Unit 2 - Blood Flow & Blood Pressure PART A & B Flashcards
Overview of Unit 1 (2 key points)
- 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.
- 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.
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
- 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
Basic vessel structure
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
Walls of blood vessels:
- layers of smooth muscle, elastic CT, & fibrous CT
- inner lining is a thin layer of ENDOTHELIUM
Tunica intima
(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.
Elastica interna
Made of elastic protein fibers (elastin) that are capable of stretch and recoil.
Tunica media
Smooth muscle - contraction and relaxation changes vessel radius.
Contraction = vasoconstriction (narrows); relaxation = vasodilation (widens it).
Elastica externa
layer of elastin
Tunica externa
outer layer of connective tissue
In most blood vessels, smooth muscle cells maintain a state of partial contraction at all times, creating the condition _____
muscle tone
Basic vessel structure from most inner to most outer order:
- Tunica intima
- Elastic interna
- Tunica media
- Elastica externa
- Tunica externa
Look at figure 15.2
done
Arteries (describe)
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.
Arteries are a pressure reservoir that helps to maintain:
i. the driving force of of blood flow during ventricular relaxation (diastole).
ii. the constant flow of blood toward capillaries.
What does elasticity of arterial walls allow?
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.
What does it mean for arteries to be low compliance?
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.
B/c of the stiffness of the fibrous tissue (in arteries)…
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
What are arteries & arterioles characterized by?
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
Describe what happens/steps during ventricular contraction
contraction of the ventricles pushes blood into the elastic arteries, causing them to stretch
- Ventricle contracts
- Semilunar valve opens. Blood ejected from ventricles flows into the arteries
- Aorta & arteries expand & store pressure in elastic walls
Describe what happens/steps during ventricular relaxation
elastic recoil in the arteries maintains driving pressure during ventricular diastole
- Isovolumic ventricular relaxation
- Semilunar valve shuts, preventing flow back into ventricle
- Elastic recoil of arteries sends blood forward into rest of circulatory system
Arterioles
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).
What is something to note about arterioles?
Note: amount of smooth muscle and innervation progressively decrease in the arterioles as they get closer to the capillaries.
What are the arterioles function?
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).
What is the arteriole diameter regulated by?
by local factors, such as tissue O2 concentrations, & by the ANS & hormones
Metarterioles
only partially surrounded by smooth muscle
- take 1 of 2 paths:
- If PRECAPILLARY SPHINCTERS (muscle rings) are relaxed, blood flowing into a metarteriole is directed into adjoining capillary beds
- If the precapillary sphincters are all constricted, metarteriole blood bypass the capillaries & goes directly to the venous circulation
Some arteries branch into vessels known as ______
metarterioles
What is part of the microcirculation?
arterioles, capillaries, & venules
Capillaries
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.
What are capillaries composed of?
Composed of a single layer of endothelial cells surrounded by thin glycoprotein/collagen matrix (basement membrane).
What are the capillaries function?
Function = Exchange (b/t blood & interstitial fluid)
- Highly permeable to small lipid soluble substances (oxygen, carbon dioxide, fatty acids, steroid hormones, anaesthetics, etc).
What facilitates exchange of materials in capillaries?
their walls consist of a flat layer of endothelium, 1 cell thick, supported on an acellular matrix called the BASAL LAMINA
What are the 2 classes of capillaries, explain. (figure 15.16)
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).
Blood flows from the capillaries into _____. Which are?
VENULES (small vessels)
- similar to capillaries, with a thin exchange epithelium & a little CT
- different than capillaries by their convergent pattern of flow
Many capillaries are closely associated with ______. What are they?
pericytes - highly branched contractile cells surrounding the capillaries forming a meshlike outer layer b/t the capillary endothelium & the interstitial fluid
What do pericytes secrete?
secrete factors that influence capillary growth, & some of them can differentiate to become new endothelium or smooth muscle cells
The more the pericytes (contribute to “tightness”), the___ ____ the capillary beds
LESS LEAKY
Veins
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
What do veins contain?
Contain LESS smooth muscle and elastin, but MORE collagen (connective tissue) than arteries.
What makes veins expand easily when filled with blood?
- thin-walled
- elastin
What are the functions of veins? Explain
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).
