Anatomy and Physiology II Unit 9 Review: Fluid & Electrolyte Balance

Key Concepts and Terminology

• Homeostasis maintains stable internal environment through regulation of fluid and electrolyte balance
• Osmolality measures solute concentration in a solution expressed as milliosmoles per kilogram (mOsm/kg)
• Osmosis movement of water across a semipermeable membrane from a region of low solute concentration to a region of high solute concentration
• Diffusion passive movement of solutes from an area of high concentration to an area of low concentration
• Electrolytes ions in body fluids that conduct electricity (sodium, potassium, chloride, bicarbonate)
• Antidiuretic hormone (ADH) regulates water reabsorption in the kidneys to maintain blood osmolality
• Aldosterone hormone that promotes sodium reabsorption and potassium excretion in the kidneys
  • Secreted by the adrenal cortex in response to low blood pressure or high blood potassium levels

Anatomy of the Urinary System

• Kidneys paired, bean-shaped organs located in the retroperitoneal space that filter blood and produce urine
  • Renal cortex outer region of the kidney containing nephrons and blood vessels
  • Renal medulla inner region of the kidney containing loops of Henle and collecting ducts
• Ureters muscular tubes that transport urine from the kidneys to the urinary bladder
• Urinary bladder hollow, muscular organ that stores urine until it is expelled from the body
• Urethra tube that carries urine from the urinary bladder to the outside of the body
• Nephrons functional units of the kidney responsible for filtering blood and producing urine
  • Renal corpuscle consists of a glomerulus and Bowman's capsule, where blood is filtered
  • Renal tubule includes the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct, where filtrate is modified and concentrated

Physiology of Fluid Balance

• Body fluids divided into intracellular fluid (ICF) and extracellular fluid (ECF)
  • ICF fluid within cells, accounts for ~60% of total body water
  • ECF fluid outside of cells, includes interstitial fluid and plasma, accounts for ~40% of total body water
• Osmotic pressure drives water movement between compartments to maintain equal osmolality
• Hydrostatic pressure force exerted by a fluid against a membrane, influences fluid movement in capillaries and glomeruli
• Oncotic pressure osmotic pressure exerted by proteins in a solution, helps retain fluid in the bloodstream
• Thirst mechanism stimulates drinking behavior when blood osmolality rises or blood volume decreases
• Urine concentration controlled by ADH and the countercurrent multiplication system in the loop of Henle
  • Allows production of urine that is more concentrated than blood plasma

Electrolyte Regulation

• Sodium (Na+) primary extracellular cation, maintains blood volume and pressure
  • Regulated by aldosterone, which promotes Na+ reabsorption in the kidneys
• Potassium (K+) primary intracellular cation, essential for nerve and muscle function
  • Regulated by aldosterone, which promotes K+ excretion in the kidneys
• Chloride (Cl-) major extracellular anion, maintains electrical neutrality and aids in acid-base balance
• Bicarbonate (HCO3-) important buffer in the blood, helps maintain acid-base balance
• Calcium (Ca2+) essential for bone mineralization, muscle contraction, and nerve function
  • Regulated by parathyroid hormone (PTH) and calcitonin
• Magnesium (Mg2+) cofactor for many enzymatic reactions, involved in muscle and nerve function
• Phosphate (PO4-) component of cell membranes, nucleic acids, and ATP, regulated by PTH and vitamin D

Hormonal Control Mechanisms

• Antidiuretic hormone (ADH) released by the posterior pituitary gland in response to increased blood osmolality or decreased blood volume
  • Increases water permeability in the collecting ducts, promoting water reabsorption
• Aldosterone secreted by the adrenal cortex in response to decreased blood pressure or increased blood K+ levels
  • Increases Na+ reabsorption and K+ excretion in the distal tubules and collecting ducts
• Atrial natriuretic peptide (ANP) released by the atria of the heart in response to increased blood volume
  • Promotes Na+ excretion and vasodilation, reducing blood volume and pressure
• Parathyroid hormone (PTH) secreted by the parathyroid glands in response to low blood Ca2+ levels
  • Increases Ca2+ reabsorption in the kidneys, Ca2+ release from bones, and Ca2+ absorption in the intestines
• Renin-angiotensin-aldosterone system (RAAS) regulates blood pressure and fluid balance
  • Renin enzyme released by the kidneys in response to decreased blood pressure or decreased Na+ delivery to the distal tubules
  • Angiotensin II potent vasoconstrictor that stimulates aldosterone secretion and ADH release

Common Disorders and Imbalances

• Dehydration occurs when fluid loss exceeds fluid intake, leading to decreased blood volume and increased osmolality
  • Causes include excessive sweating, diarrhea, vomiting, and inadequate fluid intake
• Edema abnormal accumulation of fluid in the interstitial space, often caused by increased capillary permeability or decreased oncotic pressure
• Hyponatremia low blood sodium levels (<135 mEq/L), can cause neurological symptoms and seizures
  • Causes include excessive water intake, diuretic use, and syndrome of inappropriate ADH secretion (SIADH)
• Hypernatremia high blood sodium levels (>145 mEq/L), can cause thirst, confusion, and coma
  • Causes include dehydration, excessive salt intake, and diabetes insipidus
• Hypokalemia low blood potassium levels (<3.5 mEq/L), can cause muscle weakness, arrhythmias, and paralysis
• Hyperkalemia high blood potassium levels (>5.0 mEq/L), can cause arrhythmias and cardiac arrest
  • Causes include kidney failure, tissue damage, and certain medications (ACE inhibitors, potassium-sparing diuretics)

Clinical Applications and Diagnostics

• Urinalysis examination of urine sample for color, clarity, pH, specific gravity, and presence of abnormal substances (glucose, protein, blood, leukocytes)
  • Helps diagnose urinary tract infections, kidney disorders, and metabolic diseases
• Blood tests measure electrolyte levels, kidney function (creatinine, blood urea nitrogen), and hormones (ADH, aldosterone, renin)
• Glomerular filtration rate (GFR) estimates the filtering capacity of the kidneys, calculated using serum creatinine, age, sex, and race
  • Decreased GFR indicates impaired kidney function
• Renal imaging techniques (ultrasound, CT scan, MRI) visualize kidney structure and detect abnormalities (cysts, tumors, obstructions)
• Renal biopsy removal of kidney tissue sample for microscopic examination, helps diagnose glomerular and tubular disorders
• Dialysis artificial replacement of kidney function, removes waste products and excess fluid from the blood
  • Hemodialysis uses a machine to filter blood through a semipermeable membrane
  • Peritoneal dialysis uses the patient's peritoneum as a natural filter, with dialysate solution introduced and drained from the abdominal cavity

Study Tips and Review Questions

• Create a concept map linking key terms and processes related to fluid and electrolyte balance
• Use mnemonic devices to remember the functions and regulation of major electrolytes (e.g., "NASK" for sodium, aldosterone, sweat, and kidneys)
• Practice solving clinical case studies involving fluid and electrolyte imbalances
• Review the anatomy of the urinary system using labeled diagrams and models
• Summarize the main hormones involved in fluid and electrolyte regulation and their effects on the kidneys
• Compare and contrast the composition of intracellular and extracellular fluids
• Explain the role of the nephron in filtering blood and producing urine
• Describe the countercurrent multiplication system in the loop of Henle and its importance in urine concentration
• Discuss the signs, symptoms, and treatment options for common fluid and electrolyte disorders (dehydration, edema, hyponatremia, hyperkalemia)
• Interpret sample urinalysis results and explain their clinical significance