us to take in and use food substances that the body converts to
energy and body structure. The digestive system includes all the
organs and glands involved in this process of eating and digesting.
Starting in the mouth, a long muscular tube provides continual fluid
and vital nutrients. The coiled intestines alone are about 24 feet
long. After we consume food, the body mechanically and chemically
breaks it down, then transports it for absorption and defecation
(final waste removal). The digestive glands (salivary glands, pancreas,
liver, and gallbladder) produce or store secretions that the body
carries to the digestive tract in ducts and breaks down chemically.
begins with ingestion (eating). The teeth aid in mechanical digestion
by masticating (chewing) food. Mastication permits easier deglutition
(swallowing) and faster chemical breakdown in the digestive tract.
During mastication, salivary glands secrete saliva to soften the
food into a bolus (semi-solid lump). Saliva contains the salivary
amylase enzyme, which digests carbohydrates (starches), and mucus
(a thick liquid), which softens food into a bolus. Ingestion starts
both chemical and mechanical digestion.
the tongue pushes the bolus toward the pharynx (throat) and into
the esophagus, a muscular tube that leads from the throat to the
stomach. To prevent food or liquid from entering the trachea (windpipe),
the epiglottis (a small flap of tissue) closes over the opening
of the larynx (voice box) during deglutition.
the esophagus, peristalsis (wave-like contractions) of smooth muscle
carries the bolus toward the stomach. Two layers of smooth muscle,
the outer longitudinal (lengthwise) and inner circular, contract
rhythmically to squeeze food through the esophagus. Throughout the
digestive tract, smooth muscle peristalsis aids in transporting
From the esophagus,
the bolus passes through a sphincter (muscular ring) into the stomach.
All sphincters located in the digestive tract help move the digested
material in one direction. When the stomach is empty, the walls
are folded into rugae (stomach folds), which allow the stomach to
expand as more food fills it.
In the stomach,
food undergoes chemical and mechanical digestion. Here, peristaltic
contractions (mechanical digestion) churn the bolus, which mixes
with strong digestive juices that the stomach lining cells secrete
(chemical digestion). The stomach walls contain three layers of
smooth muscle arranged in longitudinal, circular, and oblique (diagonal)
rows. These muscles allow the stomach to squeeze and churn the food
during mechanical digestion.
acid in the stomach helps break down the bolus into a liquid called
chyme. A thick mucus layer that lines the stomach walls prevents
the stomach from digesting itself. When mucus is limited, an ulcer
(erosion of tissue) may form.
Food is digested
in the stomach for several hours. During this time, a stomach enzyme
called pepsin breaks down most of the protein in the food. Next,
the chyme is slowly transported from the pylorus (end portion of
the stomach) through a sphincter and into the small intestine where
further digestion and nutrient absorption occurs.
and absorption: small intestine
The small intestine
is about 20 feet (6 meters) long and has three parts: the duodenum,
jejunum, and ileum. The duodenum is where most chemical digestion
takes place. Here, bile from the gallbladder and enzymes from the
pancreas and intestinal walls combine with the chyme to begin the
final part of digestion.
is created in the liver and stored in the gallbladder. Bile emulsifies
(breaks into small particles) lipids (fats), which aids in the mechanical
digestion of fats. The pancreas and gland cells of the small intestine
secrete digestive enzymes that chemically break down complex food
molecules into simpler ones. These enzymes include trypsin (for
protein digestion), amylase (for carbohydrate digestion), and lipase
(for lipid digestion). When food passes through the duodenum, digestion
From the duodenum,
chyme passes to the jejunum and ileum. Here, tiny villi (finger-like
projections) cover the walls of the small intestine. The cells that
line the villi are covered with small projections called microvilli
(brush border). These projections increase the surface area of the
small intestine, allowing the chyme to contact more of the small
intestine wall. The increased contact causes more efficient food
absorption, food molecules enter the bloodstream through the intestinal
walls. Capillaries (microscopic blood vessels) within the villi
absorb products of protein and carbohydrate digestion. Lymph vessels
(lacteals) within the villi absorb products of fat digestion and
eventually lead to the bloodstream.
From the small
intestine, digested products travel to the liver, one of the body's
most versatile organs. Hepatocytes (liver cells) detoxify (filter)
blood of harmful substances such as alcohol and ammonia. And, hepatocytes
store fat-soluble vitamins and excess substances such as glucose
(sugar) for release when the body requires extra energy.
Once food has
passed through the small intestine, it is mostly undigestible material
and water. It enters the colon (large intestine), named for its
wide diameter. The large intestine has six parts: the cecum, ascending
colon, transverse colon, descending colon, sigmoid colon, and rectum.
The large pouch-shaped
cecum marks the beginning of the colon. Attached near the cecum
bottom is the vermiform (worm-like) appendix. The appendix contains
lymphoid tissue and intercepts pathogenic microorganisms that enter
the digestive tract. Sometimes, fecal matter may become trapped
in the appendix, resulting in appendicitis (infection and inflammation).
The other parts
of the colon absorb water and minerals from the undigested food
and compact the remaining material into feces. Defecation is the
digestive process final stage: feces (undigested waste products)
are carried to the rectum through peristalsis and eliminated through