Functions of the Digestive System

Overview of GI processes and Controls The major functions of the digestive tract are usually summarized in 2 words: Digestion and Absorption The essential activities of the GI tract include: Ingestion Propulsion Physical food breakdown Chemical food breakdown Absorption Defecation

Ingestion/ Propulsion Ingestion: food is placed into the mouth, this is voluntary a process Propulsion: Peristalsis is involuntary and involves alternating waves of contraction and relaxation of the muscles in the wall of esophagus In segmentation, single segments of the intestine alternately contract and relax: the food is mixed rather than propelled along the tract

*Don’t try this, it’s uncomfortable * Peristalsis Because of peristalsis, you can eat upside down against the forces of gravity -- the muscles of the esophagus will propel food down towards stomach *Don’t try this, it’s uncomfortable *

Question → What do you think retroperistalsis is? reverse of the involuntary smooth muscle contractions of peristalsis. usually occurs as a precursor to vomiting

Mechanical Breakdown In the mouth by the tongue In the stomach by churning In the small intestine by segmentation

Chemical Breakdown The process by which large molecules are broken down to their building blocks: 3 macromolecules: 1) Carbohydrates 2) Proteins 3) Lipids

Chemical Breakdown to Building Blocks Carbohydrate : Monosaccharide We can digest carbs of sucrose (table sugar), lactose (milk sugar) maltose and starch Although we eat foods containing other polysaccharides, such as cellulose (cabbage), we do not have enzymes that break them down, but they help move food along the GI tract and prevent constipation start here anatomy 2

Chemical Breakdown to Building Blocks 2. Proteins: Amino Acid protein 3. Lipids: Fatty acid and glycerol start here, anatomy 1

Absorption/Defecation Absorption: The transport of digested end products from the lumen (innermost) of the GI tract to the blood or lymph The small intestine is the major absorptive site Defecation: The elimination of indigestible substances from the body in the form of feces

Control of the digestive system Digestive activity is mostly controlled by reflexes via the parasympathetic division of the autonomic nervous system The sensors involved in these reflexes are located in the walls of the alimentary canal organs and respond to a number of stimuli: Stretch of the organ by food in the lumen pH of the contents Presence of certain breakdown products of digestion

Control of the digestive system When the receptors are activated, they start reflexes that activate or inhibit: The glands that secrete digestive juices into the lumen or hormones into the blood The smooth muscles of the muscularis that mix and propel the foods along the tract What are these processes called again?

Mouth, pharynx and esophagus Food ingestion and breakdown: Food is physically broken down into smaller pieces by chewing As food is mixed with saliva, salivary amylase begins the chemical digestion of starch, breaking it down into maltose What is amylase again? No food absorption occurs in the mouth The pharynx and esophagus have no digestive function

Mouth, pharynx and esophagus Food propulsion: Deglutition, or swallowing, is a complicated process with 2 major phases: The voluntary buccal phase: (occurs in the mouth) chewing and mixing The involuntary pharyngeal-esophageal phase: Transport food through the pharynx and esophagus

Activities of the Stomach Gastric juice: secreted by the stomach glands, it is regulated by both neural and hormonal factors: Gastrin: a hormone that helps the stomach glands to produce more protein digesting enzymes -- the presence of food and falling pH in the stomach stimulates production of gastrin Hydrochloric Acid: makes stomach contents very acidic, converts pepsinogen (made by stomach walls) to pepsin, the active protein digesting enzyme What does “pepsin” remind you of?

Stomach Related Diseases Ulcers: The digestion of the stomach itself caused by insufficient mucus production Can be caused by too much aspirin, poor lifestyle, or H. pylori bacteria -- treatment includes limited use of drugs, alcohol, highly processed foods, no smoking, low stress

Stomach Related Diseases Heartburn/Acid Reflux: The inflammation of the esophagus Caused by excess stomach acid or poor closing of esophageal sphincter (leakage into esophagus) start here anatomy 2 Treatment → eating an earlier dinner, avoiding trigger foods (spicy, acidic), taking antacids, raising the head of your bed

Stomach Related Diseases Hiatal Hernia: A structural abnormality where the superior part of the stomach protrudes above the diaphragm Caused by weakened diaphragm muscle, lifting heavy objects often, persistent coughing/vomiting Treatment→ medication/surgery

Activities of the Small Intestine The microvilli of small intestine have important enzymes: brush border enzymes that break down double sugars into simple sugars and complete protein digestion Food entering the small intestine are flooded with pancreatic enzymes

Pancreatic Enzymes Pancreatic juice contains enzymes that: Along with brush border enzymes, complete the digestion of starch (pancreatic amylase) Carry out about half of protein digestion by protease enzymes ( trypsin, chymotrypsin, carboxypeptidase) Help with fat digestion → pancreas creates lipases Digest nucleic acids (nucleases) stop here anatomy 1

Activities of the Small Intestine When chyme enters the small intestine, it stimulates the mucosa cells to produce several hormones: secretin: causes the liver to increase its output of bile (think of “secrete”) cholecystokinin (CCK): causes the gallbladder to contract & release stored bile into the bile duct. Result → bile & pancreatic juice enter the small intestine together start here anatomy 1

Activities of the Small Intestine Most substances: are absorbed by active transport → explain then they enter the capillary bed in the villi to be transported in the blood Lipids: Are absorbed by diffusion → explain Then they enter both the capillary bed & lacteal in the villi → carried to the liver by blood and lymphatic fluid

Activities of the Large Intestine The colon itself produces no digestive enzymes The bacteria that live within the lumen: metabolize some of the remaining nutrients, releasing gases Produce vitamins ( vitamin K and B) What type of biological relationship is this? Symbiosis → Mutualism

Activities of the Large Intestine Two major types of propulsive movements: Colon peristalsis: sluggish and contribute very little to propulsion Mass movements: slow-moving but powerful contractile waves that move over large areas of the colon and force the contents toward the rectum

Activities of the Large Intestine The rectum is generally empty, but when feces are forced into it, the wall is stretched → defecation reflex is initiated: It is a nerve (spinal) reflex that causes the walls of the sigmoid colon & rectum to contract and the anal sphincters to relax