Human digestive system, Study notes of Biology

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The Digestive System: A Journey Through Each Organ

(Alimentary canal) and accessory organs. intestine, and large intestine Digestion is defined as the process by which food is broken down into simple chemical substances that can be absorbed and used as nutrients by the body. Most of the substances in the diet cannot be utilized as such. These substances must be broken into smaller particles, so that they can be absorbed into blood and distributed to various parts of the body for utilization. Digestive system is responsible for these functions. Digestive process is accomplished by mechanical and enzymatic breakdown of food into simpler chemical compounds. A normal young healthy adult consumes about 1 kg of solid diet and about 1 to 2 liter of liquid diet every day. All these food materials are subjected to digestive process, before being absorbed into blood and distributed to the tissues of the body. Digestive the digestion and absorption of food substances. Salivary al glands Mouth The digestive system Composed of the gastrointestinal tract (GIT) or — Esophagus 1. Alimentary canal - mouth, pharynx, esophagus, stomach, small Liver__| 2. Accessory digestive organs - teeth, tongue, gallbladder, salivary Gallbladder glands, liver, and pancreas. Duodenum ———- 7 ———— Pancreas Transverse -—- Descending coton Ascending —-- leurn Pelvic colon colon Appendix —-——— ~~... Anus FIGURE 36.1: Gastrointestinal tract Mouth Mouth is otherwise known as oral cavity or buccal cavity. It is formed by cheeks, lips and palate. It encloses the teeth, tongue and salivary glands. Mouth opens anteriorly to the exterior through lips and posteriorly through fauces into the pharynx. Digestive juice present in the mouth is saliva, which is secreted by the salivary glands. Primary function of mouth is eating and it has few other important functions also. Functions of mouth include: 1. Ingestion of food materials 2. Chewing the food and mixing it with saliva 3. Appreciation of taste of the food 4. Transfer of food (bolus) to the esophagus by swallowing 5. Role in speech 6. Social functions such as smiling and other expressions. Salivary Gland Salivary Gland — In humans, the saliva is secreted by three pairs of major (larger) salivary glands and some minor (small) salivary glands. Major glands are: 1. Parotid glands - Parotid glands are the largest of all salivary glands, situated at the side of the face just below and in front of the ear. Each gland weighs about 20 to 30 g in adults. Secretions from these glands are emptied into the oral cavity by Stensen duct. 2. Submaxillary or submandibular glands - Submaxillary glands or submandibular glands are located in submaxillary triangle, medial to mandible. Each gland weighs about 8 to 10 g. Saliva from these glands is emptied into the oral cavity by Wharton duct. 3. Sublingual glands — Sublingual glands are the smallest salivary glands situated in the mucosa at the floor of the mouth. Each gland weighs about 2 to 3 g. Saliva from these glands is poured into 5 to 15 small ducts called ducts of Rivinus. MINOR SALIVARY GLANDS i. Lingual mucus glands are situated in posterior one third of the tongue, behind circumvallate papillae and at the tip and margins of tongue. il. Lingual serous glands are located near circumvallate papillae and filiform papillae. ili. Buccal glands or molar glands are present between the mucus membrane and buccinator muscle. Four to five of these are larger and situated outside buccinator, around the terminal part of parotid duct. iv.Labial glands are situated beneath the mucus membrane around the orifice of mouth CLASSIFICATION OF SALIVARY GLANDS Salivary glands are classified into three types, based on the type of secretion: 1. Serous Glands Serous glands are mainly made up of serous cells. These glands secrete thin and watery saliva. Parotid glands and lingual serous glands are the serous glands. 2. Mucus Glands Mucus glands are mainly made up of mucus cells. These glands secrete thick, viscous saliva with high mucin content. Lingual mucus glands, buccal glands and palatal glands belong to this type. 3. Mixed glands are made up of both serous and mucus cells. Submandibular, sublingual and labial glands are the mixed glands. Stensen duct pce Parotid gland Sublingual -+- 3---- Submaxillary oe - gland FIGURE 37.1: Major salivary glands PROPERTIES OF SALIVA 1. Volume: 1000 mL to 1500 mL of saliva is secreted per day and it is approximately about 1 mL/minute 2. Reaction: Mixed saliva from all the glands is slightly acidic with pH of 6.35 to 6.85 3. Specific gravity: It ranges between 1.002 and 1.012 4. Tonicity: Saliva is hypotonic to plasma FUNCTIONS OF SALIVA Saliva is a very essential digestive juice. Since it has many functions, its absence leads to many inconveniences. 5. PREPARATION OF FOOD FOR SWALLOWING When food is taken into the mouth, it is moistened and dissolved by saliva. The mucus membrane of mouth is also moistened by saliva. It facilitates chewing. By the movement of tongue, the moistened and masticated food is rolled into a bolus. Mucin of saliva lubricates the bolus and facilitates swallowing. 6. APPRECIATION OF TASTE Taste is a chemical sensation. By its solvent action, saliva dissolves the solid food substances, so that the dissolved substances can stimulate the taste buds. The stimulated taste buds recognize the taste. 7. DIG TABLE 37.2: Digestive enzymes of saliva Source of secretion Activator Salivary amylase All salivary glands Acid medium Converts starch into maltose Maltase Major salivary glands Acid medium Converts maltose into glucose Lingual lipase Lingual glands Acid medium Converts triglycerides of milk fat into fatty acids and diacylglycerol 4. CLEANSING AND PROTECTIVE FUNCTIONS i. Due to the constant secretion of saliva, the mouth and teeth are rinsed and kept free off food debris, shed epithelial cells and foreign particles. In this way, saliva prevents bacterial growth by removing materials, which may serve as culture media for the bacterial growth. ii. Enzyme lysozyme of saliva kills some bacteria such as staphylococcus, streptococcus and brucella. ili. Proline-rich proteins present in saliva posses antimicrobial property and neutralize the toxic substances such as tannins. Tannins are present in many food substances including fruits. 5. ROLE IN SPEECH By moistening and lubricating soft parts of mouth and lips, saliva helps in speech. If the mouth becomes dry, articulation and pronunciation becomes difficult. 6. REGULATION OF BODY TEMPERATURE 7. REGULATION OF WATER BALANCE When the body water content decreases, salivary secretion also decreases. This causes dryness of the mouth and induces thirst. REGULATION OF SALIVARY SECRETION Salivary secretion is regulated only by nervous mechanism. Autonomic nervous system is involved in the regulation of salivary secretion. Unconditioned reflexes: Taste and tactile stimuli increase 8-20 times the basal rate of secretion Conditioned reflexes: Visual, olphactoric Centers: salivatory nuclei (at the juncture of the medulla and pons) Parasympathetic nerves: n.VIl, n.LX — stimulation of the salivation. Sympathetic nerves: stimulation of the secretion of the mucinous saliva Stomach Stomach is a hollow organ situated just below the diaphragm on the left side in the abdominal cavity. Volume of empty stomach is 50 mL. Under normal conditions, it can expand to accommodate 1 L to 1.5L of solids and liquids. However, it is capable of expanding still further up to 4 L. stomach has four parts: 1; wh Cardiac region - Cardiac region is the upper part of the stomach where esophagus opens. The opening is guarded by a sphincter called cardiac sphincter, which opens only towards stomach. This portion is also known as Cardiac end. Fundus - Fundus is a small dome shaped structure. It is elevated above the level of esophageal opening. Body or corpus - Body is the largest part of stomach forming about 75% to 80' es eRYands feqto jude Heldovrdatoiuihaes wo toathe pylocimregibpyloric canal. The or body of stomach ends in antrum. Junction between body and antrum is marked by an angular notch called incisura angularis. Antrum is continued as the narrow canal, which is called pyloric canal or pyloric end. Pyloric canal ro ae opens into first part of small intestine called duodenum. The opening of vce pyloric canal is guarded by a sphincter called pyloric sphincter. It opens towards duodenum. Stomach has two curvatures. One on the right side is lesser curvature and the other on left side is greater curvature. Cardiac region-- — Body --- Greater curvature Pylorus I ---- Antrum FIGURE 38.1: Parts of stomach CLASSIFICATION OF GLANDS OF THE STOMACH Gastric glands are classified into three types, on the basis of their location in the stomach: 1. Fundic glands or main gastric glands or oxyntic glands: Situated in body and fundus of stomach 2. Pyloric glands: Present in the pyloric part of the stomach 3. Cardiac glands: Located in the cardiac region of the stomach Oesophagus Cardiac sphincter Body luscularis externa: region Longitudinal laver Circular laver Pyloric fegion (a) FUNCTIONS OF GASTRIC JUICE 1. 4 DIGESTIVE FUNCTION Gastric juice acts mainly on proteins. Proteolytic enzymes of the gastric juice are pepsin and rennin etc. HEMOPOIETIC FUNCTION Intrinsic factor of Castle, secreted by parietal cells of gastric glands plays an important role in erythropoiesis. It is necessary for the absorption of vitamin B12 (which is called extrinsic factor) from GI tract into the blood. Vitamin B12 is an important maturation factor during erythropoiesis. PROTECTIVE FUNCTION —- FUNCTION OF MUCUS It protects the gastric wall by the following ways: 1.Protects the stomach wall from irritation or mechanical injury, by virtue of its high viscosity. ii. Prevents the digestive action of pepsin on the wall of the stomach, particularly gastric mucosa lili. Protects the gastric mucosa from hydrochloric acid of gastric juice because of its alkaline nature and its acidcombining power. FUNCTIONS OF HYDROCHLORIC ACID Hydrochloric acid is present in the gastric juice: iv. Activates pepsinogen into pepsin v. Kills some of the bacteria entering the stomach along with food co; bacteriolytic action vi. Provides acid medium, which is necessary for the action of horn NaHCO, - I— HCO, ff FIGURE 338.4: Secretion of hydrochloric acid in the panetal cell of gastric gland CEPHALIC PHASE: Only nervous FIGURE 38.6: Schematic diagram showing the regulation of gastric secretion CCK-PZ = Cholecystokinin-pancreozymin, GIP = Gastric inhibitory peptide, VIP = Vasoactive intestinal peptide. Pancreas Pancreas is a dual organ having two functions, namely endocrine function and exocrine function. Endocrine function is concerned with the production of hormones. The exocrine function is concerned with the secretion of digestive juice called pancreatic juice. PROPERTIES AND COMPOSITION OF PANCREATIC JUICE PROPERTIES OF PANCREATIC JUICE e Volume : 500 to 800 mL/day e Reaction : Highly alkaline with a pH of 8 to 8.3 e Specific gravity : 1.010 to 1.018 Pancreatic juice contains 99.5% of water and 0.5% of solids. The solids are the organic and inorganic substances. | Pancreatic juice — ¥ , > Water—99.5% | Solids — 0.5% I = + Organic substances | Inorganic £ I 4 substances Enzymes Other organic substances | 4. Sodium 1 2. Calcium + + | 3. Potassium Proteolytic enzymes Lipolytic enzymes 4. Magnesium 1. Trypsin 1_ Pancreatic lipase 1. Albumin i rare a pelea 2 gy ors io iia 2. Globulm 7. Phosphate arboxypepidases hydrota 4. Muclease 3. Phospholipase A SE 5. Elastase 4_ Phospholipase B 6. Collagenase 5. Colipase 6. Bile salt-activated Amylolytic enzyme lipase Bicarbonate content = 110 to 150 mEq/L Pancreatic amylase FIGURE 39.1: Composition of pancreatic juice FUNCTIONS OF PANCREATIC JUICE Pancreatic juice has digestive functions and neutralizing action. e DIGESTIVE FUNCTIONS OF PANCREATIC JUICE Pancreatic juice plays an important role in the digestion of proteins and lipids. It also has mild digestive action on carbohydrates. e DIGESTION OF PROTEINS Major proteolytic enzymes of pancreatic juice are trypsin and chymotrypsin. Other proteolytic enzymes are carboxypeptidases, nuclease, elastase and collagenase. e DIGESTION OF CARBOHYDRATE e DIGNESTION OF LIPIDS CEPHALIC PHASE: Nervous Conditioned reflex Sight, smeil, S thought of and - | Sexetan parce — [eta —- Unconditioned reflax Presence of food — in mouth GASTRIC PHASE: Hormonal ot — ci — INTESTINAL PHASE: Hormonal eaten — eee FIGURE 39.2: Schematic diagram showing the regulation of pancreatic secretion Chyme in intestine Liver Liver is a dual organ having both secretory and excretory functions. It is the largest gland in the body, weighing about 1.5 kg in man. It is located in the upper and right side of the abdominal cavity, immediately beneath diaphragm. Liver secretes the Bile Juice. PROPERTIES AND COMPOSITION OF BILE PROPERTIES OF BILE e Volume : 800 to 1,200 mL/day e Reaction: Alkaline e pH: 8to 8.6 e Specific gravity : 1.010 to 1.011 e Color : Golden yellow or green. COMPOSITION OF BILE Bile contains 97.6% of water and 2.4% of rganic substances. Inorganic substances j 1. Sodium 2. Calcium 3. Potassium 4. Chioride 5. Bicarbonate FIGURE 40.6: Composition of bile SECRETION OF BILE Bile is secreted by hepatocytes. The initial bile secreted by hepatocytes contains large quantity of bile acids, bile pigments, cholesterol, lecithin and fatty acids. From hepatocytes, bile is released into canaliculi. STORAGE OF BILE Most of the bile from liver enters the gallbladder, where it is stored. It is released from gallbladder into the intestine whenever it is required. FUNCTIONS OF BILE Most of the functions of bile are due to the bile salts. e DIGESTIVE FUNCTION e ABSORPTIVE FUNCTIONS e EXCRETORY FUNCTIONS e ANTISEPTIC ACTION Bile inhibits the growth of certain bacteria in the lumen of intestine by its natural detergent action. e MAINTENANCE OF pH IN GASTROINTESTINAL TRACT As bile is highly alkaline, it neutralizes the acid chyme which enters the intestine from stomach. Thus, an optimum pH is maintained for the action of digestive enzymes. e PREVENTION OF GALLSTONE FORMATION e LUBRICATION FUNCTION The mucin in bile acts as a lubricant for the chyme in intestine.