General Characteristics of the Alimentary Canal
Structure of the Wall and Movements of the Tube
Cheeks and Lips, Palate and Teeth
Salivary Secretions and Major Salivary Glands
Structure of the Pharynx, Swallowing Mechanism, and Esophagus
Structure of the Pancreas, Pancreatic Juice, and Regulation of Pancreatic Secretion
Liver Structure, Liver Functions, and Composition of Bile
Gallbladder, Regulation of Bile Release, and Functions of Bile Salts
Digestion is the mechanical and chemical breakdown of foods into forms that your body cells can absorb. The organs of the digestive system carry out digestion. The organs of the digestive system can be divided into two categories - 1) those of the alimentary canal and 2) the accessory organs. Organs of the alimentary canal extend from the mouth to the anus. They are the mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anal canal. The accessory organs include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.
Click here to see the organs of the digestive system
Click here to see the alimentary canal
The length of the entire canal is about 9 meters. Its wall consists of 4 layers as follows:
1) Mucosa - innermost layer of the wall; mostly made of epithelial tissue that secretes substances such as enzymes and mucous into the lumen, or passageway, of the canal. This layer also is very acitve in absorbing nutrients.
2) Submucosa - layer outside the mucosa; contains loose connective tissue, blood vessels, glands, and nerves. The blood vessels in this layer carry away absorbed nutrients.
3) Muscular layer - layer outside the submucosa; it is made of layers of smooth muscle tissue; this layer contracts to move materials through the canal.
4) Serosa - outermost layer of canal; it is also known as visceral peritoneum ( a serous membrane); it secretes serous fluid to keep the outside of the canal moist and prevents it from sticking to other organs.
Smooth muscle in the wall of the canal can contract in two ways to produce two basic types of movements of the tube. The muscle can contract in such a way as to mix substances in the tube. The muscle can contract a different way to propel the substances through the tract. Propelling movements are called peristalsis.
The mouth functions to intake food and reduces the size of food particles through chewing. The mouth also starts to chemically digest food since saliva (spit) contains an enzyme that will break down carbohydrates.
The cheeks consists of skin, adipose tissue, skeletal muscles and an inner lining of moist stratified squamous epithelium. The cheeks act to hold food in the mouth.
The lips surround the mouth opening and can open and close because of the orbicularis oris muscle found in them. The lips also contain a lot of sensory nerve fibers that can judge the temperature of food before it is eaten.
The tongue is mostly made of skeletal muscles and is covered by a mucous membrane. The body of the tongue is held to the floor of the oral cavity by a flap of mucous membrane called the lingual frenulum. The tongue acts to mix food in the mouth and to hold the food between teeth. It also contains taste buds so it functions in taste. The back of the tongue contains two lumps of lymphatic tissue called lingual tonsils. Lingual tonsils act to destroy bacteria and viruses on the back of the tongue.
The palate is the roof of the mouth. It functions to separate the oral cavity from the nasal cavity. The front of the palate is rigid because it has bony plates in it. Therefore the front of the palate is called the hard palate. The back of the palate is not rigid because it lacks bony material; therefore it is called the soft palate. The back of the soft palate hangs down into the throat; we call this portion of the soft palate the uvula. The uvula acts to prevent food and liquids from entering the nose during swallowing.
At the back of the mouth are two masses of lymphatic tissue called palatine tonsils. Just above the palatine tonsils are two more masses of lymphatic tissue called the pharyngeal tonsils (adenoids). These masses of lymphatic tissue act to protect the area from bacteria and viruses.
Click here to view the mouth and pharynx
The first set of teeth to form during development is called the primary teeth. As children and young adults we lose most of out primary teeth and they are replaced with a second set called permanent teeth. Most people end up with 32 permanent teeth (16 teeth in the upper jaw, 16 teeth in the lower jaw).
Teeth act to decrease the size of food particles. Different teeth are adapted to handle food in different ways. The most medial teeth called incisors act as chisels; they bite off pieces of food. Teeth called cuspids are the sharpest teeth and they act to tear tough food. The back teeth called biscuspids and molars are flat and they are designed to grind food.
Each tooth is divided into a crown and a root. The crown of the tooth is the portion that extends past the gums (gingiva). The portion of the tooth in the gingiva is the root. The crown of a tooth is covered with a very hard material called enamel. Enamel is the hardest substance in the body but once it is lost, the body cannot replace it. Underneath enamel is another hard substance called dentin. Dentin is found in the crown and the root of the tooth. Within the dentin is a cavity that contains blood vessels and nerves; this is where a tooth gets its nutrients. The portion of the cavity in the crown of the tooth is called the pulp cavity; the portion of the cavity in the root of the tooth is called the root canal.
The root of each tooth is covered with a bone-like substance called cementum. Ligaments called periodontal ligaments anchor the cementum of each tooth to the jaw bone. So, when you pull a tooth, the periodontal ligaments are broken.
Click here to view the structure of a tooth
Click here to view a set of secondary teeth
Salivary glands secrete saliva (spit) which is a mixture of water, enzymes and mucus.
Salivary glands are made of two types of cells - 1) serous cells and 2) mucous cells. Serous cells secrete a fluid make mostly of water but it also contain an enzyme called amylase the digests carbohydrates. Mucous cells secrete mucus.
All major salivary glands are paired; there is a right one and a left one. They are as follows:
1) Parotid glands - largest of the salivary glands; located beneath the skin anterior to the ears
2) Submandibular glands - located in the floor of the mouth just inside the surface of the mandibles
3) Sublingual glands - smallest of the salivary glands; found in the floor of the mouth beneath the tongue
Click here to view the location of salivary glands
The pharynx is more commonly called the throat. It is a long, muscular structure that extends from the area behind the nose to the esophagus. It acts to connect the nasal cavity with the oral cavity for breathing through the nose. It also acts to push food into the esophagus. The portion of the pharynx behind the nasal cavity is called the nasopharynx; the portion behind the oral cavity is called the oropharynx; and the portion behind the larynx is called the laryngopharynx. Food would normally only pass through the oropharynx and the laryngopharynx.
Swallowing is largely a reflex. In other words, it is usually an automatic response we do not have to think about doing. The following are the events that occur doing swallowing:
1) The soft palate raises causing the uvula to cover the opening between the nasal cavity and the oral cavity.
2) A flap-like structure called the epiglottis covers the opening of the larynx so food does not enter it.
3) The tongue presses against the roof of the mouth forcing food into the oropharynx.
4) The muscles in the pharynx contract forcing food toward the esophagus.
5) The esophagus opens.
6) Food is pushed into the esophagus by the muscles of the pharynx.
The esophagus is a muscular tube about 25 cm long that connects the pharynx to the stomach. It descends through the thoracic cavity through the diaphragm into the abdominal cavity where it joins the stomach. The hole in the diaphragm the esophagus goes through is called the esophageal hiatus. The is a common place for hernias to occur. A hernia occurs here when the stomach gets pushed up into the thoracic cavity through the esophageal hiatus.
The movement of food into the stomach is controlled by the esophageal sphincter. This sphincter must open for food to move into the stomach.
The stomach is below the diaphragm in the upper left region of the abdominal cavity. The stomach acts to receive food from the esophagus, mix the food with gastric juice (secretions of the stomach lining), start protein digestion, and move food into the small intestine.
The initial portion of the stomach that is attached to the esophagus is called the cardiac region of the stomach. The portion of the stomach that balloons over the cardiac portion is called the fundic region. The main part of the stomach is called the body of the stomach. The narrow portion of the stomach that is connected to the small intestine is called the pyloric region. Movement of substances from the pyloric region of the stomach into the small intestine is controlled by a sphincter called the pyloric sphincter.
Click here to view the regions of the stomach
The lining of the stomach contains gastric glands. These glands are made of the following cell types:
1) Mucous cells - secrete mucus to protect the lining of the stomach.
2) Chief cells - secrete pepsinogen which becomes pepsin in the presence of acid. Pepsin digests proteins.
3) Parietal cells - secrete hydrochloric acid which is necessary to convert pepsinogen to pepsin; also secretes intrinsic factor which is necessary for vitamin B12 absorption.
When a person smells, tastes or sees appetizing food, the parasympathetic nervous system stimulates the gastric glands to secrete their products.
A hormone called gastrin also stimulates gastric glands to become active. Gastrin is made by the stomach.
A hormone called cholecystokinin inhibits the gastric glands. This hormone comes from the small intestine.
The stomach does not absorb many nutrients. It can absorb alcohol, water and some lipid-soluble drugs.
When food enters the stomach, it is mixed with digestive enzymes and mucus. This mixture is called chyme. Once chyme is well mixed, stomach contractions will push it into the small intestine.
The pancreas is located behind the stomach. Pancreatic acinar cells are the cell types in the pancreas that produce a pancreatic juice that ultimately flows through the pancreatic duct to be emptied into the duodenum of the small intestine.
Click here to view the relationship between the pancreas and duodenum
Pancreatic juice is produced by the pancreatic acinar cells. It is emptied into the duodenum. It contains the following enzymes:
1) pancreatic amylase - digests carbohydrates
2) pancreatic lipase - digests lipids
3) nucleases - digests nucleic acids
4) trypsin, chymotrypsin and carboxypeptidase - digest proteins
The parasympathetic nervous system stimulates the pancreas to release its enzymes. The hormone secretin also stimulates the pancreas to release digestive enzymes. Secretin comes from the small intestine. Cholecystokinin also stimulates the pancreas to release digestive enzymes; recall this hormone also comes from the small intestine.
The liver is quite large and fills most of the upper right abdominal quadrant. It is reddish-brown in color.
The liver is enclosed by a tough capsule. This capsule also divides the liver into a large right lobe and a small left lobe. Each lobe is separated into smaller divisions called hepatic lobules. Each lobule contains many hepatic cells. Blood from the digestive organs is brought to hepatic lobules through branches of the hepatic portal vein. The hepatic lobules contain macrophages which will destroy bacteria and viruses in the blood. The cells of hepatic lobules also make bile which passes out of the liver through hepatic ducts.
The liver has many functions which include metabolism of carbohydrates, proteins, lipids, storage of vitamins, removal of bacteria and toxins from blood, and bile production.
Bile production is the only function related to digestion.
Bile contains water, bile salts, bile pigments, cholesterol and ions.
The gallbladder is a small sac-like structure located beneath the liver. Its only function is to store bile. It does not make bile. Bile is only made in the liver. Bile leaves the gallbladder through a duct called the cystic duct.
The hormone cholecystokinin will cause the gallbladder to release bile. Bile is delivered to the small intestine.
Bile salts carry out emulsification of fats. That is they break large fat globules into smaller ones so that digestive enzymes can digest them more quickly. Bile salts also in increase the absorption of fatty acids, cholesterol, and the fat-soluble vitamins into the blood stream. Remember bile has its action in the small intestine.
The small intestine is a tubular organ that extends from the stomach to the large intestine. It fills most of the abdominal cavity and is coiled. The small intestine carries out most digestion in the body and also is responsible for absorbing most of the nutrients into the blood stream.
The beginning of the small intestine is called the doudenum. It is C-shaped and relatively short. The middle portion of the small intestine is called the jejunum. It is coiled. The last portion of the small intestine is called the ileum. The ileum is directly attached to the large intestine.
Click here to view the small intestine
The lining of the small intestine consists of cells that contain microvilli. These microvilli greatly increase the surface area of the small intestine. The lining of the small intestine also contains intestinal glands. These glands secrete many enzymes.
There are many secretions that come from the lining of the small intestine; they are as follows:
1) Mucous - protects lining of small intestine.
2) Water - from intestinal glands; too much water secretion leads to diarrhea.
3) Peptidases - enzymes that digest proteins.
4) Sucrase, maltase, and lactase - enzymes that digest sugars
5) Intestinal lipase - digests fats
The stretching of the small intestine wall and the parasympathetic nervous system are the primary factors that trigger the small intestine to secrete its products.
Almost all nutrients (water, glucose, amino acids, fatty acids, glycerol, and electrolytes) are absorbed by the small intestine.
The wall of the small intestine contracts in such a way to mix chyme and to propel chyme towards the large intestine. If chyme moves too quickly through the small intestine, nutrients are not absorbed and diarrhea results.
At the end of the small intestine is a sphincter called the ileocecal sphincter. It controls the movement of chyme from the ileum to the cecum (beginning of the large intestine).
The large intestine extends from the ileum of the small intestine to where it opens to the outside world as the anus.
The beginning of the large intestine that is attached directly to the ileum is the cecum. Projecting off the cecum is the vermiform appendix. The appendix is mostly made of lymphoid tissue and has no significant function in humans. The cecum eventually gives rise to the ascending colon which is the portion of the large intestine that runs up the right side of the abdominal cavity. The ascending colon becomes the transverse colon as it crosses the abdominal cavity. The transverse colon becomes the descending colon as it descends the left side of the abdominal cavity. In the pelvic cavity the descending colon forms an S-sahped tube called the sigmoid colon. Eventually the sigmoid colon straightens out to become the rectum. The last few centimeters of the rectum is called the anal canal and the opening of the anal canal to the outside world is called the anus.
Click here to view the large intestine
The lining of the large intestine only secretes mucus to aid in the movement of substances through it. As chyme leaves the small intestine and enters the large intestine, the proximal portion of the large intestine will absorb water and a few electrolytes from it. The left over chyme is then called feces.
The contractions of the wall of the large intestine causes chyme to be mixed and propelled forward. Occasionally mass movements occur and the contents of the large intestine are forced towards the rectum. Eventually the defecation reflex occurs and the anal sphincters relax which allow feces to move to the outside world. The squeezing actions of the abdominal wall muscles are also important in the process of defecation.
Feces are made of undigested solid materials, a little water, ions, mucus, cells of the intestinal lining, and bacteria.
Nutrients are necessary food substances. They include carbohydrate, proteins, lipids, vitamins, minerals and water.
Humans get carbohydrates in the form of starch from foods such as pasta, potatoes, rice, and breads. Monosaccharides and dissacharides are obtained from sweet foods and fruits. Cellulose is a type of carbohydrate found in many vegetables that cannot be digested by humans. Therefore, cellulose provides fiber or bulk for the large intestine. This fiber helps the large intestine more regularly.
Most body cells use the monosaccharide glucose to make ATP. When a person has an excess of glucose, it can be stored in the liver and skeletal muscle cells as glycogen.
Humans can survive with little to large amounts of carbohydrate intakes.
Lipids are obtained through various foods. The most abundant dietary lipids are triglycerides. The are found in meats, eggs, milk and butter. Cholesterol is another common dietary lipid; it is found in eggs, whole milk, butter, and cheeses.
Lipids are primarily used by the body to make energy when glucose levels are low. Triglycerides stored in adipose tissue is simply stored energy. Cells also use phospholipids and cholesterol to make cell membranes. Cholesterol is also used to make hormones.
Humans must have the essential fatty acid linoleic acid in their diet as the body cannot make it. Humans must also take in a certain amount of fat to get the required amounts of fat-soluble vitamins.
Foods rich in protein include meats, eggs, milk, fish, chicken, turkey, nuts, cheese and beans.
Protein requirements vary from individual to individual. All people must take in proteins however that contain certain amino acids (called essential amino acids) as the body cannot make them.
The fat-soluble vitamins include vitamins A,D,E, and K
The water-soluble vitamins include all the B vitamins and vitamin C.
Minerals make up about 4% of your total body weight. They are primarily found in your bones and teeth. They have many functions in the body. Some help to make enzymes, others help to make cell membranes, others help to make hemoglobin, etc.
The most important minerals to the human body are calcium, phosphorus, sulfur, sodium, chlorine and magnesium.
Trace elements are elements needed in very small amounts by the body. They include iron, manganese, copper, iodine, and zinc.