The Lymphatic System – MCAT Biology | MedSchoolCoach

The Lymphatic System

MCAT Biology - Chapter 7 - Section 5.1 - Organ Systems - Lymphatic System
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Sample MCAT Question - Lymphatic System

One of the primary functions of the lymphatic system is to:

a) filter blood and destroy toxins and waste.

b) absorb fat in the gastrointestinal tract.

c) assist in the digestion of carbohydrates.

d)maintain proper blood sugar levels in the body.

B is correct. The lymphatic system assists in the absorption of dietary fat.


The lymphatic system has different functions. One is to recover the excess fluid lost at the capillaries in the cardiovascular system. Once the fluid enters the lymphatic vessels, it is known as lymph. Lymph travels through lymph nodes and gets filtered of pathogens before it returns to veins in the circulatory system. The lymphatic system also consists of lacteals, which function to absorb fat in the gastrointestinal tract. The liver filters the blood and destroys toxins and waste (choice A is incorrect). Carbohydrate digestion occurs in the GI tract (choice C is incorrect). The pancreas secretes insulin and glucagon which help regulate blood sugar levels (choice D is incorrect).

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What Is the Lymphatic System?

The lymphatic system is an integral part of both the circulatory and immune systems. It has a diverse array of functions, including the recovery of fluid lost during blood circulation, the filtering of pathogens, the absorption of dietary fat, and the production of certain immune cells. Figure 1 illustrates the different components of the lymphatic system. As you can see, the lymphatic system spans the entire body and includes several organs, including the spleen, the thymus, the bone marrow, and the lymph vessels and lymph nodes. 

Anatomy of the lymphatic system for the MCAT, including the lymph nodes, spleen, and thymus
Figure 1. Anatomy of the Lymphatic System

Functions of the Lymphatic System

Recall that in the cardiovascular system, there is a net loss of fluid from the capillaries. If not for the lymphatic system, this net loss of fluid would cause all of the tissues in the human body to swell. In this way, one of the primary functions of the lymphatic system is to recover excess fluid lost at the capillaries in the cardiovascular system. When fluid is taken up by the lymphatic vessels, it is called lymph rather than blood. Moreover, since this fluid is interstitial fluid, it may contain pathogens. Therefore, before this fluid is returned into circulation, it has to be filtered. Lymph nodes located all over the body are concentrated areas of lymphocytes that filter the lymph and clear it of any pathogens before it is returned into the blood supply. Lymph nodes are also important clinically because when they are enlarged, it is usually a sign of infection or inflammation. Also, depending on whether lymph nodes are painful or not, clinicians can determine what type of disease is present. A painless lymph node enlargement might point to cancer. In this way, lymph nodes are an integral part of the immune system.

The flow of fluids from the blood capillaries to the lymphatic vessels
Flow of Fluids from Blood to Lymph - MCAT Biology

Another function of the lymphatic system is to absorb fat in the gastrointestinal tract. The lymphatic vessels in the gastrointestinal system are called lacteals. Lacteals are mainly used for fat absorption. 


The spleen, a major organ of the lymphatic system, has a role in clearing the body of damaged or old red blood cells. Moreover, it is a reserve for red blood cells, as well. 


Other components of the lymphatic system are the thymus and bone marrow. The thymus is the location where T cells mature. T cells are an essential component of the adaptive immune system. The bone marrow, meanwhile, is where B cells and T cells are produced and where B cells mature. B cells are the complementary component of T cells in the adaptive immune system.

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