Fluids, Electrolytes, Acids and Bases

Fluids and Electrolytes, Acids and Bases

• Chapter focuses on the importance of maintaining electrolyte and acid-base balance for bodily functions.

Fluid Compartments

• Total Body Water accounts for 60% of body weight.
• Intracellular fluid makes up 2/3 of TBW.
• Extracellular fluid makes up 1/3 of TBW and includes:
  • Interstitial fluid (space between cells outside vessels).
  • Intravascular fluid (blood plasma), an increase can cause localized edema.
  • Transcellular fluid (lymph, synovial, intestinal, CSF, sweat, urine, pleural, peritoneal, pericardial, and intraocular fluids).

Water Movement

• Water moves between plasma and interstitial fluid through:
  • Osmosis (water being pulled).
  • Hydrostatic pressure (water being pushed).
• These occur across the capillary membrane.

Starling Forces

• Starling forces determine net filtration or reabsorption as plasma flows from the arterial to the venous end of the capillary.
• Capillary hydrostatic pressure (blood pressure) facilitates water movement from the capillary into the interstitial space.
• Capillary (plasma) oncotic pressure draws water from the interstitial space into the capillary.
• Interstitial hydrostatic pressure facilitates water movement from the interstitial space into the capillary.
• Interstitial oncotic pressure draws water from the capillary into the interstitial space.

Edema

• Edema is caused by excessive fluid accumulation in interstitial spaces.

Causation of Edema

• Increased capillary hydrostatic pressure:
  • Caused by venous obstruction or excessive salt and water retention.
• Decreased plasma oncotic pressure:
  • Results from reduced plasma proteins, which leads to fluid retention in tissues.
• Increased capillary permeability:
  • Occurs due to inflammation, immune responses, trauma, burns, neoplastic diseases, and allergic reactions, allowing proteins and fluid to leak into the interstitial space.
• Lymphatic obstruction:
  • Occurs when lymphatic channels are blocked, leading to lymphedema.

Pathophysiologic Processes of Edema

• Hydrostatic pressure increases due to venous obstruction or fluid overload, forcing fluid into the interstitial space.
• Oncotic pressure decreases when plasma proteins, such as albumin, are deficient, reducing fluid reabsorption into blood vessels.
• Capillary permeability increases, allowing proteins and fluids to escape, leading to edema.
• Lymphatic obstruction prevents proper drainage of interstitial fluid, resulting in accumulation and swelling.

Clinical Manifestations of Edema

• Pitting edema: Indentation remains after pressing on swollen areas.
• Localized edema: Occurs at specific sites.
• Generalized edema (anasarca): Widespread.
• Dependent edema: Occurs in gravity-dependent areas.
• Weight gain, swelling, and puffiness result from fluid accumulation.
• Limited joint movement and tight-fitting clothes or shoes.
• Impaired wound healing and increased infection risk due to reduced blood flow.
• Third-spacing: Fluid sequestering in interstitial spaces, pleural membranes, or pericardial spaces, potentially leading to dehydration despite fluid retention.
• Lymphedema is firm and noncompressible, while pitting edema is easily compressed.

Regulatory Processes for Sodium and Water Balance

• The body regulates sodium and water balance with antidiuretic hormone (ADH), the renin-angiotensin-aldosterone system (RAAS), and natriuretic peptides (ANP and BNP).

Antidiuretic Hormone (ADH)

• ADH, also known as vasopressin, is produced by the hypothalamus and released from the posterior pituitary.
• Secreted in response to increased plasma osmolality or low blood volume and pressure.
• Promotes water reabsorption in the renal distal tubules and collecting ducts.
• Reduces urine output and restoring fluid balance.
• Influenced by osmoreceptors and baroreceptors.
• ADH reduces plasma osmolality and increases urine concentration.

Renin-Angiotensin-Aldosterone System (RAAS)

• RAAS is a key mechanism for regulating sodium retention, blood pressure, and fluid balance.
• Renin, an enzyme, is secreted by the juxtaglomerular cells of the kidney when blood volume or pressure is low, or sodium levels are low while potassium levels are high.
• Renin converts angiotensinogen into angiotensin I, which is converted into angiotensin II by ACE in the lungs.
• Angiotensin II effects include:
  • Potent vasoconstriction.
  • Stimulates aldosterone release.
  • Stimulates ADH release.
• RAAS increases blood pressure and blood volume by conserving sodium and water.

Natriuretic Peptides (ANP and BNP)

• Atrial Natriuretic Peptide (ANP) is released by the atria.
• Brain Natriuretic Peptide (BNP) is released by the ventricles.
• Release happens in response to increased blood volume and pressure.
• These peptides oppose the RAAS by:
  • Promoting sodium and water excretion.
  • Vasodilation.
  • Inhibiting renin, aldosterone, and ADH secretion.
• ANP and BNP act as natural diuretics to reduce blood volume and pressure to prevent fluid overload.

Summary of Effects on Sodium and Water Balance

• ADH (Vasopressin):
  • Trigger: High plasma osmolality or low blood volume.
  • Effect: Increases water reabsorption, reduces urine output.
• RAAS:
  • Trigger: Low blood pressure, low sodium, high potassium.
  • Effect: Raises blood pressure and increases sodium and water retention.
• ANP & BNP:
  • Trigger: High blood volume and pressure.
  • Effect: Promotes sodium and water excretion, and lowers blood pressure.
• These systems maintain fluid balance, blood pressure, and electrolyte homeostasis.

Fluid Alterations

• Hypotonic:
  • ECF osmolality is <280 mOsm.
  • Causes: Sodium deficit or water excess leading to cellular swelling (edema).
• Sodium deficit:
  • Low ECF osmotic pressure which causes water to move into cells, which in turn leads to hypovolemia
• Water excess:
  • Leads to increased ICF & ECF volume, causing hypervolemia, water intoxication, cerebral & pulmonary edema.
• Isotonic:
  • Isotonic fluid alterations occur when total body water (TBW) changes proportionally with electrolytes.
• Isotonic fluid loss (Hypovolemia):
  • Causes: Hemorrhage, wound drainage, excessive sweating, inadequate fluid intake.
  • Symptoms: Weight loss, dry skin, decreased urine output, rapid heart rate, flattened neck veins, low blood pressure.
  • Severe cases can lead to hypovolemic shock.
  • Treatment: Isotonic IV fluids or oral rehydration solutions.
• Isotonic fluid excess (Hypervolemia):
  • Causes: Excess IV fluids, hyperaldosteronism, cortisone use.
  • Symptoms: Weight gain, distended neck veins, high blood pressure, edema, pulmonary edema, heart failure.
  • Treatment: Diuretics to remove excess fluid.
• Hypertonic:
  • ECF osmolality >294 mOsm.
    • Causes: Hypernatremia or ECF water deficit.
  • ECF hypertonicity pulls water from cells → ICF dehydration.
  • Results in: Dehydration symptoms.

Sodium Imbalance

Hypernatremia

• Central nervous system signs are serious due to alterations in membrane potentials and shrinking of brain cells.
  • Sodium cannot cross brain capillaries
• Isovolemic Hypernatremia
• Thirst
• Dry sticky tongue and mucosa

Hyponatremia

• Isovolemic Hyponatremia
• Anorexia
• Nausea
• Cramps

Hyperchloremia

• (serum chloride less than 97 mEq/L) is usually the result of hyponatremia or elevated bicarbonate concentrations.

Water Deficit (Dehydration)

• Hypertonic Fluid Alteration
  • Causes: Hypernatremia + water deficit
• Clinical Manifestation:
  • Increased serum sodium concentration (hypernatremia due to water loss)
  • Thirst
  • Headache
  • Weight loss
  • Oliguria and concentrated urine
  • Hard stools
  • Decreased skin turgor
  • Dry mucous membranes
  • Decreased sweating and tears
  • Elevated temperature
  • Soft eyeballs
  • Sunken fontanels in infants
  • Prolonged capillary refill time
  • Tachycardia
  • Weak pulses
  • Low blood pressure
  • Postural hypotension
  • Hypovolemic shock
  • Confusion
  • Coma

Potassium in the Body

• Hypokalemia
  • Potassium level less than 3.5 mEq/L.
  • Manifestations depend on rate and severity.
  • Membrane hyperpolarization causes a decrease in neuromuscular excitability, skeletal muscle weakness, smooth muscle atony, and cardiac dysrhythmias.
• Hyperkalemia
  • Potassium level greater than ~5.0 mEq/L.
  • Membrane depolarization of neurons causes increased neuromuscular irritability.
  • Mild: Tingling of lips and fingers, restlessness, intestinal cramping, and diarrhea.
  • Severe: cells unable to repolarize ➔ muscle weakness, loss of muscle tone, flaccid paralysis, can be lethal cardiac dysrhythmias.

Acid-Base Balance

• The body's ability to regulate H+ within a narrow range is a crucial function.
• Small changes in H+ dramatically influence cellular function, enzyme activity, neuromuscular membrane action potential, and ATP generation.
• Abnormal pH results in:
  • Decreased tissue oxygenation & cardiac contraction
  • Decreased vascular response to catecholamines
  • Altered neuromuscular function
  • Alters action of certain drugs
• Normal range pH 7.35-7.45
• If H+ are high in number (per volume), the pH is low (acidic)
  • pH <7.35
• If H+ are low in number (per volume), the pH is high (alkaline)
  • pH > 7.45

Regulation of Acid-Base Balance

• Chemical Buffer System
• Respiratory System:
  • Lungs regulate CO2 levels
  • Excrete CO2 & H2O
  • Chemical formula CO2 = Acid
• Renal System:
  • The Respiratory and Renal System work together to maintain pH balance
• Normal functioning kidneys
  • Conserve, reabsorb HCO3
  • Eliminate H+
  • Excrete acid urine pH 6
• In acidosis--kidneys
  • Excrete more H+
    • Combines with ammonia to form ammonium to prevent structural damage to kidneys.
  • Generate more HCO3
• In alkalosis-- kidneys
  • Retain more H+
  • Excrete more HCO3
• Renal system responds more slowly (24 hours up to 3-4 days)

Respiratory and Metabolic Acid-Base Disorders

• Respiratory Acidosis
  • Causes include cerebral edema, tumor, cerebral aneurysm, stroke, muscular dystrophy, obesity, ascites, aspiration, neck edema, pneumonia, TB, COPD, ARDS, drowning, and pulm emboli
• Metabolic Acidosis
  • Exemplar conditions include: Dehydration, electrolyte imbalances, respiratory alkalosis, respiratory acidosis, metabolic alkalosis, metabolic acidosis