Notes

Several digestive organs work
together to break down food.
Digestion is the process by which your body breaks
down the large molecules in your food into simpler
molecules that it can use. The digestive system is the
organ system that breaks food down into energy that
can be used by cells.
The organs of the digestive system include the
mouth, esophagus, stomach, pancreas, liver, gallbladder,
large and small intestines, rectum, and anus. Each
section is separated by a ring of muscle, called a
sphincter. These rings of muscles open and close to
allow food to pass through. The smooth muscles in the
walls of digestive organs contract* and relax to keep
food moving in only one direction.
Some organs, such as the mouth, stomach, and
small intestine, secrete different substances to help break
food apart.
Salivary glands in the mouth secrete an enzyme
that breaks down starches. Stomach acid includes enzymes
that break down proteins.
Once food is completely digested nutrients move into the circulatory and lymphatic systems
and are carried to all cells. Undigested material leaves the body as liquid
and solid wastes. The process takes about 24 to 33 hours.

Digestion in the Mouth
Digestion begins when you take a bite of food. Your teeth provide
mechanical* digestion as you chew your food, shredding it into small
pieces. Chemical digestion also occurs in the mouth. Your salivary glands
make saliva, which contains an enzyme called amylase. Amylase breaks
complex starch molecules into simple sugars.
When you swallow your food, it moves into your esophagus, a tube
that connects the mouth and stomach. The muscles in the walls of the
esophagus contract and relax, pushing the food down toward the stomach.
These movements of the muscles are called peristalsis.






Digestion in the Stomach
The stomach is a muscular sac that can hold about 2 liters (2 qt) of food.
It can stretch to almost twice its size. The stomach’s three layers of
muscle contract every 20 seconds, churning the food into small pieces
and mixing it with digestive juices. This action is also part of mechanical
digestion. The stomach lining secretes hydrochloric acid (HCl) and
pepsin, a digestive enzyme. Gastric acid kills bacteria, while pepsin
breaks bonds between amino acids in proteins. Together, they reduce
food to a semi-liquid mixture called chyme.
The contractions of your stomach push chyme up against the sphincter
that separates the stomach from the small intestine. Each contraction
opens the sphincter enough to allow some chyme to squirt into the small
intestine. It takes two to six hours to empty the stomach after a meal.
Digestion is completed in part of the
small intestine.
Most digestion takes place in the small intestine, a
long, narrow tube that is attached at one end to the
stomach. There, muscle contractions continue to
churn the food, and chemical digestion breaks
complex molecules into simpler molecules.
The section of small intestine closest to the
stomach is called the duodenum. The duodenum
finishes the process of digestion, with help from the
pancreas, liver, and gallbladder. The pancreas makes
an alkaline fluid to neutralize the pepsin that comes
with chyme from the stomach. The pancreas also
releases enzymes that help break down fats. The liver makes bile, a substance that also breaks down fats. The gallbladder stores the bile, releasing it into the duodenum. By the
time chyme passes through the duodenum, it has been broken into
simple molecules that the body can use.
Most absorption of nutrients occurs in the small
intestine.
Breaking food apart is only the first part of digestion. Once food is
digested, the body must absorb* its nutrients.
Absorption is the process
by which the nutrients in your food move from the digestive organs into
the circulatory and lymphatic systems. The small intestine
that help your body absorb nutrients from chyme: the lining,
the villi,
The folds in the intestine’s
lining are covered with villi, small fingerlike structures.
The villi are covered with epithelial cells that absorb nutrients.
Water is absorbed and solid wastes are
eliminated from the large intestine.
The large intestine, or colon, is about 1.5 meters (5 ft) long and wider
than the small intestine. Every day, the large intestine absorbs about 1
liter of water, along with some salts. The remaining material forms a solid