1. In the average adult, the skin covers 12-20 square feet and accounts for 12% of body weight.^b
2. There are more than 600 individual skeletal muscles in the human body.^c
3. An adult skeleton has 213 bones.^a
4. Cartilage is one of the few tissues that grows throughout life. Between ages 30 and 70, a nose might grow half an inch, and the ears grow about a quarter of an inch.^e
5. A newborn's skull contains gaps between its bony plates. In an adult, the jagged plates interlock tightly like a jigsaw puzzle.^a
6. The average human head has about 100,000 hairs.^c
7. As a person ages, the diameter of each hair on the head shrinks. Hair is thickest in the early 20s, but by age 70, it can be as fine as a baby's. Aging also causes hair to grow where it is not wanted, such as in the nose and ears, and to fall out where it is desired.^e
8. Hundreds of billions of neurons carry electrical signals that control the body from the brain and the spinal cord.^c

Alien hand syndrome occurs when a brain injury victim loses control over a hand, as if it is possessed by an alien being

9. After sustaining trauma to the brain—such as an injury, stroke, or infection—some people develop “alien hand syndrome,” a condition where the victim can feel sensation in the hand, but has no control over movement and does not sense the hand as a part of the body, as if it belonged to an alien being.^d
10. The Greek philosopher Aristotle believed that the brain exists mainly to help cool the spirit. It is now known that the brain controls nearly every function of the body and mind.^c
11. When the pituitary gland malfunctions, it can boost or reduce the amount of growth hormone in a growing child's body, resulting in gigantism or dwarfism.^a
12. The senses are highly attuned to our world, but they have limits. For example, humans cannot see in the ultraviolet spectrum as bees do, nor can they differentiate between the hundreds of millions of odors that a bloodhound can.^c
13. The appendix has no function in modern humans. It is believed to have been part of the digestive system in our primitive ancestors.^b
14. Humans smell “in stereo.” Scent signals from each nostril travel to different regions in the brain. This may help a person determine the direction the odor is coming from.^c
15. The skin contains approximately 640,000 sense receptors, scattered unevenly over the body's surface. These receptors are most abundant in the ridges of the fingertips, in the lips, at the tip of the tongue, in the palms, on the soles of the feet, and in the genitals.^e
16. An estimated five million olfactory receptors are clustered in the membrane at the upper part of our nasal passages. These receptors help us distinguish among thousands of different odors.^e
17. There are about 9,000 taste buds on the surface of the tongue, in the throat, and on the roof of the mouth.^b Taste buds contain chemoreceptors that respond to chemicals from food and other substances that are dissolved by the saliva in the mouth.^e
18. Humans produce about 10,000 gallons of saliva in a lifetime. Saliva is required for taste—until food is dissolved by saliva, we cannot taste it.^b
19. Hearing is one of the less acute senses in humans, compared to the many other animals which can detect sound at much higher and lower frequency than humans can.^c

The lens of the eye thickens as a person ages, causing many middle-aged people to need glasses

20. As humans grow older, the lens in the eye grows thicker. This is why people who once had perfect vision often need glasses in their 40s.^c
21. An adult human body contains approximately 100 trillion cells.^e
22. The body carries about 25 trillion red blood cells (erythrocytes), the most abundant cells in the body. Red blood cells make up about 45% of blood's volume.^e
23. Every hour, about 180 million newly formed red blood cells enter the bloodstream. Red blood cells are basically shells. Before being released from the bone marrow, most of a red blood cell's internal structure is ejected, creating a disc-shaped balloon that is ideal for carrying oxygen and a small amount of the body's carbon dioxide.^c
24. White blood cells, or leukocytes, make up about 1% of blood. This number can double within a day when a body responds to infection.^c
25. The circulatory system of arteries, veins, and capillaries is about 60,000 miles long.^e
26. The heart beats more than 2.5 billion times in an average lifetime.^b
27. Unlike other muscles, the heart muscle contracts without stimulus from the nervous system. Signals for the heart to beat come from the sinoatrial node near the top of the right atrium.^c
28. In a healthy adult, the small intestine can range between 18 and 23 feet long, about four times longer than the person is tall.^b About 90% of the body's nutrients are absorbed into the bloodstream in the small intestine.^c
29. At about five feet in length, the large intestine is shorter than the small intestine. However, it is more spacious so it can store and process material that will be eliminated.^c
30. There are approximately 400 feet of seminiferous tubules in the testes of a human male.^c This is where sperm is stored until an ejaculation releases 200 million to 500 million sperm, each of which is capable of fertilizing an egg.^e
31. During ovulation, the number of white blood cells in the cervical mucus drops dramatically. If it did not, the white blood cells would destroy all foreign bodies, including sperm.^e
32. Unlike other cells, which contain an individual's full DNA, the egg and sperm each contain only half of the DNA required to create a new human. Both halves must be combined for humans to reproduce.^g

A good diet helps a child's brain develop properly

33. Proper diet is critical for brain development in children. The brains of children who have died of malnutrition during the first year of life have fewer brain cells and an overall smaller size than the brains of healthy children.^f
34. DNA, the basic building block of life, is a long molecule containing four chemical bases: adenine (A), guanine (G), thymine (T), and cytosine (C).^g
35. The human genome—half the DNA contents of a single nucleus—contains about 31 billion base pairs: 31,000,000,000 A's, G's, T's, and C's.^g
36. Six billion steps of DNA are contained in a single cell. This DNA can be stretched six feet, but it is coiled up in the cell's nucleus, which measures only 1/2500 of an inch in diameter.^e
37. The maximum length of a mammal's life is generally related to its size. Thus, a man's lifespan should be somewhere between that of a goat and a horse, between 10 and 30 years. However, humans have developed ways to protect themselves from predators and disease, increasing their average lifespan to 74.7 years in the United States.^e

-- Posted March 2, 2009

References

^b Innerbody.com. "Human Anatomy Online." Accessed: February 21, 2009.

^c McMillan, Beverly. 2006. Human Body: A Visual Guide. Buffalo, NY: Firefly Books (U.S.) Inc.

^d MedicineNet.com. "Definition of Alien Hand Syndrome." Accessed: February 21, 2009.

^e National Geographic Society. 1986. The Incredible Machine. Washington, D.C.: The National Geographic Society.

^f Restak, Richard M. 1979. The Brain: The Last Frontier. Garden City, NY: Doubleday & Company, Inc.