3/25/22, 12:29 PMRequirements for Human Life – Anatomy and Physiology1/9An Introduction to the Human BodyRequirements for Human LifeLearning ObjectivesBy the end of this section, you will be able to:Discuss the role of oxygen and nutrients in maintaining human survivalExplain why extreme heat and extreme cold threaten human survivalExplain how the pressure exerted by gases and fluids influences human survivalHumans have been adapting to life on Earth for at least the past 200,000 years. Earth and its at‐mosphere have provided us with air to breathe, water to drink, and food to eat, but these are notthe only requirements for survival. Although you may rarely think about it, you also cannot liveoutside of a certain range of temperature and pressure that the surface of our planet and its atmos‐phere provides. The next sections explore these four requirements of life.
3/25/22, 12:29 PMRequirements for Human Life – Anatomy and Physiology2/9OxygenAtmospheric air is only about 20 percent oxygen, but that oxygen is a key component of thechemical reactions that keep the body alive, including the reactions that produce ATP. Brain cellsare especially sensitive to lack of oxygen because of their requirement for a high-and-steady pro‐duction of ATP. Brain damage is likely within five minutes without oxygen, and death is likelywithin ten minutes.NutrientsA nutrient is a substance in foods and beverages that is essential to human survival. The three ba‐sic classes of nutrients are water, the energy-yielding and body-building nutrients, and the mi‐cronutrients (vitamins and minerals).The most critical nutrient is water. Depending on the environmental temperature and our state ofhealth, we may be able to survive for only a few days without water. The body’s functional chem‐icals are dissolved and transported in water, and the chemical reactions of life take place in water.Moreover, water is the largest component of cells, blood, and the fluid between cells, and watermakes up about 70 percent of an adult’s body mass. Water also helps regulate our internal temper‐ature and cushions, protects, and lubricates joints and many other body structures.The energy-yielding nutrients are primarily carbohydrates and lipids, while proteins mainly sup‐ply the amino acids that are the building blocks of the body itself. You ingest these in plant andanimal foods and beverages, and the digestive system breaks them down into molecules smallenough to be absorbed. The breakdown products of carbohydrates and lipids can then be used inthe metabolic processes that convert them to ATP. Although you might feel as if you are starvingafter missing a single meal, you can survive without consuming the energy-yielding nutrients forat least several weeks.