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Does anyone know? I'm curious to find out as my human anatomy and physiology book doesn't go into much detail on how the lungs function as such

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2 Answers 2

I'll ask the lung experts I work with later today for a better answer. To my knowledge, the lungs, as an organ, have a very extensive capillary network that facilitates gas exchange. If blood clots become mobile in the blood stream, they can travel perfectly well in large vessels, but once they get down to a capillary, they are physically blocked from passing through. The blood clots don't have to be large in size, because the capillaries are only about 6 micrometres in diameter.

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But if a blot clot gets stuck in a small capillary, won't it get occluded and prevent blood flow? I know that the lungs have a mechanism to prevent ventilation-perfusion inequality, and if a blood clot did get stuck, there would be bronchioconstriction of the airway leading to that alveolus, but I don't see how just getting stuck in a capillary makes the lung a sieve for blood clots. I do appreciate you asking the lung experts. Thank you for answering! –  Halime Mar 15 '12 at 18:31
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Just as an intro...

The heart pumps deoxygenated blood from the right ventricle, through the pulmonary arteries (pic) which then eventually split into small capillary networks that surround the alveoli. The alveoli are formed by the trachea eventually branching off. So when you breathe in, the alveoli become filled with higher levels of oxygen.

The blood then becomes oxygenated and returns to the heart via the pulmonary veins to be pumped to the rest of the body.

The deoxygenated blood becomes oxygenated because there is a difference in oxygen and carbon dioxide concentration between the capillary network (O2 low / CO2 high) and the alveoli (O2 high / CO2 low) and so gas diffuses across because of the difference in concentrations (pic).

Lung Anatomy

Physiology textbooks explain these mechanisms but for blood clots in particular, you'll need to check out a pathophysiology book.

Blood clots

When a blood clot travels to the lungs, it's referred to as a pulmonary embolus or PE.

Most of the time (90%), pulmonary emboli are formed in deep veins of the lower leg. These then travel to the pulmonary circulation system.

Large emboli block larger vessels - the pulmonary arteries and their branches. The smaller ones travel further into the network.

Patients can sometimes be asymptomatic and the emboli can at times resolve on its own. The extent of the severity of a PE is determined by:

  • how much blood flow is obstructed;
  • how long the embolus has been there; and
  • presence other underlying lung or heart disease.

There are a number of things that a PE can cause physiologically.

Reduced Gas Exchange - alveolar dead space occurs when an alveolus is ventilated, but not perfused with blood. This, along with other factors, cause varying levels of hypoxemia (lack of oxygen).

Pulmonary Infarction - in a small amount of cases loss of blood flow to lung tissue can cause tissue death. This is fairly uncommon.

Right Ventricular Failure - if there is a large enough blockage (> 50 - 60%), the pressure in the pulmonary arteries increase. Subsequently, the load on the right ventricle is higher. So in acute cases, the ventricle hasn't had time to adapt (hypertrophy) and so the right heart can fail.

At any rate, the haemodynamic balance can be disrupted when a PE is present.

In the elderly or people with underlying disease where their lung function is already decreased, PEs can have a significantly larger effect because they can't compensate. There are many more important points related to this but hopefully this is a useful (very) basic overview.

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The possible advantage of this is that if the embolism being trapped in the lungs is that it does not continue on through the left heart and into the systemic circulation, where it could cause a stroke, acute myocardial infarction (heart attack) or infarction of other vital organs. Of course, if the pulmonary embolism affects a large portion of the the lung then this advantage is entirely negated. –  Rory M Feb 9 '13 at 11:45
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