Blood Clotting

Blood clotting or coagulation (also called hemostasis), is complex, but we want to try to understand this in "big picture" form first off.

When an injury occurs in a blood vessel, here are the steps we go through.

Classes.midlandstech.com

Platelets and Fibrin are the important things that we actually get as products of coagulation that seal up a wound.  But the activation of them is a complicated process.  Which is a good thing for your body!  If it were an easy reaction, we could get our blood spontaneously clotting on us and that would be BAD.

Here's another way to visualize this:

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In first forming the platelet plug, it's important to note that it's the exposed collagen fibers that are attracting platelets.  The normal, healthy state of things is to NOT have platelets sticking to blood vessels.  The ability to not stick is ensured by the blood vessel lining cells (endothelial) releasing prostacyclin to prevent platelet sticking.  But with damage, that isn't released and instead collagen causes platelets to stick.  This can be visualized here:

dc416.4shared.com

Now for the actual clotting part after the platelet plug, for now I have some videos.  If you want to cut right to the chase and a great explanation, watch the last video.



Videos
Here's a not fabulous animation but an animation nontheless...


This short clip I just found helpful to visualizing how the different factors work together to activate factor X then prothrombin, to activate thrombin which they refer to as the "thrombin burst" because it creates a large amount then creates a positive feedback loop to further increase the effect.


This is the best video I found!  I like how concise and understandable he makes it, looking at the big picture and working backward from there.  Hope it's helpful for you too.

All clotting factors are made by the liver, except 8 and PAF3. (Platelet activating factor is made by white blood cells.)

Blood

We had the Hematology lab in Physiology yesterday.  I love blood, it's so fascinating.  I posted previously about the buffer system in the blood, just one amazing part of this liquid tissue.  In this lab, we got to do blood typing and hematocrit which is a percentage of formed elements in the blood.

http://science.howstuffworks.com/environmental/life/human-biology/blood.htm

Hematocrit
Hematocrit is measured by centrifuging a sample of blood until it separates into plasma and formed elements.  The formed elements (mostly erythrocytes, aka red blood cells) sink to the bottom because they are more dense than the plasma.  A layer of white blood cells (leukocytes) and platelets (thrombocytes) forms between the plasma and RBCs, and this is called the "buffy coat".  (That will be easy to remember because of Buffy the Vampire Slayer haha.)

http://cellbiologyolm.stevegallik.org/node/73

Normal hematocrit figures are 45-52% for males and 37-48% for females.  These figures can be altered by health, environment and lifestyle.  Living at high altitude increases it because there is less oxygen there so the body compensates by making more RBC's.  That is why Kenyan men are so superior at running, because (among other factors) they live at very high altitude.  "Blood doping" is basically raising the hematocrit illegally so athletes can carry more oxygen and perform better.  Training at high altitude is the legal way to have this advantage.

Blood Typing
The alleles that determine blood type are co-dominant, so both the A and B are dominant and the recessive trait results in type O blood.  Blood cells make certain antigens which are on the surface of the cell to identify themselves, and they make antibodies found in the plasma to fight off anything foreign. 

• Type A cells  - A antigens on the surface of the cell, anti-B antibodies
• Type B cells - B antigens on the surface of the cell, anti-A antibodies
• Type AB cells - both A and B antigens on the surface of the cell, no antibodies
• Type O cells - no antigens on surface of cell, both ant-A and anti-B antibodies

http://www.truehealthlabs.com/products/Blood-Type.html

Because of the nature of the antigens and antibodies, great care must be taken in blood donations and transfusions so as not to kill the patient due to their plasma antibodies attacking the new blood cells, causing agglutination (clumping of cells).  Only certain blood types can be paired up.  Obviously a patient can receive the same type of blood they have.  But that is not always available.  Type O blood is the universal donor because it has no antigens on its surface, so the blood of the recipient has nothing to attack.  Apparently, even though type O blood has antibodies against A and B in it, when that blood is donated to an individual, it's not enough to cause a big problem.  AB blood is considered the universal recipient because it has antigens for both so anything that gets donated to them will recognize the patient's blood as familiar and no response will take place, and perhaps more importantly, it has no antibodies to attack the incoming foreign blood.

Blood typing is done by taking samples of blood and adding antibodies.  If an A antibody is added to a sample and it agglutinates, that indicates it has type A blood (or AB).  Type O blood won't agglutinate at all.  There is also the Rh factor that gets tested for and that is what gives the + or - to blood types.  Here's a chart of the agglutination from this test:

http://viromag.wordpress.com/2009/10/13/immunology-quizz/

So even when the blood types are compatible, a "crossmatching" is done before any donation/ transfusion to ensure that the blood is compatible.  They do this by mixing red blood cells from the donor with plasma from the recipient and make sure no clumping happens.


Okay once again I must say - the human body is AMAZING!!!