15 – Blood Flow Regulation

Question 1

When will flow occur?

Answer 1

-when there is a pressure gradient
*needs to be maintained
-hard to measure flow so we usually measure pressure

Question 2

What is the equation for blood flow rate in an artery (Q)?

Answer 2

Q=delta P / R
*same formula as MAP=CO x SVR
>CO =MAP/SVR

Question 3

Flow regulation systemic vs. local;

Answer 3

-systemic: determined by blood pressure (BRAIN)
-local: each organ governs their own flow
>regulate P and R

Question 4

Resistance in an artery equation:

Answer 4

-length: constant
-viscosity of blood: constant
-pi: constant
*radius: dynamic

Question 5

What is the flow rate of an artery (Q) proportional to?

Answer 5

-MAP x r^4
*if constant MAP: = r^4
*radius is what can govern blood flow

Question 6

Flow and radius at constant pressure gradient:

Answer 6

-flow is inversely proportional to radius!
>smaller radius=greater resistance=less flow
>larger radius=lower resistance=more flow

Question 7

What regulates vascular tone?

Answer 7

-extrinsic factors
-intrinsic factors
*balance between them

Question 8

Extrinsic factors:

Answer 8

-from outside of the blood vessel and surrounding tissue
-in general, they regulate BP via modulation of SVR

Question 9

What are some examples of extrinsic factors?

Answer 9

-NE
-E
-Ang II
-BNP

Question 10

NE (extrinsic factor) and alpha-1 receptors:

Answer 10

-GPCR
-activate IP3 pathway=increase Ca2+ =vasoconstriction

Question 11

NE and alpha-1 receptors: arterial side:

Answer 11

-increase tone
-increase vascular resistance
-increase MAP

Question 12

NE and alpha-1 receptors: venous side:

Answer 12

-increase tone
-increase venous pressure
-increase venous return
>increase preload
>increase contractility
>increase CO
>increase MAP

Question 13

E (extrinsic factor) and beta-2 receptors:

Answer 13

-GPCR
-activate cAMP pathway=inhibits MLCK=dilation
*if acts with NE=increase CO=increase MAP
*high concentrations=shift to alpha-1 receptors

Question 14

E and beta-2 receptors: arterial side

Answer 14

-decrease tone
-decrease vascular resistance
-decrease MAP

Question 15

Angiotensin II (extrinsic factor):

Answer 15

-acts on AT receptors in arteries and kidneys
>constriction and Na/water retention
-act on brain centres to release aldosterone and ADH=increase blood volume

Question 16

AT receptors active:

Answer 16

-IP3 pathway=increases intracellular Ca=vasoconstriction

Question 17

AT receptors: arterial side

Answer 17

-increase tone
-increase vascular resistance
-increase MAP

Question 18

AT receptors: blood volume

Answer 18

-increase venous return
-increase preload
-increase contractility
-increase CO
-increase MAP

Question 19

Intrinsic factors:

Answer 19

-from within the blood vessel and surrounding tissue
-important for LOCAL regulation of blood flow

Question 20

What are some examples of intrinsic factors?

Answer 20

-metabolites (ex. hypoxia, CO2)
-local hormones
-mechanical
-endothelial factors

Question 21

Endothelial factors:

Answer 21

-endothelial cells can communicate with each other via paracrine signalling and gap junctions

Question 22

Autoregulation examples:

Answer 22

-mechanical/myogenic tone
-mechanical/flow-induced responses

Question 23

Transmural pressure:

Answer 23

-pressure across the vessel

Question 24

Mechanical/myogenic tone (intrinsic): autoregulation

Answer 24

-stretch detectors in arterial wall respond to changes in transmural pressure
>increase pressure (stretch): constrict=decrease diameter
>decrease pressure (stretch): dilate=increase diameter

Question 25

What is the importance of mechanical/myogenic tone?

Answer 25

-buffers against fluctuations in blood pressure
-helps maintain a constant blood flow (Q)

Question 26

What happens to cerebral blood flow if BP drops below ~50mmHg?

Answer 26

-autoregulation can no longer maintain perfusion
>impaired dilation
>artery collapse
>ischemia
*decreased cerebral blood flow

Question 27

What happens to cerebral blood flow if BP rise above ~150mmHg?

Answer 27

-autoregulation can no longer regulate perfusion
>force-mediate dilation
>increased flow
>vasogenic edema

Question 28

What is shear stress?

Answer 28

-frictional force of blood acting on the vessel wall

Question 29

Mechanical/flow-induced responses (intrinsic): autoregulation

Answer 29

-glycocalyx (shear) detectors in luminal aspect of artery respond to changes in shear stress

Question 30

What is the importance of glycocalyx (shear) detectors in the arteries?

Answer 30

-helps maintain tone
>increase shear stress causes dilation and helps enhance blood flow delivery in most organs
>increase shear stress causes constriction and helps enhance myogenic reactivity in the brain

Question 31

What can cause an increase in shear stress to lead to dilation?

Answer 31

*largely NO
-prostaglandins
-EDHF (endothelial hyperpolarizing factor)