Water + Electrolytes
March 5, 2015
Water and Life
When people talk about the possibility of
life in space, the conversation often
focuses on planets or moons where liquid
water exists in some form.
Why is water considered to be so
essential for life as we know it?
What roles does water play in
keeping us alive?
Roles of Water
Chemical Reactions: The chemical
reactions that run our body can only
happen in water! The shape and function of
proteins and DNA depend on water.
Getting Rid of Waste: To lose excess salt,
toxins, and other water-soluble waste, we
need water to make urine.
Taking in Nutrients: Without adequate
water, we can’t absorb dietary nutrients.
Temperature Regulation: By evaporating
sweat off our skin, we can carry away heat
and lower body temperature.
Why Water?
• Many features of water make
it well-suited for life.
• Liquid at room temperature,
and can absorb/release heat
well, because of hydrogen
bonds.
• Dissolves many compounds
because it has a positive and
negative side (polarity).
• Needed for many chemical
reactions that make/break
polymers, including protein
and DNA.
In the diagram above,
oxygen atoms are red and
hydrogen atoms are white.
The partially negative
oxygen of one H2O molecule
is attracted to the partially
positive hydrogen of another
molecule. This is a hydrogen
bond (dotted line).
Water-Soluble Substances
• Some substances that
dissolve in water, such as
table salt (sodium
chloride, NaCl) split into
positively and negatively
charged ions. These are
called electrolytes.
• Positive = cation
• Negative = anion
• Other molecules that
dissolve in water do
not break up, but
instead stick to a
“shell” of water
molecules.
8-12 glasses?
You may have heard people say that we need to “drink 8 to 12
cups of water a day.”
Do you actually drink this much? Is it really necessary for the
average person to drink 8 to 12 cups of water each day?
How Our Body Loses Water
4-5 cups/day lost via urine
1.5-2 cups/day lost via
sweat
1 cup/day lost via feces
1 cup/day via lungs
Add an extra 11.5 cups for
every 15
minutes of
heavy exercise
Add 2 – 3
more cups if
breastfeeding
Dehydration
Human beings need to take in 8-12 cups
of water a day. Some of this water is
from food.
Insufficient water intake or water loss
via vomiting/diarrhea can lead to
dehydration and electrolyte loss.
Signs of dehydration include low blood
volume/pressure, and dry mouth and
skin. Untreated dehydration can be
life-threatening.
Children and the elderly are at
especially high risk! Children
dehydrate quickly due to their small
volume, and the brains of older
people are less able to recognize low
water content in blood. The chart at
right shows some common signs of
dehydration in children.
Sports Drinks
Many sports drinks offer a good balance of electrolytes and
water…
However, some also include about half as much sugar as soda.
While some sugar can be helpful when exercising, and the
taste can keep people drinking and rehydrating, excessive
sugar is dehydrating. Combo “sports/energy drinks” also
contain caffeine, a diuretic (causes water loss).
A 2011 paper by the American Academy
of Pediatrics states that, especially
for children and adolescents, “water
is generally the appropriate first
choice for hydration before, during,
and after most exercise regimens.”
Most people get sufficient
electrolytes in their diet!
Where Water Comes From
The tapwater in Alameda County mostly comes from
local watersheds. In general, drinking water comes
from:
• Watersheds: areas that catch snowmelt and rainfall
• Groundwater: Underground water trapped in porous
rock or aquifers. Sometimes wells up in springs.
• Rain, desalinated saltwater, etc.
The Hetch Hetchy Reservoir, shown at right,
supplies high-quality tapwater to the Bay Area
that requires little artificial treatment. The
flooding of this valley for use as a water
supply, similar to the use of the Owens Valley
in Southern California, remains a
controversial environmental decision.
Bottled Water Debate
Do you drink bottled water regularly? What advantages
might there be to drinking bottled water? What
disadvantages might there be to drinking bottled
water? Do you think people should increase or
decrease their bottled water consumption?
Bottled Water
Bottled water may come from springs and snowmelt, or it may
simply be purified tapwater.
To be labeled as mineral water, bottled water must legally contain
more than 250 parts per million of dissolved minerals.
While water purification may remove some contaminants, it does
not fluoridate water, and accidental bottled water contamination
has occurred.
Given the relatively safe water supply in our area, it is cheaper and
equally healthy to buy a thermos and fill it with tapwater.
Americans buy tens of billions
of disposable one-use water
bottles each year, most of
which end up in landfills.
Making these bottles requires
millions of barrels of oil!
A Few Important Electrolytes
Sodium (Na+): Needed to maintain water balance and send
message in nervous system. Also increases intake of other
nutrients. However, excessive sodium intake increases risk
of hypertension (high blood pressure). Found in many
processed foods, canned foods, snacks, meat, etc. 90% of
Americans eat too much sodium each day.
Chloride (Cl-): Also regulates charge, water balance. Needed
to make stomach acid, and found with sodium.
Potassium (K+): Helps body move, eliminate excess sodium;
needed for muscle contraction and heart health. In
moderation, potassium in the diet from sources such as
bananas and potatoes can help lower blood pressure. Most
Americans don’t get enough potassium!
Diffusion
When a substance is unequally distributed in water, it will
tend to diffuse until its concentration equalizes. Example: a
drop of food coloring spreads through a glass of water.
Channel proteins in cell membranes allow electrolytes to
diffuse into and out of the cell until equilibrium.
Diffusion does not require the cell to use up ATP (turn it into
ADP) or “spend” energy – it happens on its own!
Osmosis
Osmosis is a special case where water (or a different solvent,
if you’re taking chemistry) diffuses across a semipermeable
membrane.
How do we know which way the water goes? This can get
tricky, but a good general rule is: The water goes from the
side with less concentrated solute to the side with more
concentrated solute.
Or, in shorthand: “water chases salt.”
Electrolyte Pumps
Although diffusion can move an electrolyte from an area of high
concentration to an area of low concentration, what happens when you
need to move it the other way, against its concentration gradient?
Pump proteins actively move electrolytes into or out of cells, and can
concentrate them inside or outside the cell, even if they would normally
diffuse in the opposite direction.
Example: The sodium/potassium (Na+/K+) pump trades 3 sodium ions for
2 potassium ions, using ATP for energy.
Test Your Understanding
In the picture at left, the red box
represents the bloodstream, and the blue
circle represents a cell. The grey dots are
dissolved salt
Would it be necessary to use energy
from ATP to pump the solute out of the
cell?
Would it be necessary to use energy
from ATP to pump the solute into the
cell?
If water crosses the cell membrane by
osmosis, would you expect water to go
out of the cell or into the cell? Why?
The Kidneys
The body needs to eliminate wastes
and excess electrolytes from the
body without losing too much
water.
The kidneys filter the blood, then
reabsorb needed substances and
water and release urine waste to
the bladder via the ureter. If
there is a lot of waste in the
blood, more water may be lost to
eliminate it.
The kidneys is a main regulator of
blood pressure. However, the
delicate filters of the kidneys can
be damaged by high blood
pressure.
The nephrons of the kidney consist of:
1) A filter (the glomerulus inside Bowman’s capsule) that separates
the solid parts of blood (cells) from the liquid parts.
2) A long, twisting tubule that helps the kidney reabsorb substances
into the blood by diffusion and secrete unnecessary ones via pumps.
3) A duct leading to the ureter that carries out urine.
The nephron uses
osmosis, diffusion,
and ATP-powered
pumps to recover
salt and water from
the filtrate.
Near the bottom of
the loop of Henle,
sodium is pumped
into the space
around the nephron.
Because “water chases salt,” the water in the filtrate leaves the
nephron, and can then be taken back into the bloodstream.
The kidney can regulate blood pressure by controlling how much
water we keep or lose from the filtrate. Retaining water raises blood
pressure.
Image source: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/N/nephron.png
Neurons
The nervous system carries
messages from the brain to the
rest of your body, and sensory
messages back to the brain.
Cells called neurons receive
messages on branching
dendrites, then pass them down
axons to other cells.
A single neuron can run all the
way from the tip of your toe to
your spine!
The movement of messages down
neurons requires electrolytes
such as sodium (Na+) and
potassium (K+).
The Action Potential
The action potential passes messages
down the axon of a neuron.
It starts when sodium from outside
the neuron enters through a
channel, making the inside of the
cell slightly positive.
This makes more sodium channels
open further down the axon,
“passing on” the message by
flooding the cell with sodium.
The sodium channels then gradually
close and potassium channels help
reset the cell’s charge.
The Na+/K+ pump then “resets” the
sodium and potassium levels.
Electron Transport Chain