Hemostasis | Advanced hematologic system physiology | Health & Medicine | Khan Academy

Hemostasis | Advanced hematologic system physiology | Health & Medicine | Khan Academy


Voiceover: Hi everyone. I want to start off by talking about Mary. Mary likes to play soccer, and she was at one of her games, and she was running really fast. She scored a goal, and
then she tripped and fell, and scrapped her knee, and
then she started to bleed. Now, normally our bodies want
to make sure that our blood, which flows through our blood
vessels stays in our body. If we have an injury
that allows blood to leak out of the blood vessel
and out of our body, we want to make sure we can stop that. We do that through a
process called hemostasis. It’s through hemostasis
that we create this clot. This clot allows the bleeding to stop. It stabilizes the injury
in the blood vessel, and it gives the blood
vessel time to heal. Now, let’s take a look at exactly what’s going on in Mary’s knee, and exactly how we make this clot. So, here we have a blood vessel, and in our blood vessel we have our blood. In our blood, we have many
different types of cells. One of the most important
ones is our red blood cell that carries oxygen all over
to all our tissues in our body. In this blood vessel, we make sure that blood is
flowing smoothly in one direction. Did you know that if we were to line up all of our blood vessels, the total length of our blood vessels would come out to about
150 thousand kilometers? Which, is about 93 thousand miles. This is the same distance
as if you were to travel around the Earth a little
bit less than four times. So, you can see our blood
vessels have a lot of work to do. One of the cells that helps with this are the cells that line the
inner wall of the blood vessel. The cells that communicate with the blood. These cells are called endothelial cells. It’s these cells that allow
the blood to flow smoothly, and these are the cells that
communicate with the blood whenever there’s been a injury, and hemostatisis needs to begin. Now, let’s take a look at Mary’s knee. She scrapped her knee, so,
let’s cause a break in her skin. Let’s say this is her skin. She also caused damage to
the blood vessel underneath. So, let me erase this. Let me cause some damage
to the blood vessel. You can see blood that should be
flowing in this direction now can come out of the blood vessel, and out of the skin in this direction. We want to stop this. The way we stop this
is through hemostasis. There are two stages to hemostasis. The first stage is primary hemostasis. This begins immediately
after there’s been an injury. What happens is the endothelial cells,
at that sight of injury, begin to secrete proteins. This sends signals to the blood that there’s been an injury. In our blood we have platelets, these platelets are
made in the bone marrow just like red blood cells, and their purpose is to respond
to injured blood vessels. I drew them as squares,
but their actually about this small compared to red blood cell. The reason I drew squares is I want to make a note of what
they carry inside of them. They carry these sacs or
granules that carry molecules and proteins that are
necessary for hemostasis. So, they store them, and they’re ready for use whenever needed. What happens is, the endothelial cells secrete these proteins
and send a signal to the platelets to come
to that sight of injury. Then, a sequence of events
happens with the platelets. I won’t go into details here, but it allows the platelets to
stick to that sight of injury. They get activated. They change shape. They release what they have inside, and then they start clumping together and call more platelets, and cause more platelets
to clump there as well. What we end up with is what
we call a platelet plug. This is the first step
of stopping any bleeding. This is still kind of weak, and we need to make this stronger. That’s where secondary
hemostasis comes into play. In secondary hemostasis,
we make this platelet plug stronger with a protein called fibrin. What fibrin does is it
lays over and links up on top of this platelet plug, and makes it tighter and sturdier. It’s like tying a knot. In order to prevent it from getting loose, you double-knot it or triple-knot it. This is exactly what fibrin is doing. Fibrin doesn’t flow in
our blood as fibrin, it carries around an
extra piece of protein, and when it has this
extra piece of protein, it’s called fibrinogen. So, we need a way of getting
fibrin from fibrinogen. The way we do that is with
this family of proteins called coagulation factors. I’m drawing them all in one color, but there are several different
types of coagulation factors. The injured blood vessel will send signals to
these coagulation factors to get activated, and once when one gets activated, it will activate another,
and activate another, and activate another, and then ultimately, that extra protein on
fibrinogen will get cut off. That will allow fibrin to link up on the platelet plug and make it stronger. This sequence of activation
of coagulation factors is what we call the coagulation cascade. I won’t go into the details here either. It’s just key to remember
that in secondary hemostasis, we make this platelet plug stronger by getting fibrin from fibrinogen through the coagulation cascade, and it’s with this fibrin
mesh over the platelet plug, that we create this clot. It’s this clot that stops bleeding, allows blood flow to continue
through the blood vessel, and allows the blood vessel to heal.


18 thoughts on “Hemostasis | Advanced hematologic system physiology | Health & Medicine | Khan Academy

  1. This doesn’t seem advanced at all. (Molecules and proteins, clotting factors …..) how about wills brand factor , thromboxane etc…. change title please

  2. I don’t know why people are complaining about her not going into details, obviously there will/there is an another episode for those details,
    Very helpful video, thank you.

  3. Noo! First step to stop bleeding is vasoconstriction from vascular spasms – then platelet plug formation. This is not advanced at all and is skipping steps. :/ usually khan academy is a lot better than this.

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