Digestive System

A digestive system is a group of organs working together to convert food into energy and basic nutrients to feed the entire body. food passes through a long tube inside the body known as the alimentary canal or the gastrointestinal tract (gi tract). the alimentary canal is made up of the oral cavity, pharynx, esophagus, stomach, small intestines, and large intestines. in addition to the alimentary canal, there are several important accessory organs that help your body to digest food but do not have food pass through them. accessory organs of the digestive system include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

To achieve the goal of providing energy and nutrients to the body five major functions take place in the digestive system:

• Ingestion

–       This is the taking of food into the alimentary tract, i.e. eating and drinking

• Propulsion

–       These mixes and moves the contents along the alimentary tract.

• Digestion

–       mechanical digestion of food by mastication (chewing).

–       chemical digestion of food into small molecules by enzymes present in secretions produced by glands and accessory organs of the digestive system.

• Absorption

–       This is the process by which digested food substances pass through the walls of some organs of the alimentary canal into the blood and lymph capillaries for circulation and use by body cells.

• Excretion

–       Food substances that have been eaten but cannot be digested and absorbed are excreted the alimentary canal as feces by the process of defecation.

Alimentary canal

Alimentary canal, also called digestive tract, pathway by which food enters the body and solid wastes are expelled. The alimentary canal includes:

• mouth
• pharynx
• esophagus
• stomach
• small intestine
• large intestine
• rectum and anal canal

Walls of the alimentary canal have the same four layers of tissue:

• Adventitia or serosa
• Muscle layer
• Submucosa
• Mucosa

Adventitia or serosa

This is the outermost layer. In the thorax it consists of loose fibrous tissue and in the abdomen the organs are covered by a serious membrane (serosa) called peritoneum.

Muscle layer

It consists of two layers of smooth involuntary muscle. The muscle fibers of the outer layer are arranged in a longitudinal fashion. Muscle fibers of the inner layer encircle the wall of the tube. Blood vessels, lymph vessels and a network of sympathetic and parasympathetic nerves (myenteric plexus or Auerbach’s plexus) are present between these two muscle layers.

Submucosa

The submucosa is made of loose areolar connective tissue which contains collagen and elastin fibers. It binds the muscle layer to the mucosa. The submucosa contains plexuses of blood vessels and nerves. It also contains lymph vessels and lymphoid tissues. The nerve plexus is known as submucosal plexus or Meissner’s plexus. It contains sympathetic and parasympathetic nerves that supply to the mucosal lining.

Mucosa

The mucosa (also known as mucous membrane) is the innermost layer of tissue. It’s a moist epithelial membrane that lines the alimentary canal lumen from mouth to the anus.

The mucosa has three major functions:

• Secrete mucus, digestive enzymes, and hormones.
• Absorb the end products of digestion into the blood.
• Protect against infectious disease.

3 layers of tissue:

• The epithelium is the innermost layer and it is responsible for most digestive, absorptive, and secretory processes.
• The lamina propria is a layer of connective tissue that is unusually cellular compared to most connective tissue.
• The muscular mucosa is a thin layer of smooth muscle.

Mucous membrane

In regions of the alimentary canal which are subject to mechanical stress, the mucous membrane consists of stratified squamous epithelium. Mucous secreting glands are located below the surface. In regions of the tract where the food has become soft and where secretion of digestive juices and absorption takes place, the mucous membrane consists of columnar epithelial cells interspersed with goblet cells. Goblet cells secrete mucus which provides lubrication to the walls of the alimentary canal. It also protects the walls from the digestive juices. In the regions lined with columnar epithelium, glands are situated below the surface. These glands release their secretions into the lumen of the alimentary canal. These secretions include:

• Saliva from the salivary glands.
• Gastric juice from the gastric glands.
• Intestinal juice from the intestinal glands.
• Pancreatic juice from the pancreas.
• Bile from the liver.

Digestive System (Accessory Organs)

Mouth

Food begins its journey through the digestive system in the mouth, also known as the oral cavity. Inside the mouth are many accessory organs that aid in the digestion of food—the tongue, teeth, and salivary glands. Teeth chop food into small pieces, which are moistened by saliva before the tongue and other muscles push the food into the pharynx.

Teeth

The teeth are 32 small, hard organs found along the anterior and lateral edges of the mouth. Each tooth is made of a bone-like substance called dentin and covered in a layer of enamel—the hardest substance in the body. Teeth are living organs and contain blood vessels and nerves under the dentin in a soft region known as the pulp. The teeth are designed for cutting and grinding food into smaller pieces.

Tongue

The tongue is located on the inferior portion of the mouth just posterior and medial to the teeth. It is a small organ made up of several pairs of muscles covered in a thin, bumpy, skin-like layer. The outside of the tongue contains many rough papillae for gripping food as it is moved by the tongue’s muscles. The taste buds on the surface of the tongue detect taste molecules in food and connect to nerves in the tongue to send taste information to the brain. The tongue also helps to push food toward the posterior part of the mouth for swallowing.

Salivary Glands

Surrounding the mouth are 3 sets of salivary glands. The salivary glands are accessory organs that produce a watery secretion known as saliva. Saliva helps to moisten food and begins the digestion of carbohydrates. The body also uses saliva to lubricate food as it passes through the mouth, pharynx, and esophagus.

Three pairs of major salivary glands:

• Parotid glands: these are situated one on each side of the face just below the external acoustic meatus. Each gland has a parotid duct opening into the mouth at the level of the second upper molar tooth.
• Submandibular glands: these lie on each side of the face under the angle of the jaw. The two submandibular ducts open on the floor of the mouth, one on each side of the frenulum of the tongue.
• Sublingual glands: These glands lie under the mucous membrane of the floor of the mouth in front of the submandibular glands. They have numerous small ducts that open into the floor of the mouth.

Structure of salivary gland

The glands are all surrounded by a fibrous capsule. They consist of a number of lobules made up of small acini lined with secretory cells. The secretions are poured into ductless that join up to form larger ducts leading into the mouth.

Pharynx

The pharynx, or throat, is a funnel-shaped tube connected to the posterior end of the mouth. The pharynx is responsible for the passing of masses of chewed food from the mouth to the esophagus. The pharynx also plays an important role in the respiratory system, as air from the nasal cavity passes through the pharynx on its way to the larynx and eventually the lungs. Because the pharynx serves two different functions, it contains a flap of tissue known as the epiglottis that acts as a switch to route food to the esophagus and air to the larynx.

Structures

The pharynx consists of three main divisions:

• Nasopharynx (nasal pharynx): The upper part of the pharynx, connecting with the nasal cavity above the soft palate.
• Oropharynx (oral pharynx): The oral pharynx begins at the back of the mouth cavity and continues down the throat to the epiglottis, a flap of tissue that covers the air passage to the lungs and that channels food to the esophagus. Triangular-shaped recesses in the walls of this region house the palatine tonsils, two masses of lymphatic tissue prone to infection. The isthmus connecting the oral and nasal regions is extremely beneficial in humans. It allows them to breathe through either the nose or the mouth and, when medically necessary, allows food to be passed to the esophagus by nasal tubes.
• Laryngopharynx (laryngeal pharynx): Begins at the epiglottis and leads down to the esophagus. Its function is to regulate the passage of air to the lungs and food to the esophagus.

Esophagus

The esophagus is a muscular tube connecting the throat (pharynx) with the stomach. The esophagus is about 8 inches long and is lined by moist pink tissue called mucosa. The esophagus runs behind the windpipe (trachea) and heart, and in front of the spine. Just before entering the stomach, the esophagus passes through the diaphragm.

The upper esophageal sphincter (UES) is a bundle of muscles at the top of the esophagus. The muscles of the UES are under conscious control, used when breathing, eating, belching, and vomiting. They keep food and secretions from going down the windpipe.

The lower esophageal sphincter (LES) is a bundle of muscles at the low end of the esophagus, where it meets the stomach. When the LES is closed, it prevents acid and stomach contents from traveling backward from the stomach. The LES muscles are not under voluntary control.

Stomach

Stomach, saclike expansion of the digestive system, between the esophagus and the small intestine; it is in the anterior portion of the abdominal cavity in most vertebrates. The stomach serves as a temporary receptacle for storage and mechanical distribution of food before it is passed into the intestine. In animals whose stomachs contain digestive glands, some of the chemical processes of digestion also occur in the stomach.

Structures:

The human stomach is subdivided into four regions:

• The fundus, an expanded area curving up above the cardiac opening (the opening from the stomach into the esophagus);
• The body, or intermediate region, the central and largest portion;
• The antrum, the lowermost, somewhat a funnel-shaped portion of the stomach;
• And the pylorus, a narrowing where the stomach joins the small intestine.

 

Each of the openings, the cardiac and the pyloric, has a sphincter muscle that keeps the neighboring region closed, except when food is passing through. In this manner, food is enclosed by the stomach until ready for digestion.

Like the other parts of the gastrointestinal tract, the human stomach walls consist of an outer mucosa, inner submucosa, muscular externa, and serosa.

The gastric mucosa of the stomach consists of the epithelium and the lamina propria (composed of loose connective tissue), with a thin layer of smooth muscle called the muscularis mucosae separating it from the submucosa beneath. The submucosa lies under the mucosa and consists of fibrous connective tissue, separating the mucosa from the next layer. Meissner’s plexus is in this layer.

The muscularis externa lies beneath the submucosa and is unique from other organs of the gastrointestinal tract, consisting of three layers:

• The inner oblique layer: This layer is responsible for creating the motion that churns and physically breaks down the food. It is the only layer of the three which is not seen in other parts of the digestive system. The antrum has thicker skin cells in its walls and performs more forceful contractions than the fundus.
• The middle circular layer: At this layer, the pylorus is surrounded by a thick circular muscular wall which is normally tonically constricted forming a functional (if not anatomically discrete) pyloric sphincter, which controls the movement of chyme into the duodenum. This layer is concentric to the longitudinal axis of the stomach.
• The outer longitudinal layer: is responsible for moving the bolus towards the pylorus of the stomach through muscular shortening.

The stomach also possesses a serosa, consisting of layers of connective tissue continuous with the peritoneum.

Small Intestine

The small intestine is a long, highly convoluted tube in the digestive system that absorbs about 90% of the nutrients from the food we eat. It is given the name “small intestine” because it is only 1 inch in diameter, making it less than half the diameter of the large intestine. The small intestine is, however, about twice the length of the large intestine and usually measures about 10 feet in length.

The small intestine winds throughout the abdominal cavity inferior to the stomach. Its many folds help it to pack all 10 feet of its length into such a small body cavity.

A thin membrane known as the mesentery extends from the posterior body wall of the abdominal cavity to surround the small intestine and anchor it in place. Blood vessels, nerves, and lymphatic vessels pass-through the mesentery to support the tissues of the small intestine and transport nutrients from food in the intestines to the rest of the body.

Structures:

The small intestine can be divided into 3 major regions:

• Duodenum: is the first section of intestine that connects to the pyloric sphincter of the stomach. It is the shortest region of the small intestine, measuring only about 10 inches in length. Partially digested food, or chyme, from the stomach, is mixed with bile from the liver and pancreatic juice from the pancreas to complete its digestion in the duodenum.
• Jejunum: is the middle section of the small intestine that serves as the primary site of nutrient absorption. It measures around 3 feet in length.
• Ileum: is the final section of the small intestine that empties into the large intestine via the ileocecal sphincter. The ileum is about 6 feet long and completes the absorption of nutrients that were missed in the jejunum.

Villi

The villi contain large numbers of capillaries that take the amino acids and glucose produced by digestion to the hepatic portal vein and the liver. Lacteals are the small lymph vessels that are present in villi. They absorb fatty acids and glycerol, the products of fat digestion, into direct circulation.

Large Intestine

The large intestine is wider and shorter than the small intestine (approximately 1.5 meters, or 5 feet, in length as compared with 6.7 to 7.6 meters, or 22 to 25 feet, in length for the small intestine) and has a smooth inner wall. In the proximal, or upper, half of the large intestine, enzymes from the small intestine complete the digestive process, and bacteria produce B vitamins (B12, thiamin, and riboflavin) as well as vitamin K.

The primary function of the large intestine, however, is absorption of water and electrolytes from digestive residues (a process that usually takes 24 to 30 hours) and storage of fecal matter until it can be expelled.

Churning movements of the large intestine gradually expose digestive residue to the absorbing walls. A progressive and more vigorous type of movement known as the gastrocolic reflex, which occurs only two or three times daily, propels the material toward the anus.