Lecture 10: The Digestive System PDF

Lecture 10: The Digestive System Dr. Ali Azzawri  Human Digestive System  Digestion is the ability to process food in the body into a form that can be absorbed and used or excreted.  The purpose of digestion is to break food down to mole...

Lecture 10: The Digestive System Dr. Ali Azzawri  Human Digestive System  Digestion is the ability to process food in the body into a form that can be absorbed and used or excreted.  The purpose of digestion is to break food down to molecules that can be used by cells. Digestion involves three principle processes: mechanical digestion, chemical digestion and absorption.  A nutrient is a substance the body needs for growth, repair and maintenance. Nutrients include carbohydrates, proteins, lipids, vitamins, minerals and water. Carbohydrates must be broken down into monosaccharides and are the body’s main source of energy.  Proteins are broken down to amino acids and supply the raw materials for growth and repair. The body requires 20 amino acids, 10 of which it cannot make and must obtain from outside sources.  Lipids are reduced to fatty acids and glycerin. They are used to make steroid hormones, cell membranes and also store energy. Vitamins are organic molecules that aid in the regulation of body processes.  Finally, water is required for metabolism and chemical reactions within the body, for transport of substances around the body, and for regulation of body temperature.  Approximately two-thirds of the body weight is water.  Digestion involves three principle processes: ◦ Mechanical digestion: takes place in the mouth, your teeth chew the food. Large pieces of food are broken down into smaller pieces of the same food. ◦ Chemical digestion: using chemicals to digest/ break down food, this takes place in your mouth and stomach where acid and enzymes mix with the food. Food is broken down into a different substance that can easily pass into the blood ◦ Absorption: pulling nutrients out of the food, occurs in the small intestine. Structures  The GastrointestinaI tract (GI), also called the alimentary canal is the system of organs that take in food, digest it to extract nutrients and expels the waste. These organs are the mouth, pharynx, esophagus, stomach, small intestine and large intestine. ◦ Major Functions:  Ingestion  Digestion  Absorption  Defecation or Excretion  Accessory organs: Organs that help with digestion but are not part of the digestive tract. These organs are the tongue, salivary glands, liver, gall bladder, and pancreas.  Human Digestive System Diagram Mouth Pharynx Esophagus Liver Stomach Large Intestine Small Intestine Villi Nutrient absorption sites  Mouth 1. Chemical digestion (amylase converts starch to Oesophagus maltose) Connects mouth to stomach 2. Physical digestion (teeth break food down into smaller pieces) Liver Stomach 1. Holds the food for a while Produces bile for the digestion 2. Physical digestion (food is churned of fats and mixed) 3. Chemical digestion (assisted by HCl) Small intestine Gall bladder Stores bile 1.Chemical digestion 2.Absorption of nutrients into blood Pancreas Large intestine (colon) Produces digestive juices 1. Elimination of waste 2. Absorption of water Appendix Rectum Anus Stores faeces  Following the Trail  The process begins in the mouth. ◦ Chewing initiates mechanical breakdown of food and is followed by secretion of saliva, which moistens and lubricates food for swallowing. ◦ Saliva also contains amylases (enzymes), which start the chemical breakdown of carbohydrates.  The swallowing reflex begins in the pharynx and initiates rhythmic waves of smooth muscle contractions called peristalsis. ◦ Peristaltic contractions transport food to the stomach and allow a person to swallow even if he/she are upside down. Following the Trail II  The stomach contains an extra layer of muscle that aids in mechanically mixing and churning food into a semiliquid form called “chyme.” ◦ Chemical digestion begins with proteins through the action of hydrochloric acid (HCl) and the enzyme, pepsin. ◦ Only water and a few substances, such as aspirin and alcohol, are absorbed by the lining of the stomach. Following the Trail III  As food enters the small intestine secretions from the liver, gall bladder and pancreas are added.  The small intestine completes digestion of food materials by absorbing nutrients into the blood stream  The lining of the small intestine consists of tiny folds or fingerlike projections, called villi, which, in turn, are covered by microvilli which increase surface area ◦ The villi contain capillaries and lymphatic vessels for the absorption of nutrients ◦ Microvilli have brush border enzymes to hydrolyze lactose and sucrose.  The large intestine does not contain villi and it plays no role in digestion ◦ Only water and vitamin K are absorbed from the large intestine ◦ Undigested or unabsorbed food is eliminated through the rectum and then anus. Cross-Section of small intestine Villi Microvilli Where does each nutrient get broken down? Carb- Digestion Protein Digestion Nucleic Acid Digestion Fat Digestion Mouth, Throat, Polysacch. Into Esophagus Disacch. Stomach Polypepties into DNA, RNA into Fat into glycerol, fatty smaller proteins nucleotides acids and glycerides Small Intestine Disacch. Into Small proteins Nucleotides into nitrogen Monosacch. into amino acids base,sugar and phospate What enzymes break down each nutrient ? Carb Digestion Protein Digestion Nucleic Acid Digestion Fat Digestion Salivary amylase Peptidases Nucleotidases Bile salts and Lipase (amylose is starch, a (amino acids are (nucleotides are building (fats are made of lipids) polysacch) connected by “peptide” block of DNA and RNA) bonds)  The Digestive System is irregular tube; open at both ends, called “Alimentary canal” or “Gastrointestinal (GI) Tract” ◦ 29 feet long (adults) - 9 meters ◦ Food & other substances that enter tube are not really inside body. ◦ Passageway of food: broken down (digested) and absorbed thru walls < entering body - cells  Both - Mechanical & Chemical Digestion.  Wall of Digestive Tract - ◦ Mouth to anus ◦ Four layers of tissue; surrounding the hollow space within the tube “lumen” ◦ May vary in structure in different organs ◦ Mucosa or mucous membrane - tough in esophagus, delicate, for absorption or secretion in rest of tract ◦ Submucosa - connective tissue, blood vessels & nerves ◦ Muscularis - 2 layers, responsible for wavelike, rhythmic contractions (peristalsis), moves contents, assists in mixing & mechanical breakdown ◦ Serosa - outermost covering, composed of visceral peritoneum ◦ Mesentery - double folded peritoneal tissue, anchors loops of digestive tract to posterior wall of abdominal cavity  Key Organs of the GI Tract Functions of the Tongue  Mouth -  Taste: it is covered with thousands of taste buds. ◦ The food is broken down by the teeth These are sensitive to salt, sweet, sour and and mixed with saliva. bitter chemicals in food and drink. ◦ Saliva is excreted by three pairs of They help us enjoy food and drink and warn us glands: when food, drink are off or inedible.  The parotid gland (below the ear)  Chewing: the tongue aids chewing by moving the  The submandibular (under the tongue) food around the mouth, pushing it between the  The sublingual (under the tongue) teeth and covering it with saliva, which contains ◦ Saliva contains water, mucus and the enzymes that start the digestive process. enzyme salivary amylase. The food is turned into a partially digested mass known as a bolus. Swallowing: Functions of Saliva  when the food  It lubricates food with mucus, making it is ready to travel easier to swallow. to the stomach,  It contains the enzyme salivary amylase, which acts on cooked starch turning some the tongue pushes of it into maltose. it to the back of  It keeps the mouth and teeth clean. the mouth. Tongue Taste Centres  TEETH  An adult human has 32 teeth, 16 in either jaw.  The shape and size of the tooth varies with the job it has to do.  There are four different types of tooth. Incisors These cut and bite food Canines These grasp and tear food Pre-molars These grind and chew food Molars These also grind and chew food  The type of teeth that an animal has depends on what it eats  A herbivore eats plant material and has very large incisors and molars  A carnivore eats other animals and needs very large canines  An omnivore (e.g. humans) eats all kinds of food and needs and use all of the types of tooth equally  Oesophagus  The food passes into the pharynx (a muscular tube behind the mouth) and down the oesophagus.  The epiglottis a small flap of cartilage blocks the entrance to the larynx, this stops the food going down the wrong way and prevents choking. Structure of the Oesophagus & Functions  It is a muscular tube.  It leads from the pharynx to the stomach. Function  To carry chewed food from the pharynx to the stomach.  Food moves along it by a muscular contraction known as peristalsis.  The muscle fibres contract and relax which acts like a wave on the tube, pushing the bolus forward.  It’s lining secretes mucus to lubricate the passage of food.  It is a J-shaped, elastic organ. Stomach  Food enters it from the oesophagus through the cardiac sphincter.  The cardiac sphincter, is a valve that stops back flow of the stomach`s contents.  Food leaves the stomach through the pyloric sphincter into the duodenum (first part of the small intestine).  The walls of the stomach is made up of layers of muscle.  It has an inner mucous membrane.  This membrane has lots of folds.  When the stomach is full these folds stretch out, enabling it to expand, then they contract when the stomach empties. Functions of The Stomach  It digests protein through the action of enzymes.  It churns food with the gastric juices.  It helps lubricate the food by producing mucus.  It kills bacteria by producing hydrochloric acid. Gastric Juices  Hydrochloric acid neutralises bacteria and activates pepsin.  Rennin is an enzyme that curdles milk protein in infants.  Pepsin is an enzyme that breaks down proteins into peptones.  Small Intestine  It is seven metres long. Functions of The Small Intestine Digestion  It is divided into three parts:  Pancreatic juice is secreted into the duodenum The duodenum and contains the following enzymes: The jejunum Trypsin: converts proteins into shorter chains. The ileum Lipase: converts fats into fatty acids and glycerol.  The walls has four layers: Amylase: converts starch into disaccharides. A muscular layer  Bile:emulsifies fats (breaks them into smaller droplets). A layer containing blood vessels,  Intestinal Juices have the following enzymes: lymph vessels, and nerves. Maltase, sucrase, lactase: change A submucous layer, disaccharides into monosaccharides. A mucous layer.  Peptidase:changes polypeptides into amino acids. Absorption  The inner wall is covered in villi, tiny hair like projections which increase the surface area for absorption.  Each villi contain blood vessels and lymph vessels.  Digested food is absorbed through the villi walls.  Fats, fatty acids and glycerol are passed into the lymph system.  Amino acids and sugars pass along the portal vein to the liver.  Large Intestine  It deals with waste.  It is about 1.5m long.  It consists of the following: The caecum: a small pouch; the ileum empties its contents into the caecum through the ileo-caecal valve. The colon: ascending, transverse, descending colon. The appendix: narrow tube attached to the caecum. The rectum. The anus Functions  Whatever remains of the food, is passed into the large intestine  To reabsorb water and vitamins left in digestive waste.  It secretes mucus to help the movement of faeces.  Short term storage of faeces in the rectum.  Many bacteria live in the large intestine, they are harmless in the colon and may be useful e.g. produce Vitamin K.  Defecation: peristalsis pushes waste along the colon and then it is passed out of the body.  ENZYMES  An enzyme is a biological catalyst  A catalyst speeds up chemical reactions  Enzymes speed up biological reactions  All chemical reactions that take place in living systems require the action of an enzyme  Digestive enzymes break food down into smaller, more soluble substances  This allows the food to be absorbed into the blood  An example of a digestive enzyme is amylase  Amylase is present in saliva  Amylase chemically breaks down starch Amylase converts starch into a sugar called maltose STARCH MALTOSE  The substance that an enzyme works on is known as its SUBSTRATE  The substance formed by the enzyme is known as its PRODUCT  Therefore starch is the substrate for amylase and maltose is its product