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Water
Water
• The human body can survive for weeks
without food, but only days without water.
• Water makes up between 45 and 75% of a
person’s weight.
Water
• Water has strong surface tension, high heat
capacity, and the ability to dissolve substances
due to hydrogen bonds.
• The dissolved minerals in water are called
electrolytes.
Definitions
• Hydrogen bonds - Noncovalent bonds
between hydrogen and an atom, usually
oxygen, in another molecule.
• Electrolytes - Substances that dissociate into
charged particles (ions) when dissolved in
water or other solvents and thus become
capable of conducting an electrical current.
Surface Tension
Functions of Water
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Heat Capacity
Cooling Ability
Participation in metabolism
pH Balance
Body Fluids
Functions of Water
Heat Capacity
• Heat Capacity – The amount of energy
required to raise the temperature of a
substance 1°C.
• Water has a high heat capacity; therefore, it
takes a lot of heat to change the temperature
of the body. Water dampens the effects of
extreme environmental temperatures on the
conditions in cells.
Cooling Ability
• If the body temperature gets too high, the
blood vessels dilate and we begin to sweat.
• When perspiration on the skin evaporates,
heat is transferred with the water, thus
cooling the body.
Participation in Metabolism
• Water is the solvent for many biologically
essential molecules.
• Water is either a product or reactant in many
biochemical reactions.
pH Balance
• Carbonic acid forms dissolved carbon dioxide
reacts with water (CO2 + H20  H2CO3).
• Carbonic acid can dissociate to form H+ and
HCO3- (bicarbonate)
• H+ increases acidity (lowers pH).
Body Fluids
• Water is the major component of all body
fluids.
• Functions of fluids:
– Shock absorption (i.e. IVD)
– Lubrication (i.e. synovial fluid)
– Cleansing (i.e. tears, saliva)
– Protection (i.e. amniotic fluid)
– Transportation (i.e. blood and lymph)
Electrolytes and Water
• The body controls and balances the
concentration of electrolytes.
• Salts dissolve in water and form free ions.
Electrolytes and Water
• In an electrolyte solution, the number of
positive charges always equals the number of
negative charges.
• The main positively charged ions (cations) in
the body are sodium and potassium.
• The main negatively charged ions (anions) in
the body are chloride and phosphate.
Fluid Compartments
• Intracellular fluid
– Two-thirds of body water
– Potassium is the main cation
• Extracellular fluid
– One-third of body water
– Interstitial fluid and blood plasma
– Sodium is the main cation
Major Fluid Compartments
Movement Across Cell Membranes
• Sodium-potassium pumps actively pump
sodium out of the cell, while allowing
potassium into the cell.
• Water flows across the semipermeable
membrane (water passes easily, but not
solutes).
• Movement of water across the membrane is
called osmosis.
Factors Influencing Diffusion
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Steepness of the concentration gradient.
Temperature.
Mass of the diffusing substance.
Surface area.
Diffusion distance.
Osmosis
• The net movement of a solvent (water) across
a selectively permeable membrane,
• Water moves from an area of higher water
concentration to an area of lower water
concentration.
Osmotic Pressure
• A solution containing solute particles that
cannot cross the membrane exerts a force
called osmotic pressure.
• The greater the number of solute particles the
greater the osmotic pressure.
Water Intake
• How much?
– Depends on size, body composition, activity level,
temperature and humidity of environment.
• Adequate Intake (AI) for total water is 3.7
liters per day for men, 2.7 liters per day for
women. Higher during pregnancy (3.0 liters
per day) and lactation (3.8 liters per day).
Water Intake
• Water comes from drinking water, other
beverages, and water in foods.
• A small amount (250 to 350 milliliters per day)
is produced from metabolic reactions.
Water Excretion
• Exhaled into air from lungs.
• Departs through skin by evaporation and
perspiration.
• Loss in feces.
• Kidneys excrete water in urine.
Insensible Water Loss
• The continual loss of body water by
evaporation from the respiratory tract and
diffusion through the skin.
Water Balance
• Regulation of Water Balance
– Hormonal Effects
• Antidiuretic Hormone (ADH)
• Aldosterone
– Thirst
– Substances that affect water
balance
• Alcohol, caffeine, and diuretic
medications
Regulation of Fluid Excretion
• The kidneys adjust the amount and
concentration of urine in response to the
body’s hydration status.
• When water intake is low, the kidney’s
conserve water.
Regulation of Fluid Excretion
• When water intake is high, the kidney’s
excrete water.
• Osmoreceptors (in the hypothalamus)
measure extracellular sodium concentration
to determine the body’s hydration needs.
Hormonal Effects
• Antidiuretic hormone (ADH) (released from
pituitary) – decreases water loss by causing the
kidneys to reabsorb water rather than secrete it
in the urine.
– In higher concentratons ADH is also a vasoconstrictor
(also called vasopressin).
– Nicotine and morphine stimulate release of ADH
– Caffeine and alcohol inhibit release of ADH
– Severe blood loss and vomiting release ADH
– Plasma osmolarity affects release of ADH
Hormonal Effects
• Aldosterone is a hormone from the adrenal
glands that causes the kidneys to retain
sodium (and thereby water).
Thirst
• Stimuli for thirst:
• Drinking fluids temporarily alleviates thirst.
• Heavy activity can cause dehydration.
Anticipate water needs before thirst.
• Elderly are susceptible to dehydration beause
the thirst response declines with age.
• Infants have higher water needs.
Water Reabsorption in the GI Tract
• Many liters of fluid are secreted every day
through the salivary glands, stomach, small
intestine, pancreas, and gallbladder.
• The small intestine reabsorbs most of this
fluid.
• 90% of any remaining fluid is reabsorbed from
the large intestine.
Alcohol, Caffeine, and Medications
Affect Fluid Balance
• Alcohol and caffeine are diuretics. They
suppress ADH.
• Diuretic medications are often prescribed to
lower blood pressure or to decrease swelling
caused by fluid retention.
– These medications disrupt potassium and sodium
balance.
– Potassium supplements are often necessary.
Dehydration
• Too little water.
• Major killer worldwide.
• Gastrointestinal infections are primarily
responsible.
Dehydration
• Diarrhea and prolonged vomiting can cause
excessive water loss.
• Water loss equal to 20% of total body weight
is likely to cause someone to become
comatose and die.
• Burns can cause dehydration.
Effects of Dehydration
• Diminishes physical and mental performance.
• Early signs:
– Fatigue, dry mouth, headache, dark urine with
strong odor.
Effects of Progressive Dehydration
Treatment of Dehydration
• Oral rehydration solutions for mild
dehydration.
– Clean water, sugar, and table salt.
• Intravenous fluids and hospitalization may be
necessary for moderate to severe dehydration.
Water Intoxication
• Drinking temporarily relieves thirst so we
rarely drink to the point of overhydration and
dilution of body fluids.
• Replacement of fluid losses following intensive
or prolonged exercise with plain water (and no
electrolytes) can cause overhydration and
hyponatremia (low blood sodium).
• The maximum excretion rate by the kidneys is
0.7 to 1.0 liters per hour.
Water Intoxication
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Fraternity hazing.
Psychiatric disorders.
Excessive diets.
Untreated glandular disorders can cause
excessive water retention.
Definitions of Bottled Water Terms
• Mineral Water – must contain at least 250
parts per million (ppm) of dissolved minerals
and come from a geologically and physically
protected underground water source.
Definitions of Bottled Water Terms
• Purified Water – tap or ground water that has
been treated by distallation, deionization, or
reverse osmosis. May be labeled “distilled” if
produced by steam distillation.
• Spring Water – comes from an underground
formation from which water flows naturally to
the surface; it is collected either at the spring
or from a bore hole to the underground
formation.
Definitions of Bottled Water Terms
• Artesian Water – comes from tapping a
confined underground aquifer that is below
the natural water table.
• Ground Water – comes from a surface
saturated zone and is not under the direct
influence of surface water.
• Well Water – comes from a drilled hole that
taps the water of an aquifer, and is pumped to
the surface.