Physiology Lecture 18: Bodily Fluid Compartments

Lecture 18: Bodily Fluid Compartments

• Bodily fluid is essential for homeostasis
• Kidneys control many functions, including regulating body fluid volume, fluid constituents of extracellular fluid, acid-base balance, and fluid exchange between extracellular and intracellular compartments
• Water intake comes from ingested liquids/food and the oxidation of carbohydrates
• Water loss occurs by insensible water loss (through skin, humidification of inspired air, sweat, feces, urine). Kidneys are crucial for controlling water loss
• Total body fluid is divided into extracellular (blood plasma and interstitial fluid), transcellular (synovial, peritoneal, pericardial, intraocular), and intracellular fluid
• A 70 kg person contains approximately 42 liters of water (60%) and varies based on age, sex and obesity.
• Plasma is 60% of blood, RBCs is approximately 40%
• Plasma and interstitial fluid compositions are similar, except interstitial fluid typically has low protein concentration
• Osmosis is the diffusion of water through a selectively permeable membrane to equalize solute concentration. An osmole is the total number of particles in a solution
• Osmolality is osmoles/kg solution, whereas Osmolarity is osmoles/liter solution
• Osmotic pressure is proportional to osmolarity and equals CRT (C = concentration, R = ideal gas constant, T = absolute temperature (in Kelvin), and II = osmotic pressure).
• A 0.9% sodium chloride solution has an osmolarity of 286 mOsm/L
• Isotonic solutions have an osmolarity of 280 mOsm/L (neither shrink or swell the cell)
• Hypotonic solutions have an osmolarity less than 280 mOsm/L (cause the cell to swell)
• Hypertonic solutions have an osmolarity greater than 280 mOsm/L (cause the cell to shrink)

Measuring Fluid Volumes

• Total body water - add a bolus of radioactive water, measure concentration after a few hours
• Extracellular fluid volume - add a bolus of radioactive sodium, measure concentration after a few hours
• Intracellular fluid volume - total body water minus extracellular fluid volume
• Plasma volume - add a bolus of radioactive serum albumin, measure concentration after a few hours
• Interstitial fluid volume - extracellular fluid volume minus plasma volume

Fluid Regulation in Abnormal States

• Abnormalities in composition/volumes of body fluids are common in serious illness
• Factors affecting volume include water ingestion, dehydration, intravenous infusions, fluid loss from GI tract, sweat, kidneys
• Water moves rapidly across cell membranes, making intracellular and extracellular osmolarities nearly equal
• Important examples include patient with water deficit, and fluid regulation in abnormal states (hyponatremia, hypernatremia, edema)

Edema

• Edema is the presence of excess fluid in the body, predominantly in the extracellular space
• Conditions involve intracellular swelling or extracellular swelling
• Intracellular swelling: metabolic system depression, inadequate nutrient delivery
• Extracellular swelling conditions: abnormal leakage (fluid from plasma to interstitial space), failure of lymphatic system (circulatory failure, abnormal retention of salt and water)

Fluid Regulation in Abnormal States

• Edema related to heart failure, kidney failure related to salt/water retention, and edema caused by decreased plasma proteins (renal abnormalities, cirrhosis of the liver)
• Prevention of edema depends on low compliance of interstitium, increased lymph flow, and washdown of interstitial fluid protein.