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I read that blood clot reduces blood flow from few website and from doing an A level biology question; and if this blood clot is formed in pulmonary thrombosis, this can reduce gas exchange in lung.

However, how are we so certain this is true? If there is a blood clot, then blood pressure around that region increases I believe thus implying the blood will travel faster in that region? If so, then we can't firmly conclude blood clot will reduce blood flow as although blood clot will reduce the lumen of the vessel, the blood will also be travelling faster.

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A blood clot can nearly completely or completely block an artery, in which case the blood flow will be reduced or stopped.

How do we know this?

Pulmonary Embolism (Merck Manuals):

Pulmonary infarction is when some of the lung tissue does not receive enough blood flow and oxygen and appears on imaging studies to die due to blockage of a lung blood vessel by a pulmonary embolus.

An embolus is a blood clot that usually develops in the leg veins in individuals with deep venous thrombosis, detaches and travels to a certain pulmonary artery and blocks it.

Similarly, atheroma (atherosclerotic plaque) that builds up within the artery and only partially blocks it, can reduce the blood flow to the target organ. For example, a partial blockage of a coronary artery can result in decreased blood supply to the heart and consequently in angina pectoris or myocardial infarction. It is then the investigation called coronary angiography that can show that a certain coronary artery is partially blocked.

When a clot interferes with blood flow (Harvard.edu):

In venous thromboembolism, a blood clot slows or stops the flow of blood through the veins...

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  • $\begingroup$ Thank you for the answer, but I'm not sure if your last paragraph answers the question as I understand that decrease in cross sectional area can potentially decrease the flow rate (as there is a smaller cross section for blood to travel through) but this decrease can be compensated by an increase in fluid velocity thus the end result is that flow rate is not changed - this is the idea behind the continuity equation in physics. But I get your argument in the first few paragraph. $\endgroup$ – Bøbby Leung Jan 17 at 4:34
  • $\begingroup$ I deleted the last paragraph and added one new source. Also, look at the first image in this coronary angiography article and you'll see that a part of the coronary artery is narrowed, not completely blocked, so the flow of the contrast substance is not discontinued. But that narrowing results in decreased blood flow and hence in angina pectoris or myocardial infarction. So, it is a result from which you can conclude that the blood flow is decreased. $\endgroup$ – Jan Jan 17 at 9:23

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