Control of Blood flow Flashcards
what does the pulsatile arterial flow arise from?
intermittency of ventricular ejection and the distensibility (elastic tissue) of the large arteries and the resistance (smooth muscle) in the arterioles
why does steady flow in the capillaries arise?
due to the drastic increase in cross- sectional area in the capillary beds
six factors that influence blood flow
velocity pressure / volume viscosity resistance flow pattern hematocrit
For a given volume of blood, velocity of blood flow through a SINGLE blood vessel should increase with a decrease in its diameter
However, arterial vessels usually branch out into smaller vessels thereby increasing the total cross-sectional area
Increases in total X-sec area will decrease the velocity of flow in a single vessel
Flow rate eq’n
velocity x cross-sectional area; therefore At any given section of the vasculature: Vel of blood flow α 1 / aggregated X-sec area
Greater the aggregated cross-sectional area Lower is the velocity of flow
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For a single vessel: flow α velocity
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how does blood flow relate to pressure?
Blood flow is directly proportional to the pressure gradient (pressure drop) across the vessel
flow α pressure drop
ya; Where: Q = ΔP / R and R = resistance
what if there is not pressure drop across a tissue bed?
blood flow would be zero; Therefore, it is important for the heart to contract forcefully to generate a high enough pressure head to facilitate the movement of blood
Blood pressure generated is also directly proportional to the volume of blood found within the vasculature
Aorta, arteries and arterioles combine hold ~ 11% of the total blood volume
Capillaries contains ~ 5% of the blood volume
Both venules and veins combine hold ~ 67% of the total blood volume (capacitance vessels)
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how does viscosity affect BF?
Blood flow is found to be inversely proportional to viscosity; Also as one of the factors affecting the resistance to flow; flow α 1 / viscosity
what is viscosity?
It is the frictional property of the molecules as they slide by one another
how is resistance defined?
Hydraulic resistance is defined as changes in pressure divided by flow rate; R = (Pi – P0) / Q
what is poiseuille’s law
Q = [ π ( Pi – P0 ) r 4 ] / ( 8ηl ); Where: Q = flow rate (Pi – P0) = pressure drop r = radius of the vessel η = viscosity l = length of the vessel
Radius of a vessel is the dominant factor in
Rα η determining the resistance to flow
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arterioles are considered as the primary resistance vessels that regulate arterial blood pressure and flow (also known as stopcocks of the vascular tree)
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Q α r 4
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what receptors are highly expressed in the resistance vessels?
ones that bind catecholamines, angiotensin II (vasoconstriction); and adenosine and nitric oxide (vasodilation)
laminar flow and flow rate
For laminar flow, flow rate is linearly proportional to pressure drop; Q α (Pi – P0)
turbulent flow and flow rate
For turbulent flow: flow rate is proportional to the square root of pressure drop
i.e.) Q α (Pi – P0)1/2–Or need a higher driving force to achieve the same Q as in laminar flow
what is hematocrit?
the ratio of RBC to whole blood
how does hematocrit affect BF?
The viscosity of the blood increases as the hematocrit increase and it can slow the flow rate according to Poiseuille’s law
i.e.) Q α 1 / Hc
