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Water Water in the body contains numerous dissolved minerals, called electrolytes that are kept in constant balance. To live, each cell must have just the right mix of water and electrolytes. Despite a continuous flow of molecules between intracellular fluid, extracellular fluid, and the outside environment, the body maintains its electrolyte balances through the intake and excretion of water and the movement of ions. Humans can survive for weeks without food, but can live only a few days without water. The body has no capacity to store “spare” water, so it must quickly replace lost fluid. Overall, water makes up between 50 and 75 percent of body weight. The main constituent of the body is water. Adult males have more lean tissue and less fat than adult females. An adult male is approximately 62% water, 17% protein, 15% fat, and 6% minerals and glycogen. Leaner people have proportionately more water, because muscle tissue is nearly three-fourths water by weight, while adipose tissue is only about 10 percent water. Water has a strong surface tension, high heat capacity and ability to dissolve many substances result from hydrogen bonds between a hydrogen atom of one water molecule and the oxygen atom of another water molecule. 1 Functions of Water Water moves nutrients and wastes between cells and organs. In the intestines, water solubilizes and moves nutrients to your cells and tissues, and it also carries waste out of your body in urine. Heat Capacity The heat capacity of a substance is the amount of energy required to raise its temperature 10C. Raising the temperature of a substance with a high heat capacity requires more energy than raising the temperature of a substance with a low one. Water, for instance, has about three times the heat capacity of iron. Warming or cooling a substance with a high heat capacity requires a relatively large amount of energy. Because of water’s high heat capacity, it takes a lot of heat to change the temperature of the body. Cooling Ability A rise in body temperature, whether due to exercise, environmental conditions, or illness, triggers the body’s cooling system. If you get too warm, blood vessels dilate and you begin to sweat. The perspiration evaporates on the skin, thereby cooling your body. Participation in Metabolism Nearly all the chemical reactions of metabolism involve water. Water is the solvent for many biologically essential molecules (e.g., glucose, vitamins, minerals, and amino acids), and it is a product or reactant in many biochemical reactions. pH Balance Water is also an essential component of the body’s mechanisms to maintain pH (acid-base) balance in the narrow range necessary for life. One of the major buffer systems involves carbonic acid and bicarbonate. Carbonic acid forms when dissolved carbon dioxide reacts with water (CO2 + H2O H2CO3). Carbonic acid can then dissociate to form H+ and HCO3; (bicarbonate). The resulting H+ helps increase acidity, lowering pH. 2 CO2 + H2O H2CO3 H2CO3 H+ + HCO3 Body Fluids Water is the major component of all body fluids. Water also has a protective, mechanical and cleansing functions, for example; amniotic fluid provides a gentle cushion that protects the fetus, synovial fluid allows joints to move smoothly, tears cleanse the eyes, and saliva moistens food and makes swallowing possible. Electrolytes and Water The body precisely controls and balances the concentration of electrolytes dissolved in its watery fluids. When salts, such as sodium chloride, dissolve in water they come apart and form free ions, which are positively (e.g., Na +) and negatively (e.g., Cl-) charged particles. 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, and the main negatively charged ions (anions) are chloride and phosphate. There are two major fluid compartments in the body. About two-thirds of body water is in intracellular fluid and one-third is in extracellular fluid. The major components of extracellular fluid are interstitial fluid (the fluid between cells) and blood plasma (the fluid portion of blood). Sodium is the main cation in extracellular fluid, whereas potassium is the predominant cation of intracellular fluid. To maintain the balance of sodium and potassium, all cell membranes incorporate sodium-potassium pumps that actively pump sodium out of the cell while allowing potassium back in. If solutes are more concentrated on one side of a semipermeable membrane (through which water, but not solutes, can pass 3 easily), water flows to the side of higher concentration until the concentrations on both sides are the same. This movement is called osmosis; osmotic pressure is the force that causes water to flow across a membrane to the side with a higher concentration of ions. Electrolyte: Substances that dissociate into charged particles (ions) when dissolved in water or other solvents and thus become capable of conducting an electrical current. Hydrogen bond: Non-covalent bond between hydrogen and an atom, usually oxygen, in another molecule. Heat capacity: The amount of energy required to raise the temperature of a substance 1OC. salts Compounds that result from the replacement of the hydrogen of an acid with a metal or a group that acts like a metal. ion An atom or group of atoms with an electrical charge resulting from the loss or gain of one or more electrons. cation Ion that carries a positive charge anion Ion that carries a negative charge. osmosis The movement of a solvent, such as water, through a semipermeable membrane from the low-solute to the high-solute solution until the concentrations on both sides of the membrane are equal. osmotic pressure The pressure exerted on a semipermeable membrane by a solvent, usually water, moving from the side of low-solute to the side of highsolute concentration. Plasma The fluid portion of the blood that contains blood cells and other components. sodium-potassium pump Mechanism that pumps sodium ions out of a cell, allowing potassium ions to enter the cell. 4 Conclusion: Water is the most essential nutrient. Water’s functions in the body include temperature regulation, metabolism, acid-base regulation and protection. The balance of body fluids and the amount of electrolytes dissolved in the body water are controlled precisely. Potassium is the main intracellular cation, and sodium is the main extracellular cation. Water intake recommendations Each individual needs a different amount of water depending on body size and composition, activity level, and the temperature and humidity of the environment. An “average” adult who expends 2,400 kilocalories daily loses approximately 2.4 liters (about 10 cups) of water each day. Because we cannot store water, the average adult needs to replace at least 2.4 liters of water a day. Beverages, foods, and water produced by metabolic reactions supply the water that the body needs. You should drink 8 to 10 glasses of water every day. Metabolism typically provides about 400 milliliters to 500 milliliters, so the remainder (1,500 ml to 2,500 ml) must come from beverages and foods. Water Excretion Our bodies continuously lose water through various routes: In the lungs, water evaporates and exits in exhaled air. Water also excreted through the skin by evaporation and perspiration. In the GI tract feces carry water out of the body. The kidneys excrete water in urine. Typically daily water intake and output: Intake: a) Food 700-1000ml. b) Drink 550-1500ml. c) Metabolic 200-300ml. 5 Output: a) Kidneys (urine) 500-1400ml. b) Skin 450-900 ml. c) Lungs 350ml. d) Feces 150 ml. * People with diarrhea may lose several liters of water per day in feces. Water plays a critical role in the elimination of wastes. Urea, a breakdown product of protein metabolism, is a major component of urine. If we overconsume protein and salt, the kidneys have to work harder to eliminate excess urea and sodium from the body. The more protein and sodium you consume, the more fluid you need to consume and the more urine you are likely to produce. Conclusion: Water intake comes from a combination of foods, fluids, and water produced in normal metabolism. The main method of water excretion is in urine. In addition, fluid is lost through the skin and lungs, and with feces. Losses are higher when a person perspires heavily or is ill. Water is critical in eliminating the body’s waste products. Water Balance Our bodies maintain water balance by mechanisms that control water intake (e.g., thirst) and water excretion. The body works to closely regulate total body water. Regulation of Fluid Excretion Our kidneys adjust the amount and concentration of urine in response to the body’s hydration status. The kidneys can excrete a small volume of concentrated urine or a large volume of dilute urine while maintaining a relatively constant excretion of solutes such as sodium and potassium. This ability to 6 regulate water excretion without major changes in solute excretion is an important survival mechanism, especially when water is in short supply. When water intake is low, the kidneys conserve water. While continuing to excrete solutes, they reabsorb water, thus decreasing urine volume and concentrating the urine. When the body has an excess of water, the kidneys form and excrete a large volume of dilute urine. How do the kidneys know when to conserve water? Osmoreceptors, special cells in the hypothalamus of the brain, are exquisitely sensitive to very small increases in extracellular sodium concentration and thus sense the body’s need for water. If the sodium concentration rises, these receptors signal the pituitary gland to release antiduretic hormone (ADH). ADH decreases water loss by causing the kidneys to reabsorb water rather than excrete it in the urine. ADH signals the kidney to conserve water. Regulation of Blood Volume and Pressure The kidneys themselves have sensors that detect falling blood pressure. In response, the kidneys release renin, an enzyme that splits off a small protein, angiotensin I, from the blood protein angiotensinogen. Within seconds, enzymes in the small blood vessels of the lungs convert nearly all angiotensin I to angiotensin II. While angiotensin II is a powerful vasoconstrictor, it also acts directly on the kidneys to decrease excretion of sodium and water. In addition, this protein causes the release of aldosterone, a hormone from the adrenal glands. Aldosterone also causes the kidneys to retain sodium, and since water follows sodium, water retention increases as well. All of these processes work to restore blood pressure and volume. The ability of angiotensin II to act as a vasoconstrictor and increase blood pressure is a life-saving measure. This acute short-term action helps compensate for a severe loss of blood, such as occurs during a hemorrhage. Angiotensin II’s effect on the kidney increases extracellular fluid volume and arterial blood 7 pressure over a period of hours or days. One the most important roles of the renin-angiotensin system is its response to dietary sodium. It allows a person to consume either very small or very large amounts of sodium without causing major changes in extracellular fluid volume or blood pressure. Since water follows sodium, increased sodium intake increases extracellular fluid volume and blood pressure. This reduces the secretion of renin and production of angiotensin, leading to decreased retention of sodium and water by the kidneys. The resulting excretion of water and sodium returns extracellular volume and blood pressure to normal. A low sodium intake triggers the opposite effects. insensible water loss: The continual loss of body water by evaporation from the respiratory tract and diffusion through the skin. Osmoreceptors: Neurons in the hypothalamus that detect changes in the fluid concentration in blood and regulate the release of antiduretic hormone. Antiduretic hormone (ADH): A peptide hormone secreted by the pituitary gland, it increases blood pressure and prevents fluid excretion by the kidneys. Osmolarity: The concentration of dissolved particles (e.g., electrolytes) in a solution expressed per unit of volume. Renin: An enzyme produced by the kidney that affects blood pressure by catalyzing the conversion of angiotensinogen to angiotensin I. Angiotensinogen: A circulating protein produced by the liver from which angiotensin I is cleaved by the action of renin. Angiotensin I: A 10 amino-acid peptide that is a precursor of angiotensin II. Angiotensin II: In the lungs, the 8-amino-acid peptide angiotensin II is formed from angiotensin I. Angiotensin II is a powerful vasoconstrictor that rapidly raises blood pressure. Aldosterone: A steroid hormone secreted from the adrenal glands that acts on the kidneys to regulate electrolyte and water balance. It raises blood pressure by promoting retention of sodium (and thus water) and excretion of potassium. 8 Thirst Thirst is our most important stimulus for drinking. Why do we become thirsty? The four major stimuli for thirst are: 1. Increased osmolarity of the fluid surrounding the osmoreceptors in the hypothalamus. 2. Reduced blood volume and blood pressure. 3. Increased angiotensin II. 4. Dryness of the mouth and mucous membranes lining the esophagus. * Older people and infants are particularly vulnerable to dehydration. The sensitivity of the thirst response declines with age, putting the elderly at high risk. Infants need to take in a large amount of water relative to their size because a large proportion of their body weight is water. Breast milk or infant formula provides an appropriate amount of fluid for infants. People who care for children and elders must remember to give them fluid often. Conclusion: The body has mechanisms that balance water among compartments and regulate total body water. Antidiuretic hormone (ADH) stimulates water reabsorption in the kidneys, while aldosterone stimulates the kidneys to reabsorb sodium (and thus water). Thirst is not a reliable indicator to avoid dehydration when fluid losses are high, such as during hot weather or heavy exercise. 9 Substances That Affect Fluid Balance 1) Alcohol Alcohol is a diuretic—a drug that increases fluid loss through increased urination. Alcohol increases fluid loss by suppressing ADH production, and excessive alcohol consumption can cause dehydration. Symptoms of mild to moderate dehydration include thirst, weakness, dryness of mucous membranes and dizziness. 2) Caffeine Caffeine is a strong diuretic. Like alcohol, it inhibits ADH activity. 3) Diuretic Medications Diuretics are some of the most common medications. They can help lower blood pressure, decrease high intracranial pressure, or reduce excess pressure in the eye. On the other hand, diuretics can significantly disrupt sodium and potassium balance. Dehydration Dehydration is a major killer worldwide—infants and the frail elderly are especially vulnerable. Gastrointestinal infections are primarily responsible. These infections cause diarrhea and prolonged vomiting, leading to excessive water loss. Unless treated rapidly, a person who loses an amount of water equal to 20 percent of body weight is likely to become comatose and die. Burns also can cause deadly dehydration. Extensively damaged skin cannot protect the body and prevent excessive fluid loss. Dehydration diminishes physical and mental performance. Early signs of dehydration include fatigue, dry mouth, headache, and dark urine with a strong odor Change in urine color reflects the body’s attempt to conserve water by increasing water reabsorption in the kidney. 10 Water consumption is the primary treatment for dehydration. Oral-rehydration solutions also can be used; typically these consist of simple ingredients including clean water, sugar, and table salt. Oral rehydration may be sufficient for mild dehydration, but intravenous fluids and hospitalization may be necessary for moderate to severe dehydration. Diarrhea and prolonged vomiting, which cause heavy fluid and electrolyte losses, can be fatal unless the person is rapidly rehydrated with electrolyte solutions. Conclusion: Alcohol, caffeine, and diuretic medications increase urinary fluid losses. Chronic ingestion of these substances may contribute to dehydration. Dehydration occurs when fluid loss exceeds fluid intake; it is a potential consequence of gastrointestinal disease, burns, and heavy sweating. Treatment involves replacing fluids, along with electrolytes if the condition is severe. Minerals Minerals are not changed during digestion or when the body uses them. Unlike many vitamins, minerals are not destroyed by heat, light, or alkalinity. Minerals play many essential roles in the body. Some minerals, such as magnesium, participate in the catalytic activity of enzymes. Others serve a structural function; for example, calcium and phosphorus are among the minerals that make our bones hard. Minerals are categorized as major or trace minerals, based on the amount needed in the diet and the amount of the mineral in the body. This classification of minerals is unrelated to the mineral’s biological importance. For example, iron is a trace mineral but it plays a critical role in many major metabolic reactions. Drinking water can sometimes be a significant source of several minerals such as sodium, magnesium, and fluoride. 11 Bioavailability of Minerals A variety of factors affect the bioavailability (the amount available to the body) of minerals from foods. The body absorbs many minerals in proportion to its needs. For example, a calcium-deficient person more readily absorbs calcium than does a person with normal calcium levels. Overloading one mineral by taking supplements can affect the absorption of other minerals. Minerals like calcium, iron, and magnesium (which all have similar chemical properties, including a 2+ charge) compete for absorption. Fiber content of food also can affect mineral bioavailability. High-fiber diets can reduce the absorption of iron, calcium, zinc, and magnesium. Phytate in grain fibers binds minerals, sequestering them and carrying them out of the intestine unabsorbed. Oxalate, found for example in spinach, binds calcium; consequently decrease the absorption of calcium. Major minerals Major minerals are required in the diet and present in the body in large amounts compared to trace minerals. Trace minerals Minerals required in the diet and present in the body in very small quantities, less than 100 mg each. Phytate; A phosphorus-containing compound in the outer husks of cereal grains that binds with minerals and inhibits their absorption. Oxalate: An organic acid in some leafy green vegetables, such as spinach, that binds to calcium to form calcium oxalate, an insoluble compound the body cannot absorb. Conclusion: Minerals are essential inorganic elements. Those that we need and store in larger amounts are called major minerals, and those we need in very small quantities are the trace minerals. A wide variety of foods contain minerals. Physiological needs, competition with minerals, and fiber content of food all affect mineral bioavailability. 12 Functions of Sodium Sodium is the major cation in extracellular fluid and a critical electrolyte in the regulation of body fluids. It acts in concert with potassium, the major cation in intracellular fluid, and chloride, the major anion in extracellular fluid, to maintain proper body water distribution and blood pressure. Nerve transmission and muscle function require sodium. Sodium also helps control the body’s acidity, and aids the absorption of some nutrients such as glucose. 13