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.


2 Answers 2


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...

  • $\begingroup$ 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. $\endgroup$ Jan 17, 2019 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, 2019 at 9:23
  • 1
    $\begingroup$ @BøbbyLeung This answer is correct. It looks like, though, you're looking for a physicsy physiology answer, though. I've supplemented this answer with some of the physiology in mine. $\endgroup$
    – De Novo
    Mar 2, 2019 at 19:50

Given the context of your comment on @Jan's answer above, it looks like you're asking if the continuity equation, $\rho_1A_1v_1 = \rho_2A_2v_2$, applies to blood flow through a local vessel. The answer is that it doesn't, because the local vessel is not a closed system.

The continuity equation is derived from the principle of conservation of mass (see the earlier link) and requires that everything coming in at position 1 exit at position 2. Here, a blood clot, or specifically in your example, a pulmonary embolism, increases the resistance in one of a number of vessels in parallel. Blood flow is diverted to the parallel vessels with lower resistance, the ones without the clot. This is useful in a helpful clot at the site of injury as well as an unhelpful clot like a pulmonary embolism. Thankfully, a clot in an injured vessel has the ability to slow and finally stop the loss of blood. Again, this is allowed despite the continuity equation because there is an alternative path. Blood can either flow out of the vessel at the site of damage, or flow through the vessel.

The fact that changes in vascular resistance are met with changes in flow is a principle used to beneficial effect in normal physiology, not just the response to a clot. Arterioles, for example, regulate flow through vascular beds as needed, by increasing or decreasing resistance.

There is a case, a saddle embolus, where there is no parallel path. Here, though, flow still decreases because the pump fails (and pressure is lost). The rapid increase in resistance cannot be compensated for by the heart, and sudden death results.

Generally, when applying fluid dynamic principles to blood flow and the circulatory system, you have to consider whether the assumptions hold. Generally, you can apply the continuity equation to portions of the circulatory system in series (e.g., the cardiac output of the right heart has to equal the cardiac output of the left heart), but there are still caveats (vessel walls are not rigid -- they have a capacitance).

These principles are discussed in Costanzo Physiology Ch. 4.

  • $\begingroup$ Thank you very much for the detailed answer. If you have time, could you please elaborate on why is the local vessel not a closed system? A sketch of it would be helpful. $\endgroup$ Mar 3, 2019 at 4:15

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