Excellent explanation, Peta. As you all know, activity increases the heart rate - hence 'cardio' exercises like the bike, treadmill and elliptical trainer. No matter what, giving the heart periods of increased activity and stress is good for it and for the circulation in general. You see, blood only clots when it is allowed to pool and/or be in a sluggish stream. Therefore, blood that is circulating briskly is unlikely to clot.
Now all of the situations you have cited are due to different things;
~ THR - lack of activity in the legs, maybe some other medical problems as well
~ Clot after arthroscopy - same as above.
~ DVT after labour - there are various opinions as to what causes this; increased pressure within the pelvis from the pregnant uterus slows the circulation in the leg vessels as they pass through the pelvis. Long periods in second stage labour when the effort of pushing also gets applied to the vessels in the leg - remember how it freqently results in the development of haemorrhoids? Using lithotomy poles, stirrups or leg supports, especially those where most of the leg's weight is being taken on the calf muscle.
~ Breast cancer - some types of chemo-therapy can increase the likelihood.
~ Healthy male with a clot in his heart - otherwise known as a coronary thrombosis - that would be due to athero-sclerosis in the corony arteries and nothing to do with what we are commonly talking about here. That will be caused by things like high cholesterol, obesity, smoking, drinking and high fat diet.
As Jamie pointed out, otherwise fit, young adults have suffered from DVT after a long-haul flight but theoretically it can also happen after a long car ride, etc.
Now if you have a DVT, the next port of call for the blood from your leg veins is your lungs so the logic is that the second stage complication from DVT is a pulmonary embolus or clot in the lung. Clots in the heart start in the heart as described above.
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