Chapter 20- Blood Vessels Flashcards
Three principal categories of blood vessels
arteries, veins, capillaries
Walls of arteries and veins have three layers
tunica interna, tunica media, tunica externa
Tunica interna
Lines the blood vessel and is exposed to blood
Endothelium
- Simple squamous epithelium
- Selectively permeable barrier
- Secretes chemicals that stimulate dilation or constriction
- Normally repels blood cells and platelets to prevent clotting
- When tissue around vessel is inflamed, endothelial cells produce
cell-adhesion molecules - Causes leukocytes to adhere to surface and congregate in tissues
where their defensive actions are needed
Tunica media
- Middle layer
- Consists of smooth muscle, collagen, and elastic tissue
- Strengthens vessels and prevents blood pressure from
rupturing them - Contraction of muscle controls blood vessel diameter
- This layer is more prominent in arteries compared with
veins in general
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
Conducting (elastic or large) arteries
- Biggest arteries
- Examples: aorta, common carotid, subclavian, pulmonary trunk,
and common iliac arteries - 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, keeps blood flowing
Distributing (muscular or medium) arteries
- Distribute blood to specific organs
- Examples: brachial, femoral, renal, and splenic arteries
- Smooth muscle layers constitute three-fourths of wall thickness
Resistance (small) arteries
- Thicker tunica media in proportion to their lumen than
large arteries and very little tunica externa - Arterioles: smallest of the resistance arteries
- 200 μm diameter; only 1-3 layers of smooth muscle
- Control amount of blood to various organs
Metarterioles (thoroughfare channels)
- Short vessels that link arterioles directly to venules in
some places (for example, in mesenteries) - Provide shortcuts allowing blood to bypass capillary beds
Aneurysm
- weak point in artery or heart wall
- Forms a thin-walled, bulging sac that pulsates
with each heartbeat and may rupture at any
time
Dissecting aneurysm
blood accumulates between tunics of artery and separates them, usually because of degeneration of the tunica media
Most common sites for aneurysm
abdominal aorta, renal arteries, and arterial circle at base of brain
symptoms and cause of aneurysm
- Can cause pain by putting pressure on other
structures - Can rupture causing hemorrhage
- Result from congenital weakness of blood
vessels, trauma, or bacterial infections - Most common cause is atherosclerosis and
hypertension
Carotid sinuses: baroreceptors
In walls of internal carotid artery, monitor blood pressure, allow for baroreflex, innervation is glossopharyngeal n (CN IX
Carotid bodies: chemoreceptors
- Oval bodies near branch of common carotids
- Monitor blood chemistry, CNIX 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 n.
Capillaries
Where gasses, nutrients, wastes, and hormones pass
between the blood and tissue fluid
* The “business end” of the cardiovascular system
* Composed of endothelium and basal lamina
* Absent or scarce in tendons, ligaments, epithelia, cornea, and lens of the eye
* Three capillary types distinguished by permeability
Three capillary types distinguished by permeability
- Continuous capillaries
- Fenestrated capillaries
- Sinusoids
Continuous capillaries
- Occur in most tissues
- Endothelial cells have tight
junctions - Form 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 Capillary
- Organs that require rapid absorption or filtration - Examples: kidneys, small intestine
- Endothelial cells riddled with holes called filtration pores (fenestrations)
- Spanned by very thin glycoprotein layer
- Allow passage of only small molecules
- Proteins and larger particles stay in bloodstream
Sinusoids
- Found in liver, bone marrow,
spleen - Irregular blood-filled spaces with
large fenestrations - Allow proteins (albumin), clotting
factors, and new blood cells to
enter the circulation
Veins
- Thin-walled and flaccid
- Collapse when empty, expand easily
- Greater capacity for blood containment than arteries
- At rest, about 64% of blood is in veins, 15% in arteries
- Have steady blood flow (unlike pulses in arteries)
- Have valves to prevent back flow (except in head and
most of neck region) – formed from tunica interna - Subjected to relatively low blood pressure
Postcapillary venules
- Smallest veins (10 to 20 mm diameter)
- Even more porous than capillaries
- Also exchange fluid with surrounding tissues
- Tunica interna with only a few fibroblasts around it
- No muscle
- Leukocytes (WBCs) leave bloodstream through venule
walls
Varicose Vein
Standing for long periods promotes blood pooling in the lower limbs and stretches the veins, especially
superficial ones
As veins distend more their walls weaken and become
varicose veins with irregular dilations and twisted
pathways
Simplest and most common route for blood
Heart → arteries → arterioles → capillaries → venules → veins (one capillary bed)
Portal system blood flow
- Blood flows through two consecutive capillary networks before
returning to heart - Examples: Between hypothalamus and anterior pituitary; in kidneys; between
intestines to liver
Anastomosis blood flow
- Convergence between two vessels other than capillaries
-Arteriovenous anastomosis (shunt) - Artery flows directly into vein, bypassing capillaries
Venous anastomosis (most common)
Arterial anastomosis (two arteries merge, collateral route of blood supply
Blood pressure
-Force that blood exerts against a vessel wall
-Measured at brachial artery using sphygmomanometer
-A close approximation of pressure at exit of left ventricle
Two pressures are recorded for BP
- 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
- Important measure of driving force on circulation and of stress exerted on small arteries by pressure surges generated by the heart
Mean arterial pressure (MAP)
- Diastolic pressure + (one-third of pulse pressure)
- Average blood pressure that most influences risk level for edema, fainting (syncope), atherosclerosis, kidney failure, and aneurysm
Arteriosclerosis
stiffening of arteries due to deterioration
of elastic tissues of artery walls
Atherosclerosis
build up of lipid deposits that become
plaques (a form of arteriosclerosis)
Hypertension
- High blood pressure
- Chronic resting BP >140/90
- Consequences:
- Can weaken arteries, cause aneurysms, promote atherosclerosis
Hypotension
- Chronic low resting BP
- Caused by blood loss, dehydration, anemia
BP determined by three variables
- Cardiac output
- Blood volume
- Resistance to flow
Peripheral resistance
opposition to flow that blood encounters in
vessels away from the heart
Blood viscosity (thickness)
- RBC count and albumin concentration raise viscosity the most
- Low viscosity with anemia and hypoproteinemia speeds flow
- High viscosity with polycythemia and dehydration slows flow
Resistance depends on three variables:
Blood viscosity (thickness), Vessel length, Vessel radius
Vessel length
farther liquid travels through tube, the more cumulative friction it encounters; pressure and flow decline with distance
Vessel radius
- Most powerful influence on blood flow
- Only significant way of controlling
resistance
Laminar flow
flows in layers, faster in center
* Blood flow (F) proportional to fourth power of
radius (r), 𝐹 α 𝑟4
* Small changes in blood vessel radius can
cause large changes in flow (mL/min)
Local control
Autoregulation; vasoactive chemicals; shear stress; reactive hyperemia; angiogenesis
Three ways of controlling vasomotor activity
local, neural, hormonal
Neural control
- Vasomotor center of medulla oblongata has sympathetic ANS control over
blood vessels - Integrating center for baroreflexes, chemoreflexes and medullary ischemic reflex
Hormones influence blood pressure
- Some through their vasoactive effects
- Some by regulating water balance
Aldosterone – secreted from adrenal cortex
- Promotes Na+ retention by the kidneys
- “Salt-retaining hormone”
- Supports blood pressure
- Water follows sodium osmotically
- Na+ retention promotes water retention
Epinephrine and norepinephrine (adrenal medulla)
Both are vasoconstrictors and raise blood pressure
Angiotensin II
- Raises blood pressure
- Potent vasoconstrictor
Natriuretic peptides – secreted by the heart
Lower blood pressure by: Antagonizing aldosterone; increase Na+ excretion
by the kidney which reduces blood volume
- Promoting vasodilation
Chemicals pass through capillary
wall by three routes
- Endothelial cell cytoplasm
- Intercellular clefts between
endothelial cells - Filtration pores (fenestrations) of
fenestrated capillaries
Mechanisms involved in routes of Fluid Exchange Across the Capillary Wall
Diffusion, transcytosis, filtration,
and reabsorption
Hydrostatic pressure
-Physical force exerted against a surface
by a liquid
-Blood pressure in vessels is
hydrostatic pressure
-Capillaries reabsorb about 85% of the
fluid they filter
-Other 15% is absorbed by lymphatic
system, returned to blood
Lymphedema (one type of edema)
-accumulation of excess fluid in a tissue
-fluid filters into tissue faster than it is absorbed
The flow of blood back to the heart relies on
pressure gradient, gravity, skeletal muscle pump, thoracic pump, cardiac suction
Pressure gradient
Blood pressure is most important force in venous
return
Gravity
Drains blood from head and neck
Skeletal muscle pump
Contracting limb muscles squeeze blood out of
compressed part of vein; valves
Cardiac suction
During contraction of the ventricles, valves are
pulled downward and atrial space expands, slight
suction draws blood into atria from venae cavae
and pulmonary veins
Thoracic (respiratory) pump
- Inhalation expands thoracic cavity
- Thoracic pressure on the inferior vena cava (IVC) decreases
- Abdominal pressure on the IVC increases
- Blood is forced upward toward heart
- Blood flows faster with inhalation
- Central venous pressure
- Direct measurement of BP in the right atrium and vena cava
- Fluctuates during respiration due to thoracic pump
- Inhalation: 2 mm Hg
- Exhalation: 6 mm Hg
Circulatory shock
any state in which cardiac output is insufficient to meet body’s metabolic needs
Cardiogenic shock
Inadequate pumping of heart (MI)
Obstructed venous return shock
tumor or aneurysm compresses a vein
Low venous return (LVR) shock
Cardiac output low because too little blood returns to heart; three types
-Hypovolemic shock, Obstructed venous return shock, Venous pooling (vascular) shock
Hypovolemic shock
most common; Loss of blood volume - trauma, burns,
dehydration
Venous pooling (vascular) shock
Long periods of standing, sitting, or widespread vasodilation
Neurogenic shock
- Loss of vasomotor tone, vasodilation
- Causes range from emotional shock to brainstem injury
Septic shock
Bacterial toxins trigger vasodilation and increased capillary permeability
Anaphylactic shock (severe immediate allergic reaction)
Severe immune reaction to antigen, histamine release, generalized vasodilation, increased capillary permeability
Venous pooling occurs with inactivity
- Venous pressure not enough to force
blood upward - With prolonged standing, CO may be low
enough to cause dizziness - Prevented by tensing leg muscles, activate
skeletal muscle pump - Jet pilots wear pressure suits
Transient ischemic attacks (TIAs)
- Brief episodes of cerebral ischemia
- Caused by spasms of diseased
cerebral arteries, can be early
warning of a stroke
Stroke, or cerebral vascular
accident (CVA)
- Sudden death of brain tissue
caused by ischemia - Atherosclerosis, thrombosis,
ruptured aneurysm - Effects range from unnoticeable to
fatal - Blindness, paralysis, loss of
sensation, loss of speech
common - Recovery depends on surrounding
neurons, collateral circulation
hemorrhagic stroke
when a blood vessel bursts within the brain
ischemic stroke
a blood clot blocks the blood flow in an artery within the brain
Skeletal Muscles At Rest
- Arterioles constrict
- Most capillary beds shut down
- Total flow about 1 L/min
Skeletal muscles during exercise
- Arterioles dilate in response to muscle
metabolites such as lactate, CO2, and
H+ - Blood flow can increase up to 20-fold
- Blood diverted from digestive and
urinary organs
Special Circulatory Routes: Brain
Total blood flow to the brain fluctuates less than that of any other organ (700 mL/min)
* Seconds of deprivation causes loss
of consciousness
* Four to 5 minutes causes
irreversible brain damage
Brain regulates its own blood flow
Cerebral arteries dilate as systemic BP drops, constrict as BP rises
Lungs circulatory route
Low pulmonary blood pressure (25/10 mm Hg)
* Flow slower, more time for gas exchange
* Oncotic pressure overrides blood (hydrostatic) pressure
* Pulmonary capillaries absorb fluid (almost no filtration)
* Prevents fluid accumulation in alveoli
Unique response to hypoxia
* Pulmonary arteries constrict in diseased area
* Redirects flow to better ventilated region
