Costanza

• pH


  A logarithmic term that is used to express hydrogen (H+) concentration
• The H+ concentration of body fluids is very low (e.g., 40 × 10−9 Eq/L in arterial blood), so it is more conveniently expressed as a logarithmic term, pH
• pH


  The negative sign means that pH decreases as the concentration of H+ increases, and pH increases as the concentration of H+ decreases
• Interstitial fluid


  The fluid that actually bathes the cells and is the larger of the two subcompartments of extracellular fluid
• Interstitial fluid is an ultrafiltrate of plasma, formed by filtration processes across the capillary wall
• Because the capillary wall is virtually impermeable to large molecules such as plasma proteins, interstitial fluid contains little, if any, protein
• The composition of the body fluids is not uniform. Intracellular fluid (ICF) and extracellular fluid (ECF) have vastly different concentrations of various solutes
• Mole


  6 × 10^23 molecules of a substance
• Millimole


  1/1000 or 10^-3 moles
• Equivalent


  The amount of charged (ionized) solute, equal to the number of moles of the solute multiplied by its valence
• One mole of potassium chloride (KCl) in solution dissociates into one equivalent of potassium (K+) and one equivalent of chloride (Cl-)
• Each body fluid compartment must obey the principle of macroscopic electroneutrality; that is, each compartment must have the same concentration, in mEq/L, of positive charges (cations) as of negative charges (anions)
• Major cations and anions in extracellular fluid (ECF) and intracellular fluid (ICF)

  • ECF: Na+, Cl-, HCO3-
  • ICF: K+, Mg2+, proteins, organic phosphates
• Typically, ICF has a very low concentration of ionized Ca2+ (≈10−7 mol/L), whereas the Ca2+ concentration in ECF is higher by approximately four orders of magnitude
• ICF is more acidic (has a lower pH) than ECF
• The total solute concentration (osmolarity) is the same in ICF and ECF
• Phospholipids


  Consist of a phosphorylated glycerol backbone ("head") and two fatty acid "tails"
• Phospholipid molecules


  Have both hydrophilic and hydrophobic properties and are called amphipathic
• Phospholipid orientation in cell membranes

  • The lipid-soluble fatty acid tails face each other and the water-soluble glycerol heads point away from each other, dissolving in the aqueous solutions of the ICF or ECF, creating a lipid bilayer
• Integral membrane proteins


  Embedded in, and anchored to, the cell membrane by hydrophobic interactions
• Transmembrane proteins


  Integral proteins that span the lipid bilayer one or more times, in contact with both ECF and ICF
• Peripheral membrane proteins


  Not embedded in the membrane and not covalently bound to cell membrane components, but loosely attached by electrostatic interactions
• Examples of integral membrane proteins

  • Ligand-binding receptors
  • Transport proteins
  • Pores
  • Ion channels
  • Cell adhesion molecules
  • GTP-binding proteins
• Diffusion


  Transport of substances down an electrochemical gradient, requiring no input of metabolic energy
• Facilitated diffusion


  Diffusion of substances down an electrochemical gradient, using transport proteins
• Active transport


  Transport of substances against an electrochemical gradient, requiring input of metabolic energy
• Types of active transport

  • Primary active transport
  • Secondary active transport
• The exclusion of proteins from interstitial fluid has secondary consequences, causing a redistribution of small, permeant cations and anions across the capillary wall, called a Gibbs-Donnan equilibrium
• Gibbs-Donnan ratio


  The ratio of the concentration of a small, permeant ion in plasma compared to interstitial fluid
• Generally, the minor differences in concentration for small cations and anions between plasma and interstitial fluid are ignored
• Transmembrane proteins


  Proteins in contact with both extracellular fluid (ECF) and intracellular fluid (ICF)
• Types of membrane transport

  • Simple diffusion
  • Facilitated diffusion
  • Primary active transport
  • Cotransport (secondary active)
  • Countertransport (secondary active)
• Saturation


  Carrier proteins have a limited number of binding sites for the solute
• Stereospecificity


  Binding sites for solute on the transport proteins are stereospecific
• Competition


  Binding sites may recognize, bind, and transport chemically related solutes
• Simple diffusion


  1. Random thermal motion of molecules

  2. Net diffusion from high to low concentration

  3. Continues until concentrations are equal
• Factors affecting rate of simple diffusion

  • Concentration gradient
  • Partition coefficient
  • Diffusion coefficient
  • Membrane thickness
  • Surface area
• Permeability


  Combines partition coefficient, diffusion coefficient, and membrane thickness into a single term
• Diffusion of electrolytes is affected by potential differences across the membrane
• Facilitated diffusion


  Carrier-mediated transport down an electrochemical gradient, no metabolic energy required