How do you get blood clots? What in the blood decides to clot up?

Flow/vessels/protein. Clots can form when: blood flow slows or stops, a blood vessel wall is injured or components of your blood support or promote coagulation. We are always clotting and unclotting a little. When one of the factors noted above tips the scale there is rapid propagation of clot.
Stasis. Circulating blood loves to "keep moving"!, it also is assisted by muscle contraction and movement and unfortunately a sedentary lifestyle or prolonged periods of inactivity facilitate the "clotting factors" in the blood activating causing clots that rarely "stay home" but travel to distant places like the lungs(pulmonary embolism).

Related Questions

Can you get blood clots from just sitting for a long time? I'm 21 and I workout regularly but lately I've been sitting for long periods of time and my knees and the bottom of my feet started to hurt.

Yes it is possible. It is possible but unlikely. You probably are compressing the nerve behind your knees and this is creating the pain. Read more...
Unlikely. It's pretty unlikely as you will develop a DVT (deep venous thrombosis) from just sitting....Especially at your age. However, some people do have hereditary clotting disorders, in which case it's more of a possibility. If your family does not have a history of dvts or pes, you have an extremely low chance of developing a problem. Read more...
Yes. Prolonged immobility increases ones risk for developing blood clots. Prolonged plane rides, car rides, etc can lead to clots. Staying well hydrated and getting up and moving around periodically reduces your chance of developing blood clots. While sitting for prolonged time move you feet and legs can also help reduce your risk. Read more...
So to answer your . Question definately, prolonged sitting is one of the risk factors of getting a clot. You also state you have knee and foot pain. There are other reasons you can have knee and foot pain though - do you have flatfeet? If so you can have plantar fascitis and flatfeet also put pressure on the inner knee....So since you mention a clot - that should be ruled out as that is dangerous, other causes too. Read more...

How bad is it to get blood clots & also what does unresponsive mean?

Blood clot. Blood clot is nonspecifc . It can mean clots seen due to excessive bleeding or abnormal clotting in the arteries or veins(causing cell loss with lack of blood supply due to the clot) . Unresponsiveness means loss of consciousness. Read more...

Is it common to get blood clots after surgery?

Rare. But following the treatment regiment is key to prevention. For low risk surgery early movement (walking) is the most important advise. For larger surgery where you remain in bed for a while, medications and leg pumps may be used for prevention. Getting up and out of bed as early as possible is still important however. Read more...
Risk factors. This depends on assessing your risk factors & nature of the surgery. Prior history, weight, age, meds, air travel, family history, varicose veins, recent surgery, pelvic or venous manipulation, length of surgery are all risk factors. More than 5 of these places you at higher risk. See image and: http://www.Covidien.Com/imageserver.Aspx/doc245285.Pdf?Contentid=31695&contenttype=application/pdf. Read more...