|Left: Skin-boundary human system model (one human being).  Center: Two human systems (shown in the personal space bubbles). Right: One boundaried human system (containing seven people).|
The description of boundaries of a human system, in thermodynamic terms, in the sense of the boundary (outside layer of a volumetrically-changing body of water) defined originally through the heat engine model introduced in the 1824 Carnot engine description, is very problematic. In 1990, American social entropy theorist Kenneth Bailey referred to this issue as the "boundary problem"; discussing ideas such as boundary analysis and entropy breaks. 
The original version of the first law, introduced by German physicist Rudolf Clausius in 1850, states that when, according to Boerhaave’s law, a unit of heat Q is added to a body, it will cause the body to expand, where the expansion can be quantified as the body pushing out a region of new volume V against the weight or pressure P of the atoms and molecules of the surrounding atmosphere of the earth, and that the change in the energy of the body dU will be an amount of heat added dQ less the work done dW:
In 2005, referencing the 1970 work of Danish-born American environmental engineer Ole Fanger, American Mechanical engineer Myer Kutz gave the following equation as the first law of thermodynamics for the human body:
where M is the metabolic heat production (watts per meters squared of body surface area), W is the rate of mechanical work, and Q dot represents the heat losses.  What seems to be the case here is that Fanger equates metabolism M to internal energy U and starts his derivation with the first law in this form:
To note, there are numerous issues neglected in this approximation. One, of many, being that the internal energy of a person if a function of metabolism (of simply the digestion of food). More correctly, it is a person's drives and aspirations, and the effect of surrounding forces and inputs on those drives, in conjunction with energy (bond energy) released (and absorbed) in the dynamics of human chemical bonding interactions, such as in love the chemical reaction, that better quantify the internal energy of a person. We may ask, for instance, how is the energy of the sight (electromagnetic waves) of one’s father killing one’s mother, to a child, who views this incident, as depicted in the 2003 film The Hulk, conserved in the body, or rather mind, according to the principle of the conservation of energy, in its quantification of the internal energy of the human body, as years progress?  Certainly, these were the sorts questions that Austrian psychologist Sigmund Freud was after in his development of psychodynamics.
In any event, Kutz further equates his human body first law to:
where C is the convective heat losses, R the radiative heat losses, and E the evaporative heat losses. Kutz states that the figure of the man with arms extended (above left) schematically represents the energy balance components for the control volume signified by the dashed line; thus depicting the thermodynamic system of one human body. 
American educator Dick Hammond is strong promoter of the term “human system”, albeit used in an entropological sense, which he culls the from Austrian biologist Ludwig Bertalanffy’s 1968 general system theory who defines living organisms as open systems.  Hammond states, for instance, in very simplified terms:
“When a new human system arrives at the age of eighteen, having been reared by society, and closely monitored in formal education for at least twelve of those years, and remains a rapid, high entropy producer, rather than helping society keep entropy to a minimum, then society knows they have failed … the cost of coping with randomizers is very high.”The typical description of a human system, either as one body or a system of bodies, is that it is an open system, maintained far-from-equilibrium, albiet a variation of local entropy decrease.
1. Bertalanffy, Ludwig. (1968). General Systems Theory - Foundations, Development, Applications. New York: George Braziller.
2. Hammond, Dick E. (2005). Human System from Entropy to Ethics, (section: The Human System, pgs. 57-62, 75). Publisher: Dick Hammond.
3. (a) Fanger, P.O. (1970). Thermal Comfort Analysis and Applications in Environmental Engineering. McGraw-Hill.
(b) Kutz, Myer. (2005). Mechanical Engineers’ Handbook: Energy and Power ("skin-boundary diagram" (based on Fanger, 1970), pgs. 540-41). Wiley.
4. Bailey, Kenneth D. (1990). Social Entropy Theory (terms: "boundary problem", "boundary analysis", pg. 18-19; section: Boundaries, pgs. 157-62). New York: State University of New York Press.
5. Hulk (film) – Wikipedia.