How lymph forms from filtered capillary fluid
Lymph is essentially the excess interstitial fluid (fluid lying in the spaces between cells) that the capillaries do not reabsorb, once lymphatic vessels have collected it.
Remember
In a healthy adult, the systemic capillaries filter out about 20 litres of fluid each day and most of it returns directly to the capillaries, so approximately 3 litres of leftover fluid drains into the lymphatic vessels as lymph.
In a healthy adult, fluid is filtered out of the capillaries each day. At the arteriole end (the end nearest the artery) the hydrostatic pressure, meaning the pressure of the blood, is high, so fluid is pushed out into the interstitium, the space between cells. This leaked fluid is called interstitial fluid.
Towards the venule end (the end nearest the vein) the hydrostatic pressure is lower. Combined with the oncotic pressure, the pulling force on water caused by dissolved proteins in the plasma, this draws most of the fluid back into the capillary. Of the roughly 20 litres filtered each day, about 17 litres return directly in this way.
That leaves approximately 3 litres a day in the tissues. The leftover fluid raises the pressure in the interstitium. The lymphatic capillaries, where the leftover fluid enters, have a thin wall of loosely overlapping endothelial cells (the thin cells lining a vessel), and these act as one-way valves. When tissue pressure is higher than the pressure inside the vessel, the valves open and the fluid flows in. Once inside, it is called lymph. Entry is passive, driven by that pressure difference.
Lymph then moves in one direction. Muscle contraction around the deeper limb vessels squeezes it along, and smooth muscle in the walls of the larger vessels of the abdomen and chest helps too. It is carried through lymph nodes and drains into the large veins near the heart. If lymph flow fails, fluid accumulates and the tissues swell.
Key facts
- Filtered each day
- around 20 litres of fluid are filtered out of the capillaries in a healthy adult.
- Returned directly
- about 17 litres return directly to the capillaries in a healthy adult.
- Lymph volume
- approximately 3 litres a day enter the lymphatic system as lymph in a healthy adult.
- Quoted lymph volume
- some textbooks quote 3 to 4 litres a day, so treat the figure of 3 litres as approximate.
- Entry
- the overlapping endothelial cells of the lymphatic capillaries act as valves that open when tissue pressure is higher than pressure inside the vessel.
- If drainage fails
- fluid accumulates and the tissues swell.
Exam traps
- The capillaries and the lymphatic vessels do not return similar volumes of filtered fluid, because about 20 litres is filtered a day and only about 3 litres enters the lymph, so the capillaries return far more.
- The daily volume of lymph is not negligible, because about 3 litres a day is a large volume and, if the lymphatics fail to drain it, the tissues swell.
- Lymphatic endothelial cells do not actively transport fluid inwards: entry is passive, and the overlapping endothelial cells act as valves that open when pressure in the interstitium is higher than pressure inside the vessel.
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