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Home , Duodenojejunal flexure, Duodenum, Gastric glands, Gastrointestinal tract, Taenia coli

Structural organisation and function of

(4th Sem CC4-9-TH)

Introduction :

The gastrointestinal tract (GIT) consists of a hollow muscular tube starting from the oral cavity, where food enters the , continuing through the , oesophagus, and intestines to the and , where food is expelled. There are various accessory organs that assist the tract by secreting to help break down food into its component nutrients. Thus the salivary , , and gall bladder have important functions in the digestive system. Food is propelled along the length of the GIT by peristaltic movements of the muscular walls. The primary purpose of the gastrointestinal tract is to break food down into nutrients, which can be absorbed into the body to provide energy. First food must be ingested into the mouth to be mechanically processed and moistened. Secondly, occurs mainly in the stomach and where proteins, fats and are chemically broken down into their basic building blocks. Smaller molecules are then absorbed across the of the small intestine and subsequently enter the circulation. The plays a key role in reabsorbing excess water. Finally, undigested material and secreted waste products are excreted from the body via (passing of faeces). In the case of or disorders, these functions of the gastrointestinal tract are not achieved successfully. Patients may develop symptoms of , , diarrhoea, , or obstruction. Gastrointestinal problems are very common and most people will have experienced some of the above symptoms several times throughout their lives.

Anatomy of the Digestive System

The gastrointestinal tract refers to the stomach and intestine, and sometimes to all the structures from the mouth to the anus.

Upper Gastrointestinal Tract

The upper gastrointestinal tract consists of the , stomach, and . The exact demarcation between upper and lower can vary. Upon gross , the duodenum may appear to be a unified , but it is often divided into two parts based upon function, arterial supply, or .

The upper gastrointestinal tract includes the:

• Esophagus, the fibromuscular tube that food passes through—aided by peristaltic contractions—the pharynx to the stomach.

• Stomach, which secretes -digesting enzymes called and strong acids to aid in food digestion, before sending the partially digested food to the small intestines.

• Duodenum, the first section of the small intestine that may be the principal site for absorption.

Lower Gastrointestinal Tract

The lower gastrointestinal tract includes most of the small intestine and all of the large intestine. According to some sources, it also includes the anus.

The small intestine has three parts:

• Duodenum: Here the digestive juices from the pancreas ( digestive enzymes ) and the ( ) mix together. The digestive enzymes break down proteins and bile and emulsify fats into . The duodenum contains Brunner’s glands that produce , and that contains bicarbonate to neutralize in the stomach.

• : This is the midsection of the intestine, connecting the duodenum to the . It contains the plicae circulares and villi to increase the surface area of that part of the GI tract.

• Ileum: This has villi, where all soluble molecules are absorbed into the blood ( through the and ).

The large intestine has four parts:

1. , the vermiform that is attached to the cecum. 2. Colon, which includes the , , , and sigmoid flexure. The main function of the colon is to absorb water, but it also contains that produce beneficial like K. 3. Rectum. 4. Anus.

Upper and lower gastrointestinal tract: The major organs of the human gastrointestinal system

HISTOLOGY OF THE ALIMENTARY CANAL The gastrointestinal tract is a muscular tube lined by a special layer of cells, called epithelium. The contents of the tube are considered external to the body and are in continuity with the outside world at the mouth and the anus. Although each section of the tract has specialised functions, the entire tract has a similar basic structure with regional variations.

Layers of the Alimentary Canal. The wall of the alimentary canal has four basic layers: the mucosa, , muscularis, and serosa. The wall is divided into four layers as follows: Mucosa

The innermost layer of the digestive tract has specialised epithelial cells supported by an underlying layer called the . The lamina propria contains blood vessels, , lymphoid tissue and glands that support the mucosa. Depending on its function, the epithelium may be simple (a single layer) or stratified (multiple layers). Areas such as the mouth and oesophagus are covered by a stratified squamous (flat) epithelium so they can survive the wear and tear of passing food. Simple columnar (tall) or glandular epithelium lines the stomach and intestines to aid and absorption. The inner lining is constantly shed and replaced, making it one of the most rapidly dividing areas of the body! Beneath the lamina propria is the muscularis mucosa. This comprises layers of which can contract to change the shape of the .

Epithelium—In the mouth, pharynx, esophagus, and , the epithelium is primarily a non-keratinized, stratified squamous epithelium. In the stomach and intestines, it is a simple columnar epithelium. Notice that the epithelium is in direct contact with the lumen, the space inside the alimentary canal. Interspersed among its epithelial cells are goblet cells, which secrete and fluid into the lumen, and enteroendocrine cells, which secrete into the interstitial spaces between cells. Epithelial cells have a very brief lifespan, averaging from only a couple of days (in the mouth) to about a week (in the ). This process of rapid renewal helps preserve the health of the alimentary canal, despite the wear and tear resulting from continued contact with foodstuffs.

Lamina propria—In addition to loose connective tissue, the lamina propria contains numerous blood and lymphatic vessels that transport nutrients absorbed through the alimentary canal to other parts of the body. The lamina propria also serves an immune function by housing clusters of , making up the mucosa-associated lymphoid tissue (MALT). These clusters are particularly substantial in the distal ileum where they are known as Peyer’s patches. When you consider that the alimentary canal is exposed to foodborne bacteria and other foreign matter, it is not hard to appreciate why the has evolved a means of defending against the encountered within it. Submucosa - The submucosa surrounds the muscularis mucosa and consists of fat, fibrous connective tissue and larger vessels and nerves. At its outer margin there is a specialized plexus called the or Meissner plexus. This supplies the mucosa and submucosa.

Muscularis externa - The third layer of the alimentary canal is the muscalaris (also called the muscularis externa). The muscularis in the small intestine is made up of a double layer of smooth muscle: an inner circular layer and an outer longitudinal layer. The contractions of these layers promote mechanical digestion, expose more of the food to digestive chemicals, and move the food along the canal. In the most proximal and distal regions of the alimentary canal, including the mouth, pharynx, anterior part of the esophagus, and external anal , the muscularis is made up of , which gives you voluntary control over and defecation. The basic two- layer structure found in the small intestine is modified in the organs proximal and distal to it. The stomach is equipped for its churning function by the addition of a third layer, the oblique muscle. While the colon has two layers like the small intestine, its longitudinal layer is segregated into three narrow parallel bands, the tenia coli, which make it look like a series of pouches rather than a simple tube.

Serosa/

The serosa is the portion of the alimentary canal superficial to the muscularis. Present only in the region of the alimentary canal within the , it consists of a layer of visceral overlying a layer of loose connective tissue. Instead of serosa, the mouth, pharynx, and esophagus have a dense sheath of fibers called the . These tissues serve to hold the alimentary canal in place near the ventral surface of the .

Cross-sectional of alimentary canal Individual components of the gastrointestinal system

Oral cavity

The oral cavity or mouth is responsible for the intake of food. It is lined by a stratified squamous with keratin covering those areas subject to significant abrasion, such as the , hard and roof of the mouth. Mastication refers to the mechanical breakdown of food by chewing and chopping actions of the teeth. The tongue, a strong muscular organ, manipulates the food to come in contact with the teeth. It is also the sensing organ of the mouth for touch, temperature and using its specialised sensors known as papillae.In salivation refers to the mixing of the oral cavity contents with salivary . The (a glycoprotein) in acts as a lubricant. The oral cavity also plays a limited role in the digestion of carbohydrates. The serum , a component of saliva, starts the process of digestion of complex carbohydrates. The final function of the oral cavity is absorption of small molecules such as glucose and water, across the mucosa. From the mouth, food passes through the pharynx and oesophagus via the action of swallowing.

Salivary glands

Three pairs of salivary glands communicate with the oral cavity. Each is a complex gland with numerous acini lined by secretory epithelium. The acini secrete their contents into specialised ducts. Each gland is divided into smaller segments called lobes. Salivation occurs in response to the taste, smell or even appearance of food. This occurs due to nerve signals that tell the salivary glands to secrete saliva to prepare and moisten the mouth. Each pair of salivary glands secretes saliva with slightly different compositions.

Oesophagus

The oesophagus is a muscular tube of approximately 25cm in length and 2cm in diameter. It extends from the pharynx to the stomach after passing through an opening in the diaphragm. The wall of the oesophagus is made up of inner circular and outer longitudinal layers of muscle that are supplied by the oesophageal nerve plexus. This nerve plexus surrounds the lower portion of the oesophagus. The oesophagus functions primarily as a transport medium between compartments.

Stomach The stomach is a J shaped expanded bag, located just left of the midline between the oesophagus and small intestine. It is divided into four main regions and has two borders called the greater and lesser curvatures. The first section is the cardia which surrounds the cardial orifice where the oesophagus enters the stomach. The fundus is the superior, dilated portion of the stomach that has contact with the left dome of the diaphragm. The body is the largest section between the fundus and the curved portion of the J. This is where most are located and where most mixing of the food occurs. Finally the is the curved base of the stomach. Gastric contents are expelled into the proximal duodenum via the pyloric sphincter. The inner surface of the stomach is contracted into numerous longitudinal folds called rugae. These allow the stomach to stretch and expand when food enters. The stomach can hold up to 1.5 litres of material. The functions of the stomach include:

The short-term storage of ingested food. Mechanical breakdown of food by churning and mixing . Chemical digestion of proteins by acids and enzymes. Stomach acid kills bugs and germs. Some absorption of substances such as .

Most of these functions are achieved by the secretion of stomach juices by gastric glands in the body and fundus. Some cells are responsible for secreting acid and others secrete enzymes to break down proteins.

Small intestine The small intestine is composed of the duodenum, jejunum, and ileum. It averages approximately 6m in length, extending from the pyloric sphincter of the stomach to the ileo-caecal valve separating the ileum from the caecum. The small intestine is compressed into numerous folds and occupies a large proportion of the abdominal cavity. The duodenum is the proximal C-shaped section that curves around the head of the pancreas. The duodenum serves a mixing function as it combines digestive secretions from the pancreas and liver with the contents expelled from the stomach. The start of the jejunum is marked by a sharp bend, the . It is in the jejunum where the majority of digestion and absorption occurs. The final portion, the ileum, is the longest segment and empties into the caecum at the ileocaecal junction. The small intestine performs the majority of digestion and absorption of nutrients. Partly digested food from the stomach is further broken down by enzymes from the pancreas and bile salts from the liver and gallbladder. These secretions enter the duodenum at the Ampulla of Vater. After further digestion, food constituents such as proteins, fats, and carbohydrates are broken down to small building blocks and absorbed into the body’s blood stream. The lining of the small intestine is made up of numerous permanent folds called plicae circulares. Each plica has numerous villi (folds of mucosa) and each villus is covered by epithelium with projecting microvilli (brush border). This increases the surface area for absorption by a factor of several hundred. The mucosa of the small intestine contains several specialised cells. Some are responsible for absorption, whilst others secrete digestive enzymes and mucous to protect the intestinal lining from digestive actions.

Large intestine

The large intestine is horse-shoe shaped and extends around the small intestine like a frame. It consists of the appendix, caecum, ascending, transverse, descending and , and the rectum. It has a length of approximately 1.5m and a width of 7.5cm. The caecum is the expanded pouch that receives material from the ileum and starts to compress food products into faecal material. Food then travels along the colon. The wall of the colon is made up of several pouches (haustra) that are held under tension by three thick bands of muscle (). The rectum is the final 15cm of the large intestine. It expands to hold faecal matter before it passes through the anorectal canal to the anus. Thick bands of muscle, known as , control the passage of faeces. The mucosa of the large intestine lacks villi seen in the small intestine. The mucosal surface is flat with several deep intestinal glands. Numerous goblet cells line the glands that secrete mucous to lubricate faecal matter as it solidifies. The functions of the large intestine can be summarised as:

The accumulation of unabsorbed material to form faeces. Some digestion by bacteria. The bacteria are responsible for the formation of intestinal gas. Reabsorption of water, salts, sugar and vitamins.

Liver

The liver is a large, reddish-brown organ situated in the right upper quadrant of the . It is surrounded by a strong and divided into four lobes namely the right, left, caudate and quadrate lobes. The liver has several important functions. It acts as a mechanical filter by filtering blood that travels from the intestinal system. It detoxifies several metabolites including the breakdown of bilirubin and oestrogen. In addition, the liver has synthetic functions, producing albumin and blood clotting factors. However, its main roles in digestion are in the production of bile and of nutrients. All nutrients absorbed by the intestines pass through the liver and are processed before traveling to the rest of the body. The bile produced by cells of the liver, enters the intestines at the duodenum. Here, bile salts break down lipids into smaller particles so there is a greater surface area for digestive enzymes to act.

Gall bladder The gallbladder is a hollow, pear shaped organ that sits in a depression on the posterior surface of the liver’s right lobe. It consists of a fundus, body and . It empties via the into the biliary duct system. The main functions of the gall bladder are storage and concentration of bile. Bile is a thick fluid that contains enzymes to help dissolve fat in the intestines. Bile is produced by the liver but stored in the gallbladder until it is needed. Bile is released from the gall bladder by contraction of its muscular walls in response to signals from the duodenum in the presence of food.

Pancreas Finally, the pancreas is a lobular, pinkish-grey organ that lies behind the stomach. Its head communicates with the duodenum and its tail extends to the . The organ is approximately 15cm in length with a long, slender body connecting the head and tail segments. The pancreas has both exocrine and endocrine functions. Endocrine refers to production of hormones which occurs in the Islets of Langerhans. The Islets produce , glucagon and other substances and these are the areas damaged in diabetes mellitus. The exocrine (secretrory) portion makes up 80-85% of the pancreas and is the area relevant to the gastrointestinal tract. It is made up of numerous acini (small glands) that secrete contents into ducts which eventually lead to the duodenum. The pancreas secretes fluid rich in carbohydrates and inactive enzymes. Secretion is triggered by the hormones released by the duodenum in the presence of food. Pancreatic enzymes include carbohydrases, , nucleases and proteolytic enzymes that can break down different components of food. These are secreted in an inactive form to prevent digestion of the pancreas itself. The enzymes become active once they reach the duodenum.

Functions of Gastrointestinal tract (Digestive system) :

The function of the digestive system is digestion and absorption. Digestion is the breakdown of food into small molecules, which are then absorbed into the body. The digestive system is divided into two major parts:

• The digestive tract (alimentary canal) is a continuous tube with two openings: the mouth and the anus. It includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. Food passing through the internal cavity, or lumen, of the digestive tract does not technically enter the body until it is absorbed through the walls of the digestive tract and passes into blood or lymphatic vessels. • Accessory organs include the teeth and tongue, salivary glands, liver, gallbladder, and pancreas.

The treatment of food in the digestive system involves the following seven processes:

1. is the process of . 2. Propulsion is the movement of food along the digestive tract. The major means of propulsion is , a series of alternating contractions and relaxations of smooth muscle that lines the walls of the digestive organs and that forces food to move forward. 3. Secretion of digestive enzymes and other substances liquefies, adjusts the pH of, and chemically breaks down the food. 4. Mechanical digestion is the process of physically breaking down food into smaller pieces. This process begins with the chewing of food and continues with the muscular churning of the stomach. Additional churning occurs in the small intestine through muscular constriction of the intestinal wall. This process, called segmentation, is similar to peristalsis, except that the rhythmic timing of the muscle constrictions forces the food backward and forward rather than forward only. 5. Chemical digestion is the process of chemically breaking down food into simpler molecules. The process is carried out by enzymes in the stomach and small intestines. 6. Absorption is the movement of molecules (by passive diffusion or active transport) from the digestive tract to adjacent blood and lymphatic vessels. Absorption is the entrance of the digested food (now called nutrients) into the body. 7. Defecation is the process of eliminating undigested material through the anus.