MCAT Basics Podcast

The Lymphatic System

Sam Smith and Alex Starks review key information about the lymphatic system for the Bio/Biochem portion of your MCATs. They outline the structure and major functions of the lymphatic system.

  • [02:23] What is the Lymphatic System?
  • [02:53] Lymph, Lymphatic Vessels, and Ducts
  • [06:57] Primary and Secondary Lymphoid Organs
  • [14:41] Lymphoid Tissues
  • [16:11] How the Lymphatic System Returns Fluid to Blood
  • [22:12] How the Lymphatic System Supports the Immune System
  • [25:06] Absorption of Fat and Fat Soluble Nutrients
  • [27:54] The Lymphatic System’s Connection to the Brain

The lymphatic system is a network of vessels, organs, and tissues that have three primary functions which are: to return fluid to blood, to support the immune system, and to absorb fat and fat-soluble nutrients.

Lymph, Lymphatic Vessels, and Ducts

  • Lymph – Fluid that flows through the lymphatic system which contains protein, excess fluids, and pathogens. It is white in color and somewhat resembles plasma.
  • Lymphatic vessel – Thin tubes that transport lymph fluid. Lymphatic vessels below the cisterna chyli converge at it and then flow up from the cisterna chyli.
  • Right lymph duct – It dumps lymph into the bloodstream via the right subclavian right jugular veins.
  • Thoracic duct – Similar to the right lymph duct but larger in size. It helps return fluid into the bloodstream via the left subclavian and left jugular veins.

Lymphoid Organs

Primary lymphoid organs are sites where lymphocytes form and mature. These consist of the bone marrow and thymus.

  • Bone marrow – Spongy tissue inside of bones. B cells, T cells, natural killer cells, and other immune cells are produced here. It is also the site where B cell maturation occurs.
  • Thymus – A lymphoid organ that sits below the sternum. Lymphatic vessels only travel away from it. The thymus is the site for T cell maturation. Mature T cells move on to lymph nodes through lymphatic vessels. Interestingly, the thymus is the only organ that shrinks and disappears as we age.

Secondary lymphoid organs are where the cells actually fight off germs. These consist of the lymph nodes, tonsils, spleen, Peyer’s patches, and mucosa-associated lymph tissue (MALT).

  • Lymph nodes – Small bean-shaped structures that are distributed all over the body. These glands are concentrated around the neck, armpits, groin, and gut. Lymph nodes are connected to each other through a network of lymph vessels. The lymph nodes serve as a reservoir of immune cells.
  • Spleen – An organ located in the upper left abdomen. It removes old blood cells and stores blood. The spleen also contains lymphocytes.
  • Tonsils – Lymph nodes that can be found near the back of the throat. It has white blood cells and is the body’s first line of defense against respiratory pathogens.

Lymphoid Tissues

  • Mucosa-associated lymphoid tissue (MALT) – Spread out a system of lymphoid tissue in various sites of the body.
  • Gut-associated lymphoid tissues (GALT) – The GALT is an example of MALT. It consists of isolated or aggregated lymphoid tissues that make up Peyer’s patches.
  • Peyer’s patches – Immune sensors of the intestine. They look for pathogens in the gut and can respond to them very quickly.

How the Lymphatic System Returns Fluid to Blood

Blood vessels and capillaries constantly leak out fluids that allow nutrients from the blood to move into periphery cells. Bear in mind that the leaked fluid contains blood plasma, proteins, ions, pathogens, and other molecules. Too much-leaked fluid can build up in the interstitium and may cause edema.

To avoid this, the body needs to get rid of the excess fluid. Capillaries re-absorb 80% of the fluid while the rest is absorbed by lymphatic capillaries. Once absorbed by the lymphatic capillaries, the fluid turns into the lymph. The lymph will make its way into bigger lymphatic vessels which will flow upwards towards the cisterna chyli. Eventually, the lymph will pass the ducts and reach the subclavian and jugular veins. This is how the lymphatic systems return material to the blood and equalizes fluid distribution.

How the Lymphatic System Supports the Immune System

We already know that immune cells grow and mature in lymphoid organs. Another way in which the lymphatic system supports the immune system is by monitoring lymph. Pathogens can enter the interstitial fluid. Lymph that was once interstitial fluid will travel through the different lymphoid organs and tissues which have the ability to detect pathogens. If a pathogen is found, immune cells are activated. Lymphocytes can start looking for the virus and produce antibodies.

Absorption of Fat and Fat Soluble Nutrients

Fats do not dissolve easily since they are hydrophobic. The small intestine secretes bile to break down fat into micelles. Micelles are absorbed by enterocytes. Once inside, the micelles are assembled into triglycerides and are packaged with phospholipids, cholesterol, protein to make chylomicrons.

Most chylomicrons exit enterocytes via exocytosis. The chylomicrons enter lacteal which is lymphatic capillaries. At this point, fat has entered the lymphatic system. Fat-soluble nutrients are absorbed in this way too. Fats enter the blood via the lymphatic system when lymph is drained back into the bloodstream.

Sam Smith

Sam completed his Bachelors of Science in Chemical and Biological Engineering at the University of Colorado Boulder. Following his graduation, he worked at the National Institutes of Health Vaccine Research Center studying HIV. Meanwhile, with a microphone in his garage, Sam founded the MCAT Basics podcast. The podcast has grown to become the top rated MCAT podcast on iTunes. In addition to podcasting, Sam enjoys the outdoors, sports, and his friends and family.

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