Digestion
 |
| Digestion in Human |
What do you understand by Digestion?
INTRODUCTION
Digestive System, series of connected
organs whose purpose is to break down, or digest, the food we eat. Food is made
up of large, complex molecules, which the digestive system breaks down into
smaller, simple molecules that can be absorbed into the bloodstream. The simple
molecules travel through the bloodstream to all of the body's cells, which use
them for growth, repair, and energy.
All animals have a digestive
system, a feature that distinguishes them from plants. Plants produce their own
food in a process called photosynthesis, during which they use sunlight to
convert water and carbon dioxide into simple sugars. But animals, including
humans, must take in food in the form of organic matter, such as plants or
other animals.
Digestion generally involves two phases: a mechanical
phase and a chemical phase. In the mechanical phase, teeth or other structures
physically break down large pieces of food into smaller pieces. In the chemical
phase, digestive chemicals called enzymes break apart individual molecules of
food to yield molecules that can be absorbed and distributed throughout the
body. These enzymes are secreted (produced and released) by glands in
the body.
The digestive system of most animals consists
mainly of a long, continuous tube called the alimentary canal, or digestive
tract. This canal has a mouth at one end, through which food is taken in, and
an anus at the other end, through which digestive wastes are excreted. Muscles
in the walls of the alimentary canal move the food along. Most digestive organs
are part of the alimentary canal. However, two accessory digestive organs, the
liver and pancreas, are located outside the alimentary canal. These organs
contribute to chemical digestion by releasing digestive juices into the canal
through tubes called ducts.
DIGESTION IN HUMAN
If a human adult’s digestive
tract were stretched out, it would be 6 to 9 m (20 to 30 ft) long. In humans,
digestion begins in the mouth, where both mechanical and chemical digestion
occur. The mouth quickly converts food into a soft, moist mass. The muscular
tongue pushes the food against the teeth, which cut, chop, and grind the food.
Glands in the cheek linings secrete mucus, which lubricates the food, making it
easier to chew and swallow. Three pairs of glands empty saliva into the mouth
through ducts to moisten the food. Saliva contains the enzyme ptyalin, which
begins to hydrolyze (break down) starch—a carbohydrate manufactured by
green plants.
Once food has been reduced
to a soft mass, it is ready to be swallowed. The tongue pushes this mass—called
a bolus—to the back of the mouth and into the pharynx. This cavity between the
mouth and windpipe serves as a passageway both for food on its way down the
alimentary canal and for air passing into the windpipe. The epiglottis, a flap
of cartilage, covers the trachea (windpipe) when a person swallows. This action
of the epiglottis prevents choking by directing food from the windpipe and
toward the stomach.
THE ESOPHAGUS
The presence of food in
the pharynx stimulates swallowing, which squeezes the food into the esophagus.
The esophagus, a muscular tube about 25 cm (10 in) long, passes behind the
trachea and heart and penetrates the diaphragm (muscular wall between the chest
and abdomen) before reaching the stomach. Food advances through the alimentary
canal by means of rhythmic muscle contractions (tightenings) known as
peristalsis. The process begins when circular muscles in the esophagus wall
contract and relax (widen) one after the other, squeezing food downward toward
the stomach. Food travels the length of the esophagus in two to three seconds.
A circular muscle called the
esophageal sphincter separates the esophagus and the stomach. As food is
swallowed, this muscle relaxes, forming an opening through which the food can
pass into the stomach. Then the muscle contracts, closing the opening to
prevent food from moving back into the esophagus. The esophageal sphincter is
the first of several such muscles along the alimentary canal. These muscles act
as valves to regulate the passage of food and keep it from moving backward.
THE STOMACH
The stomach, located in
the upper abdomen just below the diaphragm, is a saclike structure with strong,
muscular walls. The stomach can expand significantly to store all the food from
a meal for both mechanical and chemical processing. The stomach contracts about
three times per minute, churning the food and mixing it with gastric juice. This
fluid, secreted by thousands of gastric glands in the lining of the stomach,
consists of water, hydrochloric acid, an enzyme called pepsin, and mucin
(the main component of mucus). Hydrochloric acid creates the acidic environment
that pepsin needs to begin breaking down proteins. It also kills microorganisms
that may have been ingested in the food. Mucin coats the stomach, protecting it
from the effects of the acid and pepsin. About four hours or less after a meal,
food processed by the stomach, called chyme, begins passing a little at a time
through the pyloric sphincter into the duodenum, the first portion of the small
intestine.
THE SMALL INTESTINE
Most digestion, as well as
absorption of digested food, occurs in the small intestine. This narrow,
twisting tube, about 2.5 cm (1 in) in diameter, fills most of the lower
abdomen, extending about 6 m (20 ft) in length. Over a period of three to six
hours, peristalsis moves chyme through the duodenum into the next portion of
the small intestine, the jejunum, and finally into the ileum, the last section
of the small intestine. During this time, the liver secretes bile into the
small intestine through the bile duct. Bile breaks large fat globules into
small droplets, which enzymes in the small intestine can act upon. Pancreatic
juice, secreted by the pancreas, enters the small intestine through the
pancreatic duct. Pancreatic juice contains enzymes that break down sugars and
starches into simple sugars, fats into fatty acids and glycerol, and proteins
into amino acids. Glands in the intestinal walls secrete additional enzymes
that break down starches and complex sugars into nutrients that the intestine
absorbs. Structures called Brunner’s glands secrete mucus to protect the
intestinal walls from the acid effects of digestive juices.The small
intestine’s capacity for absorption is increased by millions of fingerlike
projections called villi, which line the inner walls of the small intestine.
Each villus is about 0.5 to 1.5 mm (0.02 to 0.06 in) long and covered with a
single layer of cells. Even tinier fingerlike projections called microvilli
cover the cell surfaces. This combination of villi and microvilli increases the
surface area of the small intestine’s lining by about 150 times, multiplying
its capacity for absorption. Beneath the villi’s single layer of cells are
capillaries (tiny vessels) of the bloodstream and the lymphatic system. These
capillaries allow nutrients produced by digestion to travel to the cells of the
body. Simple sugars and amino acids pass through the capillaries to enter the
bloodstream. Fatty acids and glycerol pass through to the lymphatic system.
THE LARGE INTESTINE
A watery residue of indigestible
food and digestive juices remains unabsorbed. This residue leaves the ileum of
the small intestine and moves by peristalsis into the large intestine, where it
spends 12 to 24 hours. The large intestine forms an inverted U over the coils
of the small intestine. It starts on the lower right-hand side of the body and
ends on the lower left-hand side. The large intestine is 1.5 to 1.8 m (5 to 6
ft) long and about 6 cm (2.5 in) in diameter.The large intestine serves several
important functions. It absorbs water—about 6 liters (1.6 gallons) daily—as
well as dissolved salts from the residue passed on by the small intestine. In
addition, bacteria in the large intestine promote the breakdown of undigested
materials and make several vitamins, notably vitamin K, which the body needs
for blood clotting. The large intestine moves its remaining contents toward the
rectum, which makes up the final 15 to 20 cm (6 to 8 in) of the alimentary
canal. The rectum stores the feces—waste material that consists largely of
undigested food, digestive juices, bacteria, and mucus—until elimination. Then,
muscle contractions in the walls of the rectum push the feces toward the anus.
When sphincters between the rectum and anus relax, the feces pass out of the
body.
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