Anatomy and Physiology I Unit 26 ReviewFluid, Electrolyte & Acid-Base Homeostasis

Key Concepts

• Fluid, electrolyte, and acid-base homeostasis maintains the internal environment of the body within narrow limits
• Fluid compartments include intracellular fluid (ICF) and extracellular fluid (ECF), which consists of interstitial fluid and plasma
• Electrolytes are ions that conduct electricity in solution and play crucial roles in various physiological processes (sodium, potassium, chloride, calcium)
• Acid-base balance refers to the regulation of hydrogen ion (H+) concentration in body fluids
  • pH is a measure of the concentration of H+ ions in a solution
• Homeostatic mechanisms involve various organs and systems (kidneys, lungs, endocrine system) to maintain fluid, electrolyte, and acid-base balance
• Disorders of fluid, electrolyte, and acid-base balance can lead to serious health consequences (dehydration, edema, acidosis, alkalosis)
• Diagnostic tests help assess the status of fluid, electrolyte, and acid-base balance (blood tests, urinalysis, arterial blood gas analysis)
• Clinical applications involve the management of fluid, electrolyte, and acid-base imbalances in various medical conditions (renal failure, heart failure, burns)

Fluid Compartments

• The human body is composed of approximately 60% water, which is distributed among different fluid compartments
• Intracellular fluid (ICF) is the fluid within cells and accounts for about 40% of total body water
• Extracellular fluid (ECF) is the fluid outside cells and makes up around 20% of total body water
  • ECF includes interstitial fluid (fluid between cells) and plasma (fluid component of blood)
• The composition of ICF and ECF differs in terms of electrolyte concentrations
  • ICF has a higher concentration of potassium (K+) and a lower concentration of sodium (Na+) compared to ECF
• The movement of water between fluid compartments is governed by osmosis, which is the diffusion of water across a semipermeable membrane from a region of low solute concentration to a region of high solute concentration
• Hydrostatic pressure and oncotic pressure also influence fluid movement between compartments
• Fluid balance is maintained by matching fluid intake (drinking, eating) with fluid output (urine, sweat, respiration)

Electrolyte Balance

• Electrolytes are essential for maintaining fluid balance, nerve and muscle function, and acid-base balance
• The main electrolytes in the body include sodium (Na+), potassium (K+), chloride (Cl-), calcium (Ca2+), magnesium (Mg2+), and bicarbonate (HCO3-)
• Sodium is the primary cation in ECF and plays a key role in determining osmolarity and water balance
  • Sodium concentration is regulated by the kidneys through the action of hormones such as aldosterone and antidiuretic hormone (ADH)
• Potassium is the main intracellular cation and is crucial for maintaining resting membrane potential and cell excitability
• Chloride is the major anion in ECF and helps maintain electrical neutrality and acid-base balance
• Calcium is important for bone mineralization, muscle contraction, and blood clotting
• Magnesium is involved in enzyme activation, neuromuscular function, and bone metabolism
• Electrolyte imbalances can occur due to various factors (excessive loss, inadequate intake, impaired regulation) and can lead to symptoms such as muscle cramps, weakness, and cardiac arrhythmias

Acid-Base Balance

• Acid-base balance refers to the regulation of hydrogen ion (H+) concentration in body fluids
• The normal pH range of arterial blood is 7.35-7.45, which is slightly alkaline
• Acids are substances that donate H+ ions, while bases accept H+ ions
• The body produces acids through various metabolic processes (cellular respiration, protein metabolism) and eliminates them through the lungs (as CO2) and kidneys (as H+ ions)
• Buffers are substances that minimize changes in pH by accepting or donating H+ ions
  • The main buffer systems in the body include bicarbonate (HCO3-), phosphate, and proteins (hemoglobin, albumin)
• Respiratory regulation of acid-base balance involves adjusting the rate and depth of breathing to alter the elimination of CO2
• Renal regulation of acid-base balance involves the excretion or reabsorption of H+ ions and the generation of new bicarbonate
• Acid-base disorders can be classified as acidosis (pH < 7.35) or alkalosis (pH > 7.45) and can be of respiratory or metabolic origin

Regulatory Mechanisms

• The body employs various regulatory mechanisms to maintain fluid, electrolyte, and acid-base homeostasis
• The kidneys play a central role in regulating fluid and electrolyte balance by adjusting the excretion or reabsorption of water and ions
  • The renin-angiotensin-aldosterone system (RAAS) regulates blood pressure and sodium balance
  • Antidiuretic hormone (ADH) promotes water reabsorption in the collecting ducts of the nephrons
• The lungs contribute to acid-base balance by adjusting the elimination of CO2
  • Hyperventilation decreases CO2 levels and leads to respiratory alkalosis
  • Hypoventilation increases CO2 levels and leads to respiratory acidosis
• The endocrine system secretes hormones that influence fluid and electrolyte balance
  • Aldosterone promotes sodium reabsorption and potassium excretion in the distal tubules and collecting ducts of the nephrons
  • Parathyroid hormone (PTH) regulates calcium and phosphate homeostasis
• Thirst and salt appetite are behavioral mechanisms that drive fluid and sodium intake when the body senses dehydration or sodium depletion
• Baroreceptors and osmoreceptors detect changes in blood pressure and osmolarity, respectively, and trigger appropriate responses to maintain homeostasis

Common Disorders

• Dehydration occurs when fluid loss exceeds fluid intake, leading to a decrease in total body water
  • Causes include excessive sweating, diarrhea, vomiting, and inadequate fluid intake
  • Symptoms may include thirst, dry mouth, decreased urine output, and dizziness
• Edema is the accumulation of excess fluid in the interstitial space, causing swelling
  • Causes include heart failure, liver cirrhosis, kidney disease, and venous insufficiency
• Hyponatremia is a low serum sodium concentration (< 135 mEq/L) that can result from excessive water intake, diuretic use, or syndrome of inappropriate antidiuretic hormone secretion (SIADH)
• Hypernatremia is a high serum sodium concentration (> 145 mEq/L) that can be caused by dehydration, excessive salt intake, or diabetes insipidus
• Hypokalemia is a low serum potassium concentration (< 3.5 mEq/L) that can lead to muscle weakness, constipation, and cardiac arrhythmias
  • Causes include diuretic use, diarrhea, and renal tubular acidosis
• Hyperkalemia is a high serum potassium concentration (> 5.5 mEq/L) that can cause muscle paralysis, cardiac conduction abnormalities, and potentially fatal arrhythmias
  • Causes include kidney failure, adrenal insufficiency, and tissue damage (rhabdomyolysis)
• Metabolic acidosis is a decrease in blood pH due to an accumulation of acids or loss of bicarbonate
  • Causes include diabetic ketoacidosis, lactic acidosis, and renal failure
• Metabolic alkalosis is an increase in blood pH due to a loss of acids or gain of bicarbonate
  • Causes include vomiting, diuretic use, and excessive alkali ingestion

Diagnostic Tests

• Serum electrolyte measurements provide information about the concentrations of sodium, potassium, chloride, calcium, and magnesium in the blood
• Blood urea nitrogen (BUN) and creatinine levels assess kidney function and can indicate dehydration or renal failure
• Arterial blood gas (ABG) analysis measures the pH, partial pressure of oxygen (PaO2), partial pressure of carbon dioxide (PaCO2), and bicarbonate concentration in arterial blood
  • ABG results help diagnose and classify acid-base disorders
• Urine osmolality and specific gravity provide information about the concentration of solutes in urine and the kidney's ability to concentrate or dilute urine
• Urine electrolyte measurements can help identify the cause of electrolyte imbalances (renal losses, extrarenal losses)
• Fractional excretion of sodium (FENa) is a calculated value that helps differentiate between prerenal (dehydration) and intrinsic renal causes of acute kidney injury
• Plasma osmolality is a measure of the concentration of solutes in plasma and can be used to assess hydration status and diagnose disorders such as diabetes insipidus and SIADH

Clinical Applications

• Fluid replacement therapy is used to correct dehydration and restore fluid balance
  • Oral rehydration solutions (ORS) containing water, electrolytes, and glucose are used for mild to moderate dehydration
  • Intravenous fluids (isotonic saline, Ringer's lactate) are administered for severe dehydration or when oral intake is not possible
• Electrolyte replacement is necessary for treating specific electrolyte imbalances
  • Potassium supplementation is used for hypokalemia, while potassium-binding resins (sodium polystyrene sulfonate) are used for hyperkalemia
  • Calcium gluconate is administered for hypocalcemia, while bisphosphonates and calcitonin are used for hypercalcemia
• Diuretics are medications that promote the excretion of water and electrolytes by the kidneys
  • Loop diuretics (furosemide) are used to treat edema and hypertension
  • Thiazide diuretics (hydrochlorothiazide) are used for hypertension and calcium stone prevention
• Acid-base disorders are managed by addressing the underlying cause and correcting the imbalance
  • Metabolic acidosis can be treated with sodium bicarbonate administration or addressing the underlying cause (insulin for diabetic ketoacidosis)
  • Respiratory acidosis is managed by improving ventilation (mechanical ventilation, bronchodilators)
  • Metabolic alkalosis is treated by replacing volume deficits with isotonic saline and correcting electrolyte imbalances (potassium chloride)
• Dialysis is a treatment option for patients with end-stage renal disease or severe electrolyte and acid-base imbalances
  • Hemodialysis involves the filtration of blood through an artificial kidney machine
  • Peritoneal dialysis uses the patient's peritoneum as a semipermeable membrane for fluid and solute exchange