Digestion is the complex process of turning the food you eat into the energy you need to survive. The digestion process also involves creating waste to be eliminated.
The digestive tract (or gut) 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 three other organs that are needed for digestion: the liver, gallbladder and the pancreas.
The mouth is the beginning of the digestive tract, 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 to begin the process of breaking down food into a form your body can absorb and use. In addition, “juices” are produced that will help to further break down food.
The Pharynx & 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, which causes heartburn and regurgitation (the feeling of food coming back up).
The Stomach & 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. 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” since 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 in the small intestine nutrients from food are absorbed through the walls of the intestine 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 below is the lower GI tract.
The Large Intestine & the 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 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. The 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: Receives stool from the colon Lets the person know there is stool to be evacuated Holds the stool until evacuation happens 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: 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.
Liver: 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.
Gallbladder: 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.