Control of Blood flow

Question 1

what does the pulsatile arterial flow arise from?

Answer 1

intermittency of ventricular ejection and the distensibility (elastic tissue) of the large arteries and the resistance (smooth muscle) in the arterioles

Question 2

why does steady flow in the capillaries arise?

Answer 2

due to the drastic increase in cross- sectional area in the capillary beds

Question 3

six factors that influence blood flow

Answer 3

velocity
pressure / volume   
viscosity
resistance
flow pattern
hematocrit

Question 4

For a given volume of blood, velocity of blood flow through a SINGLE blood vessel should increase with a decrease in its diameter

Answer 4

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

Question 5

Flow rate eq’n

Answer 5

velocity x cross-sectional area; therefore At any given section of the vasculature: Vel of blood flow α 1 / aggregated X-sec area

Question 6

Greater the aggregated cross-sectional area Lower is the velocity of flow

Answer 6

ya

Question 7

For a single vessel: flow α velocity

Answer 7

ya

Question 8

how does blood flow relate to pressure?

Answer 8

Blood flow is directly proportional to the pressure gradient (pressure drop) across the vessel

Question 9

flow α pressure drop

Answer 9

ya; Where: Q = ΔP / R and R = resistance

Question 10

what if there is not pressure drop across a tissue bed?

Answer 10

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

Question 11

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)

Answer 11

ya

Question 12

how does viscosity affect BF?

Answer 12

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

Question 13

what is viscosity?

Answer 13

It is the frictional property of the molecules as they slide by one another

Question 14

how is resistance defined?

Answer 14

Hydraulic resistance is defined as changes in pressure divided by flow rate; R = (Pi – P0) / Q

Question 15

what is poiseuille’s law

Answer 15

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

Question 16

Radius of a vessel is the dominant factor in

Rα η determining the resistance to flow

Answer 16

ya

Question 17

arterioles are considered as the primary resistance vessels that regulate arterial blood pressure and flow (also known as stopcocks of the vascular tree)

Answer 17

ya

Question 18

Q α r 4

Answer 18

ya

Question 19

what receptors are highly expressed in the resistance vessels?

Answer 19

ones that bind catecholamines, angiotensin II (vasoconstriction); and adenosine and nitric oxide (vasodilation)

Question 20

laminar flow and flow rate

Answer 20

For laminar flow, flow rate is linearly proportional to pressure drop; Q α (Pi – P0)

Question 21

turbulent flow and flow rate

Answer 21

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

Question 22

what is hematocrit?

Answer 22

the ratio of RBC to whole blood

Question 23

how does hematocrit affect BF?

Answer 23

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