questions in EK 1001 Question in biology

[batista_123]

#647. i really dont understand what table 1 is talking about. these passages just throw tables at you without explaining. i hope the real mcat isnt like that. but i have a feeling it is.
#710. i think the answer could also be C, no? the back says "a DECREASE in pulmonary pressure or systemic pressure is caused by an increase in the resistance in blood flow.
i thought increased resistance increases pressure??

---

Members don't see this ad.

 

[JohnnytheNomad]

#647. i really dont understand what table 1 is talking about. these passages just throw tables at you without explaining. i hope the real mcat isnt like that. but i have a feeling it is.
#710. i think the answer could also be C, no? the back says "a DECREASE in pulmonary pressure of systemic pressure is caused by an increase in the resistance in blood flow.
i thought increased resistance increases pressure??

I don't know if you quoted the answer correctly, but an increase in viscosity of system blood flow or an increase in resistance (via vaso constriction) will cause pulmonary pressure to be low because the blood is caught in the systemic tissue. I might be wrong so you should ask the EK webline

 

[rocketbooster]

I don't know if you quoted the answer correctly, but an increase in viscosity of system blood flow or an increase in resistance (via vaso constriction) will cause pulmonary pressure to be low because the blood is caught in the systemic tissue. I might be wrong so you should ask the EK webline

or you can just think of it in terms of physics as a fluid not at rest using constant volumetric flow and Bernoulli's equation.

volumetric flow = cross-sectional area (A) * velocity (v). VF is constant, so as you decrease the A of the artery due to vasoconstriction or increased resistance (due to a blockage, viscosity, whatever), you increase the velocity of the fluid.

based on Bernoulli's equation, constant= P + 0.5(density)v^2 + (density)gh, as velocity increases, pressure decreases.

voila, pulmonary pressure decreases.

 

[batista_123]

or you can just think of it in terms of physics as a fluid not at rest using constant volumetric flow and Bernoulli's equation.

volumetric flow = cross-sectional area (A) * velocity (v). VF is constant, so as you decrease the A of the artery due to vasoconstriction or increased resistance (due to a blockage, viscosity, whatever), you increase the velocity of the fluid.

based on Bernoulli's equation, constant= P + 0.5(density)v^2 + (density)gh, as velocity increases, pressure decreases.

voila, pulmonary pressure decreases.

I agree with you, since A*V is constant, if A goes down, V goes up, and pressure goes down. but wikipedia says if area goes down, you have more resistance, therefore pressure goes up:

"Resistance. In the circulatory system, this is the resistance of the blood vessels. The higher the resistance, the higher the arterial pressure upstream from the resistance to blood flow. Resistance is related to vessel radius (the larger the radius, the lower the resistance), vessel length (the longer the vessel, the higher the resistance), as well as the smoothness of the blood vessel walls. Smoothness is reduced by the build up of fatty deposits on the arterial walls. Substances called vasoconstrictors can reduce the size of blood vessels, thereby increasing blood pressure. Vasodilators (such as nitroglycerin) increase the size of blood vessels, thereby decreasing arterial pressure. Resistance, and its relation to volumetric flow rate (Q) and pressure difference between the two ends of a vessel are described by Poiseuille's Law."

PS i dont really understand how Poiseuille's Law is related to all this??

 

[0Complications]

I agree with you, since A*V is constant, if A goes down, V goes up, and pressure goes down. but wikipedia says if area goes down, you have more resistance, therefore pressure goes up:

"Resistance. In the circulatory system, this is the resistance of the blood vessels. The higher the resistance, the higher the arterial pressure upstream from the resistance to blood flow. Resistance is related to vessel radius (the larger the radius, the lower the resistance), vessel length (the longer the vessel, the higher the resistance), as well as the smoothness of the blood vessel walls. Smoothness is reduced by the build up of fatty deposits on the arterial walls. Substances called vasoconstrictors can reduce the size of blood vessels, thereby increasing blood pressure. Vasodilators (such as nitroglycerin) increase the size of blood vessels, thereby decreasing arterial pressure. Resistance, and its relation to volumetric flow rate (Q) and pressure difference between the two ends of a vessel are described by Poiseuille's Law."

PS i dont really understand how Poiseuille's Law is related to all this??

If you decrease the radius you DO increase the resistance/pressure. Think about it conceptually. If you increase the radius there is going to be a larger volume of fluid that does not "rub" against the walls of the lumen. This fluid is going to be less impeded or experience less resistance from the wall than fluid flowing next to the wall of the lumen. So, as your radius increases the resistance decreases, overall per your volume of fluid.

 

[batista_123]

If you decrease the radius you DO increase the resistance/pressure. Think about it conceptually. If you increase the radius there is going to be a larger volume of fluid that does not "rub" against the walls of the lumen. This fluid is going to be less impeded or experience less resistance from the wall than fluid flowing next to the wall of the lumen. So, as your radius increases the resistance decreases, overall per your volume of fluid.

i agree with what you say, i just need an explanation for this contradiction:
Q=AV
Q is constant
if area decreases, velocity increases, which means pressure DECREASES
if area decreases, resistance increases, which means pressure INCREASES

 

[rocketbooster]

look below

 

[rocketbooster]

i agree with what you say, i just need an explanation for this contradiction:
Q=AV
Q is constant
if area decreases, velocity increases, which means pressure DECREASES
if area decreases, resistance increases, which means pressure INCREASES

Okay, I figured it out...I didn't put it in terms of resistance, but I point out when pressure increases or decreases. the Q=AV doesn't give the pressure relationship, Bernoulli's equation does. basically, the resistance concept doesn't apply when looking at one specific site of a vessel. when the resistance does apply, Bernoulli's equation does not work because that equation is based on an ideal fluid, which has 0 resistance. just read below.

Based on Bernoulli's equation and volumetricflowrate=Av, velocity increases, cross-sectional area decreases, and pressure decreases. This can be applied only in one specific region of a vessel. In a question that compares a health artery to a nonhealthy artery with a blockage due to atherosclerosis, you use the Bernoulli relationship between pressure, velocity, and CX-area when looking at the exact region of the 2 arteries. So, the blockage decreases CX-area, velocity goes up, and blood pressure decreases AT the site of the blockage; however, systemic blood pressure increases because the heart is working to pump out more blood volume in order to maintain the volumetric flow rate.

When it comes to comparing different areas of a vessels to one another, you cannot use Bernoulli's equation. In this case, pressure and CS-area do not both increase or decrease. Compare an artery to the capillaries. Based on Bernoulli's equation, the capillaries have a larger surface area, thus they should have higher blood pressure. We know this is not true. Capillaries have the least amount of blood pressure of any blood vessel. Therefore, we CANNOT apply Bernoulli's equation when comparing different areas of vessels. This is because Bernoullli's equation is based on an ideal fluid flow, and blood is not an ideal fluid. HOWEVER, the constant volumetric flow rate relationship with CX-area and velocity does still apply. Because volumetric flow rate is constant, capillaries have larger surface area thus should have a slower velocity, which is true. Blood moves the slowest in the capillaries compared to any other vessel.

When it comes to vasoconstriciton, CX-area decreases and blood pressure increases. This also means Bernoulli's equation cannot be applied with vasoconstriction and vasodilation. You can only apply it in blood vessels when you are looking at one specific region of a vessel. Constantvolumetricflowrate=Av always applies, to my knowledge.

Now, if the passage tells you to assume blood is an ideal fluid, then you do apply Bernoulli's equation.

Does this help?

 

[batista_123]

Does this help?

yes, thank you

 

[SBR249]

The way I always thought of it is that as blood vessels constrict, resistance obviously goes up. However, the stroke volume of the heart is relatively constant in a person. That means the heart now has to pump the same amount of blood against greater resistance, which means the heart has to use more force which leads to greater pressure.

Since I'm an engineer, I can also think of it using a circuit analogy and Ohm's law. If the heart is the voltage source (blood pressure = voltage), the blood vessels the circuit with a resistance, and the blood flow the current, then if we assume constant current and an increase in resistance, the voltage would also need to increase.

Edit: Poiseuille's Law models the fully developed flow of a newtonian fluid (constant viscosity) through a tube (or channel) under a constant pressure gradient and is used as a simple model of in vivo arterial blood flow. It's better than Bernoulli's equation at predicting local velocity profiles because it doesn't assume idealities like no viscosity.