Regulation of Solutes and Water

Na+/K+-ATPase (Na+-K+ pump)

Membrane pump that exchanges 3 Na+ out for 2 K+ in per ATP, maintaining low intracellular Na+ and high intracellular K+; essential for cell volume regulation and many transport processes.

Na+/Ca2+ exchanger (NCX)

Membrane transporter that uses the Na+ gradient to extrude Ca2+ from the cytosol, keeping intracellular Ca2+ about 10^4 times lower than extracellular.

PMCA (Plasma Membrane Ca2+-ATPase)

Ca2+-ATPase that pumps Ca2+ out of the cytosol across the plasma membrane to help maintain low intracellular Ca2+.

SERCA (Sarcoplasmic/Endoplasmic Reticulum Ca2+-ATPase)

Ca2+-ATPase on organelle membranes (SR/ER) that pumps Ca2+ into stores, reducing cytosolic Ca2+.

Na+/K+/2Cl− cotransporter (NKCC)

Cotransporter that brings Na+, K+, and 2 Cl− into cells, contributing to intracellular Cl− accumulation and cell volume regulation.

Cl−/HCO3− exchanger

Membrane exchanger that swaps Cl− and bicarbonate to help regulate intracellular Cl− levels and pH.

Na+-driven HCO3− cotransporter

Na+-coupled transporter that moves HCO3− (bicarbonate) with Na+ to help regulate intracellular pH and bicarbonate concentration.

Na+/H+ exchanger (NHE)

Exchanges Na+ into the cell for H+ out, contributing to intracellular pH regulation.

Na+-driven HCO3− cotransporter (Na+-HCO3− cotransporter)

Cotransporter that uses Na+ to bring bicarbonate (HCO3−) into the cell, aiding pH and bicarbonate balance.

V-type H+-ATPase (V-ATPase)

ATP-driven proton pump on organelle membranes that acidifies compartments and helps maintain proton gradients.

Resting membrane potential

Baseline electrical potential across the cell membrane, typically near -60 to -40 mV, set by ionic gradients and conductances.

Intracellular Ca2+ concentration gradient

Intracellular Ca2+ is kept at ~10^4 times lower concentration than extracellular Ca2+ by pumps and exchangers.

Osmolarity

Total concentration of osmotically active particles in a solution (typical plasma ~300 mOsm/L).

Isotonic solution

Solution with osmolarity similar to plasma; examples include 0.9% NaCl; 5% glucose is initially isotonic but glucose is rapidly metabolized.

Hypotonic solution

Solution of lower osmolarity than plasma; causes water to enter cells and swell.

Hypertonic solution

Solution of higher osmolarity than plasma; draws water out of cells, causing shrinkage.

Tonicity

Effective osmotic pressure gradient across a semipermeable membrane that drives water movement; depends on solute permeability.

Osmolarity

Measure of solute concentration (osmoles per liter) in a solution; relates to water movement across membranes.

Albumin

Major liver-produced plasma protein that contributes to plasma oncotic pressure; deficiency lowers oncotic pressure and can cause edema.

Oncotic pressure

Osmotic pressure exerted by plasma proteins (primarily albumin) drawing water into capillaries.

Edema

Excess fluid accumulation in the interstitial space, often due to decreased oncotic pressure or other fluid shifts.

5% glucose solution

Isotonic glucose-containing IV fluid that is metabolized, so water remains to maintain volume and the solution provides calories.

0.9% NaCl (Normal saline)

Isotonic IV fluid that distributes in the extracellular fluid and does not alter osmotic movement across membranes.

Dextrose in saline

Maintenance IV fluid combining dextrose with saline for caloric supply and fluid balance.

SGLT1 (Na+/glucose cotransporter)

Apical transporter that uses the Na+ gradient to cotransport glucose into enterocytes from the intestinal lumen.

GLUT2

Basolateral facilitated diffusion glucose transporter that moves glucose from enterocytes into the blood.

Transcellular transport

Solute movement through a cell, crossing both apical and basolateral membranes.

Paracellular transport

Solute movement between cells, through tight junctions and lateral intercellular spaces.

Tight junction (claudins)

Junctional complexes between epithelial cells that form a selective barrier to paracellular ion movement.

Adherens junction (cadherins)

Cell–cell adhesion junctions stabilized by cadherin proteins, supporting tissue integrity.

Gap junction (connexons)

Channels between adjacent cells formed by connexons that allow direct cytosolic exchange and signaling.

Volume regulatory decrease (RVD)

Cellular response to swelling by releasing solutes to reduce cell volume and promote water efflux.

Volume regulatory increase (RVI)

Cellular response to shrinkage by increasing solute uptake to restore cell volume.

Ouabain

Inhibitor of Na+/K+-ATPase that disrupts Na+ and water balance, leading to cell swelling.

Water movement by osmosis

Passive movement of water across membranes toward higher solute concentration, following osmotic gradients.