Did You Know?

1. Did You Know? • Each day an average heart “beats” (expands and contracts) 100,000 times and pumps about 2,000 gallons of blood. In a 70-year lifetime, an average human heart beats more than 2.5 billion times.

2. Did You Know? • The cycle of hair growth is different in different parts of the body. Scalp hair grows steadily and continuously for 2-6 years. Growth stops, and after three months, the hair is shed. Three months later a new hair starts to grow from the same follicle. On the other hand, eyebrows have a growing phase of only about 10 weeks, which is why eyebrow hair is not very long.

3. Did You Know? • Scientists first saw cells by using traditional light microscopes. In fact, it was Robert Hooke (1635-1703), looking through a microscope at a thin slice of cork, who coined the word “cell.” He chose the word to describe the boxlike holes in the plant cells because they reminded him of the cells of a monastery.

4. Did You Know? • Stress sets off an alarm in the brain, which responds by preparing the body for defensive action. The nervous system is aroused and hormones are released to sharpen the senses, quicken the pulse, deepen respiration, and tense the muscles.

5. Did You Know? • The skin is commonly thought to have the greatest exposure to air of any body part. In reality, the lungs have the greatest exposure, with a surface area exposed to air of 300 square feet at rest and up to 1,000 square feet during a deep breath.

6. Did You Know? • There may be between 100 trillion and a quadrillion synapses in the brain.

7. Did You Know? • Alexander Fleming discovered penicillin, the first naturally occurring antibiotic, in 1927 (the same year that Charles Lindbergh made his solo flight to Paris).

8. Did You Know? • At rest, the skin and skeletal muscles receive about 20 percent of the cardiac output. During exercise, more blood is sent to the active skeletal muscles, and, as body temperature increases, more blood is sent to the skin.

9. Did You Know? • The immune system is amazingly complex. It can recognize and remember millions of different enemies, and it can produce secretions and cells to match up with and wipe out each one of them.

10. Did You Know? • In ancient China as early as 3000 to 1000 B.C., the classic Yellow Emperor’s Book of Internal Medicine first described the principle that human harmony with the world was the key to prevention and that prevention was the key to long life.

11. Did You Know? • In 1675, Anton van Leeuwenhoek of Holland—the inventor of the microscope—first described protozoa as viewed under his instrument.

12. Did You Know? • The word cortex is from the Latin word for bark. • The cortex is about as thick as a stack of 2 or 3 dimes.

13. Did You Know? • Muscle tissues produce heat by shivering, which produces heat from contractions that are not synchronized (and, as a result, produce no motion).

14. Did You Know? • As we age, lung tissue loses much of its elasticity, and the muscles of the rib cage shrink. As a result, maximum vital breathing capacity progressively diminishes in each decade of life, beginning at about age 20.

15. Did You Know? • Bacteria are so small, a line of 1,000 could fit across the eraser of a pencil. • More than 500 types of bacteria live in the human mouth.

16. Did You Know? • Stretched end-to-end, the arteries, veins, and other vessels of the human circulatory system would measure about 60,000 miles. • That’s enough to encircle the earth more than twice.

17. Did You Know? • Blood accounts for 7-9% of the body’s weight. • About 50% of your blood is in your veins at any time.

18. Did You Know? • A fungus is actually a primitive vegetable. Thousands, perhaps millions, of different types of fungi exist on Earth. The most familiar ones are mushrooms, yeast, mold, and mildew. Some live in the human body, usually without causing illness. In fact, only about half of all types of fungi cause disease in humans.

19. Did You Know? • A person blinks once every 5 seconds—a total of almost 30 minutes while you are awake each day.

20. Did You Know? • The average adult inspires approximately 10,000 to 20,000 liters of air per day.

21. Did You Know? • The axon of a single nerve cell can run half the length of a human body.

22. Did You Know? • X-rays were discovered in 1895 by Wilhelm Konrad Roentgen, a German physicist.

23. Did You Know? • A tomato is 95% water. One ear of corn is 80% water, a chicken is 75% water, and the human body is 66% water.

24. Did You Know? • French hatters of the 17th century discovered that mercuric nitrate aided greatly in the felting of fur. Such use led to chronic mercury poisoning so widespread among members of that trade that the expression “mad as a hatter” entered our folk language.

25. Did You Know? • You are not as tall in the evening as you are in the morning. You become about one inch shorter during a day of standing and sitting. (This is less true of older people.) • After a good night’s rest in a horizontal position, your discs regain their original thickness (and you regain your original height).

26. Did You Know? • Nearly 500 species of microbes in all, most of which are not harmful, will colonize the oral cavity. The microbes form a biofilm, in which their numbers greatly exceed the number of humans on Earth.

27. Did You Know? • Old bone breaks down and new bone is formed on a continuous basis. Modeling and remodeling continue throughout life so that most of the adult skeleton is replaced about every 10 years.

28. Did You Know? • The skin is the largest organ in the body—both in weight and surface area.

29. Did You Know? • There are 5,000-10,000 white blood cells per cubic millimeter of blood. • A cubic millimeter of normal adult blood contains approximately 5,000,000 red blood cells.

30. Did You Know? • Urination accounts for the greatest fluid loss daily. About 1300 millileters of urine are excreted by the normal person each day.

31. Did You Know? • Hawks and eagles have vision almost 8 times better than humans’ vision.

32. Did You Know? • Normal pupils are rounded, centrally placed, and generally equal in size. About 25% of normal individuals have pupils slightly unequal in size.

33. Did You Know? • There are countless numbers of bacteria on Earth—most are harmless and many are even beneficial to humans. In fact, less than 1% of them cause diseases in humans. For example, Lactobacilli acidophilus live in human intestines, where they help to digest food, destroy disease-causing microbes, fight cancer cells, and give the body needed vitamins.

34. Did You Know? • Every cell in an organism contains the same set of instructions encoded in DNA.

35. Did You Know? • The first nursing schools, formed in the late 1800s, included both men and women. Men dominated the field in the South and in the military. In the early 1900s, there was a push to professionalize the field, efforts that were often led by women who were also active in the suffragette movement. Over the course of the 20th century, nursing became professionalized and a predominantly female field.

36. Did You Know? • A human being has some 100 trillion cells.

37. Did You Know? • Enamel is the hardest substance of the human body.

38. Did You Know? • About 300 million alveoli are found in the human lungs—equal to the surface area of a tennis court!

39. Did You Know? • One human eyeball weighs ¼ ounce.

40. Did You Know? • Unlike taste cells, olfactory cells can respond to many thousands of odorants, including ones that have been newly synthesized. Millions of olfactory receptors line a postage-stamp-sized area in the upper part of the nose, the olfactory epithelium.

41. Did You Know? • Respiration occurs around-the-clock, but most people are not aware that they are exhaling fluid in the form of water vapor every time they breathe. We become aware of the vapor loss during cold weather when the vapor can be seen as it is exposed to low temperatures. About 450 milliliters of fluid are lost during a 24-hour period by respiration.

42. Did You Know? • Toxicology is the study of the nature and action of poisons. The term comes from a Greek word referring to the poison in which arrows were dipped.

43. Did You Know? • The heart and blood vessels, with the blood they contain, are the efficient internal transportation system of the body. The system is dual: the right side of the heart pumps blood only to the lungs, the left side pumps the freshly aerated blood to the rest of the body.

44. Did You Know? • In an average lifetime, the heart pumps approximately one million barrels of blood—enough to fill more than 3 supertankers—through the circulatory system.

45. Did You Know? • Even as you rest, your heart is working twice as hard as your leg muscles would if you were running at full speed.

46. Did You Know? • In 1903, Dutch physiologist Willem Einthoven invented the electrocardiograph, a machine that measures the minute electrical currents generated by the heart. Initially, it was a cumbersome and costly device, taking five technicians to operate. During the procedure, patients had to place both hands and both feet in buckets of water.

47. Did You Know? • The FDA defines drugs as products intended for treating or preventing disease or for affecting the structure or any function of the body. Antiperspirants are considered drugs because they affect the function of the body by reducing the amount of sweat that reaches the skin. • Deodorants are regulated as cosmetics because they promote attractiveness only by masking odor, not by reducing sweat.

48. Did You Know? • In June 2004, the Food and Drug Administration cleared the first application for leeches (Hirudo medicinalis) to be used in modern medicine as medical devices.

49. Did You Know? • Early attempts at transfusion led to serious adverse reactions. However, in 1901, Austrian scientist Karl Landsteiner discovered that individuals belonged to one of four different blood groups (O, A, B, and AB), and that transfusions between people in different blood groups could be unsafe.

50. Did You Know? • Since its introduction in 1899, aspirin (acetylsalicylic acid) has been the most popular drug of all time.