Vasculature

Question 1

What governs vascular resistance?

Answer 1

Vessel length + internal vessel radius: determines surface area in contact with blood

blood viscosity: friction between molecules of a flowing fluid and proprotional to hematocrit

Question 2

What determines hemodynamics?

Answer 2

1) pressure differences between two points in the cardiovascular system

2) vascular resistance impeding blood flow

Question 3

What does poiseuille’s law state about the relationship between flow, resistance and radius?

Answer 3

Resistance and radius are INVERSELY proporrtional

Resistance and flow are INVERSELY proportional

Flow and Radius are DIRECTLY proportional.

Question 4

What are the layers of the artery from lumen and out?

Answer 4

1. Tunica Interna: inner single layer of specialized endothelial cells continuous with endocaridium
2. Elastica Interna: inner layer of stretchable elastin fibres
3. Tunica Media: thick layer of smooth muscle whose contraction and relaxation alters vessel radius
4. Elastica Externa: outer layer of elastin
5. Tunica Externa: sheath of tough collagen surrounding vessel to prevent overstretching.

Question 5

What are arteries?

Answer 5

thick walled elastic vessels that act as rapid transit passageways between heart and organs, and provide a pressure reservoire.

Question 6

What does the arterial pressure reservoire do?

Answer 6

Maintains a driving force for blood flow during diastole and a constant flow of blood to the capillaries.

Question 7

How does the pressure reservoire maintain a constant and driving force for blood flow?

Answer 7

1. Semilunar valve opens and blood is ejected from ventricle in systole.
2. Aorta expands to hold excess volume of ejected blood, which holds potential energy.
3. Semilunar valve closes due to pressure of aorta in diastole.
4. Passive elastic recoil releases potential energy as kinetic energy and returns to original shape, the narrowing of walls pushing excess blood downstream.

Question 8

What are artieries?

Answer 8

thin vessels with little elastic tissue and lots of smooth muscle rechly innervated by sympathetic nerves that act as as resistance vessels of the body.

Question 9

What are the 3 important functions of arteries?

Answer 9

1. determine the relative blood flow to indvidual tissues/organs
2. help regulate mean arteriole pressure
3. Eliminate pulse pressure prior to it reaching the capillaries

Question 10

How do arterioles adjust flow rate to their organs?

Answer 10

1. Radius of arterioles at different parts of the body vasodilate/contstrict independently from one another
2. Precapillary sphincters relax/constrict to open up access to the capillary bed or close off the bed and redirect through a metarteriole.

Question 11

How is vasodilation and vasoconstriction modulated in arterioles?

Answer 11

Smooth muscle is in a state of partial contraction (vascular tone) independent of neural/chemical input, but altered by external signals that decrease or increase smooth muscle’s [Ca++]

Question 12

What is the difference between intrinsic and extrinsic autoregulation of blood vessel caliber?

Answer 12

Intrinsic: Signals are local factors (metabolic, myogenic) that come from and control individual tissues/organs to match metabolic needs

Extrinsic: Signals originate outside the local tissue, typically from the nervous system or endocrine system and control to maintain homeostasis BP

Question 13

What are the three mechanisms of intrinsic autoregulation?

Answer 13

Myogenic control: responds to changes in mean arterial pressure

Local metabolic control: metabolically driven release of paracrine agents

Non metabolic chemical mediators: endothelin-1, histamine, serotonin

Question 14

How does myogenic control of blood vessels work during increase in BP, very low BP, and very high BP?

Answer 14

Increase in blood pressure: stretch of arteriole opens mechanically gated Ca++ channels, inducing constriction via Ca induced Ca release in smooth muscle

Very low blood pressure: lack of blood results in emptier vessels, inducing passive collapse

Very high blood pressure: excess blood results in excess filling of vessels, inducing passive dilation

Question 15

How does metabolic control take part in blood vessel autoregulation?

Answer 15

Rapidly contracting muscle leads to local hypoxia and local increase in CO2 (cellular respiration), H+ (lactic acid fermentation), K+ (rapid consecutive depolarizations) and adenosine (precursor to ATP). This induces NO release from vascular endothelium.

Question 16

What is NO’s role in autoregulation?

Answer 16

[NO] reduces Ca2+ entry into adjacent smooth muscle, inducing localized vasodilation of arterioles and precapillary sphincters.

Question 17

What is the difference between active hyperemia and reactive hyperemia?

Answer 17

Active hyperemia: increase in blood flow to tissues in response to metabolic demands from increase in activity

Reactive hyperemia: increase in blood flow to tissues following period of low blood flow sensed by diminished stretch (induces relaxation) and localized hypoxia/metabolic byproduct accumulation (induces vasodilation)

Question 18

How does endothelin-1 act as a non-metabolic chemical mediator?

Answer 18

Vasoconstrictor released by arteriolar endothelium that opens non-stretch sensitive Ca++ channels, inducing Ca induced Ca release

Question 19

How does histamine act as a non-metabolic chemical mediator?

Answer 19

From Mast cells: vasodilator released during allergic reactions (quick onset)

From Basophils: vasodilator released during injury or infection (2-8 hr onset)

Question 20

How does serotonin act as a non-metabolic chemical mediator?

Answer 20

Released within platelets in the blood upon sensing a wound, which activates release and induces vasoconstriction to lower blood flow at the site of the injury.

Question 21

How do sympathetic neurons control arteriolar radius?

Answer 21

Sympathetic neurons in the medulla oblongata tonically release NE to arteriolar SM a1-adrenergic receptors. Increased firing causes vasoconstriction superimposed on basal tone, and decreased firing leads to vasodilation.

Question 22

How do hormones control arteriolar radius primarily?

Answer 22

Sympathetically induced epinephrine release from adrenal glands bind to SM a1-receptors, reinforcing vasoconstriction.

However heart, liver, and skeletal muscle arteriolar SM membranes predominantly have B2-adrenergic receptors that bind to epinephrine with a higher affinity, resulting in dilation.

Question 23

What are other hormones other than epinephrine involved in extrinsic regulation of blood vessel diameter?

Answer 23

atrial natriuretic peptide, vaspressin, angiotensin II, angiotensin 1-7

Question 24

How is atrial natriuretic peptide (ANP) involved in regulation of blood vessel diameter?

Answer 24

Atria stretch due to increased pressure and volume from preload, which causes specialized cells in atria to synthesize ANP.

It causes vascular SM relaxation (lowers resistance, increases flow) and encourages kidneys to excrete water and Na+ (decreases blood volume)

Question 25

How is vasopressin involved in regulation of blood vessel diameter?

Answer 25

Released by posterior pituitary in response to decrease in blood volume/MAP sensed in atrial receptors.

It acts as a vasoconstrictor (to maintain BP), as well as an antidiuretic promoting water conservation (increases blood volume)

Question 26

How is angiotensin II involved in regulation of blood vessel diameter?

Answer 26

Low perfusion pressure in kindeys triggers release of renin which converts angiotensinogen to angiotensin I which is converted to angiotensin II in the blood.

Angiotensin II acts as a vasoconstrictor and causes the kidneys to retain sodium and water, which increases blood volume and blood pressure.

Question 27

What is a severe hemorrhage’s effect on the body?

Answer 27

• loss of blood causes MAP to decrease
• decreases arteriolar stretch, decreasing flow
• decreased O2 and increased metabolite at capillary beds due to lack of flow

Question 28

What is the concurrent systemic response of the body against a severe hemorrhage?

Answer 28

• Sympathetic stimulation releases norepinephrine causing vasoconstriction of SM in non-essential organs
• Sympathetic stimulation induces epinephrine release from adrenal medulla binding to a1-receptors inducing constriction in non essential areas, as well as b2-receptors inducing dilation in skeletal muscle
• increased plasma vasopressin and angiotensin II (conservation of water and Na+ via kidneys)