Human Body Fluid Compartments and Transport Mechanisms

Fluid Compartments

• The human body contains three fluid compartments: intracellular fluid (ICF), extracellular fluid (ECF), and transcellular fluid.
• ICF makes up 2/3 of total body water, while ECF makes up 1/3.
• ECF consists of interstitial fluid, plasma, and lymph.

Diffusion and Osmosis

• Diffusion is the movement of particles from an area of high concentration to an area of low concentration.
• Osmosis is the movement of water across a semipermeable membrane from an area of high water concentration (low solute concentration) to an area of low water concentration (high solute concentration).
• Urine specific gravity reflects the kidney's ability to concentrate or dilute urine based on serum osmolality.
• Osmolarity refers to the number of osmoles of solute per liter of solution, depending on the number of particles suspended in a solution.
• Tonicity describes the effect of osmotic pressure on cell size due to water movement across the cell membrane.
• Intravenous fluid solutions can be isotonic, hypotonic, or hypertonic:
  • Isotonic solutions have the same tonicity as blood (e.g., 0.9% NaCl), causing no cell shrinkage or swelling.
  • Hypotonic solutions have less tonicity than blood (e.g., 0.45% NaCl), causing water to move into the cell and cells to swell.
  • Hypertonic solutions have greater tonicity than blood (e.g., 3% NaCl), causing water to move out of the cell and cells to shrink.

Isotonic Fluid Volume Deficit

• Etiology can include inadequate fluid intake, excessive GI loss, excessive skin loss, and third-space fluid loss.
• Clinical manifestations include acute weight loss, decreased urine output, increased osmolality and specific gravity, thirst, increased BUN/Creatinine, hypotension, tachycardia, thready pulse, and sunken eyes.

Isotonic Fluid Volume Excess

• Etiology can include inadequate sodium and water elimination, excessive sodium intake, excessive fluid intake, and administration of parenteral fluids at an excessive rate.
• Clinical manifestations include acute weight gain, edema, full, bounding pulse, venous distension, and pulmonary edema.

Sodium Imbalance

• Serum osmolality (275-295 mOsm/kg) changes with plasma sodium concentration (135-145 mEq/L)
• Sodium imbalance is a common electrolyte disorder.
• Hypertonic/Hypernatremia (high sodium concentration) can be caused by osmotic shifts of water from ICF to ECF or extracellular-intracellular shifts.

Hypokalemia

• Etiology can include inadequate intake, excessive renal loss, GI loss, and transcompartmental shifts.
• Clinical manifestations include cardiac arrhythmias, muscle cramps, paralysis, confusion, metabolic alkalosis, and ECG changes (depression of ST segment, U wave).

Hyperkalemia

• Etiology can include decreased renal elimination, excessive rapid administration, and transcellular shift from ICF to ECF.
• Clinical manifestations include paresthesia, diarrhea, ECG changes (widened QRS complex, peaked T waves), cardiac arrest, and metabolic acidosis.

Calcium, Phosphorus, and Magnesium Balance

• Vitamin D facilitates calcium absorption from the gastrointestinal tract.
• Parathyroid hormone regulates calcium and phosphate levels.
• Calcium and phosphorus are the major mineral contents of bone.
• They are ingested in the diet, absorbed from the intestine, filtered in the kidney, and eliminated in the urine.
• There is a reciprocal relationship between calcium and phosphate: high calcium leads to low phosphate, and low calcium leads to high phosphate.

Calcium Imbalances

• Hypocalcemia (< 8.5 mg/dL) is associated with increased neuromuscular excitability and can cause laryngeal spasm, tetany, ventricular dysrhythmias, Chvostek's sign, Trousseau's sign, and bone pain.
• Hypercalcemia (> 10.5 mg/dL) is associated with decreased neuromuscular excitability and can cause muscle weakness, ataxia, coma, lethargy, hyperreflexia, flank pain/kidney stones, osteoporosis, and nausea/vomiting.
• Hypophosphatemia (< 2.5 mg/dL) can be caused by decreased absorption, antacid use, increased renal loss, alcoholism, and hyperparathyroidism.
• Clinical manifestations include intentional tremor, hyporeflexia, seizures, and reciprocal calcium changes.
• Hyperphosphatemia (> 4.5 mg/dL) can be caused by renal failure, massive trauma, and hypoparathyroidism.
• Clinical manifestations include paresthesia, tetany, cardiac dysrhythmias, and reciprocal calcium changes.

Magnesium Imbalances

• Hypomagnesemia (< 1.8 mg/dL) can result from malabsorption/malnutrition, increased renal excretion, and chronic alcoholism.
• Clinical manifestations include tetany, positive Babinski sign, nystagmus, and cardiac dysrhythmias.
• Hypermagnesemia (> 3.0 mg/dL) can be caused by renal failure, magnesium laxatives/antacids, and pre-eclampsia.
• Clinical manifestations can include hyporeflexia, muscle weakness, calcium-channel blocking effects (e.g., cardiac arrest), and respiratory muscle paralysis.

Acid-Base Imbalances

• pH is a measure of the acidity or alkalinity of a solution.
• Normal pH range for blood is 7.35-7.45.
• Acid-base imbalances can be respiratory or metabolic.

Respiratory Acidosis

• Etiology includes lung disease, depression of the respiratory center, and airway obstruction.
• Clinical manifestations include headache, confusion, stupor/coma, serum pH < 7.35, and PaCO2 > 45 mm Hg.

Respiratory Alkalosis

• Etiology includes anxiety, hypoxia, pain, fever, and salicylate intoxication.
• Clinical manifestations include dizziness, numbness/tingling of fingers, panic, muscle spasms, serum pH > 7.45, and PaCO2 < 35 mm Hg.

Metabolic Acidosis

• Etiology includes lactic acidosis, ketoacidosis, diarrhea, and chronic kidney disease.
• Clinical manifestations include Kussmaul breathing, coma, dysrhythmias, hypotension, serum pH < 7.35, and HCO3 < 22 mEq/L.
• Metabolic acidosis causes H+ to enter cells, replacing potassium, leading to hyperkalemia.

Metabolic Alkalosis

• Etiology includes excessive bicarbonate ingestion, vomiting of excess gastric acid, and gastric suction.
• Clinical manifestations include asymptomatic, volume depletion, confusion, hypotension, decreased rate/depth of respirations, serum pH > 7.45, and HCO3 > 26 mEq/L.
• Metabolic alkalosis causes potassium to enter the cell, leading to hypokalemia.

Summary Table of Acid-Base Imbalances

This quiz explores essential concepts about the fluid compartments of the human body, including intracellular and extracellular fluids. It also covers the principles of diffusion, osmosis, urine specific gravity, osmolarity, and tonicity in relation to cellular functions. Test your understanding of these critical physiological processes.