digestion.docx

**Human digestive system** - Digestion is the process of conversion of complex non absorbable nutrients into simpler absorbable form by the activity of digestive or hydrolytic enzymes. - It may be intracellular (within the cells in food vacuoles.eg. Amoeba) or extracellular (outsid...

**Human digestive system** - Digestion is the process of conversion of complex non absorbable nutrients into simpler absorbable form by the activity of digestive or hydrolytic enzymes. - It may be intracellular (within the cells in food vacuoles.eg. Amoeba) or extracellular (outside the cells, in a lumen or cavity.eg. higher animals) - The process of digestion involves five steps: i) ingestion ii) Digestion, iii) absorption, iv) assimilation and v) Egestion - Human digestive system consists of Alimentary canal or gastrointestinal tract (GI tract) and digestive glands. **General functions of human digestive system** - It receives ingested food materials. - Temporary storage of food materials and slowly pushes down along due to the contraction of muscles. - Helps in breaking down larger food materials into simpler forms with the help of teeth and contraction of muscles. - Food particles are further broken down chemically due to the activities of hydrolytic enzymes. - It also helps in the absorption of digested food particles and transport to the body cells through blood stream. - It expels out undigested solid food particles. - It is a long tube of varying diameter. It is about 8 to 10 metres in length. - It is divided into following parts: i. Mouth- - It is a transverse slit or opening bounded by two soft, movable lips i.e the upper lip and the lower lip. - It is covered and protected by skin and helps in ingestion of food. ii. Vestibule - It is a narrow space enclosed between the lips and cheeks externally and a pair of jaws internally. - Its lining contains mucous glands and leads into the buccal cavity. iii. Buccal cavity/ oral cavity - The space to inner side of the mouth bounded above by palate, below by throat and sides by jaws. - Throat provides support to the tongue and jaws bear teeth. - Palate forms the roof part of the oral cavity which separates oral cavity from nasal chamber. - Palate is divided into two parts a. Hard palate -- it is supported by bones which are lined by mucous membrane consisting of transverse ridges called rugae, which keeps the food in place during mastication. b. Soft palate- it forms the posterior part with smooth surface which makes swallowing easy. iv. **Tongue** - It is a large, muscular and highly mobile tissue located on the floor of buccal cavity. - The posterior part of the tongue is attached to the floor of buccal cavity with the help of ligament called frenulum linguae. - Dorsal part of the tongue contains numerous papillae called lingual papapillae. - Lingual papaillae are of three types; a. Vallate papillae- arranged in an inverted V at posterior part and bear about 100 taste buds. b. Fungiform papillae- mushroom shaped and are located near the tip of tongue bearing taste buds. c. Filiform papillae- thread like structure and distributed over the anterior two third of the tongue. **Taste buds** a. Taste buds located near the tip of tongue, taste for sweet. b. Taste buds located on the sides, taste for sour and salt. c. Taste buds located on the posterior, taste for bitter. Picture of the Human Tongue General functions of tongue a. Helps in ingestion and tastes the food b. Helps in mixing food with saliva and makes it soft and slippery. c. While chewing, it moves the food in buccal cavity. d. It makes food into spherical mass called food bolus. e. It also helps in speech. v. Teeth/ Dentes - Human teeth are described as a. Thecodont- Human teeth is embedded within the socket or pits of jaw bones called maxillae in upper jaw and mandibles in lower jaw. b. Diphyodont- Because these are formed in two sets. The first set of teeth that erupt in childhood are called milk teeth or temporary or deciduous teeth. Milk teeth are later replaced by permanent teeth. - Milk teeth - There are 20 milk teeth - Milk teeth appears during first few years (1-2 yrs) after birth from 6 months. - Permanent teeth - Milk teeth are later replaced by permanent teeth which appear between 6-12 years except last molar teeth (wisdom teeth) after 18 yrs. c. Heterodont - Human teeth are of different size, shape and structure to adapt for various functions. Based on this, there are four different types of teeth. 1. Incisors- they are chisel shaped and are responsible for cutting and chopping. 2. Canines- they are pointed and responsible tearing and shredding. 3. Premolars and 4. Molars --They are broader and responsible for crushing and grinding. Number of teeth i. Milk teeth - 20 teeth. In each jaw, two pairs of incisors, one pair of canine, no premolar and two pairs of molar/ wisdom teeth. - Dental formula for milk teeth; (I=2/2, C=1/1,M=2/2)X 2 =20 ii. Permanent teeth - 32 teeth. In each jaw, two pairs of incisors, one pair of canine, two pairs of premolars and three pairs of molars. - Dental formula for permanent teeth ; (I= 2/2,C=1/1,M= 3/3, P= 2/2)X 2 = 32 Structure of tooth - Generally tooth consists of three parts a. Root- basal part embedded in socket. b. Neck- junction between root and crown. c. Crown-it forms the outer exposed part. - Vertical section of tooth consists of a. Enamel - It is white in colour and formed of calcium phosphate and carbonate. - It is the hardest part in human body. b. Dentine - It forms the major portion of the body and is yellow in colour. - It is also made of calcium phosphate and carbonate. c. Pulp cavity - It is the central space of the tooth consisting of nerve fibres, connective tissues and blood capillaries. They are sensitive to pain. Fig. Structure of tooth. Some common Diseases of teeth **Periodontitis**- The inflammation of gums and periodontal membrane. **Pyorrhoea**- Caused by the growth of microorganisms in the space between adjacent teeth where food accumulates. The teeth sockets get swollen, roots of teeth become weak and mouth gives a foul smell. **Riggs**- caused by *Entamoeba gingivalis*. It act upon tooth enamel and gradually dissolve it, forming cavity in the presence of CO2 and H2O. When the cavity reaches deeper into the dentine and pulp cavity we feel pain. **Falling of teeth in old age** is due to accumulation of cholesterol in the root canal through which artery enters the pulp cavity. It closes the opening of pulp cavity and cuts off blood supply and nourishment of dentine and enamel, causing fall of teeth. **Scurvy**-It is a disease of gum caused by the deficiency of vitamin C. vi. **Pharynx** - It is divided into three parts 1. Nasopharynx - It is the uppermost part present behind the nasal chamber. 2. Oropharynx - It lies behind the buccal cavity and helps in passing down food bolus. 3. Laryngopharynx - It forms the lowermost part and has two openings; - Glottis-anterior slit (opening of trachea) - Gullet- posterior slit(opening of oesophagus) Note; when food is swallowed, the tracheal opening (glottis) gets closed with the help of leaf like cartilage called epiglottis which prevents entry of food into the trachea. If a small particle of food enters the tracheal opening, it is thrown out by coughing. And during inhalation, gullet remains closed. vii. **Tonsil** - The lymphatic tissue in the buccal cavity and pharynx forms tonsil. - It is also called waldeyer's lymphatic ring. - They are of oval shaped patches located on either side of buccal cavity. - It contains WBCs and lymphocytes. Hence helps in preventing the entry of microorganisms along with food particles. - viii. **Oesophagus** - It is a muscular tube about 25cm long in length. - Food passes down through it by peristaltic movements of its muscular wall into the stomach - Peristaltic movement is the wave like contraction and relaxation of muscular wall of digestive tract which helps in the process of digestion. - **Function**: *Oesophagus transfers food from the pharynx to the stomach.* https://hi-static.z-dn.net/files/d5b/fc355d914f93fb720e11302652685031.jpgfig.oesophagus showing peristaltic movement. ix. **Stomach** - It is an elongated J shaped digestive tract located behind diaphragm. - It is divided into following parts 1. Cardiac - It receives the opening of oesophagus called cardiac aperture and it is guarded by cardiac sphincter. - **Function of cardiac sphincter**; *it checks regurgitation of food.* 2. Fundus - It is the upper projected part of cardiac filled with air or gas. 3. Corpus or body - Forms the main, middle part of the stomach. 4. Pyloric - It is the lower narrow part of stomach which opens into the duodenum by pyloric aperture which is guarded by pyloric sphincter. - Functions of pyloric sphincter; it prevents pre-digested food to enter duodenum. Note;Inner walls of stomach consists of number of folds known as gastric rugae. Function of gastric rugae; it dilates the stomach for the storage of food particles and increases surface area for digestion.  **Functions of stomach** - Temporary storage of food particles - It secretes gastric juice which contains digestive enzymes such as gastric amylase, gastric lipase, proenzymes pepsinogen and prorennin - It also produces HCl which helps in killing micro-organisms present in the food particles. x. **Small intestine** - It is tube like coiled structure located within the abdominal cavity. - It is about 6 metres long and forms the longest part of the alimentary canal - It is divided into three parts; a. Duodenum - It is about 25cm long and U-shaped in appearance. - It receives the hepatopancreatic duct formed by the union of bile duct and pancreatic duct. b. Jejunum - It forms the middle part of the small intestine and is about 2.5m long and narrower than duodenum. c. Ileum - It forms the lowermost and the largest part of the small intestine and is about 3.5 metres long. - It opens into first part of the large intestine Note; inner mucosa of small intestine consists of finger like projections called villi and microvilli which helps in the process of digestion and absorption of food particles. **Functions of small intestine** - Completes digestion - Absorbs digested food - Secretes hormones like cholecystokinin, secretin,enterogastrone, duocrinin, enterocrinine and vikinin that control the secretion of pancreatic juice, bile and intestinal juice. - Intestinal juices are collectively known as succus entericus. xi. **Large intestine** - It is about 1.5 to1.8 metres long and is wider than small intestine - It is divided into following 1. Caecum - It receives the opening of ileum - It is a small pouch like structure connected to a tubular part called vermiform appendix (in man it has no function (vestigial organ)). Note; sometimes, mucous lining of appendix gets inflamed, causing pain and discomfort. This condition is called appendicitis. 2. Colon - It forms the middle part of large intestine - It is divided into ascending colon, transverse and descending colon. 3. Rectum - Forms the lowermost part which opens into the exterior through anus. Functions of large intestine - Absorption of water from food - Secretion of mucus - Egestion of undigested waste matter. **[Digestive Gland]** **a. [Salivary Gland]:** is the gland which is associated with the secretion of saliva. - It is divided into **three parts:** a. **Parotid gland**It forms the largest part of the salivary gland located at the side of the face, in front of the ears. **b. Sublingual Gland:** - It is located below the front part of the tongue. - Its duct opens below the tongue - The duct of the Subingual gland is called *Rivinus duct.* **c. Submaxilliary Gland:** - It is located at the angles of the lower jaw and its duct opens under the tongue. - The duct of Submaxilliary gland is known as *Wharton's duct*. **[Functions of Salivary Gland]** 1. It secretes saliva (1.5L daily) 2. Saliva contains 98 -- 99% of water, 0.2% of salt, mucus, starch Hydrolysing enzyme called *Salivary amylase* or *Ptyalin.* 3. Saliva is slightly acidic and its pH value is 6.8 -- 7. **\>** An enzyme ptyalin hydrolyses larger starch into simpler forms such as isomaltose, maltose and α-dextrins at pH 7. Starch [\$\\frac{\\text{Ptyalin}}{(pH\\ 7)}\$]{.math.inline}\> Maltose + Isomaltose + Dextrins **\>** Starch is further broken down into glucose in the presence of enzyme *maltase*. Maltose [\$\\frac{\\text{Maltase}}{}\$]{.math.inline}\> Glucose 4. Saliva along with tongue makes food soft and slippery. 5. Saliva also helps in making food into spherical mass called *bolus**.*** 6. Saliva helps in keeping the tongue moist. 7. Saliva also contains an enzyme *lysozyme* which helps in killing bacteria present in the food particles. 8. It also helps in speech. **b. [Gastric Gland]:** is the gland associated with the stomach. - It is divided into 3 parts: a\. Cardiac gland b\. Pyloric gland c\. Fundic gland Fundic Gland is composed of ; **1. Peptic cells/Chief Cells/ Zymogen:** It secretes pro-enzymes such as pepsinogen, prorennin, and gastric lipase. **2. Oxyntic cells:** It secretes stomach acid called *hydrochloric acid **(HCl)***. **3. Argentaffin cells:** It secretes hormone called *serotonin* (vasoconstrictor) **4. Goblet cells:** It secretes mucus. **5. Endocrine cells:** It secretes gastrin which stimulates gastric gland to secrete other enzymes. **c. [Intestinal glands]:** is the gland associated with intestine. - It is located in the inner surface of the intestinal walls in between villi. - It is divided into 2 types: 1. **Crypts of Lieberkuhn:** Present in the mucusa of small intestine and responsible for secreting intestinal juices. 2. **Brunner's Gland:** Located in the submucosa of the duodenum and secretes watery alkaline fluid. **d. [Liver]:** It forms the largest gland in the body. - It is brownish in color and weighs about 1.5 Kg. **[Functions of Liver]** 1. **Bile secretion:** - Bile consists of bile pigments called bilirubin and biliverdin and bile salts called sodium glycocholate and sodium taurocholate. - Bile salts are responsible for emulsification of fats. - Bile is secreted by hepatic cells (liver cells) and are stored in gall bladder. - Bile is transported with the help of bile duct which later unites with pancreatic duct to form *hepatopancreatic duct*. - Hepopancreatic duct is guarded by a valve called *sphincter of oddi* which opens to duodenum. 2. **Metabolism of glucose: [ ]** a. **Glycogenesis** Excessive glucose in the blood is converted into glycogen. b. **Glycogenolysis** Glycogen is broken down into glucose whenever required. c. **Glyconeogenesis:** Synthesis of glucose from amino acids, fatty acids and glycerol. d. **Lipogenesis:** Conversion of glucose into fats that gets stored in adipose tissue. 3. **Phagocytosis:** 4. **Protein formation:** 5. **It acts as a centre of heat production.** 6. **Helps in detoxification** 7. **Synthesis of urea** 8. **Production of heparin which prevents clotting of blood within the blood vessels.** **[Pancreas]** - **It is leaf shaped glands consisting of two parts:** - **Endocrine gland** - **Exocrine gland** **Exocrine gland** - **Made of small lobules** - **Mainly responsible for the secretion of pancreatic juice containing proenzymes such as trypsinogen,chymotrypsinogen and steapsin** **Endocrine gland** - **Made of patches of cells called islet of langerhans.** - **Islets of langerhans are made of following types of cells.** a. **Alpha cells- it produces hormone glucagon which converts glycogen to glucose.** **Activity of Glucagon increases the blood sugar level** b. **Beta cells- it produces hormone insulin which converts glucose into glycogen.** **Activity of insulin decreases the blood sugar level and prevents from getting diabetes mellitus.** c. **Delta cells- produce hormone somatostatin which inhibits the secretion of glucagon and insulin.**