Chapter 21
Water, Electrolyte, and Acid-Base Balance
Teresa Schlager
1. Explain how water balance and electrolyte balance are interdependent.
Electrolytes are dissolved in the water of body fluids. Meaning anything that alters the concentrations of the electrolytes will alter the concentration of the water by adding solutes to it or by removing solutes from it. Anything that changes the concentration of the water will change the concentration of the electrolytes by concentrating or diluting them. Page 827
2. Name the body fluid compartments, and describe their locations.
Intracellular fluid compartment-The intracellular fluid compartment includes all of the water and electrolytes that a cell membranes enclose. Intracellular fluid is the fluid within cells. In an adult, it represents about 63 percent by volume of the total body water.
Extracellular fluid compartment-The extracellular fluid compartment includes all of the fluid outside the cells. To include tissue spaces (interstitial fluid), the blood vessels (plasma), and the lymphatic vessels (lymph). Epithelial layers separate a specialized fraction of the extracellular fuild from other extracellular fluids. It’s compartment constitute is about 37 percent of the total body water. Page 828
3. Explain how the fluids within these compartments differ in composition.
Extracellular fluids are similar, including relatively high concentrations of sodium (Na+), Calcium (Ca+2), Chloride (Cl-) and bicarbonate ions (HCO3-), and less concentrations of potassium (K+), magnesium Mg+2), phosphate (PO4-3), and sulfate ions (SO4-2). The blood plasma fraction contains considerably more protein than either interstitial fluid or lymph. Intracellular fluid contains relatively high concentrations of potassium (K+), magnesium Mg+2), phosphate (PO4-3), and sulfate ions (SO4-2), a somewhat greater concentration of sulfate ions (SO4-2), and lesser concentrations of sodium (Na+), Chloride (Cl-) and bicarbonate ions (HCO3-). It also has a greater concentration of protein than plasma. Page 828, 829
4. Describe how fluid movements between the compartments are controlled.
There are two main factors that regulate fluid and electrolyte movement between the compartments. They are hydrostatic pressure and osmotic pressure. Net movements of fluids between compartments result from differences in hydrostatic and osmotic pressure.
Hydrostatic pressure – Pressure exerted by fluids, such as blood pressure. It is the amount of force that water is exerting on the blood vessel walls. It is the main reason that fluid enters the interstitial space from the capillaries. Osmotic pressure, the “attraction” of water to solutes, is the main factor in water and electrolyte movement. Any changes in the amounts of water will cause the movement of solutes in the appropriate direction until the concentrations on both sides are stabilized. Any change in the concentration of the solutes will cause water to move in the same way. Thus equilibrium must be achieved by the osmotic movement of water alone. Page 829
5. Prepare a list of sources of normal water gain and loss to illustrate how the input of water equals the output of water. Water is taken in by drinking water or beverages, eating moist foods, and from oxidative metabolism of nutrients which is called water metabolism. Water is lost in urine, through skin and lungs, feces, and sweat. The average amount of water taken in daily is 2,500 ml and the average daily ouput of water is 2,500 ml – this is water balance. Page 830
6. Define water of metabolism.
Normal oxidative metabolism of nutrients produces water, which is called water of metabolism. Page 830
7. Explain how water intake is regulated.
The primary regulator of water intake is thirst. The intense feeling of thirst derives from the osmotic pressure of extracellular fluids and a thirst center in the hypothalamus of the brain. As the body loses water, the osmotic pressure increases and the osmoreceptors of the thirst center cause the person to feel thirsty and to seek water. This mechanism is triggered when as little as one percent of the total body water is lost. The act of drinking and the result of the stomach being distended trigger nerve impulses that inhibit the thirst mechanism. So, a person usually stops drinking long before the water is actually absorbed. Page 831
8. Explain how the nephrons function in the regulation of water output.
The distal convoluted tubules and collecting ducts of the nephrons regulate the volume of water excreted in the urine. When water volume is low, the osmoreceptors in the hypothalamus signal for the hormone ADH to be released. ADH, in the kidneys, causes the distal convoluted tubules and collecting ducts to become more permeable to water, and water is reabsorbed into the bloodstream. When the water volume is excessive, the osmoreceptors inhibit ADH secretion, and the distal convoluted tubules and collecting ducts remain impermeable to water. This prevents reabsorption, and the excess water is excreted in the urine. Page 831
9. List the most important electrolytes in body fluids.
The most important electrolytes are the sodium, potassium, calcium, magnesium, chloride, sulfate, phosphate, and bicarbonate and hydrogen ions. Page 834
10. Explain how electrolyte intake is regulated.
A person obtains sufficient electrolytes by responding to hunger and thirst. However, a severe electrolyte deficiency may produce salt craving, which is a strong desire to eat salty foods. Page 834
11. List the routes by which electrolytes leave the body.
The body loses some electrolytes by perspiration, varying amounts are lost in feces and the greatest electrolyte output occurs as a result of kidney function and urine production. Page 834
12. Explain how the adrenal cortex functions in the regulation of electrolyte output.
Sodium ions account for nearly ninety percent of the positively charged ions in extracellular fluids.
As the concentration of sodium ions decrease, the adrenal cortex secretes the hormone aldosterone to increase sodium ion reabsorption in the DCT and collecting ducts of the nephrons. Aldosterone also regulates potassium ions. Aldosterone functions to increase the secretion of potassium ion concentration. Page 835
13. Describe the role of the parathyroid glands in regulating electrolyte balance.
Parathyroid glands secrete parathyroid hormone when the calcium concentration drops below normal. The parathyroid hormone increases activity in bone-resorbing cells, which increase both calcium and phosphate ions in the extracellular fluids. This hormone also causes the kidneys to conserve calcium ions and increases the urinary excretion of phosphate ions. Page 835
14. Describe the role of the renal tubules in regulating electrolyte balance.
The permeability of the renal tubule aids in the control of electrolyte balance by passively reabsorbing negatively charged ions that follow positively charged ones. When sodium (Na+) is reabsorbed, chloride (Cl-) follows because negatively charged chloride ions are electrically attracted to the positively charged sodium ions and accompany them as they are reabsorbed. Page 835
15. Distinguish between an acid and a base.
Electrolytes that ionize in water and release hydrogen ions are called acids. Substances that combines with hydrogen ions are bases. Page 836
16. List five sources of hydrogen ions in the body fluids, and name an acid that originates from each source.
Aerobic respiration of glucose -carbonic acid.
Anaerobic respiration of glucose -lactic acid.
Incomplete oxidation of fatty acids -acidic ketone bodies.
Oxidation of amino acids containing sulfur -sulfuric acid.
Breakdown of phospho- and nucleoproteins - phosphoric acid.
Page 836, 837
17. Distinguish between a strong acid and a weak acid, and name an example of each.
Acids that ionize more completely (release more H+) are strong acids, and those that ionize less completely are weak acids. Hydrochloric acid (HCl) of the gastric juice ionizes completely and is a strong acid. Carbonic acid (H2CO3) produced when carbon dioxide reacts wit water is a weak acid because it oxidizes less completely. Page 837
18. Distinguish between a strong base and a weak base, and give an example of each.
Strong bases, such as bicarbonate ions (HCO3-), are those that combine more readily with hydrogen ions. Weak bases, such as chloride ions (Cl-), are those that combine less readily with hydrogen ions. Page 838
19. Explain how an acid-base buffer system functions.
They are in all body fluids and are based on chemicals that combine with excess acids or bases. Buffers are substances that stabilize the pH of a solution, despite the addition of an acid or a base. The chemical components of a buffer system can combine with strong acids to convert them into weak acids, and can combine with strong bases to convert them into weak bases. This activity helps minimize pH changes in the body fluids. The three buffer systems are: Bicarbonate buffer system, Phosphate buffer system and Protein buffer system. Page 838, 839
20. Describe how the bicarbonate buffer system resists changes in pH.
The bicarbonate buffer system occurs in both intracelluar and extracellular fluids. It consists of carbonic acid (H2CO3) and sodium bicarbonate (NaHCO3). If a strong acid is present, it reacts with sodium bicarbonate to produce carbonic acid and sodium chloride, minimizing the increasing concentration of hydrogen ions. If a strong base is present, it reacts with carbonic acid, producing sodium bicarbonate and water, minimizing the alkaline shift. Page 838
21. Explain why a protein has acidic as well as basic properties.
The protein buffer system consists of the plasma proteins such as albumin and certain proteins within the cells, including the hemoglobin of red blood cells. Because some amino acids have freely exposed carboxyl groups (-COOH), under some conditions these groups can become ionized and a hydrogen ion is released. Other amino acids have a freely exposed amino group (-NH2), which can accept hydrogen ions. Thus, protein molecules can function as acids by releasing hydrogen ions, or as bases by accepting hydrogen ions. This property allows protein molecules to act as a self-controlling acid-base buffer system. Page 839
22. Describe how a protein functions as a buffer system.
Altered carboxyl groups can now accept hydrogen ions in an acidic environment, while the altered amino groups can release a hydrogen ion in a basic environment. In this way, a protein can act as a buffer system when necessary. Page 839
23. Describe the function of hemoglobin as a buffer system.
Red blood cells contain an enzyme called carbonic anhydrase that speeds the reaction between carbon dioxide and water. This reaction produces carbonic acid, which quickly dissociates into bicarbonate and hydrogen ions. Hemoglobin can bind the hydrogen ions generated within red cells, thus acting as a buffer to minimize the pH change that would otherwise occur. Page 839
24. Explain how the respiratory center functions in the regulation of the acid-base balance.
The respiratory center in the brain stem helps to control hydrogen ion concentration by controlling the rate and depth of breathing. If body cells increase their production of carbon dioxide, carbonic acid production increases. When carbonic acid dissociates, the concentration of hydrogen ions increases and the pH drops. Page 840
25. Explain how the kidneys function in the regulation of the acid-base balance.
Nephrons help regulate excess hydrogen ion concentration of the body fluids by excreting hydrogen ions in the urine. The tubular secretion of hydrogen ions is linked to tubular reabsorption of bicarbonate ions. Page 840, 841
26. Describe the role of ammonia in the transport of hydrogen ions to the outside of the body.
Cells in the renal tubules, through deamination of certain amino acids, produce ammonia. Ammonia diffuses easily through the tubules into the urine. Because ammonia is a weak base, it can accept hydrogen ions to form ammonium ions, which are trapped in the urine because renal tubules are impermeable to them. Page 841
27. Distinguish between a chemical buffer system and a physiologic buffer system.
A chemical buffer system is one that uses only chemical reactions to convert acids or bases almost immediately. These are acid-base buffer systems. A physiological buffer system is one that causes a change in the excretion of acids and bases by influencing the cells of an organ. Two examples are the respiratory mechanism and the renal mechanism. Page 841
Electrolytes are dissolved in the water of body fluids. Meaning anything that alters the concentrations of the electrolytes will alter the concentration of the water by adding solutes to it or by removing solutes from it. Anything that changes the concentration of the water will change the concentration of the electrolytes by concentrating or diluting them. Page 827
2. Name the body fluid compartments, and describe their locations.
Intracellular fluid compartment-The intracellular fluid compartment includes all of the water and electrolytes that a cell membranes enclose. Intracellular fluid is the fluid within cells. In an adult, it represents about 63 percent by volume of the total body water.
Extracellular fluid compartment-The extracellular fluid compartment includes all of the fluid outside the cells. To include tissue spaces (interstitial fluid), the blood vessels (plasma), and the lymphatic vessels (lymph). Epithelial layers separate a specialized fraction of the extracellular fuild from other extracellular fluids. It’s compartment constitute is about 37 percent of the total body water. Page 828
3. Explain how the fluids within these compartments differ in composition.
Extracellular fluids are similar, including relatively high concentrations of sodium (Na+), Calcium (Ca+2), Chloride (Cl-) and bicarbonate ions (HCO3-), and less concentrations of potassium (K+), magnesium Mg+2), phosphate (PO4-3), and sulfate ions (SO4-2). The blood plasma fraction contains considerably more protein than either interstitial fluid or lymph. Intracellular fluid contains relatively high concentrations of potassium (K+), magnesium Mg+2), phosphate (PO4-3), and sulfate ions (SO4-2), a somewhat greater concentration of sulfate ions (SO4-2), and lesser concentrations of sodium (Na+), Chloride (Cl-) and bicarbonate ions (HCO3-). It also has a greater concentration of protein than plasma. Page 828, 829
4. Describe how fluid movements between the compartments are controlled.
There are two main factors that regulate fluid and electrolyte movement between the compartments. They are hydrostatic pressure and osmotic pressure. Net movements of fluids between compartments result from differences in hydrostatic and osmotic pressure.
Hydrostatic pressure – Pressure exerted by fluids, such as blood pressure. It is the amount of force that water is exerting on the blood vessel walls. It is the main reason that fluid enters the interstitial space from the capillaries. Osmotic pressure, the “attraction” of water to solutes, is the main factor in water and electrolyte movement. Any changes in the amounts of water will cause the movement of solutes in the appropriate direction until the concentrations on both sides are stabilized. Any change in the concentration of the solutes will cause water to move in the same way. Thus equilibrium must be achieved by the osmotic movement of water alone. Page 829
5. Prepare a list of sources of normal water gain and loss to illustrate how the input of water equals the output of water. Water is taken in by drinking water or beverages, eating moist foods, and from oxidative metabolism of nutrients which is called water metabolism. Water is lost in urine, through skin and lungs, feces, and sweat. The average amount of water taken in daily is 2,500 ml and the average daily ouput of water is 2,500 ml – this is water balance. Page 830
6. Define water of metabolism.
Normal oxidative metabolism of nutrients produces water, which is called water of metabolism. Page 830
7. Explain how water intake is regulated.
The primary regulator of water intake is thirst. The intense feeling of thirst derives from the osmotic pressure of extracellular fluids and a thirst center in the hypothalamus of the brain. As the body loses water, the osmotic pressure increases and the osmoreceptors of the thirst center cause the person to feel thirsty and to seek water. This mechanism is triggered when as little as one percent of the total body water is lost. The act of drinking and the result of the stomach being distended trigger nerve impulses that inhibit the thirst mechanism. So, a person usually stops drinking long before the water is actually absorbed. Page 831
8. Explain how the nephrons function in the regulation of water output.
The distal convoluted tubules and collecting ducts of the nephrons regulate the volume of water excreted in the urine. When water volume is low, the osmoreceptors in the hypothalamus signal for the hormone ADH to be released. ADH, in the kidneys, causes the distal convoluted tubules and collecting ducts to become more permeable to water, and water is reabsorbed into the bloodstream. When the water volume is excessive, the osmoreceptors inhibit ADH secretion, and the distal convoluted tubules and collecting ducts remain impermeable to water. This prevents reabsorption, and the excess water is excreted in the urine. Page 831
9. List the most important electrolytes in body fluids.
The most important electrolytes are the sodium, potassium, calcium, magnesium, chloride, sulfate, phosphate, and bicarbonate and hydrogen ions. Page 834
10. Explain how electrolyte intake is regulated.
A person obtains sufficient electrolytes by responding to hunger and thirst. However, a severe electrolyte deficiency may produce salt craving, which is a strong desire to eat salty foods. Page 834
11. List the routes by which electrolytes leave the body.
The body loses some electrolytes by perspiration, varying amounts are lost in feces and the greatest electrolyte output occurs as a result of kidney function and urine production. Page 834
12. Explain how the adrenal cortex functions in the regulation of electrolyte output.
Sodium ions account for nearly ninety percent of the positively charged ions in extracellular fluids.
As the concentration of sodium ions decrease, the adrenal cortex secretes the hormone aldosterone to increase sodium ion reabsorption in the DCT and collecting ducts of the nephrons. Aldosterone also regulates potassium ions. Aldosterone functions to increase the secretion of potassium ion concentration. Page 835
13. Describe the role of the parathyroid glands in regulating electrolyte balance.
Parathyroid glands secrete parathyroid hormone when the calcium concentration drops below normal. The parathyroid hormone increases activity in bone-resorbing cells, which increase both calcium and phosphate ions in the extracellular fluids. This hormone also causes the kidneys to conserve calcium ions and increases the urinary excretion of phosphate ions. Page 835
14. Describe the role of the renal tubules in regulating electrolyte balance.
The permeability of the renal tubule aids in the control of electrolyte balance by passively reabsorbing negatively charged ions that follow positively charged ones. When sodium (Na+) is reabsorbed, chloride (Cl-) follows because negatively charged chloride ions are electrically attracted to the positively charged sodium ions and accompany them as they are reabsorbed. Page 835
15. Distinguish between an acid and a base.
Electrolytes that ionize in water and release hydrogen ions are called acids. Substances that combines with hydrogen ions are bases. Page 836
16. List five sources of hydrogen ions in the body fluids, and name an acid that originates from each source.
Aerobic respiration of glucose -carbonic acid.
Anaerobic respiration of glucose -lactic acid.
Incomplete oxidation of fatty acids -acidic ketone bodies.
Oxidation of amino acids containing sulfur -sulfuric acid.
Breakdown of phospho- and nucleoproteins - phosphoric acid.
Page 836, 837
17. Distinguish between a strong acid and a weak acid, and name an example of each.
Acids that ionize more completely (release more H+) are strong acids, and those that ionize less completely are weak acids. Hydrochloric acid (HCl) of the gastric juice ionizes completely and is a strong acid. Carbonic acid (H2CO3) produced when carbon dioxide reacts wit water is a weak acid because it oxidizes less completely. Page 837
18. Distinguish between a strong base and a weak base, and give an example of each.
Strong bases, such as bicarbonate ions (HCO3-), are those that combine more readily with hydrogen ions. Weak bases, such as chloride ions (Cl-), are those that combine less readily with hydrogen ions. Page 838
19. Explain how an acid-base buffer system functions.
They are in all body fluids and are based on chemicals that combine with excess acids or bases. Buffers are substances that stabilize the pH of a solution, despite the addition of an acid or a base. The chemical components of a buffer system can combine with strong acids to convert them into weak acids, and can combine with strong bases to convert them into weak bases. This activity helps minimize pH changes in the body fluids. The three buffer systems are: Bicarbonate buffer system, Phosphate buffer system and Protein buffer system. Page 838, 839
20. Describe how the bicarbonate buffer system resists changes in pH.
The bicarbonate buffer system occurs in both intracelluar and extracellular fluids. It consists of carbonic acid (H2CO3) and sodium bicarbonate (NaHCO3). If a strong acid is present, it reacts with sodium bicarbonate to produce carbonic acid and sodium chloride, minimizing the increasing concentration of hydrogen ions. If a strong base is present, it reacts with carbonic acid, producing sodium bicarbonate and water, minimizing the alkaline shift. Page 838
21. Explain why a protein has acidic as well as basic properties.
The protein buffer system consists of the plasma proteins such as albumin and certain proteins within the cells, including the hemoglobin of red blood cells. Because some amino acids have freely exposed carboxyl groups (-COOH), under some conditions these groups can become ionized and a hydrogen ion is released. Other amino acids have a freely exposed amino group (-NH2), which can accept hydrogen ions. Thus, protein molecules can function as acids by releasing hydrogen ions, or as bases by accepting hydrogen ions. This property allows protein molecules to act as a self-controlling acid-base buffer system. Page 839
22. Describe how a protein functions as a buffer system.
Altered carboxyl groups can now accept hydrogen ions in an acidic environment, while the altered amino groups can release a hydrogen ion in a basic environment. In this way, a protein can act as a buffer system when necessary. Page 839
23. Describe the function of hemoglobin as a buffer system.
Red blood cells contain an enzyme called carbonic anhydrase that speeds the reaction between carbon dioxide and water. This reaction produces carbonic acid, which quickly dissociates into bicarbonate and hydrogen ions. Hemoglobin can bind the hydrogen ions generated within red cells, thus acting as a buffer to minimize the pH change that would otherwise occur. Page 839
24. Explain how the respiratory center functions in the regulation of the acid-base balance.
The respiratory center in the brain stem helps to control hydrogen ion concentration by controlling the rate and depth of breathing. If body cells increase their production of carbon dioxide, carbonic acid production increases. When carbonic acid dissociates, the concentration of hydrogen ions increases and the pH drops. Page 840
25. Explain how the kidneys function in the regulation of the acid-base balance.
Nephrons help regulate excess hydrogen ion concentration of the body fluids by excreting hydrogen ions in the urine. The tubular secretion of hydrogen ions is linked to tubular reabsorption of bicarbonate ions. Page 840, 841
26. Describe the role of ammonia in the transport of hydrogen ions to the outside of the body.
Cells in the renal tubules, through deamination of certain amino acids, produce ammonia. Ammonia diffuses easily through the tubules into the urine. Because ammonia is a weak base, it can accept hydrogen ions to form ammonium ions, which are trapped in the urine because renal tubules are impermeable to them. Page 841
27. Distinguish between a chemical buffer system and a physiologic buffer system.
A chemical buffer system is one that uses only chemical reactions to convert acids or bases almost immediately. These are acid-base buffer systems. A physiological buffer system is one that causes a change in the excretion of acids and bases by influencing the cells of an organ. Two examples are the respiratory mechanism and the renal mechanism. Page 841
