Lecture 23 - Human Digestive System PDF

Human Digestive System KHUSHAL KHAN DEMONSTRATOR CARDIOLOGY KHYBER MEDICAL UNIVERSITY – INSTITUTE OF PARAMEDICAL SCIENCES   The digestive system consists of;  Alimentary Canal or Gastrointestinal tract (GIT)  Ac...

Human Digestive System KHUSHAL KHAN DEMONSTRATOR CARDIOLOGY KHYBER MEDICAL UNIVERSITY – INSTITUTE OF PARAMEDICAL SCIENCES   The digestive system consists of;  Alimentary Canal or Gastrointestinal tract (GIT)  Accessory organs of digestion  Alimentary Canal or Gastrointestinal tract (GIT)  begins at the mouth  passes through the thorax, abdomen and pelvis Digestive  ends at the anus  Accessory organs of digestion System  They are: 1. Teeth 2. Tongue 3. Salivary glands 4. Pancreas They are ducted organs – connected to the alimentary 5. Liver and biliary tract canal.   Ingestion  Taking of food into the alimentary tract, i.e. eating and drinking.  Secretion  Secretion of mucous and enzymes i.e. digestive enzymes.  Propulsion (Peristalsis)  Mixing and movement of food along the alimentary tract.  Digestion 1) Mechanical digestion - breakdown of food by, e.g. Mastication (chewing). Functions 2) Chemical digestion - conversion of food into small molecules by enzymes.  Absorption  Passage of digested food substances through the walls of the alimentary canal into the blood and lymph capillaries like alcohol, nutrients, vitamins, water and electrolytes.  Elimination  The removal of undigested and non absorbable food from the alimentary canal.  Defaecation - the discharge of faeces from the body.   The digestive system consists of; 1) The gastrointestinal tract (GI tract, digestive tract, alimentary canal) 2) Accessory organs of digestion 1) Alimentary canal Physiologica  Alimentary canal also known as the gastrointestinal (GI) tract. l Anatomy  This is a long tube through which food passes (about 9 meters).  It starts at the mouth and terminates at the anus.  It consists of;  Mouth, pharynx, oesophagus, stomach, small intestine, large intestine, rectum and anal canal. 2) Accessory Organs of digestion  These organs helps primary digestive organs in the process of digestion.  The organs and glands are linked physiologically as well as anatomically to the alimentary canal.  They consist of:  Teeth  Tongue  Three pairs of salivary glands  Pancreas  Liver and biliary tract   The basic structure of the walls of the alimentary tract (GIT) are formed by 4 layers of tissue: 1) Adventitia or The Serosa 2) The Muscle Layer 3) The Submucosa Layers of 4) The Mucosa Gastrointestin al Tract   This is the outermost layer.  In thorax - it consists of loose fibrous tissue  In the abdomen - the organs are covered by a 1- Serosa serous membrane (serosa) called peritoneum.  The peritoneum serves to support the organs of the abdomen and acts as a conduit for the passage of nerves, blood vessels, and lymphatics.   The largest serous membrane of the body.  It is a closed sac, containing a small amount of serous fluid, Peritoneum within the abdominal cavity.  It has two layers:  Parietal peritoneum - lines the abdominal wall.  Visceral peritoneum - covers the organs (viscera) within the abdominal and pelvic cavities.  It is richly supplied with blood and lymph vessels, and contains many lymph nodes.  It provides a physical barrier to local spread of infection, and can isolate an infective focus such as appendicitis, preventing involvement of other abdominal structures.  Peritoneal Cavity – space between visceral and parietal peritoneum.  Contains peritoneal fluid – secreted by peritoneal cells  Prevent friction between two layers of peritoneum.   The muscle layer consists of two layers of smooth (involuntary) muscle.  The muscle fibres of the outer layer are arranged longitudinally.  The muscle of inner layer encircle the wall of the tube.  Between these two muscle layers are blood vessels, lymph vessels and a plexus (network) 2-Muscle of sympathetic and parasympathetic nerves, called the myenteric plexus (Auerbach’s plexus). layer  The Myenteric Plexus (Auerbach’s Plexus)  Neurons of this plexus regulate peristaltic waves that move digestive products from the oral to the anal opening.  Myenteric neurons control local muscular contractions that are responsible for stationary mixing and churning.  These nerves supply the adjacent smooth muscle and blood vessels. Myenteric Plexus   The sub-mucosal layer consists of loose areolar connective tissue.  It contains collagen and elastic fibres, which binds the muscle layer to the mucosa.  The submucosal layer contains;  blood vessels 3-  nerves plexuses - Meissner’s plexus  lymph vessels Submucosa  lymphoid tissue  Meissner’s plexus  Meissner's plexus, a neural network located within the submucosa.  It transmits sensory (afferent) stimuli to the central nervous system through both parasympathetic and sympathetic pathways. Meissner’s Plexus   The mucosa consists of 3 layers of tissue: 1)Mucous Membrane  The innermost layer - formed by columnar epithelium  It has three main functions: protection, secretion and absorption 2)Lamina Propria 4-Mucosa   It consists of loose connective tissue. It supports the blood vessels that nourish the inner epithelial layer, and lymphoid tissue that has a protective function. 3) Muscularis Mucosa  It is the thin, outer layer of smooth muscle.  It provides involutions of the mucosa layer, e.g. gastric glands, villi. Enteric Nervous System   The gastrointestinal tract has a nervous system all its own called the enteric nervous system.  It lies entirely in the wall of the gut, beginning in the esophagus and extending all the way to the anus. Enteric  The number of neurons in the enteric system is greater than 100 million. Nervous  Its plays a vital role in controlling gastrointestinal System movements and secretion.  The enteric nervous system is composed mainly of two plexuses; 1. Myenteric plexus or Auerbach’s plexus 2. Submucosal plexus or Meissner’s plexus Enteric Nerve Plexus  Myenteric plexus Submucosal plexus  also known as  also known as Auerbach’s plexus. Meissner’s plexus.  It lies between the  It lies in the longitudinal and submucosa. circular muscle  The submucosal plexus layers. controls mainly  The myenteric plexus gastrointestinal controls mainly the secretion and local gastrointestinal blood flow. movements.  Parasympathetic Supply Sympathetic Supply 1) Vagus Nerves 1) Celiac Ganglion Sympathetic 2) Sacral Nerves 2) Superior Mesenteric Ganglion &  The effects of 3) Inferior Mesenteric Ganglion Parasympath parasympathetic  The effects of sympathetic stimulation are: etic Nerve  Increased peristalsis - stimulation are:  decreased peristalsis - Supply to GIT through increased activity of the because there is less stimulation of the myenteric plexus myenteric plexus  increased glandular  decrease glandular secretion - through secretion - as increased activity of stimulation of the the submucosal submucosal plexus is plexus reduced Splanchnic Circulation   The term ‘splanchnic circulation’ describes the blood flow to the abdominal gastrointestinal organs including the stomach, liver, spleen, pancreas, small intestine, and large intestine. Splanchnic  It comprises 3 major branches of the abdominal aorta; Circulation 1. Celiac artery 2. Superior mesenteric artery (SMA) 3. Inferior mesenteric artery (IMA)  The hepatic portal circulation delivers the majority of the blood flow to the liver. Components of Digestive System   The mouth – also called oral cavity (buccal cavity).  Parts:  Vestibule - part of the mouth between the gums and the cheeks  Uvula - curved fold of muscle, hanging down from middle of soft palate  The oral cavity is lined with mucous membrane - Mouth stratified squamous epithelium  Borders:  Anterior – the lips.  Posterior – it is continuous with the oropharynx.  Lateral – muscles of the cheeks.  Superior – bony hard palate and muscular soft palate.  Inferior – muscular tongue and soft tissues of the floor of the mouth.   The tongue is a voluntary muscular structure.  Occupies the floor of the mouth.  Attachments:  Hyoid bone Tongue  Frenulum  Superior surface consists - papillae (little projections).  Papillae contain sensory receptors (specialised nerve endings) for the sense of taste in the taste buds.   The tongue plays an important part in:  Chewing (mastication) Functions  Swallowing (deglutition)  Speech  Taste   The teeth are embedded in the alveoli or sockets of the alveolar ridges  The mandible and the maxilla  Babies are born with two sets of teeth (dentitions)  Temporary teeth (Deciduous teeth) Teeth  There are 20 temporary teeth, 10 in each jaw.  Begin to erupt about 6 months, and complete by 24 months of age.  Permanent teeth  The permanent teeth replace the deciduous teeth in the 6th year of age.  This dentition, consisting of 32 teeth, is usually complete by the 21st year.   Teeth have different shapes depending on their functions.  Incisors and canine (cutting teeth) - used for Functions biting off pieces of food.  Premolar and molar teeth - used for grinding or chewing food.   Salivary glands make saliva, which aids in digestion.  It keeps the mouth moist and supports healthy teeth. Salivary  Salivary glands release their secretions into Glands ducts that lead to the mouth.  There are three main pairs: 1) Parotid Glands 2) Submandibular Glands 3) Sublingual Glands   Chemical digestion of polysaccharides  Saliva contains the enzyme amylase that begins the breakdown of complex sugars, including starches, reducing them to the disaccharide maltose.  The optimum pH for the action of salivary amylase is 6.8 (slightly acid). Functions  Enzyme action continues during swallowing until terminated by the strongly acidic pH (1.5 to 1.8) of the gastric juices, which degrades the amylase.  Lubrication of food  The high water content means that dry food entering the mouth is moistened and lubricated by saliva before it can be made into a bolus ready for swallowing.  Cleaning and lubricating the mouth  An adequate flow of saliva is necessary to clean the mouth, and to keep it soft, moist and pliable.  It helps to prevent damage to the mucous membrane by rough or abrasive food.  Non-specific defence  Lysozyme, immunoglobulins and clotting factors present in saliva combat invading microbes.  Taste  The taste buds are stimulated only by chemical substances in solution and therefore dry foods only stimulate the sense of taste after thorough mixing with saliva.  The senses of taste and smell are closely linked and involved in the enjoyment of food.   The pharynx is hollow tube inside the neck that starts behind the nose and ends at the top of the trachea (windpipe) and oesophagus.  It is divided into three parts;  Nasopharynx  Oropharynx Pharynx  Laryngopharynx  The oropharynx and laryngopharynx are passages common to both the respiratory and the digestive systems.  Food passes from the oral cavity into the pharynx then to the oesophagus below, with which it is continuous.  The walls of the pharynx consist of three layers of tissue.  Inner Layer - (mucosa)  Provide lining of the mouth - stratified squamous epithelium  Stratified epithelial tissue provides a lining well suited to the wear and tear of swallowing.  Middle layer  Made of connective tissue  contains blood and lymph vessels and nerves  Outer Layer  consists of involuntary muscles - involved in swallowing  When food reaches the pharynx swallowing is no longer under voluntary control.   The oesophagus is a hollow, muscular tube that carries food and liquid from throat to the stomach.  Also called: Food pipe or gullet  It is about 25 cm (10 inch) long and about 2 cm in diameter. Oesophagu  It begins at the back of the mouth, passing downward through the rear part of the s mediastinum, through the diaphragm, and into the stomach.  Location:  It lies in the median plane in the thorax  in front of the vertebral column  behind the trachea and the heart  It is continuous with the pharynx above and just below the diaphragm it joins the stomach.  The oesophagus is one of the upper parts of the digestive system.  The oesophagus starts around the level C-6 behind the cricoid cartilage of the trachea, enters the diaphragm at about the level of T-10, and ends at the cardia of the stomach, at the level of T-11.  The wall of the oesophagus consists of;  Outer layer of connective tissue  Muscular layer  Submucosa (connective tissue)  Mucosa  Sphincters:  The upper and lower ends of the oesophagus are closed by sphincters.  Upper oesophageal sphincter (cricopharyngeal)  prevents air passing into the oesophagus during inspiration  prevents the aspiration of oesophageal contents  The cardiac or lower oesophageal sphincter  Physiological – (not anatomical) i.e. no thickening of the circular muscle  prevents the reflux of acid gastric contents into the oesophagus   The stomach is a J-shaped dilated portion of the alimentary tract.  It is situated in the epigastric, umbilical and left hypochondriac regions of the abdominal cavity.  Organs associated with the stomach Stomach  Anteriorly – left lobe of liver and anterior abdominal wall  Posteriorly – abdominal aorta, pancreas, spleen, left kidney & adrenal gland  Superiorly – diaphragm, oesophagus and left lobe of liver  Inferiorly – transverse colon and small intestine  To the left – diaphragm and spleen  To the right – liver and duodenum  The stomach is continuous with the oesophagus at the cardiac sphincter and with the duodenum at the pyloric sphincter.  Curvatures  It has two curvatures. 1) Lesser Curvature  short, lies on the posterior surface of the stomach  It is the downward continuation of the posterior wall of the oesophagus. 2) Greater Curvature  Where the oesophagus joins the stomach the anterior region angles acutely upwards  Curves downwards forming the greater curvature and then slightly upwards towards the pyloric sphincter.  Regions  The stomach is divided into four regions: 1) Cardia – entry of food 2) Fundus – storage of food 3) Body – chemical digestion + mixing of food 4) Pylorus – emptying of food  At the distal end of the pylorus is the pyloric sphincter, guarding the opening between the stomach and the duodenum.  When the stomach is inactive the pyloric sphincter is relaxed and open, and when the stomach contains food the sphincter is closed.  Walls of the Stomach  The four layers of tissue that comprise the basic structure of the alimentary canal are found in the stomach but with some modifications.  Muscle layer  This consists of 3 layers of smooth muscle fibres: 1. an outer layer - longitudinal fibres 2. a middle layer - circular fibres 3. an inner layer - oblique fibres  This arrangement allows for the churning motion - characteristic of gastric activity, as well as peristaltic movement.  Circular muscle is strongest in the pylorus and pyloric sphincter.  Mucosa of the stomach  When the stomach is empty the mucous membrane lining is thrown into longitudinal folds or rugae.  When the stomach is full the rugae are pressed and the surface has a smooth, velvety appearance.  Numerous gastric glands are situated below the surface in the mucous membrane and open on to it.  They consist of specialised cells that secrete gastric juice into the stomach.  1. Temporary storage of food  Allowing time for the digestive enzymes, pepsins, to act 2. Chemical digestion Functions  Pepsins convert proteins to polypeptides 3. Mechanical breakdown  The three smooth muscle layers enable the stomach to act as a churn, gastric juice is added and the contents are liquefied to chyme.   The small intestine is continuous with the stomach at the pyloric sphincter.  The small intestine is about 5 metres long.  It leads into the large intestine at the ileocaecal valve. Small  It lies in the abdominal cavity surrounded by the large Intestine intestine.  The small intestine comprises three parts:  Duodenum  Jejunum  Ileum  3 Parts of small intestine; 1) Duodenum  1st part of small intestine  about 25 cm long, curves around the head of the pancreas  Secretions from the gall bladder and pancreas merge in a common structure – the hepatopancreatic ampulla – and enter the duodenum at the duodenal papilla.  The duodenal papilla is guarded by a ring of smooth muscle, the hepatopancreatic sphincter (of Oddi). 2) Jejunum  middle part, about 2 meters long 3) Ileum  about 3 metres long, ends at the ileocaecal valve   The walls of the small intestine are composed of the four layers of tissue.  Some modifications occurs at peritoneum and mucosa and described as;  Peritoneum – (Mesentery)  Short, fan shaped Structure of  A double layer of peritoneum Small  Attaches the jejunum and ileum to the posterior abdominal wall. intestine  The large blood vessels and nerves lie on the posterior abdominal wall and the branches to the small intestine pass between the two layers of the mesentery.  Mucosa  The surface area of the small intestine mucosa is greatly increased by permanent circular folds, villi and microvilli.  The permanent circular folds, unlike the rugae of the stomach, are not smoothed out when the small intestine is distended.  They promote mixing of chyme as it passes along.   Chemical Digestion  In the small intestine the chemical digestion of food is completed. Functions  Absorption  Absorption of most nutrients takes place in small intestine.   The large intestine is about 1.5 metres long.  It begins at the caecum in the right iliac fossa and terminate at the rectum and anal canal deep in the pelvis.  Its lumen is about 6.5 cm in diameter, larger than Large that of the small intestine.  It forms an arch round the coiled-up small Intestine intestine.  The large intestine is divided into;  Caecum  Colon  Rectum  Anal canal   Caecum is the first part of the large intestine.  It is a dilated region and is continuous with the ascending colon. Just below the junction of the two the ileocaecal valve opens from the ileum.  Vermiform Appendix  a fine tube, closed at one end, which leads from the caecum.  It is usually about 8 to 9 cm long. Caecum  It has the same structure as the walls of the large intestine but contains more lymphoid tissue.   The colon has four parts;  Ascending colon  This passes upwards from the caecum to the level of the liver where it curves acutely to the left at the hepatic flexure to become the transverse colon.  Transverse colon  This is a loop of colon that extends across the abdominal cavity in front of the duodenum and the stomach to the area of Colon the spleen where it forms the splenic flexure and curves acutely downwards to become the descending colon.  Descending colon  This passes down the left side of the abdominal cavity then curves towards the midline. After it enters the true pelvis it is known as the sigmoid colon.  Sigmoid colon  This part describes an S-shaped curve in the pelvis that continues downwards to become the rectum.   This is a slightly dilated section of the large intestine about 13 cm long. Rectum  It leads from the sigmoid colon and terminates in the anal canal.   This is a short passage about 3.8 cm long in the adult.  It leads from the rectum to the exterior.  Two sphincter muscles control the anus; 1. Internal sphincter Anal canal  It consists of smooth muscle.  It is under the control of the autonomic nervous system. 2. External sphincter  It is formed by skeletal muscle.  It is under voluntary control.   Absorption  absorption of water - by osmosis, until semisolid consistency of faeces is achieved.  Mineral salts, vitamins and some drugs are also absorbed.  Microbial activity  Escherichia coli, Enterobacter aerogenes, Streptococcus faecalis, Clostridium perfringens - synthesise vitamin K and folic acid  Mass movement Function  a wave of strong peristalsis sweep along the transverse colon forcing its contents into the descending and sigmoid colons – mass movement  Defaecation  Thus defaecation involves involuntary contraction of the muscle of the rectum and relaxation of the internal anal sphincter.  Contraction of the abdominal muscles and lowering of the diaphragm increase the intra-abdominal pressure (Valsalva’s manoeuvre) and so assist the process of defaecation. THANK YOU