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Chapter 19

ANP 1105 Chapter Notes - Chapter 19: Tunica Intima, Tunica Externa, Blood Vessel

Anatomy and Physiology
Course Code
ANP 1105
Jacqueline Carnegie

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Chapter 19 The Cardiovascular System: Blood Vessels
Blood Vessel Structure and Function
- Arteries carry blood away from the heart
o Are said to branch, diverge, or fork as they form smaller and smaller divisions
- Veins carry blood toward the heart
o Are said to join, merge and converge into successively larger vessels approaching
the heart
- Capillaries are the only blood vessels which come into direct contact with tissue cells and
serve cellular needs
Structures of Blood Vessel Walls
**View figure on pg. 700
- Walls of all blood vessels, except the smallest, have three distinct layers (tunics)
- Walls surround the central blood-containing space called the lumen
- Tunica intima (innermost tunic): single layer of endothelial cells, (same as the structure
of the heart and capillaries), some smooth and elastic muscle is present in its structure
o It has the most intimate contact with blood
- Tunica media: has smooth muscle (gives the muscle its ability to regulate diameter)
- Tunica adventita (externa): wrapping around the outside of the vessel so that it is
anchored in place and protected
o Composed largely of loosely woven collagen fibers that protect and reinforce the
Types of Arterial Vessels
1. Elastic (conducting) arteries:
- Thick-walled, large-diameter located near the heart
- Highest amount of elastin smooth muscle out pressure fluctuations
- Relatively inactive in vasoconstriction
- They are pressure reservoirs, expanding and recoiling as the heart ejects blood
- Allowing blood to flow continuously in stead of back and forth as the heart beats
- 2.5cm to 1cm in diameter
2. Muscular (distributing arteries:
- Deliver blood to specific organs
- Contain more smooth muscle than elastin = are more active in vasoconstriction and less
capable of stretching
- 0.3mm to 1cm in diameter
3. Arterioles:
- Larger arterioles have all 3 tunics, but their tunica media is primarily smooth muscle
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- A single layer in smallest arterioles determine which capillary beds flushed minute-to-
- Changing diameter changes resistance to blood flow, meaning arterioles are resistance
- 10 um to 0.3mm in diameter
Types of Capillaries
- They all only possess a thin tunica intima
- Length about 1 mm; with a lumen diameter of 8-10 um
- Their function = exchange
- Have tight junctions to join endothelial cells
o They are usually incomplete and leave gaps of unjoined membrane (called
intercellular clefs)
1. Continuous capillaries
a. Skin and muscle
b. Uninterrupted lining intercellular clefs allow limited passage of fluids
2. Fenestrated capillaries
a. Similar but endothelial cells riddled with pores
b. Increased permeability to fluids/small solutes
c. Occur in areas of active filtration or absorption (small intestine, endocrine organs,
3. Sinusoidal capillaries
a. Highly modified, leaky capillaries (liver, bone marrow, lymphoid tissues,
endocrine organs)
b. Large irregular lumens and usually fenestrated
c. Fewer tight junctions and large intercellular clefts for passage of proteins, RBCs
Capillary Beds
- Capillaries do not function independently; form interweaving networks called capillary
- Microcirculation: flow of blood from an arteriole to a venule through a capillary bed
- Normally consist of two types of vessels:
o Vascular shut: (metarteriole-thoroughfare channel)
o True capillaries: surrounds root of each true capillary
Structure and Functions of Venules & Veins
- 8-100 um in diameter
- Smallest venules: postcapillary venules
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o Consist entirely of endothelium around which pericytes congregate
- Extremely porous
- Venules join to form veins
- Normally have 3 distinct tunics, but their walls are always thinner and their lumens larger
than those of corresponding arteries
- Relacitvely little elastin/smooth muscle in tunica media; tunica adventitia is heaviest
- Up to 65% of blood in veins at any one time = capacitance vessels or blood reservoirs
- Structural adaptations of veins that promote blood return:
o Large-diameter lumens, which offer relatively little resistance to blood flow
o Valves: keeping the blood from backing up on itself
Define: Blood flow, blood pressure, resistance, peripheral resistance
Blood Flow: the volume of blood going past a certain point per unit time
- Considering the entire vascular system, blood flow is equivalent to cardiac output, and
under resting conditions is relatively constant
- Blood flow through individual body organs may vary widely according to their
immediate needs
Blood pressure: force per unit area exerted on the wall of a blood vessel by its contained blood
(mm Hg)
- Refers to systemic arterial blood pressure in the largest arteries near the heart
- Flow occurs in response to a pressure gradient
Resistance: the major determinant of blood flow because a change in blood vessel radius
increases resistance to the 4th power
- Measures total of frictional forces that impede flow; flow and resistance are inversely
Arterioles: can regulate their diameters
Three important sources of resistance:
- Blood viscosity: due to formed elements; plasma proteins
- Total blood vessel length: more length = the more resistance
- Blood vessel diameter: can be regulated; fluid not touching walls move faster
Velocity of Blood Flow
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