Digestion - The Human Digestive System Part 1

All animals, fungi and humans are heterotrophs. A heterotroph is an organism that obtains their nutrition by breaking down organic molecules in foods. The digestive system varies depending on the type of organisms; Fungi have the simplest system and humans having the most elaborate. Like the digestive systems in all organisms, the digestive system of the human is vital for survival.

Most of the organs in the human body play a roll in the digestion of nutrients. A nutrient basically is any element or compound essential to maintain life and health. Nutrients can be split up into 2 parts; Essential nutrients and Non-essential nutrients. Essential nutrients include vitamins, minerals, amino acids, fatty acids and some carbohydrates as a source of energy. Non-essential nutrients are nutrients which the body has the ability to synthesis from other compounds, as well as, from food sources. Macro nutrients are nutrients that the body needs to acquire daily in large amounts to sustain good health, and micro nutrients are nutrients the body needs very little of and are often stored in the body; both are vital. These nutrients are found in the food we eat, this is where the body must break down and digest the food and "extract" the nutrients from it.

Believe it or not, gravity helps the body digest food. Think of the digestive system as an organic assembly line that starts at the mouths and ends at the anus and urethra. There is anywhere from 10 main parts to 100's of parts of the digestive system, depending on how detailed it needs to be explained. The digestive system is truly a marvelous attribute we have and should not be taken for granted, or be neglected. This is why a good general knowledge of the digestive system is important, not only for self/health awareness, healthy living and BIO class but because it is indeed, truly fascinating.

It all starts in the brain. The brain sends signals to the organs involved in digesting, to the muscles, mouth, glands and every thing that is living in the body. It produces hormones that make us hungry and do an assortment of other tasks that aid the digestive system. The hypothalamus (part of the brain) has two centers that control hunger. One is the appetite center, the other the satiety center. The body produces 3 main hormones that control digestion. The hormones that control digestion are gastrin, secretin, and cholecystokinin (CCK).

Gastrin causes increased stomach acid secretion and mobility of the digestive tract to move food. It is also necessary for the normal growth of the lining of the stomach, small intestine, and colon.

Secretin causes the pancreas to send out a digestive juice that is rich in bicarbonate. It stimulates the stomach to produce pepsin, an enzyme that digests protein, and it also stimulates the liver to produce bile.

CCK causes the pancreas to grow and to produce the enzymes of pancreatic juice, and it causes the gallbladder to empty.

Two others that control appetite are; Ghrelin which is produced in the stomach and upper intestine in the absence of food in the digestive system and stimulates appetite and Peptide YY which is produced in the GI tract in response to a meal in the system and inhibits appetite. They both help the brain regulate the intake of food for energy.

The Mouth:

The moment you take a bite of that apple or pull that tasty piece of chicken off of your fork with your teeth, into your mouth, the digestive system kicks in. As the mouth chews the body starts signaling the rest of the digestive system, preparing it for the work ahead. The salivary glands secrete Salivary Amylase and Ptyalin enzymes begin the chemical breakdown of starch into glucose. IgA antibodies are also found in saliva, also found present is lysozyme; an anti bacterial enzyme. Bicarbonate ions in saliva neutralize the acids in foods. Mucus moistens the food and lubricates the esophagus and then we swallow.

Esophagus:

Not much happens in the Esophagus. Its major role in digestion is directing the food into our stomachs and not our lungs, and keeping stomach content from spilling upwards into it ("heart burn" -> "esophagus burn"). The Esophagus has a sphincter that closes the stomach off from itself. Without it standing on our heads would be a messy and painful maneuver. It also has a small flap that directs water and food down into our stomach and air into our lungs. Carbohydrate digestion, begun by salivary amylase in the mouth, continues in the bolus (word for chewed up food) as it passes thru the esophagus into the stomach. The food reaches the stomach and that’s where the body really starts to work.