The Digestive System
At its simplest, the digestive system is a tube running from mouth to anus. Its chief goal is to break down huge macromolecules (proteins, fats and starch), which cannot be absorbed intact, into smaller molecules (amino acids, fatty acids and glucose) that can be absorbed across the wall of the tube, and into the circulatory system for dissemination throughout the body.
Regions of the digestive system can be divided into two main parts: the alimentary tract and accessory organs. The alimentary tract of the digestive system is composed of the mouth, pharynx, esophagus, stomach, small and large intestines, rectum and anus. Associated with the alimentary tract are the following accessory organs: salivary glands, liver, gallbladder, and pancreas.
The alimentary tract is a musculomembraneous canal about 8.5 m in length. It extends from the oral cavity to the anus. It consists of the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The liver with gallbladder and pancreas are the large glands of the alimentary tract.
The first division of the alimentary tract is formed by the mouth. Important structures of the mouth are the teeth and the tongue, which is the organ of taste. The soft and hard palates and the salivary glands are also in the oral cavity.
From the mouth food passes through the pharynx to the esophagus and then to the stomach.
The stomach is a dilated portion of the alimentary canal. It is in the upper part of the abdomen under the diaphragm. It measures about 21-25 cm in length, 8-9 cm in its greatest diameter. It has a capacity of from 2.14 to 4.28 liters (l).
The small intestine is a thin-walled muscular tube about 6.5 metres long. It is located in the middle portion of the abdominal cavity. The small intestine is composed of the duodenum, jejunum and ileum.
The large intestine is about 1.5 metres long. It is divided into caecum, colon, sigmoid colon and rectum.
The liver is the largest gland in the human body. It is in the right upper part of the abdominal cavity under the diaphragm. The weight of the liver is 1,500 g.
The gall bladder is a hollow sac lying on the lower surface of the liver.
The pancreas is a long thin gland lying behind the stomach.
proteins, fats and starch
be absorbed intact
amino acids, fatty acids and glucose
dissemination throughout the body
organ of taste
soft and hard palates
sigmoid colon ingestion
deciduous (primary) teeth
permanent (secondary) set
The mouth, or oral cavity, is the first part of the digestive tract. It is adapted to receive food by ingestion, break it into small particles by mastication, and mix it with saliva. The lips, cheeks, and palate form the boundaries. The oral cavity contains the teeth and tongue and receives the secretions from the salivary glands.
The lips and cheeks help hold food in the mouth and keep it in place for chewing. The lips contain numerous sensory receptors that are useful for judging the temperature and texture of foods.
The palate is the roof of the oral cavity. It separates the oral cavity from the nasal cavity. The anterior portion, the hard palate, is supported by bone. The posterior portion, the soft palate, is skeletal muscle and connective tissue. Posteriorly, the soft palate ends in a projection called the uvula. During swallowing, the soft palate and uvula move upward to direct food away from the nasal cavity and into the oropharynx.
The tongue manipulates food in the mouth and is used in speech. The surface is covered with papillae that provide friction and contain the taste buds.
A complete set of deciduous (primary) teeth contains 20 teeth. There are 32 teeth in a complete permanent (secondary) set. The shape of each tooth type corresponds to the way it handles food.