Abstract
The first and second laws of thermodynamics were applied to biochemical reactions typical of human metabolism. An open-system model was used for a human body. Energy conservation, availability and entropy balances were performed to obtain the entropy generated for the main food components. Quantitative results for entropy generation were obtained as a function of age using the databases from the U.S. Food and Nutrition Board (FNB) and Centers for Disease Control and Prevention (CDC), which provide energy requirements and food intake composition as a function of age, weight and stature. Numerical integration was performed through human lifespan for different levels of physical activity. Results were presented and analyzed. Entropy generated over the lifespan of average individuals (natural death) was found to be 11,404 kJ/ºK per kg of body mass with a rate of generation three times higher on infants than on the elderly. The entropy generated predicts a life span of 73.78 and 81.61 years for the average U.S. male and female individuals respectively, which are values that closely match the average lifespan from statistics (74.63 and 80.36 years). From the analysis of the effect of different activity levels, it is shown that entropy generated increases with physical activity, suggesting that exercise should be kept to a “healthy minimum” if entropy generation is to be minimized.
Highlights
The biosphere, a thin layer around earth, contains all living species including plants, and all the living organisms owe their existence to atmosphere, hydrosphere and lithosphere.The quest for a longer, healthier lifespan of biological species (BS) is the subject of intensive research and publications [1, 2, 3]
The reasoning is that for a human in resting condition, most of the energy output from the metabolism of food appears as heat, and neglecting other causes of irreversibility, they approximated the internal entropy production rate by the basal metabolic rate (BMR) divided by the average body temperature and estimated the entropy exchange with the surrounding by the change of entropy in the breathing air
This is the approach taken in the current paper, in which the estimated energy requirements (EER) of the BS were used to calculate the metabolic rates, and an availability analysis was applied to the metabolic reactions of the main nutrient groups to obtain the entropy generated
Summary
The biosphere, a thin layer around earth, contains all living species including plants, and all the living organisms owe their existence to atmosphere (air), hydrosphere (water) and lithosphere (land). Living organisms constantly need to expend energy to perform vital life functions This includes running the heart, operating the neural computer (brain), performing mechanical functions (e.g. lifting weights, walking, etc), and eating food to maintain body temperature in the presence of heat loss to the environment, driving other chemical reactions, replicating, and repairing tissues and removing wastes. This energy is provided by oxidation of organic substances called metabolism: carbohydrates (CH), fats (F) and proteins (P) introduced to the organism by feeding. Prior to proceeding with a rigorous analysis, a brief literature review is presented
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