Hemodynamics Flashcards
What happens to pressure as we move through circulation?
It decreases. Biggest drop occurs in arterioles.
How do flow, pressure and resistance relate?
Flow = change in hydrostatic pressure / Resistance Q=P/R
How does velocity relate to flow and vessel area?
Flow = velocity X cross sectional area
Poiseuille’s Equation for Vascular Resistance
Blood viscosity increases with…
Increased hematocrit! More force needed to move fluid.
Vessel compliance
Compliance = Volume/Pressure
Low compliance: stiff, difficult to fill
High Compliance: loose, easy to fill (veins)
Compliance is like capacitance, the ability for a vessel to store fluid.
Flow in series
Q1=Q2=Q3
Flow through aorta = flow through all capillaries = flow through all veins
Flow in parallel
Q total = Q1 + Q2 + Q3
Resistance in series
R total = R1 + R2 + R3
If one resistance is much greater than other two, the total resistance is effectively that number.
Resistance in series
1/Rtotal = 1/R1 + 1/R2 + 1/R3
Overall resistance is less than each individual resistance.
Why do capillaries have such low resistance if they have such tiny radii?
Because there are 3 billion of them wired in parallel. The total resistance is less than each individual resistance.
Which vascular section presents the greatest resistance?
Arterioles! Greatest pressure drop equals greatest resistance because flow doesn’t change.
Is blood flow through capillaries slow or fast?
Because v=Q/A, we’d suspect that the flow would be fast, but not so. It’s very slow. Why? Because need to consider total area, or need to divide flow by three billion.
How does the body control blood flow to different organs?
By increasing the resistance in some regions, diverting flow to others. Organs are wired in parallel, so increased resistance at one means reduced resistance at the other!
Why do arterioles have such high resistance?
Not due to the presence of smooth muscles, more due to the fact that they’re not much bigger than capillaries, and there aren’t nearly as many.
MAP
Mean arterial pressure = Cardiac output x Total peripheral resistance.
This is a causal relationship. Changes in cardiac output and total peripheral resistance cause changes in MAP.
MAP relationship to diastolic and systolic BP
MAP = 2/3 DP + 1/3 SP
Because diastole is longer than systole.
Baroreceptors
Located in carotid sinus and aortic arch. When they stretch, they send more impulses to the medulla to regulate BP.
Three phases of the baroreceptor response
Direct response that changes MAP. The reflex response, which attempts to return MAP to where it was initially. A new steady state is achieved.
Direct Response After Hemorrhage
Decreased blood volume leads to reduced ventricular filling. Reduced ventricular filling leads to reduced stroke volume. Reduced stroke volume causes decreased cardiac output, which decreases MAP.
Cardiac Output = ?
CO = HR X Stroke Volume
The Reflex Response after hemorrhage
Decreased MAP due to decreased cardiac output causes reduced baroreceptor firing to medulla. This causes increased sympathetic output to SA node, ventricles, arterioles, veins, and decreased parasympathetic output to SA node.
Increased sympathetic output to SA node increases HR, to ventricles increases contractility,
How does increased sympathetic output and decreased parasympathetic output affect hr on a molecular level?
Increased Symp: Beta adrenergic activation of cAMP which increases If.
Decreased Para: Decreased activation of Ik,Ach by BG subunit of mAChR.
How does increased sympathetic output to ventricles increase contractility on a molecular level?
Increased B adrenergic activation causes PKA activation, which P’s phospholamban to increase amount of Ca in SR and which P’s L type Ca channels to increase amount of Ca in cell.
