Re: Why do arteries get clogged more often than veins?

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