MA5 - Blood and Lymphatic Vessels Flashcards
Describe the organization of blood vessels.
blood vessels comprise closed circulatory system
Describe the difference between the left and right sides of the heart in the context of circulation.
right side = pulmonary circulation between heart and lungs
left side = systemic circulation between heart and peripheral tissue
What is the function of lympathic vessels?
return excess tissue fluid from tissues to blood
Describe how the pressure and rate of blood flow changes while progressing from arteries to capillaries.
both pressure and rate of flow drop
Differentiate between elastic arteries and other arterial structures in terms of primary composition.
elastic arteries = elastic substance predominates
muscular arteries/small arteries/arterioles = smooth muscle predominates
Which structure regulates the amount of blood entering capillaries?
arterioles
Where is total blood vessel area highest?
capillary beds
Where is vessel permeability highest?
capillary beds
Through which structure do white blood cells migrate to reach tissues?
postcapillary venules
What are the three layers of arterial/venous walls?
tunica intima
tunica media
tunica adventitia
[from innermost to outermost]
Define tunica intima. (2)
innermost layer
includes endothelial lining
Define tunica media. (2)
middle layer
rich in smooth muscle (vascular smooth muscle)
Define tunica adventitia. (2)
outermost layer
connective tissue layer that provides support
What are the components of the tunica intima? (4)
endothelium
basal lamina of endothelium
subendothelial connective tissue [in larger arteries]
internal elastic lamina
Describe the endothelium of the tunica intima.
single layer of squamous epithelial cells
Describe the composition of the subendothelial connective tissue in the tunica intima of larger arteries. (7)
non-contractile smooth muscle cells
macrophages
lymphocytes
mast cells
type I/III collagen
elastic fibers
ground substance
What is the internal elastic lamina?
component of tunica intima that is a fenestrated lamina (layer) of elastin
Where is the internal elastic lamina visible?
visible in arteries and larger veins
The internal elastic lamina is synthesized by what cells?
synthesized by smooth muscle cells and endothelial cells of tunica intima and tunica media
Generally speaking, what is the function of the tunica media?
resists pressure within vessel
What are the components of the tunica media?
circular smooth muscle
fenestrated elastic laminae [in elastic arteries]
external elastic lamina
Describe the circular smooth muscle found in the tunica media.
circumferentially organized in extracellular matrix of fibers and ground substance
What produces the fibers and ground substance found in the tunica media?
vascular smooth muscle cells
What is the most prominent feature of the arterial wall?
tunica media
[note: tunica media in veins is much smaller, so it is potentially useful to distinguish between arteries and veins]
What is the external elastic lamina?
one or more fenestrated elastic laminae near the border of tunica media/tunica adventitia
Most vascular smooth muscle in arteries is found in what layer?
tunica media
What are the components of the tunica adventitia? (7)
mostly type I collagen
some elastic fibers organized longitudinally
fibroblasts
macrophages
smooth muscle [in large veins only — organized longitudinally]
vasa vasorum
nervi vascularis
What is the function of the tunica adventitia?
provides external mechanical support
What is the most prominent feature of the venous wall?
tunica adventitia
[note: this is in contrast to the arterial wall, whose most prominent layer is the tunica media]
What synthesizes the elastic fibers and collagen found in the tunica adventitia?
fibroblasts
What is the function of macrophages in the tunica adventitia? (2)
maintain tissue homeostasis
serve as immune sentinels
(T/F) Smooth muscle is present in all venous structures.
False. Smooth muscle is only present in large veins, and if it is present, is organized longitudinally.
What is the vasa vasorum?
plexus of blood vessels that supplies the outer aspects of larger blood vessels (both large veins and arteries)
In which layer of the arterial/venous wall is the vasa vasorum found?
usually most abundant in tunica adventitia near the border with the tunica media, but can penetrate into the tunica media
What is the nervi vascularis?
plexus of autonomic nerves at border of tunica media and tunica adventitia that regulates contraction of smooth muscle of the tunica media
Where is the nervi vascularis found?
border of media/adventitia only in arteries and arterioles
[tip: nervi vascularis is for contraction, and only arteries and arterioles contract]
What kinds of junctions are present in the endothelial cells of blood vessels?
zonula adherens
zonula occludens
Describe how endothelial cells can regulate blood pressure and vascular tone.
endothelial cells produce paracrine factors that regulate smooth muscle contraction (and therefore regulate blood pressure/vascular tone) — e.g. nitric oxide
Describe the role of endothelial cells in leukocyte extravasation.
local inflammation increases expression of adhesion
receptors for leukocytes on luminal surface of endothelial cells
Describe the role of endothelial cells in regulating blood coagulation.
endothelial cells release factors that modulate platelet function + clot formation/dissolution
(T/F) Endothelial cells of blood vessels are capable of division.
True. They are capable of proliferation.
Endothelial cells are the result of differentiation from
vascular stem cells
What stimulates stem cells for the endothelium to proliferate and differentiate?
vascular endothelial growth factor (VEGF)
What is vascular endothelial growth factor (VEGF)?
growth factor released by hypoxic tissues (wound healing, recovery from ischemia) to promote angiogenesis
What are pericytes?
support cells for capillaries and postcapillary venules
List the features of pericytes. (2)
have contractile functions required for blood flow
required for angiogenesis
How do pericytes change in response to wound repair?
pericytes differentiate into myofibroblasts during wound repair
Where are pericytes found?
bound to/encapsulated by basal lamina of endothelium of tunica intima
What structure helps prevent/reduce spontaneous ruptures in capillaries and venules?
pericytes
Define vasculogenesis.
de novo generation of vessels through migration/proliferation of angioblasts, the endothelial cell precursors
How does vasculogenesis differ from angiogenesis?
vasculogenesis does not require connection to pre-existing vasculature to generate new vessels (build first, then connect)
What are the two types of angiogenesis?
sprouting angiogenesis
splitting angiogenesis
Describe the steps of sprouting angiogenesis. (6)
low O2 tension → VGEF production → endothelial cell proliferation → closed-end tubes → closed-end tubes fuse → new vessel
What is sprouting angiogenesis?
new vessels sprout from existing vessels
What is splitting angiogenesis?
splitting of existing vessels
Describe the steps of splitting angiogenesis.
endothelial cells on opposite sides of capillary form contact → existing junctions become leaky → bone marrow-derived myofibroblasts enter → myofibroblasts + pericytes separate connection into two vessels
Differentiate between vasculogenesis and angiogenesis in the context of embryonic development.
vasculogenesis occurs frequently during embryonic development
angiogenesis is a post-embryonic process
Order the arterial structures in terms of diameter, from largest to smallest.
elastic arteries > muscular arteries > small arteries > arterioles
What is the first structure to receive blood from the heart?
elastic arteries
Describe the features of elastic arteries.
thick tunica intima
thick tunica media with elastic laminae interspersed among smooth muscle
thick adventitia w/ vasa vasorum
Describe the features of muscular arteries. (4)
thin intima
well-defined internal elastic lamina
many layers of smooth muscle cells
adventitia w/ collagen fibers
Describe the features of small arteries. (2)
branches of muscular arteries
up to 10 layers of smooth muscle in tunica media
How do you differentiate between arterioles, capillaries, venules, muscular venules, and small veins in section?
arterioles = lumen 1-2 RBCs wide + media w/ 1-2 layers of smooth muscle
capillaries = lumen 1 RBC wide but no media
venules = lumen 2-4 RBCs wide but no media
muscular venules = lumen >4 RBCs wide + media w/ 1-2 layers of smooth muscle but no tunica adventitia
small veins = lumen >4 RBCs wide + definitve media and adventitia
What are metarterioles?
branches of arterioles (and metarterioles themselves branch into capillaries)
Describe the role of metarterioles in blood flow from the arteriole to the capillary.
precapillary sphincter (rings of smooth muscle) located where capillary branches from metarteriole
if sphincter open → blood flows through
if sphincter closed → blood flows through thoroughfare channel connecting metarteriole to venule (bypass capillary)
Blood pressure is a combination of what two factors?
cardiac output
total peripheral resistance
Peripheral resistance is regulated by (3)
local factors (nitric oxide)
systemic factors (epinephrine)
autonomic innervation (nervi vascularis)
Through which structure do hormones of endocrine glands enter the circulation?
capillaries
Describe the composition of capillaries.
layer of endothelial cells
basal lamina
pericytes
What are the three types of capillaries?
continuous
fenestrated
discontinuous (a.k.a. sinusoid)
Of the three types of capillaries, which is most common?
continuous capillaries
What junctions are present in continuous capillaries?
leaky zonula occludens (important for paracellular movement of nutrients, waste)
The leaky zonula occludens junctions between endothelial cells found in continuous capillaries permit the passage of
small molecules (nutrients, waste)
**but not protein
Describe how fluid moves on the arterial side of the capillary bed.
on arterial side, hydrostatic pressure > colloidal pressure, so fluid leaves the arterial side, allowing for delivery of nutrients/O2 to tissue
Describe how fluid moves on the venous side of the capillary bed.
on venous side, colloidal pressure > hydrostatic pressure so fluid enters venous side, allowing for removal of waste products from tissue
The leaky zonula occludens junctions in continuous capillaries permit the passage of small molecules but not protein. How does protein cross endothelial cells?
proteins and lipoproteins cross by transcytosis via numerous caveolae and pinocytotic vesicles
Most continuous capillaries have leaky zonula occludens junctions, but there are some exceptions. What are the exceptions?
capillaries of nervous system and thymic cortex have very tight zonula occludens (i.e. not leaky) and therefore do not permit any movement via paracellular pathway
What are the characteristics of fenestrated capillaries?
presence of 50-100 nm fenestrations (holes in endothelium) that allow small molecules and proteins to pass through
Fenestration in capillaries commonly replaces what transport mechanism?
fenestration commonly replaces transcytosis (and fenestration increases protein flux by ~100 fold)
In fenestrated capillaries, how does the basal lamina modulate protein flux?
basal lamina repels negatively charged proteins
Give three examples of tissues/organs in which fenestrated capillaries would be found.
endocrine glands
kidney
liver
Describe the features of discontinuous/sinusoid capillaries. (4)
gaps between endothelial cells for protein flux
discontinuous basal lamina
fast rate of exchange
conform to underlying parenchyma
Which of the three types of capillaries have the fastest rate of exchange?
discontinuous/sinusoid — the fast rate of exchange means there is reduced ability to regulate protein movement
Describe the features of venules.
continuous basal lamina
pericytes
no smooth muscle
Describe the role of junctions in venules.
junctions become leaky in response to inflammatory mediators such as histamine
Describe the relationship between venules and leukocytes.
venules are the sites where leukocytes bind (margination) and extravasate (diapedesis) out of blood circulation
Differentiate between muscular venules, small veins, and medium veins.
muscular venules = receive blood from postcapillary venules
small veins = receive blood from muscular venules
medium veins = receive blood from small veins
How are muscular venules, small veins, and medium veins similar in section?
all have 1-2 layers of smooth muscle in media
Describe the characteristics of large veins.
2-15 layers of smooth muscle in media
thin intima
thin media
thick adventitia
longitudinally-oriented smooth muscle in adventitia
Large veins are also called
hanging veins
What are valves?
paired, thin flaps of tunica intima that project into lumen of vessel
Valves are reinforced by
elastic fibers
collagen fibrils
Describe the function of valves.
flip back and close lumen to prevent backflow away from heart
(T/F) The lymphatic system is bidirectional.
False. The lymphatic system is unidirectional.
What is the function of the lymphatic system?
removes excess fluid from tissues
Describe the order of structures in the lymphatic system. (4)
fluid goes through lymphatic capillaries → collecting vessels → lymphatic ducts → subclavian veins
Describe the characteristics of lymphatic capillaries. (4)
discontinuous endothelium
discontinuous basement membrane
lack pericytes
anchoring microfibrils
Describe the function of anchoring microfibrils in lymphatic capillaries. (2)
maintain openness of capillaries
tissue swelling pulls on microfibrils to help open gaps between endothelial cells to facilitate intravasation of fluid
Describe the characteristics of collecting vessels. (4)
have colloidal material
thin walls compared to lumen size
valves to prevent backflow of lymph
small amount of spiraling muscle
What is the function of spiraling muscle in collecting vessels/thoracic duct?
contractions of spiraling muscle drive lymph toward lymphatic ducts
Describe the characteristics of the thoracic duct. (3)
3-layered wall structure
spiraling smooth muscle in media
thick adventitia compared to media/intima
Describe how atherosclerosis arises.
buildup of lipid (from VLDL, LDL) in tunica intima that can result in fatty plaque
Describe the resulting events when lipids build up in the intima of arteries. (9)
macrophages and SMCs modify/phagocytose LDL → macrophages converted into foam cells → foam cells die → resulting hypoxia promotes angiogenesis → weakening of vessel wall → expansion of necrotic core → plaque rupture → thrombosis → infarction
How do lipids gain access to the intima of an artery in the first place?
transcytosis
leaky tight junctions weakened by oscillatory blood flow
What causes aneurysms?
weakining of tunicae in wall, allowing bulge/balloon to extend from vessel
(T/F) Aneurysms are restricted to arteries.
False. They can occur in any vessel, but are most common in arteries.
What are three risk factors for aneurysms?
atherosclerosis
smoking
diabetes
What causes hypertension?
increased systemic resistance at level of small arteries and especially arterioles
Why is hypertension a significant risk factor for heart failure?
hypertension promotes ventricular hypertrophy (stiffens ventricular wall through vibrosis) which disrupts connections between cardiomyocytes and leads to arrhythmias
Describe how aging leads to changes in large/medium arteries.
aging causes diameter of large/medium arteries to increase
makes walls thicker/stiffer (more type I collagen replacing elastic laminae)
Describe how aging leads to changes in small arteries/arterioles.
become more tortuous (twisty)
Describe how aging leads to changes in all arteries.
walls have dimished compliance → higher systolic BP
Describe the relationship between aging and nitric oxide. (3)
aging → reduction in nitric oxide → BP rises to compensate
