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

Body Fluids • Water content – ~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

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

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

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

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

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

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

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

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

Sodium imbalance • Hyponatremia – 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

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

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

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

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

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)

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

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+

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

Respiratory pH imbalance

• Respiratory acidosis – CO2 accumulation in blood – Shallow breathing, emphysema, cystic fibrosis,

pneumonia • Respiratory alkalosis– CO2 eliminated too quickly – Shallow breathing

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)