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- [Instructor] With red blood cells down,

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white blood cells down,
we're on to platelets.

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Platelets really are not actually cells,

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they're small fragments of a cell.

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What ends up happening is,

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this larger cell called a megakaryocyte

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is going to allow little, tiny pieces

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or fragments to break off.

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It's these fragments

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that are going to be
called your platelets.

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So we have these megakaryocytes

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that allow fragments to
break off to form platelets.

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The platelets are really
important in hemostasis,

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or better known as blood clotting.

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So if you need your blood to clot,

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you'd better have some platelets around.

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The way hemostasis works,

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there's three main
phases to blood-clotting.

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First off, a vascular spasm,

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or a constriction of blood
vessels to reduce blood flow.

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Actually, the spasm is
going to increase blood flow

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momentarily, then decrease it.

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It's going to increase
blood flow for a little bit,

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to help get more platelets

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and more clotting factors to the site.

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Then it's going to help
reduce the blood flow,

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slow down the blood
inside the blood vessel.

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The platelet plug forms.

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The ruptured vessel will
have this makeshift plug

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that's shoved into the opening,

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referred to as the platelet plug.

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The platelets start to attach

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to the edges of the torn vessel,

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and slowly work their way inward

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to create a whole network or
mesh covering the wounded site.

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Coagulation, now once you get
the platelet plug in there,

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the blood vessel is pretty much sealed up,

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it's not totally sealed yet.

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You end up with a blood clot.

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The blood clot is a series of reactions.

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I mean, we're talking, I
want to say, 24 or 25 steps,

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not important for us.

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What we're looking at is the fact

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that you now have stopped the bleeding,

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you've reached this coagulation phase.

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Three different steps to stop bleeding,

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vascular spasm, platelet plug formation,

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and then coagulation.

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When we're all done it should
look something like this.

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What you're seeing here is

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the reddish capped almost structure,

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those are red blood cells.

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They're stuck in this,
basically, formation

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of fibers, red blood cells, and platelets.

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This is your coagulation.

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This is how your blood clots.

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You form a network of platelets,

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which help to trap all these fibers,

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which in turn trap red blood cells,

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sure, some white blood cells, too,

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but by allowing all this material

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to get kind of pushed up into one space,

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it helps prevent a bleed-out,

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helps create a clot.

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Here's the pictorial view
of what we're looking at.

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First off, number one at
the top, vessel damage.

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Artery, vein, capillary, whatever.

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You've damaged the blood vessel.

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Number two, what ends up happening is

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it'll increase a little bit,

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and then shrivel down, a vascular spasm.

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Less blood flow to the site momentarily.

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The platelet plug will form now.

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See all those blue
platelets having stuffed,

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or basically stuck in
where the opening was.

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Then you get your actual clot formation.

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The platelet plug still has pores.

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Fluid can still get out.

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But with the particles and fibers,

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nothing else is getting out.

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That means now the platelets
are trapping red blood cells,

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the platelets are trapping
strands of fibrin, or fibers,

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all this trapped creates that plug

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to stop bleeding completely.

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Now can this work on any
sized cut no matter what?

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No, if it did, you wouldn't need stitches.

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But only working on cuts

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that have a small to moderate size

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where the body can help mend itself.

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Any large, gaping openings
need some medical intervention.

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The platelet plug cannot be big enough.

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On this picture, you can see that

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that red and black colored bruise

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is not a typical bruise.

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This is the disorder hemophilia.

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Heme is blood.

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Hemophilia is a genetic disorder.

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That means unfortunately
right from birth this occurs.

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You're missing one or
more clotting factors.

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Remember I said, somewhere in the 20s,

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well, again, don't worry
about all this stuff

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or anything like that,

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but the fact of one missing

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means this whole cascade will break down.

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From this case, that bruise there

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could have been a simple
bump into the counter.

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The problem is, with not having
all the clotting factors,

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the body cannot stop the
bleeding as efficiently.

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So in this case it looks like it probably

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ruptured a blood vessel or
two, maybe some capillaries,

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because it could not clot,

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the blood kept flowing and
flowing under the skin,

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creating this very large
area of inflammation

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and this very large area of redness.

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It's hemophilia.