Video Transcript
All of the following constituents exist in the lymphatic vessels except (A) blood
plasma, (B) erythrocytes, (C) antibodies, or (D) fatty acids.
This question asks us about the composition of the fluid that the lymphatic vessels
carry. To answer it, let’s review the key facts about the lymphatic system.
The lymphatic system is extremely important for circulation, the absorption of
certain nutrients, and immunity. Capillaries are the smallest vessels of the circulatory system. Their walls are only one cell thick, and they are leaky, allowing fluid to move out
and fill the space around the body cells called the interstitial space. This fluid called interstitial fluid is blood plasma, the fluid part of the
blood. It contains oxygen and nutrients, which enter the cells to enable them to keep
functioning. Waste pass out of the cells into the fluid to be taken away when the fluid is
reabsorbed.
About 90 percent of the fluid is reabsorbed by the capillaries. The other 10 percent is absorbed by the lymphatic capillaries that you can see
here. Again, these are very small vessels which link up to larger vessels of the lymphatic
system.
It is important to note here that red blood cells, which are also called
erythrocytes, are too large to move out of the capillaries into the interstitial
space. So they remain in the blood. Once the fluid has entered the lymphatic capillaries, it is called lymph. It passes up the capillaries and into the vessels to be carried around the body to
lymph nodes. Here, the lymph is filtered for damaged and cancerous cells, as well as for
pathogens. White blood cells are found in the lymph nodes, including T and B lymphocytes. These respond to any pathogens in the lymph by activating an immune attack. B lymphocytes produce antibodies against pathogens, which attach to the pathogens to
try to prevent them from harming the body.
We have mentioned the circulatory system and the role the lymphatic system plays in
immunity. But what about absorption of nutrients? Let’s have a look.
In the villi of the small intestine there are blood capillaries to absorb the
majority of digested nutrients, but there are also special lymph capillaries called
lacteals. These lacteals take up fats and fat-soluble vitamins. Once the fatty acids have been absorbed by the epithelial cells of the villi, they do
not pass straight into the lymph. Instead, they are converted back into triglycerides. They then combine with other molecules, such as phospholipids and cholesterol, to
form particles called chylomicrons. Fat-soluble vitamins are also carried in these chylomicrons, if present. The chylomicrons are then transported through the lymphatic system and delivered into
the circulatory system to be used by the body.
After reviewing the lymphatic system and the fluid that it carries, we should be able
to answer our question correctly. We just learned that the lymphatic system absorbs interstitial fluid, that is, blood
plasma that has leaked out of the capillaries. Erythrocytes do not leak out as they are too large. The lymph nodes contain B lymphocytes, which produce antibodies against
pathogens. Fatty acids are absorbed from the small intestine and carried in the lymph in
particles called chylomicrons.
Therefore, the correct response to this question is (B). The option that is not a constituent existing in the lymphatic vessels is
erythrocytes.
