The Human Body
1. An adult skeleton has 213 bones.
2. The average human head has about 100,000 hairs
3. Hundreds of billions of neurons carry electrical signals that control the body from the brain and the spinal cord
4. 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.
5. 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.
6. 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.
7. 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.
8. 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.
9. An adult human body contains approximately 100 trillion cells.
10. 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.
11. 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.
12. White blood cells, or leukocytes, make up about 1% of blood. This number can double within a day when a body responds to infection.
13. The circulatory system of arteries, veins, and capillaries is about 60,000 miles long.
14. 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.
15. DNA, the basic building block of life, is a long molecule containing four chemical bases: adenine (A), guanine (G), thymine (T), and cytosine (C).
a. 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.
b. 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.
1. The average adult heart beats 72 times a minute; 100,000 times a day; 3,600,000 times a year; and 2.5 billion times during a lifetime.
2. Though weighing only 11 ounces on average, a healthy heart pumps 2,000 gallons of blood through 60,000 miles of blood vessels each day.
3. The volume of blood pumped by the heart can vary over a wide range, from five to 30 liters per minute.
4. Every day, the heart creates enough energy to drive a truck 20 miles. In a lifetime, that is equivalent to driving to the moon and back.
5. Because the heart has its own electrical impulse, it can continue to beat even when separated from the body, as long as it has an adequate supply of oxygen.
6. The heart pumps blood to almost all of the body’s 75 trillion cells. Only the corneas receive no blood supply.
7. The heart begins beating at four weeks after conception and does not stop until death.
8. The heart pumps oxygenated blood through the aorta (the largest artery) at about 1 mile (1.6 km) per hour. By the time blood reaches the capillaries, it is moving at around 43 inches (109 cm) per hour.
9. A woman’s heart typically beats faster than a man’s. The heart of an average man beats approximately 70 times a minute, whereas the average woman has a heart rate of 78 per minute.
10. Blood is actually a tissue. When the body is at rest, it takes only six seconds for the blood to go from the heart to the lungs and back, only eight seconds for it to go the brain and back, and only 16 seconds for it to reach the toes and travel all the way back to the heart.