hi let's talk about vasculature or in other words the vessels that connect your heart to
the metabolically active tissues of your body so leaving the heart our arteries these are the
vessels that conduct blood to the metabolically active tissues now most vessels are comprised of
three layers or tunics there's the Tunica externa tunica media and deep the Tunica
interna the Tunica externa of the artery is the outermost layer that consists of collagen and
other connective tissues and this is going to surround the tunica media the tunica media in
an artery consists of layers of smooth muscle and these can be quite variable with respect to
the number of layers and then an external elastic lamina which provides a little bit of tone to the
vessel the Tunica interna consists of a layer of simple squamous epithelium supported by a basement
membrane surrounded by an internal elastic lamina together these three layers provide the vessel
with enough support and tone and elasticity to move blood away from the heart so that it may
be delivered to those metabolically active tissues now there are two major types of arteries elastic
and muscular the elastic arteries typically are larger more major arteries such as the aorta
and the pulmonary trunk these are arteries with a high amount of a particular type of protein called
elastin and they're walls that are able to serve as pressure reservoirs to absorb the relatively
large pressure put on the blood as it exits the heart and then squeeze back into it's more typical
anatomical position muscular arteries or distrib arteries are branches from elastic arteries these
are the other typically named arteries that are able to distribute the blood throughout the body
arteries feed into arterioles arterioles then feed into thoroughfare channels which control blood
into capillaries and those capillaries can then go back into thoroughfare channels into muscular
venules into veins and return to the heart micro circulation is all of the vessels between the
arterial and the vein yule so as we can see here from the met arterial through the thoroughfare
channel into the venule is microcirculation in the action of the cardiovascular system the
filtrate from the blood into the interstitial tissues occurs at the level of the capillary bed
the exchange of nutrients the exchange of gases the movement of hormones the movement of waste
products and even by proxy the movement of heat is a phenomenon that is controlled by the capillaries
on the left here we can see capillaries that are fully engorged with blood and that's a
function of these pre capillary sphincters from the thoroughfare channel being relaxed
so that blood can flow through the capillary beds on the right we can see what happens when
those pre capillary sphincters are shut tight that prevents the blood from moving into the capillary
beds and it forces it through the thoroughfare channel back to the venule and then that will
eventually be returned at the heart capillaries themselves as the sites of exchange are remarkably
simple they consist of two layers an endothelium which is simple squamous epithelia supported by
a basement membrane now if you recall those two layers correspond with the Tunica interna of the
artery the capillary is generally about 8 to 10 micrometers in diameter and that's approximately
the diameter of an erythrocyte or a red blood cell so these red blood cells are typically going to
move through capillary beds one at a time and have ample opportunity to unload oxygen pick up
carbon dioxide and participate in that exchange the muscular venules feed into veins in veins by
definition our vasculature that returns blood to the heart like arteries veins have three layers
a Tunica externa which is very similar to the Tunica externa of arteries a tunica media which
unlike arteries is typically very very very very thin and a Tunica interna which is very similar to
arteries it's simple squamous epithelium supported by a basement membrane what you'll notice is that
there are no elastic lamina as part of veins and also there's an elaboration of the Tunica interna
in that some veins have involutions or in foldings of that simple squamous epithelium so that a valve
is formed these valves promote the unidirectional flow of blood this is helpful because pressure
within veins is extraordinarily low relative to the pressure of arteries blood pressure in the
vessels after a capillary bed is very low and as a consequence of that blood needs some extra help to
be moved along there are a few ways in which the body can help promote the flow of blood through
veins one very easy way is by means of gravity so for instance any of the veins that are superior
to the heart that aren't involved in the upper limb may be passively drained via gravity but
this in and of itself is insufficient in order to return blood even from those areas because you can
imagine that astronauts in a microgravity environment still have the need to return
blood back from their head and so we have other means by which we can encourage the flow or the
movement of blood in veins all of these means exert extrinsic pressure on the veins and the
blood within the veins so a few of these include muscle milking via skeletal muscles and so what
happens here is that muscles that are adjacent to veins as they contract they squeeze the veins
pushing the blood through one valve to the next compartment another means of extrinsic pressure
on veins is through respiration by changing the volume of the abdominal and the thoracic cavity
by the diaphragm depressing and elevating you change the relative pressures of these spaces
and by changing the relative pressures of these spaces you can exert force on the inferior vena
cava to help promote the flow of blood the third way that we can provide extrinsic pressure on a
vein is rather ingenious and that is veins can accompany arteries and as arteries are pulsatile
vessels that is they throb that pulsation exerts adjacent pressure on the veins which helps move
blood through the veins veins which accompany arteries and make use of their their pulsatile
nature are referred to as vena comitantes and finally there are times when the valves of veins
fail and so typically what we'll see is if there's any backflow from a vein the valve catches it and
closes it and the blood at least remains within that particular segment of the vein but if the
valve were to fail if we have any incompetence of that valve blood may flow in reverse or retrograde
from one compartment to the next and then pressure can build up on the walls of these veins that
enlarges the vein and it gives the overall appearance of larger more torturous sometimes
curved vanes people refer to these as varicosities or varicose veins this is typically a phenomenon
within the superficial veins of the lower limb it may be painful it may produce a burning sensation
it's not generally medically significant and some people may elect to treat these cosmetically so
we've talked about the vessels that carry blood away from the heart the vessels that exchange
nutrients gases and waste products and then the vessels that return blood to the heart let's look
at some of these individual vessels and think about the tissues that they supply thank you