27-1

Fluid, Electrolyte and Acid-Base Homeostasis

• Body fluid– all the water and dissolved

solutes in the body’s fluid compartments

• Mechanisms regulate– total volume

– distribution

– concentration of solutes and pH

27-2

Balance Between Fluid Compartments

• Only 2 places for exchange between compartments:– cell membranes separate intracellular from interstitial fluid.– only in capillaries are walls thin enough for exchange between

plasma and interstitial fluids

Volume of fluid in each is kept constant. Since water follows electrolytes, they must be in balance as well

27-3

Body Water Gain and Loss

• 45-75% body weight– declines with age since fat

contains almost no water

• Gain from ingestion and metabolic water formed during aerobic respiration & dehydration synthesis reactions (2500 mL/day)

• Normally loss = gain– urine, feces, sweat, breathe

27-4

Regulation of Water Gain• Formation of metabolic water is not regulated

– function of the need for ATP

• Main regulator of water gain is intake regulation

• Stimulators of thirst center in hypothalamus– dry mouth, osmoreceptors in hypothalamus,

decreased blood volume causes drop in BP & angiotensin II

• Drinking occurs– body water levels return to normal

27-5

Dehydration Stimulates Thirst

• Regulation of fluid gain is by regulation of thirst.

27-6

Regulation of Water and Solute Loss

• Elimination of excess water or solutes occurs through urination

• Consumption of very salty meal demonstrates function of three hormones

• Demonstrates how– “water follows salt”– excrete Na+ and water will follow

and decrease blood volume

27-7

Hormone Effects on Solutes

• Angiotensin II and aldosterone promote reabsorption of Na+ and Cl- and an increase in fluid volume– stretches atrial volume and promotes release of ANP– slows release of renin & formation of angiotensin II

• increases filtration rate & reduces water & Na+ reabsorption• decreases secretion of aldosterone slowing reabsorption of

Na+ and Cl- in collecting ducts

• ANP promotes natriuresis or the increased excretion of Na+ and Cl- which decreases blood volume

27-8

Hormone Regulation of Water Balance• Antidiuretic hormone (ADH) from the posterior

pituitary– stimulates thirst– increases permeability of principal cells of collecting

ducts to assist in water reabsorption– very concentrated urine is formed

• ADH secretion shuts off after the intake of water• ADH secretion is increased

– large decrease in blood volume– severe dehydration and drop in blood pressure– vomiting, diarrhea, heavy sweating or burns

27-9

27-10

Movement of Water

• Intracellular and interstitial fluidsnormally have the same osmolarity,so cells neither swell nor shrink

• Swollen cells of water intoxicationbecause Na+ concentration of plasmafalls below normal– drink plain water faster than kidneys can

excrete it – replace water lost from diarrhea or vomiting

with plain water– may cause convulsions, coma & death unless oral rehydration

includes small amount salt in water intake

27-11

Enemas and Fluid Balance

• Introduction of a solution into the bowel to stimulate activity and evacuate feces

• Increase risk of fluid & electrolyte imbalance unless isotonic solution is used

27-12

Concentrations of Electrolytes

• Functions of electrolytes– control osmosis between fluid compartments– help maintain acid-base balance– carry electric current– cofactors needed for enzymatic activity

• Concentration expressed in mEq/liter or milliequivalents per liter for plasma, interstitial fluid and intracellular fluid

27-13

Comparison Between Fluid Components

• Plasma contains many proteins, but interstitial fluid does not– producing blood colloid osmotic pressure

• Extracellular fluid contains Na+ and Cl-• Intracellular fluid contains K+ and phosphates (HPO4 -2)

27-14

Sodium

• Most abundant extracellular ion– accounts for 1/2 of osmolarity of ECF

• Average daily intake exceeds normal requirements• Hormonal controls

– aldosterone causes increased reabsorption Na+

– ADH release ceases if Na+ levels too low--dilute urine lost until Na+ levels rise

– ANP increases Na+ and water excretion if Na+ levels too high

27-15

Edema, Hypovolemia and Na+ Imbalance• Sodium retention causes water retention

– edema is abnormal accumulation of interstitial fluid

• Causes of sodium retention– renal failure– hyperaldosterone

• Excessive loss of sodium causes excessive loss of water (low blood volume)– due to inadequate secretion of aldosterone– too many diuretics

27-16

Chloride• Most prevalent extracellular anion

• Moves easily between compartments due to Cl- leakage channels

• Helps balance anions in different compartments

• Regulation– passively follows Na+ so it is regulated indirectly by

aldosterone levels– ADH helps regulate Cl- in body fluids because it

controls water loss in urine

• Chloride shift & hydrochloric acid of gastric juice

27-17

Potassium

• Most abundant cation in intracellular fluid

• Helps establish resting membrane potential & repolarize nerve & muscle tissue

• Exchanged for H+ to help regulate pH in intracellular fluid

• Control is mainly by aldosterone which stimulates principal cells to increase K+ secretion into the urine– abnormal plasma K+ levels adversely affect cardiac

and neuromuscular function

27-18

Bicarbonate• Common extracellular anion

• Major buffer in plasma

• Concentration increases as blood flows through systemic capillaries due to CO2 released from metabolically active cells

• Concentration decreases as blood flows through pulmonary capillaries and CO2 is exhaled

• Kidneys are main regulator of plasma levels– intercalated cells form more if levels are too low– excrete excess in the urine

27-19

Calcium• Most abundant mineral in body (skeleton &

teeth)

• Abundant extracellular cation in body fluids

• Important role in blood clotting, neurotransmitter release, muscle tone & nerve and muscle function

• Regulated by parathyroid hormone– stimulates osteoclasts to release calcium from bone– increases production of calcitriol (Ca+2 absorption

from GI tract and reabsorption from glomerular filtrate)

27-20

Phosphate• Present as calcium phosphate in bones and teeth, and in

phospholipids, ATP, DNA and RNA

• HPO4 -2 is important intracellular anion and acts as buffer of H+ in body fluids and in urine– mono and dihydrogen phosphate act as buffers in the blood

• Plasma levels are regulated by parathyroid hormone & calcitriol– resorption of bone releases phosphate– in the kidney, PTH increase phosphate excretion– calcitriol increases GI absorption of phosphate

27-21

Magnesium

• Found in bone matrix and as ions in body fluids– intracellular cofactor for metabolic enzymes,

heart, muscle & nerve function

• Urinary excretion increased in hypercalcemia, hypermagnesemia, increased extracellular fluid volume, decreases in parathyroid hormone and acidosis

27-22

Acid-Base Balance• Homeostasis of H+ concentration is vital

– proteins 3-D structure sensitive to pH changes– normal plasma pH must be maintained between

7.35 - 7.45– diet high in proteins tends to acidify the blood

• 3 major mechanisms to regulate pH– buffer system

– exhalation of CO2 (respiratory system)

– kidney excretion of H+ (urinary system)

27-23

Actions of Buffer Systems

• Prevent rapid, drastic changes in pH

• Change either strong acid or base into weaker one

• Work in fractions of a second

• Found in fluids of the body

• 3 principal buffer systems– protein buffer system– carbonic acid-bicarbonate buffer system– phosphate buffer system

27-24

Carbonic Acid-Bicarbonate Buffer System

• Acts as extracellular & intracellular buffer system– bicarbonate ion (HCO3-) can act as a weak base

• holds excess H+

– carbonic acid (H2CO3) can act as weak acid• dissociates into H+ ions

• At a pH of 7.4, bicarbonate ion concentration is about 20 times that of carbonic acid

• Can not protect against pH changes due to respiratory problems

27-25

27-26

Exhalation of Carbon Dioxide• Breathing plays a role in the homeostasis

of pH• pH modified by changing rate & depth of

breathing– faster breathing rate, blood pH rises– slow breathing rate, blood pH drops

• H+ detected by chemoreceptors in medulla oblongata, carotid & aortic bodies

• Respiratory centers inhibited or stimulated by changes is pH

27-27

Kidney Excretion of H+

• Metabolic reactions produce 1mEq/liter of nonvolatile acid for every kilogram of body weight

• Excretion of H+ in the urine is only way to eliminate huge excess

• Kidneys synthesize new bicarbonate and save filtered bicarbonate

• Renal failure can cause death rapidly due to its role in pH balance

27-28

Acid-Base Imbalances

• Acidosis---blood pH below 7.35

• Alkalosis---blood pH above 7.45

• Compensation is an attempt to correct the problem– respiratory compensation– renal compensation

• Acidosis causes depression of CNS---coma

• Alkalosis causes excitability of nervous tissue---spasms, convulsions & death

27-29

Summary of Causes

• Respiratory acidosis & alkalosis are disorders involving changes in partial pressure of CO2 in blood

• Metabolic acidosis & alkalosis are disorders due to changes in bicarbonate ion concentration in blood

27-30

Respiratory Acidosis

• Cause is elevation of pCO2 of blood

• Due to lack of removal of CO2 from blood

– emphysema, pulmonary edema, injury to the brainstem & respiratory centers

• Treatment– ventilation therapy to increase exhalation of

CO2

27-31

Respiratory Alkalosis

• Arterial blood pCO2 is too low

• Hyperventilation caused by high altitude, pulmonary disease, stroke, anxiety, aspirin overdose

• Renal compensation involves decrease in excretion of H+ and increase reabsorption of bicarbonate

• Treatment– breathe into a paper bag

27-32

Metabolic Acidosis

• Blood bicarbonate ion concentration too low– loss of ion through diarrhea or kidney dysfunction– accumulation of acid (ketosis with dieting/diabetes)– kidney failing to remove H+ from protein metabolism

• Respiratory compensation by hyperventilation

• Treatment– IV administration of sodium bicarbonate– correct the cause

27-33

Metabolic Alkalosis

• Blood bicarbonate levels are too high

• Cause is nonrespiratory loss of acid– vomiting, gastric suctioning, use of diuretics,

dehydration, excessive intake of alkaline drugs

• Respiratory compensation is hypoventilation

• Treatment– fluid and electrolyte therapy– correct the cause

27-34

Diagnosis of Acid-Base Imbalances

• Evaluate – systemic arterial blood pH– concentration of bicarbonate (too low or too high)

– PCO2 (too low or too high)

• Solutions– if problem is respiratory, the pCO2 will not be normal

– if problem is metabolic, the bicarbonate level will not be normal

27-35

Homeostasis in Infants• More body water in ECF so more easily disrupted• Rate of fluid intake/output is 7X higher• Higher metabolic rate produces more metabolic wastes• Kidneys can not concentrate urine nor remove excess H+• Surface area to volume ratio is greater so lose more

water through skin• Higher breathing rate increase water loss from lungs• Higher K+ and Cl- concentrations than adults

27-36

Impaired Homeostasis in the Elderly• Decreased volume of intracellular fluid

– inadequate fluid intake

• Decreased total body K+ due to loss of muscle tissue or potassium-depleting diuretics for treatment of hypertension or heart disease

• Decreased respiratory & renal function– slowing of exhalation of CO2

– decreased blood flow & glomerular filtration rate– reduced sensitivity to ADH & impaired ability to produce dilute urine– renal tubule cells produce less ammonia to combine with H+ and excrete as

NH+4