BIO 226 Chap 25: Fluid and Electrolyte Balance

Fluid and Electrolyte Balance

• Learning outcomes include naming body fluid compartments.
• Learning outcomes include discussing fluid and mineral balance importance for homeostasis.
• Learning outcomes include summarizing the relationship between sodium and water.
• Learning outcomes include explaining the factors that control potassium balance, hypokalemia, and hyperkalemia.

Body Composition

• Water distributes in fluid compartments, creating distinct environments with differing ionic concentrations.
• Extracellular fluid (ECF) includes interstitial fluid of peripheral tissues, plasma, lymph, cerebrospinal fluid (CSF), synovial fluid, serous fluids, aqueous humor, perilymph, and endolymph.
• Intracellular fluid (ICF) includes the cytosol inside cells.
• Solid body components account for 40–50% of body mass.
• Solid body components include proteins, lipids, carbohydrates, and minerals.

Fluid Balance

• Fluid balance exists when water content remains stable.
• Water gain occurs via absorption along the digestive tract and metabolic processes.
• Urination accounts for over 50% of water loss.
• Other water losses include feces and evaporation from skin and lungs.
• Water moves by osmosis and passive flow down osmotic gradients.
• ICF and ECF compartments have different compositions but exist at osmotic equilibrium.
• Fluid shift involves rapid water movement between ECF and ICF.
• Equilibrium between ECF and ICF is reached in minutes to hours.
• Dehydration happens when water losses are greater than water gains.
• Water loss from ECF increases osmotic concentration in ECF
• Water moves from ICF to ECF for osmotic equilibrium; both fluids become more concentrated.
• Continued fluid imbalance and water loss from ICF lead to severe thirst, dryness, and skin wrinkling.
• Continued fluid loss drops blood volume and blood pressure, potentially causing circulatory shock.

Mineral Balance

• A mineral is an inorganic substance.
• An electrolyte is an ion released when mineral salts dissociate.
• Mineral balance occurs when ion absorption and excretion are equal.
• Absorption happens across the lining of the small intestine and colon.
• Excretion primarily happens at the kidneys, with variable loss at sweat glands.
• The body maintains reserves of key minerals.
• Daily mineral intake should average the amount lost to maintain balance.

Electrolyte Solutions

• Electrolytes movement in solution can be summarized as follows:
  • Sodium (Na+) uses channel-mediated diffusion, cotransport, or active transport.
  • Calcium (Ca2+) uses active transport.
  • Potassium (K+) uses channel-mediated diffusion.
  • Magnesium (Mg2+) uses active transport.
  • Iron (Fe2+) uses active transport.
  • Chloride (Cl-) uses channel-mediated diffusion or carrier-mediated transport.
  • Iodide (I-) uses channel-mediated diffusion or carrier-mediated transport.
  • Bicarbonate (HCO3-) uses channel-mediated diffusion or carrier-mediated transport.
  • Nitrate (NO3-) uses channel-mediated diffusion or carrier-mediated transport.
  • Phosphate (PO43-) uses active transport.
  • Sulfate (SO42-) uses active transport.

Mineral and Mineral Reserves

• Bulk minerals and their values:

  • Sodium (Na+): major cation in body fluids; essential for normal membrane function, is 110 g primarily in body fluids and its route of excretion is through urine, sweat and feces.
  • Potassium (K+): major cation in cytosol; essential for normal membrane function, is 140 g primarily in cytosol with the route of excretion being urine.
  • Chloride (Cl-): Major anion in body fluids; functions in forming HCl, is 89g primarily in body fluids, and the excretion route being primarily urine and sweat.
  • Calcium (Ca): is essential for muscle and neuron function and normal bone structure, with 1.36 kg primarily in skeleton with the excretion route through urine and feces.
  • Phosphorus (P): a high energy compound, found in nucleic acids, and bone matrix (as phosphate), typically 744 g, primarily in the skeleton via urine and feces.
  • Magnesium (Mg): cofactor of enzymes, required for normal membrane function, comes in at 29 g (17 g is cytosol and body fluids, and 12 g is in the skeleton) excreted through urine.

• Trace elements, their function and values include:

  • Iron (Fe); component of hemoglobin, myoglobin, and cytochromes; 3.9 g (1.6 g stored as ferritin or hemosiderin), excreted through urine (traces).
  • Zinc (Zn); cofactor of enzyme systems, notably carbonic anhydrase; 2 g, excreted through urine and hair (traces).
  • Copper (Cu); required as cofactor for hemoglobin synthesis; 127 mg, excreted through urine and feces (traces).
  • Manganese (Mn); cofactor for some enzymes; 11 mg, excretion is through feces and urine (traces).
  • Cobalt (Co); cofactor for transaminations; mineral in vitamin B12 (cobalamin); 1.1 g, excreted through feces and urine.

Sodium and Water Balance

• Sodium balance is when sodium gains equal sodium losses.
• Regulatory mechanisms change the ECF volume while keeping sodium concentration stable.
• When sodium gains exceed losses, ECF volume increases.
• When sodium losses exceed gains, ECF volume decreases.
• The primary hormone involved is ADH.
• Small changes in ECF volume do not cause adverse physiological effects.
• When changes in ECF volume are extreme, homeostatic mechanisms are utilized.
• Increased ECF volume means increased blood volume and pressure therefore mechanisms respond to lower blood volume and blood pressure.
• Decreased ECF volume means decreased blood volume and blood pressure therefore mechanisms respond to increased blood volume and pressure.
• Sustained sodium imbalances in ECF only occur with severe fluid balance problems.
• Sodium imbalances are serious and potentially life-threatening.
• Hyponatremia (natrium, sodium) is low ECF sodium concentration (<136 mEq/L). It results from overhydration or inadequate salt intake.
• Hypernatremia is high ECF sodium concentration (>145 mEq/L). Dehydration is the most common cause.

Potassium Balance

• Key factors to maintaining potassium balance: the rate of K+ entry across the digestive epithelium (~100mEq or 1.9–5.8g/day), and the rate of K+ loss into urine.
• Potassium ion concentration is highest in ICF due to the Na+/K+ exchange pump (~135 mEq/L in ICF vs ~5 mEq/L in ECF).
• The kidneys mainly drive K+ concentration in ECF, with dietary intake of K+ being relatively constant and K+ loss controlled by aldosterone.
• Na+/K+ exchange pumps; Aldosterone stimulates Na+ reabsorption and K+ excretion.
• Low pH in ECF causes H+ to be substituted for K+.
• Hypokalemia (kalium, potassium) is potassium levels below 2 mEq/L in plasma (normal levels 3.5–5.0 mEq/L).
• Hypokalemia can be caused by diuretics or aldosteronism (excessive aldosterone secretion).
• Symptoms include muscular weakness, paralysis, and becoming lethal to the heart.
• Treatment includes increased dietary intake of potassium.
• Hyperkalemia is potassium levels above 5 mEq/L in plasma.
• Hyperkalemia can be caused by chronically low pH, kidney failure, or drugs promoting diuresis by blocking Na+/K+ pumps.
• Symptoms include Muscular spasm, including heart arrhythmias.
• Treatment includes Dilution of ECF with a solution low in K+, stimulating K+ loss in urine with diuretics, adjusting pH of the ECF, and restricting dietary K+ intake. If caused by renal failure, dialysis may be required.

Acid-Base Balance

• The body is in acid-base balance when H+ production equals H+ loss, and the pH of body fluids is within normal parameters.
• Buffer systems temporarily store H+ and provide short-term pH stability.
• H+ production derives from CO2 (to carbonic acid) from aerobic respiration and lactic acid from glycolysis.
• Constant production by these processes creates a primary challenge to acid-base homeostasis.
• Respiratory system eliminates CO2
• Kidneys excrete H+
• Buffers temporarily store H+
• Storage removes H⁺ from circulation, affecting pH
• Fixed acids do not leave the solution and remain in body fluids until kidney excretion, e.g., sulfuric and phosphoric acid and are generated during metabolism.
• Metabollic acids are products or byproducts of cell metabolism; metabolized quickly. Examples: pyruvic acid, lactic acid, and ketones.
• Volatile acids are are able to leave the body by entering the atmosphere, e.g carbonic acid
• Buffers in body fluids temporarily neutralize the acids produced by metabolic operations.
• pH refers to the negative exponent (negative logarithm) of the hydrogen ion concentration [H+] in a solution; pH is a measure of how acidic or basic a solution is
• Neutral (a solution with a pH of 7) contains equal numbers of hydrogen ions (H+) and hydroxide ions (OH)
• Acidic solution means the solution with a pH below 7; in this solution, hydrogen ions predominate
• Basic (alkaline) is the solution with a pH above 7; in this solution, hydroxide ions predominate
• Acids refer to a substance that dissociates to release hydrogen ions, decreasing pH
• Base, in contrast, is a substance that dissociates to release hydroxide ions or to remove hydrogen ions, increasing pH
• Salt is an ionic compound consisting of a cation other than a hydrogen ion and an anion other than a hydroxide ion
• Buffer is a substance that tends to oppose changes in the pH of a solution by removing or replacing hydrogen ions; in body fluids, buffers maintain blood pH within normal limits (7.35–7.45)
• Normal pH of the ECF is 7.35–7.45.
• Changes in H+ concentrations alters the stability of plasma membranes, structure of proteins, activities of enzymes, and have major effects on the nervous and cardiovascular systems
• pH below 6.8 or above 7.7 is quickly fatal.
• Acidosis is a physiological condition caused by plasma pH < 7.35 (acidemia). Severe acidosis (pH < 7.0) can be deadly because CNS function deteriorates potentially causing coma, Cardiac contractions become weak and irregular, and Peripheral vasodilation causes BP drop, potentially leading to circulatory collapse.
• Alkalosis is a physiological condition caused by plasma pH > 7.45 (alkalemia) . It is can be dangerous but is relatively rare as it relates to pH.

Carbon Dioxide and pH

• Partial pressure of carbon dioxide (PCO2) is the most important factor affecting pH of body tissues.
• Carbon dioxide (CO2) combines with water to form carbonic acid (H2CO3), which can dissociate into hydrogen ions (H+) and bicarbonate ions (HCO3-).
• An inverse relationship exists between PCO2 and pH.
• Increase in PCO2 results in a decrease in pH.
• Decrease in PCO2 results in an increase in pH.
• A buffer system in body fluids generally consists of a weak acid (HY) and an anion released by its dissociation (Y-); this anion functions as a weak base.
• With equilibrium of weak acids and anions addition of of H+ ions disrupts equilibrium resulting in formation of more weak acid molecules (and fewer free H+ ions)
• With equilibrium of weak acids and anions removing of H+ ions also disrupts equilibrium and Results in more dissociation (and more free H+ ions)
• Three major body buffer systems include the phosphate buffer system (buffers pH of ICF and urine), protein buffer system and Carbonic acid – bicarbonate buffer system
• In the Hemoglobin buffer system; Only intracellular buffer system having immediate effect on pH and Red blood cells absorb CO2 from plasma to the lungs where the process is reversed, and CO2 are released into the alveoli
• Proteins contribute to regulation of pH in ECF and ICF and bind excess H+ ions

Amino Acids and Carbonic Acids

• The amino acid buffers bind excess H+ ions through carboxylate group (COO–), forming carboxyl group (–COOH), Amino group (–NH2), forming an amino ion (–NH3+), and R-groups, forming RH+ and provides the most of the buffering capacity
• The carbonic acid protects the body against the effects of acids - the released generating carbonic acid H+
• Metabolic acid-base disorders Result from the production or loss of excessive amounts of fixed or organic acids. The carbonic acid–bicarbonate buffer system protects against these disorders
• The respiratory acid base disorder results from an imbalance of CO2 generation, carbonic acid–bicarbonate buffer system cannot protect against respiratory disorders. Imbalances must be corrected by depth and rate of respiration.

Homeostatic Responses

• Metabolic acidosis develops when large numbers of H+ are released by organic or fixed acids and pH decreases

• Responses to restore homeostasis are mainly comprised of increased respiratory rate and PCO2 levels. The converting more carbonic acid to water

• Metabolic renal response: secretion of more H+ ions into urine

• Tubular cells secrete + into tubular fluid including PCT, DCT and the collecting duct, and removal of CO2 and reabsorption

• Metabolic alkalosis - Develops when large numbers of H are removed from body fluids, raising pH- Kidney responses of secretion declines and collect bicarbonate from tubular fluid

• Decreasing respiratory rate and P are other response mechanism, and converting2 increase levels

Respiratory Acid-Base

• Disorders result from an imbalance between the rate of CO2 generation in body tissues and the rate of CO2

• Respiratory acidosis and alkaline can be remedied, but cannot be fully corrected.

• The 3 stages of respiratory acidosis are: shifting carbonic acid with subsequent H+ levels are achieved, 3 goes to the bicarbonate preserve and + levels are"tied". The issue requires correction with an increase in the respiratory rate.

• Respspiraroty acidosis and the increased and with a resulting increase, the buffer increase of HCO and O accept h+

• +ions230

• Respiratory alkalosis a condition where the rate of Co2 is eliminated, the the left. H ions create water and

• 2. is ebaled during respiration for the conditions with anxiety and most of the time the condition self corrects and return to balance.