1. Chapter 27 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
Tortora & Grabowski 9/e 2000 JWS

2. Balance Between Fluid Compartments
Volume of fluid in each is kept constant. Since water follows electrolytes, they must be in balance as well
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
Tortora & Grabowski 9/e 2000 JWS

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
Tortora & Grabowski 9/e 2000 JWS

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
Tortora & Grabowski 9/e 2000 JWS

5. Dehydration Stimulates Thirst
Regulation of fluid gain is by regulation of thirst.
Tortora & Grabowski 9/e 2000 JWS

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
Tortora & Grabowski 9/e 2000 JWS

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
Tortora & Grabowski 9/e 2000 JWS

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
Tortora & Grabowski 9/e 2000 JWS

9. Movement of Water
Intracellular and interstitial fluids normally have the same osmolarity, so cells neither swell nor shrink
Swollen cells of water intoxication because Na+ concentration of plasma falls 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
Tortora & Grabowski 9/e 2000 JWS

10. 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
Tortora & Grabowski 9/e 2000 JWS

11. 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
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12. 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)
Tortora & Grabowski 9/e 2000 JWS

13. 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
Tortora & Grabowski 9/e 2000 JWS

14. 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
Tortora & Grabowski 9/e 2000 JWS

15. 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
Tortora & Grabowski 9/e 2000 JWS

16. 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
Tortora & Grabowski 9/e 2000 JWS

17. 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
Tortora & Grabowski 9/e 2000 JWS

18. 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)
Tortora & Grabowski 9/e 2000 JWS

19. 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
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20. 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
Tortora & Grabowski 9/e 2000 JWS

21. Acid-Base Balance Homeostasis of H+ concentration is vital
proteins 3-D structure sensitive to pH changes
normal plasma pH must be maintained between
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)
Tortora & Grabowski 9/e 2000 JWS

22. 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
Tortora & Grabowski 9/e 2000 JWS

23. Protein Buffer System Abundant in intracellular fluids & in plasma
hemoglobin very good at buffering H+ in RBCs
albumin is main plasma protein buffer
Amino acids contains at least one carboxyl group (-COOH) and at least one amino group (-NH2)
carboxyl group acts like an acid & releases H+
amino group acts like a base & combines with H+
some side chains can buffer H+
Hemoglobin acts as a buffer in blood by picking up CO2 or H+
Tortora & Grabowski 9/e 2000 JWS

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
Tortora & Grabowski 9/e 2000 JWS

25. Phosphate Buffer System
Most important intracellularly, but also acts to buffer acids in the urine
Dihydrogen phosphate ion acts as a weak acid that can buffer a strong base
Monohydrogen phosphate acts a weak base by buffering the H+ released by a strong acid
Tortora & Grabowski 9/e 2000 JWS

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
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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
Tortora & Grabowski 9/e 2000 JWS

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
Tortora & Grabowski 9/e 2000 JWS

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
Tortora & Grabowski 9/e 2000 JWS

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
IV administration of bicarbonate (HCO3-)
ventilation therapy to increase exhalation of CO2
Tortora & Grabowski 9/e 2000 JWS

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
Tortora & Grabowski 9/e 2000 JWS

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
Tortora & Grabowski 9/e 2000 JWS

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
Tortora & Grabowski 9/e 2000 JWS

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
Tortora & Grabowski 9/e 2000 JWS

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
Tortora & Grabowski 9/e 2000 JWS

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
Tortora & Grabowski 9/e 2000 JWS