BMS100 ClinPhys FlowDown.docx

BMS100: CLINICAL PHYSIOLOGY REVIEW NOTES FLOW DOWN GRADIENTS Flow: movement of a substance from one point in system (A) to another (B) Measured by amount of substance that moves over time Energy gradient: driving force for the flow of a substance Amount of flow is directly related to energy gradie...

BMS100: CLINICAL PHYSIOLOGY REVIEW NOTES FLOW DOWN GRADIENTS Flow: movement of a substance from one point in system (A) to another (B) Measured by amount of substance that moves over time Energy gradient: driving force for the flow of a substance Amount of flow is directly related to energy gradient between A and B All systems will have factors that resist flow Essential for life – fluids and gases moving in the body Poiseuille’s Law Poiseuille’s Law: describes the flow of a viscous fluid through a tube; depends on the following parameters: Hydrostatic pressure - causing gas or liquid to flow from point A to B Physical structures resist flow (resistance) – dimensions Viscosity of the fluid F = flow; volume of liquid that passes through a tube per unit time P = hydrostatic pressure; force that a substance exerts on the walls of its container R = radius of the tube that the fluid is moving through Resistance is inversely related to the 4th power of radius = greatest impact L = length of the tube U = viscosity of the fluid Less viscous fluid is more runny = less friction More viscous fluid is more syrupy = more friction Flow INCREASES when: P (hydrostatic pressure) – increases R (radius) – increases Flow DECREASES when: L (length) – increases U (viscosity) – increases Respiratory tract and cardiovascular system: Flow of gas or blood to tissues (in terms of mL/min) Body controls flow through vessels by: Controlling the pressure in large vessels Controlling the radius of small vessels Poiseuille’s Law exceptions: Only accurate for rigid, simply shaped tubes with non-turbulent flow Harder to quantify when: Branched or irregularly shaped Flow is turbulent Tube is flexible Radius is still most important factor Fick’s Law Fick’s law: quantifies how the rate of diffusion is affected by various parameters Diffusion is the movement of a solute or a gas from an area of high concentration to low concentration; usually across a membrane barrier F = flow/flux; number of molecules of a substance diffusing from point A to point B over time (CA-CB) = concentration gradient A = surface area of the membrane K = constant that increases when: *do not need to worry about this* Substance is a smaller molecule and dissolves better Permeability of barrier to the substance increases T = thickness of the membrane Flow INCREASES when: (CA-CB) (concentration gradient) – increases A (surface area) – increases K (constant) – increases Flow DECREASES when: T (thickness) – increases Fick’s Law in the body Typical capillary lined with endothelial cell membrane – some molecules can diffuse Thickness for diffusion needs to be very small = slow over distances greater than 0.1mm Adaptation to allow other molecules to diffuse Channels or transporters to increase permeability of the membrane Need for transporters/channels depends on solubility of the substance in the membrane Structural features that increase the surface area to volume ratio Bodies manipulate concentration gradients Ex. metabolism, transporters that increase gradients (ex. sodium potassium pump) Ohm’s Law Ohm’s Law: defines the movement of a dissolved, charged particle across a barrier; rate of flow of charges across a membrane (current) Movement depends on: Charge of particle – opposites attract; like charges repel Difference in concentration of charges across membrane = voltage Type of potential energy (how much work it takes to move particle) Permeability of the membrane to the charged particle I = current; number of charges or charged particles that move across the membrane per unit time V = voltage; energy generated by separating charges across the cell membrane Particles move down a gradient of voltage - established by electric field R = resistance; anything that impedes movement of particle More channels for a charged particle; less resistant Current INCREASES when: V (voltage) – increases Current DECREASES when: R (resistance) – increases Ohm’s Law in the body: Most useful when thinking about unequal distributions of charges very close on either side of a membrane Overall – positive/negative charges are balanced Electric field declines very rapidly as charges are separated by distance (needs to be close on either side of the membrane) Starling Forces In physiology, more than on force acts on the same substance Ex. filtration through a capillary – diffusion and hydrostatic pressure Ex. distribution of ions across a membrane – diffusion and electrostatic forces Starling’s Law: set of principles that describe the balance of forces responsible for the movement of fluids across the walls of the capillaries Forces influence the exchange of fluids such as – plasma, tissue (interstitial) fluid, solutes between blood and tissues Purpose of capillaries are to transport substances Water uses – hydrostatic pressure and diffusion All other molecules uses – diffusion, protein-mediated transport, endocytosis Can help describe tissue swelling in a wide variety of situations Ex. inflammation/infections, changes in pressure within the circulation LP – leakiness of capillary wall to water; inverse of resistance P – hydrostatic pressure Π - osmotic pressure Cap – fluid within the capillary ISF – fluid within the interstitial space σ - how much protein leaks through the capillary wall Nernst Potential Nernst Potential: concept that represents the electrical potential difference (voltage) across a cell membrane when a specific ion is in equilibrium (no net movement across cell membrane) Voltage at which the concentration gradient of an ion is balance by the electrical gradient = no net flow Charged particles move across a membrane based on electrostatic forces Dissolved particles move across a membrane based on concentration gradient Describes the voltage across a membrane that is permeable to P given the ratio of [P] inside : outside The equation accounts for: Diffusional forces and electrical fields Charge of the particle Ratio of the particle’s concentration intracellular to extracellular Does NOT include flow (current) or resistance Ep – membrane voltage at which a particle (P) moves into and out of the cell at the same rate = equilibrium Zp – charge and valence of P [P]I / [P]o – ratio of intracellular to extracellular concentrations of P (particle) Nernst potential in the body: Living cells have membrane potential – charge and ion balance serves important functions in: Cellular signaling Transport of substances Regulation of cell volume Goldman Field Equation: describes the membrane potential (voltage) of a cell membrane when multiple ions contribute to that potential; extends the Nernst equation Different ions with different permeabilities across concentration gradients Pk – membrane permeability to K+, PNa+ - membrane permeability to Na+, etc.