Life involves the process of metabolism by means of enzymes in one cell or in multiple cells that are linked together, also called organisms. An organism lives in an environment and uses that environment for its metabolism. In the case of metabolism, a distinction is made between the build-up of substances while energy is used (anabolism) and the decomposition of biomolecules in which energy is released (catabolism). The human body is instinctively or intuitively stimulated, consciously or unconsciously, that it needs something, mainly from its environment, to maintain (protect) and to grow, such as; oxygen, nutrition, rest, defecation, a body temperature of 37 degrees Celsius and reproduction by sexual intercourse. For example, a person needs an adequate intake of food to generate enough energy to grow and to keep his metabolic process going. The feeling of an empty stomach and fatigue ensure that you are stimulated to take in food.
The autonomic nervous system regulates the functioning of internal organs. A large part of this process happens unconsciously to the host. Examples are; breathing, digestion, widening and narrowing of blood vessels and the heartbeat. The autonomic nervous system is regulated by the sympathetic (fight-or-flight response) and parasympathetic (rest and digest response) nervous system. Both these nervous systems ensure that a balance is found for the functioning of the organs in our body, so that the body can function as a whole.
The functioning of the autonomic nervous system occurs through the function of neurotransmitters. Neurotransmitters ensure the contact between the various organs along the nerves through synapses. In addition, the autonomic nervous system works closely with the internal glands. The production of hormones in the internal glands and their release into the blood is regulated by the autonomic nervous system. These hormones themselves again act upon the nervous system. The autonomic nervous system, for example, causes sweat glands to release fluid if our body gets too hot, so that we cool down. Almost all cells in the body need oxygen to function. Oxygen from the environment is absorbed into the blood via the lungs, after which the oxygen is transported through the blood vessels into the cells. Breathing is also regulated by the autonomic nervous system, but unlike the other processes, you can temporarily influence it.
Hormones in the body provide sexual characteristics and mating urge (libido). Known hormones in humans related to reproduction are estrogen for woman and testosterone for man. Although the sleep-wake rhythm is not fully understood in science, evidence from studies suggests that certain substances in the brain, the most well-known of which is adenosine, affect the sleep-wake rhythm. Adenosine acts as a neuromodulator in the brain and inhibits body processes associated with being awake. Adenosine levels in the brain rise when the time that you’re awake increases since the last sleep period and fall rapidly during the next sleep period.
Humans depend on their environment for survival by taking the necessary energy and releasing unnecessary substances. Without an environment, there would be no life, but without life, there would be no environment that would surround life. Life is thus mutually dependent on its environment.
 . Dr. Bruker, Lebensbedingte krankheiten, p47-53, editie 19, 2014
 . https://www.howsleepworks.com/how_homeostasis.html