1.4: Requirements for Human Life
The Earth and its atmosphere have provided humans with air, water, and food, but these are not the only requirements for survival. Humans also require a specific range of temperature and pressure that the Earth and its atmosphere provides.
Oxygen
Atmospheric air is only about 20 percent oxygen, but that oxygen is a key component of the chemical reactions that keep the body alive, including the reactions that produce ATP. Brain cells are susceptible to a lack of oxygen because they require a high-and-steady ATP production. Brain damage is likely within five minutes without oxygen, and death is likely within ten minutes.
Nutrients
A nutrient is a substance in foods and beverages essential to human survival. The three basic classes of nutrients are water, energy-yielding and body-building nutrients, and micronutrients– vitamins and minerals.
The most critical nutrient is water. The body's functional chemicals are dissolved and transported in water, and the chemical reactions of life take place in water. Moreover, water is the most significant component of cells, blood, and the fluid between cells and water makes up about 70 percent of an adult's body mass.
The energy-yielding nutrients are primarily carbohydrates and lipids, while proteins supply the amino acids that are the body's building blocks. Humans ingest these in plant and animal foods and beverages, and the digestive system breaks them down into molecules small enough to be absorbed. The breakdown products of carbohydrates and lipids can then be used in the metabolic processes that convert them to ATP. Water and energy-yielding nutrients are also called macronutrients because the body needs them in large amounts.
The micronutrients participate in many essential chemical reactions and processes, such as nerve impulses, and some, such as calcium, also contribute to the body's structure. The body can store some of the micronutrients in its tissues and draw on those reserves if they are missed in the diet for a few days or weeks. Some other micronutrients, such as vitamin C and most of the B vitamins, are water-soluble and cannot be stored; as a result, they need to be consumed regularly in a balanced amount.
Narrow Range of Temperature
The chemical reactions upon which the body depends can only occur within a narrow body temperature range, from just below to just above 37°C (98.6°F). When the body temperature rises well above or below normal, certain proteins (enzymes) that facilitate chemical reactions lose their normal structure and their ability to function, and the chemical reactions of metabolism cannot proceed.
However, the body can respond effectively to short-term exposure to heat or cold. One of the body's responses to heat is, sweating. As sweat evaporates from the skin, it removes some thermal energy from the body, cooling it.
The body can also respond effectively to short-term exposure to cold. One response to cold is shivering, which is a random muscle movement that generates heat.
Narrow Range of Atmospheric Pressure
Atmospheric pressure does more than just keep blood gases dissolved. The ability to breathe also depends upon precise atmospheric pressure. Altitude sickness occurs because the atmosphere at high altitudes exerts less pressure, reducing the exchange of these gases, and causing shortness of breath, confusion, headache, lethargy, and nausea.
This text is adapted from Openstax, Anatomy and Physiology 2e, Section 1.4: Requirements for Human Life
