|MadSci Network: Medicine|
Both arteries and veins can develop thrombosis ("blood-clot") within them. I do not have any accurate figures to go by but venous thrombosis is at least as common as arterial thrombosis. According to the famous Virchow's triad, any disturbances to one or more of the three components of our circulation would lead to thrombosis, they are:- (1) constituents of the blood, (2) blood vessels, and (3) blood flow. A range of pathological processes can affect one of more of the three and results in the development of thrombosis and there are preferential difference of the effects of the three factors that affect either the arteries and the veins. Arteries are under greater hydraulic pressure than veins and the shearing force of blood flowing through arteries, esp. at branching points, often results in damage to the surface lining of the vessels (endothelium), i.e. affecting the second components of the triad. In addition, when the body tries to repair the damage, a process known as atherosclerosis often takes place. The fatty atherosclerotic plaques gradually build up in size; and if the plaques are disrupted by further damage, then a rapid thrombosis can develop within the arteries and occlude the blood flow completely. This is often the case when a patient with known ischaemic heart disease sudden develops a myocardial infaction (i.e. heart attack). Venous thrombosis is very common in or out of hospital. Deep vein thrombosis (DVT) is especially common in bed-bound patients and those travellers after long-haul flights. This time, it is the third components of the triad being disturbed, i.e. blood flow. Venous drainage depends on muscular contraction of the draining part of the body. If one is immobile for a long time, e.g. sitting on a plane for hours without moving about, then venous drainage becomes more and more sluggish. This slow flow then favours the process of thrombosis in e.g. the veins in the calves. One potentially fatal consequence of DVT is that if the blood clot is dislodged from the leg veins, it might get thrown up into our lungs. And if a large clot blocks up a main vessel in the lung, a condition known as pulmonary embolism can occur, whereby the blood is unable to travel to the lung to pick up the oxygen, with sometimes fatal outcome. You might then ask what happen to the first component of the Virchow's triad? A number of medical conditions, some genetic some autoimmune, can affect the blood constituents and results in thrombosis. These conditions alter the proportion of functional clotting factors and anti-clotting factors in the blood and are collectively known as thrombophilias. Both arteries and veins can be affected in thrombophilias. As to why we use venous grafts in arterial bypass surgery. One of the reasons is that synthetic material is more prone to thrombosis under arterial pressure. Hence biological material is preferred. If we were, then, to harvest biological conduits for bypass grafts, we need vessels that are (relatively) easily accessible, of comparable sizes, and which functions can be substituted by a parallel system. Veins in the legs have been used as conduits for coronary artery bypass surgery for a long time because of these reasons. There are 2 venous drainage systems in the legs, deep and superficial. Taking out one of them will not have too significant an effect on the overall venous drainage of the legs. Having said that, the life-span of a venous graft is only about 10-15 years before re-narrowing occurs. Therefore, we are always on the look out for better conduits for future bypass surgery. I hope this helps:-) Sincerely yours, Dr. Joshua Chai Medical House Officer
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