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