CHAPTER 25 Summary

• Living organisms have an anatomical structure that is adapted to suit their physiological functions.
• Humans and other multicellular organisms are organized hierarchically: cells assemble to make up tissues; tissues congregate to form organs; organs work together as part of organ systems.
• Humans have many different organ systems that cooperate to accomplish basic physiological tasks, such as obtaining energy, taking in nutrients to build new molecules during growth and repair, and ridding themselves of wastes.
• Most organisms cannot tolerate wide fluctuations in their internal environment; their bodies work to maintain homeostasis, a stable internal environment.
• The process whereby organisms maintain a relatively constant internal temperature is called thermoregulation.
• The body responds to cold temperatures in two main ways: by conserving the heat it has through vasoconstriction, and by generating more heat through shivering. When overheated, the body releases heat by vasodilation and sweating.
• Maintaining homeostasis requires both sensors and effectors. Sensors include nervous system receptors that detect changes in a variety of internal states (for example, temperature and blood pressure). Effectors include the glands and muscles that respond to an abnormal state in an effort to correct it.
• Sensors and effectors work together as part of a circuit or feedback loop. Negative feedback loops are important in homeostasis.
• Hormones are chemical messengers that travel through the bloodstream, bind to receptors on a target cell, and effect a change in that cell. Insulin and glucagon are hormones that regulate blood-glucose levels.
• Osmoregulation is the control of water balance in the body. Sensors detect blood pressure, blood volume, and solute concentration. Kidneys are important effectors in maintaining water balance.
• Maintaining homeostasis is work and requires adequate energy and oxygen to power cellular respiration.
• Humans (and other mammals) are endotherms: we generate heat internally. Many other organisms, such as reptiles and fish, are ectotherms: they rely on behavior and the environment to maintain a temperature compatible with life.

• MORE TO EXPLORE

  • Jon Krakauer (1996) “Into Thin Air,” Outside. Sept 1.
  • Krakauer, J. (1997) Into Thin Air: A Personal Account of the Mt. Everest Disaster. New York: Random House.
  • PBS (2008) Frontline: Storm over Everest. http://www.pbs.org/wgbh/pages/frontline/everest/
  • Firth, P. G., et al. (2008) Mortality on Mount Everest, 1921–2006: descriptive study. British Medical Journal 337:a2654.
  • Moore, G. W. K., et al. (2006) Weather and death on Mount Everest: an analysis of the Into Thin Air storm. Bulletin of the American Meteorological Society 87:465–480.