Local Regulation of Blood Flow

Question 1

How do changes in metabolic demand affect local blood flow?

Answer 1

Vasodilator intermediates produced by local tissues reduce arteriole smooth muscle tone and induce vasodilation to metabolically active tissue. (called active hyperemia)

Question 2

What metabolites promote vasodilation?

Answer 2

Potassium, adenosine, nitric oxide, prostaglandins, hydrogen peroxide

Question 3

How can a tissue overcome arteriole occlusion?

Answer 3

Vasodilating metabolites can increase locally and force the arteriole to open enough to allow an occlusion to pass (reactive hyperemia)

Question 4

What is autoregulation?

Answer 4

A tissue can increase peripheral resistance in response to an increase in blood pressure (or vice versa) so that there is no change to local blood flow

Question 5

In what two types of tissue is autoregulation especially strong?

Answer 5

Coronary and cerebral

Question 6

What are the metabolic mechanisms of autoregulation?

Answer 6

If arterial pressure increases, local blood flow will increase. This causes local vasodilating metabolites to be washed away. The decrease then permits an increase in smooth muscle tone and vasoconstriction results

Question 7

What is the myogenic mechanism of autoregulation?

Answer 7

Increase in arterial pressure will cause an increase in the stretching of local smooth muscle. This opens mechanical calcium channels which trigger contractions that induce vasoconstriction

Question 8

From what compound is nitric oxide produced?

Answer 8

L-arginine

Question 9

What are two means of triggering an increase in nitric oxide production?

Answer 9

Chemical (serotonin, Ach) or mechanical (blood flow shear forces)

Question 10

What are two functions of nitric oxide?

Answer 10

1) Serves as a tonic vasodilator which modulates resistance on a local level
2) Inhibits platelet activity

Question 11

What are two main local vasodilators?

Answer 11

Nitric oxide and prostacyclin

Question 12

What are three main local vasoconstrictors (and two platlet activators)?

Answer 12

Endothelin (no platelet activity though)
Serotonin
Thromboxane

Question 13

What are the two main neurotransmitters that the sympathetic nervous system uses in its postganglonic fibers?

Answer 13

Norepinephrine

Neuropeptide Y

Question 14

What is the receptor target of sympathetic system in the heart? in the vasculature?

Answer 14

1) beta-adrenergic receptors

2) alpha receptors

Question 15

How is the magnitude and duration of sympathetic activation limited?

Answer 15

Postganglionic nerve fibers have a norepinephrine reuptake mechanism in their nerve terminals; norepinephrine’s presence will inhibit further release. The presence of vasodilator metabolites will also prevent release

Question 16

What are the main effects of sympathetic innervation in the arterioles and venules?

Answer 16

In arterial system: promotes vasoconstriction (increases resistance)
in venous system: reduces compliance

Question 17

In which tissues is sympathetic vasoconstrictive tone active?

Answer 17

Skin, inactive skeletal muscles, viscera, kidneys

Question 18

In which tissues is sympathetic vasoconstrictive tone inactivated?

Answer 18

Brain, heart, and active skeletal muscle

Question 19

What are the effects of the parasympathetic nervous system on blood flow?

Answer 19

Essentially none in the vasculature itself;

In the heart, lowers heart rate, atrial contractility, and A-V conduction

Question 20

What is the effect of hormonal norepinephrine on blood flow?

Answer 20

increases heart rate and contractility (beta-adrenergic)
increases vasoconstriction (alpha receptor)

Question 21

What is the effect of hormonal epinephrine on blood flow?

Answer 21

increases heart rate and contractility (beta-adrenergic)
increases vasoconstriction (alpha receptor–high concentrations)
increases vasodilation in skeletal muscle (beta receptor)

Question 22

What is the effect of hormonal renin on blood flow?

Answer 22

Responds to decreased arterial pressure to promote conversion of angiotensinogen to angiotensin I (which in turn becomes angiotensin II); angiotensin II promotes vasoconstriction

Question 23

What is the effect of hormonal vasopressin on blood flow?

Answer 23

Released from the posterior pituitary in response to low pressure; promotes vasoconstriction and in the kidneys promotes sodium and water reabsorption (ADH)

Question 24

What is the upstream pressure for blood delivery to the heart?

Answer 24

Since most coronary vessels are closed during ventricular contraction, aortic diastolic pressure is the driving force for blood flow

Question 25

How is coronary oxygen consumption calculated?

Answer 25

Coronary O2 consumption = Coronary blood flow * (PO2 of coronary arteries-PO2 of coronary veins)

Question 26

How is coronary work calculated?

Answer 26

Systolic arterial pressure x Heart Rate

Question 27

How are mismatches between oxygen delivered and oxygen required resolved?

Answer 27

Since the heart extracts most of the oxygen delivered to it, it must increase flow to gain more O2; thus it will increase vasodilating metabolites to promote more flow into the coronary vasculature

Question 28

How does ‘coronary steal’ work?

Answer 28

If a coronary vessel is partially occluded, it will not respond normally to local metabolite production, whereas normal vessels will. Normal vessels then will receive additional blood whereas the occluded vessels will actually lose blood due to their higher resistance

Question 29

What are the main local blood flow regulators for the brain?

Answer 29

Potassium, H+, NO, Adenosine, low O2

CO2 is an especially strong signal

Question 30

What type of control mechanisms predominate in exercising skeletal muscle?

Answer 30

Local (i.e. metabolic vasodilators)

Question 31

How does the body compensate for the increased demand to skeletal muscle that is working?

Answer 31

Sympathetics will
1) vasoconstrict inactive muscles and viscera
2) decrease venous compliance and force blood into heart [increase cardiac output]
3) increase heart rate and contractility
4) decrease parasympathetic involvement to heart
Active skeletal muscle also helps pump venous reservoir to heart

Question 32

What is the main role of vascular control in the skin?

Answer 32

Skin has low nutrient needs, so its vascular control is for body temperature control; vasoconstriction promotes retaining heat, vasodilation promotes heat dissipation