Merck Manual

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Overview of Hydration
Overview of Hydration
Overview of Hydration

    Water is essential for human life, (shocking I know) but it’s why human civilizations historically sprouted up along the banks of rivers, lakes, and oceans. Water is the main substance in our bodies, making up more than 50% of a person’s body weight, and it’s directly involved in every biochemical reaction in each cell in our body. Ultimately maintaining the right balance of water is what keeps us alive.

    Water is a V-shaped molecule made up of two hydrogen atoms that bind to a single oxygen atom, and it’s commonly referred to by its chemical composition of H20. The bond between hydrogen and oxygen is a way of representing the fact that the two atoms share a single electron that zips around in the space between them. The space where it moves around is called an electron cloud and it’s a bit lopsided, since the sharing isn’t completely balanced. Because the electron spends a bit more time on the side nearest the oxygen, the oxygen has a partial negative charge and the hydrogens have a partial positive charge. That’s called a dipole, with the hydrogen end of the bond having a slight positive charge, and the oxygen end having a slight negative charge.

    In fact, it’s this dipole that really explains the magic of water, because it allows the slightly positive hydrogens to line up with slightly negative oxygen atoms from other water molecules. That attraction between water molecules is called a hydrogen bond, and ultimately it’s the reason that water molecules huddle up together. Think about the dew droplets that form on leaves early in the morning, that bead is huddled up because of millions of hydrogen bonds within it. Also, having lots of slightly positive hydrogens and slightly negative oxygens is what allows water to be a great solvent for other molecules like sugar and salt which can easily dissolve right into it.

    Total body water can be subdivided into two major compartments, intracellular fluid which is fluid inside cells, and extracellular fluid which is fluid outside of cell like in the blood and in the interstitial tissue between cells. Let’s say that a person’s total water makes up 60% of their body weight. Two-thirds of that 60%, or 40% of body weight, is intracellular fluid. And the other 1/3 or 20% of body weight is extracellular fluid.

    Both inside and outside the cells, water acts as a solvent for electrically charged molecules called ions or electrolytes. When water dissolves electrolytes, the slightly negatively charged oxygen attracts positive ions like sodium and the slightly positively charged hydrogen attracts negative ions like chloride. That’s how table salt or NaCl dissolves into water.

    In our bodies, the main positive electrolytes are sodium, potassium, calcium, and magnesium, and the main negative electrolytes are chloride, bicarbonate, phosphate, and sulfate. These are kept at very specific concentrations both within and outside of the cell, through a variety of processes. One mechanism is osmosis, where water moves from the more dilute compartment or one with low concentration, to the more concentrated compartment.

    So blood osmolarity, which is the overall concentration of all substances dissolved in the blood like electrolytes, glucose, and urea, is a good measure of hydration status, and it’s normally around 300 (milliosmoles) mOsm per liter. When blood osmolarity is high, a common reason is that there’s not enough water in the body, like in dehydration. When blood osmolarity is low, a common reason is that there’s too much water — like when it’s being retained by the kidneys.

    Normally, the amount of total body water is balanced through ingestion and elimination of water — ins and outs. About 80% of our water intake comes from drinking fluids — the other 20% comes from food we eat. Water content in food varies — but some fruits and vegetables, like watermelon or strawberries, are 90% water by weight. As far as water output goes, we eliminate water through breathing, as humidified air leaves the body, as well as through sweating, urinating, and with bowel movements. In addition to the water we get from foods, the recommended daily amount of fluid intake for women is around 11 glasses of water, or 2.2 L, and for men it’s about 13 glasses, or 3L. But as we’ll soon see, the amount of water needed per day can vary depending on many different factors. Plain water is the ideal choice when it comes to hydration, but all fluids, including caffeinated drinks like coffee and tea, or flavored waters and juices, contribute to water intake.

    After we drink water, it travels all the way through our digestive tract until it reaches the small and large intestines, where water is absorbed into the bloodstream. When we’re at rest, each heartbeat propels about 25% of our blood to the kidneys, where millions of nephrons filter it to produce urine. When we’re properly hydrated, the kidneys produce between 800 and 2000 milliliters of urine every day, and the urine has a pale yellow shade – like lemonade. (I hope you’re not drinking any right now) Some water, around 200 milliliters per day, is also lost during bowel movements. Sweat glands in the skin produce small amounts of sweat, and their production increases when we’re nervous, when it’s really hot outside, or during exercise. The amount of sweat we lose each day varies quite a lot based on the level of activity and the person, so let’s say that on average it’s 500-700 milliliters per day, even though some athletes can sweat more than a liter in an hour when it’s really hot! Finally, there’s the “insensible” water losses. They’re called insensible because we’re not aware of them. When we breathe in, water inside our body is used to humidify the air, and that water vapor is then lost when we breathe out. Water also constantly diffuses through the layers of our skin, keeping them elastic and nourished, but also evaporating at the skin surface. This is in addition to losing water through our sweat glands. All in all, insensible losses account for an incredible 600-900 milliliters per day — which is a lot of water to lose without even really sensing it.

    I don’t want to shock you again, but hydrated is important, because water does so many important things in the body. Water makes up tears, mucus, saliva, and other secretions that protect or lubricate passageways in and out of the body like the eyes, nose, mouth, and genitals. Lubrication is also important in the pleural and pericardial cavities in the chest and the peritoneal cavity in the abdomen, where internal organs touch and slide over one another. It’s also needed at joints, where it helps form synovial fluid that keeps our bones from rubbing against one another. Water is critical for digestion: the water in saliva moistens food when we chew, while gastric and intestinal juices are a fluid environment in which digestive enzymes break down our meals. Water forms the bulk of blood which allows oxygen and glucose to move around the body, and plays a role in eliminating toxins from the body through urination. Water also helps regulate body temperature: when we’re hot, like during a vigorous workout, the capillaries in our skin dilate and sweat glands produce more sweat to dissipate heat. On the other hand, when we’re cold, our blood vessels constrict, retaining heat.

    Water can also help with weight loss and maintaining a healthy body weight. Replacing sweetened drinks with water reduces calorie intake, and drinking water before and during a meal can increase our sense of fullness and prevent overeating.

    Now, when water losses are greater than the intake, it results in dehydration. There are many causes of dehydration, ranging from vigorous exercise or simply not drinking enough fluids throughout the day, to vomiting, diarrhea, excessive sweating, or an inability to swallow. Sometimes dehydration can result from using diuretics, or substances like alcohol or certain medications. Dehydration typically causes thirst, dry mouth and lips, nausea, fatigue, and lightheadedness, as well as a darkening of the urine color or a decrease in urination. A loss of as little as 2% of our body weight due to water losses can lead to irritability, difficulty concentrating, and headaches.

    Some groups like children and the elderly are at increased risk of dehydration. Compared to adults, children have lower body stores of water to begin with, and they also have a higher surface area to body mass ratio, so they end up losing more water through their skin. And children’s thirst sensors are not fully developed, so they are less inclined to drink water. Further, kids often depend on caregivers to provide fluids, which makes it challenging for them to meet their hydration needs. Children between the ages of 4 and 13 need about 1.7 liters of fluid daily, and research shows that well hydrated children have improved concentration and ability to focus. In fact, studies have shown drinking more water can boost children’s school performance. The elderly are also more susceptible to dehydration. Like children, they also have a decreased thirst sensation, may be taking medications that alter their hydration status, and oftentimes they have chronic diseases that affect their kidneys’ ability to maintain a healthy water balance.

    There are some circumstances in which a person, regardless of age, might become dehydrated — like travelling on an airplane or during extended strenuous physical activity. Air inside airplanes is drier than the air on the ground, so a flight over 2 hours can lead to dehydration. Drinking fluids before and during a flight can help prevent that. Playing sports or doing heavy physical labor — both of which make us sweat more — can lead to a loss of both water and electrolytes. In the majority of situations, water and electrolyte-containing foods can help replace the losses, but replenishing with an electrolyte-containing drink may help avoid dehydration in longer-duration activities like running a marathon or working outside in hot weather.

    Alright, as a quick recap: Most of our water intake comes from fluids, but we can also get some of it from food. We lose water in a number of ways – such as sweating, breathing, urinating or defecating – and when those losses are greater than our intake, dehydration can settle in. The first signs of dehydration are a sensation of thirst, dry mouth and lips, dark urine, difficulty concentrating, and irritability. The best way to avoid becoming dehydrated is to monitor your urine color and drink fluids before you get thirsty – once the thirst sensation is present, dehydration is already underway. Generally, drinking around 2 liters of water per day is recommended – with more likely needed more for males, people in dry or hot environments, people who exercise, or people who perform heavy physical labor.

Video credit: Osmosis (

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