What does the composition of blood vessels depend upon?
mechanical factors- such as blood pressure and metabolic factors reflecting the local needs of tissues
What structure do all blood vessels have?
A specialised epithelium that acts as a semi-permeable barrier between two major internal compartments, the blood and interstitial fluid
Structure of endothelial cell epithelium
squamous, polygonal and elongated, with the long axis in the direction of blood flow
nonthrombogenic surface, vascular tone and blood flow, inflammation and local immune response, growth, strength
Explain nonthrombogenic surface
actively secrete agents that control local clot formation- such as heparin, von Willebrand agent and tissue plasminogen activator. Prevents blood from clotting
explain the control of vascular tone and blood flow
secrete various factors that stimulate smooth muscle contraction, such as endothelin-1, NO and angiotensinconverting enzyme
explain inflammation and local immune responses
Endothelial cells in venules induce specific white cells to stop and undergo transendothelial migration at sites of injury or infection. Under those conditions, p-selectin is expressed rapidly.
secrete various growth factors that include inducing the proliferation of immune cells and cells that make up the vascular wall
Important growth factor and function
VEGF- vascular endothelial growth factor
stimulates formation of vascular system from embryonic mesenchyme (vasculogenesis) and angiogenesis
What enzyme is present + function?
membrane bound angitoensin converting enzyme ACE- converts angiotensin 1 to angiotensin 11, which is a vasoconstrictor
Functions of blood vessels
O2 delivery, CO2 removal
nutrient delivery and waste removal
delivery of hormone signals to tissues
regulate blood flow, with specialised functions in certain tissues such as kidneys
Two components of the circulatory system
lymphatic system and blood vessels
lymphatic system function
collects lymph (extracellular tissue fluid) and delivers it back to he cardiovascular system
vessels that are greater than 0.1mm in diameter, such as arteries and veins
vessels that are less than 0.1mm in diameter, such as arterioles, venules and capillaries
2 different circuits and their passage
pulmonary- carries blood to and from the lungs
systemic circuit- carries blood to and from the organs and tissues of the body
what is an exception to this?
A portal system
portal system definition
blood carried from one site to another without directly involving the heart
2 examples of portal systems
hepatic portal system- gastrointestinal tract to liver- through the hepatic portal vein
hypothalamic-pituitary portal system- hypothalmus to anterior pituitary
Explain how blood flows through a portal system
occurs when a capillary bed pools into another capillary bed through veins, without first going to the heart.
How is the hepatic portal vein formed?
From the superior mesenteric vein and the splenic vein
resistance and pressure in both ciruits
pulmonary- low resistance, low pressure
systemic- high resistance, high pressure
main blood vessels + brief explanation
arteries- thick walled vessels that transport blood away from the heart, branch into smaller and smaller arterioles
capillaries- thin walled vessels that form capillary beds where substances pass between blood and tissue
veins- drain capillary beds- venules. Form larger and larger blood vessels that lead back to the heart, also have pocket valves
what are the layers of vessels called?
tunica intima, tunica media and tunica adventitia
structure and location of tunica intima
squamous epithelium endothelium adjacent to lumen of blood vessel
sub-endothelial layer of connective tissue
internal elastic lamina
tunica intima main function
acts as a principal barrier to plasma from exiting lumen of blood vessels
structure of connective tissue + function
collagen fibres in subendothelial layer
elastic fibres provide the resiliency required for the vascular wall to expand under pressure
various levels of proteoglycans and hyaluronates dependent on vessel function
Internal elastic lamina of tunica intima structure
composed of elastin, with holes allowing the better diffusion of substances from the blood deeper into the wall
tunica media structure and location
middle layer, composed of concentric layers of helically arranged smooth muscle
may also contain elastic fibres, elastic lamellae, reticular fibres and proteoglycans
what produces the non muscular components of the tunica media?
smooth muscle cells
what does the tunica media contain in arteries?
an external elastic lamina
function of tunica media
collagen provides restraint
Tunica adventitia structure
fibroelastic connective tissue comprised predominantly of type-1 collagen, small blood vessels and elastic fibres
arranged longitudinally with adipose tissue
adventitia is continuous with and bound to the stroma of the organ through which the blood vessels run
autonomic nerve fibres synapse, vasomotor nerves, that release vasoconstrictor norepinephrine
How do arteries compare to veins under a TEM ?
veins appear collapsed
Vasa vasorum definition
the microvasculature of large vessels
vasa vasorum function
formed of small arteries that enter the vessel and branch profusely
serve cells located in the tunica media and adventitia with blood supply
which has more vasa vasorum: arteries or veins? + why?
veins- they carry deoxygenated blood, so less oxygen able to diffuse from the blood within the lumen
where does the tunica intima receive oxygen?
the blood carried in the vessels
Two different types of arteries
where are the elastic arteries situated relative to the muscular?
elastic close to the heart, further from the heart the amount of elastin decreases and media becomes more muscular
examples of elastic arteries
pulmonary and aorta
examples of muscular arteries
include most vessels originating from the aorta, such as femoral and carotid
structure of elastic arteries
thick layer of elastic tissue formed of concentric layers of elastin within the tunica media
function of elastic arteries
make blood flow more uniform by stretching during systole and rebounding passively during diastole
structure of muscular arteries
thick tunica media composed of mainly smooth muscle, with up to 40 layers of cells
prominent internal elastic lamina
when is an external elastic lamina present?
only in larger muscular arteries
function of muscular arteries
regulate blood pressure
levels of vasculature during blood flow
from artery to arteriole to capillary to venule to vein
2 common features of microvasculature
discontinuous smooth muscle cells- individual cells
what are pericytes?
multitasking cells with remarkable plasticity and range of activities-
functions of pericytes
support blood vessels, regulate vascular tone and tissue perfusion, act as macrophage like scavangers
detect that blood is not going through the vessel which leads to their strangulation
angiogenesis, wound healing
Explain how pericytes aid wound healing
can differentiate into fibroblasts, smooth muscle cells and macrophage
what can a loss of pericytes cause?
vessels to become hyperdilated and haemorrhagic - lead to oedema and diabetic retinopathy
normal and vital proess in growth and development, wound healing and granulation tissue
fundamental step in transition from benign to malignant tumours
smallest of arterial branches, with around three to four layers of tunica media smooth muscle
lumen often less than 0.1mm in diameter, lumen as wide as the wall is thick
subendothelial layer is very thick, elastic laminae are absent
very thin adventitia
function of arterioles
principal resistance vessel- control arterial blood pressure
controls distribution of blood to whole capillary beds
what is present at the arteriole and capillary junction?
structure and function of pre-capillary sphincters
a ring of muscle surrounding the blood vessel
controls the dilation and constriction of resistance vessels and can effectively open or close the capillary, thus facilitating shunting
what are arterioles controlled by?
sympathetic autonomic nerves
three different types of capillaries
continuous (most popular), fenestrated and sinusoidal
structure of capillaries
approximately 50 micrometres in length
squamous epithelium and pericytes
continuous capillary structure+ function
have junctional complexes, tight junctions to control trans-endothelial transport
prevent passage of many molecules
substances may move across by active transport
important in blood brain barrier
fenestrated capillaries structure and function
pores that are 60-80nm in diameter
permeable to small ions and other molecules
pores bridged by diaphragms formed of proteoglycans
sinusoidal capillaries structure and function
large pores that are not linked to diaphragms
enhances exchange between blood and tissue
allows leukocytes to pass between organs where they are produced to mature and to mediate a response in the tissues
irregular channels conforming to a shape of structure in which they are located
highly discontinuous basement membrane
large diameter 30-40 μm which slows blood flow
structures of capillary beds
supplied by terminal arteriole branches called metarterioles
metarterioles are continuous thoroughfare channels connected with postcapillary venules
the metarteriole smooth muscles act as precapillary sphincters
How is blood pushed?
sphincters contract and relax cyclically, with 5-10 cycles per minute causing blood to pass through in a pulsatile manner
Where are each capillaries located?
continuous- connective tissue, lungs, exocrine glands and nervous tissue
fenestrated- found in organs with rapid exchange of materials- kidneys, intestine, choroid plexus
sinusoidal- liver, spleen and some endocrine organs and bone marrow
What enables capillary beds to be bypassed?
How are veins classified?
on the basis of their diameter and wall thickness- small, medium, large and low pressure
Brief structure compared to arteries
same 3 layers as arteries but elastic not as well developed
connective tissue more pronounced
boundary between tunica media and intima not clear
medium sized veins have valves
walls similar to capillaries- thin endothelium surrounded by reticular fibres and pericytes
function of venules
site of migration of neutrophils macrophages- especially during diapedesis
medium vein structure and function
less than 1cm in diameter
smooth muscle cells of tunica media are loosely organised with collagen fibres and fibroblasts
tunica adventitia is the thickest of the tunicas
large veins structure and function
low pressure, high capacitance system
have pocket valves to prevent back flow
thin tunica media
few have a well developed smooth muscle layer
superficial veins have well developed muscle to resist distention caused by gravity
well developed tunica adventitia