Structural organisation and function of Gastrointestinal tract (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 mouth, continuing through the pharynx, oesophagus, stomach and intestines to the rectum and anus, where food is expelled. There are various accessory organs that assist the tract by secreting enzymes to help break down food into its component nutrients. Thus the salivary glands, liver, pancreas 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, digestion occurs mainly in the stomach and small intestine where proteins, fats and carbohydrates are chemically broken down into their basic building blocks. Smaller molecules are then absorbed across the epithelium of the small intestine and subsequently enter the circulation. The large intestine plays a key role in reabsorbing excess water. Finally, undigested material and secreted waste products are excreted from the body via defecation (passing of faeces). In the case of gastrointestinal disease or disorders, these functions of the gastrointestinal tract are not achieved successfully. Patients may develop symptoms of nausea, vomiting, diarrhoea, malabsorption, constipation 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 human 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 esophagus, stomach, and duodenum. The exact demarcation between upper and lower can vary. Upon gross dissection, the duodenum may appear to be a unified organ, but it is often divided into two parts based upon function, arterial supply, or embryology. 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 protein -digesting enzymes called proteases 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 iron 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 gallbladder ( bile ) mix together. The digestive enzymes break down proteins and bile and emulsify fats into micelles. The duodenum contains Brunner’s glands that produce bicarbonate, and pancreatic juice that contains bicarbonate to neutralize hydrochloric acid in the stomach. • Jejunum: This is the midsection of the intestine, connecting the duodenum to the ileum. 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 capillaries and lacteals). The large intestine has four parts: 1. Cecum, the vermiform appendix that is attached to the cecum. 2. Colon, which includes the ascending colon, transverse colon, descending colon, and sigmoid flexure. The main function of the colon is to absorb water, but it also contains bacteria that produce beneficial vitamins like vitamin 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 tissue layers: the mucosa, submucosa, 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 connective tissue layer called the lamina propria. The lamina propria contains blood vessels, nerves, 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 secretion 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 smooth muscle which can contract to change the shape of the lumen. Epithelium—In the mouth, pharynx, esophagus, and anal canal, 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 mucus and fluid into the lumen, and enteroendocrine cells, which secrete hormones 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 gut). 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 lymphocytes, making up the mucosa-associated lymphoid tissue (MALT). These lymphocyte 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 immune system has evolved a means of defending against the pathogens 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 nerve plexus called the submucosal plexus 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 sphincter, the muscularis is made up of skeletal muscle, which gives you voluntary control over swallowing 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/mesentery The serosa is the portion of the alimentary canal superficial to the muscularis. Present only in the region of the alimentary canal within the abdominal cavity, it consists of a layer of visceral peritoneum overlying a layer of loose connective tissue. Instead of serosa, the mouth, pharynx, and esophagus have a dense sheath of collagen fibers called the adventitia. These tissues serve to hold the alimentary canal in place near the ventral surface of the vertebral column. Cross-sectional histology 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 oral mucosa with keratin covering those areas subject to significant abrasion, such as the tongue, hard palate 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 bolus to come in contact with the teeth. It is also the sensing organ of the mouth for touch, temperature and taste using its specialised sensors known as papillae.In salivation refers to the mixing of the oral cavity contents with salivary gland secretions. The mucin (a glycoprotein) in saliva acts as a lubricant. The oral cavity also plays a limited role in the digestion of carbohydrates. The enzyme serum amylase, 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.