Cell Physiology & Body Fluids MCQ

Every drug, anesthetic, and ion the dentist administers crosses cell membranes and equilibrates in body fluid compartments. Membrane transport (passive, facilitated, primary active, secondary active) decides which molecules get in. The resting membrane potential, set mostly by K+ permeability, decides how easily nerves fire when a local anesthetic blocks sodium channels. The body fluid compartments (the 60-40-20 rule) decide what dose reaches the tissue, and the Starling forces decide where edema appears.

Cell Membrane and Transport

• The cell membrane is a phospholipid bilayer with embedded proteins (channels, transporters, receptors); the lipid core is selectively permeable to lipophilic molecules (gases, lipid-soluble drugs) and impermeable to polar molecules (ions, glucose) that need transporters.
• PASSIVE DIFFUSION moves molecules down a concentration gradient without ATP; FACILITATED DIFFUSION uses a transporter (saturable, e.g., GLUT1-4 for glucose) but still moves down the gradient without ATP.
• PRIMARY ACTIVE transport moves molecules against the gradient using ATP directly; the Na+/K+ ATPase pumps 3 Na+ out and 2 K+ in per ATP, creating the gradients that power neurons and many other transporters.
• SECONDARY ACTIVE transport uses the Na+ gradient (set up by the Na+/K+ ATPase) to move a second molecule against its gradient; SGLT1 in the intestine and SGLT2 in the kidney are the classic glucose examples (and the SGLT2 inhibitors are the diabetes drugs).

Resting Membrane Potential

• The resting membrane potential in most cells is about -70 mV (inside negative relative to outside); it is set by the asymmetric distribution of K+ (high inside) and Na+ (high outside) and the membrane's permeability to those ions.
• At rest, the membrane is much more permeable to K+ than to Na+, so the resting potential sits near the K+ equilibrium potential (about -90 mV) but is slightly less negative because of small Na+ permeability.
• The NERNST equation gives the equilibrium potential of one ion (Eion); the GHK equation gives the resting potential considering multiple permeable ions.
• Action potentials are driven by voltage-gated Na+ channels (depolarization) and voltage-gated K+ channels (repolarization); LOCAL ANESTHETICS block voltage-gated Na+ channels from inside the nerve, preventing depolarization.

Body Fluid Compartments

• Total body water is about 60 percent of body weight in adult men (and about 50-55 percent in women, who carry more adipose); the 60-40-20 rule says 60 percent water, 40 percent intracellular fluid (ICF), and 20 percent extracellular fluid (ECF).
• The ECF (20 percent) further splits into PLASMA (about 5 percent of body weight) and INTERSTITIAL fluid (about 15 percent); transcellular fluids (CSF, GI secretions, synovial fluid) are small.
• ICF and ECF differ in composition: K+ is the principal intracellular cation, Na+ is the principal extracellular cation, and the Na+/K+ ATPase maintains the asymmetry.
• Estimating compartments uses tracers (e.g., D2O for total body water, inulin for ECF, radioiodinated serum albumin for plasma); the dental relevance is dose-volume reasoning for IV drugs and assessing dehydration.

Osmolality, Tonicity, and Cell Volume

• OSMOLALITY is the total concentration of all solute particles per kg of water; plasma osmolality is normally about 285-295 mOsm/kg.
• TONICITY is the effective osmolality created by solutes that do NOT cross the cell membrane (impermeable solutes); a solution is hypertonic, isotonic, or hypotonic relative to the cell.
• A cell placed in HYPERTONIC solution shrinks (water leaves); a cell in HYPOTONIC solution swells (water enters and can lyse the cell); ISOTONIC saline (0.9% NaCl) is the dental and surgical default for IV fluid.
• Plasma osmolality can be estimated as 2(Na+) + glucose/18 + BUN/2.8; a measured-minus-calculated osmolar gap suggests an additional osmotically active solute (e.g., methanol, ethylene glycol, mannitol).

Capillary Exchange and Edema (Starling Forces)

• Fluid moves across capillary walls according to Starling forces: hydrostatic pressure inside the capillary pushes fluid out; oncotic pressure inside the capillary (from plasma proteins) pulls fluid back in; interstitial hydrostatic and oncotic pressures act in the opposite directions.
• Net filtration occurs when the sum of out-forces exceeds the sum of in-forces; most filtered fluid is reabsorbed in venules, and the small remaining excess is returned by the lymphatics.
• EDEMA occurs when filtration exceeds the capacity for reabsorption and lymphatic drainage; causes include raised hydrostatic pressure (heart failure), lowered oncotic pressure (low albumin), increased capillary permeability (inflammation, allergy, ACE inhibitor angioedema), and lymphatic obstruction.
• Dental relevance: postoperative facial edema, periapical inflammatory edema, and angioedema from drugs (ACE inhibitors) all illustrate Starling-force disturbances; treatment targets the underlying mechanism (anti-inflammatory measures, drug cessation, airway support).