Physiology 21: Digestive System PDF

Chapter 21 -- Digestive system **[21.1 Anatomy of the Digestive System]** 1\. [Trace a piece of undigested food from mouth to anus] - Oral cavity GI tract Accessory organs - GI tract = continuous tube from mouth to rectum - Accessory organs add exocrine secretions but don't direct...

Chapter 21 -- Digestive system **[21.1 Anatomy of the Digestive System]** 1\. [Trace a piece of undigested food from mouth to anus] - Oral cavity GI tract Accessory organs - GI tract = continuous tube from mouth to rectum - Accessory organs add exocrine secretions but don't directly house food or chyme - [Salivary glands, liver, gallbladder, pancreas] - Mouth Pharynx Esophagus Stomach (fundus, body, antrum) small intestine (duodenum, jejunum, ileum) large intestine (colon, rectum) anus - **Chyme** = soupy substance created as ingested food is broken down [2. Describe the 4 layers of the GI tract wall] - Modifications can increase surface area to help function - 4 layers of GI tract wall - Mucosa - Innermost layer facing the lumen - Lamina propria (connective tissue) - Contains immune cells - Submucosa - Beneath mucosa - Contains blood vessels and lymph vessels - Submucosal plexus (part of enteric nervous system) - Muscularis externa - 2 layers of smooth muscle - Myenteric plexus (part of enteric nervous system) - Serosa - Continuation of the peritoneal membrane, which forms sheets of mesentery - Outermost layer of GI tract -- visceral peritoneum A diagram of a human body Description automatically generated **[21.2 Digestive Function and Processes]** 3\. Describe the primary function of the digestive system - [Move nutrients, water, and electrolytes from external internal environment] - 4 basic processes - Digestion - Chemical and mechanical breakdown of food into absorbable units - Absorption - Transfer of substances from lumen of GI tract to the ECF - Secretion - Transfer of fluid and electrolytes from ECF to lumen - Motility - Movement of material through GI tract via muscle contractions (**peristalsis**) - Mechanically mix food to break it into small particles (**segmentation**) [4. Explain the challenges of autodigestion, mass balance, and defense] - [These are the **3 challenges** of the digestive system] - **Autodigestion** = We want to ensure we digest the food we eat... not ourselves - Digestive enzymes are secreted as zymogens (inactive form) - Mass balance = need to maintain balance - 2L enter via food + drink / 7L via secretion - Nearly all of it is reabsorbed to maintain balance - Defense = against pathogens - Contains largest collection of lymphoid tissue \-- **GALT** [5. Describe and compare secretion, digestion, absorption, and motility] - Big idea is volume entering GI tract must = volume exiting leaving the lumen - Ingest about 2L of fluid a day & secrete about 7L of fluid = total of 9L entering GI tract - We secrete way more fluid than we ingest! - 9L leaves the lumen -- (0.1L from poop / 7.5L absorption in SI / 1.4L absorption in LI) - 4 basic processes - **Digestion** - Chemical and mechanical breakdown of food into absorbable units - **Absorption** - Transfer of substances from lumen of GI tract to the ECF - **Secretion** - Transfer of fluid and electrolytes from ECF to lumen - **Motility** - Movement of material through GI tract via muscle contractions (**peristalsis**) - Mechanically mix food to brake it into small particles (**segmentation**) [6. Describe single-unit smooth muscle, slow wave potentials, tonic and phasic contractions] - GI smooth muscle contracts spontaneously - Slow wave potentials = spontaneous depolarization - Contractions differ on regions of body - **Tonic contractions** -- minutes or hours - **Phasic contractions** -- seconds  [7. Describe and compare peristalsis, segmentation, and the migrating motor complex] - **Movement** of material through GI tract via [muscle contractions] (**peristalsis**) - Mechanically **mix food** to break it into small particles (**segmentation**) - **Migrating motor complex** = moves food between meals from upper GI tract to lower regions - is inhibited during ingestion / FED state \*Peristalsis & MMC = move food along digestive tract A diagram of a structure Description automatically generated with medium confidence **[21.3 Regulation of GI Function ]** [8. Compare the enteric nervous system to the central nervous system] - **Enteric nervous system** - Controls motility, secretion, & growth in the digestive system - Can act independently - **Intrinsic neurons** = neurons completely within the ENS - **Extrinsic neurons** = neurons from the CNS to the digestive system - Shared features with the CNS - Neurotransmitters - Glial cells - Diffusion barrier (like blood brain barrier) - Integrating center [9. Contrast long reflexes, short reflexes, and control involving GI peptides] - **Short reflexes** = integrate & originate in the ENS - **Long reflexes** = integrate in the CNS / "cephalic reflexes" - Some originate in the ENS - Other originate outside the GI tract - **GI peptides** excite or inhibit GI motility and secretion [10. GI hormones]  - This is the only one released in the stomach - The rest are released from the small intestine - Inhibited by somatostatin - Primary effects: gastric acid food breakdown; mucosal growth maintain stomach lining integrity A screenshot of a computer screen Description automatically generated - CCK = hormone that has broad systemic effects so emphasis on its blood-borne action! - Secreted by small intestine once FA's and AA's enter duodenum - **Inhibits** gastric emptying in stomach **(acid secretion**) slowing rate of chyme entering more time for digestion - CCK = makes you feel full regulating food intake  - Released by small intestine in response to low pH - Acidic chyme from stomach is entering the small intestine - Pancreas releases bicarbonate to neutralize the acid & protect the intestinal lining - Stomach **inhibits** more gastric emptying (**acid secretion**) so acidic chyme will stop entering until the existing acidic load is neutralized - Overall is protective for intestine & maintains optimal pH for enzyme activity A screenshot of a computer screen Description automatically generated - Secreted in small intestine when glucose, FA, & AA's enter - Responds to basically anything we eat - Stimulates pancreas to secrete insulin -\> FEED FORWARD - Also inhibits gastric emptying & acid secretion - Allows more time to absorb & process nutrients - Overall big role in insulin release, slowing gastric activity, & preparing body to absorb nutrients  - Released in response to mixed meal with carbs or fats - Enhances insulin secretion from beta cells in pancreas (endocrine pancreas) - Suppresses/inhibits alpha cells from releasing glucagon - Insulin will lower BGL - **Inhibits acid secretion** - Promotes satiety tells brain to stop eating... I'm full! - Overall regulates BGL & helps appetite control A screenshot of a computer screen Description automatically generated - Unique because it responds to fasting... NOT something in small intestine like the others - Released periodically during fasting in cyclical manner - NOT triggered by food intake - Triggers MMC peristaltic waves that sweep undigested food through GI tract - Ingestion/FED state inhibits Motilin - Overall ensures GI tract is cleaned between meals & ready for next round! **[21.4 Integrated Function: The Cephalic Phase]** **Cephalic phase controlled by CNS** [11. Explain the feedforward control in digestion] - Anticipatory mechanism that prepares digestive system to process food - Stimuli of food vagus nerve salivary glands, stomach, pancreas & gallbladder produce enzymes/hormones to prepare for digestion - Basically, primes the body for food consumption  [12. Map the processes and control pathways of the cephalic phase] - Smelling, seeing, or thinking about food **initiates long reflexes** that prepare the stomach for the arrival of food - **Reflex starts in brain** (medulla) - Chemical & mechanical digestion begins in mouth - Mechanical digestion starts with chewing = mastication - **Salivary secretions** - Exocrine - Autonomic control - **Function to**: soften & lubricate food / digestion of starch (chemical) - Swallowing moves food from mouth to stomach - Deglutition = swallowing reflex - Stretch from food sensory neurons contraction of muscles [13. Explain the functions of saliva] - Just know functions from above 14\. SKIP **[21.5 Integrated Function: The Gastric Phase]** **[15. Map the processes and control pathways of the gastric phase]** - Gastric phase = occurs in the **stomach** - **3 functions of the stomach**: storage, digestion, & defense - Digestion begins with [long vagal reflex of cephalic phase] - [Food in stomach initiates **short reflexes of gastric phase**] A diagram of a human body Description automatically generated  [16. Describe the gastric secretions and their major actions] A diagram of the human body Description automatically generated  **[21.6 Integrated Function: The Intestinal Phase]** [17. Compare and contrast digestion and motility in the large and small intestines] - Small intestine - Most digestion and absorption occur in the small intestine - 7.5L of the 9L is reabsorbed here - [Pancreatic & brush border enzymes complete digestion] - Maximal surface area - **Peristalsis and segmentation** - **Lots of Nutrients are absorbed** from intestinal epithelium & most go through circulation to be distributed - Hepatic portal system - Movement is faster and more dynamic - Large intestine - Absorbs 1.5L - 0.1L is lost in feces - Not many nutrients absorbed... mostly water - **Segmental contractions** mix chyme - **Mass movement** moves chyme forward & triggers defecation - Movement is slower & more powerful for mixing & mass movements [18. Describe the anatomy and function of the hepatic portal system] - Capillaries of the digestive tract - Allows absorbed nutrients to be delivered to organs A diagram of the internal organs Description automatically generated [19. Describe the major secretions of the pancreas and liver] - Liver secretes bile - Bile is stored and concentrated in the gall bladder - Bile helps digest lipids/fats - Bile salts facilitate fat digestion - Most nutrients absorbed in the intestine pass through the liver **get filtered before systemic circulation**  - Pancreas secretes enzymes and bicarbonate - Exocrine pancreas secretes digestive enzymes & sodium bicarbonate - Enzyme secretion: **enteropeptidase** in brush border converts trypsinogen to **trypsin activates other enzymes (zymogens)** - **Bicarbonate** secretion neutralizes gastric acid A diagram of a pancreas Description automatically generated [20. Diagram the cellular mechanisms for secretion or absorption of water and ions] - Intestine absorbs ions and water [21. Diagram the digestion and absorption of carbohydrates, proteins, and fats] - Fats - Lipid digestion begins with gastric & lingual lipases & is assisted by pancreatic lipase in the duodenum - Absorbed in apical side of mucosal cells - Cholesterol needs to be actively transported - Combine cholesterol with absorbed fat chylomicrons - **Chylomicrons enter lymph**  - Carbohydrates are **absorbed as monosaccharides** - Absorbed in the small intestine blood stream - Absorbed with sodium through secondary active transport A diagram of a structure Description automatically generated - Proteins are digested into small peptides & amino acids - Digestion begins in stomach with pepsin - Amino acids are **co transported with sodium ions to be absorbed** - **Absorbed in blood**  **[21.7 Immune Functions of the GI Tract:]** 22\. Explain the neural and hormonal control of the intestinal phase of digestion A diagram of a process Description automatically generated