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What are blood vessels?
Dynamic structures that carry blood, beginning and ending at the heart. They work with the lymphatic system to circulate fluids.
What is the role of arteries?
Carry blood away from the heart. They are oxygenated except in the pulmonary circulation.
What do capillaries do?
are in direct contact with tissue cells and directly serve cellular needs.
What is the role of veins?
carry blood toward the heart. They are deoxygenated except in the pulmonary circulation.
What do all blood vessels have in common?
consist of a lumen (central blood-containing space) surrounded by a wall.
What are the three layers (tunics) of blood vessel walls, except for capillaries?
Tunica intima
Tunica media
Tunica externa
What are capillaries made of?
an endothelium with a sparse basal lamina.
What is the tunica intima?
the innermost layer of blood vessels.
What type of cells make up the endothelium in the tunica intima?
simple squamous epithelium.
What is the function of the endothelium in the tunica intima?
reduces friction by providing a slick surface, allowing smooth blood flow
What is the tunica media?
the middle layer of blood vessels, composed mostly of smooth muscle and elastin sheets.
What controls the tunica media?
Sympathetic nerve fibers innervate the tunica media, controlling vasoconstriction and vasodilation.
What is vasoconstriction and vasodilation?
Vasoconstriction: Decreased lumen diameter.
Vasodilation: Increased lumen diameter.
Why is the tunica media important?
It is the bulkiest layer and is responsible for maintaining blood flow and blood pressure
What is the tunica externa?
the outermost layer of the blood vessel wall.
What is the composition of the tunica externa?
composed mostly of loose collagen fibers
What are the functions of the tunica externa?
It protects and reinforces the blood vessel wall and anchors it to surrounding structures.
What additional features are found in the tunica externa?
its infiltrated with nerve fibers and lymphatic vessels
What do large veins contain in the tunica externa?
elastic fibers
How are arteries divided?
divided into three groups based on size and function: Elastic arteries, Muscular arteries, Arterioles
What are elastic arteries?
thick-walled arteries with a large, low-resistance lumen, primarily found near the heart
Where is elastin found in elastic arteries?
found in all three tunics of elastic arteries, but mostly in the tunica media
How much smooth muscle do elastic arteries contain?
contain substantial smooth muscle, but it is mostly inactive in vasoconstriction
What is the function of elastic arteries?
act as pressure reservoirs. They expand and recoil as blood is ejected from the heart, allowing for continuous blood flow downstream, even between heartbeats
What do elastic arteries give rise to?
muscular arteries also known as distributing arteries because they deliver blood to body organs
What is the size range of muscular arteries?
range in diameter from pinky-finger size to pencil-lead size.
What is special about the tunica media in muscular arteries?
have the thickest tunica media with more smooth muscle, but less elastic tissue compared to elastic arteries.
What is the role of muscular arteries in blood flow?
active in vasoconstriction, helping to regulate blood flow.
What are arterioles?
smallest of all arteries
What is the structure of larger arterioles?
contain all three tunics (intima, media, and externa)
What is the structure of smaller arterioles?
mostly made up of a single layer of smooth muscle
What is the function of arterioles?
control blood flow into capillary beds through vasodilation and vasoconstriction of smooth muscle.
Where do arterioles lead to?
capillary beds
What are capillaries?
are microscopic vessels with very small diameters, allowing only a single red blood cell to pass through at a time
What makes up the walls of capillaries?
thin tunica intima. In the smallest vessels, one cell forms the entire circumference
What do capillaries supply?
supply almost every cell in the body, except for cartilage, epithelia, cornea, and the lens of the eye
What are the functions of capillaries?
responsible for the exchange of gases, nutrients, wastes, hormones, and other substances between blood and interstitial fluid
What is a capillary bed?
an interwoven network of capillaries located between arterioles and venules
What are the two types of vessels in a capillary bed?
Vascular shunt, True capillaries
What is a vascular shunt?
a channel that directly connects an arteriole to a venule, also known as a metarteriole-thoroughfare channel
What are true capillaries?
the actual vessels involved in the exchange of gases, nutrients, and wastes between blood and tissues
What is a vascular shunt in a capillary bed?
a direct channel that connects an arteriole to a venule, bypassing the capillary network.
What is the pathway of a vascular shunt?
Terminal arteriole
Metarteriole (intermediate between arteriole and capillary)
Thoroughfare channel (intermediate between capillary and venule)
Postcapillary venule
What is the role of the postcapillary venule in the vascular shunt?
drains the capillary bed after the thoroughfare channel
How many true capillaries are in a capillary bed?
10 to 100 true capillaries per capillary bed
Where do true capillaries branch from?
from the metarteriole or terminal arteriole
What regulates blood flow into true capillaries?
Precapillary sphincters
Can blood flow through true capillaries or a shunt?
Blood may flow into true capillaries or bypass them via the vascular shunt, depending on the regulation
What controls the blood flow into true capillaries?
regulated by local chemical conditions and vasomotor nerves
What is the role of veins?
carry blood toward the heart
How do veins form?
when capillary beds unite in postcapillary venules and merge into larger veins
What forms venules?
when capillaries unite to create postcapillary venules
What are venules made of?
consist of endothelium and a few pericytes
Why are venules porous?
Allow fluids and white blood cells (WBCs) to move into tissues
What do larger venules contain?
have one or two layers of smooth muscle cells
What is the structure of veins?
have all three tunics, but their walls are thinner with larger lumens compared to corresponding arteries
How does the tunica media and tunica externa differ in veins?
In veins, the tunica media is thin, and the tunica externa is thick
Why are veins considered good storage vessels?
have a large lumen and thin walls, making them ideal for storing blood
Why are veins called blood reservoirs?
hey can contain up to 65% of the blood supply
What is the goal of veins?
to return blood to the heart.
What challenge do veins face in returning blood to the heart?
having lower blood pressure than arteries
How do veins overcome low blood pressure?
have adaptations like large-diameter lumens and venous valves to help return blood to the heart
How do large-diameter lumens help veins?
offer little resistance to blood flow, making it easier for blood to move through veins
What do venous valves do?
prevent the backflow of blood, ensuring blood moves toward the heart
Where are venous valves most abundant?
in the veins of the limbs.
What are venous sinuses?
are flattened veins with extremely thin walls, composed only of endothelium
Where are venous sinuses found?
The coronary sinus of the heart and the dural sinuses of the brain
What are varicose veins?
dilated and painful veins caused by incompetent (leaky) valves
What factors contribute to varicose veins?
heredity and conditions that hinder venous return, such as prolonged standing, obesity, or pregnancy
How do varicose veins form?
Blood pools in the lower limbs, weakening the valves and causing veins to dilate and become painful.
How common are varicose veins?
Varicose veins affect more than 15% of adults.
What are vascular anastomoses?
interconnections of blood vessels that provide alternate pathways for blood flow.
What is the function of arterial anastomoses?
provide collateral channels, ensuring continuous blood flow even if one artery is blocked
Where are arterial anastomoses common?
in joints, abdominal organs, brain, and heart
Where are arterial anastomoses absent?
retina, kidneys, or spleen
What are arteriovenous anastomoses?
shunts that bypass capillaries, such as the metarteriole–thoroughfare channel
What are venous anastomoses?
connections between veins that are so abundant that occluded veins rarely block blood flow
Blood flow
volume of blood flowing through a vessel, organ, or entire circulation in a given period
How is blood flow measured?
measured in milliliters per minute (ml/min) and is equivalent to cardiac output (CO) for the entire vascular system
How constant is blood flow?
relatively constant when at rest but varies at the individual organ level based on needs.
What is blood pressure (BP)?
The force per unit area exerted on the wall of a blood vessel by blood.
How is blood pressure expressed?
expressed in millimeters of mercury (mm Hg)
Where is blood pressure typically measured?
as systemic arterial BP in large arteries near the heart
What drives blood flow in the circulatory system?
The pressure gradient, which provides the driving force that keeps blood moving from higher- to lower-pressure areas
What is resistance (peripheral resistance)?
The opposition to blood flow, which is a measure of the amount of friction blood encounters with vessel walls
Where is resistance typically measured?
in the peripheral (systemic) circulation
What are the three important sources of resistance?
Blood viscosity
Total blood vessel length
Blood vessel diameter
What is blood viscosity?
refers to the thickness or "stickiness" of blood, which is influenced by formed elements and plasma proteins
How does blood viscosity affect resistance?
Increased blood viscosity leads to increased resistance
How does total blood vessel length affect resistance?
The longer the blood vessel, the greater the resistance encountered
Which factor has the greatest influence on resistance?
Blood vessel diameter
How do frequent changes in diameter affect resistance?
alter peripheral resistance
How do viscosity and blood vessel length compare to diameter in affecting resistance?
Viscosity and blood vessel length are relatively constant, while blood vessel diameter frequently changes and has a larger impact on resistance
What is laminar flow?
the pattern where fluid moves more slowly near the walls of the vessel and faster in the middle of the tube.
How does blood vessel diameter affect resistance?
Resistance varies inversely with vessel radius. If the radius increases, resistance decreases, and vice versa
What are the major determinants of peripheral resistance?
Small-diameter arterioles
How do abrupt changes in vessel diameter affect resistance?
dramatically increase resistance
What is an example of an obstacle that can increase resistance?
Fatty plaques from atherosclerosis
What is the relationship between cholesterol and atherosclerosis?
Atherosclerosis is caused by cholesterol buildup in the arteries, leading to plaque formation.
What are the two types of cholesterol?
LDL (low-density lipoprotein) and HDL (high-density lipoprotein)
