Chapter 19 Blood Vessles

Arteries and Veisns Have How Many Layers?

3 Layers

Innermost layer of an artery or vein?

tunica interna

middle layer of an artery or vein?

tunica media

outermost layer of an artery or vein?

tunica externa

Tunica interna (tunica intima)

• Lines the blood vessel and is exposed to blood

• Endothelium: simple squamous
  • Acts as a selectively permeable barrier
  • Secretes chemicals that stimulate dilation or constriction of
  the vessel
  • Repels blood cells and platelets that may adhere to it and
  form a clot
  • When tissue around a vessel is inflamed, produces cell-adhesion
  molecules (CAMs), induce leukocytes to adhere to the surface

Basement Membrane

• Basal lamina
  – From epithelial cells
  – Collagen fibers (IV), laminins
  • Reticular lamina
  – Secreted by connective tissue
  cells
  – Reticular fibers (collagen III),
  fibronectin
  • Functions
  - Structural support: provides a framework for cells to attach to and anchor in
  tissues
  - Cell signaling: regulates cell behavior, such as proliferation, differentiation,
  migration
  - Tissue integrity: protect tissues from mechanical stress and define tissue
  boundaries

Tunica media

Middle layer
– Consists of smooth muscle, collagen, and elastic
tissue
– Strengthens vessels and prevents high blood pressure
from rupturing them
– Regulates the diameter of the blood vessel

Tunica externa (tunica adventitia)

Outermost layer
– Consists of loose connective tissue that often merges
with that of neighboring blood vessels, nerves, or
other organs
– Anchors the vessel and provides passage for small
nerves, lymphatic vessels
• Vasa vasorum: small vessels that supply blood to outer
part of the larger vessels

Vasa vasorum

small vessels that supply blood to outer
part of the larger vessels

Arteries

classified by size
– Conducting (elastic or large) arteries
– Distributing (muscular or medium) arteries
– Resistance (small) arteries
• Arterioles
• Metarterioles

Conducting (elastic or large) arteries

Biggest arteries
• Aorta, common carotid, subclavian, pulmonary trunk, and
common iliac arteries
• Have a layer of elastic tissue, internal elastic lamina, at
the border between interna and media
• External elastic lamina at the border between media and
externa
• Expand during systole, recoil during diastole
– Expansion takes pressure off smaller downstream vessels
– Recoil maintains pressure during relaxation and keeps blood
flowing

Distributing (muscular or medium) arteries

Distributes blood to specific organs
• Brachial, femoral, renal, and splenic arteries
• Smooth muscle layers constitute three-fourths of
wall thickness

Resistance (small) arteries

Arterioles: smallest arteries
– Control amount of blood to various organs
• Thicker tunica media in proportion to their lumen than large
arteries and very little tunica externa
– Metarterioles
• In some places, short vessels that link arterioles to capillaries
• Muscle cells form a precapillary sphincter around entrance
to capillary
– Constriction of these sphincters reduces blood flow through
their capillaries
– Diverts blood to other tissues

Arterioles

smallest arteries
– Control amount of blood to various organs

Metarterioles

In some places, short vessels that link arterioles to capillaries

Aneurysm

weak point in artery or heart wall

Arterial Sense Organs

Sensory structures in walls of major vessels
that monitor blood pressure and chemistry

Carotid sinuses

baroreceptors
• In walls of internal carotid artery
• Monitor blood pressure
– Transmit signals through glossopharyngeal nerve
– Allow for baroreflex

Carotid bodies

chemoreceptors
• Oval bodies near branch of common carotids
• Monitor blood chemistry
• Transmit signals through glossopharyngeal nerve to
brainstem respiratory centers
• Adjust respiratory rate to stabilize pH, CO2, and O2

Aortic bodies

chemoreceptors
• One to three bodies in walls of aortic arch
• Same structure and function as carotid bodies, but
innervation is by vagus nerve

Capillaries

exchange vessels: site where gasses,
nutrients, wastes, and hormones pass between the
blood and tissue fluid

Three capillary types

continuous capillaries
• fenestrated capillaries
• sinusoids

Continuous capillaries

occur in most tissues
• Endothelial cells have tight junctions forming a
continuous tube with intercellular clefts
• Allow passage of solutes such as glucose
• Pericytes wrap around the capillaries and contain the
same contractile protein as muscle
– Contract and regulate blood flow

Fenestrated capillaries

kidneys, small
intestine
• Organs that require rapid absorption or filtration
• Endothelial cells riddled with holes called filtration
pores (fenestrations)
– Spanned by very thin glycoprotein layer
– Allow passage of only small molecules

Sinusoids (discontinuous capillaries)

liver, bone marrow, spleen
• Irregular blood-filled spaces with large
fenestrations
• Allow proteins (albumin), clotting factors, and
new blood cells to enter the circulation

Capillary beds

are networks of 10-100 capillaries

Veins

Greater capacity for blood containment than arteries
• Thinner walls, flaccid, less muscular and elastic tissue
• Collapse when empty, expand easily
• Have steady blood flow
• Merge to form larger veins
• Subjected to relatively low blood pressure
– Averages 10 mm Hg with little fluctuation

Postcapillary venules

smallest veins
– Even more porous than capillaries so also exchange
fluid with surrounding tissues
– Tunica interna with a few fibroblasts and no muscle
fibers
– Most leukocytes emigrate from the bloodstream
through venule walls

Muscular venules

up to 1 mm in diameter
– One or 2 layers of smooth muscle in tunica media
– Have a thin tunica externa

Medium veins

up to 10 mm in diameter
– Thin tunica media and thick tunica externa
– Tunica interna forms venous valves
– Varicose veins result in part from the failure of these
valves
– Skeletal muscle pump propels venous blood back
toward the heart

Venous sinuses

Veins with especially thin walls, large lumens, and no
smooth muscle
– Dural venous sinus and coronary sinus of the heart
– Not capable of vasomotor responses

Large veins

diameter larger than 10 mm
– Some smooth muscle in all three tunics
– Thin tunica media with moderate amount of smooth
muscle
– Tunica externa is thickest layer
• Contains longitudinal bundles of smooth muscle
– Venae cavae, pulmonary veins, internal jugular veins,
and renal veins

Varicose Veins

• Blood pools in the lower legs of people who
  stand for long periods stretching the veins

• Hereditary weakness, obesity, and pregnancy
  also promote problems

Hemorrhoids

are varicose veins of the anal
canal

Circulatory Routes

• Simplest and most common route for blood

• Heart → arteries → arterioles → capillaries → venules → veins

• Passes through only one network of capillaries from the time it leaves the heart until the time it returns

Portal system

Blood flows through
two consecutive
capillary networks
before returning to
heart

Anastomosis

convergence point
between two vessels
other than capillaries

Arteriovenous
anastomosis (shunt)

Artery flows directly into
vein, bypassing capillaries

Venous anastomosis

Most common
– One vein empties directly into
another
– Reason vein blockage is less
serious than arterial blockage

Arterial anastomosis

Two arteries merge
– Provides collateral
(alternative) routes of blood
supply to a tissue
– Coronary circulation and
common around joints

Blood flow

the amount of blood flowing through an
organ, tissue, or blood vessel in a given time (mL/min.)

Perfusion

the flow per given volume or mass of tissue in
a given time (mL/min./g)

Blood Pressure,
Resistance, and Flow

Important for delivery of nutrients and oxygen,
and removal of metabolic wastes

Hemodynamics

Physical principles of blood flow based on pressure
and resistance
• The greater the pressure difference between two
points, the greater the flow; the greater the resistance,
the less the flow

Blood pressure (BP)

the force that blood exerts
against a vessel wall

Systolic pressure

peak arterial BP taken during
ventricular contraction (ventricular systole)

Diastolic pressure

minimum arterial BP taken during
ventricular relaxation (diastole) between heart beats

Pulse pressure

difference between systolic and
diastolic pressure

Mean arterial pressure (MAP)

the mean pressure
one would obtain by taking measurements at several
intervals throughout the cardiac cycle

Hypertension

high blood pressure
– Chronic resting BP > 140/90

Hypotension

chronic low resting BP

Peripheral resistance

the opposition to flow that blood
encounters in vessels away from the heart

Resistance hinges on three variables

blood viscosity, vessel
length, and vessel radius

Vasoreflexes

changes in vessel radius

Vasoconstriction

when smooth muscle of tunica media
contracts

Vasodilation

relaxation of the smooth muscle, allowing
blood pressure to expand vessel

Arterioles

are most significant point of control over
peripheral resistance and flow

Capillary exchange

two-way movement of fluid
across capillary walls

Water, oxygen, glucose, amino acids, lipids, minerals,
antibodies, hormones, wastes, carbon dioxide, ammonia

Chemicals pass through the capillary wall by
three routes

Through endothelial cell cytoplasm
– Intercellular clefts between endothelial cells
– Filtration pores (fenestrations) of the fenestrated
capillaries

Diffusion

is the most important form of capillary
exchange

Trancytosis

endothelial cells pick up material on one side of
their membrane by pinocytosis or receptor-mediated
endocytosis, transport vesicles across cell, and discharge
material on other side by exocytosis

Blood hydrostatic pressure

drives fluid out of capillary
• High on arterial end of capillary, low on venous end

Colloid osmotic pressure (COP)

draws fluid into
capillary

Oncotic pressure

net COP (blood COP − tissue COP)

Hydrostatic pressure

Physical force exerted against a surface by a liquid

Capillary filtration

at
arterial end

Capillary reabsorption

at venous end

Venous return

the flow of blood back to the heart;
relies on: pressure gradient, gravity, skeletal muscle
pump, thoracic pump, and cardiac suction

Thoracic (respiratory) pump

Inhalation—thoracic cavity expands and thoracic pressure
decreases, abdominal pressure increases, forcing blood
upward

Cardiac suction

of expanding atrial space

Venous pooling

occurs with inactivity
– Venous pressure not enough to force blood upward

Circulatory shock

any state in which cardiac output is
insufficient to meet the body’s metabolic needs

Cardiogenic shock

inadequate pumping of heart (MI)

Low venous return (LVR)

cardiac output is low because
too little blood is returning to the heart

Hypovolemic shock

most common
– Loss of blood volume: trauma, burns, dehydration

Obstructed venous return shock

Tumor or aneurysm compresses a vein

Venous pooling (vascular) shock

Long periods of standing, sitting, or widespread
vasodilation

Neurogenic shock

loss of vasomotor tone,
vasodilation

Septic shock

Bacterial toxins trigger vasodilation and increased
capillary permeability

Anaphylactic shock

Severe immune reaction to antigen, histamine release,
generalized vasodilation, increased capillary permeability