Which of the following is a primary function of lymphatic vessels?

Lymphatic vessels play a crucial role in maintaining fluid balance within the body, and one of their primary functions is to return excess tissue fluid to the bloodstream. This excess fluid, known as lymph once it enters the lymphatic system, accumulates in tissues due to various factors such as filtration at capillary beds, and if not returned to the circulatory system, it can lead to swelling, known as edema. The lymphatic system collects this excess fluid, along with cellular waste and other materials, and transports it through the lymphatic vessels back to the venous circulation, effectively preventing fluid overload in tissues and supporting overall homeostasis.

The other options focus on different physiological processes. Transporting oxygen is primarily a function of the circulatory system via red blood cells. Distributing hormones is mainly performed by the endocrine system through the bloodstream, while regulating blood pressure involves a complex interplay of the cardiovascular system and neural mechanisms. These functions are distinct and do not pertain to the essential role of lymphatic vessels in fluid balance and immune function.