The digestive system includes the digestive tract and its accessory organs, which process food into molecules that can be absorbed and utilized by the cells of the body. Food is broken down, bit by bit, until the molecules are small enough to be absorbed and the waste products are eliminated. The digestive tract, also called the alimentary canal or gastrointestinal (GI) tract, consists of a long continuous tube that extends from the mouth to the anus. It includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The tongue and teeth are accessory structures located in the mouth. The salivary glands, liver, gallbladder, and pancreas are major accessory organs that have a role in digestion. These organs secrete fluids into the digestive tract.
Food undergoes three types of processes in the body:
- Digestion
- Absorption
- Elimination
Digestion and absorption occur in the digestive tract. After the nutrients are absorbed, they are available to all cells in the body and are utilized by the body cells in metabolism.
The digestive system prepares nutrients for utilization by body cells through six activities, or functions.
Ingestion
The first activity of the digestive system is to take in food through the mouth. This process, called ingestion, has to take place before anything else can happen.
Mechanical Digestion
The large pieces of food that are ingested have to be broken into smaller particles that can be acted upon by various enzymes. This is mechanical digestion, which begins in the mouth with chewing or mastication and continues with churning and mixing actions in the stomach.
Chemical Digestion
The complex molecules of carbohydrates, proteins, and fats are transformed by chemical digestion into smaller molecules that can be absorbed and utilized by the cells. Chemical digestion, through a process called hydrolysis, uses water and digestive enzymes to break down the complex molecules. Digestive enzymes speed up the hydrolysis process, which is otherwise very slow.
Movements
After ingestion and mastication, the food particles move from the mouth into the pharynx, then into the esophagus. This movement is deglutition, or swallowing. Mixing movements occur in the stomach as a result of smooth muscle contraction. These repetitive contractions usually occur in small segments of the digestive tract and mix the food particles with enzymes and other fluids. The movements that propel the food particles through the digestive tract are called peristalsis. These are rhythmic waves of contractions that move the food particles through the various regions in which mechanical and chemical digestion takes place.
Absorption
The simple molecules that result from chemical digestion pass through cell membranes of the lining in the small intestine into the blood or lymph capillaries. This process is called absorption.
Elimination
The food molecules that cannot be digested or absorbed need to be eliminated from the body. The removal of indigestible wastes through the anus, in the form of feces, is defecation or elimination.
Topic Resources The Digestive System
How the Esophagus Works
As a person swallows, food moves from the mouth to the throat, also called the pharynx (1). The upper esophageal sphincter opens (2) so that food can enter the esophagus, where waves of muscular contractions, called peristalsis, propel the food downward (3). The food then passes through the diaphragm (4) and lower esophageal sphincter (5) and moves into the stomach.
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"Just a spoonful of sugar... " goes the song. But what happens to that sugar once you swallow it? In fact, how is it that you are able to swallow it at all? Your digestive system performs amazing feats every day, whether you eat a double cheeseburger or a stalk of celery. Read on to learn what exactly happens to food as it makes its
way through your digestive system. Digestion is the complex process of turning the food you eat into nutrients, which the body uses for energy, growth and cell repair needed to survive. The digestion process also involves creating waste to be eliminated. The digestive tract (or
gastrointestinal tract) is a long twisting tube that starts at the mouth and ends at the anus. It is made up of a series of muscles that coordinate the movement of food and other cells that produce enzymes and hormones to aid in the breakdown of food. Along the way are other 'accessory' organs that are needed for digestion: liver and the pancreas.What Is Digestion?
Food's Journey Through the Digestive System
Stop 1: The Mouth
The mouth is the beginning of the digestive system, and, in fact, digestion starts here before you even take the first bite of a meal. The smell of food triggers the salivary glands in your mouth to secrete saliva, causing your mouth to water. When you actually taste the food, saliva increases.
Once you start chewing and breaking the food down into pieces small enough to be digested, other mechanisms come into play. More saliva is produced. It contains substances including enzymes that begin the process of breaking down food into a form your body can absorb and use. Chew your food more -- it also helps with your digestion.
Stop 2: The Pharynx and Esophagus
Also called the throat, the pharynx is the portion of the digestive tract that receives the food from your mouth. Branching off the pharynx is the esophagus, which carries food to the stomach, and the trachea or windpipe, which carries air to the lungs.
The act of swallowing takes place in the pharynx partly as a reflex and partly under voluntary control. The tongue and soft palate -- the soft part of the roof of the mouth -- push food into the pharynx, which closes off the trachea. The food then enters the esophagus.
The esophagus is a muscular tube extending from the pharynx and behind the trachea to the
stomach. Food is pushed through the esophagus and into the stomach by means of a series of contractions called peristalsis. Just before the opening to the stomach is an important ring-shaped muscle called the lower esophageal
sphincter (LES). This sphincter opens to let food pass into the stomach and closes to keep it there. If your LES doesn't work properly, you may suffer from a condition called GERD, or reflux, which causes heartburn and regurgitation (the feeling of food coming back up). Stop 3: The Stomach and Small Intestine The
stomach is a sac-like organ with strong muscular walls. In addition to holding food, it serves as the mixer and grinder of food. The stomach secretes acid and powerful enzymes that continue the process of breaking the food down and changing it to a consistency of liquid or paste. From there, food moves to the small intestine. Between meals, the non-liquefiable remnants are released from the stomach and ushered through the rest of the
intestines to be eliminated. Made up of three segments -- the duodenum, jejunum, and ileum -- the small intestine also breaks down food using enzymes released by the pancreas and bile from the
liver. The small intestine is the 'workhorse' of digestion, as this is where most nutrients are absorbed. Peristalsis is also at work in this organ, moving food through and mixing it up with the digestive secretions from the pancreas and liver, including
bile. The duodenum is largely responsible for the continuing breakdown process, with the jejunum and ileum being mainly responsible for absorption of nutrients into the bloodstream. A more technical name for this part of the process is "motility," because it involves moving or emptying food particles from one part to the next. This process is highly dependent on the activity of a large network of nerves, hormones, and muscles. Problems with any of these components can cause a variety of
conditions. While food is in the small intestine, nutrients are absorbed through the walls and into the bloodstream. What's leftover (the waste) moves into the large intestine (large bowel or colon).
Everything above the large intestine is called the upper GI tract. Everything including the large intestine
and below is the lower GI tract. Stop 4: The Colon, Rectum, and Anus The colon (large intestine) is a five- to seven -foot -long muscular tube that connects the small intestine to the rectum. It is made up of the cecum, the ascending (right) colon, the transverse (across) colon, the descending (left) colon and the sigmoid colon, which connects to the rectum. The
appendix is a small tube attached to the ascending colon. The large intestine is a highly specialized organ that is responsible for processing waste so that defecation (excretion of waste) is easy and convenient. Stool, or waste left over from the digestive process, passes through the colon by means of peristalsis, first in a liquid state and ultimately in solid
form. As stool passes through the colon, any remaining water is absorbed. Stool is stored in the sigmoid (S-shaped) colon until a "mass movement" empties it into the rectum, usually once or twice a day. It normally takes about 36 hours for stool to get through the colon. The stool itself is mostly food debris and bacteria. These bacteria perform several useful functions, such as synthesizing various
vitamins, processing waste products and food particles, and protecting against harmful bacteria. When the descending colon becomes full of stool, it empties its contents into the rectum to begin the process of elimination. The rectum is an eight-inch chamber that connects the colon to the anus. The rectum: When anything (gas or stool) comes into the rectum, sensors send a message to the brain. The brain then decides if the rectal contents can be
released or not. If they can, the sphincters relax and the rectum contracts, expelling its contents. If the contents cannot be expelled, the sphincters contract and the rectum accommodates so that the sensation temporarily goes away. The anus is the last part of the digestive tract. It consists of the muscles that line the pelvis (pelvic floor muscles) and two other muscles called anal sphincters (internal and external). The pelvic floor
muscle creates an angle between the rectum and the anus that stops stool from coming out when it is not supposed to. The anal sphincters provide fine control of stool. The internal sphincter is always tight, except when stool enters the rectum. It keeps us continent (not releasing stool) when we are asleep or otherwise unaware of the presence of stool. When we get an urge to defecate (go to the bathroom), we rely on our external sphincter to keep the stool in until we can get to the toilet.
Pancreas Liver GallbladderAccessory Digestive Organs
Among other functions, the pancreas is the chief factory for digestive enzymes that are secreted into the duodenum, the first segment of the small intestine. These enzymes break down protein, fats, and carbohydrates.
The liver has multiple functions, but two of its main functions within the digestive system are to make and secrete an important substance called bile and to process the blood coming from the small intestine containing the nutrients just absorbed. The liver purifies this
blood of many impurities before traveling to the rest of the body.
The gallbladder is a storage sac for excess bile. Bile made in the liver travels to the small intestine via the bile ducts. If the intestine doesn't need it, the bile travels into the gallbladder, where it awaits the signal from the
intestines that food is present. Bile serves two main purposes. First, it helps absorb fats in the diet, and secondly, it carries waste from the liver that cannot go through the kidneys.