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

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

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 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

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)