I would love to shrink down to microscopic size and greet the cells in my own body face-to-face, like something out of the movie Fantastic Voyage.  Of course, I won’t “hold my breath” waiting for such an opportunity to come along.  But even if there were some way to do this, I would quickly drown without SCUBA equipment!  This underscores the fact that our internal environment is an aquatic one.  Our living cells depend on the special properties of water, both to exchange gases, nutrients and wastes, and to facilitate the chemical reactions necessary for life.  Evolution has not yet found a way to allow life to thrive without internal water (a few creatures are capable of surviving complete desiccation, but they do so in suspended animation).

Indeed, a cell out of water is like a fish out of water.  This means that the dry skin surface of the human body is all dead tissue.  It also means that all our internal tissues are moist.  Even bone tissue, despite being largely composed of calcium phosphate, is permeated with water and houses living bone cells that live comfortably within tiny water-filled “lagoons” known as lacunae.

Some tissues contain more water than others.  Blood is the tissue with the greatest water content, but all other tissues have, between their cells, a fluid known as tissue fluid, or interstitial fluid.  The tissue depicted above might represent areolar connective tissue, a ubiquitous tissue found in many parts of the body, which contains a large amount of interstitial fluid. Because it’s found adjacent to all blood vessels, it plays an important “middleman” role allowing molecules to make their way between the blood and nearby tissues.  The two typical cells shown here would be fibroblasts, which produce the protein fibers giving structure to the tissue (collagen fibers are shown in gray, and reticular fibers in magenta); these in turn provide attachment points for the cells, as they go about living their aquatic lives. Unfortunately, the interstitial fluid is also a benign environment for bacteria (shown in pink) — but that’s a story for another day.