Physiology 3120 Lecture Notes - Lecture 36: Theca Interna, Tunica Externa, Smooth Muscle Tissue
2 May 2018
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Human Physiology Lecture 36
Blood Vessel Structure
- Blood vessel structure relates to their function
Blood Vessel Structure
The walls of Arteries and Veins contain three layers:
- Outer layer (tunica externa)
o Fibrous connective tissue
▪ Holding things together
- Middle Layer (tunica media)
o Find specialized tissue: circularly arranged smooth muscle & elastic tissue
o Smooth muscle arranged in a ring like fashion
▪ If it contracts, it causes the blood vessel to constrict
▪ If it relaxes, it causes the blood vessel to dilate
▪ CONTRACT = CONSTRICT, RELAX = DILATE
- Inner Layer (tunica interna)
o Endothelial cells
o Thin
- Content & thickness of each of the layers are different depending on what the blood vessel is
- Almost every blood vessel contains all three layers
- Veins also contain valves
o Ensure blood flows in only one direction
o They have valves because the blood pressure in the venous side of circulation is VERY low;
there is the possibility that blood could reverse direction
- Capillaries have one layer of endothelial cells
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- The relative amount of each tissue: elastic, smooth muscle and fibrous connective tissue
- KNOW RELATIVE AMOUNTS & GENERAL IDEA OF RELATIVE SIZE & THICKNESS
- Aorta:
o Large diameter vessel – 25mm
o Thin wall thickness
o Walls contain A LOT of elastic tissue, some smooth muscle, and fibrous connective tissue
holding everything together
- Arteriole:
o VERY thick wall relative to their size
o Wall contains predominantly smooth muscle tissue. Smooth muscle is important for
regulating the diameter of the vessel
- Capillary:
o Smallest vessels – diameter & thin wall (one cell thickness, no outer or inner layer)
o Endothelial cell has no elastic, smooth muscle, or fibrous CT
- Vena cava:
o Larger diameter than aorta
o Wall thickness is thinner than the aorta
o Contains some smooth muscle
o Note: to increase SV, have to increase the filling of the heart, and it can be done by increasing
venous return activate SYN – activates the smooth muscle in the wall, causing the vessels
to constrict and help pump blood back to the heart & increase venous return
▪ Increase EDV to increase SV
- How does the blood vessel structure relate to its function?
Structure relates to function
- Aorta & large arteries – BP is very pulsatile
o Heart contracts, ejects blood out to the vessels
o As the vessel is getting the ejection of blood, they EXPAND
- When the ventricles contract during systole, they eject some amount of blood
o Even during rest, you eject 70mL of blood
o Blood enters the aorta and artery, since they are thin walls with a lot of elastic tissue – they
can expand to accommodate the in rush of blood
▪ VERY COMPLIANT!
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- Compliance impacts pulse pressure
- Vessels can accommodate rush of blood = aorta, arteries
o Structure allows for in rush of blood, expand
o When the heart is relaxing, they may begin to contract & help continue blood along
o Since they can expand, there is VERY LITTLE resistance = small drop in BP
o Large arteries are very good at distributing blood throughout the body because of the small
BP drop= distribution vessels
o Maintain high BP throughout the vessels
- Arterioles:
o Largest drop of BP is there because it is the site of HIGHEST resistance
▪ Resistance is dependent on the size of the blood vessel & arterioles are very small
▪ Thick wall with very little elastic tissue in it – therefore it can not expand as much as
the arteries
▪ Don’t expand to accommodate in rush of blood
o Site of HIGH resistance = resistant vessels
o Have a lot of smooth muscle – allows vessels to relax, dilation or contract, constriction
o Vessels help regulate blood flow because we are altering blood flow with pressure gradient +
radius
▪ Change radius by contracting or relaxing smooth muscle of the walls
- Capillaries:
o Smallest vessels
o Short diffusion distance
o Largest total cross sectional area of ALL blood vessels
o Blood is moving at is SLOWEST = slow blood velocity
o Exchange vessels – site where gas exchange, nutrient exchange, etc.
▪ Low blood velocity, high total SA, short diffusion distance
- Veins
o Where you find most of your volume at rest – 70% of total blood volume is found in the vein
o Larger diameter than their arteriole counterparts – therefore can accommodate more blood
o Thinner walls than arterioles counterparts
o Easy to expand due to the elastic tissue content
o Capacitance/collection vessels – contain majority of the total blood volume
o Contain smooth muscle – can be controlled by ANS
▪ Can control size, constrict
o Have valves to ensure when you do squeeze them, blood can only go in one direction: back to
the heart
o Help regulate cardiac output by changing venous return
▪ Increase venous return increase EDV increase SV increase CO
CV Regulatory Mechanisms
- Purpose: homeostasis (maintain relatively constant conditions of the internal environment)
o 1. To increase blood flow to active tissues
▪ CO is matched to your oxygen demands in your body – as the demand goes up, you
have to increase blood flow/ CO
• If a tissue needs more oxygen, the tissue can regulate its own blood flow
o 2. To increase/decrease heat loss from the body
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Document Summary
Blood vessel structure relates to their function. The walls of arteries and veins contain three layers: Outer layer (tunica externa: fibrous connective tissue, holding things together. Middle layer (tunica media: find specialized tissue: circularly arranged smooth muscle & elastic tissue, smooth muscle arranged in a ring like fashion. If it contracts, it causes the blood vessel to constrict. If it relaxes, it causes the blood vessel to dilate: contract = constrict, relax = dilate. Inner layer (tunica interna: endothelial cells, thin. Content & thickness of each of the layers are different depending on what the blood vessel is. Almost every blood vessel contains all three layers. Veins also contain valves: ensure blood flows in only one direction, they have valves because the blood pressure in the venous side of circulation is very low; there is the possibility that blood could reverse direction. The relative amount of each tissue: elastic, smooth muscle and fibrous connective tissue.
