Humans can live about 30 to 45 days without food but only 10 to 14 days without water. Dry mouth or thirst indicates low water level in our body or blood electrolyte concentrations too high and sends signals to the brain stimulating the feeling to drink.
Water is a neutral molecule and has a pH of 7.0 meaning it is neither acidic nor basic.50 to 60% of the body weight of normal adults is composed of water.
Fluid and Electrolyte Balance
Water in our body is not pure, rather a mixture of cells, proteins, glucose, lipoproteins, electrolytes, and other dissolved substances. Electrolytes are minerals that have either positive or negative charge and the principal electrolyte in intracellular fluid and extracellular fluid, respectively.
Sensible and insensible water loss must be replaced in terms of both volume and electrolyte content. Body maintains the fluid and solutes balance in different compartments through homeostatic mechanism.
Water can move freely across cell membranes by osmosis from an area of low to an area of high solutes concentration. The purpose of movement of water across the cell membranes is to maintain the same osmolality inside and outside of the cells.
Functions of Water
Of all the nutrients, water is considered as the most critical for our body as its absence produces lethal effects within a few days. Water performs the following four basic functions
Transports Substances
Water is called the universal solvent because more substances are dissolved in it than any other solvent. The water molecule has different electrical charges-one end is positive and another end is negative. This property allows water to surround other charged molecules and disperse them.
Water serves as a transport medium to deliver substances to cells and remove wastes. For example, blood which contains 90% water transports oxygen, nutrients, hormones, drugs, and other substances to cells and removes metabolic waste products such as CO2,urea,ketones etc from the cells and transports to the lungs or kidneys for excretion.
Regulates body temperature
Water plays an important role in fixing body temperature at around set point 98.6 Fahrenheit or 37 Celsius. The water in blood helps to regulate body temperature at around a set point by increasing or decreasing the amount of heat lost at the body surface. Body can operate properly in a narrow range of temperature and metabolism is halted which can be life threatening. When body temperature starts to raise, the blood vessels in the skin dilate which increases blood flow to the skin and allows more heat to be released into the environment. The most obvious way that water helps to regulate body temperature is through the evaporation of sweat. When body temperature increases, the brain triggers the sweat glands in the skin to produce sweat, which is mostly water. AS the sweat evaporates from the skin, heat is lost, cooling the body.
Works as a medium for chemical reactions
Water acts as a medium for all biochemical reactions in the body. Among the roles it plays is as a medium in which all of the body’s metabolic reactions occur. Water also participates directly in a number of chemical reactions, many of which are involved in energy production. The addition of water to a large molecule can break it into two smaller ones.
Acts as lubricant
Water protects the body by acting as a cushion. For example, fluids inside the eyeballs and spinal cord cushion against shock; the fluid between joints cushions, lubricates and eases the movement of articulated bones.
Daily water input
Total water output per day is about 2.5 L. This must be balanced through water input. Our body tissues can produce around 300mL of water per day through metabolic processes. The remaining water output must be balanced by drinking fluids and eating solid foods. The average fluid consumption approximately per day is 1.5 L through beverages and 700 mL through solid foods.
Recommended water intake daily
The recommendation for total water intake is about 2.7 L per day for women and 3.7L per day for men. It is important to note that recommended water intake includes water from all sources such as drinking pure water, milk, juice and other beverages. Water and other beverages account for about 80% of adult fluid intake. The other 20% comes from water in foods.
Water needs are also higher for pregnant women to allow for the increase in maternal blood volume, the production of amniotic fluid, and the needs of the fetus. During lactation, fluid needs are increased because the fluid secreted in milk, about 3 cups per day, must be restored by the mother’s fluid intake.