Fluid, Electrolyte, and Acid-Base Balance

• Fluid, Electrolyte, and Acid-Base Balance are critical for homeostasis
• It is important to understand water compartments, mechanisms of fluid balance, electrolyte roles and distributions, causes and symptoms of imbalances, acid-base regulation, and compensation mechanisms

Fluid Compartments

• Water constantly circulates within the body and moves between compartments
• Maintaining water balance in these compartments is essential for homeostasis

Body fluid compartments

• Intracellular fluid (ICF) is the fluid within cells

• Extracellular fluid (ECF) exists outside cells, including:

  • Interstitial fluid (tissue fluid)
  • Blood plasma
  • Lymph
  • Specialized fluids like CSF, synovial fluid, aqueous humor, and serous fluid

• Water intake should equal water loss to maintain water balance

• Water sources include ingested fluids, food, and cell respiration

• Water exits the body through urine, perspiration, exhaled air, and feces

Regulation of Water Intake and Output

• The hypothalamus contains a thirst mechanism
  • Osmoreceptors in the hypothalamus detect fluid volumes and concentrations, promoting fluid intake as needed
• Antidiuretic hormone (ADH)
  • ADH promotes water reabsorption from kidney tubules into the blood, reducing water loss in urine
• Aldosterone
  • Aldosterone increases sodium reabsorption from kidney tubules
  • Water then passively follows sodium via osmosis
• Atrial natriuretic peptide
  • This promotes sodium and water loss in urine

Electrolytes

• These are chemicals that dissociate into ions (charged particles) when dissolved in water
  • Most electrolytes are inorganic minerals
  • Cations are positively charged ions
  • Anions are negatively charged ions
• Electrolytes create osmotic pressure, which regulates the movement of water between body compartments
  • Areas with higher electrolyte concentrations have higher osmotic pressure

Electrolyte regulation

• Electrolytes are ingested through food and drinks
• Electrolytes are excreted via urine, sweat, and feces

Hormones

• Some hormones regulate extracellular fluid (ECF) electrolyte concentrations
  • Aldosterone increases sodium reabsorption and potassium excretion in the kidneys, increasing blood sodium and decreasing blood potassium levels
  • Atrial Natriuretic Peptide increases sodium excretion by the kidneys; decreasing blood sodium levels
  • Parathyroid hormone (PTH) increases calcium reabsorption from bones and absorption in the small intestines, decreases phosphate reabsorption, increasing blood calcium, while decreasing blood phosphate
  • Calcitonin promotes calcium and phosphate removal from blood to form bone matrix, decreasing blood calcium and phosphate

Electrolytes in Body Fluids

• Intracellular Fluid (ICF)
  • Potassium is the most abundant cation
  • HPO₄²⁻ is the most abundant anion
  • Protein anions are also abundant
• Extracellular Fluids (ECF) Plasma
  • Sodium is the most abundant cation
  • Chloride is the most abundant anion
  • Protein anions are significant
• Interstitial (tissue) Fluid
  • Similar to plasma, but with fewer protein anions

Electrolyte Concentrations

• Concentrations are expressed in milliequivalents per liter (mEq/L)
• Sodium (Na+)
  • Plasma level: 136-142 mEq/L
  • ICF level: 10 mEq/L
  • Functions include: osmotic pressure in ECF and essential for electrical activity of neurons/muscle cells
• Potassium (K+)
  • Plasma level: 3.8-5.0 mEq/L
  • ICF level: 141 mEq/L
  • Functions include: osmotic pressure in ICF and essential for electrical activity of neurons/muscle cells
• Calcium (Ca²⁺)
  • Plasma level: 4.6-5.5 mEq/L
  • ICF level: 1 mEq/L
  • Functions include: normal excitability of neurons/muscle cells and essential for blood clotting
• Magnesium (Mg²⁺)
  • Plasma level: 1.3-2.1 mEq/L
  • ICF level: 58 mEq/L
  • Functions include: essential for ATP production, neural and muscle activity
• Chloride (Cl⁻)
  • Plasma level: 95-103 mEq/L
  • ICF level: 4 mEq/L
  • Functions include: Diffuses easily in & out of cells, osmotic pressure in ECF, part of HCI in gastric juice
• Bicarbonate (HCO₃⁻)
  • Plasma level: 28 mEq/L
  • ICF level: 10 mEq/L
  • Part of bicarbonate buffer system
• Phosphate (HPO₄²⁻)
  • Plasma level: 1.7-2.6 mEq/L
  • ICF level: 75 mEq/L
  • Functions include: Primarily an ICF anion, part of DNA, RNA, ATP, phospholipids, phosphate buffer system
• Sulfate (SO₄²⁻)
  • Plasma level: 1 mEq/L
  • ICF level: 2 mEq/L
  • Functions include: Part of some amino acids & proteins

Movement of Fluid

• Water and ions are constantly moving to maintain fluid-electrolyte homeostasis
• Relative proportions remain constant in compartments
• Fluid circulates through the body via filtration and osmosis
  • Filtration moves fluid from high to low pressure
  • Osmosis moves fluid from low to higher solute concentration
• Fluid movement depends on relative hydrostatic and osmotic pressures within compartments
  • Hydrostatic pressure is the "push" force to move fluid out
  • Osmotic pressure is the "pull" force to draw fluid in
• Changes in either force alter fluid movement and volumes within a compartment

Fluid Imbalance

• Fluid excess can be caused by kidney disease, liver disease, or hypoalbuminemia
  • This may be due to decreased liver protein production, kidney disorder causing excess protein excretion, malabsorption syndromes, or low dietary protein intake
  • Symptoms can include: Localized edema/swelling (feet, hands, ascites), weight gain, lethargy; possible seizures, pale or red skin color, slow, bounding pulse, high BP, pulmonary congestion, cough, decreased hematocrit, increased urine output; low specific gravity
• Fluid deficit is potentially caused by dehydration, severe hemorrhage, or 3rd spacing after trauma
  • Dehydration includes losses via vomiting & diarrhea, excessive sweating, insufficient water/fluid intake, or diabetic ketoacidosis
  • Symptoms can include: Sunken eyes, fatigue, weakness, possible confusion, pale skin, decreased skin turgor, dry mucous membranes, weight loss, rapid, weak, thready pulse, low BP & orthostatic hypotension, increased hematocrit, decreased urine output, high specific gravity

Electrolyte Imbalances

• Sodium Imbalance: Hyponatremia, is caused by excessive sweating, diarrhea, or vomiting, renal failure, excessive water intake and hormone imbalance
• Sodium Imbalance; Hypernatremia, possible causes are excessive sodium ingestion w/o increased water intake, loss of thirst mechanism, hormone imbalance, and severe watery diarrhea
• Potassium Imbalance: Hypokalemia, this is caused by excessive losses by diarrhea or vomiting, diuresis associated with some medications, endocrine dysfunction, decreased dietary intake and kidney disease
• Potassium Imbalance: Hyperkalemia, is caused by renal failure, endocrine dysfunction (aldosterone deficit), "potassium-sparing" diuretics, and extensive tissue damage
• Calcium imbalance: Hypocalcemia, possible causes are hypoparathyroidism, malabsorption syndrome, and renal failure
• Calcium imbalance: Hypercalcemia, possible causes are hyperparathyroidism, demineralization caused by immobility, increased calcium and vitamin D intake, and malignant bone tumors

Acid-Base Balance

• Normal blood pH range: 7.35-7.45
• Normal intracellular fluid pH range: 6.8-7.0
• An acid-base balance is essential to homeostasis
  • Cell enzymes can only effectively function within a narrow pH range

Control of Serum pH

• There are 3 mechanisms to control pH
  • Buffer systems work fastest
  • The respiratory system alters CO2 (carbonic acid) levels to change pH
  • The kidneys modify the excretion rate of acids and modify production and absorption of bicarbonate ions

Buffer Systems

• Buffer systems prevent drastic changes in pH
• Weak acid & base pairs react with strong acids or bases to prevent large shifts in pH
• Buffer systems gives the fastest response, but lowest capacity
  • They React within a fraction of a second but also a limited number of molecules in body fluids
• Major systems: Bicarbonate buffer system, Phosphate buffer system, and Protein buffer system

Bicarbonate Buffer System

• This is an important one in blood and tissue fluid
• Components
  • Carbonic acid (H₂CO₃) & sodium bicarbonate (NaHCO3)
  • H2CO3 reacts with strong bases, producing weaker base
  • NaHCO3 reacts with strong acids, producing weaker acid
• There is more sodium bicarbonate needed than H₂CO₃
  • Ratio of NaHCO3 to H₂CO₃~ 20:1

Phosphate Buffer System

• Components include sodium dihydrogen phosphate (NaH2PO4) and sodium monohydrogen phosphate (Na2HPO4)
• This is important in regulation of blood pH by kidneys
• Cells can remove excess H+ ions, which is then excreted in urine.
• Na+ ions returned to blood in peritubular capillaries, along with bicarbonate
• Bicarbonate ions are synthesized by renal cells

Protein Buffer System

• This is the most important intracellular buffer system
  • Example: Hemoglobin buffers the H+ formed during CO2 transport
• Amino acids have carboxyl and an amine group
  • The COOH group can act as an acid
  • The NH2 group can act as a base

Role of the Kidneys in Acid-Base Balance

• The greatest role is in maintaining pH of blood & tissue fluid
• When body fluids are too acidic, the kidneys secrete more H+ ions and return more bicarbonate to blood
• When body fluids are too alkaline, the kidneys return H+ ions and also excrete more bicarbonate in urine

Respiration & Acid-Base Balance

• Respiration plays a role in regulating the amount of CO2 in body fluids
• Chemoreceptors in the medulla detect pH and CO₂ levels
  • RR & depth increase when pH lowers, so more carbonic acid exhaled and pH increases
  • RR decreases & breaths become shallower when pH is high, so the body retains more carbonic acid to decrease pH

Acid-Base Imbalance

• Acidosis; either a respiratory or metabolic issue
  • Excess H+ ions will be apparent
  • There will therefore be a decrease in serum pH
• Alkalosis; either a respiratory or metabolic issue
  • There is a deficit of H+ ions will be apparent
  • There will therefore be an increase in serum pH

Acid-Base Imbalances - Potential Causes and Compensation Mechanisms

• Metabolic acidosis has potential causes of kidney disease, diarrhea/vomiting and diabetic ketoacidosis
  • The compensation mechanism will involve increased respiratory rate with deeper respirations
• Metabolic alkalosis has potential causes including over ingestion of bicarbonate meds, gastric suctioning
  • The compensation mechanism will involve decreased respiratory rate and shallow respirations
• Respiratory acidosis has potential causes including decreased respiratory efficiency, paralysis of respiratory muscles and drug overdose
  • The compensation mechanism will involve the kidneys excreting more H+ ions and reabsorbing more Na+ and HCO₃⁻ ions
• Respiratory alkalosis has potential causes including hyperventilation and increased respiratory rate at high altitudes
  • The compensation mechanism will involve the kidneys retain more H+ ions and excrete more Na+ and HCO₃⁻ ions