A number of factors stimulate us to eat. Our senses of sight, smell, and taste are stimulated by
foods. The texture of foods can also stimulate us to eat or may cause some foods to be
unappealing. These four factors interact to motivate us to eat.
Appetite is thought to be a psychological desire to consume certain foods and is generally related
to pleasant sensations associated with food. Appetite typically involves cravings for foods in the
absence of hunger. Environments and moods contribute to appetite. For people trying to lose
weight, it is important to ignore the cues of appetite to avoid overeating.
In contrast to appetite, hunger is a physiologic sensation triggered by the hypothalamus in
response to cues about stomach and intestinal distension, levels of energy substances in the
blood, and the release of certain hormones and hormone like substances. High-protein and high-
fat foods make us feel satiated for longer periods, and bulky meals fill us quickly, causing the
distension that signals us to stop eating.
Our bodies are made up of atoms, which are small units of matter. Atoms group together to form
molecules. The food we eat is composed of molecules. The ultimate goal of digestion is to break
food into molecules small enough to be passed through the gastrointestinal tract walls and easily
transported to the cells as needed.
Cells are the smallest unit of life. They perform all critical body functions, such as reproducing
new cells, utilizing nutrients, transmitting nervous impulses, and excreting waste products. Cells
are encased in a cell membrane that acts as a gatekeeper for the cell. Cells contain organelles,
which are tiny structures that perform many functions, such as making proteins, storing nutrients,
and producing energy.
Different cells types give rise to different tissue types and ultimately to different kinds of organs.
Body systems, such as the gastrointestinal system, depend on many different organs to carry out
all the varied functions they perform.
Digestion is the process by which foods are broken down into molecules. Absorption is the
process of taking the products of digestion across the GI track wall and into the body.
Elimination is the process by which undigested food, unabsorbed nutrients, and waste products
are excreted from the body. These processes take place in the gastrointestinal tract. The organs of
the GI tract include the mouth, esophagus, stomach, small intestine, and large intestine (also
called the colon or large bowel). Accessory organs, such as the pancreas, gallbladder and live
assist with digestion and absorption of nutrients.
The cephalic phase of digestion involves hunger and appetite working together to prepare the GI
tract for digestion and absorption before you take your first bite of food. Chewing initiates
mechanical digestion of food by breaking it into smaller components and mixing all nutrients
together. Chewing also stimulates chemical digestion through the secretion of digestive juices,
such as saliva. Saliva moistens food and starts the process of carbohydrate digestion through the
action of the enzyme salivary amylase. This action continues during transport of food through
the esophagus and stops when food mixes with the stomach acid. Swallowing causes our nasal passages to close and the epiglottis to cover our trachea to prevent
food from entering our sinuses and lungs. The esophagus opens as the trachea closes. The
esophagus is a muscular tube that transports food from the mouth to the stomach. The rhythmic
waves of muscles surrounding the esophagus, called peristalsis, push food toward the stomach.
Gravity helps move food toward the stomach to lesser extent. Once food reaches the stomach,
the lower esophageal sphincter (or cardiac sphincter) opens to allow food into the stomach.
The stomach prepares itself for digestion by secreting gastric juice. Gastric juice contains
substances that assist in digestion, including hydrochloric acid and the enzymes pepsin and
gastric lipase. The stomach wall also secretes mucus to protect the stomach’s lining from
digestion by HCL and enzymes. Digestion of proteins and minimal digestion of fats begins in the
stomach. The stomach mixes food into a liquid substance called chime, which is more easily
digested than solid food. The stomach holds the acidic chime and releases it periodically into
small intestine through the pyloric sphincter in very small amounts.
Most digestion and absorption occurs in the small intestine. The small intestine comprises three
sections: the duodenum, the jejunum, and the ileum. The gallbladder concentrates and stores bile,
which is produced by the liver. Bile emulsifies fat into pieces that are more easily digested. The
pancreas synthesizes and secretes digestive enzymes that break down carbohydrates, fats and
proteins into their basic building blocks: sugars, fatty acids and amino acids. The lining of the
small intestine is heavily folded, with the surface area expanded by villi and microvilli. Nutrients
are absorbed across the mucosal membrane by various processes and enter either the lymph or
the bloodstream. The liver processes all nutrients absorbed from the small intestine and stores
and regulates monosaccharides, fatty acids and amino acids.
The large intestine comprises seven sections: the cecum, ascending colon, transverse colon,
descending colon, sigmoid colon, rectum, and the anal canal. Small amounts of undigested food,
indigestible food material, bacteria, and water enter the large intestine from the small intestine.
The bacteria assist with final fermentation of any remaining digestible food products; this
fermentation causes some gas production. The main functions of the large intestine are to hold
the digestive mass and absorb any remaining nutrients and water over a 12 to 24 hour period.
The remaining substance, a semisolid mass called feces, is then eliminated from the body
through the anus.
The coordination and regulation of digestion is directed by the neuromuscular system. The
muscles of the GI tract mix food and move it from the mouth to the anus. Voluntary muscles,
which are under our conscious control, assist us with chewing and swallowing and with
expelling waste from our bodies. Once our food is swallowed, the involuntary muscles along the
entire length of the GI tract function together so that materials are moved in one direction in a
coordinated manner. The enteric nerves of the GI tract work with the central nervous system to
achieve digestion, absorption and elimination of wastes.
Heartburn is caused by the seeping of gastric juices into the esophagus. Gastroesophageal reflex
disease (or GERD) is a painful type of heartburn that occurs more than twice per week. Factors
contributing to GERD include a hiatal hernia; cigarette smoking; overweight; alcohol use;
pregnancy; spicy, acidic and fatty foods; large meals; lying down after a meal. GERD can be
treated by changing these factors and with medications. GERD can cause serious health
consequences, such as esophageal bleeding, ulcers, and cancer. Peptic ulcers are located in the stomach or duodenum and are caused by erosion of the GI tract
by hydrochloric acid and pepsin. Peptic ulcers are painful and can lead to serious health
consequences, such as