1. Chapter 21 Lecture Outline
See separate PowerPoint slides for all figures and tables pre-inserted into PowerPoint without notes.

2. 21.1: The Balance Concept
The term balance suggests a state of constancy
For water and electrolytes, this means that equal amounts enter and leave the body
Balance is maintained by mechanisms that replace lost water and electrolytes and excrete excesses
This results in stability in water and electrolytes at all times
This state of balance helps maintain homeostasis
Water and electrolyte balance are interdependent

3. 21.2: Distribution of Body Fluids
Body fluids are not uniformly distributed
They occupy compartments of different volumes that contain varying compositions
Water and electrolyte movement between these compartments is regulated to stabilize their distribution and the composition of body fluids

4. Fluid Compartments
An average adult female is about 52% water by weight, and male is about 63% water
Females have more adipose tissue (low in water), and males have more muscle (high in water)
There are about 40 liters of water, with dissolved electrolytes, in the body, distributed into 2 major compartments:
Intracellular fluid:
Fluid inside cells; contains 63% of body water
Extracellular fluid:
Fluid outside cells; contains 37% of body water

5. Fluid Compartments Extracellular Fluid Compartment:
Consists of various fluids in different spaces and vessels
Interstitial fluid, in tissue spaces
Blood plasma, in blood vessels
Lymph, in lymphatic vessels
Transcellular fluid: Separated from other extracellular fluids by epithelial layers; consists of cerebrospinal fluid, aqueous and vitreous humors in eye, synovial fluid in joints, serous fluid

6. Body Fluid Composition
All body fluids are solutions of electrolytes in water
Extracellular fluids are similar in composition, having high concentrations of N a+, Cl−, C a +2, and HC O3− ions
Blood plasma has more proteins than interstitial fluid or lymph
Intracellular fluids have high concentrations of K+, Mg+2, PO4−3, and SO4−2 ions

7. Movement of Fluid Between Compartments
2 major factors regulate the movement of water and electrolytes from one fluid compartment to another:
Hydrostatic pressure
Osmotic pressure
Hydrostatic pressure in cells and interstitial fluids remains equal and stable. Most fluid movement results from changes in osmotic pressure.

8. 21.3: Water Balance
Water balance exists when water intake (and metabolic production) equals water output
Homeostasis requires control of both water intake and water output
Water intake is controlled by the thirst centers in the brain
Water output is controlled by the kidneys

9. Water Intake
The volume of water gained each day varies among individuals, averaging about 2,500 mL daily for an adult:
60% from drinking fluids
30% from moist foods
10% as a bi-product of oxidative metabolism of nutrients, called water of metabolism

10. Regulation of Water Intake
The primary regulator of water intake is thirst.

11. Water Output Water normally enters the body only through the mouth
Water output is about 2500 mL/day, equal to water intake
Water can be lost by a variety of methods:
60% loss in urine
6% loss in feces
6% loss in sweat, or sensible perspiration
28% loss through a combination of evaporation from the skin (insensible perspiration) and from the lungs during breathing
Water output varies with temperature, humidity, activity level
Water output can be adjusted if intake is changed

12. Regulation of Water Output
The osmoreceptor-ADH mechanism in the hypothalamus regulates the concentration of urine produced in the kidney.
During dehydration, excess water loss causes osmoreceptors to lose water and shrink. This stimulates ADH secretion. ADH increases permeability of renal distal convoluted tubules and collecting ducts to water, which increases reabsorption and conservation of water.

13. Water Balance Disorders
Clinical Application 21.1
Water Balance Disorders
Dehydration:
H2O deficiency, in which output exceeds intake
Extracellular fluid becomes concentrated, and H2O leaves cells by osmosis
Wastes accumulate in extracellular fluid, leading to nervous system issues
Temperature regulation mechanism can fail, due to lack of H2O for sweating, resulting in hyperthermia
Water Intoxication:
Excess fluid intake can result in hyponatremia, low blood level of N a+
Creates an imbalance between H2O and electrolytes
Edema:
Abnormal accumulation of extracellular fluid in the interstitial spaces
Caused by decrease in plasma proteins, obstruction in lymphatic vessels, increased capillary permeability or venous pressure, inflammation

14. 21.4: Electrolyte Balance
Electrolyte balance exists when the quantities of electrolytes the body gains equals those lost

15. N a+, K+, C a+2, Mg+2, Cl−, SO4−2, PO4−3, HC O3−, and H+
Electrolyte Intake
The most important electrolytes for cellular functions release the following ions:
N a+, K+, C a+2, Mg+2, Cl−, SO4−2, PO4−3, HC O3−, and H+
These ions are obtained mainly from foods, but some are from water and other beverages, and some are by-products of metabolism

16. Regulation of Electrolyte Intake
Usually, a person obtains sufficient electrolytes by responding to hunger and thirst
A severe electrolyte deficiency may cause salt craving

17. Electrolyte Output
Some electrolytes are lost by perspiring, more on warmer days and during strenuous exercise
Some are lost in the feces
The greatest electrolyte output is a result of urine production
Kidneys regulate composition of body fluids and maintain homeostasis, by adjusting electrolyte losses in the urine

18. Regulation of Electrolyte Output
The concentrations of positively charged ions, such as N a+, K+ and C a+2 are of particular importance
These ions are vital for nerve impulse conduction, muscle fiber contraction, and maintenance of cell membrane permeability
K+ maintains resting potential in nerve and cardiac muscle cells
N a+ ions account for nearly 90% of the positively charged ions in extracellular fluids
The hormone aldosterone regulates both N a+ and K+ ion concentrations; high K+ concentration stimulates secretion of aldosterone, which increases tubular reabsorption of N a+ and tubular secretion of K+
Decrease in plasma C a+2 level stimulates secretion of parathyroid hormone, which causes an increase in the plasma C a+2 level
Some negative ions, such as Cl−, are transported along with positive ions, such as N a+

19. Regulation of Electrolyte Output

20. Sodium and Potassium Imbalances
Clinical Application 21.2
Sodium and Potassium Imbalances
Hyponatremia (low blood [N a+]):
Caused by prolonged sweating, vomiting, drinking too much water
Effects are hypotonic extracellular fluid, uptake of water by cells by osmosis
Hypernatremia (high blood [N a+]):
Caused by excess water loss, as in fever or diabetes insipidus
Effects are central nervous system disturbances: confusion, stupor, coma
Hypokalemia (low blood [K+]):
Caused by some diuretics, kidney disease, decrease in extracellular H+
Effects are muscle weakness or paralysis, cardiac disturbances, breathing problems
Hyperkalemia (high blood [K+]):
Caused by renal disease, aldosterone deficiency
Effects are skeletal muscle paralysis, cardiac disturbances

21. 21.5: Acid-Base Balance
Acids are electrolytes that ionize in water and release hydrogen ions
Bases are substances release ions that combine with hydrogen ions
Acid-base balance involves regulation of the H+ ion concentrations in body fluids
Important because slight changes in H+ ion concentrations can alter the rates of enzyme-controlled metabolic reactions, shift the distribution of other ions, or alter hormone actions
pH number indicates the degree to which a solution is acidic or basic (alkaline).
The more acidic the solution, the lower its pH
The more basic/alkaline the solution, the higher its pH
Normal pH of internal environment is 7.35 – 7.45

22. Sources of Hydrogen Ions
Most H+ ions in body are by-products of metabolic reactions, but digestive tract also absorbs some.

23. Strengths of Acids and Bases
Strong acids ionize more completely and release more H+
Example: HCl in gastric juice
Weak acids ionize less completely and release fewer H+
Example: H2C O3 , produced when C O2 combines with H2O
Bases:
Release ions that can combine with H+ ions, and therefore lower their concentration
Strong bases ionize more completely and release more O H− or other negative ions
Example: N a O H (sodium hydroxide)
Weak bases ionize less completely and release fewer O H− or other negative ions
Examples: NaHC O3 (sodium bicarbonate), HC O3− can act as base

24. Regulation of Hydrogen Ion Concentration
Either an acid shift or an alkaline (basic) shift in the body fluids could threaten the internal environment
Normal metabolic reactions generally produce more acid than base
The reactions include cellular metabolism of glucose, fatty acids, and amino acids
Maintenance of acid-base balance usually eliminates acids in one of 3 ways:
Acid-base buffer systems
Respiratory excretion of carbon dioxide
Renal excretion of hydrogen ions

25. Chemical Buffer Systems
Chemical buffer systems are in all body fluids, and are based on chemicals that combine with excess acids or bases.
Buffers: Substances that stabilize pH of a solution, even when an acid or base is added
Buffers minimize pH changes in body fluids
Buffer components combine with strong acids to convert them to weak acids, and with strong bases to convert them to weak bases
Bicarbonate buffer system:
Found in intracellular and extracellular fluids
The bicarbonate ion acts as weak base; converts a strong acid to a weak acid
Carbonic acid acts as weak acid; converts a strong base to a weak base
H+ + HC O3− ↔ H2C O3
If conditions are acidic, reaction proceeds toward right; H+ ions are taken up
If conditions are basic, reaction proceeds toward left; H+ ions are released

26. Chemical Buffer Systems
Phosphate buffer system:
Found in intracellular and extracellular fluids
Important in intracellular fluid, renal tubular fluid, urine
The monohydrogen phosphate ion (HPO4−2) converts a strong acid to a weak acid
The dihydrogen phosphate ion (H2PO4−) converts a strong base to a weak base
H+ + HPO4−2 ↔ H2PO4−
Acidic conditions cause reaction to proceed toward right, binding H+ ions; basic conditions shift reaction toward left, releasing H+ ions
Protein buffer system:
Consists of plasma proteins (albumin), hemoglobin, certain cell proteins
–NH2 groups and –C O O H groups buffer excess acidity or alkalinity by accepting / releasing H+ ions

28. Respiratory Excretion of Carbon Dioxide
The respiratory center in the brainstem helps regulate H+ ion concentrations in the body fluids by controlling the rate and depth of breathing
Increased production of C O2 by body cells leads to formation of carbonic acid, which then dissociates into H+ and HC O3−.
The increase in free H+ lowers the pH in the body fluids:
C O2 + H2O ↔ H2C O3 ↔ H+ + HC O3−
In response to increased acidity, the respiratory center increases rate & depth of breathing

29. Renal Excretion of Hydrogen Ions
Nephrons help regulate the H+ ion concentration of body fluids by excreting H+ ions in the urine
H+ ions are added into the renal tubules by tubular secretion
H+ ion secretion is linked to tubular reabsorption of HC O3– ions
When acids appear in the body fluids as a result of metabolic processes, reabsorbed HC O3– ions bind to the free H+ ions, and form H2C O3. This buffering mechanism prevents body fluids from becoming too acidic
Breakdown of certain amino acids produces phosphoric or sulfuric acids; so a high-protein diet can process excess acids
Kidneys compensate for extra acidity by increasing tubular secretion of H+ ions, and increased excretion in the urine

30. Renal Excretion of Hydrogen Ions

31. Time Course of H+ Ion Regulation
Various regulators of H+ ion concentration function at different rates
Acid-base (chemical) buffers function rapidly, almost immediately
Respiratory and renal mechanisms (physiological buffers) function more slowly:
Respiratory: minutes
Renal: 1 – 3 days

32. 21.6: Acid-Base Imbalances
Chemical and physiological buffer systems usually maintain the H+ ion concentration of body fluids within very narrow pH range (7.35 – 7.45 in arterial blood)
Abnormal conditions may disturb the acid-base balance

33. Acid-Base Imbalances
Acidosis results from the accumulation of acids or loss of bases, both of which cause abnormal increases in the H+ ion concentration in body fluids, and lower the pH below 7.35
Alkalosis results from a loss of acids or an accumulation of bases accompanied by a decrease in H+ ion concentration, which increases the pH above 7.45

34. Acidosis 2 major types of acidosis:
Respiratory acidosis: Caused by increase in C O2 and H2C O3 levels
Metabolic acidosis: Caused by accumulation of other acids or loss of bases

35. Alkalosis 2 major types of alkalosis:
Respiratory alkalosis: Caused by excess loss of C O2 and H2C O3
Metabolic alkalosis: results from excess loss of H+ ions or a gain in bases

36. 21.7: Compensation Compensation:
Resistance to a pH shift during an acid-base imbalance
Accomplished by changes in the action of various chemical buffers, respiratory C O2 excretion, and renal H+ ion excretion
Example: Compensation for metabolic alkalosis due to excess antacids:
Chemical buffers release H+ ions into body fluids
Rate and depth of breathing decreases, retaining C O2 and lowering pH
Kidneys decrease tubular secretion of H+ ions, retaining the H+ ions
Example: Compensation for respiratory acidosis due to pulmonary disease:
Chemical buffers bind to H+ ions, to remove them from body fluids
Respiratory system is unable to help with compensation
Increased H+ ion secretion in kidneys, and increased excretion in urine