Food must be broken down into a form that usable by the body's cells. This is done by the digestive system. Mechanical digestion begins in the mouth when food is broken down into smaller pieces. Chemical digestion breaks the food down into molecules that can move through the cell membrane. The digestive system can be broken down into two groups: the alimentary canal and its accessory organs.
The Alimentary Canal
The alimentary canal, or digestive tract, is where food travels and is broken down and absorbed. It is made up of the mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anus. The wall of the alimentary canal consists of four layers, although their characteristics may vary from one organ to another.
The innermost lining of the alimentary canal is the mucous membrane, or mucosa. It is a protective layer that may have invaginations that are used for the secretion of mucus and enzymes. Some areas of the mucosa have projections or folds that increase the surface area of the canal for better absorption.
The submucosa lies beneath the mucosa. It contains blood vessels that feed the surrounding tissues and carry away nutrients absorbed by the mucosa. It also contains glands, nerves, and lymphatic vessels.
The muscular layer contains two types of muscles that are responsible for contracting and relaxing the parts of the canal. These muscular movements are responsible for two kinds of movement within the alimentary canal. Mixing movements, like that of the stomach, combine food with digestive enzymes. Peristalsis is the process of moving food forward within the canal. This is done within the small and large intestines.
The outermost layer of the alimentary canal is the serous layer, or serosa. It serves as a protective covering and secretes serous fluid so that the abdominal organs can move freely within the abdomen.
Accessory Organs
The digestive system also consists of accessory organs including the salivary glands, gallbladder, liver, and pancreas. Salivary glands secrete saliva into the mouth. Saliva contains enzymes that begin chemical digestion. Saliva also contains mucus which moistens food and makes it easier to move through the alimentary canal.
Bile contains salts that break down fats in food. The liver makes bile and stores it in the gallbladder. When food is emptied into the duodenum from the stomach, the gallbladder releases bile that is mixed with the food. The gallbladder often fully contracts after a meal. This is one of the reasons why fasting is necessary before medical examinations such as abdominal ultrasound. The gallbladder cannot be seen if it is fully contracted.
The pancreas secretes pancreatic juice into the duodenum at about the same time the gallbladder releases bile. Pancreatic juice contains enzymes that break down proteins, fats, nucleic acids, and carbohydrates. The pancreas also serves as an endocrine organ because it releases insulin and glucagon, which are two hormones responsible for regulating blood sugar levels.
The digestive system consists of several organs that are responsible for breaking food down into a form that the body can use for cellular processes. Nutrients and water are absorbed along the length of the alimentary canal. The components of food that the body cannot break down are excreted from the body in the form of feces.
Reference
Shier, D., Butler, J., Lewis, Ricki. Hole's Human Anatomy & Physiology. New York: McGraw-Hill, 2002.
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