Histology of Blood Vessels Flashcards
Why does velocity of blood flow decrease along the arterial tree?
Greater and greater branches increase overall cross sectional area of blood flow despite smaller individual vessels
Where is there higher arterial pressure: systemic or pulmonary circulation?
systemic
What are the three basic layers of blood vessels?
intima, media, adventitia
Which basic blood vessel layers are present in capillaries?
only intima
4 substructures of the tunica intima
endothelial cells, basal lamina, subendothelial connective tissue, internal elastic lamina (arteries and veins)
3 substructures of the tunica media
smooth muscle cells, elastic fibers, collagen
5 substructures of the tunica adventitia
external elastic lamina in arteries, smooth muscle cells in large veins, loose connective tissue, vasa vasorum, nerves/nervi vascularis
Why do large conduit arteries contain so many elastic fibers?
to create tension that can “snap” and propel blood through conduit to smaller arteries/arterioles
Boundary between intima and media
internal elastic lamina
Arteries tend to have thicker/thinner walls relative to their luminal diameter than corresponding veins
thicker
In comparison to arteries of similar size, veins tend to have narrower/wider lumens, thinner/thicker walls with adventitia thicker/thinner than media.
wider lumens, thinner walls, thicker adventitia than media (vs arteries)
Role of muscular arteries
distributing vessels that control regional blood flow as needed (e.g. exercise, meal)
Defining features of muscular arteries
media: spiral smooth muscle with collagen fibers and fewer elastic fibers than elastic arteries w/o organized lamellae
How is blood flow regulated in muscular arteries?
by controlling luminal diameter and resistance to flow by contraction/relaxation of vascular smooth muscle under control of ANS, catecholamines (adrenal medulla), and local mediators
Role of small arteries/arterioles
resistance vessels that are the major determinant of arterial resistance to blood flow and therefore blood pressure (esp diastolic) and to control flow to microvascular bed
What regulates contraction/relaxation of medial smooth muscle in arterioles?
ANS, adrenal catecholamines, angiotensin II, local factors (e.g. NO, prostacyclin), pH, oxygen, carbon dioxide, adenosine
Role of capillaries
exchange vessels
Ultrastructure of capillaries
thin attenuated endothelial cells with a basal lamina that may be shared with pericytes
Role of pericytes
contractile function to modulate capillary flow, progenitors of vascular smooth muscle during angiogenesis
3 modes of exchange between capillary lumen and extravascular space
- passive diffusion
- active transport (pinocytic vessels or receptor mediated pinocytosis)
- passage between adjacent cells
Are all capillaries receiving blood flow at the same time in a given bed?
No. there is a smooth muscle sphincter for each capillary that regulates when it gets blood
What component of capillary beds always are perfused?
thoroughfare channels
3 classes of capillaries
continuous < fenestrated < discontinuous (with relation to exchange-ability)
Most common type of capillary
continuous
How are continuous capillaries sealed?
tight junctions, continuous basal lamina –> requires direct diffusion of gases or channel-mediated pinocytosis (carrier or active transport)
Where do we find fenestrated capillaries?
gut, kidney, hormonal structures
Where do we find discontinuous capillaries?
spleen, liver, lymph nodes
Do fenestrated capillaries have a complete basal lamina?
yes. only discontinuous has incomplete lamina
T/F veins and venules can store variable volume of blood
T –> capacitance vessels can readily distend
Do venules have an internal elastic lamina?
no
Do veins have an internal elastic lamina?
may be incomplete
Do veins/venules have an external elastic lamina?
no
Do veins/venules have a vasa vasorum?
only largest veins
What kind of capillaries are sinusoids?
disocntinuous
What are the 2 exceptions to normal microcirculation principles?
- renal portal circulation: afferent arteriole –> cap bed –> efferent arteriole –> cap bed –> vein
- portal venous circulation: liver, hypothalamic venous circulation
What structures prevent backflow in veins?
semilunar valves
Smaller veins have higher/lower pressure than larger veins
higher
Defining histologic features of veins
smaller media, larger adventitia than artery
____ are an ultrastructural marker of endothelial cell differentiation.
Weibel-palade bodies: contain VWF (factor VIII) –> exposure during injury promotes platelet adherence and activation
How do endothelial cells promote platelet adherence and activation during injury?
- reduced prostacycline and thrombomodulin
2. increased vWF and tissure factor exposure
How do endothelial cells modulate immune cell migration?
- alteration of surface adhesion molecules (E-selectins and ICAMs) under influence of cytokines from other inflammatory cells (TNF alpha, IL1)
- alteration of immune cell function by release of PDGF and cytokines 1,6,8
What products of endothelial cells promote vessel relaxation?
NO, prostacyclin
What products of endothelial cells promote vessel contraction?
endothelin
Can vascular smooth muscle contract tonically?
yes it has to –> constant tension –> constant stimulation by sympathetic ANS
How does smooth muscle differ from striated muscle?
- consists of individual spindle cells with single nuclei
- no organized sarcomeres, z lines, t tubules, or troponin
- capacity to maintain tension for long periods with little energy expense
- slower contraction/relaxation
- longer time of linking of actin/myosin cross bridges
How does smooth muscle contract?
intracellular calcium –> activates myosin –> move along actin filaments anchored to dense bodies ( which themselves are tethered to the cell membrane by intermediate filaments) scattered in cell cytoplasm
What structure replaces z bands in smooth muscle?
dense bodies attached to actin filaments
What tethers dense bodies in smooth muscle?
intermediate filaments connect dense bodies to cell membrane and to each other –> are compressed during contraction and provide restorative force during relaxation
What feature of smooth muscle allows for greater degrees of contraction and maintenance of maximal contractile force at various cell lengths?
the actin:myosin ratio is higher than in striated muscle, which allows myosin to switch between actin filaments as it progresses –> thereby increasing it’s ability to “travel” along the muscle –> greater contraction distance
How do myosin differ between smooth and striated muscle?
in smooth muscle myosin heads form sidepolar cross bridges with one actin filament per head –> contraction pulls two actin filaments in opposite directions
How does calcium activate contraction of smooth muscle?
combines with calmodulin which activates myosin light chain kinase (MLC kinase) –> phosphorylates the MLC in the myosin head –> changes the shape of the head activating the actin binding site of the head and the atpase site
What is required for ATP binding and actin binding in smooth muscle?
MLC phosphorylation
What allows smooth muscle to hold tension without extensive energy burn?
slower cross bridge cycling means myosin head holds onto actin longer than in striated muscle (where atp releases the head faster) == latch state
What can trigger vascular smooth muscle contraction?
- electromechanical coupling (AP)
- pharacochemical coupling (G proteins for norepi, angiotensin II, vasopressin, thromboxane A2)
- stretch channels
What inactivates myosin in vascular smooth muscle?
MLC phosphatase
What activates myosin in vascular smooth muscle?
MLC kinase
What determines vascular smooth muscle tone?
balance of MLC kinase/phosphatase
What mechanisms promote relaxation of vascular smooth muscle by regulating the balance of MLC kinase/phosphatase activity?
- reduction of intracellular calcium –> decrease MLC kinase activity
- stimulation of g proteins (b2 for epi, histamine receptors),
- NO mediated increase in cGMP and consequent activation of MLC phosphatase
T/F there is a net loss of fluid as blood moves across capillary beds
T –> this is the liquid picked up by lymph vessels
Do lymphatic capillaries have complete basal lamina?
no –> incomplete lamina without tight junctions –> kept open by anchoring filaments –> drain into larger vessels with one way valves (valves divide lymphatic vessels into segments called lymphangions) –> think lock system for boats (fill one lymphangion first and then push it all into the next)
What is chylous lymph?
lymph with high lipid content (normally clear w/o red blood cells)
Where do the thoracic duct and right lymphatic duct end?
left and right innominate veins
What is the source of force that propels lymph through the lymphatics?
skeletal muscle or arterial contraction/external compression
