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 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 (CO 2 + H 2 0  H 2 CO 3 ). Carbonic acid can dissociate to form H + and HCO 3 - (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 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 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.