Fluid, Electrolyte, and Acid-Base Balance

I. Body Fluids

A. Fluid Compartments

• Water occupies three main locations within the human body:
  • Intracellular Fluid (ICF) compartments --> cytoplasm within a cell, accounts for 2/3 volume of body fluids
  • Extracellular Fluid (ECF) compartments --> water outside cells (plasma & interstitial fluid)
  • Other - lymph, CSF, humors of eye, serous fluid, and GI secretions

B. Composition of Body Fluids

• Water is a universal solvent and dissolves various ionic and covalent bonded compounds that are classified as either an electrolyte or nonelectrolyte
• Nonelectrolytes contain covalent bonds that prevent them form dissociating in solution and therefore have no electrical charge (egs. glucose, lipids, and urea)
  
• Electrolytes dissociate into ions (ionize) in water; ions are charged particles and conduct an electrical current
• Electrolyte examples --> Mg⁺, Na⁺, Cl^-, K⁺
  
• Dissolved solutes increase osmotic activity of a fluid; electrolytes have higher osmotic power than nonelectrolytes because each electrolyte molecule dissociates into at least two ions
  • NaCl ---------> Na⁺ + Cl^-
  • MgCl₂ ---------> Mg²⁺ + Cl^- + Cl^-
  • glucose ---------> glucose
• Therefore electrolytes have a greater ability to cause fluid shift
• NOTE: review the chemical properties of water and functions of ions covered previously (i.e. AP I)
  

C. Movement of Fluids Between Compartments

• Exchange of body fluids regulated by osmotic and/or hydrostatic pressures
• Osmotic pressure - pressure resulting from the movement of water through a membrane and against its concentration gradient (i.e. the process of osmosis)
• Hydrostatic pressure - pressure exerted to counteract the process of osmosis
  
• Solutes are unequally distributed because of molecular size, electrical charge, or dependence on active transport; therefore changes in solute concentration cause net water flows
• Exchanges between plasma and interstitial fluid occur across capillary membranes (driven by hydrostatic pressure of blood); plasma filters into interstitial space, water is reabsorbed and any leakage is picked up by lymphatic vessels
  
• Exchanges between the interstitial and intracellular fluids depend upon selective permeability of the cell and forms of transport (active transport)
  
• Plasma servers as link between the external and internal environments

II. Water Balance

A. Overview

• Water intake = Water output
• Intake = 2500 ml/day, water ingested as fluids (60%), foods (30%), and produced from cell metabolism or also called metabolic water (10%)
  
• Water output - vapor in lungs/diffusion from the skin (28%), perspiration (8%), and feces (4%); rest is excreted by kidneys as urine (60%)
• Rise in plasma osmolarity (soute concetration) triggers:
  • Thirst, provoking water intake
  • ADH release, causing the kidneys to excrete concentrated urine

B. Regulation of Water Intake (Thirst Mechanism)

• Thirst Mechanism
  • Decrease in plasma volume and increase in plasma osmolarity causes dry mouth, in stimulating the hypothalamic thirst centers
  • Dry mouth results from a decrease in water filtered from the bloodstream (therefore increased osmolarity) and therefore salivary gland receives less water, in turn producing less saliva
  • Hypothalamic stimulation occurs when water moves (due to hypertonic ECF) out of thirst center osmoreceptors by osmosis, causing osmoreceptors to become irritable and depolarize (therefore sensation of thirst)
    
• Thirst is imperfect - osmoreceptors provide feedback and inhibit hypothalamic thirst centers BEFORE enabling osmotic changes to occur (thereby prematurely stopping intake of water)

C. Regulation of Water Output

• Obligatory water losses - insensible water loss from lungs and through skin, undigested food, feces, and urine
• Kidneys can concentrate urine, but only a minimum of 500 ml of water is lost in urine/day and therefore concentration and volume of urine excreted depends on fluid intake

D. Disorders of Water Balance

• Dehydration - water loss exceeds water intake
  
• Hypotonic hydration - ECF is diluted; sodium concentration is normal but there is an increase in water, causing ECF sodium levels to lower (hyponatremia), increase in osmosis occurs and tissue cells swell (edema)

III. Electrolyte Balance

A. Role of Sodium in Fluid and Electrolyte Balance

• Sodium most abundant cation in the ECF and is the only one exerting a significant osmotic pressure
• Sodium does not easily cross cellular membranes, it must be pumped across; therefore, abundance, osmotic effect, and transport of sodium are controlling factors of ECF volume and water distribution
  
• While the sodium content of the body may be altered, its concentration in the ECF remains stable because of immediate adjustments in water volume; WATER FOLLOWS SALT
• Because all body fluids are in osmotic equilibrium, a change in plasma sodium levels affects not only the plasma volume and blood pressure (intravascular compartment), but also the fluid volumes of the other two compartments (ICF and ECF)
  

B. Regulation of Sodium Balance (and sodium-water balance, BP, and Blood Volume)

i. Influence of Aldosterone

• 75-80% of sodium (NaCl) in renal filtrate is reabsorbed in proximal tubules of kidneys
• Aldosterone aids in actively reabsorbing remaining Na⁺Cl^- in distal convoluted tubule/collecting tubule by increasing tubule permeability; therefore aldosterone promotes both sodium and water retention
• Mechanism:
  • increase in K or decease in Na in blood plasma renin-angiotensin Mechanism
  • stimulates adrenal cortex to release aldosterone
  • aldosterone targeted towards the kidney tubules
  • increase in Na reabsorption increase in K secretion
  • restores homeostatic plasma levels of Na and K
• Influences on aldosterone synthesis and release:
  • Elevated potassium levels in ECF directly stimulates adrenal cells to secrete aldosterone
  • Juxtaglomerular apparatus of renal tubes release renin in response to:
    1. decreased stretch (due to decrease in blood pressure)
    2. decreased filtrate osmolarity
    3. sympathetic nervous system stimulation

ii. Cardiovascular system

• As blood volume (and pressure) rises, the baroreceptors in the heart and in the large vessels of the neck and thorax (carotid arteries and aorta) communicate to the hypothalamus
• Sympathetic nervous system impulses to kidneys decrease, allowing afferent arterioles to dilate; as the glomerular filtration rate rises, sodium and water output increases (causing pressure diuresis)
• Reduced blood volume and pressure results

iii. Influence of ADH

• Amount of water reabsorbed in the distal segments of the kidney tubules is proportional to ADH release (increase in ADH secretion = increase in water resorption)
• Osmoreceptors of the hypothalamus sense the ECF solute concentrations and trigger or inhibit ADH release from the pituitary
• Mechanism:
  • decrease in sodium concentration in plasma (decreased osmolarity)
  • stimulates osmoreceptors in hypothalamus
  • stimulates posterior pituitary to release ADH
  • ADH targeted toward distal and collecting tubules of kidney
  • the effect is increased water resorption
  • plasma volume increases, osmolarity decreases
  • scant urine produced

iv. Influence of atrial natriuretic factor (ANF)

• Reduces blood pressure and blood volume by inhibiting nearly all events that promote vasoconstriction and sodium and water retention
• In essence, inhibits ADH and Aldosterone production
  

C. Regulation of Potassium Balance

• Potassium is the chief intracellular cation
• Relative intracellular-extracellular potassium concentrations directly affects a cell's resting membrane potential, therefore a slight change on either side of the membrane has profound effects (egs. on neurons and muscle fibers)
• Potassium is part of the body's buffer system, which resists changes in pH of body fluids; ECF potassium levels rise with acidosis (decrease pH) as potassium leave cells and fall with alkalosis (increase pH) as potassium moves into cells
  
• Potassium balance is maintained primarily by renal mechanisms (i.e. influenced by Aldosterone)
  
• Potassium reabsorption from the filtrate is constant - 10-15% is lost in urine regardless of need; because potassium content of ECF is low (compared to sodium concentration), potassium balance os accomplished by changing amount of potassium secreted into the filtrate; therefore regulated by collecting tubules

D. Regulation of Calcium Balance

• 99% of calcium found in bones as an apatite
• Calcium needed for blood clotting, nerve transmission, enzyme activation, etc...
• Calcium ion concentration is regulated by interaction of two hormones: parathyroid hormone and calcitonin
  
• Calcium ion homeostasis: effects of PTH and calcitonin
  
  • PTH - released by the parathyroid cells, promotes increase in blood calcium levels by targeting...
    • Bones - PTH activates osteoclasts, which breakdown the matrix
    • Small intestines - PTH enhances intestinal absorption of calcium ions indirectly by stimulating the kidneys to transform vitamin D to its active form which is a necessary cofactor for calcium absorption
    • Kidneys - PTH increases calcium reabsorption by renal tubes while simultaneously decreasing phosphate ion reabsorption
  • Calcitonin - targets bone to encourage deposition of calcium salts and inhibits bone reabsorption (therefore an antagonist of PTH and decreases blood calcium levels)

E. Other Major Electrolytes: Magnesium and Chloride

• Magnesium - cation, cofactor of many enzymes and is need for both sodium-potassium pump and calcium ion channel function
  
• Chloride - anion, part of hydrochloric acid (chemical digestion) and involved in chemical digestion and blood chemistry (e.g. chloride shift in oxygen/carbon dioxide circulation)
  

IV. Acid-Base Balance

• All biochemical reactions are influenced by pH of their fluid environment, therefore optimum conditions and balance (acid-base) is required
• Optimal pH of various body fluids differ but not by much (pH of body fluids: arterial blood = 7.4, venous blood and interstitial fluid = 7.35, intracellular fluid = 7.0)
• Changes in pH in blood: arterial blood >7.45 = alkalosis and <7.35 = physiologic acidosis
  
• Body has buffer systems: Bicarbonate (blood ph), Phosphate (urine pH), and Protein buffers (e.gs., albumins, amino acids, hemoglobin, regulation of blood pH)