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I am thinking the more reasons why sitting much lead to thrombosis i.e. clotting of blood in blood vessels and then hypoxia in the deep tissues eventually.

What are the physiological mechanisms for this to happen?

One student said hormones. It can be right, but I am thinking how.

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Venous return from the limbs is largely facilitated by the action of neighbouring skeletal muscles compressing the veins and forcing the blood proximally past the valves which prevent back flow. If you are inactive for a long time by sitting, or are sitting in such a way as to impede blood returning from your calves (i.e. with the back of your knees pressing into the edge of the chair) blood movement in this manner is greatly slowed.

Blood that is moving more slowly or turbulently has a greater propensity to coagulate and form a thrombus. This is similar to the rationale for prescribing anti coagulants to patients with atrial fibrillation - the turbulent and slow movement of blood through their atria leaves them at greater risk of throwing a clot to the brain causing a cerebrovascular accident.

Hypoxia isn't really that relevant in the case you describe as arterial supply is unaffected (much higher pressure) meaning tissues do not become ischaemic. There is also collateral venous drainage from the limb although this can subsequently cause problems in the form of post thrombotic syndrome. The most significant risk would be that of the thrombus breaking off and lodging in the lungs - a pulmonary embolism.

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I think the oxygen supply is deprived in some tissues too. So therefore I think some local hypoxia can occur. –  Masi Feb 8 at 8:32
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@Masi one hypothesis for the original formation of the VTE is that venous stasis causes the haemoglobin in the static blood to desaturate resulting in hypoxic insult to the vein endothelium that then triggers the coagulation cascade. However, there isn't much of an evidence base behind this and it would be a cause of VTE rather than a consequence. –  Rory M Feb 9 at 16:31
    
Thank you very much for your addition! –  Masi Feb 9 at 17:00
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