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Introduction to the lymphatic system

Introduction to the lymphatic system


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USMLE® Step 1 style questions USMLE

1 questions

A woman presents to the physician because of pain and swelling in the right breast. The woman has been lactating for 2-weeks. Physical examination reveals tenderness, erythema, and swelling in the right breast and enlarged, tender right axillary lymph nodes. Which of the following best describes the lymphatic drainage pathway from the inflamed lymph nodes?  


The lymphatic system may be the most underrated system of the human body. It protects the body from invaders, maintains the fluid level in the body, and absorbs dietary fat from the intestine.That’s pretty amazing for a system that is really small, sometimes even microscopic!

Now, the lymphatic system is composed of lymph, lymphocytes, lymphatic vessels and plexuses, lymph nodes and other lymphoid organs. Lymphoid organs are the part of the lymphatic system that are directly involved in the immune system because they produce lymphocytes, as is the case with the thymus and bone marrow and house lymphocytes - and other white blood cells - that are ready to respond to pathogens, as is the case with lymph nodes, the spleen and tonsils.

But we are going to focus on the role of the lymphatic system in returning leaked fluid to the blood, so let’s start by turning our attention to lymph and lymphatic vessels.

Lymph is a clear - or slightly yellow - fluid that is transported through tubes similar to blood vessels, called lymphatic vessels. Now, you may be wondering, where does this lymph come from? Well, it all starts at a network of tiny blood vessels called the capillary beds where nutrient and gas exchange occurs. During this process, around 10% of this fluid is lost and ends up in the extracellular space, which is where the lymphatic system kicks in. This fluid will be taken up first by lymphatic capillaries which are small, porous capillaries throughout the body that form lymphatic plexuses. Once this fluid enters the lymphatic capillaries, it’s called lymph. Now, the small lymphatic capillaries merge to form the next part of the lymphatic system, called lymphatic vessels. Lymphatic vessels drain lymph through lymph nodes and eventually into lymphatic trunks, which drain large regions of the body - like the subclavian trunks that drain the upper limbs. Finally, lymphatic trunks converge to form two bigger channels called lymphatic ducts, specifically the right lymphatic duct and the thoracic duct.

The right lymphatic duct collects the lymph from the right side of the head and neck, the right side of the chest, and the right upper limb. This duct drains its lymph into the right venous angle, which is the meeting point of the right internal jugular vein and the right subclavian vein.


The lymphatic system consists of a network of lymphatic plexuses and vessels, lymph nodes, and other lymphoid organs such as the spleen and tonsils. The lymphatic system helps to absorb dietary fat from the intestine, protects the body from foreign invaders, and maintains extracellular fluid volume by returning excess tissue fluid to the blood. Lymph moves through lymphatic vessels and finally drains into the venous system through the thoracic duct and the right lymphatic duct.

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  2. "Physiology" Elsevier (2017)
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  4. "Principles of Anatomy and Physiology" Wiley (2014)
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