Human factor, Part 1

In the average adult, the skin covers 12-20 square feet and accounts for 12% of body weight.

There are more than 600 individual skeletal muscles in the human body.

An adult skeleton has 213 bones.

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.

The average human head has about 100,000 hairs.

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.

Hundreds of billions of neurons carry electrical signals that control the body from the brain and the spinal cord.

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.

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.

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.

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.

The appendix has no function in modern humans. It is believed to have been part of the digestive system in our primitive ancestors.

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.

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.

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.

There are about 9,000 taste buds on the surface of the tongue, in the throat, and on the roof of the mouth. Taste buds contain chemoreceptors that respond to chemicals from food and other substances that are dissolved by the saliva in the mouth.

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.

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.