The digestive system is composed of 2 main components: the gastrointestinal tract, or
GI tract, where digestion and absorption take place; and accessory organs which secrete
various fluids/enzymes to help with digestion.
The GI tract is a continuous chain of organs where food enters at one end and waste gets
out from the other.
These organs are lined with smooth muscles whose rhythmic contractions generate waves
of movement along their walls, known as peristalsis.
Peristalsis is the force that propels food down the tract.
Digestion is the process of breaking down food into smaller, simpler components, so
they can be absorbed by the body.
Basically, carbohydrates such as sugars and starch are broken down into glucose, proteins
into amino acids, and fat molecules into fatty acids and glycerol.
Digestion starts in the oral cavity where the food is moistened with saliva and chewed,
food bolus is formed to facilitate swallowing.
Saliva is secreted by the salivary glands and contains the enzyme amylase.
Amylase breaks down starch into maltose and dextrin which are processed further in the
small intestine.
The food bolus is propelled down the esophagus into the stomach, the major organ of the GI
tract.
The stomach produces gastric juice containing pepsin, a protease, and hydrochloric acid
which act to digest proteins.
At the same time, mechanical churning is performed by muscular contraction of the stomach wall.
The result is the formation of chyme, a semi-liquid mass of partially digested food.
Chyme is stored in the stomach and is slowly released into the first part of the small
intestine, the duodenum.
The duodenum receives the following digestive enzymes from accessory organs:
- Bile, produced in the liver and stored in the gallbladder; bile emulsifies fats and
makes it easier for lipases to break them down.
- Pancreatic juice from the pancreas.
This mixture contains proteases, lipases and amylase, and plays major role in digestion
of proteins and fats.
The small intestine also produces its own enzymes: peptidases, sucrase, lactase, and
maltase.
Intestinal enzymes contribute mainly to the hydrolysis of polysaccharides.
The small intestine is where most of digestion and absorption take place.
The walls of the small intestine absorb the digested nutrients into the bloodstream, which
in turn delivers them to the rest of the body.
In the small intestine, the chyme moves more slowly allowing time for thorough digestion
and absorption.
This is made possible by segmentation contractions of the circular muscles in the intestinal
walls.
Segmentation contractions move chyme in both directions.
This allows a better mixing with digestive juices and a longer contact time with the
intestinal walls.
The large intestine converts digested left-over into feces.
It absorbs water and any remaining nutrients.
The bacteria of the colon, known as gut flora, can break down substances in the chyme that
are not digestible by the human digestive system.
Bacterial fermentation produces various vitamins that are absorbed through the walls of the
colon.
The semi-solid fecal matter is then stored in the r. until it can be pushed out from
the body during a bowel movement.