Interstitiаl hydrоstаtic pressure remаins nearly cоnstant and clоse to 0 mm Hg due to the activity of the lymphatic system, which siphons excess interstitial fluid into lymphatic vessels. Lymphatic vessels are not associated with a positive-pressure pump, so lymph in these vessels in always under very low pressure. Like veins, lymphatic vessels possess _____ to ensure that lymphatic flow is one-way (toward the heart).