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By Luis Gustavo. See how we research and review our content.
Researchers using carbon-14 dating have concluded that neurons in the cerebral cortex are not replaced, while the lining of the intestine turns over in a matter of days. Your body is a mix of very new and very old parts.
The five facts below can sound like exaggerations, so each comes with the caveat scientists attach to it. This is general information about anatomy, not a guide to your own health.
1. A large part of your cells get replaced over the years
The idea that every cell in your body is replaced every seven years is an urban myth, according to Karolinska Institutet in Sweden, which notes that some cell types renew within a week, others never, and for many the rate is still unknown. The intestinal lining turns over in a matter of days, the outer layer of skin over a few weeks, and red blood cells last on the order of 120 days.
Heart muscle shows how slow renewal can be. Using carbon-14 left in the atmosphere by Cold War nuclear tests as a date stamp, a 2009 study in Science found that about 1 percent of heart muscle cells are renewed each year at age 20, falling to about 0.5 percent by age 75. Most heart cells therefore last a lifetime.
2. Your gut has its own “nervous system”
The enteric nervous system is a mesh of neurons lining the digestive tract. It manages muscle contractions, the mixing of food and the movement of waste. Estimates of its size run from about 100 million to several hundred million neurons, depending on the source, which is more than the spinal cord holds.
It can run gut functions without orders from the brain, but the two are linked. The vagus nerve is the main connection, and about 90 percent of its fibers carry signals from gut to brain rather than the other way around. Researcher Michael Gershon, who helped popularize the "second brain" nickname, notes that it does not take part in thinking, which is why the label is informal.
3. The heart beats about 100,000 times a day
A resting rate of 60 to 100 beats per minute is the figure commonly cited, including by the American Heart Association, as normal for adults. Over 24 hours, that works out to roughly 86,000 to 144,000 beats, so "about 100,000" is a convenient round number rather than a measurement. People who are very physically fit often have lower resting rates, and a doctor can interpret any individual figure.
Resting cardiac output of about 5 liters per minute adds up to roughly 7,200 liters of blood per day, close to 1,900 gallons. That calculation is where the popular "2,000 gallons a day" figure comes from.
4. Bones are stronger than many metals, relative to their weight
Bone is a composite of type I collagen, a flexible protein, and hydroxyapatite, a hard calcium-phosphate mineral. Neither is strong alone. The mineral resists crushing, and the collagen gives the material some give, which helps bone absorb energy before it breaks.
The comparison with steel holds only in a narrow sense. One materials overview puts cortical bone’s compressive strength at roughly 100 to 230 megapascals against about 250 to 400 for mild structural steel, but steel is nearly four times denser. Pound for pound, bone can therefore match or beat it in compression. In tension and stiffness steel is far ahead, and bone is strongest along the direction it normally carries load.
5. The human eye can distinguish an impressive number of colors
Color vision relies on three types of cone cells in the retina, each most sensitive to a different range of wavelengths. The brain compares their signals to construct color. Cone counts vary between studies, with figures from roughly 3 million to 6 million per eye, and rods, which handle dim light, are far more numerous.
How many colors that allows depends on the method. A calculation by Pointer and Attridge published in 1998 arrived at about 2.3 million discernible surface colors, other models land between one million and several million, and a 2010 study of natural scenes argued that counting methods overstate the number of perceptually distinct colors by a wide margin.
Why a skeleton is never finished
Bone is living tissue that is constantly broken down and rebuilt. Cells called osteoclasts remove old bone and osteoblasts lay down new bone. Adults renew roughly 10 percent of their bone each year by one overview, and other estimates say 5 to 10 percent, which is why the often-quoted "a new skeleton every ten years" is an average and not a clock.
The process repairs microdamage from everyday activity, adapts bone to the loads it carries and helps keep calcium levels steady. When the balance between removal and building tips, conditions such as osteoporosis can develop, and a clinician can assess bone health. Related reading: facts about water and hydration.
When a cone type is missing or shifted
The National Eye Institute says about 1 in 12 men have color vision deficiency, and men are at much higher risk than women. Most people with it are born with it, because the most common types are inherited.
The most common form makes red and green hard to tell apart, a rarer form makes blue and yellow look alike, and complete color blindness is rare. It can also result from injury to the retina, optic nerve or brain, from some treatments, or from cataracts as people age. An eye-care professional can test for it.
Other reads that fit this theme: facts about water and hydration and facts about diseases and medicine.
Want to better understand this incredible machine?
Netter Atlas of Human Anatomy: Classic Regional Approach — a comprehensive visual guide to human anatomy.
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A gut with its own neurons, a skeleton rebuilt a little each year and a heart that renews about half a percent of its cells by age 75 make a good case that you are never quite the same body twice.