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).
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.
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.
HISTOLOGY OF THE ALIMENTARY CANAL
The gastrointestinal tract is a muscular tube lined by a special layer of cells, called epithelium. The wall of the alimentary canal has four basic tissue layers: the mucosa, submucosa, muscularis, and serosa.
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.
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.
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.
The oesophagus functions primarily as a transport medium between compartments.
Stomach: 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 gastric glands are located and where most mixing of the food occurs. Finally the pylorus is the curved base of the stomach. The functions of the stomach include:
The short-term storage of ingested food.
Mechanical breakdown of food by churning and mixing motions.
Chemical digestion of proteins by acids and enzymes.
Stomach acid kills bugs and germs.
Some absorption of substances such as alcohol.
The gastric glands in the wall of the stomach secrete secrete gastric juices which is composed of water, mucin salts, Hcl and three protein splitting enzymes i.e pepsin, renin and gastric lipase.
Small intestine
The small intestine is composed of the duodenum, jejunum, and ileum. 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 duodenojejunal flexure. 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 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.
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 sigmoid colon, 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 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 abdomen. It is surrounded by a strong capsule 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 metabolism 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 neck. It empties via the cystic duct 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 hormone signals from the duodenum in the presence of food.
Pancreas : 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 spleen. 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 insulin, glucagon and other substances and these are the areas damaged in diabetes mellitus. The exocrine (secretrory) portion makes up 80-85% of the pancreas 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, lipases, 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
Mechanical Digestion of Food: Food can be digested by a combination of two methods – mechanical digestion and chemical digestion
- In mechanical digestion, food is physically broken down into smaller fragments via the acts of chewing (mouth), churning (stomach) and segmentation (small intestine)
Mechanical Digestion Mechanical digestion begins in our mouth with chewing, then moves to churning in the stomach and segmentation in the small intestine. Peristalsis is also part of mechanical digestion. This refers to involuntary contractions and relaxations of the muscles of our esophagus, stomach, and intestines to break down food and move it through our digestive system.
Chewing (Mouth)
- Food is initially broken down in the mouth by the grinding action of teeth (chewing or mastication)
- The tongue pushes the food towards the back of the throat, where it travels down the esophagus as a bolus
- The epiglottis prevents the bolus from entering the trachea, while the uvula prevents the bolus from entering the nasal cavity
Churning (Stomach)
- The stomach lining contains muscles which physically squeeze and mix the food with strong digestive juices ('churning’)
- Food is digested within the stomach for several hours and is turned into a creamy paste called chyme
- Eventually the chyme enters the small intestine (duodenum) where absorption will occur
Movement of Food
Peristalsis
- Peristalsis is the principal mechanism of movement in the oesophagus, although it also occurs in both the stomach and gut
- Continuous segments of longitudinal smooth muscle rhythmically contract and relax
- Food is moved unidirectionally along the alimentary canal in a caudal direction (mouth to anus)
Segmentation
- Segmentation involves the contraction and relaxation of non-adjacent segments of circular smooth muscle in the intestines
- Segmentation contractions move chyme in both directions, allowing for a greater mixing of food with digestive juices
- While segmentation helps to physically digest food particles, its bidirectional propulsion of chyme can slow overall movement
Chemical digestion
Chemical digestion involves the secretions of enzymes throughout your digestive tract. These enzymes break the chemical bonds that hold food particles together. This allows food to be broken down into small, digestible parts.
Chemical Digestion of Carbohydrate: (Starch + Water-----------sa------------ Maltose+ Dextrin--------maltase----Glucose) Mouth
Intestine_-pancreatic Juice—(Trypsin ,Chymotrypsin,Carboxypeptidases,Amylase)
Amylase----maltose---Glucose
Limit dextrin---Limit dextrinase----Glucose
Digestion of Protein: