Biology 12 Unit 3A. The Digestive System

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1 The Digestive System The digestive system consists of a long passageway that starts at the mouth and continues through more than 20 feet (7m) of muscular tubes and chambers where it ends at the anus. It takes approximately 24 hours for the food we eat to travel along this passageway as it is mechanically and chemically broken down into the nutrients that build and fuel our cells. Anatomical Overview of the Digestive System The organs of the digestive system fall into 2 main groups: 1

2 1) Gastrointestinal Tract (GI Tract) The GI tract is a continuous muscular digestive tube that winds through the body FUNCTION: food is digested broken down into small nutrient molecules, nutrients are absorbed into the bloodstream, and indigestible wastes are eliminated from the body. The organs of the GI tract include the mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anus Food in the GI tract is technically OUTSIDE the body because the canal is open to the external environment at both ends. 2) The Accessory Digestive Organs The accessory organs of the digestive tract do not have food pass through them, but they are involved in the mechanical or chemical digestion of food e.g. Digestive Processes The processing of food by the digestive system involves six essential activities: 1) Ingestion the intake of food and fluids (usually through the mouth) 2) Propulsion the movement and mixing of food along the digestive tract 3) Mechanical Digestion Physical preparation of food e.g. chewing, churning 4) Chemical Digestion In a variety of catabolic steps macromolecules such as proteins, starches, and fats are broken down into their chemical building blocks by enzymes that are secreted into the digestive tract 5) Absorption The passage of digested end products, vitamins, minerals and water from the GI tract through the walls of the small and large intestines and into the blood stream so they can be transported throughout the body 6) Elimination Indigestible food material such as fiber and other wastes are eliminated from the body The Mouth (Oral Cavity) The mouth (oral cavity) is the site of ingestion mechanical and chemical breakdown of food begins when it is chewed and mixed with saliva Teeth & Tongue teeth tear and grind food and break it down into smaller fragments (mechanical digestion) providing more surface area for digestive enzymes (chemical digestion) to work 2

3 We masticate (chew) by opening and closing our jaws and moving them side to side During chewing, the tongue mixes food with saliva and constantly repositions it between the teeth food is formed into a compact mass called a bolus that can be easily swallowed Salivary Glands Three pairs of salivary glands secrete up to 1.5 L of saliva into the oral cavity every day! Functions of Saliva: 1) Cleanses the mouth Saliva contains antimicrobial peptides called defensins that kill bacteria and other microorganisms that are ingested 2) Dissolves food chemicals so they can be tasted 3) Moistens food and lubricates the mouth to make chewing and swallowing easier. 4) Produces Salivary Amylase - a hydrolytic enzyme that breaks down starch molecules into the disaccharide maltose Swallowing The process of swallowing begins as a voluntary action when a bolus of food is pushed by the tongue to the back of the mouth towards the pharynx Deep at the back of the throat the pharynx branches off into 2 separate tubes: 1) Trachea The passage way that carries respiratory gases (air) to and from the lungs 2) Esophagus A muscular passage way approximately 25 cm long (10 inches) that carries food to the stomach 3

4 The Steps of Swallowing The swallowing process begins when a bolus is pushed towards the pharynx by the tongue. As the bolus arrives at the pharynx the soft palate blocks the upper pharynx to prevent food from entering the nasal cavity. Voluntary muscle contractions in the neck and throat continue to push the bolus safely down the pharynx towards the epiglottis At this time the epiglottis, a flap of cartilage, flips downward to cover the entrance to the trachea The epiglottis functions to direct food into the esophagus and prevented from going down the wrong tube and into the lungs Once inside the esophagus food is moved along its 10 inch length using a series of involuntary wave-like muscle contractions in a process called peristalsis 4

5 The Stomach At the end of the esophagus there is a ring of circular muscle called the cardiac sphincter which relaxes to allow the bolus to enter the stomach. the cardiac sphincter remains closed to prevent the stomach s acidic contents from backing up the esophagus (acid reflux or heart burn) the esophagus expands to form the J-shaped stomach the stomach functions as a temporary storage tank where the chemical and mechanical digestion of food continues when the stomach is empty, it collapses inward on itself creating specialized folds called rugae - the rugae give the stomach incredible expansive properties Empty stomach = 50 ml Full stomach 4 L and may extend nearly all the way to the pelvis! The wall of the stomach is very muscular and functions to churn, mix and mechanically break down the food into even smaller fragments in a process called churning The stomach wall is lined with mucus to protect the stomach from the HCl The bolus entering the stomach causes cells in the stomach lining to release gastric juice from gastric glands found deep within gastric pits. Rugae Gastric Pit Goblet Cells Muscular Wall of the Stomach 5

6 Gastric juice contains: 1) Hydrochloric acid (HCl) ph 2 (strong acid) HCl works to kill any bacteria present in food HCl denatures the proteins found in the food we have eaten so they can be chemically digested by hydrolytic enzymes. HCl does NOT DIGEST FOOD, it breaks down the connective tissues of meat 2) Pepsinogen HCl in gastric juice converts Pepsinogen into the active enzyme Pepsin Pepsin, breaks down large proteins into smaller polypeptide chains. Pepsin is secreted in an inactive form to protect the cells that secrete it from auto-digesting 3) Mucus Goblet cells secrete alkaline mucus onto the stomach lining to protect it from HCl burns Homeostatic Imbalance Anything that breaks through the stomach s protective mucus lining causes inflammation of the stomach wall, a condition called gastritis. Persistent damage to the underlying tissues can promote gastric ulcers, erosions of the stomach wall. The danger posed by ulcers is that they can potentially lead to massive hemorrhages that can be fatal. For years the blame for stomach ulcers was put on things that caused high HCl production or low mucus production such as high doses of Aspirin, non-steroidal anti-inflammatory drugs (Ibuprofen), smoking, alcohol, caffeine, and stress. Recently, the Nobel Prize winning research of one doctor has changed the way the doctors treat ulcers. It is now known that up to 90% of all gastric ulcers are caused by a corkscrew-shaped bacterium called Helicobacter pylori that burrows beneath the mucus lining of the stomach and destroys its protective mucosal layer. This leaves these areas vulnerable to acid burns. Recent studies also show that this bacteria may also be responsible for causing some types of stomach cancer. A 1-2 week course of complementary antibiotics is usually all it takes to cure a patient of this troublesome microbe. Digestive Processes that Occur in the Stomach 1) Protein Digestion Protein to small peptide chains with the aid of pepsin 2) Absorption of lipid soluble substances such as alcohol and some drugs NO FOOD is absorbed by the stomach however lipid soluble substances such as alcohol and Aspirin can easily pass though the stomach wall and into the bloodstream 6

7 3) Churning (Mechanical Digestion) Contractions of the smooth muscle layer in the stomach wall cause a churning action that mixes all of the stomach s contents with gastric juices. Thanks to churning, the bolus eventually becomes mixed with HCl, Pepsin and mucus to form a semi-liquid mass called acid chyme At the bottom end of the stomach the pyloric sphincter acts as a pump that delivers small amounts of the acid chyme into the duodenum of the small intestine The stomach usually empties completely within 4 hours after a meal, however if you eat a meal high in fats, food may remain in the stomach 6 hours or more The Small Intestine The small intestine is named for its small diameter (2.5-4 cm) when compared to the large intestine (5 8 cm); but is 5-6 m long Function of the small intestine: chemical digestion, and virtually all nutrients are absorbed into the bloodstream The small intestine is divided into 3 sections: 1) Duodenum is the first 25 cm of the small intestine. Produces and secretes digestive enzymes: Maltase breaks maltose to glucose Peptidase breaks peptides amino acids Nucleosidase breaks nucleotides N-base, phosphate group and pentose sugar Receives digestive juices from the pancreas, liver and gallbladder Completes chemical digestion 2) Jejunum is approximately 2.5 m long Absorption of nutrients 3) Ileum approximately 2-4 m long, connects to the large intestine Absorption of nutrients 7

8 Absorption of Nutrients in the Small Intestine The small intestine has an incredible amount of surface area and this makes it a specialized organ for absorption The majority of nutrient absorption occurs in the Ileum and the Jejunum The following features of the small intestine contribute to its large surface area: 1) Small finger-like projections called villi cover the inner surface of the intestine 2) Each villi also contains microvilli thousands of microscopic projections that further increase the overall surface area of the small intestine. Each villus is richly supplied with capillaries, small blood vessels that absorb glucose, amino acids, and dipeptides into the blood stream At the center of each villus there is a lacteal a small lymphatic vessel capable of absorbing fats into the lymphatic system Microvilli contain additional intestinal digestive enzymes that aid in the digestive process and increase the efficiency of absorption Much of the absorption of nutrients is via active transport (against the concentration gradient) however, passive transport also occurs. As a result, microvilli have many mitochondria and use a lot of ATP organs_of_digestion.html 8

9 The Accessory Organs: The Pancreas As an accessory digestive organ, the pancreas produces enzymes that break down all major macromolecules found in the food we eat i.e. carbohydrates, proteins, and fats. The pancreas behaves like an exocrine gland when it releases pancreatic juice into the small intestine. The arrival of acid chyme (ph 2) from the stomach into the duodenum stimulates the release pancreatic juice Pancreatic juice is a basic fluid with ph 8 that is composed of: 1) Sodium bicarbonate (NaHCO 3 ) - why do you think this is important?? 2) water 3) pancreatic enzymes (proteases) Pancreatic Digestive Enzymes (Proteases) 1. Trypsin polypeptides to smaller peptide chains 2. Pancreatic amylase starch to maltose 3. Lipase fats (triglycerides) to fatty acids and glycerol 4. Nuclease nucleic acids (DNA & RNA) to nucleotides 9

10 The Pancreas & Maintenance of Blood Sugar Levels In addition to being an important exocrine gland the pancreas is also considered to be an endocrine gland because it produces hormones that travel throughout the body and affect particular target organs such as the liver. Specialized cells in the pancreas called Islets of Langerhans secrete 2 important hormones that help regulate blood glucose levels 1. Insulin Lowers blood sugar levels by increasing a cell s ability to uptake glucose When blood sugar levels are HIGH, insulin promotes the storage of glucose in the form of glycogen in the liver and muscle cells 2. Glucagon Raises blood sugar levels by activating enzymes that hydrolyze glycogen stored in the liver and muscle cells to release glucose monomers The glucose molecules are then released into the blood to raise blood sugar levels 10

11 The Liver & Gallbladder The liver is one of the body s most important organs as it has many regulatory and metabolic roles. The liver sits in the upper right quadrant of the abdominal cavity underneath the ribcage where it is protected from trauma Large Lobe of the Liver Esophagus Small Lobe of the Liver Gall Bladder Stomach Common Bile Duct Duodenum Ileum Pancreas Important Functions of the Liver 1) Regulation of Blood Glucose As we have already learned, the pancreas plays a vital role in the maintenance of blood glucose levels by secreting insulin and glucagon, 2 endocrine hormones that affect the liver There are 3 main processes the liver uses to regulate blood sugar levels: i) Glycogenesis When there is excess glucose in the blood the pancreas releases insulin. Insulin stimulates liver cells (hepatocytes) to combine glucose monomers into glycogen molecules The glycogen molecules are then stored in the liver until glucose is needed to raise blood sugar levels again. 11

12 ii) Glycogenolysis When blood glucose levels drop glucagon is secreted by the pancreas. Glucagon stimulates liver enzymes to split molecules of glycogen into free glucose molecules. Because glucose is a small molecule, it can readily diffuse from liver cells into the blood and raise blood sugar levels. iii) Gluconeogenesis When dietary sources and glucose reserves in the body are depleted and blood sugar levels drop, the liver forms new glucose molecules from amino acids and glycerol in response to glucagon release from the pancreas. This process occurs during periods of fasting, starvation, or intense exercise. Gluconeogenesis protects the body, the nervous system in particular, from the damaging effects of low blood sugar by ensuring that ATP synthesis can continue. 2) The liver destroys old blood cells Red blood cells have an average life span of 4 months millions die every day in our bodies. The liver disassembles the RBC and recycles useful components, such as iron from hemoglobin. Homeostatic Imbalance When red blood cells are broken down or recycled hemoglobin, the oxygen carrying protein found in red blood cells, is broken down and bilirubin is produced. In a healthy individual the liver removes bilirubin from the blood and excretes it as a component of bile. If the liver is not functioning properly (there are many possible reasons) bilirubin will build up in the tissues. The build-up of bilirubin results in a condition called jaundice. Because bilirubin is a bright yellowgreen color, the skin and whites of the eyes appear yellow. 3) The liver produces bile salts that emulsify particles of dietary fat The digestive function of the liver is to produce ml of bile per day for export to the duodenum of the small intestine When protein-rich and/or fat-rich chyme arrives in the duodenum, the gall bladder secretes bile into the duodenum via the bile duct Bile mechanically digests fat molecules by emulsifying them into microscopic particles that have a greater surface area Excess bile produced by the liver is sent to the gallbladder where it is concentrated and stored Did You Know? Bile contains a pigment called bilirubin that gives it a yellow-green color. When bilirubin is broken down it turns brown. This is what gives our feces a brown color. 12

13 4) The liver makes non-essential amino acids and blood proteins The liver uses amino acid monomers to build important blood proteins such as albumin and fibrinogen 5) The liver helps regulate the level of cholesterol in the blood When cholesterol levels are high the liver will convert cholesterol into bile salts 6) The liver detoxifies the blood Unlike foods, alcohol needs no digestion and is quickly absorbed. About 20% is absorbed directly across the walls of an empty stomach and the rest is rapidly absorbed in the duodenum Alcohol-laden blood travels to the liver via the veins and capillaries of the digestive tract, which affects nearly every liver cell. Liver cells produce alcohol dehydrogenase, an enzyme that oxidizes alcohol and breaks it down into fatty acids Homeostatic Imbalance As alcohol is oxidized by the liver, fatty acids are released and deposited in liver tissue. Over time these fatty deposits cause the liver to enlarge and scar tissue forms. Eventually, tough connective tissue will replace healthy liver cells where scar tissue was formed and liver function decreases. Many alcoholics have cirrhosis, and it can eventually lead to liver failure and death. The Large Intestine (Colon) The large intestine is shorter than small intestine, but is almost 2.5 times larger in diameter. By the time chyme has reached the end of the small intestine, most nutrients have been absorbed into the bloodstream. Indigestible materials such as water and cellulose, are moved into the large intestine through the ileocecal valve and into the cecum 13

14 The colon is divided into 4 sections: 1) Ascending colon 2) Transverse colon 3) Descending colon 4) Sigmoid colon The colon has 2 main roles in digestion: 1) Water is re-absorbed from food material 2) It is home to symbiotic bacteria such as E.coli that provide us with some minerals, vitamins and essential amino acids In the large intestine there is a tremendous number of goblet cells cells that produce mucus to protect the intestinal wall from irritating acids and gases released by the bacteria that live in the colon The large intestine has a thriving flora of intestinal bacteria that help us with the final stages of digestion These bacteria ferment some of the indigestible carbohydrates such as cellulose and produce about 500 ml of gas as a byproduct each day One example of healthy bacteria found in the colon is E.coli. that metabolize some macromolecules our body cannot and as a result they release minerals, vitamins (Vit K), and amino acids into the gut Thanks to E.coli, these nutrients can be absorbed along with water into our circulatory system The Appendix The appendix is a tiny worm-like projection that extends down from the cecum(large intestine) and is thought to be a vestigial organ it has no known function Recent medical studies have shown there is some evidence that the appendix may play a role in fighting infection. 14

15 Homeostatic Imbalance Acute inflammation of the appendix, or appendicitis, results from a blockage (often by feces) that traps infectious bacteria in its lumen. Because it is unable to empty its contents due to the blockage, the appendix swells, and squeezes off venous drainage. The lack of circulation that results may lead to death and decay of the appendix. If the appendix ruptures, feces containing bacteria spray over the abdominal contents, causing inflammation of other organs. Loss of appetite, nausea, vomiting, and pain in the lower-right abdomen are common symptoms of appendicitis. The Rectum The sigmoid colon empties into the rectum The semi-solid product (¾ water and ¼ solid) delivered to the rectum via peristalsis as feces contains undigested food residues, mucus, millions of bacteria and just enough water to allow its smooth passage out of the anus. The rectum is usually empty, but when feces are forced into it, stretching of the rectal wall initiates the defecation reflex As feces are forced into the anal canal, messages reach the brain allowing us to decide whether the external (voluntary) anal sphincter should remain open or be constricted to stop the passage of feces through the anus. If defecation is delayed, the reflex contractions end within a few seconds, and the rectal walls relax. During defecation the muscles of the rectum contract and the anal sphincter relaxes Did you Know? Some laxatives act as oily lubricants which literally grease up the lumen of the large intestine to allow feces to slide past. Some laxatives such as Milk of Magnesia, act osmotically and prevent water from being absorbed by the large intestine. If people aren t careful with the dosage, they can actually cause water to be deposited into the intestine from intestinal cells, and promote dehydration. Some laxatives are irritants which cause inflammation in the intestinal wall. The irritation triggers peristalsis and feces are then eliminated from the body. The safest and most effective laxative are the ones which contain natural sources of fiber such as cellulose that simply increases the bulk of the feces and makes it easier to eliminate wastes. 15