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 USMLE Step 1 Forum USMLE Step 1 Discussion Forum: Let's talk about anything related to USMLE Step 1 exam

#1
05-22-2015
 USMLE Forums Scout Steps History: Not yet Posts: 15 Threads: 7 Thanked 1 Time in 1 Post Reputation: 11
Help in physiology

Hi
Could u explain this for me ?
Driving pressure = Q* R
What do we mean by Q ( blood flow ) ?

Also in total R in paralel , how the pressure is the same between all vesseles ??

#2
05-22-2015
 USMLE Forums Scout Steps History: Not yet Posts: 84 Threads: 1 Thanked 49 Times in 35 Posts Reputation: 59

Quote:
 Originally Posted by Sana ri Hi Could u explain this for me ? Driving pressure = Q* R What do we mean by Q ( blood flow ) ? Also in total R in paralel , how the pressure is the same between all vesseles ??
Q is the volume of blood that passes a certain area in the cardiovascular system per unit time. For example, renal blood flow would be volume of blood (milliliters, liters, gallons) going through the kidney per unit time (minute, hour, day). Cardiac output is also an example of flow, since it is the amount of blood passing through the left ventricle (or the right ventricle, does not matter) per unit time (usually milliliters of blood per minute). It is like if you open your faucet and let it run for one minute, collect all the water that comes out of it and measure it. Let's say you measured 5 liters, then the flow through your faucet would be 5L/min.

Regarding the second question. I think the important thing here is to compare it to the situation where resistors are in series. When resistors are in series pressure in the resistor downstream is determined entirely by the resistor upstream. Since pressure is directly related to fluid volume and fluid volume is directly related to flow, if we constrict the first resistor (increasing the resistance), we decrease the flow, so less volume is going toward the second resistor, decreasing its pressure. When you vasoconstrict the arterioles, upstream pressure increases (because there is less flow through the arterioles and the fluid "builds up" behind it) and downstream pressure decreases (because there is less flow to the other side of the arteriole decreasing volume of fluid).

Pressure in the parallel resistors in harder to explain but I will try. You just have to use your imagination a little bit. Pressure is directly related to volume so more volume equals more pressure. When there is blood in the aorta it distributes among all the major arteries and let's say it stops at the level of arterioles because they are completely closed. In this situation, blood has filled all the major arteries and generated the same pressure everywhere. Now why the same pressure. In our hypothetical example arterioles are completely closed so the there is no blood flowing through them, meaning that the blood flow has stopped, right? Now if there were a pressure gradient between the aorta and the major arteries the blood flow would not have stopped, it would have flown down the pressure gradient. Since the flow stopped, pressure must be equal throughout. The system will always try to equilibrate, solutes are going to diffuse to maintain equal concentration everywhere, fluids are going to distribute in a way that makes pressure same everywhere. Again you just have to use your imagination a little bit. I could probably explain better if I drew it all out. If you try to imagine all that in your head it will start making sense. The first site of significant resistance is the arterioles and thats where the biggest pressure drop happens due to markedly reduced flow. So before the arterioles (resistors) there is almost no resistance to flow and the pressure is same throughout.
 The above post was thanked by: blueboy3056 (05-22-2015), CleverFOX (05-22-2015), Sana ri (05-23-2015)
#3
05-23-2015
 USMLE Forums Scout Steps History: Not yet Posts: 15 Threads: 7 Thanked 1 Time in 1 Post Reputation: 11

Quote:
 Originally Posted by Nodo Q is the volume of blood that passes a certain area in the cardiovascular system per unit time. For example, renal blood flow would be volume of blood (milliliters, liters, gallons) going through the kidney per unit time (minute, hour, day). Cardiac output is also an example of flow, since it is the amount of blood passing through the left ventricle (or the right ventricle, does not matter) per unit time (usually milliliters of blood per minute). It is like if you open your faucet and let it run for one minute, collect all the water that comes out of it and measure it. Let's say you measured 5 liters, then the flow through your faucet would be 5L/min. Regarding the second question. I think the important thing here is to compare it to the situation where resistors are in series. When resistors are in series pressure in the resistor downstream is determined entirely by the resistor upstream. Since pressure is directly related to fluid volume and fluid volume is directly related to flow, if we constrict the first resistor (increasing the resistance), we decrease the flow, so less volume is going toward the second resistor, decreasing its pressure. When you vasoconstrict the arterioles, upstream pressure increases (because there is less flow through the arterioles and the fluid "builds up" behind it) and downstream pressure decreases (because there is less flow to the other side of the arteriole decreasing volume of fluid). Pressure in the parallel resistors in harder to explain but I will try. You just have to use your imagination a little bit. Pressure is directly related to volume so more volume equals more pressure. When there is blood in the aorta it distributes among all the major arteries and let's say it stops at the level of arterioles because they are completely closed. In this situation, blood has filled all the major arteries and generated the same pressure everywhere. Now why the same pressure. In our hypothetical example arterioles are completely closed so the there is no blood flowing through them, meaning that the blood flow has stopped, right? Now if there were a pressure gradient between the aorta and the major arteries the blood flow would not have stopped, it would have flown down the pressure gradient. Since the flow stopped, pressure must be equal throughout. The system will always try to equilibrate, solutes are going to diffuse to maintain equal concentration everywhere, fluids are going to distribute in a way that makes pressure same everywhere. Again you just have to use your imagination a little bit. I could probably explain better if I drew it all out. If you try to imagine all that in your head it will start making sense. The first site of significant resistance is the arterioles and thats where the biggest pressure drop happens due to markedly reduced flow. So before the arterioles (resistors) there is almost no resistance to flow and the pressure is same throughout.
Thank u very much
I found myself weak in basic physiology , any advise ??
#4
05-23-2015
 USMLE Forums Scout Steps History: Not yet Posts: 84 Threads: 1 Thanked 49 Times in 35 Posts Reputation: 59

Quote:
 Originally Posted by Sana ri Thank u very much I found myself weak in basic physiology , any advise ??
What I do for physiology are Kaplan video lectures for step 1 and Costanzo's physio textbook. Kaplan gives me basic ideas for what I should expect, gives good step 1 advice, does a lot of integration (great value in Kaplan course I believe) and Costanzo develops those fundamental ideas and concepts. I am planning to take step 1 in 18 months so I still have time to work with moderate size textbooks. I highly recommend Costanzo, amazing resource. Very well written, very concise, gets to the point very quickly, very easy to read. BRS Physiology is also written by Costanzo and it works great along with the textbook. I am in my second year of a six year medical program at my university and I have the luxury to spend a good amount of time understanding physiology (understanding being the key word).

Depending on when you are planning to take the exam I would advise a different approach. If you are taking step 1 in the next few months I would not use a textbook, I would stick to Kaplan (or maybe Becker) video lectures (the classroom anywhere live lectures). If you have at least a year and you feel that you are weak in physio I would recommend going through Costanzo very thoroughly. It might not be super high yield but it is worth investing your time in. You will develop understanding of concepts and you will have to do less memorization. Even if your exam is in a few months but you are pretty strong in other subjects, still take a look at the textbook. I know people on here usually do not like textbooks (myself included) but Costanzo is just a great resource.

Hope that helps.
 The above post was thanked by: Sana ri (05-23-2015)
#5
05-23-2015
 USMLE Forums Scout Steps History: Not yet Posts: 15 Threads: 7 Thanked 1 Time in 1 Post Reputation: 11

Quote:
 Originally Posted by Nodo What I do for physiology are Kaplan video lectures for step 1 and Costanzo's physio textbook. Kaplan gives me basic ideas for what I should expect, gives good step 1 advice, does a lot of integration (great value in Kaplan course I believe) and Costanzo develops those fundamental ideas and concepts. I am planning to take step 1 in 18 months so I still have time to work with moderate size textbooks. I highly recommend Costanzo, amazing resource. Very well written, very concise, gets to the point very quickly, very easy to read. BRS Physiology is also written by Costanzo and it works great along with the textbook. I am in my second year of a six year medical program at my university and I have the luxury to spend a good amount of time understanding physiology (understanding being the key word). Depending on when you are planning to take the exam I would advise a different approach. If you are taking step 1 in the next few months I would not use a textbook, I would stick to Kaplan (or maybe Becker) video lectures (the classroom anywhere live lectures). If you have at least a year and you feel that you are weak in physio I would recommend going through Costanzo very thoroughly. It might not be super high yield but it is worth investing your time in. You will develop understanding of concepts and you will have to do less memorization. Even if your exam is in a few months but you are pretty strong in other subjects, still take a look at the textbook. I know people on here usually do not like textbooks (myself included) but Costanzo is just a great resource. Hope that helps.
z

Thanks , hope to u the best of the best
Im reading from BRS rightnow , planning to make step 1 next year
Can i found kapaln v online ? Is it nessesary to get kapaln book also ?
#6
05-23-2015
 USMLE Forums Scout Steps History: Not yet Posts: 84 Threads: 1 Thanked 49 Times in 35 Posts Reputation: 59

Quote:
 Originally Posted by Sana ri z Thanks , hope to u the best of the best Im reading from BRS rightnow , planning to make step 1 next year Can i found kapaln v online ? Is it nessesary to get kapaln book also ?
BRS is good but nothing beats Costanzo.

I found them online a while back and downloaded them but I can not find them anymore. Post in the "For Sale and Wanted Items" forum and maybe somebody has a working link. You do not need the books in my opinion.
#7
05-23-2015
 USMLE Forums Scout Steps History: Step 1 Only Posts: 99 Threads: 2 Thanked 238 Times in 39 Posts Reputation: 248

Quote:
 Originally Posted by Nodo BRS is good but nothing beats Costanzo. I found them online a while back and downloaded them but I can not find them anymore. Post in the "For Sale and Wanted Items" forum and maybe somebody has a working link. You do not need the books in my opinion.
constanzo wrote the brs physio book so all the high yield info is in there
 The above post was thanked by: dukecool44 (05-23-2015), Sf2008 (05-23-2015)
#8
05-23-2015
 USMLE Forums Scout Steps History: Not yet Posts: 84 Threads: 1 Thanked 49 Times in 35 Posts Reputation: 59

Quote:
 Originally Posted by blueboy3056 constanzo wrote the brs physio book so all the high yield info is in there
I agree. But I think that the format of BRS does not make it the best choice for the first exposure to the subject. Great for review, not good for learning.

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