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How many cells are in the human body?

Depending on the source, that figure varies from 50 to 75 trillion cells.

Sources: Asimov, Isaac. The Human Body, New rev. ed., p. 79; New Encyclopaedia Britannica, vol. 6, p. 134; Van Amerogen, C. The Way Things Work Book of the Body, p. 13.

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1. Characteristics And Functions
   1. What chemical elements are found within the human body?
   2. What is the normal pH of blood, urine, and saliva?
   3. How hard does the heart work?
   4. How does the body introduce oxygen to the blood and where does this happen?
   5. How does the immune system work?
   6. How much blood is in the average human body?
   7. What are the seven endocrine glands?
   8. How many cells are in the human body?
   9. How many miles of blood vessels are contained in the body?
   10. How many chromosomes are there in a human body cell?
   11. How long does it take food to digest?
   12. What causes people to sweat when they eat spicy foods?
   13. How much heat is lost through the head when a person is not wearing a hat?
   14. What regulates body temperature in humans?
   15. In the United States, what is the average height and weight for a man and a woman?
   16. What percent of human body weight is water?
   17. What are considered healthy weights for men and women?
   18. Who is the heaviest person that ever lived?
2. Parts Of The Body
   1. How many muscles are in the human body?
   2. Who is the world's oldest person?
   3. Why does excessive exercise cause muscles to become stiff and sore?
   4. What is the purpose of goose bumps?
   5. How many bones are in the human body?
   6. What is the likely purpose of the human appendix?
   7. What is the funny bone?
   8. What is the basic unit of the brain?
   9. How much skin does an average person have?
   10. In addition to left- or right-handedness, what other left or right preferences do people have?
   11. How many hairs are on an average person's head?
   12. Who first used fingerprints as a means of identification?
3. Senses And Sleep
   1. What is the difference in the functions of the rods and cones found in the eyes?
   2. What is the length of the human intestine?
   3. Are more people nearsighted or farsighted?
   4. What does it mean to have 20/20 vision?
   5. How often does the human eye blink?
   6. Why do eyes tear when we work with onions?
   7. What are the primary sensations of taste?
   8. What is REM sleep?
   9. How many calories does a person bum while sleeping?
   10. Why do people snore and how loud can snoring be?