PY1.1-7 | General Physiology — Glossary

The phospholipid bilayer with embedded proteins that forms the outer boundary of every cell, regulating transport, signal transduction, and cell recognition

The Singer-Nicolson (1972) model describing the cell membrane as a fluid phospholipid bilayer with proteins floating within it like icebergs in a lipid sea

An amphipathic lipid with a hydrophilic phosphate head and two hydrophobic fatty acid tails that spontaneously forms the bilayer structure of cell membranes

Double-membraned organelles that generate ATP through oxidative phosphorylation; contain their own circular DNA and are inherited maternally

Membrane-bound organelle containing acid hydrolases that digest worn-out organelles, ingested bacteria, and cellular debris; deficiency of specific lysosomal enzymes causes storage diseases

The fluid within cells, comprising approximately 28 L (two-thirds of total body water) in a 70 kg adult; characterised by high K+ and low Na+ concentrations

All fluid outside cells, comprising approximately 14 L (one-third of TBW); includes interstitial fluid (11 L) and plasma (3 L); characterised by high Na+ and low K+ concentrations

The total concentration of osmotically active solutes per litre of solution; normal plasma osmolarity is 280-295 mOsm/L, maintained primarily by Na+ and its associated anions

The net movement of water across a semipermeable membrane from a region of low solute concentration to high solute concentration, driven by the osmotic pressure gradient

The effective osmolarity of a solution relative to the cell interior; isotonic solutions maintain cell volume, hypotonic solutions cause swelling, hypertonic solutions cause shrinkage

Passive transport of small nonpolar molecules (O2, CO2, steroids) directly through the lipid bilayer down their concentration gradient, governed by Fick's Law

Passive transport of polar or charged molecules down their concentration gradient using membrane proteins (channels or carriers) without ATP expenditure

The primary active transport pump that uses ATP to move 3 Na+ out and 2 K+ into the cell per cycle, maintaining the electrochemical gradients essential for all cell function; consumes ~30% of cellular ATP

Transport that uses the electrochemical gradient of one ion (usually Na+) created by primary active transport to drive another solute against its gradient; includes symport and antiport

The maintenance of a stable internal environment within narrow physiological ranges through continuous monitoring and feedback mechanisms involving sensors, control centres, and effectors

A control mechanism where the output of a system inhibits further output, counteracting any deviation from the set point and maintaining stability; the predominant mechanism of homeostasis

A control mechanism where the output amplifies the original stimulus, creating an escalating cycle that continues until an external event breaks the loop (e.g., oxytocin in parturition)

The percentage of blood volume occupied by red blood cells; normal values are 40-54% in males and 36-48% in females; elevated in polycythaemia, decreased in anaemia

The osmotic pressure exerted by plasma proteins (mainly albumin) that tends to pull fluid from the interstitium into the capillary; approximately 25 mmHg

A chemical system that resists changes in pH by absorbing or releasing H+ ions; the bicarbonate buffer system (CO2/HCO3-) is the most important extracellular buffer in the body

pH = 6.1 + log([HCO3-]/[0.03 × PCO2]); relates blood pH to the ratio of bicarbonate to dissolved CO2, showing that pH depends on the 20:1 ratio of HCO3- to CO2

A peptide hormone secreted by parathyroid chief cells in response to low plasma calcium; raises Ca2+ by stimulating bone resorption, renal reabsorption, and vitamin D activation

A peptide hormone from thyroid C (parafollicular) cells secreted in response to high plasma calcium; lowers Ca2+ by inhibiting osteoclasts and increasing renal calcium excretion

A large multinucleated cell derived from monocyte-macrophage lineage that resorbs bone by secreting acid and cathepsin K into the resorption lacuna (Howship's lacuna)

A bone-forming cell derived from mesenchymal stem cells that synthesises and deposits osteoid matrix (collagen type I) which subsequently mineralises with hydroxyapatite crystals