Naked Science Forum
Life Sciences => Physiology & Medicine => Topic started by: Chivalrous on 23/02/2015 19:22:29
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Hi all!
My question is related to "Frank-Starling law", which can be illustrated by the following graph:
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fs6.postimg.org%2F9usp2x12p%2FLenth_tension.jpg&hash=8367e8d042e83565a58250a09476379f)
( image link: s6.postimg.org/9usp2x12p/Lenth_tension.jpg )
I've read that:
- When the heart is over stretched, it will not be able to contract, there will be cardiac standstill. This occurs at the point of meeting of the 2 curves of active and passive tension, this doesn't occur except at time of death.
- the previously mentioned point happens when passive tension = active tension, where the heart can not contract. ( i.e. death )
My Questions:
- Can you clarify the difference between active and passive tensions ?
- Why when the passive tension = active tension, the heart will not contract ?
- What does the "the point of meeting of the 2 curves" mean ?
Finally:
Thanks in advance for any help which I'd greatly appreciate.
I hope I've explained my question well.
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My Questions:
- Can you clarify the difference between active and passive tensions ?
- Why when the passive tension = active tension, the heart will not contract ?
- What would the graph be like if that occurs ? ( i.e. What does the "the point of meeting of the 2 curves" mean or look like ? )
Hadn't come across this graph before.
As I understand it Frank-Starling law is about an automatic system where the heart muscle stretches as it fills with blood and then contracts to force the blood out. I say automatic because the heart can do this outside the body without any connection to the brain. I'll try to answer the other questions, but you will need to search or ask others for more info.
- Active stretch is when the muscle is being stretched is in an active state. An example is walking where the muscle resists and controls the stretching phase as the leg extends. If not the leg muscle would stretch out of control and be damaged.
Passive stretch is when the muscle is not being activated by nerve impulses but is being stretched by an external force. A bit like when you touch your toes to stretch the hamstrings.
- When active and passive force are equal they balance each other out and so there is no motion. If active is greater they will contract, if passive greater they will stretch. Although this balance might happen at point of death in the heart, I would be surprised if it was the only time the balance happened.
- the graph shows total tension vs volume during a normal heart cycle, i asume active vs pasive might look similar and meet at 180/70. I suggest you search the net, perhaps look at blood pressure mesurements.
Heart muscle can be overstretched in congestive heart failure and become weak. It might be worth searching for heart disease or overstretching injuries.
Sorry, not a lot of help but might get you started until others reply.
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Colin2B,
I'm so glad to receive your reply.
- First of all, thanks a lot, you have clarified it a bit for me. :)
- Concerning:
Hadn't come across this graph before.
I've modified the topic and made my questions in bold.
- Concerning my first and second questions: ( i.e. the difference between the passive and active tensions)
I've understood from some articles and videos in addition to your reply that:
The passive tension is the tension produced when the muscle is being stretched ( i.e. the desire of the muscle to oppose the stretching, if it's being stretched by an external force ), while the active tension is the tension produced when the muscle is being contracted ( i.e. the movement of actin along myosin filaments, which produces a tension or pressure opposing the contraction, depending on the initial length of the muscle fiber). If that is right, Hence when the passive tension = the active one, they will balance or cancel each other, as both tensions oppose each other, as you have mentioned before. I hope that that's right, isn't it?
- And concerning my third question: I think you had already answered it perfectly. ( i.e. the 2 curves never meet each other -which means that they never become equal as illustrated in the graph- except at Cardiac standstill.
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Colin2B,
I'm so glad to receive your reply.
- First of all, thanks a lot, you have clarified it a bit for me. :)
Glad to hear that, I was concerned that because my reply wasn't exactly what you were asking for, it might have been unhelpful.
Not sure about the conclusion that the 2curves never meet. We might have something in textbook, will have a look and get back if I find it.
Hope someone more knowledgeable comes along
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And concerning my third question: I think you had already answered it perfectly. ( i.e. the 2 curves never meet each other -which means that they never become equal as illustrated in the graph- except at Cardiac standstill.
Hi Chivalrous
can't find anything on this so in absence of other replies I'll give some thought that might help your thinking.
The graph you have shows total tension on the top curve and passive on the lower one. If we assume there are only active and passive tensions operating then the active pressure curve can be drawn by subtracting the passive from total.
Doing this by eye suggests that these 2 curves will cross during normal heart cycle. As I said before I don't have a problem with this. If you think about it, the heart will fill while active tension is relaxed and at some point will contract - active tension - and the active tension will increase until it overcomes the passive tension. Active tension will then continue increasing until the contraction has finished.
If we go back to eg of congestive heart failure, the heart is so weak it cannot overcome the passive tension and will stop. The first point at which this will occur is when active = passive.
These thoughts are not derived from knowledge but just thinking through how the system might work, so don't take them as gospel. However, I hope they might help your search until you find a full answer.
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Colin2B,
Sorry for my late reply.
I'd like to thank you a lot for your replies and efforts to help.
I've also found some good answers, I hope others can also benefit from it.
http://www.thescienceforum.com/biology/48523-question-concerning-cardiovascular-system-frank-starling-law.html (http://www.thescienceforum.com/biology/48523-question-concerning-cardiovascular-system-frank-starling-law.html)
(Sorry for cross-posting. I really needed a fast reply, and you were the first to help. Thanks.)