Final answer:
The incorrect statement about the lymphatic system is A. The cells of lymphatic capillaries are tightly joined, as they actually have overlapping edges which function as one-way valves for fluid entry.
Step-by-step explanation:
All of the following statements about the lymphatic system are true EXCEPT: A. The cells of lymphatic capillaries are tightly joined. This statement is not accurate as the lymphatic capillaries are formed by a one cell-thick layer of endothelial cells with overlapping edges that function as one-way valves, allowing interstitial fluid to flow into them but preventing backflow.
B. Lymphatic vessels operate under low-pressure. This is true because unlike the cardiovascular system, the lymphatic system is not pressurized by the heart.
C. Lymphatic vessels belong to a one-way system that drains lymphatic capillaries. True, they carry lymph in a single direction toward the heart.
D. Lymph flow is driven in part by contraction of smooth muscle in the walls of lymph-collecting vessels. This is correct; the lymph moves through the vessels due to contractions of the vessel walls and external forces such as muscle movements.