CHAPTER 5
A Walking, Talking Water Balloon
PART 1
Under Pressure
The water within our bodies is sourced almost exclusively by the liquids we consume. Once ingested, water circulates in the bloodstream and is rationed to the body’s tissues in an egalitarian system. Every organ requires water, whether directly or indirectly, though none receives more than the fair share needed for healthy development and functioning. READ MORE
Vital Pressure
As in an actual water balloon, a closed system saturated with liquid will be under pressure. All of the organs and systems in our body rely either directly or indirectly on water-based pressure for their form and function. In fact, the four primary vital signs — the indications that we are indeed alive — are all pressure readings:
- Blood pressure: Is there enough pressure within vessels to send blood and oxygen to the body’s organs? Is there too much pressure on arteries when the heart contracts or rests?
- Pulse: Is the heart beating fast enough to maintain pressure? Is it racing to recover pressure?
- Respiratory rate: Is enough pressure being exerted on the thoracic cavity to draw and exhale air?
- Temperature: Is blood and oxygen being circulated to warm the body adequately?
Pressure: The Smaller Picture
At the molecular level, pressure is partly responsible for exchanging water between blood and tissue. Though water molecules don’t exit large blood vessels, higher blood pressure in the narrow passageways of capillaries expels the molecules into neighboring tissues. Capillaries are the smallest blood vessels; and you may remember that water pressure increases when a constant volume is passed through a narrower channel, like water flowing from a drainpipe versus spraying from a hose. That’s the pressure that strains water from the capillaries.
The exchange of water between blood and tissue is crucial to life, because (a) cells need the oxygen and solutes carried by water; and (b) if water couldn’t get into cells they would dehydrate, resulting in cell death. LESS
PART 2
Hydrostasis
About a third of the water in our bodies exists within cells (intracellular) and the other two thirds is outside the cells (extracellular, as in the blood, in the digestive system, in other organs, and between individual cells). In order for intracellular and extracellular fluids to be balanced — a state known as hydrostasis — the concentration of solutes within cells has to match the concentration of solutes outside. If a cell has a low concentration, water will cross through the cell’s semi-permeable membrane in order to match the concentration outside the cell. Water achieves this transfer in and out of tissue cells, and from tissue into the bloodstream, by the process of osmosis. READ MORE
It’s not simple, but the concept is a bit like water seeking its own level in a pot; it will travel from higher to lower until it’s level. Similarly, water will travel in and out of cells until solute concentrations are even. When concentration is higher inside a cell, water moves into the cell via osmosis to make things level. The reverse is true as well: if the environment outside a cell has a higher concentration, water will move via osmosis into the extracellular environment. LESS
PART 3
Water Babies
We begin life completely submerged. While in utero, a baby is protected 360 degrees around by fluid under pressure, which pillows her against bumps from the outside world and softens jolts and jostles. The life-giving environment of the womb is almost entirely water: the chemical composition of amniotic fluid is 98% to 99% water, varying slightly throughout stages of gestation. READ MORE
The baby not only floats in the sphere of protective fluid, but “breathes” it; every three hours, amniotic fluid is circulated as the baby swallows the fluid and releases it through urine. Until a child is born, all the oxygen and nutrients she needs come through the umbilical cord and placenta.
Photo courtesy of Verena Vonasek
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