Final answer:
Lymph flow is maintained by changes in abdominal pressure during activities that involve the Valsalva's maneuver, such as breathing, defecation, urination, and childbirth, which engage the diaphragm and abdominal muscles.
Step-by-step explanation:
Lymph flow is maintained by changes in abdominal pressure that occur during breathing, defecation, urination, and childbirth. These activities involve the diaphragm and abdominal muscles, which increase intra-abdominal pressure, assisting the movement of lymph through the lymphatic vessels. This increase in abdominal pressure is known as the Valsalva's maneuver, during which the diaphragm and abdominal wall muscles contract while the glottis closes. This maneuver is particularly important for propelling lymph fluid through the body as it doesn't have a central pump like the heart in the circulatory system.