Principles of Blood Flow Flashcards
Stages 1-4 of blood flow and types of pressure
1: steady laminar flow in rigid vessels with static driving pressure
2. high Reynold’s number flow (turbulence) with dynamic pressure
3. elastic vessel walls with pulsatile pressure
4. microcirculation with diffusion pressure
flow in series VS in parallel and where such things are the case
series: Q1=Q2=Q3… - in organ systems
parallel: Q=Q1+Q2 (if Q1 and Q2 are in parallel) - in general circulation
why the parallel architecture of circulatory system permits redistribution of blood flow
- rate of blood flow to each tissue is almost always recisely controlled by tissue need
- cardiac output is controlled mainly by sum of all local tissue flows
- arterial pressure regulation is generally independent of either local blood flow or cardiac output control
- blood is redistributed by adjustment of resistances (precapillary sphincters and arterioles) prior to capillary beds
blood to which organs increase/decrease during exercise?
increase: lungs, heart, brain, bone, muscle
decrease: digestive organs, kidneys
how does linear velocity vary with X-sectional area change?
linear velocity varies inversely as X-sectional area increases
v = Q/A
what is the general cardiac output?
5 L/minute
transit time
time required for a blood cell to travel between 2 points in the system
-t = length/velocity = volume/flow
blood as a fluid (approximations)
steady flow of incompressible fluids in rigid, straight, cylindrical tubes (good except for smaller vessels)
- not always true: flow is laminar with no slippage at wall, and viscosity is constant across diameter of vessel
- -in reality, they don’t slide past each other (stay in adjascent paths) but NOT so at boundary of vessel
- -also, viscosity is highest at the borders
is blood compressible or incompressible?
incompressible
where does resistance to blood flow come from?
- walls of the vessel (drag)
- viscosity of the blood
both are frictional forces
what does mean linear velocity equal in regards to peak velocity?
MLV = 1/2 peak velocity
what is the major physiological variable that determines resistance to blood flow?
radius of blood vessels (mostly arterioles), then viscosity
relationship between temperature and viscosity
temperature is inversely related to viscosity (so colder = more viscous, less blood flow)
-vasoconstriction supplements temperature by reducing blood flow
relationship between pressure drop and radius, viscosity
the larger the radius, the smaller the pressure drop (inverse relationship)
the more viscous, the larger the pressure drop
turbulent flow
laminar flow that breaks down when velocity reaches critical point (for Re>3000; between 2000-3000 is transitional period)
-causes significant losses of kinetic energy
viscosity
measure of intermolecular attractions in liquid
- determines steepness of velocity gradient
- NOT density
Reynolds Number (Re)
Re = disruptive forces/cohesive forces
4 factors that generate pressure
- gravity
- compliance of vessels
- viscous resistance
- inertia
murmurs
audible sounds due to vibrations in heart or vessel walls (also called “bruit”)
-don’t occur under resting conditions
how does gravity affect blood pressure?
adds or subtracts from pressure generated by heart
- exists whether or not the heart is beating
- doesn’t affect flow of blood in circuit of distensible vessels b/c gravitational pressure in arteries is exactly counter-balanced by same gravitational pressure at same level in corresponding veins
- does affect distribution of blood throughout system of distensible vessels
conclusions from Bernouli effect
- as velocity decreases, dynamic pressure becomes smaller fraction of total pressure
- as vessel radius narrows, dynamic component increases significantly
sheer stress
created by flowing blood on endothelial wall directed along long axis of vessel
-sheer stress on vessel wall is proportional to viscosity and shear rate (rate at which axial velocity changes from wall to lumen)
Poiseuille flow
sheer stress is directly proportional to viscosity and flow rate, and inversely proportional to cube of vessel radius
-measured in units of pressure
anomalous viscosity of blood
increase in viscosity at low flow rate (blood is non-newtonian, so viscosity changes)
-at low flow rates, rouleaux form, creating higher resistance
what does “to yield shear stress” mean?
blood behaves anomalously, meaning at low flow rates they need a threshold force to get moving
what does blood viscosity depend on?
- fibrinogen (increased clots)
- hematocrit
- vessel radius (at diameters less than 0.3 mm, viscosity decreases)
- velocity (higher flow = lower viscosity)
- temperature (colder = higher viscosity)
Fahraeus-Lindqvist effect
at tube diameters less than 0.3 mm (arterioles, capillaries, venules) the apparent viscosity of blood decreases
-due to axial streaming (RBC accumulate in rapidly flowing axial lamina)
does plasma/saline viscosity depend on tube diameter?
no, they are independent
plasma skimming
tendency of cell-free plasma to be skimmed off at a branch point of microcirculation
-cause altered hematocrit in small vessels
how do the structure and function of microcirculation differ in different tissues?
- nutritional source and waste removal in most vascular beds
- filtration in renal glomeruli
- thermoregulation in skin
where do microcirculatory circuits extend from?
from an arteriole to a venule
- arterioles are surrounded by single layer of vascular smooth muscle cells
- venules are surrounded by discontinuous layer of vascular smooth muscle cells
precapillary sphincters
control local flow w/in capillary network
-not innervated, but responsive to local conditions of O2, CO2, acidity
meta-arterioles
provide shunt that bypasses capillary network (not present in all tissues)
continuous capillaries
most common, with interendothelial junctions 10-15 nm wide
-absent in brain capillaries, which have narrow tight junctions that form blood-brain-barrier
fenestrated capillaries
surround exocrine glands or epithelial membranes like in small intestine
-endothelial cells have conduits that permit flow of fluid and solutes across capillary endothelial membrane
discontinuous capillaries
in liver sinusoids
-have fenestrations and large gaps between endothelial cells
how are substances carried between organs w/in cardiovascular system?
convective transport (carried with flow of blood) -transport rate of X = flow rate * concentration of X =flow of O2 in arteries = CO * arterial O2 content
what are the only 2 methods to alter rate at which a substance is carried to an organ?
- change flow rate through organ (like CO)
2. change arterial concentration
FICK principle
-what is the transcapillary efflux rate, and what does it mean if transcapillary efflux rate is negative?
tissue rate of utilization or production of substance X measured from transport rate in and out of the tissue (transcapillary efflux rate = Q * (arterial X - venous X))
–if TER of a substance is negative, then the tissue is producing it
what 4 factors determine diffusion rate of a substance between blood and interstitial fluid?
- concentration difference
- surface area for exchange
- diffusion distance
- permeability of capillary wall to diffusing substance
osmotic pressure
hydrostatic pressure needed to balance movement of solute
what is the total osmotic pressure of normal plasma?
5000 mmHg
what are the 4 pressures of capillary fluid balance?
- capillary hydrostatic pressure (Pc; drives out of capillary)
- interstitial fluid hydrostatic pressure (Pif; drives out of capillary)
- capillary osmotic pressure (Pi c; drives inside capillary)
- interstitial fluid osmotic pressure (Pi if; drives into capillary)
what is the net pressure equation?
Pnet = (Pc - Pif) - (Pi c - Pi if) = net hydrostatic pressure - net osmotic pressure
what does a positive hydrostatic pressure difference mean?
drives a positive flow of water out of the capillaries into the interstitial fluid space
what does a positive osmotic pressure difference mean?
drives a negative flow of water into the capillaries from the interstitial fluid space
hydraulic conductivity
constant of proportionality that relates the amount of flow to net driving force
-also called net filtration pressure, which is the difference between hydrostatic and osmotic pressure differences
what is net capillary flow proportional to? and what is its equation?
Jv is proportional to filtration pressure
Jv = Lp * Pnet
Lp = hydraulic conductivity
what does high Pc (capillary hydrostatic pressure) favor?
filtration
-higher in renal glomeruli (50 mmHg), lower in capillaries (5-15 mmHg, so no filtration or pulmonary edema)
effect of arteriolar dilation or venular constriction
increases capillary hydrostatic pressure (Pc)
- resistance post > resistance pre
- drives water out of capillaries, promoting edema (tracks Pa)
effect of arteriolar constriction or venular dilation
decreases capillary hydrostatic pressure (Pc)
- resistance pre > resistance post
- favors absorption of fluid back into capillaries (tracks Pv)
what is total osmotic pressure of plasma due to?
both salts and proteins
- higher concentrations of PRO in plasma is primary factor
- colloid osmotic pressure of plasma is due to PRO + excess salt caused by Gibbs Donnan effect by PRO
what is oncotic pressure
portion of the solutions total osmotic pressure that is due to particles that do not move freely across the membrane
what is interstitum made of?
both solid and liquid phases, with only a small fraction of water “free” to move
where is interstitial fluid hydrostatic pressure positive and negative?
+ rigid enclosed compartments (BM, brain) and encapsulated organs (kidney); drives fluid absorption
- loose tissue (drives tissue filtration)
what is interstitial fluid hydrostatic pressure sensitive to?
addition of fluids to interstitial compartment
- leads to disruption of solid phase collagen fibers and proteoglycan gel
- especially true in loose subcutaneous tissue, that can accommodate edema fluid
low and high compliance systems
low: if add interstitial fluid, raise Pif, opposing further filtration
high: if add interstitial fluid, some tissues can accommodate large volume of edmatous fluid w/o rise in pressure
Starling forces along a capillary
net filtration per day
2-4 L/day from plasma to interstitium (neglecting renal glomular filtration)
- total capillary filtration: 20 L/day at arteriolar ends
- total absorption: 16-18 L/day at venular ends
what are initial lymphatics similar to?
similar to capillaries, but with primary one-way lymph valves
what are collecting lymphatics similar to?
small veins with sparse smooth muscle and secondary lymph valves
- lymph nodes are located along the path
- large collecting lymphatics drain into right and left subclavian veins
what is pulmonary edema caused by?
left heart failure (CHF), pulmonary HTN
what is edema in lower extremities and abdominal viscera caused by?
right heart failure
ascites
fluid from hepatic and interstitial capillaries moves from interstitum to peritoneal cavity
what is peripheral edema caused by?
liver disease and inappropriate secretion of ADH by lung tumors (SIADH)
-liver disease also causes hypoalbuminemia
what does lymphatic blockage cause?
malignant neoplasms that cause local edema upstream of sites of blockage
