Fluid Balance and Electrolytes

• Objectives of the material include identifying water compartments, fluid balance mechanisms, roles/distribution of electrolytes.
• Other objectives are, causes/symptoms of fluid imbalances, mechanisms of acid-base regulation, causes of acid-base imbalance.
• Concluding objectives are compensation mechanisms that occur with altered blood pH levels.

Fluid Compartments

• Water constantly moves between and circulates within the body’s compartments.
• Water balance in these compartments is crucial for maintaining homeostasis.

Types of Fluid Compartments

• Intracellular fluid (ICF) is the fluid inside the cells.
• Extracellular fluid (ECF) is the fluid outside the cells, including:
  • Interstitial fluid that surrounds tissue.
  • Blood plasma.
  • Lymph.
  • Specialized fluids like cerebrospinal fluid (CSF), synovial fluid, aqueous humor, and serous fluid.

Water Intake and Output

• Water intake should equal water output to maintain balance.
• Water sources are from ingested fluids/foods and cell respiration.
• Water is lost through urine, perspiration/sweat, exhaled air (water vapor), and feces.

Regulation of Water Intake and Output

• The thirst mechanism in the hypothalamus regulates water intake.
  • Osmoreceptors detect fluid volumes and concentrations, promoting fluid intake as needed.
• Antidiuretic hormone (ADH) regulates water output.
  • ADH promotes reabsorption of water from kidney tubules into the blood, reducing fluid loss in urine.
• Aldosterone regulates water output.
  • Aldosterone increases the reabsorption of sodium from kidney tubules; water follows sodium via osmosis.
  • Aldosterone conserves fluid when there is a deficit.
• Atrial natriuretic peptide (ANP) regulates water output.
  • ANP promotes the loss of sodium ions and water in urine.

Electrolytes

• Electrolytes dissolve in water and dissociate into ions.
  • Most electrolytes are inorganic minerals.
  • Cations are positively charged ions.
  • Anions are negatively charged ions.
• Electrolytes create osmotic pressure that regulates the movement of water between compartments.
  • Areas with higher electrolyte concentrations have higher osmotic pressure.

Electrolyte Regulation

• Electrolytes are ingested through food and drinks.
• Electrolytes are lost via urine, sweat, and feces.
• Hormones regulate some extracellular fluid (ECF) concentrations like aldosterone, ANP, parathyroid hormone (PTH), and calcitonin.

Hormonal Regulation of Electrolytes

• Aldosterone increases sodium reabsorption and potassium excretion by the kidneys, increases blood sodium, and decreases blood potassium.
• ANP increases sodium excretion by the kidneys and decreases blood sodium.
• Parathyroid hormone increases calcium reabsorption from bones and increases absorption in the small intestines and blood calcium levels, it also decreases phosphate reabsorption at convoluted tubules and blood phosphate levels.
• Calcitonin promotes calcium & phosphate removal from blood to form bone matrix and decreases blood calcium and phosphate levels.

Electrolytes in Body Fluids

• Intracellular fluid
  • Potassium (K+) is the most abundant cation.
  • HPO42- is the most abundant anion.
  • Protein anions are also abundant.
• Extracellular fluids - Plasma
  • Sodium (Na+) is the most abundant cation.
  • Chloride (Cl-) is the most abundant anion.
  • Protein anions are significant.
• Interstitial (tissue) fluid
  • Same as plasma except with fewer protein anions.

Plasma, ICF, Functions of Electrolytes

• Sodium (Na+):
  • Plasma Level: 136-142 mEq/L
  • ICF Level: 10 mEq/L
  • The most abundant cation in ECF regulates osmotic pressure in ECFand is essential for electrical activity of neurons & muscle cells.
• Potassium (K+):
  • Plasma Level: 3.8-5.0 mEq/L
  • ICF Level: 141 mEq/L
  • The most abundant cation in ICF regulates osmotic pressure in ICF and is essential for electrical activity of neurons & muscle cells.
• Calcium (Ca2+):
  • Plasma Level: 4.6-5.5 mEq/L
  • ICF Level: 1 mEq/L
  • This element is mostly found in bones & teeth, maintains normal excitability of neurons & muscle cells and is essential for blood clotting.
• Magnesium (Mg2+):
  • Plasma Level: 1.3-2.1 mEq/L
  • ICF Level: 58 mEq/L
  • Mostly found in bone, and more abundant in ICF than in ECF, it is essential for ATP production, as well as neural & muscle activity.
• Chloride (Cl-):
  • Plasma Level: 95-103 mEq/L
  • ICF Level: 4 mEq/L
  • The most abundant anion in ECF diffuses easily in & out of cells helps regulate osmotic pressure and is part of HCl 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
  • Mostly found in bones & teeth, primarily an ICF anion, it is part of DNA, RNA, ATP, phospholipids and the phosphate buffer system.
• Sulfate (SO42-):
  • Plasma Level: 1 mEq/L
  • ICF Level: 2 mEq/L
  • Part of some amino acids & proteins.

Movement of Fluid

• Water and ions are constantly moving to maintain fluid-electrolyte homeostasis.
  • Relative proportions in compartments remains constant.
• Fluid circulates throughout the body via filtration and osmosis.
  • Filtration from high pressure to low pressure.
  • Osmosis from low solute concentration to higher solute concentration.
• Fluid movement depends on relative hydrostatic and osmotic pressures within compartments.
  • Hydrostatic pressure is the "push" force that moves fluid out of an area, regulating filtration.
  • Osmotic pressure is the "pull" force that draws fluid into an area, regulating osmosis.
• Changes in either force alter fluid movement and volumes within a compartment.

Fluid Imbalance - Excess

• Potential causes:
  • Kidney disease
  • Liver disease
  • Hypoalbuminemia
  • Possible causes include decreased production in the liver, kidney disorder causing excess protein excretion, malabsorption syndromes and low dietary intake of protein.
• Electrolyte imbalance

Signs and Symptoms of Fluid Excess

• 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
• Signs & symptoms may vary depending upon the cause of the fluid imbalance.

Fluid Imbalance - Deficit

• Potential causes:
  • Dehydration due to vomiting & diarrhea, excessive sweating (affects Na+ & H2O levels), insufficient water/fluid intake, diabetic ketoacidosis.
  • Loss of fluid, electrolytes, & glucose in urine.
  • Severe hemorrhage.
  • 3rd spacing following trauma.

Signs and Symptoms of Fluid Deficit

• 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
• Signs & symptoms may vary depending upon the cause of the fluid imbalance

Electrolyte Imbalances - Sodium

• Hyponatremia
  • Possible causes - Excessive sweating, diarrhea, or vomiting, renal failure, excessive water intake, hormone imbalances.
• Hypernatremia
  • Possible causes - Excessive sodium ingestion w/o increased water intake, loss of thirst mechanism, hormone imbalance (insufficient ADH), severe watery diarrhea

Electrolyte Imbalances - Potassium/Calcium

• Hypokalemia
  • Possible causes include excessive losses caused by diarrhea or vomiting, diuresis associated with some medications, endocrine dysfunction, decreased dietary intake, kidney disease.
  • May occur with alcoholism, eating disorders.
• Hyperkalemia
  • Renal failure
  • Endocrine dysfunction (aldosterone deficit)
  • "Potassium-sparing" diuretics
  • Extensive tissue damage (burns or crush injuries)
    • leakage of intracellular potassium into ECF
• Hypocalcemia
  • Hypoparathyroidism
  • Malabsorption syndrome resulting in decreased intestinal absorption of calcium or vit. D resulting
  • Renal failure
• Hypercalcemia
  • Hyperparathyroidism
  • Demineralization caused by immobility will decrease stress on bone leads to increased osteoclast activity.
  • Increased calcium and vitamin D intake.
  • Malignant bone tumors will result in uncontrolled release of calcium ions from bones.

Acid-Base Balance

• Normal pH range of blood is 7.35 - 7.45.
• Normal pH range of intracellular fluid is 6.8 - 7.0.

Relationship Between Hydrogen Ion and pH Scale

• Acid-base balance is essential to homeostasis.
• Cell enzymes can only effectively function within a narrow pH range.

Mechanisms for the Control of Serum pH

• Buffer systems are the fastest mechanism.
• The respiratory system alters CO2 (carbonic acid) levels to change pH.
• The kidneys modify the excretion rate of acids, modify the production & absorption of bicarbonate ions, are the most significant control mechanism, and are the slowest mechanism.

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.
• This is the fastest response, but also has the lowest capacity.
  • Reactions happen within a fraction of a second.
  • There is a limited number of molecules in body fluids.
• Major systems include the bicarbonate buffer system, phosphate buffer system and protein buffer system.

Bicarbonate Buffer System

• The bicarbonate buffer system is important in blood & tissue fluid.
• The components are carbonic acid (H2CO3) & sodium bicarbonate (NaHCO3).
• H2CO3 reacts with strong bases, producing weaker bases that do not affect pH as much.
• NaHCO3 reacts with strong acids, producing weaker acids that do not affect pH as much.
• Normally, more sodium bicarbonate is needed than H2CO3.
  • The ratio of NaHCO3 to H2CO3 is ~ 20:1.

Phosphate Buffer System

• Components are sodium dihydrogen phosphate (NaH2PO4) and sodium monohydrogen phosphate (Na2HPO4).
• This system is important in the regulation of blood pH by kidneys.
  • Cells of kidney tubules can remove excess H+ ions by forming NaH2PO4, which is excreted in urine.
  • Retained Na+ ions are returned to blood in peritubular capillaries, along with bicarbonate ions.
  • Bicarbonate ions are synthesized by renal cells from CO2 and H2O.

Protein Buffer System

• The protein buffer system is the most important intracellular buffer system.
• An example is Hemoglobin, which buffers the H+ formed during CO2 transport.
• Amino acids have carboxyl group (COOH) & an amine (or amino) group (NH2).
  • COOH group can act as acid (can "donate” hydrogen).
  • NH2 group can act as a base (can "pick up” excess hydrogen).

Role of the Kidneys in Acid-Base Balance

• The kidneys play the greatest role in maintaining the pH of blood & tissue fluid.
• Kidney response when body fluids are too acidic:
  • The kidneys will secrete more H+ ions into renal filtrate.
  • The kidneys will return more bicarbonate to blood increasing blood pH back to normal.
• Kidney response when body fluids are too alkaline:
  • The kidneys will return H+ ions to blood.
  • The kidneys will excrete more bicarbonate in urine decreasing blood pH back to normal.

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, resulting in more carbonic acid exhaled and pH increases.
• RR decreases & breaths become shallower when pH is high, resulting in the body retains more carbonic acid to decrease pH.

Acid-Base Imbalance

• Acidosis can be respiratory or metabolic.
  • Excess H+ ions and a decrease in serum pH.
• Alkalosis can be respiratory or metabolic.
• Deficit of H+ ions and an increase in serum pH.

Acid-Base Imbalances - Metabolic

• Metabolic acidosis:
  • Potential causes - kidney disease, diarrhea or vomiting, diabetic ketoacidosis
  • Compensation mechanisms - Increased respiratory rate and deeper respirations.
• Metabolic alkalosis:
  • Potential causes - Over ingestion of bicarbonate meds and gastric suctioning.
  • Compensation mechanisms - Decreased respiratory rate and shallow respirations.

Acid-Base Imbalances - Respiratory

• Respiratory acidosis:
  • Potential causes - Decreased respiratory efficiency (lung infections, COPD, etc.), paralysis of respiratory muscles drug overdose.
  • Compensation mechanisms - Kidneys excrete more H+ and reabsorb more Na+ & HCO-3 ions into blood.
• Respiratory alkalosis:
  • Potential causes - Hyperventilation, Increased respiratory rate at high altitudes
  • Compensation mechanisms - Kidneys retain more H+ ions and excrete more Na+ & HCO-3 ions in urine.