Which of the following statements is/are true of the pulmonary lymphatic system?

1. It consists of both superficial and deep vessels.
2. It drains into the right lymphatic or thoracic duct.
3. Vessels begin as dead-end lymphatic channels in the lung.
4. With phagocytes, it defends against foreign material.
a. 1, 3, and 4 only
b. 2, 3, and 4 only
c. 2 only
d. 1, 2, 3, and 4

ANS: D
The lymphatic system plays an important role in the specific defenses of the immune system. It removes bacteria, foreign material, and cell debris via the lymph fluid and through the action of various phagocytic cells (e.g., macrophages) that provide defense against foreign material and cells that are able to penetrate deep into the lung. It also produces a variety of lymphocytes and plasma cells to aid in defense. Both roles are essential for maintaining normal function of the res-piratory system. Most of the pulmonary lymphatic system consists of superficial and deep ves-sels. The superficial (pleural) vessels that drain the lung surface and pleural space are more nu-merous over the lower half of the upright lung. Both drain the blind lymphatic capillaries in the respective regions. The deeper lymph vessels are closely associated with the small airways but do not extend into the walls of the alveolar-capillary membranes. Lymph fluid is collected by the loosely formed lymphatic capillaries and drains through the lymph vessels toward the hilum. The lymph fluid rejoins the general circulation after passing through the right lymphatic or thoracic duct, which drains into the jugular, subclavian, and/or innominate veins. The lymph fluid then mixes with blood and returns to the right heart.