The nervous system is the master controller of the body. Each thought, each emotion, each action—all result from the activity of this system.
The reproductive system makes life possible. An individual does not need the system to survive, but the human race does.
Breathing, controlled by the respiratory system, is a continuous process of which a person is normally unaware. If breathing stops, however, a person becomes acutely aware of the fact.
The word skeleton comes from the Greek word skeletos, meaning "dried up." The parts of the skeletal system—the bones and other structures that make up the joints of the skeleton—are anything but dried up. Strong yet light, the skeletal system is made of living material, with networks of blood vessels running throughout.
When cells in the body break down proteins into forms they can utilize, they produce ammonia wastes that the liver turns into urea (a chemical compound of carbon, hydrogen, nitrogen, and oxygen). When cells break down carbohydrates, they produce water and carbon dioxide as waste products.
The senses connect humans to the real world, allowing them to interpret what is happening around them and respond accordingly. The color of the sky at dusk, the sound of laughter at a party, the scent of eucalyptus and pine, the taste of freshly baked bread—all would be meaningless without the senses.
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