Hemodynamics (L15) Flashcards
Organization of the circulatory system: Flow of blood starting with Left ventricle
Blood goes from left ventricle, to major arteries, to arterioles, to capillaries (this was the high pressure system)
then, after capillaries (where gas exchange occurs), it goes into venules, to veins, to major veins (low pressure system; does not drive flow). This is eventually returned to vena cava
After vena cava, blood is dumped into right atrium, then into RV, which puts it in the pulmonary circulation. Here, blood is re-oxygenated and then returned via the pulmonary veins into the left atrium. Note that the pulmonary circuit does not oxygenate the lungs, this is done through the bronchial circuit of the systemic circulation.
What are the 3 major pressure types in a vessel when considering the fluid itself?
Hydrostatic
Driving
Transmural
Hydrostatic pressure
Pressure generated by gravity (think of the column of blood, like in a leg when you’re standing). Gravity affects pressure differentially, so your legs are at higher pressure in certain situations than others.
Driving pressure
This is pressure driven from the flow. This is the pressure gradient and is determined by transmural pressure at location 1 minus transmural pressure at location 2.
Transmural pressure
Consider the cross-sectional area of a vessel. Transmural pressure is the pressure pushed by the liquid onto the vessel wall. This is what we think of as blood pressure.
Relationship between blood velocity, blood flow, and cross-sectional area.
v=Q/A
where v=velocity of blood (as in a single blood cell), Q=flow (as in the unit of blood, flowing in volumes/second), and A=Cross sectional area of the vessel
What is bernoulli’s principle?
Given that you cannot destroy or create energy in a flow system,
we can consider blood flow to be kinetic energy and blood pressure to be potential energy.
Therefore,
you’re basically trading out energy as blood moves from one tube to another. This also means that if you have an area of stenosis, the cross-sectional area has decreased. To compensate, kinetic flow increases (because velocity is inversely proportional to CSA; v=Q/A). This has to be converted from potential energy, so this potential energy decreases, meaning pressure drops.
This also means that after the drop the blood can flow back into an area that isn’t stenotic anymore and pressure will rise. This doesn’t mean that the blood goes bckward however because the total energy in the downstream segment is still reduced from upstream areas
Is blood a newtonian, or non newtonian fluid, and why?
Blood is a non-newtonian fluid. Newtonian fluids are supposed to be homogenous, whereas blood is a heterogenous mix
Pouiseuille’s law
Flow is inversely proportional to viscosity and length, but directly proportional to the radius of the tube and the pressure gradient
The contribution of the radius is exponential
This is analagous to ohm’s law (Q (I) = delta P (V) / R (still R) )
How did poisseuille describe resistance?
Resistance is directly proportional to viscosity and length of tube but inversely proportional to the radius (whose contribution is exponential)
How is it possible that the both cardiac systems (pulmonary and systemic) have the same cardiac output, despite the pulmonary side being a shorter circuit?
The pulmonary system has a much lower resistance. This means for the same flow, you have different pressures.
Resistance and flow of a circuit in series
Total resistance equals sum of individual resistances
Flow should remain constant throughout; we’re in series and it has nowhere else to go.
The pressure however should be additive (therefore the pressure drop in aorta to the RV (over whole system) is the summation of the pressure drop at each point in the series, summated).
Resistance, flow, and pressure in a parallel vascular circuit
The total flow is going to equal that of the branches summed. The pressure remains constant.
However, resistance is now, in total, less than individual resistances.
What happens if an area of a vessel becomes stenotic? What are the two possible outcomes?
This is a decrease in radius, and the vessel has two options:
- ) Redirect flow. Vasoconstrict the region of poor radius and redirect blood down another branch.
- ) Redirection is not possible (this is a possibility in vessels in the brain, for instance). Blood MUST go across. In this case, you increase pressure upstream and decrease pressure downstream. This increases the driving pressure to help flow.
How can a stenosis get worse
A stenosis is a narrowing of a certain region, which according to bernoullie’s principle means ther ewill be a decrease in pressure in this region
Axial streaming
This is the tendency of objects to want to move in the fastest part of the liquid, thus RBC’s flow in the parabolic pattern that the fluid itself moves. this leaves a layer of border zone on the edge of the vessel that is cell-free. While the absolute size of this area remains constant, as the radius of the vessels become smaller, this means the relative size becomes larger. This thins out the blood in that area and means it is less viscous
Plasma skimming
As vessels branch, the branching part is going to be closer to the border zone which is cell free, which means more cell-free stuff gets sucked inside than cell-inclusive stuff
