Blockage of the lymph vessels, as occurs in filariasis, causes swelling of the limb because
- A. blood cannot leave the limb through the veins
- B. excess tissue fluid cannot be drained away and accumulates
- C. the arteries dilate and overfill the tissue
- D. plasma proteins are digested in the tissue spaces
Explanation
The lymphatic system is the only route by which the surplus tissue fluid returns to the blood, so blocking it lets fluid build up in the spaces between cells, and the result is the oedema seen in elephantiasis. Venous return is unaffected in this condition, which is why the swelling is soft and painless at first. The same reasoning explains the oedema of severe protein deficiency, where too little fluid is drawn back osmotically.