Introduction to the Digestive System:
The digestive tract and auxiliary digestive organs make up the digestive system. Digestive tract is also called as gastro-intestinal tract or alimentary canal. It is tubular structure that includes following parts.
Parts of the Alimentary Canal:
- Mouth and buccal cavity (Oral cavity)
- Pharynx
- Oesophagus
- Stomach
- Small intestine (Duodenum, jejunum and ileum)
- Large intestine (Cecum, colon and rectum)
Mouth:
- Entry of the alimentary canal is through mouth or oral cavity. This opening is guarded two jaws, immovable upper jaw and movable lower jaw.
- Roof of buccal cavity is called palate. It separates oral cavity from nasal passage. Anterior part of palate is hard palate and posterior part is soft palate.
- Uvula is the structure hanging down from the rear end of the palate. It seals the nasal passage during swallowing.
- Tongue is present at the floor of the oral cavity. It guides the food for chewing. It also helps in swallowing. Taste buds are embedded in the upper epithelium of the tongue that help to detect the taste of the food.
- On both jaws teeth are present. Teeth are formed from dentine and covered by enamel. The dentition in human is thecodont as teeth are firmly embedded in the sockets of jaw bone, diphyodont as humans have two sets of teeth and heterodont, having different types of teeth. Four types of teeth, Incisors, canines, premolars and molars are present.
- Three pairs of major salivary glands (parietal gland, sub-mandibular gland and sublingual glands) are associated with mouth. Salivary glands are tubular alveolar glands located outside the oral cavity and pour their secretions, saliva in the oral cavity through their ducts.
- Secretion of salivary gland is called saliva. Salivary amylase is a carbohydrate digesting enzyme. It hydrolyzes polysaccharides to produce monosaccharides. Salivary mucus holds food particles together, lubricates them, and moistens them, making swallowing easier. Lysozymes in saliva exerts antibacterial action, destroy certain bacteria by breaking down their cell wall. v. Saliva keeps the mouth moist and help in speech.
Pharynx and Oesophagus:
Pharynx is a common passage for digestive and respiratory tract. The oesophageal and tracheal openings at the base of the pharynx are known as the gullet and glottis, respectively. With the exception of swallowing, the respiratory tube is always open. During swallowing the glottis is closed by a cartilaginous flap called epiglottis.
Oesophagus is a tube which carries food from pharynx to stomach. It merely acts as passage for food. Food is swept through the oesophagus by a swift peristaltic wave. Oesophageal secretions mostly consist of protective mucus, which shields the wall from food particles’ sharp edges. In the throat or oesophagus, neither digestion nor absorption occurs.
Stomach:
Stomach is the most dilated part of alimentary canal. Anatomically it is divided into fundus, body and pyloric antrum. It has a pyloric sphincter at the connection with the duodenum and a cardiac sphincter at the junction with the oesophagus. The cardiac sphincter prevents the retrieval of food from stomach to oesophagus. The rate and flow of stomach emptying into the oesophagus are controlled by the pyloric sphincter.
The wall of stomach is thick muscular. It has an extra muscle layer called oblique muscle layer inner to the circular muscle layer. These muscles help in churning of food.
Functions of stomach
- Storage of the food
- Mechanical digestion and mixing of food: food is mixed with gastric juice by the muscle action. Food churning is also accomplished by mixing the movement of the stomach wall.
- Enzymatic digestion: This is carried out by the action of gastric juice secreted by gastric gland. Gastric glands are mucosal glands embedded in mucosa of stomach. They secrete mucus, hydrochloric acid and pepsinogen.
- Mucus neutralizes HCl and protects stomach wall from its action.
- Pepsin is protein digesting enzyme. It is secreted in inactive form pepsinogen. HCl converts inactive pepsinogen to active pepsin Pepsin cleaves peptide bond between hydrophobic and perfectly aromatic amino acids.
- Gastrin is a proteinous hormone secreted by endocrine cells of gastric glands at antral region. The hormone is secreted in blood. It acts on the gastric gland and enhances the secretion.
Small Intestine:
Small intestine is a coiled, long, narrow tube. The common hepatopancreatic duct passes through the duodenum, the first segment of the small intestine. This duct transports pancreatic juice from the pancreas and bile from the gall bladder.
Duodenum is the proximal C-shaped region that curves around the head of the pancreas. The duodenum serves a crucial function as it receives digestive secretions from the pancreas (pancreatic juices) and bile from the liver at the greater duodenal papilla that immediately mix with the contents passing through the pyloric sphincter from the stomach.
Jejunum
It is the start of the jejunum is marked by a sharp bend, the duodeno-jejunal flexure. It is in the jejunum where the majority of digestion and absorption occurs.
Ileum
It is the final and longest segment and continues the absorption of nutrients. The ileum empties into the cecum at the ileocecal junction. The majority of nutritional absorption and digestion takes place in the small intestine. Pancreatic enzymes and bile salts from the liver and gallbladder further break down partially digested food from the stomach. The larger duodenal papilla is where these secretions enter the duodenum. Proteins, lipids, and carbohydrates are among the food components that are broken down into tiny building blocks and absorbed into the bloodstream after additional digestion. Numerous folds known as plicae circulares make up the lining of the small intestine. Each villus is coated by columnar epithelium with projecting microvilli (brush border), and each plica includes several villi (folds of mucosa).
All of these anatomical arrangements increase the surface area for absorption by a factor of several hundred.
Role of bile and pancreatic juice:
Bile is continuously secreted by liver cells and carried by bile canaliculi to bile ducts which carry it to gall bladder; in gall bladder it is stored until it receives a signal. The bicarbonate ions in bile neutralise the acidic chyme entered in duodenum from stomach. Bile salts help in emulsification of lipids.
Pancreatic juice is secreted by exocrine pancreas. It contains proteolytic enzymes: typsinogen, chymotypsinogen and procarboxypeptidase that digest proteins; pancreatic amylase that digests carbohydrates and pancreatic lipase that digests fat.
Large Intestine:
Food items are compressed into fecal material by the cecum, an enlarged pouch that takes material from the ileum. Food then travels along the colon.
Colon is composed of 4 regions:
- Ascending Colon
- Transverse Colon
- Descending Colon
- Sigmoid Colon
The wall of the colon is made up of several pouches (haustra) that are held under tension by three thick bands of muscle (taenia coli). The rectum expands to hold fecal matter before it passes through the anorectal canal to the anus. Fecal transit is regulated by sphincters, which are thick bands of muscle. The glands that release mucus to lubricate the solidified fecal matter are lined with many goblet cells. The functions of the large intestine are to reabsorb water and compact feces, absorb vitamins and electrolytes and helps make Vitamin K and stores fecal matter. Undigested food materials are egested through anus.