Lymphatic System

Lymphatic vessels infiltrate tissues that are bathed in fluid released from blood into those tissues. Pockets of fluid collect in the tissues, and increased pressure allows the fluid to seep into the lymphatic vessels. Whereas blood vessels return deoxygenated blood to the heart to be pumped to the lungs for oxygen, lymphatic vessels return fluid that has leaked out of the capillaries into various tissues. However, before this lymphatic fluid is rejoined with venous fluid at the thoracic duct, it is filtered through the lymph nodes to remove infectious agents.

Lymphatic vessels are made up of single-cell epithelial layers that drain fluid away from tissue. Smooth muscles controlled by the autonomic nervous system direct the fluid away from tissues toward the lymph nodes and, eventually, the heart. The vessels contain one-way valves that close behind fluid traveling back to the heart so that lymphatic fluid cannot go backward. Lymphatic fluid is usually returned to the circulation within 24 hours. When the lymphatic vessels become clogged, stopped up, or blocked, severe edema (bloating due to water retention) can result in a condition known as lymphedema.