Basal metabolism refers to energetic needs of a person in the morning while lying in the bed, at the temperature of the 84 F degrees and while both mental and physical activity are at minimum (which is exactly when we wake up) and this has to be 12 hours after the last meal.
Basal metabolism interacts with different needs of different systems in organism (cardiovascular, respiratory and digestive); it actually interacts with all cells in the body. Under usual circumstances basal metabolism includes 60% to 70% of total energy need in organism. Energetic needs are defined through spent energy, which should satisfy basal metabolism, keeping the body temperature at normal levels, and digestion and physical activity.
In children, pregnant women and breastfeeding mothers, energy values should be enough for growth and proper tissue development (including normal lactation). Energetic values and basal metabolism are influenced by sex, age, physiological state of the organism and pathological conditions that affect the organism (in that situation, energy values needed are much higher). Also, energy is needed for mental activity. Brain for its normal functioning uses oxygen (20%) even though the brain mass occupies only 2% of the entire body.
Proper distribution of energy in a day ensures constant level of glucose in blood, which is the only substance that the brain can use. As said, basal metabolism is different in opposite sexes and that difference is caused by the physiological characteristics of both genders (increased muscle mass in men, for example). Women have a bit more fat tissue, which does not require that much energy because it is an energy source itself. Fat tissue is metabolically less active than tissue with small amounts of fat. This means that basal metabolism is a great indicator of fat tissue amount in organism, analyzing if there is more or less fat than needed.
In older people, because of the smaller amounts of muscle mass the need for basal metabolism is decreased, which means that in children, required energy values are higher because of the constant growth, while in adults, with decrease of non-fat tissue (muscles for example), required energy is decreased. Gender affects the energy values because of the body mass differences. Big people have more need for basal metabolism and higher amount of energy for daily activities. Eating also affects the basal metabolism, whether it is immediately after the meal or after a while (creation of warmth in organism and on surface).