Unit 2 - Blood Flow & Blood Pressure PART A & B

Question 1

Overview of Unit 1 (2 key points)

Answer 1

1. As seen in Unit 1, blood flow is dependent on pressure gradients, the properties of the vessels through which blood is flowing (vessel radius & length) and the viscosity of the blood.
2. Blood flow to specific organs can therefore be modulated by changing the pressure gradient, the blood vessel radius, blood vessel length, or blood viscosity. Since the vessel length and blood viscosity are relatively constant under normal conditions, changes in vessel radius and blood pressure are typically used to change blood flow.

Question 2

Describe the key points of Figure 15.1 - the functional model of the CV system showing the heart & blood vessels as a single closed loop

Answer 2

• the elastic systemic arteries are a pressure reservoir that maintains blood flow during ventricular relaxation
• the arterioles, shown with adjustable screws that alter their diameter, are the site of variable resistance
• exchange b/t the blood & cells takes place at the capillaries
• systemic veins serve as an expandable volume reservoir
• each side of the heart functions as an independent pump

Question 3

Basic vessel structure

Answer 3

depending on the vessel, they will have some or all of the following components:

a. Tunica intima
b. Elastica interna
c. Tunica media
d. Elastica externa
e. Tunica externa

Question 4

Walls of blood vessels:

Answer 4

• layers of smooth muscle, elastic CT, & fibrous CT

- inner lining is a thin layer of ENDOTHELIUM

Question 5

Tunica intima

Answer 5

(innermost)

i. Single layer of endothelial cells.
ii. Continuous with endothelial lining (endocardium) of heart.
iii. These cells can secrete paracrines that regulate blood flow.

Question 6

Elastica interna

Answer 6

Made of elastic protein fibers (elastin) that are capable of stretch and recoil.

Question 7

Tunica media

Answer 7

Smooth muscle - contraction and relaxation changes vessel radius.

Contraction = vasoconstriction (narrows); relaxation = vasodilation (widens it).

Question 8

Elastica externa

Answer 8

layer of elastin

Question 9

Tunica externa

Answer 9

outer layer of connective tissue

Question 10

In most blood vessels, smooth muscle cells maintain a state of partial contraction at all times, creating the condition _____

Answer 10

muscle tone

Question 11

Basic vessel structure from most inner to most outer order:

Answer 11

1. Tunica intima
2. Elastic interna
3. Tunica media
4. Elastica externa
5. Tunica externa

Question 12

Look at figure 15.2

Answer 12

done

Question 13

Arteries (describe)

Answer 13

a. Thick walls with lots of elastic tissue.
b. Large elastic arteries like aorta have very large diameters (radii) that offer little resistance to blood flow (F = ∆P/R)
c. Are a pressure reservoir
d. Elasticity of arterial walls
e. These are low compliance (aka low capacitance) vessels.

Question 14

Arteries are a pressure reservoir that helps to maintain:

Answer 14

i. the driving force of of blood flow during ventricular relaxation (diastole).
ii. the constant flow of blood toward capillaries.

Question 15

What does elasticity of arterial walls allow?

Answer 15

allows them to stretch out (EXPAND) to accommodate blood coming from the ventricle during ejection, but also acts to maintain pressure on that blood due to the thickness of the wall and the PASSIVE ELASTIC RECOIL that occurs, pushing excess blood downstream.

Question 16

What does it mean for arteries to be low compliance?

Answer 16

these are LOW COMPLIANCE (aka low capacitance) vessels. I.e. the ability of the vessel to stretch and increase volume with increasing transmural pressure is low (∆ volume / ∆ transmural pressure = low). Like trying to blow up a tire vs a balloon.

Question 17

B/c of the stiffness of the fibrous tissue (in arteries)…

Answer 17

substantial amounts of energy are req. to stretch the walls of an artery outward, but that energy can be stored by the stretched elastic fibers & released through elastic recoil

Question 18

What are arteries & arterioles characterized by?

Answer 18

a divergent pattern of blood flow
- as major arteries divide into smaller & smaller arteries, the character of the wall changes, becoming less elastic & more muscular

Question 19

Describe what happens/steps during ventricular contraction

Answer 19

contraction of the ventricles pushes blood into the elastic arteries, causing them to stretch

1. Ventricle contracts
2. Semilunar valve opens. Blood ejected from ventricles flows into the arteries
3. Aorta & arteries expand & store pressure in elastic walls

Question 20

Describe what happens/steps during ventricular relaxation

Answer 20

elastic recoil in the arteries maintains driving pressure during ventricular diastole

1. Isovolumic ventricular relaxation
2. Semilunar valve shuts, preventing flow back into ventricle
3. Elastic recoil of arteries sends blood forward into rest of circulatory system

Question 21

Arterioles

Answer 21

THICK walls with little elastic tissue and lots of smooth muscle that is innervated by sympathetic nerves (except penis and clitoris which have innervation from PSNS).

Question 22

What is something to note about arterioles?

Answer 22

Note: amount of smooth muscle and innervation progressively decrease in the arterioles as they get closer to the capillaries.

Question 23

What are the arterioles function?

Answer 23

i. Determine the relative blood flow to individual organs/tissues through vasoconstriction and vasodilation.
ii. Help maintain mean arterial pressure (MAP).
iii. Eliminate the pulse pressure prior to it reaching the capillaries (eliminates pulsatile changes in flow/pressure in capillaries).

Question 24

What is the arteriole diameter regulated by?

Answer 24

by local factors, such as tissue O2 concentrations, & by the ANS & hormones

Question 25

Metarterioles

Answer 25

only partially surrounded by smooth muscle
- take 1 of 2 paths:

1. If PRECAPILLARY SPHINCTERS (muscle rings) are relaxed, blood flowing into a metarteriole is directed into adjoining capillary beds
2. If the precapillary sphincters are all constricted, metarteriole blood bypass the capillaries & goes directly to the venous circulation

Question 26

Some arteries branch into vessels known as ______

Answer 26

metarterioles

Question 27

What is part of the microcirculation?

Answer 27

arterioles, capillaries, & venules

Question 28

Capillaries

Answer 28

Smallest, thinnest, most numerous blood vessels in body.

• 5-10 μm in diameter, thus huge surface area (~6000 m2) for exchange
• Contain 5% of circulating blood at rest.

Question 29

What are capillaries composed of?

Answer 29

Composed of a single layer of endothelial cells surrounded by thin glycoprotein/collagen matrix (basement membrane).

Question 30

What are the capillaries function?

Answer 30

Function = Exchange (b/t blood & interstitial fluid)
- Highly permeable to small lipid soluble substances (oxygen, carbon dioxide, fatty acids, steroid hormones, anaesthetics, etc).

Question 31

What facilitates exchange of materials in capillaries?

Answer 31

their walls consist of a flat layer of endothelium, 1 cell thick, supported on an acellular matrix called the BASAL LAMINA

Question 32

What are the 2 classes of capillaries, explain. (figure 15.16)

Answer 32

Two classes: (allow differing levels of exchange)

i. CONTINUOUS CAPILLARIES
- Cells connected by leaky junctions (small intercellular spaces)

ii. FENESTRATED CAPILLARIES
- Large intercellular spaces (junctions) between cells and large channels (pores) passing through cells.
- Found in kidney (glomerulus), intestines, liver, bone marrow (in liver and bone marrow = discontinuous capillaries).

Question 33

Blood flows from the capillaries into _____. Which are?

Answer 33

VENULES (small vessels)

• similar to capillaries, with a thin exchange epithelium & a little CT
• different than capillaries by their convergent pattern of flow

Question 34

Many capillaries are closely associated with ______. What are they?

Answer 34

pericytes - highly branched contractile cells surrounding the capillaries forming a meshlike outer layer b/t the capillary endothelium & the interstitial fluid

Question 35

What do pericytes secrete?

Answer 35

secrete factors that influence capillary growth, & some of them can differentiate to become new endothelium or smooth muscle cells

Question 36

The more the pericytes (contribute to “tightness”), the___ ____ the capillary beds

Answer 36

LESS LEAKY

Question 37

Veins

Answer 37

Thin-walled (THINNER THAN ARTERIES), valved (ENSURE THAT BLOOD PASSING THE VALVE CANNOT FLOW BACKWARD), high compliance vessels (expand easily with
increasing pressure).

• from venules, blood flows into veins that become larger in diameter as they travel toward the heart
• more numerous than arteries & have a larger diameter, therefore, hold more than 1/2 the blood in the CS, making them the VOLUME RESERVOIR of the CS

Question 38

What do veins contain?

Answer 38

Contain LESS smooth muscle and elastin, but MORE collagen (connective tissue) than arteries.

Question 39

What makes veins expand easily when filled with blood?

Answer 39

• thin-walled

- elastin

Question 40

What are the functions of veins? Explain

Answer 40

i. Low resistance (high compliance) vessels for blood flow
- Mean driving pressure for blood flow from peripheral veins to right atrium is only 10-15 mmHg. Combined with the large cross-sectional area, this leads to a low velocity of flow and blood can end up pooling/collecting in the veins, which leads to veins acting as a…….

i. Volume reservoir
- Hold a much greater volume of blood than arteries, even though pressure is much lower
- This reservoir can rapidly be drawn on when required (constriction of veins prevents pooling/collection of blood inside of them and increases venous return; dilation of veins allows blood to pool/collect inside of them and decreases return).