Arterial thromboembolism is a serious and painful disease of cats.  In the most common form, a thrombus (clot) flows from the heart, down the aorta and lodges where it divides to supply the back legs.  It straddles the "Y" like a saddle straddles a horse.  Hence the name, "saddle thrombus".  Symptoms of this problem include lameness and/or paralysis of the back legs. 

Clots can occur in other areas as well.  Dr. George Kramer's notes from IVECCS 2008, reminded me that the right brachial artery is the second most common site for clots to lodge.  These poor cats lose blood flow to their right front leg.  They present with lameness and sometimes paralysis of the right front leg. 
 
Some cats generate mini clots intermittently.  Because of their small size, they reduce but do not completely block blood flow to the effected limb.  I imagine the leg feels numb or like it is "asleep".  Cats with this condition often shake their leg.  

As a cat owner, it is important for you to recognize the signs of thromboembolism.  Lameness, pain, vocalizing, rapid breathing, paw shaking and paralysis are the most common symptoms.  If you observe any of these, please seek veterinary care immediately.