Bio 230 Lecture Notes- THE DIGESTIVE SYSTEM
Be sure to read these notes while following along with the powerpoints I posted. Those 2 powerpoints should cover all that you need. I have “predigested” the material for you. So you finish churning it in your head, and absorb as much as you can.
INTRODUCTION:DIGESTION is the breakdown of large food molecules into smaller molecules that the body cells can absorb and use.
There are 6 essential digestive processes. 1. Ingestion
2. Propulsion of food: swallowing & peristalsis
3. Secretion of water, acid, buffers, and digestive enzymes. 4. Mechanical and chemical (enzymatic) digestion 5. Absorption of nutrients.
6. Defecation of wastes.
ORGANIZATION OF DIGESTIVE SYSTEM ORGANS: The Digestive system is made up of the organs of the GI tract AND the accessory structures that aid in its function.
1. Gastrointestinal (GI) Tract (aka alimentary canal) is about 30 feet long
Organs of the GI Tract: MOUTH (AKA oral cavity), PHARYNX, ESOPHAGUS, STOMACH, SMALL INTESTINE, & LARGE INTESTINE.
2. The accessory structures (support players) that aid the GI tract in the mechanical & chemical digestion of food include the TEETH, TONGUE, SALIVARY GLANDS, LIVER, GALL BLADDER, & PANCREAS.
HISTOLOGY OF THE GI TRACT
There are 4 layers in the GI tract: MUCOSA, SUBMUCOSA, MUSCULARIS EXTERNA, and SEROSA/ADVENTICIA.
1. The MUCOSA (MUCOUS MEMBRANE) is a moist epithelial tissue membrane that lines the LUMEN of the GI tract from the mouth to the anus. Made up of 3 layers:
a. The EPITHELIAL LAYER of the mucous membrane of the MOUTH, PHARYNX, ESOPHAGUS, & ANAL CANAL is composed of NONKERATINIZED STRATIFIED SQUAMOUS EPITHELIUM The epithelium of the mucous membrane of the STOMACH, SMALL INTESTINE & LARGE INTESTINE consists of NON-CILIATED SIMPLE COLUMNAR EPITHELIUM rich in mucus Secreting GOBLET CELLS. The slippery mucus protects the organs from being digested by Digestive enzymes and eases the passage of foods along the GI tract “lubes the tube”. The mucosa of these organs are also involved in secretion of digestive enzymes and absorption of nutrients.
b. The LAMINA PROPRIA is the connective tissue layer of the MUCOUS MEMBRANE and it consists of AREOLAR CONNECTIVE TISSUE.(Remember those wondering cells! Good place for them in case bacteria are ingested.)
c. The MUSCULARIS MUCOSAE is a thin layer of smooth muscle. Local action only.
2. The SUBMUCOSA is a DENSE IRREGULAR CONNECTIVE TISSUE layer that binds the
mucosa to the third layer, the muscularis externa. The submucosa contains a rich supply of blood and lymphatic vessels, as well as digestive glands and a nerve supply that controls digestive secretions.
3. The MUSCULARIS EXTERNA =Muscle Layer:
The Muscularis externa of the MOUTH and PHARYNX upper 1/3 of esophagus is SKELETAL MUSCLE.
INTESTINES is SMOOTH MUSCLE.
The smooth muscle is arranged as an INNER CIRCULAR LAYER and an OUTER LONGITUDINAL LAYER. The smooth muscle muscularis externa is important in peristalsis, the alternating waves of contractions and relaxation that move food along the GI tract. The muscularis externa also contains a nerve supply found between these two muscle layers called the MYENTERIC PLEXUS (aka
Auerbach Plexus) that controls these smooth muscle contractions. (parasympathetic and sympathetic fibers are here!)
4 .The SEROSA (the visceral peritoneum) is the protective outermost layer of the stomach, small intestine and large intestine. Rememeber your serous membranes from that long ago first day in class? In the abdominal cavity we have the PERITONEUM, with a visceral peritoneum that covers the external surfaces of most digestive organs and the parietal peritoneum that lines the abdominal body walls. The space in-between the visceral and parietal peritoneum is the Peritoneal Cavity, with serous fluid!!
Function is to reduce friction between the organs of the digestive system. The serosa layer (visceral peritoneum) is only found if the organ is in the peritoneal cavity. If the organ is found outside the peritoneal cavity this 4th layer is a fibrous C/T. that is called the ADVENTTITIA.
The esophagus thus only has the adventitia has its 4th layer.
ORGANS OF THE DIGESTIVE SYSTEM
1. THE MOUTH (aka oral cavity) AND ASSOCIATED ORGANS:
The ORAL CAVITY is lined with MUCOUS MEMBRANE. The walls of the mouth have to With stand abrasion, so the mucous membrane consists of NONKERATINIZED STRATIFIED SQUAMOUS EPITHELIUM
A. THE TEETH -The teeth tear and grind the food, breaking it down mechanically into smaller pieces (chewing or mastication)
B. THE TONGUE- is composed of skeletal muscle covered by mucous membrane. It is anchored at its inferior base to the HYOID bone The muscles of the tongue allow the tongue to change shape and to move from side to side and in and out. These movements are involved in speech, maneuvering food for chewing, forming the food into a BOLUS (ball of food), and forcing food to the back of the mouth for swallowing.
C. THE PALATE -forms the roof of the mouth. The HARD PALATE serves as a rigid surface
against which the tongue can force food during chewing & swallowing. The posterior SOFT PALATE dips inferiorly at its free edge to form the UVULA. Remember the function of the uvula?
D. THE SALIVARY GLANDS secrete saliva, a fluid that contains mucus, salivary amylase (a digestive enzyme that breaks down starch into sugar), and enzyme lysozyme (a enzyme that breaks down bacterial cell walls).
Saliva Functions:
(1) keeps the mucous membranes moist between meals
(2) dissolves food chemicals so that they can be tasted by taste buds (3) moistens food and aids in its compaction into a bolus
(4) begins the chemical breakdown of starchy foods. (5) defense from bacterial ingestion
Most of the saliva is secreted by the three paired salivary glands:
1. PAROTID GLANDS: “para - otic” (around. d the ear) inferior and anterior to the ears. Its ducts open into the cheeks.
2. SUBMANDIBULAR GLANDS: deep to the mandible, just anterior to the angle of the mandible. Its ducts open into the floor of the mouth.
Swallowing is the mechanism that moves food from the mouth to the stomach. The tongue pushes on the hard palate, pushing the bolus of food from the ORAL CAVITY to the
OROPHARYNX. The SOFT PALATE & UVULA rise to close off the NASOPHARYNX, so that food doesn't go up into your nose when you swallow.
Food moves from the OROPHARYNX, LARYNGEAL PHARYNX (mucosa Stratified Squamous) into the ESOPHAGUS as the LARYNX is closed off by the EPIGLOTTIS.
2. THE ESOPHAGUS:
Is a muscular, collapsible tube about 10 in long and posterior to the trachea It passes through the mediastinum and then pierces the diaphragm through the opening called the ESOPHAGEAL HIATUS. Where the esophagus joins the stomach is called the cardia.. The esophagus has in its mucosa N.K. Stratified Squamous epithelium. Mucus secreted by glands in the esophagus prevents friction when food passes through the esophagus during peristalsis (alternating waves of
contractions). The smooth muscle sphincter called the GASTROESOPHAGEAL SPHINCTER is found where the esophagus enters the stomach. It relaxes during peristalsis, allowing the bolus of food to enter the stomach.
- The esophagus has No digestive or absorptive function. The MUSCULARIS EXTERNA of the esophagus is skeletal muscle its upper 1/3, mixed skeletal & smooth muscle middle 1/3 and smooth muscle only in lower 1/3.
3. THE STOMACH
-about10 inches long, just under the diaphragm in the LUQ/epigastric area of the abdominopelvic cavity.
The stomach serves as a temporary "storage tank" for food and an important site for mechanical breakdown of food and some chemical breakdown of food (proteins.)
GROSS ANATOMY OF THE STOMACH: Know the regions of the stomach:
1. CARDIA/CARDIAC ORIFICE 2. FUNDUS (the rounded top of the stomach) 3. BODY 4. PYLORUS (aka pyloric region) 5. LESSER & GREATER CURVATURES
The smooth muscle sphincter at the junction of the esophagus and stomach is the GASTROESOPHAGEAL SPHINCTER. The sphincter at the junction of
the stomach and small intestine is the PYLORIC SPHINCTER.
When the stomach is empty the MUCOSA (stomach lining) forms large longitudinal folds called RUGAE. Rugae allow the stomach to stretch to comfortably hold 1.5 quarts of food, and when really distended, the stomach can hold as much as 4 quarts!! Anyone know of Mr. Creosote? (Monty Python)
HISTOLOGY OF THE STOMACH:
The epithelium that lines the stomach MUCOSA is composed entirely of SIMPLE COLUMNAR EPITHELIUM.
Located in the MUCOSA are GASTRIC GLANDS, which open into the stomach through the GASTRIC PITS. The gastric glands produce GASTRIC JUICE: The GASTRIC GLANDS consist of the following cell types:
1. CHIEF CELLS in the gastric glands secrete pepsinogen. Pepsinogen is an inactive digestive enzyme & is converted to its active form, pepsin, under the acidic conditions of the stomach. Pepsin digests (breaks down) proteins into amino acids.
2. PARIETAL CELLS in the gastric glands secrete hydrochloric acid (HCl), which makes the stomach contents very acidic (pH 1.5-3.5). This acidic pH helps destroy bacteria in food; it is also necessary to convert pepsinogen to pepsin (see #1 above). PARIETAL CELLS also secrete intrinsic factor, which is necessary for absorption of vitamin B12 by the small intestine. Vitamin B12 is
3. MUCOUS CELLS (goblet cells) in the gastric glands secrete a sticky, alkaline mucus, which clings to the stomach wall and shields it from the damaging effects of HCl.
4. G CELLS-produce the hormone GASTRIN. Gastrin triggers increased secretion of the chief and parietal cells and stimulates smooth muscle contraction.
The stomach mucosa does not absorb many nutrients. It does absorb some water, electrolytes, aspirin, and alcohol (why you should eat with alcohol).
The stomach MUSCULARIS EXTERNA, unlike the rest of the GI tract, consists of 3 layers of smooth muscle :
LONGITUDINAL (outermost muscle layer), CIRCULAR (middle layer) which are both found
throughout the GI tract, and it has an additional layer, OBLIQUE (inner most muscle layer). These 3 muscle layers allow the stomach to mechanically CHURN the food and mix the food with the gastric secretions. Once the food is broken down physically and mixed with gastric juices, it has been converted to a creamy, semifluid mass called chyme.
Food moves from the stomach into the small intestine through a ring of circular smooth muscle called the PYLORIC SPHINCTER. As the food churns in the stomach, only small amounts of chyme squirt through the sphincter into the small intestine. It takes about 2-6 hours for the stomach to completely empty.
4. SMALL INTESTINE
Is a convoluted tube extending from the pyloric sphincter of the stomach to the large intestine. The longest section of the GI tract, with an average length of 21 ft, 1 inch diameter.
The small intestine is divided into 3 segments:
1. The DUODENUM is the first section of the small intestine. It is about 10 inches long. 2. JEJUNUM is about 8 feet long
3. ILEUM is about 12 feet long
The jejunem and ileum are held (supported) in place by a highly vascular membrane called MESSENTARY which anchors the SI to the posterior wall of the Abdominal Cavity.
DUODENUM:
-C shaped tubular structure It receives bile secretions from the liver and gallbladder through the BILE DUCT (aka common bile duct) and pancreatic secretions through the PANCREATIC DUCT. These 2 ducts unite to form the HEPATOPANCREATIC AMPULLA (aka Ampulla of Vater) DUODENAL GLANDS in the SUBMUCOSA of the duodenum produce alkaline mucus that helps neutralize the acidic chyme that comes in from the stomach.
JEJUNUM: This region has the most number of Circular Folds aka Plicae Circularis (increase surface area). The jejunum is the primary area of absorption.
ILEUM: joins the large intestine at the ILEOCECAL VALVE, empties into the cecum (first region of the large intestines).
The completion of enzymatic digestion takes place in the small intestine. About 90% of all nutrient absorption takes place in the small intestine. The anatomy of the small intestine makes digestion and absorption more efficient. For example, the small intestine is the longest portion of the GI tract; this gives a larger surface area for nutrient absorption. Also, 3 specializations are found in the SI that further increase the surface area.
1. PLICAE CIRCULARIS (aka Plicae Circulares) (large macroscopic circular folds) increase the surface area exposed to the food. The chyme spirals down the plicae circularis, exposing more nutrients to the intestinal wall, where absorption of nutrients takes place.
2. INTESTINAL VILLI- fingerlike macroscopic folds that arise from the Plicae Circularis
3. MICROVILLI (microscopic)- Each intestinal villi is covered by a simple columnar epithelium, and the apical surfaces of these epithelial cells have microvilli (aka brush boarder).
microscopic projections of the plasma membrane, located on the apical (exposed) surface of the mucosal epithelial cells. These microvilli of the epithelial cells of the small intestine are called the BRUSH BORDER because they have a fuzzy appearance in high magnification images. The plasma membrane of the brush border cells contains several digestive enzymes called brush border
enzymes. The brush border enzymes digest nutrients at the cell surface and the cell then absorbs the nutrients.
The areolar connective tissue layer of the mucosa is called the LAMINA PROPRIA. The lamina propria contains a dense blood capillary bed, which receives absorbed nutrients from the small intestine and carries it to the hepatic portal vein circulation. Also within the lamina propria, next to the blood capillaries is where a wide lymphatic capillary called a LACTEAL resides. Remember from the Lymphatic System what lacteals are? It is through the lacteals where fat is absorbed. The implications of this are significant. The blood vessels that drain the small intestine carry absorbed nutrients to the liver via the hepatic portal vein. Absorbed fats however do not go directly to the liver, but rather travel through the lymphatic system and empty into the blood circulation through the Thoracic Duct which empties into the left subclavian vein at the junction of the left internal jugular vein (see your Lymphatic System notes). Why does this matter? Well any ingested and absorbed fat-soluble toxins such as pesticides or herbicides can enter our blood circulation without first being detoxified by our mighty LIVER.
ILEUM
The last segment of the small intestines. It is here where we find aggregated lymphoid nodules
(Peyer’s Patches) which are located in the submucosa layer of the Ileum. These Peyer’s patches help protect the small intestines from any bacteria that may enter from the Large Intestines.
The MUSCULARIS EXTERNA of the small intestine consists of a circular (inner) and longitudinal (outer) layer of smooth muscle. These two layers contract and relax in waves, moving the chyme through the small intestine by peristalsis.
The indigestible & unabsorbable material in the chyme enters the large intestine through the ILEOCECAL SPHINCTER (VALVE). Ileo=ileum, cecal=cecum
THE LARGE INTESTINE: is about 5 ft long, 2.5 inches in diameter. The large intestine is divided into four STRUCTURAL REGIONS: 1. CECUM 2. COLON 3. RECTUM and 4. ANAL CANAL
1. The CECUM is a pouch inferior to the ILEOCECAL VALVE. Hanging from its inferior/posterior surface is the VERMIFORM APPENDIX. The appendix contains lymphatic tissue.
2. The cecum merges with the COLON (longest part of the LI), which has 4 distinct regions: ASCENDING COLON, TRANSVERSE COLON, DESCENDING COLON, & SIGMOID COLON.
3. RECTUM (“straight”): last 8 inches of the large intestine.
4. ANAL CANAL, the last 1 inch of the rectum which opens to the exterior at the ANUS. The mucosa of the anal canal is composed of NONKERATINIZED STRATIFIED SQUAMOUS
EPITHELIUM, which resists abrasion. Anal sinuses release mucus when they are compressed by feces, which aids in emptying the anal canal. The INTERNAL ANAL SPHINCTER is composed of circular smooth muscle (involuntary) and the EXTERNAL ANAL SPHINCTER is voluntary skeletal muscle.
HISTOLOGY OF THE LARGE INTESTINE
The large intestinal MUCOSA is SIMPLE COLUMNAR EPITHELIUM (except for the anal canal), but there are no villi like there are in the small intestine. The simple columnar cells function primarily in water absorption. Goblet cells are present in the mucosa and the mucus helps ease the passage of feces (solid waste) to the end of the GI tract. The mucosa of the large intestine produces no digestive enzymes.
Bacteria in the lumen of the large intestine digest any remaining carbohydrates and amino acids left in the chyme, releasing hydrogen, carbon dioxide, methane, and hydrogen sulfide gases into the lumen (yes -that's where that gas comes from!). The bacteria in the lumen of the large intestine are beneficial because they make vitamin K and some B vitamins.
The mucosa of the large intestine absorbs the vitamins synthesized by intestinal bacteria, some electrolytes (salts) and most of the remaining water in the chyme. So, the large intestine is important in maintaining water and electrolyte balance in the body.
DEFECATION REFLEX-occurs with increase rectal pressure (from the feces filling the rectum/anal canal). Internal and external sphincters relax. Internal sphincter smooth muscle (involuntary), external sphincter skeletal muscle (voluntary…most of the time!!). Oh that Parasymphathetic System!
The Large Intestine can be distinguished from the small intestines by the following:
1. TENIAE COLI- 3 thickened bands of longitudinal muscle fibers. The L.I. still has both a circular and longitudinal muscle layer (muscularis externa) but this outer muscle layer has been modified and gives the colon a distinct appearance.
2. HAUSTRA-sacculations (pouches) of the colon between the teniae coli.
3. CALIBER- the larger internal diameter of the L.I. which is about 3 inches (small intestines diameter about 1 inch).
OTHER ACCESSORY ORGANS OF THE DIGESTIVE SYSTEM
Liver, gallbladder and pancreas- aid in the chemical breakdown of food. Function as EXOCRINE GLANDS (i.e. their secretions are transported to the lumen of the GI tract via “ducts”). Remember “crine” means secretion. Exo=through ducts
1. THE PANCREAS
-is the principal digestive enzyme-producing organ of the digestive system. It is 5-6 inches long and 1 in thick. Located behind the stomach. It is shaped like a tadpole with 3 regions: head, body & tail, The pancreas is both an endocrine & exocrine gland. The pancreatic endocrine cells that secrete hormones that control blood sugar levels (insulin and glucogon) are located in PANCREATIC
ISLETS. The pancreatic exocrine cells (ACINAR CELLS) secrete about 1200-1500 ml of pancreatic juice into ducts each day. Pancreatic juice is an alkaline fluid that contains many digestive enzymes (about 22 different kinds) that aid digestion (lipases, carbohydrases, nucleases, proteolytic (protein) enzymes etc.)
These pancreatic juices leaves the pancreas via the PANCREATIC DUCT.
The pancreatic duct joins with the COMMON BILE DUCT to form the HEPATOPANCREATIC AMPULLA.
The hepatopancreatic ampulla empty into the duodenum of the small intestine.
2. THE LIVER
– is the largest organ in the body, weighing about 3.5-4 lbs in the average adult. The liver is located just under the diaphragm, in the RUQ and EPIGASTRIC regions.
The livers reddish brown color is due to its great vascularity. It receives 25-30 % of the resting cardiac output, and is responsible for 20% resting oxygen consumption.
GENERAL GROSS ANATOMY
-The liver consists of 2 principal lobes, the larger right lobe and the smaller left lobe.
-The FALCIFORM LIGAMENT is fibrous tissue that separates the right and left lobes and attaches part of the liver to the anterior abdominal wall and the underside of the diaphragm..
umbilicus…a remnant of the umbilical vein of the fetus.
FUNCTIONS OF THE LIVER : Over 200! Including the following: __ 1. SYNTHESIS: proteins, clotting factors, bile and cholesterol. 2. STORAGE: Fe (iron), glycogen, blood, fat soluble vitamins
3. METABOLIC FUNCTIONS: The liver converts the simple sugar glucose into glycogen (a complex carbohydrate similar to plant starch), and stores glycogen for later use. When blood sugar levels are low, the liver breaks down stored glycogen into glucose and releases glucose into the bloodstream. This keeps blood sugar levels constant so that body cells always have a steady supply of glucose. We need our glucose fix, remember neurons must have glucose!
Along with the spleen, the liver will take worn out RBC’s from the circulation.
4. DETOXIFICATION of many drugs/ and poisons. The liver converts toxic ammonia NH3 (from
protein breakdown) to a less toxic waste product, urea. Urea is peed out with your urine. The liver also detoxifies antibiotics such as penicillin, ampicillin and erythromycin and excretes them in the bile. The liver detoxifies other drugs as well, such as Tylenol, alcohol and amphetamines.
VASCULAR SUPPLY TO and FROM THE LIVER:- the liver receives a dual(two) vascular supply. 1. HEPATIC PORTAL VEIN- Brings ALL the blood which comes from the GI tract. Nutrient rich, oxygen poor, not detoxified blood. Remember the hepatic portal vein brings blood to the liver (think a ship coming into port).
2. HEPATIC ARTERY- brings fresh (oxygen rich) blood from the abdominal aorta (celiac trunk). Nutrient rich. Remember, the abdominal aorta brings O2 rich and nutrient rich blood to all abdominal organs. Hepatic artery is a branch of the celiac trunk.
3. HEPATIC VEIN- blood leaves the liver (after being detoxified) from the hepatic vein which then drains right into the Inferior Vena Cava. This blood in nutrient rich, oxygen poor and detoxified.
LIVER TERMS TO KNOW: You are expected to be able to draw a liver lobule (we will draw this in class) that includes the following:
-Hepatocytes (liver cells)
-Hepatic Lobules – these are the fundamental functional units of the liver. The liver contains over million individual hexagon shaped lobules.
Be sure you can draw the following structures and understand their functions. (We will go over this is class):
Portal Triade, Sinusoids, Central Vein, Bile canaliculi , Kupffer Cells.
The liver is an exocrine gland that forms/secretes bile, a yellow-to-green fluid containing bile salts, bile pigments & cholesterol. The bile is secreted by HEPATOCYTES (liver cells) into the BILE CANALICULI. The bile canaliculi empty into bile ducts. The bile eventually leaves the liver in the COMMON HEPATIC DUCT. The common hepatic duct merges with the CYSTIC DUCT of the gall bladder to form the COMMON BILE DUCT. The common bile duct and the pancreatic duct merge to form the HEPATOPANCREATIC AMPULLA, which empties the bile into the duodenum of the small intestine.
Bile's digestive function is fat emulsification. In other words, bile breaks up fats and oils into small particles which are easier to digest with enzymes.
Bilirubin, a product of hemoglobin breakdown from our RBC’s, is excreted in bile and is one of the bile pigments that gives bile its yellow-neon green color. When bilirubin is broken down in the large intestine by bacteria, the by-product gives the waste material(feces) its brown color.
Jaundice, yellow coloring of skin and whites of the eyes, is often associated with liver disease. If bile cannot be properly made/excreted bilirubin will build up in the blood and gives that jaundice glow.
3. THE GALLBLADDER
the gallbladder contains smooth muscle.
The gallbladder's function is to store bile. Although the liver continuously secretes bile, a sphincter of the hepatopancreatic ampulla guards the entry of bile into the duodenum of the small intestine. The sphincter is closed when bile is not needed for digestion. The bile backs up the CYSTIC DUCT into the gallbladder, where it is stored. Water is reabsorbed from the bile, making the bile more
concentrated. During digestion, the smooth muscle in the gallbladder wall contracts and pushes bile into the cystic duct. The hepatopancreatic ampulla sphincter relaxes, releasing the bile into the duodenum.
BE sure you know your ducts! Common Hepatic Duct, Cystic Duct, Common Bile Duct.
