7.3: Water’s Importance to Vitality

1. 
2. Buy Print CopyDonate

• 
• Anonymous
• LibreTexts

Learning Objectives

• Describe the four major functions of water in the human body

You get up in the morning, flush wastes down the toilet, take a shower, brush your teeth, drink, eat, drive, wash the grime from your windshield, get to work, and drink coffee. Next to a fountain you eat lunch and down it with a glass of water, you use the toilet again and again, drive home, prepare dinner, etc. Add all the ways you use water every day and you still will not come close to the countless uses water has in the human body. Of all the 

nutrients

, water is the most critical as its absence proves lethal within a few days. Organisms have adapted numerous mechanisms for water conservation. 

Water uses in the human body can be loosely categorized into four basic functions: transportation vehicle, medium for chemical reactions, lubricant/shock absorber, and temperature regulator.

Figure 7.3.1$\mathrm{7.3.}1$: Water is the foundation of all life—the surface of the earth is 70 percent water; the volume of water in humans is about 60 percent. "The Blue Marble" is a famous photograph of the Earth taken on December 7, 1972 by the crew of the Apollo 17 spacecraft en route to the Moon at a distance of about 29,000 kilometers (18,000 statute miles). It shows Africa, Antarctica, and the Arabian Peninsula. (Public Domain; NASA)

Water As a Transportation Vehicle

Water is called the “universal solvent” because more substances dissolve in it than any other fluid. Molecules dissolve in water because of the hydrogen and oxygen molecules ability to loosely bond with other molecules. Molecules of water (H2O) surround substances, suspending them in a sea of water molecules. The solvent action of water allows for substances to be more readily transported. A pile of undissolved salt would be difficult to move throughout 

tissues

, as would a bubble of gas or a glob of fat. Blood, the primary transport fluid in the body is about 78 percent water. Dissolved substances in blood include 

proteins

, lipoproteins, glucose, electrolytes, and metabolic waste products, such as carbon dioxide and urea, all of which are either dissolved in the watery surrounding of blood to be transported to cells to support basic functions or are removed from cells to prevent waste build-up and toxicity. Blood is not just the primary vehicle of transport in the body, but also as a fluid tissue blood structurally supports blood vessels that would collapse in its absence. Similarly, the brain is 75 percent water, with the water in the brain providing structure.

Water As a Medium for Chemical Reactions

Water is required for even the most basic chemical reactions. Previously, you learned that 

enzymes

 are 

proteins

 and 

proteins

 fold into their functional shape based on how their amino-acid sequences react with water. Once formed, 

enzymes

 must conduct their specific chemical reactions in a medium, which in all organisms is water. Water is an ideal medium for chemical reactions as it can store a large amount of heat, is electrically neutral, and has a pH of 7.0, meaning it is not acidic or basic. Additionally, water is involved in many enzymatic reactions as an agent to break bonds or, by its removal from a molecule, to form bonds.

Water As a Lubricant/Shock Absorber

Many may view the slimy products of a sneeze as gross, but sneezing is essential for removing irritants and could not take place without water. Mucus, which is not only essential to discharge nasal irritants, but also required for breathing, transportation of 

nutrients

 along the gastrointestinal tract, and elimination of waste materials through the rectum is composed of more than 90 percent water. 

Mucus is a front-line defense against injury and foreign invaders. 

It protects 

tissues

 from irritants, entraps pathogens, and contains immune-system cells that destroy pathogens. 

Water is the main component of the lubricating fluid between joints and eases the movement of articulated bones.

The aqueous and vitreous humors, which are fluids that fill the extra space in the eyes and the cerebrospinal fluid surrounding the brain and spinal cord are primarily water and buffer these 

organs

 against sudden changes in the environment. 

Watery fluids surrounding 

organs

 provide both chemical and mechanical protection. Just two weeks after fertilization water fills the amniotic sac in a pregnant woman providing a cushion of protection for the developing embryo.

Water As a Temperature Regulator

Another homeostatic function of the body, termed thermoregulation is to balance heat gain with heat loss and body water plays an important role in accomplishing this. Human life is supported within a narrow range of temperature, with the temperature set point of the body being 98.6°F (37°C). Too low or too high of a temperature causes 

enzymes

 to stop functioning and metabolism is haulted. At 82.4°F (28°C) muscle failure occurs and hypothermia sets in. At the opposite extreme of 111.2°F (44°C) the 

central nervous system

 fails and death results. Water is good at storing heat, an attribute referred to as 

heat capacity

 and thus helps maintain the temperature set point of the body despite changes in the surrounding environment.

There are several mechanisms in place that move body water from place to place as a method to distribute heat in the body and equalize body temperature (Figure 7.3.2$\mathrm{7.3.}2$). The hypothalamus in the brain is the the rmoregulatory center. The hypothalamus contains special protein sensors that detect blood temperature. The skin also contains temperature sensors that respond quickly to changes in immediate surroundings. In response to cold sensors in the skin, a neural signal is sent to the hypothalamus, which then sends a signal to smooth muscle tissue surrounding blood vessels causing them to constrict and reduce blood flow. This reduces heat lost to the environment. The hypothalamus also sends signals to muscles to erect hairs and shiver and to endocrine glands like the thyroid to secrete 

hormones

 capable of ramping up metabolism. These actions increase heat conservation and stimulate its production in the body in response to cooling temperatures.

Figure 7.3.2$\mathbf{7.3.}\mathbf{2}$: Thermoregulation is the ability of an 

organism

 to maintain body temperature despite changing environmental temperatures.

When body temperature rises, as what occurs during exercise, the hypothalamus detects an increase in blood temperature. In response, the hypothalamus sends a signal to the smooth muscle of blood vessels causing them to dilate so that more blood flows to the body’s periphery. Once on the skin, extra heat exits the body via perspiration (sweat), which is 98 to 99 percent water. Water on the skin’s surface evaporates, a process that takes 

energy

 and results in the loss of heat, thereby cooling the body. Perspiration is a process that intertwines temperature homeostasis with fluid and electrolyte homeostasis. Water and electrolytes lost in sweat need to be replenished in order to maintain fluid and electrolyte balance.

Key Takeaways

• Uses of water in the human body can be loosely categorized into four basic functions: transportation vehicle, medium for chemical reactions, lubricant/shock absorber, and temperature regulator.
• Water has been termed the “universal solvent” because more substances dissolve in it than any other fluid. Water is essential to form 
  enzymes
   and serves as a medium for chemical reactions. Water as a component of body fluids acts as a lubricant and shock absorber.
• Water is good at storing heat and buffers the body against extreme variations in temperature.
• There are several mechanisms that move body water from place to place as a method of heat distribution and temperature equalization.

Discussion Starters

Reflection on Water's Importance to Vitality

Water is fundamental to life, playing irreplaceable roles in the human body. This reflection explores the four primary functions of water: as a transportation vehicle, a medium for chemical reactions, a lubricant and shock absorber, and a temperature regulator.

1. Transportation Vehicle: Water's ability to dissolve more substances than any other liquid earns it the title of "universal solvent." This property is crucial for transporting various substances throughout the body. For example, blood, which is about 78% water, carries nutrients, waste products, and other critical elements to and from cells. This capability supports essential bodily functions and removes toxins.

2. Medium for Chemical Reactions: Water is not just a substance within which chemical reactions occur; it actively participates in these reactions. Enzymes, which are vital for almost all biochemical reactions in the body, require water to function. Water helps maintain the proper pH and temperature that enzymes need to catalyze reactions efficiently.

3. Lubricant and Shock Absorber: Water is also crucial for protecting body structures. It acts as a lubricant in joints, facilitating smooth movement. Moreover, water is a major component of mucus and other bodily fluids that protect tissues by trapping pathogens and cushioning organs and tissues against mechanical damage.

4. Temperature Regulator: Water's high heat capacity plays a critical role in maintaining body temperature. It absorbs and redistributes body heat, allowing humans to adapt to varying temperatures. Through processes like perspiration and vasodilation, water helps dissipate excess heat to maintain a stable internal environment suitable for enzyme function and cellular activities.

Understanding these functions highlights how essential water is beyond just quenching thirst. It is integral to our biological systems, facilitating countless processes that sustain life. Maintaining adequate hydration is not just a daily necessity but a foundational aspect of health and well-being