Under stress, the base ATP store can supply the cell with energy for up to perhaps 10 seconds. After that time, the primary anaerobic system kicks in deriving more energy from muscle-stored glycogen. This process lags by a few seconds because it requires inter-cell transfer of glycogen. The muscles can store enough glycogen to meet the body’s cellular requirements for several tens of seconds. This anaerobic system creates significant byproducts in the form of lactate and hydrogen ions. Saturation of the tissue with hydrogen ions then limits the continuation of the operation of the anaerobic system. Depending on the capabilities of the circulatory system, the hydrogen ions and lactate will either be removed from the muscle or will be stored within the muscle in the form of lactic acid; this latter mechanism being what causes the muscles to be sore after an unusual amount of strenuous activity.

Of course, physical activity of the body generally requires an energy system that will function continuously over a much longer time period than a minute or two. This is the domain of the aerobic system. At the base cellular level, this system still works through the short term storage and retrieval of energy using the ATP molecule. However, the aerobic system draws fatty acids from adipose tissue found beneath the skin and around internal organs within the body. When excess food is taken in to the body, the amount beyond what is needed for current organic processes can be stored as fat within this tissue. When operational demands of the body exceed that found in local stores within individual cells and muscle tissue, the aerobic system responds by conveying the fatty acids to the cells where it is used to continually replenish the supply of ATP. The aerobic system is limited by the amount of stored body fat, but in most instances it can support the body’s operations for days and longer.

The body requires a continuing supply of several materials in order for it to continue operating (living). The most urgently required such materials are air, food and water. Food encompasses a significant range of materials in its own right due to the many systems within the body, many of which require very specific materials for their continued operation. For most of the required materials, there is a sensory feedback mechanism through the brain that indicates deficiencies whenever there is a shortage. The dominant of these indicators are hunger, thirst and a feeling of suffocation, indicating respectively, of course, a lack of sufficient food, water or air. If we consider these indicators in terms of the needs hierarchy, then these needs reflect physiological requirements of the body. At this level, it is difficult to differentiate the specific indicator from an emotional state on the part of the person. Beyond these primary indicators, the mind will sometimes evoke cravings, specifically regarding food, that indicate mounting deficiencies in very specific substances needed by the body. Such cravings for things sweet, sour, salty or bitter are indicators of deficiencies of various minerals that are needed by the body.

The central gut gives rise to the alimentary canal, the basis of the digestive tract that provides for the processing of raw food materials into a form suitable for storage and use within the body. Much like the reduction of base ore into finished metal, the digestive process involves first mechanical deconstruction of raw foodstuffs through mastication (chewing) and preliminary chemical breakdown by saliva within the mouth. Then, the food is transported to the stomach and the intestines where further chemical breakdown reduces all foods to a variety of chemical components, primarily sugars and lipids that can be directly used by the body or stored for later use. As we noted in the brief consideration of the aerobic and anaerobic energy systems, the human body makes use of several distinct energy provision systems in order to meet its requirements for the necessary materials. As have many other species, humans have developed an ability to store excess food internally within the body in the form of fat. This capability allows for significant decoupling of the acquisition and subsequent use of the various forms of sustenance

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