Chapt21 fluid & electrolytes

Hole’s Human Anatomyand PhysiologyTwelfth Edition

Shier w Butler w Lewis

Chapter 21

Water, Electrolyte, andAcid-Base Balance

21.1: Introduction

• The term balance suggests a state of equilibrium• For water and electrolytes that means equal amounts enter and leave the body• Mechanisms that replace lost water and electrolytes and excrete excesses maintain this balance• This results in stability of the body at all times• Keep in mind water and electrolyte balance are interdependent

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

Fluid Compartments

• Of the 40 liters of water in the body of an average adult, about two-thirds is intracellular fluid and one-third is extracellular fluid• An average adult female is about 52% water by weight, and an average male about 63% water by weight

Extracellularfluid(37%)

Intracellularfluid(63%)

323028262422

201816

14121086420

Total body water

Interstitial fluid

Plasma

Transcellular fluid

Extracellular fluid(37%)

Membranes ofbody cells

Intracellular fluid(63%)

Body Fluid Composition

• Extracellular fluids are generally similar in composition including high concentrations of sodium, calcium, chloride and bicarbonate ions

• Intracellular fluids have high concentrations of potassium, magnesium, phosphate, and sulfate ions

Relative concentrations and ratios of ions in extracellular and intracellular fluids

14:1Ratio(Extracellular: intracellular)

1:28K+

5:1Ca+2

1:19Mg+2

26:1Cl

3:1HCO3

1:19PO4

1:2SO4

Extracellular fluid

Intracellular fluid

Movement of Fluid Between Compartments

• Two major factors regulate the movement of water and electrolytes from one fluid compartment to another

• Hydrostatic pressure• Osmotic pressure

Interstitial fluid

Capillary wall

Transcellularfluid

Plasma

Serousmembrane

Intracellularfluid

Cellmembrane

Lymphvessel

Fluid leaves plasmaat arteriolar end ofcapillaries becauseoutward force ofhydrostatic pressurepredominates

Fluid returns toplasma at venularends of capillariesbecause inward forceof colloid osmoticpressure predominates

Hydrostatic pressurewithin interstitialspaces forces fluidinto lymph capillaries

Interstitial fluid isin equilibrium withtranscellular andintracellular fluids

21.3: Water Balance

• Water balance exists when water intake equals water output• Homeostasis requires control of both water intake and water output

Water Intake• The volume of water gained each day varies among individuals averaging about 2,500 milliliters daily for an adult:

• 60% from drinking• 30% from moist foods• 10% as a bi-product ofoxidative metabolism ofnutrients called water ofmetabolism

Water inbeverages(1,500 mL or 60%)

Average daily intake of waterWater lost in sweat(150 mL or 6%)

Total output(2,500 mL)

Average daily output of water

(a) (b)

Total intake(2,500 mL)

Water inmoist food(750 mL or 30%)

Water ofmetobolism(250 mL or 10%)

Water lost in feces(150 mL or 6%)Water lost throughskin and lungs(700 mL or 28%)

Water lost in urine(1,500 mL or 60%)

Regulation of Water Intake

Water Output

• Water normally enters the body only through the mouth, but it can be lost by a variety of routes including:

• Urine (60% loss)• Feces (6% loss)• Sweat (sensible perspiration) (6% loss)• Evaporation from the skin (insensible perspiration)• The lungs during breathing(Evaporation from the skin and the lungs is a 28% loss)

Regulation of Water Output

21.4: Electrolyte Balance• An electrolyte balance exists when the quantities of electrolytes the body gains equals those lost

Electrolyte intake

Fluids

Electrolyte output

Perspiration

Metabolicreactions

Electrolyte Intake• The electrolytes of greatest importance to cellular functions release sodium, potassium, calcium, magnesium, chloride, sulfate, phosphate, bicarbonate, and hydrogen ions• These ions are primarily obtained from foods, but some are from water and other beverages

Regulation of Electrolyte Intake

• Ordinarily, a person obtains sufficient electrolytes by responding to hunger and thirst• A severe electrolyte deficiency may cause salt craving

Electrolyte Output

• The body loses some electrolytes by perspiring typically on warmer days and during strenuous exercise• Some are lost in the feces• The greatest output is as a result of kidney function and urine output

Regulation of Electrolyte Output

• The concentrations of positively charged ions, such as sodium (Na+), potassium (K+) and calcium (Ca+2) are of particular importance• These ions are vital for nerve impulse conduction, muscle fiber contraction, and maintenance of cell membrane permeability

Aldosterone is secreted

Adrenal cortex is signaled

Potassium ionconcentration increases

Renal tubulesincrease reabsorption ofsodium ions and increasesecretion of potassium ions

Sodium ions areconserved and potassiumions are excreted

Calcium ionconcentration decreases

Calcium ion concentrationreturns toward normal

Normal phosphateconcentration is maintained

Addition of phosphateto bloodstream

Renal tubules conservecalcium and increasesecretion of phosphate

Parathyroid glandsare stimulated

Parathyroid hormoneis secreted

Intestinal absorptionof calcium increases

Activity of bone-resorbingosteoclasts increases

Increased phosphateexcretion in urine

21.1 Clinical Application

Water Balance Disorders

21.2 Clinical Application

Sodium and Potassium Imbalances

21.5: Acid-Base Balance• Electrolytes that ionize in water and release hydrogen ions are acids• Substances that combine with hydrogen ions are bases• Acid-base balance entails regulation of the hydrogen ion concentrations of body fluids• This is important because slight changes in hydrogen ion concentrations can alter the rates of enzyme-controlled metabolic reactions, shift the distribution of other ions, or modify hormone actions

Sources of Hydrogen Ions

Aerobicrespirationof glucose

Anaerobicrespirationof glucose

Incompleteoxidation offatty acids

Oxidation ofsulfur-containingamino acids

Hydrolysis ofphosphoproteinsand nucleic acids

Carbonicacid

Lacticacid

Acidic ketonebodies

Sulfuricacid

Phosphoricacid

Internal environment

Strengths of Acids and Bases

• Acids:• Strong acids ionize more completely and release more H+

• Weak acids ionize less completely and release fewer H+

• Bases:• Strong bases ionize more completely and release more OH-

• Weak bases ionize less completely and release fewer OH-

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 three ways:

• Acid-base buffer systems• Respiratory excretion of carbon dioxide• Renal excretion of hydrogen ions

• Bicarbonate buffer system• The bicarbonate ion converts a strong acid to a weak acid• Carbonic acid converts a strong base to a weak base H+ + HCO3

- H2CO3 H+ + HCO3-

• Phosphate buffer system• The monohydrogen phosphate ion converts a strong acid to a weak acid• The dihydrogen phosphate ion converts a strong base to a weak base H+ + HPO4

-2 H2PO4- H+ + HPO4

• Protein buffer system• NH3

+ group releases a hydrogen ion in the presence of excess base• COO- group accepts a hydrogen ion in the presence of excess acid

Acid-Base Buffer Systems

• The respiratory center in the brainstem helps regulate hydrogen ion concentrations in the body fluids by controlling the rate and depth of breathing• If body cells increase their production of CO2…

More CO2 is eliminated through lungs

Rate and depth of breathing increase

Respiratory center is stimulated

Cells increase production of CO2

CO2 reacts with H2O to produce H2CO3

H2CO3 releases H+

Respiratory Secretion of Carbon Dioxide

• Nephrons help regulate the hydrogen ion concentration of body fluids by excreting hydrogen ions in the urine

High intake of proteins

Increased concentrationof H+ in urine

Increased secretionof H+ into fluid ofrenal tubules

Increased concentrationof H+ in body fluids

Increased metabolismof amino acids

Increased formationof sulfuric acid andphosphoric acid

Concentration of H+

in body fluids returnstoward normal

Renal Secretion of Hydrogen Ions

• Various regulators of hydrogen ion concentration operate at different rates• Acid-base (chemical) buffers function rapidly• Respiratory and renal (physiological buffers) mechanisms function more slowly

Phosphatebuffer system

Proteinbuffer system

First line of defenseagainst pH shift

Second line ofdefense againstpH shift

Chemicalbuffer system

Physiologicalbuffers

Bicarbonatebuffer system

Respiratorymechanism(CO2 excretion)

Renalmechanism(H+ excretion)

Time Course of pH Regulation

21.6: Acid-Base Imbalances

• Chemical and physiological buffer systems ordinarily maintain the hydrogen ion concentration of body fluids within very narrow pH ranges• Abnormal conditions may disturb the acid-base balance

Survival range

Normal pH range

pH scale

7.45 8.07.06.8 7.8

Acidosis Alkalosis

Acidosis

• Acidosis results from the accumulation of acids or loss of bases, both of which cause abnormal increases in the hydrogen ion concentrations of body fluids• Alkalosis results from a loss of acids or an accumulation of bases accompanied by a decrease in hydrogen ion concentrations

pH rises

pH drops

Increased concentration of H+

pH scale7.4

Acidosis

Alkalosis

Decreased concentration of H+

Accumulationof acids

Loss ofbases

Loss ofacids

Accumulationof bases

• Two major types of acidosis are respiratory acidosis and metabolic acidosis

Accumulation of CO2

Respiratoryacidosis

Decreased rateand depth ofbreathing

Obstruction ofair passages

Decreasedgas exchange Accumulation of nonrespiratory acids

Metabolic acidosis

Excessive loss of bases

Kidney failureto excrete acids

Excessive production of acidicketones as in diabetes mellitus

Prolonged diarrheawith loss of alkalineintestinal secretions

Prolonged vomitingwith loss of intestinalsecretions

Acidosis

Alkalosis• Respiratory alkalosis develops as a result of hyperventilation• Metabolic alkalosis results from a great loss of hydrogen ions or from a gain in bases, both accompanied by a rise in the pH of blood

Excessive loss of CO2

Hyperventilation

Decrease in concentration of H2CO3

Decrease in concentration of H+

Respiratory alkalosis

• Anxiety

• Fever

• Poisoning

• High altitude

Loss of acids

Net increase in alkaline substances

Metabolic alkalosis

Gastricdrainage

Vomiting with lossof gastric secretions

Important Points in Chapter 21:Outcomes to be Assessed

21.1: Introduction

Explain the balance concept.

Explain the importance of water and electrolyte balance.

21.2: Distribution of Body Fluids

Describe how body fluids are distributed in compartments.

Explain how fluid composition varies among compartments and how fluids move from one compartment to another.

21.3: Water Balance

List the routes by which water enters and leaves the body.

Explain the regulation of water input and water output.

21.4: Electrolyte Balance

List the routes by which electrolytes enter and leave the body.

Explain the regulation of the input and the output of electrolytes.

21.5: Acid-Base Balance

Explain acid-base balance.

Identify how pH number describes the acidity and alkalinity of a body fluid.

List the major sources of hydrogen ions in the body.

Distinguish between strong acids and weak acids.

Explain how chemical buffer systems, the respiratory system, and the kidneys keep the pH of body fluids relatively constant.

21.6: Acid-Base Imbalances Assignment

Describe the causes and consequences of increase or decrease in body fluid pH. Place your question and answer in your journal.

Chapt21 fluid & electrolytes

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