What organ makes lymph?

Answer 1

No particular organ makes lymph. If you have a look at your blood circulatory system, you will see that it seems to be a closed loop: blood flows from your heart through your arteries to smaller arteries and then arterioles, into your practically microscopic capillary vessels. From there it flows into venules, small, then larger veins, and finally back into the heart. In other words, it looks much like a closed loop.

If you study the lymph vessels on the other hand, you find that it flows from the tissues into tiny, then larger lymph vessels till it joins the large subclavian veins and mixes into the bloodstream. In other words, unlike the bloodstream, the lymph vessels are not a closed circuit but seem to be a one-way system.

Actually, they are part of a larger closed circuit, which we only find in mammals and birds. Mammals and birds have a high pressure system of blood circulation which permits more efficient circulation and greater endurance; we can keep up a sprint or a marathon far longer than any reptile for example.

However, there is a penalty. The higher pressure means that part of the liquid, the plasma, of blood leaks out of the capillaries into the surrounding tissues. If nothing is done about this the tissues swell up (we call such swelling oedema and it can be a very nasty medical condition and a symptom of various diseases) and the blood loses its necessary volume. To prevent oedema, the lymph vessels absorb liquid from the tissues; we call such liquid "tissue fluid", and once it has entered the lymph vessel, we call it lymph. Afterwards, as I said, it enters our blood circulation system, thereby closing the lymph circuit as well, as part of the body's fluid circulation.

This circulation of fluid among our tissue cells is nearly as important as our blood circulation; we simply cannot function without it. It cycles a lot of substances that the blood cannot otherwise reach as such. What is more, some of our immune cells that grow from stem cells in our marrow and enter the bloodstream, sneak out between the cells of our capillary vessels into the tissues. There they look for and fight various kinds of germs and generally wind up and enter lymph vessels. On the way to the bloodstream they pass through organs such as the spleen and lymph nodes to pass on information about the germs that they found, if any. These cells we call lymphocytes.

So no particular organ makes lymph. Most of the lymph is tissue fluid that is collected to send it back to the bloodstream. Most of the rest is cells that came from the blood , but were made in the marrow.