INTRODUCTION :
DIGESTION : Digestion is the process of breakdown of complex food molecules into simple water :soluble molecules so that it can be easily absorbed by the body. Digestion is a catabolic process, that is, it is a breakdown process to release energy.
Various types of Nutrients in food- Nutrients may be organic or inorganic and are of 2 types
Macro nutrients : 1.they include organic constituents such as carbohydrates , proteins, lipids , and vitamins
2. also known as proximate principles of food because they are energy sources and used in the production of heat and different organic functions.
Micro nutrients : 1. They include inorganic constituents such as minerals, vitamins and water.
2.also known as protective principles of food as they do not provide any energy but their deficiencies can lead to certain abnormalities and deficiencies in man.
3. major micro elements – are sodium, potassium, calcium, sulphur, phosphorous, magnesium, and chloride requeried in large amounts
4.trace micro elements – are iron, iodine, zinc,manganese, cobalt, copper, molybdenum etc…
Digestive system:Consists of Alimentary canal and digestive glands.
I. Alimentary Canal- Begins with the mouth which is anteriorly located and associated with lip called orbicularis oris and opens through posteriorly located anus, which further leads to the Buccal or oral cavity containing teeth and muscular tongue
1. Buccopharyngeal cavity– Divided into two parts-
a) Buccal Vestibule– space b/w gums and cheeks where food stored temporarily.
b) Main Oral Cavity– inner and central part lined by stratified squamous epithelium and
surrounded by upper and lower jaw.
TEETH :teeth appears once in life are monophyodont e.g, premolars and last molars of man.
a) Human beings and a majority of mammals contain 2 sets of teeth one temporary milk or
deciduous teeth replaced by permanent and this type of dentition is diphyodont appears twice in life. E.g, incisors, canines, 1st and 2nd molars. When the tooth is embedded in the socket of the jaw bone such dentition is thecodont e.g, Incisors, Canines, 1st, and 2nd molars.
b) Human has 32 permanent teeth differentiated into incisors (I), canine (C), premolars (PM), and Molars (M), such dentition is known as heterodont. In mammals including human except for premolar and last molar, all type teeth appear twice in life.
c) The hardest chewing surface of teeth in humans made up of enamel secreted by ameloblast cell involved in mastication of food.
d) The arrangement of teeth in the jaw is dentition. The human dental formula= 2123/2123 i.e
I2/2C1/1PM2/2M3/3=8/8*2=16/16=32, similarly dental formula of the child is 20 and a 17-year-
old is 28.
e) Human teeth are thecodont, heterodont, diphyodont and bunodont.
f) Acrodont when teeth attached to upper part of jaw e.g, fishes, amphibians and reptiles.
Pleurodont when teeth fixed to the lateral surface of jaw ridge e.g, fangs of snakes(reptiles). Bunodont consists of small, blunt and rounded cusps e.g, humans.
3.Tongue– a voluntary, muscular and glandular structure present on the floor of the oral cavity
connected through a flexible membrane called frenulum linguae.V shaped sulcus called sulcus terminalis divides the dorsal surface of the tongue into two unequal parts i.e anterior part which is free and posterior part connected to hyoid bone. Its function is the reception of taste and taste buds are the modifications of the epithelium. Lingual frenulum prevents the tongue from falling backwards.
Papillae – 3 functional papillae found in the posterior part in which taste receptors are present in form of taste buds, these are :
a) Filiform papillae– have no taste buds, smallest and most abundant.
b) Fungiform papillae– have taste buds, appears as red dots in the tongue.
b) Foliate papillae- absent in man.
c) Circumvallate papillae– have taste buds, largest and knob-like
4.Pharynx- Oral cavity leads into pharynx, common passage for food and air.
5. Oesophagus– Pharynx leads into the oesophagus. The opening of the oesophagus into the
stomach is regulated by muscular (gastro-oesophageal) sphincter. The passing site of the oesophagus at Diaphragm called hiatus.
6.Stomach– The Widest part of the alimentary canal, J-shaped muscular structure covered by a layer called peritoneum. When fat and lymph tissues accumulated on peritoneum it is known as Omentum. Divided into 3 parts- cardiac portion into which oesophagus opens, fundic region and the pyloric region which opens into 1st part of small intestine.
7.Small intestine– categories into three parts-
a) Duodenum– retroperitoneal and initial part of small intestine, C-shaped, and 25 cm long.
b) Jejunum– long and coiled middle portion.
c)Ileum – longest portion and highly coiled.
(The opening of the stomach into the duodenum is regulated by Pyloric sphincter.)
( Brunner’s glands in the duodenum located in sub-mucosa secrete mucus.)
8. Large intestine- differentiated into 3 parts-
a) Caecum- small blind sac, Vermiform appendix arises from the caecum is vestigial in a human being and well developed in the rabbit. Further opens into the colon.
b) Colon– divided into 3 parts- ascending, transverse and descending part. Descending part opens into rectum while ascending part is the smallest and lacks mysentry. Mass action
contraction only occurs in the colon. Small fat filled projections called epiploic appendages are present. Colon wall has 3 bands of longitudinal muscle called taeniae coli.
c)Rectum- largest part and repository of fecal matter. Opens through the anus.
HISTOLOGY OF ALIMENTARY CANAL
Wall of alimentary canal possesses 4 layers- serosa, muscularis, sub-mucosa and mucosa.
1. Serosa – outermost layer made up of thin mesothelium with some connective tissues. Absent
in upper part of the oesophagus.
2. Muscularis layer– made up of the circular inner layer and the longitudinal outer layer of
smooth muscles. Maximum peristalsis thickest layer found in the stomach and minimum
peristalsis thinnest layer found in the rectum. Additional oblique muscle found inner to circular muscle in the stomach.
3. Sub-mucosa – composed of areolar connective tissue layer containing blood vessels, lymph vessels and nerves.
4. Mucosa– innermost layer forms irregular folds called rugae in the stomach and finger-like projections in small intestine also contain absorptive and secretory cells. It is further divided into
3 parts-
a) Mucosa muscularis– formed of smooth muscles, exposed surface area for absorption and supporting the folds of the mucosa.
b) Lamina propria– made up of areolar connective tissue contains mucosa-associated lymphoid tissue provides immunity e.g, Peyer’s patches.
c) Epithelial mucosa– this layer in oesophagus made up of non- keratinised stratified squamous epithelium. Except for oesophagus, it is made up of columnar mucous epithelium and single layer thick.
(Gastric glands and crypts in between the bases of villi called crypts of Lieberkuhn in the
intestine. are formed by mucosa in the stomach. All 4 layers show some modifications in the alimentary canal.)
• Myenteric plexus/ plexus of Auerbach- present between longitudinal and circular muscle fibres of muscle coat and contain fibres from both autonomic divisions mostly controls
peristaltic movements in the alimentary canal.
• Meissner plexus/ Submucosal plexus– present b/w mucosa and muscle coat, controls
different digestive glands secretion.
Digestive glands
Accessory digestive glands are Salivary glands, Liver and Pancreas.
1. Salivary gland- Consists of 3 glands.a)Parotids- These are largest glands opens through Stenson’s Duct, found near cheek.b)Sub-maxillary/Sub-mandibular– opens through the Whartons duct, present at lower jaw.c)Sub-lingual– opens through Rivinius duct, smallest salivary glands and found near the tongue.
These glands secrete salivary juice into buccal cavity and located outside the buccal cavity.
Saliva- daily secretion-1.5 litres, ph- 6.7, salivary amylase- ptyalin and lysozyme, secreted by-
parasympathetic innervation, activation of salivary amylase by- chlorine atoms, viral infection-mumps.
2. Liver– largest gland, developed from endoderm, located in the abdominal cavity below the diaphragm has right and left lobes which are separated from each other through the falciform ligament.
• Structural and functional unit of the liver is hepatic lobules containing hepatic cells
arranged in forms of cords. Thin connective tissue sheath called Glisson’s Capsule
covers each capsule (characteristic of the mammalian liver).
• Kupper cells are phagocytic cells (kill RBCs, WBCs and bacteria) present in the
mammalian liver.
• Bile is secreted by hepatic cells which are transported to the gallbladder (where bile is
stored and concentrated) through hepatic ducts.
• Cystic ducts (ducts of the gallbladder) along with hepatic duct forms bile duct.
• Hepatopancreatic duct (bile duct + pancreatic duct) guarded by the sphincter of Oddi
to open into the duodenum.
3. Pancreas– both exocrine and endocrine, exocrine part secretes alkaline pancreatic juice
containing enzymes while the endocrine part secretes hormones, insulin and glucagon. It has 2 ducts, 1st is duct of Santorini (non-functional) and 2nd is duct of wirsung (functional).
TYPES OF DIGESTION :
There are two types of Digestion: Mechanical Digestion and Chemical Digestion.
Mechanical Digestion involves the physical breakdown of large food molecules into small molecules.
Chemical Digestion involves breakdown of complex food molecules into small molecules in presence of digestive enzyme.
DIGESTIONAND ABSORPTION OF CARBOHYDRATES :
Digestion of carbohydrates begins in the mouth with the help of an enzyme salivary amylase. It digests the starch into maltose. Rest of the digestion of carbohydrates occurs in small intestine with the help of the enzyme pancreatic amylase secreted by pancreas. The simplest form of carbohydrates is glucose. So all the carbohydrates consumed is finally converted into glucose for absorption.
DIGESTION AND ABSORPTION OF PROTEINS :
Digestion of Proteins begins in the stomach. Stomach produce an enzyme Pepsinogen. Pepsinogen is an inactive form of enzyme pepsin. Inactive form of an enzyme is known as Zymogens. Pepsinogen is converted into pepsin by Hydrochloric Acid present in the stomach. Pepsin begins the process of digestion of proteins.
DIGESTION AND ABSORPTION OF LIPIDS IN HUMAN BEINGS :
Liver secretes bile that helps in digestion of fats (a form of lipid). The process of digestion of fats by bile is known as Emulsification of fats. Fat digestion is also promoted by lipase enzyme from the pancreas known as Pancreatic Lipase. The simplest form of fats are Fatty Acid and Glycerol. Lipase or bile breakdown fats into fatty acids and glycerol so that they can be absorbed by the body.
DIGESTION OF VITAMINS AND MINERALS :
Digestion of Vitamins and Minerals begins in the mouth. Further digestion of vitamins and minerals occurs in the small intestine where they are also absorbed and taken to the blood stream.
APPENDIX :
Appendix is located at the junction of the small intestine and large intestine. It is considered as a vestigial organ. Cattles use appendix for fodder digestion. It helps in digestion of food containing plant matter such as cellulose.
REVERSE PERISTALSIS :
The most common example of Reverse Peristalsis is Vomiting. Medulla oblongata is the vomiting center in the brain. When it is activated due to some local irritation in the stomach, the food reverts back and results in vomiting. This is known as Reverse Peristalsis.
DISEASE OF DIGESTIVE SYSTEM :
The common diseases of Digestive System are given below-
Gastroesophageal Reflux Disease (GERD) – A disease in which the ingested food leaks back from the stomach into the esophagus. It happens when the esophageal sphincter does not close well.
Esophagitis – The inflammation of the esophagus commonly caused by infection. It is also a complication of GERD.
Peptic Ulcer – A painful lesion of the stomach of small intestine.
Gastritis – A disease that causes the inflammation of the lining of the stomach. It has many causes such as alcohol abuse, cigarette smoking, excessive consumption of coffee and aspirin.
Colon Cancer – The uncontrolled growth of abnormal cells in the large intestine. It is third most common cause of cancer in Philippines.
