Lecture notes on Structure of Human Digestive System

Digestion is the process by which complex food materials are broken down to simple absorbable molecules, so that they can be directly used by thecells of the body. The digestive system is the organ system that processes food, extracts nutrients from it, and eliminates the residue. The study of the digestive tract and the diagnosis and treatment of its disorders is called gastroenterology.

General structure  of Alimentary Canal

The digestive system has two anatomical subdivisions, the digestivetract (alimentary canal) and the accessory organs. The digestive tract is a tube extending from mouth to anus, measuring about 9 meters in length. It includes the oral cavity, pharynx, oesophagus, stomach, small intestine and large intestine. Of these, the stomach and intestines constitute the gastrointestinal (GI) tract. The accessory organs are the teeth, tongue, salivary glands, liver, gall bladder and pancreas.

Histology of gut:

Most of the regions of the digestive tract have the same basic structural plan , with a wall composed ofthe following tissue layers,  in order from outer  to inner surface:

1.Serosa (serous coat): The serosa is composed of a thin layer of areolar tissue covered by a

simple squamous mesothelium. Serosa is present from about the lower 3 to 4 cm of the oesophagusand ends at the sigmoid colon. The other regions are covered by a fibrous connective tissue called the adventitia.The oesophagus, stomach and intestines have a nervous network called the enteric nervous system, which regulates the motility of the digestive tract. The stomach and intestines are enfolded and suspended from the body wall by extensions of the peritoneum which form the mesenteries.

2.Muscularis externa: It comprises smooth  muscle layers; an inner layer of circular muscles and an outer layer of longitudinal muscles. Contractions of this layer help in the thorough mixing of food with digestive enzymes in the lumen of the gut  .

3. Submucosa: It is a thicker layer of loose connective tissue containing blood vessels, lymphatic vessels, nerve plexus, and in some places, glands that secrete mucus.

Mucosa: This is the innermost layer and consists of an inner layer of epithelium, a loose connective tissue layer called lamina propria, and a thin layer of smooth muscle called the muscularis mucosae. The epithelium is simple columnar in most areas but stratified squamous type from oral cavity to oesophagus and in the lower anal canal.

Mouth

It receive food and is guarded by lips and it leads to the oral or buccal cavity. The sides of the buccal cavity are formed by the cheeks.There are the upper and iower jaws bearing teeth, 32 in number. There are two incisors, one canine, two premolars and three molars on each half of the jaw to cut, tear and grind the food.

Teeth: All teeth are basically similar in structure with three regions -— the crown is the portion above the gum, the root is the portion below the gum held in the socket of the jaw bone and neck is the portion where the crown, root and gum meet. innermost region of a tooth is the pulp cavity, filled with a tissue, the pulp and blood vessels and nerve fibres. The odontoblasts lining the pulp cavity secrete the dentine, which is a hard yellowish connective tissue that forms the bulk of the teeth. Dentine of the crown is coated with white enamel. Enamel is composed primarily of calcium phosphate and is the hardest substance in the body. Root is fixed to the socket by the vascular periodontal membrane and the cementum. Collagen fibres from the periodontal membrane pass to both the cementum and jaw bones to fix the tooth firmly. Roof of the buccal cavity is formed by the palate. The anterior region is formed by the hard palate and the posterior regionlby the soft palate. Posterior end of the soft palate hangs down as a muscular flap called the uvula. The floor of the oral cavity is formed by the movablemuscular tongue, the anterior portion of which is free. It is lined by squamous epithelium and the dorsal side bears numerous papillae on which the taste  buds are present. Tongue also helps in mastication process and in swallowing the food. Three pairs of extrinsic salivary glands - parotid, sublingual and submaxillary glands  open into the buccal cavity, in addition to the numerous intrinsic salivary glands seen on the tongue, lips and cheeks.

Pharynx

The buccal cavity opens to the pharynx, which is divided into a dorsal nasopharynx, middle oropharynx and lower laryngopharynx. Paired internal nostrils and openings of Eustachian tubes are located in the nasopharynx. 'At this region the tonsils are seen. At the posterior end ofn the tongue and at floor of the oropharynx, there is a slit like opening, then glottis. Glottis opens to the respiratory tract. It is guarded by the cartilaginous flap called epiglottis. Behind the glottis is located another opening, the gullet, through which the buccal cavity opens into the oesophagus.

Oesophagus .

Oesophagus is a straight muscular tube 25 to 30 cm long. lt leads from the gullet, pierces the diaphragm and opens into the stomach. It is lined internally by tough stratified squamous epithelium. It conducts food from mouth to stomach. Each swallowing initiates a peristaltic wave in the oesophagus, which pushes the food to the stomach.

Stomach

The stomach is a large muscular J shaped sac with a capacity of 0.9 to 2 litres . It lies on the left side of the body below the diaphragm. The medial concave border is known as the lesser curvature and the lateral convex border is known as the greater curvature. The stomach is divided into four regions:

(1) the cardiac region (cardia) is the dilated upper region into which the oesophagus opens;

(2) the fundic region (fundus)vis the dome shaped portion superior to the oesophageal attachment;

 (3) the body (corpus) is the large central portion of the stomach;

(4) the pyloric region is the funnel shaped terminal portion. The pylorus is the narrow passage from stomach to duodenum. The flow of food into and from the stomach is controlled by the cardiac sphincter and pyroric sphincter respectively. There are several longitudinal folds in the mucosa of the stomach, known as gastric rugae. This helps stomach distension and they are not seen in a fully distended stomach. The mucosa contains simple columnar epithelium and it has small infoldings which form the gastric pits and gastric glands.

Small intestine:

The small intestine is about 7 to 9 m in length and 2.5 cm inldiameter. It has three regions, the duodenum, jejunum and ileum. Duodenum is a curved portion following the stomach. It begins at the pyloric sphincter and ends in a sharp bend called the duodenojejunal flexure. Secretions of liver and pancreas are emptied into the duodenum. The duodenum hasb prominent duodenal (Brunner) glands in the submucosa. They secrete bicarbonate rich mucus, which neutralises the acidic food from the stomach.The duodenumis followed byjejunum, which is a lesser coiled region than the small intestine. ileum is the final region ofthe small intestine. It is highly coiled and forms about 3.6 m in length. The mucosa of intestine has numerous finger-like mucosal folds called villi (singular: villus) to increase the surface area . A villus is covered with two kinds of epithelialcells —columnar absorptive cells and mucus secreting goblet cells. The surface of absorptive cells facing  surface of absorptive cells facing the intestinal lumen is again produced into numerous microscopic projections, known as microvilli or brush border. This again increases the absorptive area. Interior of the villus is formed of rich capillary network and a lymph vessel called lacteal. Lipid contents of the digested food are absorbed by the lacteal.

Food movement in the intestine is aided by peristalsis and also by pendular movements. In the ileum, the submucosa has nodules of lymphatic tissue known as Peyer's patches. The intestine is protected from the lytic activity of digestive enzymes by a lining of glycoprotein called glycocalyx.

Large Intestine and Rectum

The small intestine opens to the colon or large intestine through the ileocolonic valve. The colon is about 1.5 m long. It begins with a caecum, a blind pouch inferior to the ileocaecal valve . Attached to the lower end of caecum is the appendix (vermiform appendix), a blind tube 2 to 7 cm long. lt is densely populated with lymphocytes. The appendix sometimes may get infected and inflammatory causing appendicitis. The colon is divisible into four regions: the ascendingcolon, transverse colon, descending colon and sigmoid colon. The ascending colon begins at the ileocaecal valve and runs upward through the right side of the abdominal cavity. It makes a 90° turn near the right lobe of the liver and runs horizontally as the transverse colon. lt again makes a 90° turn downward near the spleen and passes downas the descending colon. The descending colon turns medially and downward, forming a roughly  S — shaped sigmoid colon. The large intestine straightens in the pelvic cavity and ends in the rectum.

Rectum is a wide tubular area where faecal matter is temporarily stored. The rectum opens into the anal canal. The rectal opening is guarded by a sphincter. The large intestine is devoid ofvilli but has numerous mucosa folds. The anus is regulated by two sphincters -— an internal anal sphincter formed of smooth muscles and an external anal sphincter formed of skeletal muscles. The main function of the large intestine is the re- absorption of water. Sometimes the large intestine may become irritated and discharge its contents into the rectum and’ out of anus before the absorption of water. This condition is known as diarrhoea.

Digestive Glands

Digestive glands in man are the salivary glands, gastric glands, pancreas, liver and the digestive glands in small intestine.

Salivary Glands

There are three pairs of external salivary glands that open into the buccal cavity .

They are:

i.The parotid glands located in front of the ear lobes.he stensons of it duct opens near the second upper molar. Mumps is the inflammation and swelling of the parotid gland caused by a virus.

ii.The submandibular glands are located inferior to the mandible and its duct(Whartons duct) opens in the floor of the buccal cavity near the incisors.

iii.The sublingual glands lie under the mucosa in the floor of the buccal cavity on the side of the

tongue. It opens to the floor of buccal cavity through several small ducts.

The collective secretion of salivary glands is the saliva. lt has a pH of 6.8 to 7.2and contains a carbohydrate splitting enzyme salivary amylase or ptyalin. Another enzyme, lysozyme is also seen in small quantities.

Gastric Glands

They are groups of tubular secretory cells opening in the inner lining of the stomach. There are several different types of cells in the gastric glands that secrete different products:

(1) goblet cells, which secrete mucus

(2) parietal cells, which secrete hydrochloric acid

 (3) chief cells, which secrete pepsinogen,

 (4) argentaffen cells, which secrete serotonin and histamine and

(5)g-cells, which secrete the hormone gastrin. The secretions of gastric glands are together known as the gastric juice.

Liver

Liver is the largest gland in the body that weighs about 1.5 kg in anadult. It is placed immediately beneath the diaphragm on the right side.Liver has four lobes - the right, left, quadrate and caudate lobes. The basic structural unit of liver is the liver lobule, which is a cylindrical structure measuring 2 mm in length and 1 mm in diameter. A lobule consists of a central vein, surrounded by radiating sheets of cuboidal cells called hepatocytes. The spaces between the sheets of hepatocytes are blood filled channels called hepatic sinusoids. The sinusoids contain hepatic macrophages called Kupffer cells. The liver secretes bile into narrow channels, the bile canaliculi, which join to form larger bile ducts. They, in turn, drain into the right and left hepatic ducts and these converge to form the common hepatic duct. The hepatic duct joins with the cystic duct from the gall bladder to form the common bile duct. Near the duodenum the bile duct joins the pancreatic duct to form a hepatopancreatic ampulla, which opens to the duodenal wall.

Bile contains no digestive enzymes, but its alkalinity helps to neutralize the acidic chyme from the stomach. The pigments, bilirubin and biliverdin, give a greenish yellow colour to the bile. Sodium taurocholate and sodium glycocholate are the bile salts, which help in the emulsification of lipids in the chyme.

Pancreas

Pancreas is the primary source of digestive enzymes that acts in the small intestine. It has both exocrine and endocrine functions. The exocrine portion secretes the pancreatic juice and the endocrine portion secretes hormones, insulin, glucagon and somatostatin. The exocrine portion of pancreas contains secretory acini, which open into ducts thatjoin together to form the pancreatic duct. This duct runs through the middle ofthe gland and joins the bile duct at hepatopancreatic ampulla. The pancreatic juice has a pH of 7.5 to 8.8. It contains many digestive enzymes: trypsin, chymotrypsin and carboxypeptidase (proteolytic), pancreatic amylase (amylolytic), pancreatic lipase (fat digesting) ribonuclease and deoxyribonuclease (digest RNA and DNA).

Intestinal Glands

The intestinal wall has two types of secretory cells. They are glands of Brunner, which secrete mucus and crypts of Lieberkuhn, which secrete a variety of digestive enzymes. The secretions of intestinal glands are collectively called succus entericus with a pH of 7 to 8. The brush border of microvilli contains brush border enzymes, which are integral proteins of the plasma membrane. They are not released into the lumen but they act on the chyme when it comes in contact with the microvilli. This process is known as contact digestion.