CHAPTER TWENTY-ONE
The Lymphatic System and Immunity
The immune system is the complex collection of cells and organs that destroys or neutralizes pathogens that would otherwise cause disease or death. The lymphatic system is the system of vessels, cells, and organs that carries excess fluids to the bloodstream and filters pathogens from the blood.
21.1 Functional Anatomy of the Lymphatic and Immune Systems A. Functions of the Lymphatic System: 1. The lymphatic system drains excess interstitial fluid from the interstitial space and transports it to the bloodstream. Once this tissue fluid enters into the lymphatic vessels, it is no longer called interstitial fluid; it is now referred to as lymph. Any blockage in the normal drainage of lymph produces lymphedema. 2. The lymphatic system houses the phagocytic cells and lymphocytes that clean the tissue fluid before it is dumped into circulation. 3. The lymphatic system absorbs digested fats from the intestine by specialized lymph vessels called lacteals. The fatty lymph is known as chyle. B. Structure of the lymphatic system 1. The lymphatic system includes the vessels, cells, tissues, and organs responsible for defending the body against both environmental hazards (such as various pathogens) and internal threats (such as cancer cells). 2. Lymph Vessels: carry lymph from the peripheral tissues to the venous system. The lymphatic network begins with lymphatic capillaries, which merge to form progressively larger vessels as they make their way towards circulation. a. Lymph capillaries: present in almost every tissue and organ in the body; are believed to be as abundant as blood capillaries. . Lymph capillaries are absent, however, in areas of the body that lack a blood supply, such as the cornea of the eye, and are also absent from the central nervous system and bone marrow. ii. Lymphatic capillaries differ from blood capillaries in that they 1) are blind-end tubes, 2) have larger diameters with lower resistance and pressure, 3) have
irregular shaped lumen. iii. Although lymphatic capillaries are lined by simple squamous epithelium, the basal lamina is incomplete or absent. Furthermore, the endothelial cells overlap forming a type of one-way valve called a mini-valve. The mini-valves permit the entry of fluids and solutes (such as proteins) as well as viruses, bacteria, and cell debris, but prevent their return to the intercellular spaces. b. Lymph collecting vessels: from the lymph capillaries, lymph flows into larger lymph collecting vessels with valves that lead toward the body's trunk. C. Lymph trunks: the superficial and deep lymph collecting vessels converge to form larger vessels known as lymphatic trunks and are named by the areas of body they drain: i. The jugular trunks are located in the neck and drain the head. ii. The subclavian trunks are located in the shoulders and drain the arms. iii. The bronchomediastinal trunks are located in the chest and drain the thoracic cavity and lungs. iv. The lumbar trunks are located in the lower back and drain the pelvis and lower limbs.
V.
The intestinal trunk is located in the abdomen an