Okay let's get back to the small intestine,
where we left off. We talked about digestion from beginning to end. But, we still have to talk about
absorption. The small intestine is a very long organ with folds on the inside. On the folds are
villi and microvilli. Now, why in the world does a small intestine need to have this very long organ
with folds and villi? The answer is surface area. Imagine if your small intestine was flat inside.
It would look something like this. But, the villi with the microvilli and folds,
it looks something like this. Now, if you were to pull this line taut, it would
stretch out much further. See how the surface area increases by having the folds and villi We need
to have the increased surface area because we need every chance for absorption to occur.
Absorption occurs in the lining of the villi. The nutrients cross the cells of the
intestine into the transport systems inside. It does us no good to eat food if the nutrients
are not going to be absorbed into the body. The digestive tract is one big open tube from mouth
to anus. We have some sphincters along the way, but the tube is open. It does us no good
if it goes in one end and out the other. The cells, organs, and processes necessary to
life are not located inside the GI tract. We have to get the nutrients out of the GI tract and into
the transport systems to carry nutrients to cells in the body. That is what absorption is about.
We want every chance we can to absorb nutrients. Hence the long tube with folds and villi. The good
news is that the cells on the brush border of the intestine are very picky. They will only allow
entrance to very specific things it recognizes, that are in the form it requires. Therefore,
undigested particles will not pass through. There are some disease states that may interfere
with this, including some inflammatory conditions that lead to what is called leaky gut. When
the nutrients are absorbed they enter into one of two transport systems. We have the
circulatory system represented in red and blue. This is the primary route to transport
nutrients to cells, and the lymphatic system is represented in yellow. The nutrients
that are water soluble, such as amino acids, sugars, and water-soluble vitamins will enter
into the bloodstream or circulatory system. Fats and fat soluble vitamins will
enter into the lymphatic system This graphic shows an overview of the two systems. The blue and red shows the circulatory system.
If you look closely at the digestive system, you will see that all of the bloodstream
coming from the digestive tract goes to the liver first. There is a specific reason for
this that we will discuss later. After the liver blood goes to the heart and lungs to be sent
to all of the tissue in the body. The lymphatic system is designated as a light peach color.
It bypasses the liver from the digestive tract. Bats will be packaged up in the lymphatic system
so they can be transported in blood. Eventually, the nutrients in the lymphatic system will enter
the bloodstream near the heart and those nutrients will join the others in the bloodstream so it can
be pumped from the heart to the cells of the body.