In this video, we're going to talk about the transport or the propulsion of lymph through the lymphatic vasculature. And so unlike the cardiovascular system, which we know has the heart that serves as a pump to propel blood through the blood vessels, the lymphatic system actually lacks a direct pump like the heart to keep the fluid moving. And so the question becomes, well, how is the lymph transported or propelled through the lymphatic vasculature? And the answer is that a multitude of mechanisms contribute to lymph flow. And so notice that down below in the image, we've got these 4 boxes, each with a mechanism that contributes to lymph flow. And so in the box on the far left, we have one of the greatest contributors to lymph flow, and that is going to be skeletal muscle contractions. So recall that skeletal muscles are muscles that we have voluntary control over their contractions. And so whenever you extend, bend, flex, or move a muscle, the shape of those muscles changes in such a way that it can compress nearby lymphatic vessels and propel the lymph through them. And so that's exactly what we're showing you down below in this image. Notice that the skeletal muscle is contracting and changing its shape in such a way that it's actually compressing this nearby lymphatic vessel, and that is propelling the lymph through the lymphatic vessel.
And another thing that you'll notice is that these lymphatic vessels have lots of valves in them that you can see here, and these valves ensure the one way flow of the lymph. So lymph is able to flow past the valves, but the valves prevent the backflow of lymph, and so it keeps the lymph flowing in one direction, the correct direction, which is towards the veins that are near the heart. In fact, it's well known that physical exercise, which requires skeletal muscle contractions, greatly increases the rate of lymph return back to the cardiovascular system. Now the next box that we have here refers to smooth muscle contractions, and recall that smooth muscles are muscles that we do not have voluntary control over their contractions. And so especially for the larger lymphatic vessels, they can actually have some smooth muscle in their walls. And so notice that the pink that you can see down below in this image around the larger lymphatic vessels represents the smooth muscle in the walls. And so when these larger lymphatic vessels stretch from having a large amount of lymph in them, the smooth muscles can detect that stretching and contract, and the contraction can narrow down the lumens of these lymphatic vessels and that can propel the lymph through them. And, again, the valves are going to be there to ensure that there is no backflow of the lymph. Now these smooth muscles, again, are really only found in the larger lymphatic vessels, so the smaller lymphatic capillaries do not have smooth muscle in them. Now the next box that we have here are the pulsation of nearby arteries. Now recall that pulsation refers to the rhythmic contraction and dilation of the arteries, And so it turns out that these lymphatic vessels are often nearby arteries, and these arteries, because they can pulsate, again, contract and dilate in a rhythmic fashion, the movement of the arteries can actually compress some nearby lymphatic vessels, and that can propel the lymphatic or the lymph through the lymphatic vessels.
Now last but not least, over here on the far right, what we have is the respiratory pump, and this is referring to our breathing. And so we are constantly breathing, inhaling and exhaling. And whenever we inhale, take a deep breath, the pressure in our abdomen changes where there's increased pressure, and that increased pressure changes the microenvironment around the lymphatic vessels that are in our abdomen, and that can actually compress those lymphatic vessels that are in the abdomen, and that can propel the lymph through the, the lymphatic vessels that are in our, abdomen. And then, of course, when we exhale, the pressure in the abdomen is going to decrease. However, the valves that are present all throughout the lymphatic vessels are going to prevent the backflow of lymph. And so the inhale is really going to change the pressure and help to propel the lymph through the lymphatic vessels that are in our abdomen and thoracic regions. So really, this here concludes our lesson on the transport and the propulsion of lymph, and we'll be able to get some practice applying these concepts as we move forward. So I'll see you all in our next video.