Flashcards in Two B Deck (8)
What are two ways to calculate flow?
Flow (Q) (Vol/time) = Cross sectional area x velocity of flow
Also, Q=change in pressure/resistance
How is the velocity of flow affected by area? How does this affect the different vessels?
The larger the area, the slower the velocity. Therefore, since capillaries (arterioles, venules) have the largest area, they have the slowest flow whereas the aorta, with its small area will have the fastest flow.
Explain the different factors involved in Poiselles law?
Q = (πr^4ΔP) / ((8 η L) How are they used to calculate resistance?
Q = volume flow rate
r = radius
L = length of tube
ΔP = change in pressure over the length of the tube
η = visscosity of blood
Since Q = ΔP/R, R = (8ηL)/(pi*r^4)
explain how viscosity relates to laminar flow/velocity of flow, etc. What is Reynold's number? What will increase it?
When the velocity of blood flow increases, the RBCs move the center of the vessel, and apparent viscosity decreases. Reynolds number is a number that shows the likelihood of turbulent flow which will lead to murmurs. Under 2000-->laminar flow. Over 2000, might have turbulence. Over 3000, likely turbulence.
Density of blood will increase it. Increasing diameter will increase it. Incr. velocity will increase it. Decreasing viscosity will increase it.
What are two common clinical examples that cause turbulence? explain.
Anemia - decreased hematocrit results in a decreased blood viscosity, increased Re and
turbulence that is often reflected as functional murmur. In addition, cardiac output
often is also elevated resulting in an increased blood velocity.
Thrombi - narrowing of blood vessel resulting in an increase in velocity at the site of
the thrombus, increasing Re and inducing turbulence.
What differences in resistance occur between series and parallel circuits?
When multiple vessels are in series, it increases the effective resistance. When vessels are in parallel, it decreases the effective resistance.
What are the differents elements of fluid flux in capillaries? describe them.
a. Capillary hydrostatic pressure (Pc): moves fluid outward through the membrane. It is determined by both arterial and venous pressures. Although the value is closer to arterial pressure, it is more affected by venous pressure.\
b. Interstitial fluid pressure (Pif): tends to move fluid inward when Pif ispositive and outward when Pif is negative.
c. Plasma colloid osmotic (oncotic) pressure (πc) which causes osmosis offluid inward though the membrane. Normal plasma has a colloid osmotic pressure of 28 mmHg with 80% of the total colloid osmotic pressure
resulting from the albumin fraction (4.5 g/dl; πc=21.8 mmHg), 20% from the globulin fraction (2.5 g/dl; πc = 6.0 mmHg) and almost none from the fibrinogen fraction (0.3 g/dl; πc= 0.2 mmHg). Due to fluid transfer out of the capillary, πc may increase along the length of the capillary.
d. Interstitial fluid osmotic (oncotic) pressure (πif) which causes osmosis of fluid outward through the membrane. The concentration of proteins in the interstitial fluid of muscles is 1.5 g/dl; in subcutaneous tissues 2 g/dl; in
intestine 4 g/dl; in liver 6 g/dl. The average protein concentration in the interstitial fluid of the body is about 3 g/dl (40% of that in the plasma) which results in a πif of ~ 8 mmHg.
e. Kf is the filtration coefficient which is determined by the capillary permeability and surface area of a particular capillary. Although it can be markedly altered under pathological conditions, physiologically it rarely changes for a particular capillary bed. Fluid movement for a given pressure difference is largest for capillaries with a high Kf (e.g., glomerular capillaries) and lowest in capillaries with a low Kf (e.g., cerebral capillaries).