Water Intake and Output: Mechanisms of Regulation

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Did you know that more than 60% of the human body is water? Likely yes, it is one of the well-known anatomical facts, but the extent of it and complex mechanisms which regulate water in the bodies are often underestimated. It is important to start with statistics, with 73% of the brain and heart made up of water, 83% of the lungs, 64% of the skin, 79% of muscles and kidneys, and even bones contain up to 31% water (USGS, 2019). Water is the critical nutrient to life in every cell which acts as building material for the organs, as well as serving secondary functions such as being a coolant for body temperature and transport for proteins in the bloodstream. It is evident that without water, humans cannot survive, with a health adult needing between 2.2-3 liters (2-3 quarts) per day of water consumption. Water has many unique aspects such as being a universal solvent that is able to participate solutions and non-covalent interactions of human cells, that is why it is a universal presence in the various body compartments and organs that play a role in human physiology (Riveros-Perez & Riveros, 2018).

For healthy function, the human body requires water balance as one of the key mechanisms, where the average daily water intake and output are relatively equal. The balance is maintained by combined workings of various systems including respiratory, digestive, endocrine, cardiovascular, urinary, and lymphatic. There are two major mechanisms of water intake metabolic and preformed. Metabolic water is a by-product of aerobic respiration and dehydration synthesis. Preformed is water consumed with food and drink. Meanwhile, the main mechanisms of water output are urine (1500 mL/day), feces (200 mL/day), expired breath (300 mL/day), and sweat (100 mL/day). At the same time, as much as 400 mL/day are lost through cutaneous transportation which is water that diffuses through the epidermis and evaporates, similar to glandular secretion (Saladin, 2003). The body seeks to maintain water balance by regulation of excretion as well as driving fluid intake through intrinsic survival mechanisms of thirst.