Final answer:
Lymphatic vessels have one-way valves to ensure lymph flows in the proper direction towards the cardiovascular system, assisted by skeletal muscle contractions and the intrinsic contractile activity of the vessels.
Step-by-step explanation:
Lymphatic vessels have one-way valves to help ensure the flow of lymph towards the veins of the cardiovascular system. These valves, often referred to as semi-lunar valves, are crucial for maintaining the unidirectional flow of lymph, as the lymphatic system lacks a central pump like the heart in the cardiovascular system. Instead, lymph is moved through the vessels by the contraction of skeletal muscles during body movements and breathing, along with the intrinsic contractions of the lymphatic vessels themselves.
Lymphatic capillaries begin as closed-end tubes that absorb interstitial fluid, which then becomes lymph. This lymph moves through progressively larger lymphatic vessels until it reaches major lymphatic ducts, which empty the lymph into the subclavian veins near the heart. Throughout its journey, lymph passes through lymph nodes where it is filtered, and pathogens are removed.
Because the lymphatic system is not pressurized by the heart, these valves are essential in preventing lymph from backflowing and ensuring that it progresses toward the cardiovascular system, thus maintaining fluid balance and immune functions within the body.