1. Chapter 25 – Fluid, Electrolyte, and Acid-Base Balance

2. Body Fluids Water content Fluid compartments
~60% in males; ~50% in females
Females have more adipose tissue, which has a low water content
Fluid compartments
2/3 of water is intracellular fluid (ICF)
Water present inside the cell
1/3 of water is extracellular fluid (ECF)
2 main divisions
Plasma
Interstitial (IF)
Other fluids include CSF, serous, synovial, digestive secretions

3. Composition of body fluids
Solvent (most abundant molecule type) + solutes (usually solid) = solution
Water is the universal solvent
Lots of different molecules dissolve in it
Electrolytes – dissociate into charged ions/molecules
Conduct electricity
Cations are positively charged; anions are negatively charged
Na+ is the main cation in extracellular fluid; K+ is the main cation in intracellular fluid
Non-electrolytes - do not dissociate
Glucose, albumin

5. Osmolality Total concentration of all solute particles in a solution
Tonicity – comparing ECF to ICF
Isotonic – no concentration gradient
Hypotonic – ECF has lower concentration of solutes
Cell gains water
Hypertonic – ECF has higher concentration of solutes
Cell loses water
Water moves between extracellular and intracellular compartments based on osmotic pressure

7. Osmolality cont
Increase of plasma osmolality (not enough water; too many solutes)
Triggers hypothalamus
Thirst center
ADH – aquaporins in nephrons to recapture more water
Decrease of plasma osmolality
Inhibits hypothalamus

8. Disorders of water balance
Dehydration
Output exceeds input
Fever, mental confusion, hypovolemic shock
Treated by increase of water intake – orally, intravenously
Hypotonic hydration
Overhydration
Renal problems, excess of ADH excessive intake
Neural damage, death
Treated by administration of hypertonic solutions

9. Disorders of water balance cont
Edema
Excess water in interstitial fluids
Fluid leaves bloodstream at a greater rate than the return
Due to increase pressure in capillaries
Congestive heart failure, high blood volume
Plasma proteins aid with regulation of osmotic pressure
Hypoproteinemia – low level of plasma proteins
Liver failure
Fluid gets pushed out of capillaries by blood pressure, and normally returns due to osmotic pressure
Low protein levels decrease osmolality, and water is not transported back into bloodstream

10. Electrolyte balance Controls movements of fluids
Required for body functions
Na+ and K+ for nervous impulses, Ca2+ for muscle contraction
Required for structure
Ca2+ for bones

11. Regulation of sodium
~90% of sodium ions in filtrate in proximal convoluted tubule and loop of Henle
Angiotensin II causes the release of aldosterone from adrenal glands
Aldosterone causes additional sodium to be reabsorbed in distal convoluted tubule
Increased sodium reabsorption causes additional water to be reabsorbed
Increases blood volume/pressure

12. Regulation of sodium cont
Cardiovascular baroreceptors
Stretch receptors in aorta and carotid bodies send signal to brainstem – blood pressure too high
Nervous system causes afferent arteriole to dilate
Increases glomerular filtration rate (GFR)
Increased speed through nephron = less time to be reabsorbed
Increased water and salt output in urine
Estrogen acts like aldosterone
Increases water and sodium reabsorption
Water retention during menstruation

13. Sodium imbalance Hyponatremia Hypernatremia
Dilutional of hypertonic (hyperglycemia causes osmotic movement of water)
Muscle cramps, weakness, disorientation
SLOW administration of hypertonic saline solution
Hypernatremia
More common in individuals unable to obtain water (infants) or elderly (hypodipsia); enema abuse
Dryness of mucous membranes, coma, seizures
Replacement fluids

14. Regulation of potassium/imbalance
Maintains intracellular osmolality
Needed for sodium-potassium pump; exchanged for H+ for buffer changes in blood
Hyperkalemia
Rare in healthy individuals (excess secreted into renal tubules)
Raises neuron resting potential – causes nerves to fire more easily
Muscle weakness, ventricular fibrillation, cardiac arrest
Dietary restriction, dialysis, IV of insulin and glucose

15. Potassium imbalance cont
Hypokalemia
Inadequate intake, excessive losses (Mg depletion, vomiting, diuretic use), redistribution from ECF to ICF (decongestants, insulin)
Lowers neuron resting potential away from threshold
Cells fire less easily
Weakness/fatigue, cramps, paralysis, respiratory insufficiency
Oral supplements, diet, IV

16. Calcium imbalance Hypercalcemia
Increased bone breakdown (hyperparathyroidism), prolonged immobilization
Blocks sodium gates; cells fire less easily
Weakness, muscle flaccidity, ventricular arrhythmia
Diuretics and NaCl (increases calcium excretion in kidneys; inhibition of osteoclasts

17. Calcium imbalance cont
Hypocalcemia
Abnormal loss from kidney, impaired access to bone calcium, hypoparathyroidism
Unable to block sodium gates; cells fire more easily
Oral calcium and vitamin D

18. Magnesium imbalance Cofactor of enzymes, can block calcium channels
Hypermagnesemia
Rare due to kidney excreting excess (seen in kidney disease)
Overdose of Mg containing compounds (antacids, supplements)
IV calcium (antagonistic to Mg)
Hypomagnesemia
Insufficient intake, abnormal losses in kidneys
Increases muscle excitability (tremors, arrhythmia)

19. Acid-Base balance Acid – contributes H+/protons to solution
Base – removes H+ from solution
Buffer – resists change in pH
Blood pH 7.35 – 7.45
Alkalosis – pH above 7.45
Acidosis – pH below 7.35

20. Blood buffering H2O + CO2 ↔ H2CO3 ↔ H+ + HCO3-
Increase of carbon dioxide (depressed ventilation) creates H+
Causes pH to drop/become more acidic
Bicarbonate ions will join with hydrogen ions to form carbonic acid
Decrease in carbon dioxide causes pH to rise/become more alkaline
Carbonic acid dissociates to release H+

22. Blood pH imbalance Respiratory (due to CO2 levels) or metabolic
When pH is lower than 7.0
Central nervous system is depressed
Coma, death
When pH is higher than 7.8
Nervous system is overexcited
Convulsions, respiratory arrest

23. Respiratory pH imbalance
Respiratory acidosis
CO2 accumulation in blood
Shallow breathing, emphysema, cystic fibrosis, pneumonia
Respiratory alkalosis
CO2 eliminated too quickly
Shallow breathing

24. Metabolic pH imbalance
Metabolic acidosis
Excessive alcohol intake (metabolized to acetic acid), loss of anions due to diarrhea, lactic acid from excessive exercise, ketosis – diabetic crisis or starvation
Metabolic alkalosis
Loss of stomach acids from vomiting, excessive intake of bases (antacids)