How does lymph fluid return from the legs to the circulatory system?

Lymph fluid returns from the legs to the circulatory system primarily through a combination of one-way valves and the action of surrounding muscles. The lymphatic system is designed with valves that ensure unidirectional flow of lymph, preventing backflow as it travels upwards toward the thoracic duct, and ultimately enters the circulatory system. When muscles contract during movement, especially in the legs, they exert pressure on nearby lymphatic vessels, helping to push the lymph fluid upwards. This mechanism is crucial for overcoming gravity, particularly when returning fluid from the lower extremities to the upper body.

In contrast, the options regarding pumping through arteries and direct diffusion do not accurately reflect the functioning of the lymphatic system. The lymphatic system works independently of blood circulation in arteries and does not involve direct diffusion from interstitial fluids into tissues without a transport mechanism. Additionally, while there may be some contribution from veins, the primary role of the lymphatic system is to transport lymph fluid, which is distinct from blood circulation. Thus, the actions of muscle contraction and the presence of valves are essential components of the lymphatic return mechanism.