If you're seeing this message, it means we're having trouble loading external resources on our website.

如果你被网页过滤器挡住,请确保域名*.kastatic.org*.kasandbox.org 没有被阻止.



走进神秘的身体第二大循环系统.学习它输送体液的动力.讲解-帕特里克. . Patrick van Nieuwenhuizen 创建




the heart pumps blood throughout the body and it pumps so powerfully that it actually squeezes some fluid out of the capillaries and that fluid becomes lymph and it's the lymphatic system that gathers all that fluid and brings it back into circulation but this idea might bother you you might have an objection which is these lymph vessels don't have a heart of their own I didn't draw one and it's because they don't have one so how is it that they're able to pump this fluid in one direction and moreover how is it that they're able to pump it back into this high-pressure system given that they don't have a pump that must be quite difficult there are two answers to this question really and the first is a little simpler so let's start there the first answer is essentially in the location of reentry so the reason why lymph is able to get back into the high pressure system is because lymph is quite intelligent about where it tries to re-enter the high-pressure system so I'm drawing a star here because that's where the lymph enters I didn't draw that by accident it reenters at the very end of the venous circulation system and the pressure there is much lower than the pressure in the arteries and even the pressure in the capillaries the numbers are actually pretty striking the pressure here in the early arteries can be about 120 millimeters of mercury that's kind of like your average blood pressure but over here just before it enters the heart the pressure is more like five millimeters of mercury so the fact that the lymph fluid enters circulation at the end here makes it much easier it has to fight against much less if the lymph vessels ended up trying to drain a year let's say into the aorta or into a big artery probably none of this would work so that makes the job easier but it doesn't explain how lymph moves sort of one directionally into the blood vessels what is actually forcing the lymph what is getting to move in this direction why wouldn't it just come back as much as it would go forward and the reason why that's not the case is because of valves so if valves our structures within the lymph vessels which prevent fluid from going back so let's draw the lymph vessel here and a valve might look something like this and what this valve accomplishes is that if fluid gets pushed that way it'll part the two leaves of the valve and it'll get through but let's say that fluid tried to come back then it tries to come back it'll push the valves back the leaves of the valve back together and it will create an obstacle now this is a really smart system and actually it's not only used in your lymph vessels it's you might know that it's also used actually in the heart you have four big valves in the heart that operate on this exact principle and it's also used in veins because the pressure to push fluid through veins is sometimes not sufficient and so having a mechanism to prevent backward flow is very helpful but still you might not be satisfied because you might say well what causes the lymph to move at all here why wouldn't it just sit where it is and there are actually two ways that the motion begins one is that you actually have a little bit of smooth muscle smooth muscle attached to your lymphatic vessels and that can contract and just by squeezing it'll cause motion of the fluid forwards so that's the first mechanism by which the motion starts and the second is skeletal muscle skeletal muscle just refers to basically all the big muscles in your body that are under voluntary control think about your you know your leg muscles your arm muscles and so on and the reason why this does something is that just throughout the course of your average day you use your muscles you move around you have a certain amount of jostling in your body and so it's inevitable that you're gonna squeeze certain parts of your body at some times and when you squeeze a part of the body that has a lymph vessel like this it's gonna start the motion it's gonna squeeze it just like the smooth muscle did and you'll get fluid moving ahead in the direction that the valves allowed them to and so in addition to the valves which we can draw in to our diagram here in addition to those there's an interesting contraption where the limb first enters the lymphatic vessels so I drew it here as an open tube but in reality it's it's closed at least it looks closed but it's porous of course and that's what allows fluid to get in let me just draw this a little bigger but basically this is where the lymph vessel starts and the walls of this lymph vessel are kind of like valves in and of themselves and that they allow fluid to come in but when the pressure inside rises it prevents fluid from going out so the lymph cannot go back out so finally let's just look at a human body to get a sense of where everything that we're doing fits in so here's another ugly human let's quickly give him some arms a head and now we can actually show where the lymph we enter circulation so let's say that that's the heart well you probably know that one of the huge veins leading into the heart is the superior vena cava and right near the superior vena cava you have what are called the subclavian veins and you have one on the right and one on the left they're called subclavian veins because they pass just under the clavicle and it's actually right into these that the lymph is re-entered so I'm drawing that in green so that it corresponds with that at this point that we have very low pressure and it's there that all the limbs from the body goes and so that includes lymph from down in your legs and from your arms and from your neck and so on and it's pretty remarkable if you think about it that all the lymph in your body traveled up this entire path and got dumped back into circulation there and all of it was without an active pump it was all done through this method of a little bit of squeezing with smooth muscle but mostly just the ingenuity of these one-way valves that take advantage of the fact that you're always moving around to sort of squirt the lymph back up all the way up your body to essentially your neck