Need to know before starting Lesson 2: It is easy to think hormones are just related to the female reproductive system, but they aren't. In fact there are many, many hormones surging through your body at this moment, and there are many places in your body that produce them. Because this tutorial is just a broad overview, we will say hormones deliver telegrams from one place in the body to another. You may not realize that insulin and adrenaline are hormones. Steroids, too.
Say you take a bite of a hamburger. You use your teeth to get the hamburger into little pieces. The saliva your mouth produces has an enzyme in it that already begins to break down any starches in the bread into sugar. Absorption of nutrients can begin in the mouth. (B12 supplements and nitroglycerin are common examples of people using the mouth as the delivery system for chemicals.) When you swallow the hamburger mush, it moves to the next part of the digestive system to the pharynx. The pharynx is the part you see if you open your mouth really wide in a mirror and look down your throat. It's where swallowing happens. No absorption of nutrients happens in the pharynx.
The pharynx passes the wad of hamburger mush to the esophagus, which moves automatically to push food down to the stomach. The hormone gastrin delivers the message to the stomach to start secretion of gastric juices. The stomach acids use chemical reactions break down the food more. The acids also stop the enzyme action that occurred to the bread in the mouth, and begins another enzyme action to start breaking up the hamburger. Also, this acid attacks the chains of amino acids (proteins). This allows the nutrients to become of use to the rest of the digestive tract. Very little absorption of nutrients happens here. Incidentally, the bacteria left behind by the fly that landed on your hamburger bun when you weren't looking also get destroyed.
Once the used-to-be-a-hamburger enters the small intestine from the stomach, other hormones deliver the message that gastric juices can start to lessen. Now the biggest part of breaking down and absorbing nutrients begins. In the first foot of the small intestine, the pancreas secretes stuff to help the nutrients further break down. Also, the gall bladder puts stuff the liver has given it into that intestine to help break down the fats in the former hamburger, and helps the body absorb them.
There are a vast number of other secretions, hormones, and actions that occur in the small intestine. The most important thing to know is that the sludge from that burger covers the inside of the small intestine and hair-like things on the wall take the nutrients and put them into the bloodstream and lymph system, which will transport them to the other areas of the body. This process has taken about 4 hours.
Finally, the large intestine receives whatever is left, and over the next 24 hours, minerals, some vitamins, and water are further absorbed. "Good" bacteria also help break down certain parts of the sludge, which the body also absorbs.
The rest is history.