Hemodynamics Flashcards
What is the function of the cardiovascular system?
defense, temperature regulation, transport hormones and other messengers between various regions of the body, and delivers nutrients and removes wastes from each organ.
What is the function of all 12 body systems?
to maintain the homeostatic mechanism (maintain internal body environment).
What is the principle mechanism for circulatory system exchange?
diffusion
What drives diffusion?
concentration gradient
Is diffusion fast or slow?
fast but only over a short distance (microns= 1x10^-6)
What is the medium for exchange of diffusion?
liquid tissue, which consists of plasma, blood cells, and platelets.
What is the diameter of a capillary?
10 microns
At what level of the cardiovascular system do these exchange processes occur?
capillaries
What is hemodynamics?
the study of the interrelationships among the various forces and parameters that are required to move this liquid tissue through the cardiovascular system.
How does force manifest itself in a liquid system?
via pressure
What are the 2 types of pressure in the cardiovascular system?
- hydrostatic pressure= pressure of a liquid at rest (pressure increases with depth). Aka caused by effects of gravity on the fluid.
- hydrodynamic pressure= the cardiac pump adds energy to the blood, which we measure as pressure. This energy moves blood through blood vessels and is dissipated as a result of movement of fluid through the vessels (rubbing against the walls of the vessels and the viscosity of the blood itself).
What is the pressure equation?
P= hpg, where h= height, p= density (g/cm^3), g= gravitational acceleration (980 cm/s^2).
What exactly is pressure?
the force applied perpendicular to the surface of an object per unit area over which that force is distributed (Pressure= force/area).
How do we measure pressure?
on a column of fluid. Height of column (P2-P1). If the column is mercury, pressure is mm Hg. If the column is water, pressure is cm H2O.
Why do we use mercury to measure BP?
because it is more dense than water and therefore doesn’t require a long tube.
Who is credited with taking the first blood pressure?
Stephen Hales
How do pressures change from lying to when you stand up?
lying= about 100 mmHg
standing= 51 mmHg in carotids and 183 mmHg in feet
*there are baroreceptors that function to offset this however, and ensure equal pressures.
What is the equation for flow (Q)?
Q= (change in P)/R
Q= flow (ml/min)
**change in P= pressure gradient **(dynes/cm^2)
R= resistance to flow (dynes/cm^2)/(ml/min)
What law is the equation for flow similar to?
Ohm’s law: I=V/R
I= current (flow of electrons) V= voltage R= resistance
What is the pressure gradient?
the DIFFERENCE in pressure between the beginning and end of a vessel. Thus, flow is directly proportional to the difference in pressure, and inversely proportional to resistance.
What resistance factors influence fluid flow through vessels?
- fluid viscosity
- vessel length (preset so this doesn’t change)
- vessel diameter
What is viscosity?
the friction developed between the molecules of a fluid as they slide over each other during flow of the fluid. In general, the thicker the liquid, the more viscous.
How did newton define viscosity?
as a lack of slipperiness between adjacent laminae. It is a measure of the fluidity of a liquid.
Is the velocity greatest at the center or on the peripheries?
at the center.
What does each layer of fluid laminae do?
retards the movement of the adjacent layer, generating a velocity gradient.
What is a primary determinant of blood viscosity?
hematocrit (% RBCs)
What is the normal range for hematocrit?
34-45.
Where do cyclists who blood dope, want their hematocrit?
around 51 to increase the O2 carrying capacity. You do not want to go over this however, because the higher you increase the hematocrit, the more viscous the blood and the harder the heart has to work to pump it.
What happens to flow if you double length?
flow decreases by half. Thus, flow is inversely proportional to length.
How does vessel diameter affect flow?
as diameter increases, flow (volume per time; not velocity) increases (because there is more fluid able to flow through the center, with less surface area contact around the walls)
Is vessel diameter proportional to resistance?
NO. It goes up by increments. So flow (Q) is proportional to radius^4 or resistance (R) is proportional to 1/(r^4)
What factor has the largest effect on flow?
resistance via vessel diameter.
What is Poiseuille’s Law?
R= (8Lviscosity)/(3.14*r^4)
Thus resistance is proportional to the length and viscosity of a vessel, and inversely proportional to the radius^4).
So incorporating resistance into flow gives you Poiseuille’s Law:
F (Flow) or Q= (3.14change in Pr^4)/(8Lviscosity)
Thus flow is directly proportional to the change in pressure and radius^4, and inversely proportional to length and viscosity
An angiogram reveals an atherosclerotic coronary artery that is 50% occluded. Blood flow through this artery is approximatelY?
about 10% of maximum
(0.50.50.5*0.5)= 0.0625 or 6.25%
How much blood do we have in our bodies?
5-6 L
What would happen if we dilated every vessel in the body?
you would drop over. Thus the job of the cardiovascular system is to direct blood to areas that need it more (i.e. more to skeletal muscles during exercise and less to the GI tract).
What is the difference between flow and velocity?
flow is a VOLUME per time (ml/sec) and velocity is a DISTANCE per time (cm/sec)
How are flow and velocity related?
Flow (ml/sec)= v (cm/sec) * A (cm^2)
A=cross sectional area (pi*r^2)
**What happens to velocity as diameter (cross sectional area) decreases?
velocity increases. However, this increase in velocity is offset by the many branches that occur as you go through the cardiovascular system. This means that even though individual vessels are decreasing in diameter, because there as so many when added together, the overall cross sectional area is actually increasing, thus decreasing overall velocity (this is why blood flow through capillaries is slowest).
What happens as you add resistance to the vascular system in series vs. in parallel?
in SERIES= the total of individual resistances sums together, thus INCREASING total resistance (R total= R1 + R2 + R3…)
in PARALLEL= the total resistant will DECREASE with each additional resistor added in parallel (1/R total= 1/R1 + 1/R2 + 1/R3…)
What is a more convenient way to look at resistance in parallel vasculature?
conductance = 1/R. So now parallel resistance equation becomes: C total= C1 + C2 + C3…
Thus, this is why we look at conductance for capillaries. As we add more capillaries, flow increases and resistance decreases. This accounts for the lower total resistance through capillaries despite their smaller caliber and thus individual higher resistance.
**What should happen to total peripheral resistance, blood flow, cardiac output, and heart rate respectively in a patient who donates a kidney?
USMLE question
Think of the cardiovascular system as a circuit with all parallel circuits. Thus, because we are removing a kidney (aka a parallel circuit), total peripheral resistance will INCREASE (because remember the more parallel circuits, the lower the resistance or greater the conductance).
Blood flow will DECREASE because there is now MORE resistance.
Cardiac output will DECREASE because this is the flow in the cardiovascular system.
Heart rate will DECREASE
When does uniform laminar flow become turbulent?
when its rate becomes too great, it encounters a turn, obstruction, or rough surface.
Is turbulent flow more or less efficient than laminar flow?
less efficient because it contains cross currents and eddies. Thus a greater pressure gradient is required to push turbulent blood through a vessel.
Can you hear turbulent flow?
YES
At what velocity does laminar flow become turbulent?
the critical velocity along with other factors included in the reynolds number (NR= v* diameter* blood density/viscosity
When the reynolds number (NR) exceeds 2000, you have turbulent flow.
Is the reynolds number absolute?
NO, because you could have atherosclerosis, which reduces the reynolds’ number (aka you have turbulent flow at a lower reynolds number)
Are thrombi more likely to develop with turbulent or laminar flow?
turbulent
Are branch points in the cardiovascular system prone to more or less damage?
more due to more turbulence at these points.
What happens as we inflate the blood pressure cuff and then release air?
As the cuff pressure falls below artery pressure, blood begins to “spurt” past the cuffed region, resulting in TURBULENCE that produces an audible sound known as Korotkoff sounds.
What is Newtonian fluid?
Viscosity is constant, independent of flow rate, and independent of tube size. BLOOD IS NOT A NEWTONIAN FLUID.
What happens to the viscosity of blood?
as it goes through smaller and smaller vessels, viscosity decreases due to axial streaming of RBCs (aka the spinning of the RBCs and the plasma).
With respect to what is viscosity measured?
water
What is the relative viscosity of blood with a hematocrit of 45%?
3.8
What is the relative viscosity of plasma?
1.5
What accounts for most of the blood’s viscosity?
RBCs
What is blood doping?
remove about 1/2 a liter of blood, spin it down, and then reinfuse plasma immediately. You freeze the hematocrit (RBCs) and then infuse them before a high endurance event.
Is there a price to pay for blood doping (aka higher hematocrits)?
YES. It forces the heart to work harder to drive more viscous blood and can increase risks for heart failure.
What is compliance?
the change in volume per change in transmural pressure.
C=change in V/change in P
So a vessel that is very compliant is easily distensible and a vessel that is not compliant requires a lot of pressure to change its volume.
What is transmural pressure?
The pressure gradient across the vessel wall (aka the difference in pressure between the outside and inside).
Pi-Po
Are extraluminal pressures generally greater or less than intraluminal pressures?
less, except in situations like the coronary arteries where the extraluminal pressure is greater during systole, preventing blood flow to these vessels.
Is flow directly proportional to the difference in pressure?
YES, but it does so disproportionately, because it is not a linear, increase (ex. blood flow at 100 mm Hg arterial pressure is usually four to six times as great as blood flow at 50 mm Hg, not two times). This occurs, because the diameter of vessels will increase a bit (due to compliance) and we know that when diameter increases, flow also increases :)
What happens to the tone of smooth muscle of arteries with sympathetic stimulation?
increases the resistance (stiffens the vessel), thus INCREASING the critical closing pressure (i.e. the internal pressure at which a blood vessel collapses and closes completely). So it is harder to collapse the vessel.
What happens to the tone of smooth muscle of arteries with inhibition of sympathetic activity?
dilates the vessels, DECREASING critical closing pressure (i.e. the internal pressure at which a blood vessel collapses and closes completely). So it is easier to collapse the vessel.
What is the critical closing pressure?
internal pressure at which a blood vessel collapses and closes completely. If blood pressure falls below critical closing pressure, then the vessels collapse.
What is Laplace’s Law?
In a hollow cylinder, the tension in the wall (T) equals the product of the pressure across the wall (P) and the radius (r).
T= P * r
So for a given pressure, the tension increases proportionally to the radius. This is why larger arteries have thicker walls.
How does Laplace’s Law apply to the aorta and capillaries, respectively?
the aorta has a large radius and therefore tremendous tension.
capillaries have a very small radius and therefore very low tension.
*You will see a drop in pressure between these two by about a third but a HUGE drop in tension of about 10,000.
What does Laplace’s law tell you about the amount of material required to hold the wall of capillaries together?
you don’t need a lot, because tension is low :)
How does Laplace’s law apply to dilated cardiomyopathy of the left ventricle?
The left ventricle becomes dilated (increased radius), thus increasing tension of the wall itself because it has to work harder to overcome the pressure due to more area.
How does Leplace’s law apply to aneurysms?
Same as dilated cardiomyopathy; you have an increase in radius, leading to an increase in tension of the wall itself, putting you at a higher risk for dissection :(
