Microcirculation Flashcards
how is blood viscosity anomalous? is it newtonian under physiological conditions?
it is not constant under all conditions
yes
what graph is viscosity the slope of?
shear stress vs shear rate plot
under what conditions is blood not a newtonian fluid?
at lower flow rates
what is a yield shear stress?
at lower flow rates, blood requires a threshold force to get moving
what is polycythemia?
abnormally elevated hematocrit
what are the five determinants of blood viscosity?
fibrinogen, hematocrit, vessel radius, velocity and temperature
what is the relationship between the flow and pressure of a newtonian fluid?
linear
why do low flow rates of blood appear to have more resistance?
formation of rouleaux at low velocities that break up as blood flow increases
what happens to the viscosity of blood when hematocrit increases from 40-60%?
it doubles
what is a phlebotomy?
removal of excess red blood cells
what is the fahraeus-lindqvist effect?
that the apparent viscosity of blood depends on the diameter of the tube through which it is flowing
what happens to blood in tubes with a diameter less than 0.3 mm?
the apparent viscosity of the blood decreases
what is axial streaming?
the tendency for red blood cells to flow in the fast moving laminar flow part of the stream
what is the result of axial streaming?
RBC move through small vessels faster than plasma resulting in lower hematocrit in those vessels and therefore lower viscosity
what are rouleaux?
chain like aggregates of RBC which form at low flow rates
what is plasma skimming?
tendency of plasma to be skimmed off into a branch of the circulation because it flows near the edges of the flow due to axial streaming of red blood cells
what prevents plasma skimming?
arterial cushions near the branch points that cause turbulence before the blood moves into the branch
what is cellular deformability?
the ability for RBC to bend in order to squeeze through small capillaries
what occurs if RBC are unable to deform when moving though small capillaries?
it may lead to blockage of flow and ischemia
what are the three functions of microcirculation?
nutritional source
filtration in renal glomeruli
thermoregulation in the skin
what do capillaries connect?
arterioles and venules
what is one possible function of a metarteriole?
to shunt blood to bypass a capillary network
what controls local flow within the capillary network?
precapillary sphincters
how are precapillary sphincters signaled to contract or release? what, in general, results in their opening?
they respond to local conditions to oxygen, CO2 and acidity mainly
hypoxia leads to opening
how does the flow react in microcirculation to varying pressure? why?
subtle changes affect flow velocity and direction because the pressure differences at the beginning and end are not very substantial
what is a metarteriole?
a blood vessel that connects from the arteriole to the venule and has inward blood flow from some capillaries. may shunt blood if the capillary bed is bypassed
what is the structure of a capillary?
a single layer of endothelial cells surrounded by a basement membrane
what is the diameter range of a capillary? of a RBC?
4-10 microns
8 microns
how do capillaries vary?
in their degree of leakiness
which are the most common capillary subtype? what is a defining feature?
continuous capillary
contain interendothelial junctions
what is different about continuous capillaries in the brain?
there are no interendothelial junctions but tight junctions instead
what are fenestrated capillaries and where are they located?
their endothelial cells have conduits that permit flow of fluid and ions across the endothelium
located in exocrine glands or epithelial membranes (small intestine)
what are discontinuous capillaries? where are they found?
have fenestrations and large gaps between endothelial cells
found in liver sinusoids
where is capillary density low?
low O2 consumption tissues like the joints and cartilage
what percentage of muscle capillaries are perfused at erst? how does this increase?
20%
increases with precapillary sphincter relaxation
what is convective transport?
transport of substances between organs within the cardiovascular system
what does the transport rate of a substance in the blood depend on?
flow rate and concentration
what is the fick principle?
transcapillary efflux rate= flowx concentration in arteries-concentration in veins
what causes the transcapillary efflux rate to be negitive?
the tissue is producing a substance
what is the farthest distance between a cell and a capillary?
10 micrometers
what four factors determine the diffusion rate of a substance from ECF to capillaries?
concentration difference, surface area for exchange, diffusion distance and permeability of the capillary to that substance
what does fick’s first law of diffusion sate?
the flux is directly proportional to the area of the membrane and to the concentration across the membrane
what is flux?
the number of moles that diffuse across the membrane during an interval of time
what is the diffusion coefficient?
the flux when the area and concentration gradient are in unity
when is flux negative and when is it positive?
it is positive with an efflux from the plasma and negative with an influx to the plasma
what increases the diffusion coefficient?
increased temperature and with small solutes
what does area represent in fick’s law of diffusion?
the area of the membrane available for diffusion
what does the presence of solute on one side of a membrane do the chemical potential of water?
decreases it
what does hydrostatic pressure in a tube do to the chemical potential of the water in a solutino?
it increases it
what is osmotic pressure?
the hydrostatic pressure that would stop the flow of water into a solution
what are the six things that net shifts between capillary and interstitial compartments are important for?
maintenence of blood volume, interstitial fluid absorption, edema formation, saliva, sweat and urine production
what four pressures are involved in capillary fluid balance?
capillary and interstitial fluid hydrostatic pressure (P) and capillary and interstitial fluid osmotic pressure (pi)
what is the starling equation?
capillary flow=L(hydraulic conductivity) x pressure summation
how does one summate pressure in the for the starling equation?
[(Pc-Pif)-(Pi c-Pi if)]
how does fluid travel out of the capillaries?
trancellularly and paracellularly
how does fluid travel through the capillary endothelium transcellularly? paracellularly?
trans- aquaporin channels in capillary endothelial membranes
para-through interendothelial clefts, fenestrae or gaps
how do gasses and other small solutes cross the capillary membrane? water?
gas-diffusion
water-convection
how is flow into and out of the capillaries quantified?
flow from capillaries to IF is positive, flow to the capillaries from the IF is negative
what does a positive hydrostatic pressure difference cause? a positive osmotic pressure difference?
hydrostatic-positive flow of water
osmotic-negative flow of water
what is hydraulid conductivity?
constant of proportionality that relates amount of flow to the driving force
how does capillary hydrostatic pressure change along the length of the capillary?
it falls linearly starting at 35 mmHg and ending at 15 mmHg
what is the Pc in the middle of a capillary?
the averages of the Pc at the venuolar and arteriolar end
besides along the length of a capillary,where does Pc vary?
in different tissues (higher in kidneys for filtration)
like other blood pressures, what other force affects Pc?
gravity
what changes in vessel diameter increase capillary hydrostatic pressure?
arteriolar dilation or venular constriction
what changes in vessel diameter decrease capillary hydrostatic pressure?
arteriolar constriction or venular dilation
what contributes to the total osmotic pressure of plasma? which is primary?
salts and proteins (primary)
what does the gibbs donnan equilibrium do? what is its cause?
increases the excess salt in the plasma
exerted by the plasma proteins that are charged and sequester other charged molecules around them (electroneutrality)
what is the total protein concentration in the blood and what does albumin contribute to that?
total-7 g/dl or 1.5 mM
albumin-5 g/dl or 1 mm
what are the three major plasma proteins?
albumin, globulins and fibrinogen
what is the colloid osmotic pressure of the plasma? what are the contributions?
25 mmHg
plasma proteins and excess salt
what is needed to correct van’t hoff’s law for actual measured osmotic pressure?
a coefficient specific for the specific solute
what is the sign of the hydrostatic pressure in loose tissue and encapsulated organs?
negative in loose tissue and positive in encapsulated organs
what can occur with the addition of fluids to the interstitial compartment? when is this a concern?
disruption of the solid phase collagen fibers and proteoglycan gel
edema
what direction does an increase in interstitial fluid hydrostatic pressure drive fluid?
negatively-absorption into capillaries
what is the body average interstitial fluid osmotic pressure? what does this drive?
3 mmHg
drives fluid into tissue and out of capillaries
what occurs at the arteriolar end of the capillary? the venule end?
arteriole- filtration out of capillary
venule- capillary absorption
what causes the change from filtration to absorption along the capillary?
decrease in capillary hydrostatic pressure and increase in IF osmotic pressure ( P if and Pi c remain the same)
what causes the net increase in IF osmotic pressure along the length of a capillary bed?
fluid leaving the capillary contains plasma protein
in the entire capillary bed, what is the net movement of fluid? what is the value?
there is a net filtration into the IF (neglecting glomerular filtration)
2-4 L/day
how are capillaries different than initial lymphatics?
there are one way valves in lymph vessels
what does lymph accommodate? how much fluid does it collect per day?
it accommodates for the net filtration into the IF by the capillaries
2-4 L/day
what types of diseases cauase edema?
relan, cardiac lung and hepatic diseases
what is ascites?
fluid from hepatic and intestinal capillaries moving from the interstitium into the peritoneal cavity
why can liver disease cause edema?
because the liver synthesizes albumin- the major factor in capillary osmotic pressure
