, which allows fluid to accumulate more easily in the middle ear. In addition, their immune systems are still developing, increasing susceptibility to infection.

Age-related hearing loss becomes more common after age 60. Roughly one in three adults over 65 experiences some degree of hearing impairment. Globally, however, the total number of people with hearing loss is far higher than 165 million; current global estimates exceed 400 million individuals with disabling hearing loss, with many more experiencing mild to moderate impairment.

Prolonged exposure to high-volume sound through headphones or earphones can cause permanent sensorineural hearing loss. Damage occurs when excessive sound levels injure cochlear hair cells. Risk increases with both volume and duration of exposure. Limiting volume and taking listening breaks significantly reduces long-term harm.

The heart is highly resistant to fatigue because cardiac muscle has an exceptional blood supply and dense mitochondria for continuous energy production. However, it is not the only muscle that “never gets tired.” Under disease conditions—such as heart failure—the heart can weaken and lose pumping efficiency.

If a person lives to around 70 years old, the heart will beat roughly 2.5 to 3 billion times, depending on average heart rate. This estimate assumes a resting rate of about 60–80 beats per minute over a lifetime.

At an average of 100,000 beats per day, the heart beats about 36–38 million times per year.

An average adult resting heart rate is typically between 60 and 100 beats per minute. A commonly cited average is around 70–75 beats per minute, though trained athletes may have significantly lower resting rates.

Heart rate can synchronize temporarily with rhythm or tempo in music, especially when emotionally engaged. Fast music may elevate heart rate, while slower music may reduce it. This effect occurs through autonomic nervous system responses rather than direct mechanical synchronization.

Over a lifetime, the heart pumps an enormous volume of blood. Estimates suggest roughly 1 to 1.5 million barrels of blood, which equals about 40–50 million gallons (150–190 million liters), assuming average lifespan and cardiac output. This figure varies by body size and health.

An Olympic pool holds about 2.5 million liters. The heart pumps roughly 7,000–8,000 liters per day, which totals about 2.5–3 million liters per year—so it is closer to the volume of one Olympic pool per year.

On average, adult women tend to have slightly higher resting heart rates than men. This difference is largely due to smaller average heart size and stroke volume, meaning the heart must beat slightly faster to maintain the same cardiac output.

The heart’s rhythm is regulated by its intrinsic electrical system, known as the cardiac conduction system. Electrical impulses originate in the sinoatrial (SA) node, often called the natural pacemaker. Signals then pass through the atrioventricular (AV) node and along specialized pathways (the bundle of His and Purkinje fibers), coordinating contraction of the atria and ventricles.

The heart can continue beating for a short time after being removed from the body, provided it still has oxygen and appropriate electrolyte balance. Cardiac muscle cells possess automaticity, meaning they can generate electrical impulses independently of the brain. This property allows the heart to beat in transplant procedures before being reconnected to the recipient’s circulation.

In 1893, Daniel Hale Williams performed one of the earliest successful heart surgeries in Chicago. The procedure repaired a stab wound to the pericardium (the sac surrounding the heart). It was not open-heart surgery in the modern sense involving a heart-lung machine, but it was a landmark achievement in cardiac surgery.

The first fully implantable pacemaker was placed in 1958. Arne Larsson received the device in Stockholm. He went on to receive multiple pacemakers over his lifetime and lived to age 86, ultimately dying from causes unrelated to his original heart rhythm condition.

Heart surgery has been performed on newborns within minutes to hours after birth for critical congenital heart defects. However, identifying a single “youngest person” is not accurate, as neonatal cardiac surgeries occur worldwide in life-threatening cases. Outcomes vary depending on the specific defect and overall health of the infant, and not all such patients necessarily require future heart transplantation.

Evidence of cardiovascular disease has been found in ancient remains, including Egyptian mummies dating back several thousand years. Imaging studies of mummified remains have revealed arterial calcification consistent with atherosclerosis, suggesting that heart disease is not solely a modern condition.

Some epidemiological studies have observed a higher incidence of heart attacks on Mondays. This pattern is often attributed to stress-related hormonal changes associated with the start of the workweek. However, the increase is modest and varies across populations.

The familiar “lub-dub” heartbeat sound is produced primarily by the closing of heart valves, not their opening. The first sound (S1) occurs when the mitral and tricuspid valves close. The second sound (S2) occurs when the aortic and pulmonary valves close.

“Broken heart syndrome” is a real medical condition known as Takotsubo cardiomyopathy. It is typically triggered by intense emotional or physical stress and causes temporary weakening of the heart muscle. Although most patients recover, severe complications and, in rare cases, death can occur.

Laughter can have short-term cardiovascular benefits. It reduces stress hormones, promotes blood vessel dilation, and may improve circulation temporarily. While it supports overall well-being, it is not a substitute for established heart-protective behaviors such as exercise, diet control, and blood pressure management.

The stylized heart symbol may have historical links to the ancient silphium plant, which was used in antiquity and associated with love and fertility. However, the true origin of the modern heart shape remains debated among historians.

The human heart is approximately the size of the individual’s clenched fist. This comparison is a practical anatomical approximation and varies slightly depending on body size.

The total surface area of an adult’s lungs is approximately 50–75 square meters, largely due to the extensive network of alveoli that maximize gas exchange.

The right lung has three lobes, while the left lung has two lobes and a cardiac notch to accommodate the heart. Because of this, the left lung is slightly smaller than the right.

Human lungs contain roughly 300–500 million alveoli. The branching airway system, including bronchi and bronchioles, forms an extensive network that collectively spans thousands of kilometers when measured microscopically.

The lungs are central organs of the respiratory system, enabling oxygen to enter the bloodstream and carbon dioxide to be expelled. At rest, an average adult breathes about 6–8 liters of air per minute, though this increases significantly during physical activity.

The lungs can float in water due to the air they contain. This characteristic has historically been used in forensic examinations, although it is not always definitive.

Average total lung capacity in healthy adult males is approximately 5,000–6,000 milliliters, while in females it is typically somewhat lower. Capacity varies based on body size, age, and fitness level.

Regular aerobic exercise improves respiratory efficiency, strengthens breathing muscles, and enhances oxygen utilization. While it may not dramatically increase structural lung capacity in adults, it improves overall pulmonary function and endurance.

Occasional coughing is a normal protective reflex that clears mucus and irritants from the airways. Healthy individuals may cough several times per day without underlying lung disease. Persistent or worsening cough, however, warrants medical evaluation.

Tobacco use is the leading cause of lung cancer and a major contributor to chronic obstructive pulmonary disease (COPD) and other respiratory illnesses. Exposure to secondhand smoke increases the risk of lung cancer, coronary artery disease, and stroke even in non-smokers.

The right lung has three lobes, while the left lung has two lobes to accommodate the heart. This anatomical difference explains why the left lung is slightly smaller.

Breathing is regulated by respiratory centers in the brainstem, primarily the medulla oblongata and pons. These centers automatically adjust breathing rate and depth based on carbon dioxide levels, oxygen levels, and blood pH. Breathing can also be voluntarily controlled for short periods.

It is possible to live with one lung. Many individuals who undergo pneumonectomy adapt well over time. Although overall lung capacity is reduced, most people can perform daily activities normally, though strenuous exercise may be more challenging.

When you inhale, air contains about 21% oxygen. When you exhale, it contains about 16% oxygen. This means the body absorbs roughly 4–5% of the inhaled air’s oxygen during each breath under resting conditions.

The lungs primarily function in gas exchange—supplying oxygen and removing carbon dioxide, a metabolic waste product. In fact, a large proportion of the body’s carbon dioxide is eliminated through breathing, making respiration a major pathway for removing gaseous waste from the body.

The lungs are located within the thoracic cavity. They are protected by the rib cage, lie on either side of the heart, and are anchored by connective tissues to structures including the diaphragm below and the mediastinum centrally. They are not directly attached to the spinal cord, but they sit anterior to the vertebral column and posterior to the sternum.

The mouth and lungs are connected through the respiratory tract. Air passes from the mouth or nose into the pharynx, then through the larynx and trachea (windpipe) into the lungs. The esophagus lies behind the trachea and carries food to the stomach, remaining separate from the airway during normal swallowing.

Before birth, a fetus’s lungs are filled with fluid and do not perform gas exchange. Oxygen is supplied through the placenta. Shortly after birth, the newborn takes its first breaths, air replaces fluid in the lungs, and independent respiration begins.

The lungs play a crucial role in speech. Air expelled from the lungs passes through the larynx (voice box), where vocal cords vibrate to produce sound. The mouth, tongue, and lips then shape these sounds into speech.

Slow, deep breathing activates the parasympathetic nervous system, promoting relaxation and reducing stress. Controlled breathing techniques are widely used in practices such as yoga and meditation to improve calmness and focus.

The lungs possess defense mechanisms to filter and remove inhaled particles. Goblet cells produce mucus that traps dust and microbes, while cilia lining the airways move this mucus upward toward the throat for removal. Immune cells such as macrophages also help eliminate contaminants.

Air pollution can damage lung tissue over time, even in non-smokers. Long-term exposure to pollutants increases the risk of respiratory conditions, reduced lung function, and cardiovascular disease.

The kidneys contain microscopic filtering units called nephrons. Together, they continuously filter blood, removing waste products and excess fluids to form urine. In healthy adults, the kidneys produce roughly 1.5 to 2 liters of urine per day, depending on hydration and physiological conditions.

The kidneys receive a remarkably high proportion of cardiac output—about 20–25% of the blood pumped by the heart at rest. Relative to their size, they receive more blood flow than most other organs.

On average, the kidneys filter about 120–125 milliliters of blood plasma per minute. Over 24 hours, this amounts to roughly 150–180 liters of filtrate, most of which is reabsorbed before urine is formed.

The kidneys are two bean-shaped organs located on either side of the spine, just below the rib cage. Each is approximately the size of a clenched fist.

During filtration, the kidneys reabsorb about 99% of the filtered fluid back into the bloodstream. Only about 1–2 liters per day remain as urine, which carries waste products such as urea, creatinine, and excess salts out of the body.

The kidneys are slightly asymmetrical in position. The left kidney is typically a bit higher and slightly larger than the right, as the right kidney sits lower to accommodate the liver.

Staying well hydrated significantly reduces the risk of kidney stones. Low fluid intake concentrates minerals in the urine, increasing the likelihood that crystals will form and grow into stones.

Exceptionally large kidney stones have been documented in medical history. In rare cases, stones weighing over 1 kilogram have been surgically removed, though such cases are extremely uncommon.

A single healthy kidney is sufficient to sustain normal life. Even partial kidney function can adequately maintain filtration, as the remaining nephrons adapt and increase their workload to compensate.

If a person is born with only one kidney—a condition known as unilateral renal agenesis—the existing kidney often enlarges over time. This compensatory growth allows it to handle the body’s filtration needs effectively.

Nephrons are the microscopic functional units of the kidneys. Each kidney contains roughly one million nephrons. They filter blood, regulate fluid and electrolyte balance, maintain acid–base stability, and remove metabolic waste products, all of which are essential for survival.

Each kidney contains roughly 800,000 to 1.5 million nephrons, depending on the individual. If all the tubules within a single kidney were placed end to end, they would extend several kilometers in length due to their highly coiled structure.

The kidneys play an essential role in activating vitamin D. Vitamin D obtained from sunlight or diet is first converted in the liver to 25-hydroxyvitamin D. The kidneys then convert it into its active form, calcitriol, which helps regulate calcium balance and bone health.

The kidneys receive about 20–25% of the heart’s output. In total, they filter approximately 150–180 liters of fluid per day from the bloodstream, though only about 1.5–2 liters are ultimately excreted as urine.

Yurii Voronoy performed the first human kidney transplant in 1933 in what is now Ukraine. The procedure was not successful, but it marked a historic milestone in transplant surgery.

The first successful long-term kidney transplant was performed in 1954 in Boston by Joseph Murray and his team. The transplant between identical twins avoided immune rejection and demonstrated the viability of organ transplantation.