Physio Homeostasis & Memb Potentials (2/3)

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  1. Lecture 2 - Homeostasis
  2. Homeostasis
    • the control of some vital parameter
    • eg. O2, glucose, H+, body temperature, blood pressure, & ion concentration gradients
  3. distribution of water in humans:
    women tend to have more fat than men → less body weight consist of water (fat cells contain less water than other cell types)

    • • 60% of total body water is intracellular
    • • 40% of total body water is extracellular
    • • 75% of extracellular water is interstitial
  4. hematocrit
    the fraction of blood volume occupied by blood cells
  5. Transcellular Fluid
    5% (~1 liter) of ECF is trapped within spaces that are completely encased by epithelial cells (eg. synovial fluid w/in joints, cerebral spinal fluid surrounding brain & spinal cord
  6. Where can most protein be found?
    • intracellularly - inside the cell
    • accounts for the negative charge inside a cell
  7. Osmolality
    • measure of solute concentration (osmoles) per liter (L) of solution
    • should be equal inside & outside a cell
    • if it becomes imbalanced, there's movement of water into or out of a compartment
  8. cations
    • eg. Na+, K+
    • move toward the CATHODE - a negatively charged pole - in an electrical system
    • cathode polarity is usually negative
  9. Steady State
    • driving forces acting on a substance distributed across a membrane are constant over time
    • the RATE of net movement (transport) of the substance is CONSTANT over time
    • how our cells exist
  10. Equilibrium
    • NO net driving force is acting on a substance to force it across a membrane
    • no net transport of the substance occurs
  11. Passive Driving Forces
    • 1. concentration gradients (differences in concentration)
    • 2. electrical gradients (differences in voltage)
  12. Active Driving Force
    energy (ATP) consuming - act AGAINST concentration & electrical gradients
  13. What is the major difference between interstitial fluid and plasma?
    • there are no plasma proteins in the interstitium
    • are the 2 major constituents of the ECF
    • are almost identical compositions of small solutes
  14. What are the 2 ways plasma proteins affect solute concentrations?
    • 1. their volume
    • 2. their electrical charge

    [plasma lipids affect solute concentrations by their volume alone]
  15. principle of bulk electroneutrality
    that the number of positive charges in the solution equal the number of negative charges
  16. Anion Gap
    • the difference between major cations & anions in plasma
    • some anions are ignored - is a normal gap
    • Anion Gapplasma = Na+plasma - (Cl-plasma + HCO3-plasma)
  17. What could be one possible cause of a larger than normal anion gap?
    • type I diabetes
    • because large amounts of negatively charged metabolites (eg. acetoacetate & β hydroxybutyrate) may be produced during an event like ketoacidosis, the gap could be larger than normal
    • the resultant excess negativity will draw Na+ into the plasma to compensate, making it look like there's excess Na+
  18. What do K+ gradients across cell membranes mainly determine?
    • electrical excitability
    • a disorder in extracellular [K+] is likely to cause cardiac arrhythmias & neuromuscular problems
  19. What can disturbances in extracellular Na+ cause?
    • water to shift into or out of neurons
    • such water movements can lead to seizures, coma, or death
  20. Passive (non-coupled) Transport Requires:
    • 1. a driving force (concentration or electrical gradient)
    • 2. a pathway through the membrane

    • hydrophobic solutes (CO2, steroids, & some drugs) can pass through lipid bilayer membrane while hydrophilic solutes (electrolytes, glucose) pass through cells through protein channels or carriers
  21. Active Transport
    a process that transfers a solute across a membrane against its electrochemical gradient
  22. Primary Active Transport
    active transport where the driving force comes from the energy change associated with an exergonic chemical reaction like ATP hydrolysis
  23. Secondary Active Transport
    active transport where the driving force is provided by coupling the uphill movement of that solute to the down hill movement (eg. Na+) of one or more other solutes for which a favorable electrochemical gradient exists
  24. Osmolality
    total concentration of all particles in solution
  25. What is the anion with the greatest extracellular concentration?
    • Chloride
    • [Na+ is a cation]
  26. Lecture 3 - Generation of Membrane Potentials
  27. What 3 conditions need to exist to generate a membrane potential?
    • 1. ionic concentration gradients
    • 2. a semi-permeable membrane
    • 3. membrane capacitance
  28. Semi-permeable Membrane
    can be achieved via ion-permeable channels, aqueous pores through the membrane, or channels that have selective permeability
  29. Membrane Capacitance
    • the ability to separate & store a charge
    • this generates the membrane potential
    • membrane becomes the capacitor across which voltage can change
  30. the overall permeability of a membrane depends on:
    the sum of the permeabilities of all open channels
  31. membrane potential
    • Em refers to the charge INSIDE a cell compared to outside
    • generally the charge outside = 0 (as a reference)
    • inside - outside charge
  32. What two entities cancel each other out in true equilibrium?
    • concentration & electrical force
    • there are equal numbers of ions moving in each direction
  33. The Nernst Equation
    • EK = RT/zF • ln [ion]out/[ion]in
    • the magnitude of electrical potential required to balance a given ion’s concentration gradient
    • this electrical voltage difference between inside & outside a cell is a bunch of constants times the ratio of concentrations of ions outside & inside the cell
  34. What will a cells membrane potential be it's only permeable to one ion type?
    the cell’s membrane potential will be equivalent to that IONS equilibrium potential
  35. Values in the Nernst Equation
    • at normal body temperature
    • RT/zF • ln = 60mV/z • log
    • R: gas constant (8.32 joules/mole • °K)
    • T: the absolute temperature (in °K, ~37)
    • z: ion valence (+1 in the case of K+)
    • F: Faraday's constant (96,500 coulombs/mole of + charge)
  36. If the concentration of an ion is much lower OUTSIDE than it is inside: (eg. K+)
    • EK = 60mV/z • log [ion]out/[ion]in
    • EK = 60mV/(+1) • log (10/100)
    • EK = 60 • -1 / +1 = -60 mV
  37. Log Rules
    if the denominator is greater than the numerator, then log(ionout/ionin) is negative

    if the numerator is greater than the denominator, then log(ionout/ionin) is positive
  38. Potassium's Effect on Membrane Potential
    • actual EK = -93 mV
    • Em = -70 mV
    • [K+]inside > [K+]outside
    • K+ wants to move OUT of the cell; this is down its electrochemical gradient
    • moving out makes the inside of the cell more negative (b/c + charge is leaving), bringing the membrane potential CLOSER to that of K+ in equilibrium (EK = -93 mV)
    • the concentration gradient is directed OUTWARDS
  39. If the concentration of an ion is much higher OUTSIDE than it is inside: (eg. Na+)
    • ENa = 60mV/z • log [ion]out/[ion]in
    • ENa = 60mV/(+1) • log (100/10)
    • ENa = 60 • +1 / +1 = 60 mV
  40. Sodium's Effect on Membrane Potential
    • actual ENa = +65 mV
    • Em = -70 mV
    • the concentration gradient is directed INWARDS
    • when Na+ moves DOWN it's concentration gradient, the membrane potential inside the cell relative to outside the cell becomes more POSITIVE (b/c Na is a cation)
    • this brings the Em closer to ENa
  41. Chloride's Effect on Membrane Potential
    • ECl = -89 mV
    • Em = -70 mV
    • higher concentration outside the cell; lower inside the cell (similar distribution as Na+)
    • this concentration gradient drives Cl- inside, making the inside of the cell more negative
    • ECl is more negative than Em; Cl- wants to move into the cell (that's down its electrochemical gradient)
  42. Equilibrium Potentials of Each Ion
    • K+: –93 mV
    • Na+: +65 mV
    • Cl-: –89 mV
    • Ca2+: +129 mV (extremely low inside the cell)
  43. Between what 2 values can a cell's actual membrane potential be found?
    • between EK & ENa
    • when a cell membrane is at rest, the net flow of sodium into the cell balances the net flow of potassium out of the cell
  44. Goldman-Hodgkin-Katz Equation
    • Em = 60 *
    • log pK[K]o + pNa[Na]o
    •      pK[K]i + pNa[Na]i
  45. What is the contribution that an ion makes to a cell’s membrane potential dependent on?
    • the contribution an ion makes is proportional to the ion’s membrane permeability
    • when multiple ions are permeable, the MORE permeable a given ion, the closer the actual membrane potential will be to that ion’s equilibrium potential
  46. What determines the magnitude of Em to injected current?
    • the passive resistance
    • small cells have a HIGHER resistance than large ones
    • small cells have less area - if the # of channels is the same (between a little & large cell) a larger change in membrane potential will come about in the small cell with the same magnitude of stimulation
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  47. λ
    • λ = Rm / Ri
    • used to quantify the distance that a graded electric potential will travel along a neurite via passive electrical conduction
    • the greater λ, the further the potential will travel
  48. Rm (membrane resistance)
    • the force that impedes the flow of electric current from the outside of the membrane to the inside
    • is a function of the number of open ion channels
  49. Ri (axial resistance)
    • the force that impedes current flow through the axoplasm, parallel to the membrane
    • is a function of the diameter of the axon (the greater the axon diameter, the LOWER the Ri)
  50. Passive Membranes
    • only have channels that are always open
    • the amplitude of the potential membrane voltage change is a linear function of the membrane current
  51. Excitable Membranes
    • have channels open at rest & have channels which can be controlled by a variety of factors (gated)
    • *can propagate signals without decrement along axons, dendrites, & muscle fibers (have specialized channels that allow conduction of all-or-none signals for long distances)
Card Set:
Physio Homeostasis & Memb Potentials (2/3)
2014-01-25 02:32:04
MBS Physiology
Exam 1
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