To quote Casley-Smith (1980), "In brief, the tissue lymphatic system is a leaky swamp through which material flows due to the vagaries of adjacent pressure changes".
At the most basic level, its just interstitial fluid
Lymphatic vessels are not present in bone, cartilage or the central nervous system.
Terminal lymphatic capillaries have generally been thought of as blind-ended tubes (Casley-Smith, 1980), but forty years later authors have generally been describing them as open-ended (Hansen et al, 2015), i.e. at the end of every terminal lymphatic capillary there is an open portal leading back into the circulatory system.
Protein and debris from the interstitial fluid are sucked up into the lymph, as well as any excess interstitial fluid. Thus, lymph is interstitial fluid with 20g/L of extra protein.
Most of the extra protein is albumin. Not much fibrinogen makes it out into the lymphatics, because of how large its molecule is. In spite of this, even without any platelets and with minimal fibrinogen, lymph can still clot if given half a chance.
The protein is sucked by the lymphatics out of the interstitial fluid so as to keep the oncotic pressure of the interstitial fluid low (Otherwise, oedema would develop).
The protein and debris from the tissues is washed past some resting antigen-presenting cells and lymphocytes.
Lymphatic vessels have flap valves to prevent backflow. They also have smooth muscle (which peristalses) and they travel next to arteries, taking advantage of their pulsation. And on top of that there is the pumping action of contracting muscles.
The liver contributes about 50% of the total body lymph.
Hepatic lymph is highly concentrated, up to 60g/L of protein is present.
All of this protein-rich lymph drains via the thoracic duct into the confluence of the left subclavian and left internal jugular veins.
83% of the total body's lymph returns to the blood in the thoracic duct; the rest is via the right lymphatic duct.
- About 3 litres of lymph are produced every day
- That makes about 120ml per hour (100mls of which returns via the thoracic duct
- This volume increases massively with exercise