The big idea: Not all tissue fluid drains back into the capillaries. The excess drains into lymph vessels and is returned to the blood.
The points to remember
- Lymph capillaries start as blind-ended tubes among the cells.
- Their walls are thin, with gaps between the cells, so excess tissue fluid flows in: it is now lymph.
- Valves in lymph vessels keep lymph moving one way.
- Lymph ducts empty lymph back into the blood, into veins near the heart.
Remember it as: Leftover fluid in, valves keep it going, back to the blood.
Real example: swelling after surgery. When lymph nodes in the armpit are removed during breast-cancer surgery, lymph can drain poorly and the arm may swell with tissue fluid (lymphoedema).
Trap: Most tissue fluid goes back into the capillaries; only the excess becomes lymph.
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How it is asked: Expect 'outline how excess tissue fluid is returned to the blood', or the features of lymph vessels.
IB-style questionOutline[3 marks]
Outline how excess tissue fluid is returned to the blood.
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See the answer broken into clear points, with guidance on how each point earns marks.