Physiology Flashcards
∆P = F x R
Ohm’s law of hydrodynamics
P = pressure
F = Flow
R = Resistance
R1 + R2 + R3 +…
1/R1 + 1/R2 + 1/R3 +….
Resistance in series is additive
Resistance in parallel
The difference in pressure between two points along the axis of a vessel
Driving pressure
The difference in pressure between a point just inside the vessel and a point just outside the vessel
Also
The difference between the intravascular pressure and the tissue pressure. This is what governs vessel diameter.
transmural pressure
Radial pressure difference
pressure in a vertical column
hydrostatic pressure
cardiac output per square meter of BSA
cardiac index
at rest = 3.0 L/min/m2
∆V/∆T
Flow
Implications of Poiseuille’s law
- flow is directly proportional to the axial pressure difference
- flow is directly proportional to r4
- Flow is inversely proportional to the length of the vessel and the viscosity of the fluid
Note: Poiseulle only applies to rigid tubes
○ F= ∆P(πr4)/8ƞƖ
Assumptions for application of Poiseuille’s law to blood flow
- fluid must not be compressible
- tube must be straight, rigid, cylindrical, unbranched and with a constant radius
- velocity of the thin layer at the wall = 0
- flow must be laminar
- flow must be steady
- viscosity of the fluid must be constant
○ Re=(2r⊽ρ)/ƞ
Reynolds number describes turbulence
> 3000 = mostly turbulent
<2000 = mostly laminar
turbulence occurs when radius is large or the velocity is high (high CO) or the viscosity is low (anemia)
Describe mean pressure in the head and in the foot
The mean arterial pressure is 95 mm Hg. For a large vein in the head the mean pressure is lower (subtract 37), and for a vein in the foot the pressure is higher (add 95 mm Hg).
Note: although the arterial and venous pressures in the different location of the body are different, the driving pressure (aka pressure moving volume of blood from arteries to veins) is the same in both supine and standing positions.
As fluid flows along a horizontal tube with a narrow central region which has half the diameter of the two ends, the pressure in the central region is lower than pressure at the distal end of the tube.
Bernoulli effect
For the flow to occur from low pressure to high, the velocity in the center must be 4-fold higher. The fluid in the central region flows along an energy gradient (kinetic energy).
the lateral pressure in an artery in reference to atmospheric pressure
blood pressure
For catheters, blood pressure measurement is accurate when the opening of the transducer is perpendicular to the flow of blood
measure of the left atrial pressure
pulmonary wedge pressure
Equation for determining CO using the Fick principle (restatement of the law of conservation of mass)
Qp = VO2/(Cpv[O2] - Cpa[O2])
VO2 = amount of oxygen taken up by the lungs in ml/min
Cpv[O2] = pulmonary vein O2
Cpa[O2] = pulmonary artery O2
What is the reason the behavior of blood is non-Newtonian?
interaction of RBCs with fibrinogen
plasma is Newtonian, and the absence of fibrinogen eliminates shear stress
Relatively low Hct has increased viscosity due to stickiness of RBCs, whereas with high Hct viscosity increases due to cell deformation.
Do capillaries have a lower Hct than arterial Hct?
Yes. Branch vessels preferentially skim plasma from the main stream of the parent vessel.
Does blood have a high apparent viscosity at low flow?
yes.
The shear rate is also low when flow rate is low. This makes blood behave in a non-newtonian manner.
In modest flow, RBCs move toward the center of the stream and lower viscosity
Flow relationship of arteries and capillaries
According to the principle of continuity (mass action again), the total volume flow of blood must be the same at any level of arborization. The mean linear velocity of flow is a mirror image of the profile of the total cross-sectional area. Because the smaller vessels (capillaries and venules) have a smaller diameter but collectively have a much larger cross-sectional area than the parent vessel, in order for flow to be conserved, the flow in each smal vessel must be significantly lower than the parent. Note: postcapillary venules have the largest total cross-sectional area.
Application of Poiseulle to radius of a vessel
The resistance on an individual unbranched vessel is inversely proportional to r4. Therefore, vessel with small radius has high resistance. BUT the number of vessels in parallel determines the overall resistance to flow. Since the diameters of arterioles are larger than capillaries, it would appear their resistance is lower. There are more capillaries in parallel than arterioles, so the resistance of the capillaries is lower. The arterioles are where the steepest pressure change occurs and these are the vessels of highest resistance.
How are smooth muscle cells arranged in elastic arteries vs muscular arteries?
In elastic arteries, the VSMCs are arranged in spirals with varying pitch. In muscular arteries, they are in concentric rings or helices. Vascular elastic tension is determined by the elastin and collagen fibers. VSMCs exert tension by contraction.
Relationship of artery and veins to transmural pressure and volume
Arteries have a low volume capacity but can withstand high transmural pressure. Veins have high volume capacity but can withstand only small transmural pressure differences.
This is the relationship of compliance.
C = (change in volume)/(change in pressure)
Laplace’s law
The transmural pressure is the distending force that increases the circumference of a vessel. Force opposing this distension is tension. The equilibrium between transmural pressure and tension depends on the radius.
T = (change in P) x r
The wall tension increases as the radius increases. High wall tension is required to withstand high pressure. In order to withstand higher tension, the vessel must contain a higher amount of elastic tissue. Aorta has a larger radius, higher wall tension, withstands high pressure and contains higher amount of elastic tissue.
Elderly have decreased elasticity of vessels due to increased collagen and crosslinking of collagen fibers.
Locations of highest capillary density
myocardium
lungs
The myocardium capillary density is 7x that of skeletal muscle
[O2]a - [O2]v/ [O2]a
Extraction ratio
the amount of O2 that the tissues remove is an expression of the A-V difference normalized to the arterial content of a substance.
The extraction ratio decreases with increased flow but increases with increased metabolic demand (greater tissue extraction).
Only 20% of capillaries are perfused at rest, but precapillary sphincters dilate in exercise.
the amount of solute that crosses a particular SA of a capillary per unit time
flux
The Px (permeability coefficient) describes the ease with which the solute crosses the capillary by diffusion.
Relationship between hydrostatic pressure and colloid osmotic pressure with movement of fluid across capillary wall
When the difference between the intravascular pressure and the extravascular pressure (hydrostatic pressure) is positive, fluid moves from the vascular space to the tissue. If the osmotic pressure exerted by the plasma proteins is greater than the extravascular pressure exerted by proteins and proteoglycans, fluid moves into the capillary lumen from the tissue space.
The ratio of colloid osmotic difference observed over the colloid osmotic difference in theory
reflection coefficient
if it is 0, water moves the solute
if it is 1 the barrier completely excludes the solute
Why is interstitial fluid pressure slightly negative in most tissues?
fluid removal by the lymphatics
lymphatics return 2-4 L of fluid and 100-200 g of proteins to the circulation per day
How does endothelin cause vasoconstriction?
It binds to ETa receptor and increases intracellular calcium, promoting vasoconstriction
Calcium influx in cardiac muscle
unlike skeletal muscle, cardiac muscle requires extracellular calcium to enter L-type calcium channels in the T tubule membrane. This influx of calcium triggers release of calcium from the SR and intiates muscle contraction
How do catecholamines exert positive inotropic effect?
make SR calcium channels more sensitive to cytoplasmic calcium
Stimulate SERCA, increasing calcium stores for later release