The role of water

This is a fairly comprehensive resource.  Think of it as a reference point for understanding the role of water.  Some of it is information you may not need, but I think it is better to have more (like a buffet where you can pick and choose!), rather than not enough information.

Where is water located in the body?  Water is either inside our body cells, or located in regions outside cells.  See the illustration on page 399, which shows a cell, and the locations of fluids inside and outside the cell.

Intracellular fluid (within each of our body's cells)= ~25 liters
Extracellular fluid (interstitial, between cells and surrounding cells; intravascular (in the blood vessels and the heart; in body fluids like tears, synovial fluid at movable joints, in cerebrospinal fluid, etc.) = ~15 liters

Water and Electrolytes: see pages 402-406:

bulletProtein channels allow free movement of water into and out of cells
bulletDecreased blood volume: water moves from cells into vessels
bulletIncreased blood volume: water moves from vessels into cells
bulletMajor minerals separate into ions in water, and are called electrolytes---electrolyte solutions are solutions of water and ions that can carry electric current
bullet+ and - electrolytes are usually distributed to ensure electrical neutrality in body cells, but the movement of electrolytes into and out of tissues enables the electrical "current" absolutely essential to nerve signals and muscle contractions (including heart muscle) to occur
bulletExamples of cations (atoms which have lost one or more electrons in chemical processes, and which now carry a positive static electrical charge): Na+, K+, Ca+, Mg+
bulletExamples of anions (atoms or clusters of atoms which have gaines extra electrons in chemical processes, and which then have a negative static electrical charge): Cl-, HCO3-, PO4-3, SO4-2

 Fluid distribution—>The role of osmotic pressure:

bulletOsmosis is the diffusion of water: click here for an illustration of osmosis and diffusion across a membrane. 
bulletOne of the more important factors determining fluid distribution among compartments is the osmotic pressure
bulletOsmotic pressure is proportional to the number of solute particles in a given volume of solution; when particle concentration goes up, osmosis (the movement of water) is in the direction of that concentration (to dilute out the particle concentration)
bulletCells control the amount of water entering and leaving the cell by pumping electrolytes in and out
bulletWater follows the flow of the solutes (electrolytes) by osmosis
bulletIntracellular water volume depends primarily on the concentration of K+ (potassium ions)and PO4- (phosphate ions)
bulletExtracellular water volume depends on the concentration of Na+ (sodium ions) and Cl- (chloride ions)
bullet(The cell works to keep Na+ on the outside of the cell, and K+ on the inside, and sodium-potassium pumps work to maintain this balance)

 Functions of water:

bulletSolubilizes compounds; water is the medium (the substance) in which nearly all compounds dissolve, and metabolic reactions take place; participates in a majority of reactions as a reactant or product (hydrolysis—>breaking apart molecules, dehydration reactions—>putting units together into larger molecules)
bulletTemperature regulation (which is assisted by water's high specific heat—>it can absorb a lot of energy without having its temperature increase—this assists us in maintaining a stable body temperature, and it pulls a lot of heat away in evaporative cooling from sweat, for example)
bulletWaste product removal (through sweat, urine, blood-delivery of carbon dioxide back to the lungs from cells)
bulletUrine volume is determined primarily by a person's fluid intake, and the amount of other substances that need to be excreted--for example, excess protein waste products (urea), sodium chloride (salt)
bulletIn diabetes, the glucose excreted in the urine pulls too much fluid from body tissues, and increases urine output---this is why excessive thirst is a symptom of diabetes
bulletWe produce about 1-2 liters day (normal); < 600 ml is too concentrated and may be an indication of dehydration or a kidney complication
bulletLubrication—>synovial (joint fluid); mucus, tears,
bulletShock absorption—>synovial fluid; cerebrospinal fluid (which also serves a nutritive function); water in tissues is also resistant to compression.
bulletWe need approximately 1- 1.5 ml of water per kcal consumed in our diet/day (infants and athletes need 1.5 ml/kcal).  So, in a 2000 kcal diet, we need about 2500 ml of water daily, or about 1/2 cup per 100 kcal.  This DOES NOT MEAN WE NEED TO DRINK 2.5 LITERS OF WATER!  It means all fluid intake, including soups, the water in fruits and vegetables, our morning coffee, the ice tea with lunch, etc. should be about 2500 ml of water.
bulletCheck out the table on page 399 of your text to see the percentages of water in selected types of foods.  There is a surprising amount of water even in foods like pizza!

Water in/water out:

bulletLarge and small intestines reabsorb almost all of the water that enters the digestive tract—>out of 10 L that enters the small intestine (from food, and from the secretions that are contributed through the various structures of the digestive tract, only 200 ml will be excreted in our fecal wastes from the large intestine---this is a remarkable rate of recovery of fluids!
bulletAnother remarkable level of recovery is what happens at our kidneys.  The kidneys filter all of our blood many times per day.  But we don't lose a lot of fluid because the kidneys reabsorb ~97% of water from blood (kidney filters toxins, and other substances from blood to excrete them in the urine, but retains most of the water that passes through the filtration process)
 

IN:

Food and drink = ~1500 to 1500 ml (though many may consume less or more depending on their dietary habits)
Metabolic water (produced by chemical reactions in cells)= ~350 ml to 500 ml

OUT:

Urine = ~500 to 1400 ml
Sweat = ~100 ml
Feces = ~200 ml
Insensible water loss (from evaporation from skin and lungs)= ~600ml as a ballpark estimate; but can range from 25% up to 50% of daily fluid loss; this depends on environmental temperatures, humidity, and level of physical activity

Increased need for water can be caused by:

bulletBeing very young (but this water should come through breast milk and/or formula for the most part--giving water to babies without a doctor's advice is potentially dangerous); and the very old, who often lose their ability to sense thirst, and easily become dehydrated (common cause of geriatric hospitalizations)
bulletDiabetes
bulletUse of diuretics, primarily used for treatment of high blood pressure
bulletTemperature and humidity can increase the need for hydration
bulletDry air indoors from furnaces; dry air from air conditioning
bulletAlcohol and caffeine are dehydrating/diuretics, respectively
bulletExercise increases the need for water (sports drinks for intensive activity that produces a lot of sweating, and/or goes on well over an hour)
bulletPregnancy and lactation
bulletSurgery, blood loss, burns

How we regulate fluid balance---the kidneys play a significant role in regulating the fluid balance of the body, and also play a key role in regulating electrolytes, blood volume and blood pressure---see the illustrations on pages 402-403:

bulletWhen we are dehydrated, the blood becomes concentrated and attracts water out of salivary glands. The result is a dry mouth—we feel thirsty. Notice that the feeling of being thirsty occurs after we are already somewhat dehydrated! (You will understand why, below).
bulletThe hypothalamus (an endocrine gland and nerve center in the middle of the brain) monitors the concentration of the blood--that is, it keeps monitoring to make sure we aren't getting dehydrated, and take steps when we are; it also monitors blood volume and gets blood pressure information via baroreceptors (pressure monitors) at various sites in major arteries of the body.
bulletWhat the hypothalamus does is cause the release of the hormone, ADH (antiduretic hormone) [which it has made, but which is stored in the pituitary gland in the brain]. ADH acts on the kidneys to reduce urine output, and acts on the hypothalamus itself to help you have the sensation of thirst so you will start drinking fluids. ADH is also a powerful vasoconstrictor (tightens/constricts blood vessels—so it is also knows as vasopressin), and can be released in response to either a drop in blood pressure or an increase in the concentration of the blood. ADH levels also go up after vomiting, to help correct the loss of fluid balance. ADH is more sensitive to changes in blood concentration than to blood pressure.  Drinking alcohol inhibits the release of ADH, so a person who has been drinking will also urinate frequently. This is why a hangover includes feeling terribly thirsty---the person is dehydrated!

 

bulletPressure receptors in the kidneys monitor fluid volume and blood pressure, and concentration of ions (salts) in the urine. Decreased fluid volume of the blood and decreased blood pressure can cause release of renin, an enzyme made  by the kidneys. This begins a cascade of reactions (see figure 12.3 on page 403):
bulletRenin, an enzyme made by the kidneys, activates angiotensinogen (a protein made by the liver, and circulating in the blood) —>converts it to angiotensin I
bulletAngiotensin I travels to the lungs where enzymes there convert it to angiotensin II (hey, the body never does anything simply or directly!)
bulletAngiotensin II traveling in the blood, triggers the release of the hormone aldosterone from the adrenal glands (located above the kidneys).
bulletAngiotensin II also acts as a vasoconstrictor, and can act to increase blood pressure (due to fluid/blood loss) in a short amount of time.
bulletAldosterone causes the kidneys to retain rather than excrete sodium (Na+ ions), and this has the effect of retaining water from the urine as well. (Where sodium goes, water follows). [Aldosterone can be secreted by the adrenal glands in response to falling Na+ levels,and rising K+ levels—interestingly, aldosterone increases the sensitivity of salt receptors on the tongue, making us seek out this flavor. The result, of course, is that our increase in salt ingestion increases our water intake, and our excretion of K+).
bulletThis system is more sensitive to changes in blood volume/blood pressure than to changes in blood concentration.

Dehydration & Water Intoxication:

For the progressive effects of dehydration, see table 12.1 on the bottom of page 398. Your body begins to respond with symptoms of dehydration at about 1-2% of body weight loss of fluid.  At a loss of about 20% of the body's weight in fluids, coma and death can result.

Water intoxication is rare. It is sometimes due to over-secretion of ADH, causing the body to retain too much fluid, diluting vital electrolytes. Some people with mental disorders may over hydrate, but the body is able to excrete 15-20 liters of urine/day, so it is difficult to do damage, though it does happen. Symptoms of water intoxication are blurred vision, confusion, cramping, convulsions, even death.