Is this AAMC bio question misleading or do I confuse myself?

[mikexima]

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Capillaries in the kidney and elsewhere in the body maintain fluid homeostatis by balancing hydrostatic and osmotic pressures. Which of the following is the initial effect of blood clot forming on the venuos side of a capillary bed?

My immediate thought process was that two things happen:

A) solutes that participate in formation of clot rush into the blood stream (fibrinogen, firbin, thrombin etc). Thus solute concentration in blood increase and as a result water rushes into the blood.

B) Clot interfers with capillary flow rate, blood backs up and hydrostatic pressure increase, water rushes out.

so which one do you go with? obviously since the question asked for initial effect i chose the answer that says "capillary osmotic pressure will decrease". but this answer is wrong.... correct answer is that net fluid flow out of the capitally will increase because of the clot formation. and guess what this translates to, because water flows out of blood, the osmotic pressure DECREASES, completly the opposite of what i concluded.

how do you go about a question like this on the test? what am I missing 🙁

 

[tendiw]

Capillary osmotic pressure should increase. When the blood clot forms, the blockage increases hydrostatic pressure. To reduce hydrostatic pressure fluid MUST be removed from the capillary. Hydrostatic pressure increases with fluid amount, to decrease it, fluid must be removed.

As water flows out of the blood to reduce hydrostatic pressure, osmotic pressure INCREASES in the capillary. Remember that more Particles and less water means more osmotic pressure. If the water/fluid decreases you get more osmotic pressure.

The only exception is in a clot in the brain where blood pressure increases in order to push the clot out

Finally, factors that participate in blood clotting are ALREADY in the blood stream. They dont rush in as they are usually present. In fact the reason you got the question wrong was because you over-thought it. The MCAT tests basic understanding. Keep it simple...usually that's the way to go for the sciences. There are maybe only one or two "deep" questions

 

[mikexima]

Capillary osmotic pressure should increase. When the blood clot forms, the blockage increases hydrostatic pressure. To reduce hydrostatic pressure fluid MUST be removed from the capillary. Hydrostatic pressure increases with fluid amount, to decrease it, fluid must be removed.

As water flows out of the blood to reduce hydrostatic pressure, osmotic pressure INCREASES in the capillary. Remember that more Particles and less water means more osmotic pressure. If the water/fluid decreases you get more osmotic pressure.

The only exception is in a clot in the brain where blood pressure increases in order to push the clot out

Finally, factors that participate in blood clotting are ALREADY in the blood stream. They dont rush in as they are usually present. In fact the reason you got the question wrong was because you over-thought it. The MCAT tests basic understanding. Keep it simple...usually that's the way to go for the sciences. There are maybe only one or two "deep" questions

thanks for the explaination. I had osmotic pressure backwards.

so you're saying the factors are already in the plasma? that makes more sense, it would take a long time to be synthesized then secreted. I should have paid attention to how these factors are zymogens that are activated. duh...... this was the source of my confusion and wasting time/energy

 

[tendiw]

Yes the body keeps various levels of different immune and clotting factors in the blood for quick responses. That why if you cut yourself a scab forms only after a few minutes. As for activation of the factors such as platelets, they aren't already activated in the blood that's why scabs don't form in your blood, there are systems that prevent them from coagulating but when your skin is damaged, the platelets are activated at the point of damage (I believe through contact with the area). But platelets are already in the blood if you get my point. Same reason you react so fast to a mosquito bite is that white blood cells are already in the blood and the full inflammatory event occurs during infection (an you get nasty bumps lol).

For the MCAT atleast just think of zymogens when dealing with the digestive system..so pepsinogen, trysinogen, and chymotrypsinogen activation.

 

[mikexima]

Yes the body keeps various levels of different immune and clotting factors in the blood for quick responses. That why if you cut yourself a scab forms only after a few minutes. As for activation of the factors such as platelets, they aren't already activated in the blood that's why scabs don't form in your blood, there are systems that prevent them from coagulating but when your skin is damaged, the platelets are activated at the point of damage (I believe through contact with the area). But platelets are already in the blood if you get my point. Same reason you react so fast to a mosquito bite is that white blood cells are already in the blood and the full inflammatory event occurs during infection (an you get nasty bumps lol).

For the MCAT atleast just think of zymogens when dealing with the digestive system..so pepsinogen, trysinogen, and chymotrypsinogen activation.

well i meant that they are present as zymogens then they are activated when needed. why just limited to digestive? i was under the impression that anything with "ogen" is a zymogen, so fibrinogen involved in clot is inactive 😀

 

[tendiw]

well i meant that they are present as zymogens then they are activated when needed. why just limited to digestive? i was under the impression that anything with "ogen" is a zymogen, so fibrinogen involved in clot is inactive 😀

Yes fibrinogen is a zymogen that get converted to fibrin when platelets are needed. You are correct with that statement. It will just create less confusion if you look at zymogens as digestive stuff for the MCAT. But keep in mind zymogens appear anywhere, for the MCAT they are mostly digestive enzymes.

 

[mikexima]

hmmm, how do you know that? i've read some of your posts on other threads and they sound intelligent, are you an mcat writer? 😎

 

[Morsetlis]

This is a fluid dynamics question, not a "zymogen" and "blood clotting cascade" question.

 

[Classof2013PreM]

yeah, its really just that hydrostatic pressure increases when the blot clot blocks the capillary, resulting in net fluid flow out of the capillary.....

simple enough.

 

[tendiw]

hmmm, how do you know that? i've read some of your posts on other threads and they sound intelligent, are you an mcat writer? 😎

No I am not an MCAT writer, just helping people out on the forums and the reason why you should know what is tested on the MCAT is they tell you what is tested and then further tell you only basic (first year level) courses are required for the MCAT. From what the MCAT tests, if you go through their list and first year courses, even some upper year courses, you will find that, the digestive zymogens from the pancreas are most likely to be tested. For example with fibrinogen, I would guess that any question on it would have to do with simply know it its a zymogen from its name ending in "ogen" or it is an important factor in clotting. Even in the 1001 Examkrackers books it is only tested as a key component in clotting along with thrombin.

Just look through the AAMC checklist on what is tested and you'll be fine🙂

This is a fluid dynamics question, not a "zymogen" and "blood clotting cascade" question.

yes but he asked about other stuff too. I provided him with an answer for his initial question.