I was working on a yard project the other day and I ended up cutting myself with a utility knife. It was more a puncture than a cut, pretty deep but not very big. Anyway, the weird thing is that it didn't bleed. Instead, this clear fluid oozed out. What was that clear fluid?
Assuming that you are not a space alien of some sort, the clear fluid that you saw was lymph. Apparently, when you cut yourself you did not puncture any sort of blood vessel, so you had a rare opportunity to actually see your lymph!
One of the amazing things about lymph is that we all have it but we generally know nothing about it. Your entire body is soaked in lymph and there is a whole lymph vessel system that is just as complex as your blood vessels!
- Oxygen (blood also picks up carbon dioxide so it can be exhaled)
- White blood cells (to find and eliminate bacteria, viruses and other foreign materials)
Capillaries flow past cells but do not actually connect to them. What happens is that the clear, watery blood plasma -- containing the oxygen, proteins, glucose and white blood cells -- "leaks" out through the capillary walls and flows around all the cells. The pores in the capillaries are too small to let red blood cells through, however -- that is why lymph is clear rather than red. All of the cells in your body are therefore bathed in lymph, and they receive their nutrients and oxygen from the lymph.
Somehow, all of this lymph has to end up somewhere, so it is recirculated. The lymph capillaries and vessels pick up the lymph fluid and start pumping it away from the cells. Lymph vessels do not have an active pump like the heart. Instead, lymph vessels have one-way valves, and muscle motion pumps the lymph. You have just as many lymph vessels and capillaries as you have blood vessels and capillaries!
Lymph in the lymph vessels eventually reaches a lymph node -- there are about 100 nodes scattered throughout the body. Lymph nodes filter the lymph and also contain large numbers of white blood cells (a big part of the immune system), which remove foreign cells and debris from the lymph. When you get certain infections, the lymph nodes swell with billions of white blood cells working to clear the foreign cells causing the infection. The filtered lymph then flows back into the blood stream at certain points.
One thing this explains is how a shot at the doctor's office works. The fluid of the shot is injected into the lymph, not the blood stream. But eventually it makes it to the bloodstream through the lymph system.
Here are some interesting links: