20.1 Structure and Function of Blood Vessels

what is the function of arteries in the circulatory system

carry blood AWAY from the heart to rest of body

what happens to arteries as they branch out

into smaller vessels, eventually forming arterioles and then capillaries

what is the function of capillaries

site of nutrient and waste exchange between the blood and the body’s tissue

what are venules and their function

small blood vessels that collect blood from capillaries and merge to form veins which return blood to the heart

what is the function of systemic arteries

deliver oxygen rich blood to the bodys tissue

what is the function of systemic veins

return oxygen depleted blood to the heart

what is the function of pulmonary arteries

carry oxygen depleted blood to the lungs for gas exchange

what is the function of pulmonary veins

return oxygenated blood to the heart

what are arterioles

smallest arteries

what is the function of the pulmonary circuit

moves blood from right side of the heart to the lungs and back to the heart

what is the function of the systemic circuit

moves blood from the left side of the heart to the head and body and returns it to the right side of the heart

what is the lumen of a blood vessel

hollow passageway through which blood flows

how do structures of arteries and veins differ

arteries have thicker walls and smaller lumens than veins due to the higher pressure of blood flowing through them

why do arteries have smaller lumens than veins

to help maintain the pressure of blood moving through the system

what gives arterial lumens a more rounded appearance in cross section

the thicker walls and smaller diameters of arteries give their lumens a more rounded appearance

what happens to blood pressure (bp) as blood flows from arteries to venules and veins

bp diminishes as blood flows from arteries to venules and veins

how do the walls of venules and veins differ from those of arteries

thinner than arteries

how do the lumens of venules and veins differ from those of arteries

lumens of venules and veins are larger in diameter than arteries

what is the purpose of the larger lumens in venules and veins

the larger lumen allow more blood to flow with less vessel resistance

what role do valves play in veins, particularly in the limbs

valves in veins assist the unidirectional flow of blood toward the heart

why are valves in veins critical

blood flow becomes sluggish in the extremities due to lower pressure and the effects of gravity

why do the walls of arteries and veins require a special blood supply

walls of arteries and veins are composed of living cells that require nourishment and produce waste, but the blood flowing through the lumen is not sufficient to provide this exchange

what is the vasa vasorum

network of small blood vessels within the walls of larger arteries and veins that provides nourishment and removes waste from the vessel’s cells

why is the vasa vasorum located in the outer layers of arteries

high pressure within arteries would collapse the vasa vasorum if it were located closer to the lumen, so it is restricted to outer layers

how does the location of the vasa vasorum differ in veins

located closer to the lumen due to the lower pressure within the vessel

why are arterial diseases thought o b more common than venous diseases

restriction of the vasa vasorum to the outer layers of arteries makes it more difficult to nourish the cells and remove waste products, potentially contributing to the higher incidence of arterial diseases

what is the function of the nervi vasorum

are minute nerves within the walls of blood vessels that control the contraction and dilation of smooth muscle

what are the 3 distinct tissue layers of arteries and veins called

tunics

what are the names of the 3 tunics in order from innermost to outermost

tunica intima, tunica media, and tunica externa

what is the general appearance of arteries

thick walls with small lumens and generally appear rounded

what is the general appearance of veins

thin walls with large lumens and generally appear flat

what is the general appearance of the tunica intima in arteries

Endothelium usually appears wavy due to constriction of smooth muscle
& Internal elastic membrane present in larger vessels

what is the general appearance of the tunica intima in veins

Endothelium appears smooth & Internal elastic membrane absent

what is the general appearance of the tunica media in arteries

Normally the thickest layer in arteries, Smooth muscle cells and elastic fibers predominate (the proportions of these vary with distance from the heart), and External elastic membrane present in larger vessels

what is the general appearance of the tunica media in veins

Normally thinner than the tunica externa, Smooth muscle cells and collagenous fibers predominate, Nervi vasorum and vasa vasorum present, & External elastic membrane absent

what is the general appearance of the tunica externa in arteries

Normally thinner than the tunica media in all but the largest arteries, Collagenous and elastic fibers, & Nervi vasorum and vasa vasorum present

what is the general appearance of the tunica externa in veins

Normally the thickest layer in veins, Collagenous and smooth fibers predominate, Some smooth muscle fibers, & Nervi vasorum and vasa vasorum present

what is the tunica intima composed of

epithelial and connective tissue layers

what type of epithelium lines the tunica intima

simple squamous epithelium called the endothelium

what is the function of the endothelium

critical role in regulating capillary exchange, altering blood flow, and releasing local chemicals that affect bp

what are endothelins and what is their role

local chemicals released by the endothelium that can constrict smooth muscle in vessel walls, increasing bp

what is the potential consequence of uncompensated overproduction of endothelin

hypertension and cardiovascular disease

what is hypertension

high bp

why is damage to the endothelial lining a concern

can expose blood to collagenous fibers, leading to clot formation

what structure is next to the endothelium in the tunica intima

basement membrane or basal lamina

what is the function of the basement membrane (basal lamina)

binds to endothelium to the connective tissue, provides strength while maintaining flexibility, and is permeable to allow materials to pass through

what type of tissue is found in the thing outer layer of the tunica intima

areolar connective tissue

what components of the areolar connective tissue provide flexibility and strength

elastic fibers

what is the internal elastic membrane

thick, distinct layer of elastic fibers found in larger arteries at the boundary with the tunica media

what is the internal elastic membrane also called

internal elastic lamina

what is the function of the internal elastic membrane

provide structure while allowing the vessel to stretch

what feature of the internal elastic membrane allows for exchange of materials between tunics

permeated with small openings that allow exchange of materials

is the internal elastic membrane found in veins

not apparent in veins

what is a distinctive feature of many veins, particularly in lower limbs

contain valves formed by sections of thickened endothelium reinforced with connective tissue

how do the appearance of vein and arteries differ under the microscope

lumen and tunica intima of a vein appear smooth

how do the appearance of arteries and veins differ under the microscope

artery appear wavy due to partial constriction of smooth muscle in the tunica media

what is the tunica media

middle layer of the vessel wall consisting of smooth muscle and connective tissue

what is the composition of the tunica media

consists of layers of smooth muscle supported by connective tissue primarily made up of elastic fibers

what is the function of the circular muscles in the tunica media

contraction and relaxation of the circular muscles decrease and increase the diameter of the vessel lumen

what is vasoconstriction

the decrease in blood flow that occurs when smooth muscle in the walls of the tunica media contracts, making the lumen narrower and increasing bp

what is vasodilation

the increase in blood that occurs when smooth muscle in the walls of the tunica media relaxes, allowing the lumen to widen and blood pressure to drop

what regulates vasoconstriction and vasodilation

small vascular nerves (nervi vasorum), hormones, and local chemicals

what is the role of nervi vasorum

small vascular nerves that run within the walls of blood vessels and regulate vasoconstriction and vasodilation

how do parasympathetic stimulation and sympathetic fibers affect blood vessels

parasympathetic stimulation triggers vasodilation, and sympathetic fibers can trigger both vasodilation and vasoconstriction depending on the neurotransmitter and receptors

in short what is vasoconstriction

narrowing of lumen

in short what is vasodilation

widening of lumen

what supports the smooth muscle layers of the tunica media

collagenous fibers

what is the function of the collagenous fibers in the tunica media

support smooth muscle layers and bind the tunica media to the inner and outer tunics

what type of fibers are found in large numbers in tunica media

elastic fibers are found in large numbers

what is the external elastic membrane

structure that separates the tunica media from the outer tunica externa in larger arteries

where is the external elastic membrane typically found

larger arteries

what is the tunica externa also caleld

tunica adventitia

what is the tunica externa

outer tunic of blood vessel composed of primarily connective tissue

what is the composition of the tunica externa

collagenous fibers, with some elastic fibers, and in VEINS, groups of smooth muscle fibers

what is the function of the tunica externa

helps hold the vessel in relative position and prevents disruption of blood flow

how does the thickness of the tunica externa compare to the tunica media in veins and larger arteries

tunica external is normally the thickest tunic in veins and may be thicker than the tunica media in some larger arteries

how does the tunica external interact with surrounding tissue

outer layers of the tunica externa blend with the surround connective tissue outside the vessel

what is a characteristic of all arteries

relative thick walls that can withstand the high pressure of blood ejected from the heart

what is an elastic artery

type of artery with thick walls containing a high percentage of elastic fibers allowing it to expand and recoil with blood flow

what is the function of elastic fibers in elastic arteries

allow the artery to expand with surge of blood and recoil after it passes, maintain blood flow and pressure

why is the elastic recoil of the vascular wall important

helps maintain the pressure gradient that drives blood through the arterial system

what is another name for an elastic artery

conducting artery

why is conducting artery another name for elastic artery

its large diameter enables it to accept and conduct a large volume of blood from the heart to smaller branches

what is a muscular artery

type of artery with a high proportion of smooth muscle in its tunica media and a decreased percentage of elastic fibers

how does the composition of a muscular artery differ from an elastic

muscular arteries have more smooth muscle in the tunica media and fewer elastic fibers compared to elastic arteries

what is the typical diameter range of muscular arteries

0.1-10 mm

what is the role of muscular arteries in vasoconstriction

play leading role due to their thick tunica media

why is elasticity less important in muscular arteries

bp has decreased by the time it reaches these vessels

what is another name for a muscular artery

distributing artery

why is distributing artery another name for muscular artery

it branches to distribute blood to the vast network of arterioles

what is an arteriole

very small artery that leads to a capillary

what is the structure of an arteriole

same 3 tunics as larger vessels, but each is diminished in thickness

what is the role of arterioles in blood flow

critical in slowing down or resisting blood flow, causing a substantial drop in bp

why are arterioles referred to as resistance vessels

they resist blood flow and cause a drop in bp

what determines the diameter of the lumen of an arteriole

neural and chemical controls

what is the importance of arterioles in regulating bp

primary site of both resistance and regulation of bp, and vasoconstriction and vasodilation in arterioles are the primary mechanisms for distribution of blood flow

what is a capillary

channel that supplies blood to tissues

what is perfusion

process by which capillaries supply blood to tissues

what occurs in capillaries

exchange of gases and other substances between the blood in capillaries and surrounding cells and tissue fluid

what is microcirculation

refers to the flow of blood through capillaries