03 - Basic Physiology of Membranes
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What is the difference in composition of extracellular fluid versus intracellular fluid in regards to Na+, K+, Ca2+, Cl-, and HCO3-?
- Na+ Extra: 142 mEq/L Intra: 10 mEq/L
- K+ Extra: 4 mEq/L Intra: 140 mEq/L
- Ca2+ Extra: 2.4 mEq/L Intra: 0.0001 mEq/L
- Cl- Extra: 103 mEq/L Intra: 4 mEq/L
- HCO3-Extra: 28 mEq/l Intra: 10mEq/L
Plasma versus Interstital Fluid
The concentration of ions between the blood plasma and the interstitial fluid is the same, except in regards to proteins; there should be no proteins in the interstitial fluids, whereas there will be ~7 g% in the blood plasma
What is Fick’s First Law?
The net rate of diffusion (flux;mmol/sec) is equal to the permeability (cm/sec) times the area (cm6) of the membrane times the concentration difference across the membrane (mmol/ml) divided by the membrane thickness (cm); J=PA (CA-CB)/ΔX
What factors affect permeability?
Lipid solubility, size and shape of diffusing molecule, temperature, and membrane thickness
What does the reflection coefficient (σ) tell you?
If σ = 1, then the solute is completely reflected and is impermeable; if σ = 0, then the solute is not reflected at all and is highly permeable
What is Lidocaine and how does it work?
Blocks voltage-gated Na+ channels and prevents the occurrence of nerve action potentials
Ca2+ channel blockers
Epithelial Sodium Channels (ENaC)
Found in Kidney, lung, and gut; aldosterone increases incorporation of ENaC into renal tubular cell membranes; Amiloride blocks ENaC and causes diuresis
What are uniporters?
Transport proteins; often called this because they transport a single substance down its concentration gradient during facilitated diffusion; has a Vmax
What are the features of carrier-mediated diffusion?
Takes place more rapidly than simple diffusion for that solute; shows saturation kinetics; high degree of chemical specifity; competitive and non-competitive mechanisms can limit binding to the carrier; works in both direction, but always down a concentration gradient; requires binding and conformational change in the carrier protein
What is the difference between primary and secondary active transport?
Primary directly moves molecule with ATP energy (Na+/K+ ATPase), while secondary uses cotransport of molecule moving down a concentration gradient set up by primary active transport
Why is the Na+/K+ ATPase pump so important?
There is a concentration gradient where Na+ is outside the cell and K+ is inside the cell, but this gradient is so great they leak across the cell membrane; the pump keeps this gradient because it is so important to other molecules who get across the membrane via specialized co-transporters
What are some examples of primary active transport and what do they do?
Na+/K+ ATPase pumps Na+ out and K+ in to balance the leak; Ca2+ pump in plasma membrane pumps Ca2+ out of the cell to lower cystolic Ca2+ levels; Ca2+ pump – SR or mitochondria pumps ca2+ into the SR to lower cytosolic Ca2+ levels
How do glycosides control cardiac contractility?
- Via an antiport; inhibit the Na+/K+ ATPase which helps keep cellular Ca2+ low by running the Na+/Ca+ exchanger off the
- Na+ gradient; blocking the Na+/K+ ATPase indirectly decreases the Na+/Ca+ exchanger which increases intracellular Ca2+
What is osmolarity?
Concentration of osmotically active particles; 1mM of NaCl contains 1mM Na+ and 1mM Cl-, and therefore have 2 milliosmoles; 2mOsmoles/liter = 2mOsmolar
What does D5W demonstrate?
5% dextrose in water, is isosmotic but is hypotonic; glucose enters the cell and the water follows the glucose, swelling the cell; saline by contrast is isosmotic and isotonic
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