The control of salivation is unique in two ways. First, it is mediated almost entirely by the nervous system. Note that both gut hormones and neuro-reflexes regulate secretion in other portions of the GI tract. Secondly, both the parasympathetic and sympathetic branches of the autonomic nervous system stimulate salivation. Note that parasympathetic and sympathetic nerves have opposing actions on most other organs. The parasympathetic division of the autonomic nervous system is the primary controller of salivation. Parasympathetic fibers are carried in the facial and glossopharyngeal nerves, cranial nerves VII and IX. Parasympathetic activity initiates and maintains salivation. In general, parasympathetic stimulation produces large amounts of watery saliva containing enzymes. The thought, smell, or taste of food, stimulates the salivary center in the medulla to increase parasympathetic activity and salivation. Acidic substances like a piece of lemon, and pressure of chewing food or non-food in the mouth, like a peach pit, are powerful stimuli for salivation. Intestinal irritation and accompanying nausea are also powerful stimulants of salivation. Fear, fatigue, sleep, and dehydration all inhibit salivation. Activity in the sympathetic nervous system produces a small volume of saliva that is thick with mucus. Because sympathetic stimulation accompanies frightening or stressful situations, the mouth may feel dry at such times. In summary, both parasympathetic and sympathetic activity increase salivation. Both also stimulate metabolism and growth of the salivary glands. The surface of the stomach has deep wells called gastric pits. Each pit leads to gastric glands. Throughout the stomach, gastric pits and glands contain cells that produce mucus and pepsinogen. In the fundus and body, gastric glands are deep and contain cells that produce both acid and intrinsic factor, and cells that produce histamine. Surface epithelium cells secrete thick, alkaline mucus. Mucous cells in the neck region of the gland secrete thin watery mucus. Parietal cells secrete acid and intrinsic factor. Chief cells secrete pepsinogen. These paracrine cells secrete the local chemical messenger histamine. In the pyloric region, gastric glands contain cells that produce gastrin and cells that produce mucus and pepsinogen. G cells secrete the hormone gastrin. Most digestion and absorption occurs in the small intestine. The essential secretions for digestion come from the pancreas and liver, not from the small intestine. We will study secretions of these accessory glands first beginning with the pancreas. The exocrine pancreas secretes a juice with two components that are regulated separately: enzymes to digest all major foodstuffs and a bicarbonate solution. CCK stimulates secretion of pancreatic enzymes, and secretin stimulates secretion of bicarbonate solution. Pancreatic juice is important for digestion and protection. Pancreatic juice is produced in structures called acini and carried through a network of ducts to enter the duodenum through the hepatopancreatic sphincter. Acinar cells secrete digestive enzymes for all major foodstuffs. They include: proteases to digest protein, pancreatic amylase to digest starch, and pancreatic lipase to digest fat. The major pancreatic proteases are trypsinogen, chymotrypsinogen, and procarboxypeptidase. Like pepsin, they are secreted in an inactive form. Enterokinase, an enzyme embedded in the intestinal cell membranes, activates trypsinogen. Trypsin activates more trypsinogen. This process is similar to activation of pepsinogen in the stomach, first by hydrochloric acid, then by pepsin. Trypsin also activates the other proteases. Carbohydrates and protein are partially digested prior to arriving in the small intestine. Fat is digested primarily by pancreatic lipase in the small intestine. Therefore, if the pancreas fails to secrete adequate amounts of digestive enzymes, the major disruption in digestion is the failure to digest fat. Duct cells secrete a watery solution containing bicarbonate. Bicarbonate neutralizes acidic chyme in the duodenum. A neutral-to-slightly alkaline intestinal environment is essential for maximal activity of pancreatic enzymes. Remember that the pancreas is a mixed gland with both exocrine and endocrine secretions. The endocrine pancreas secretes hormones including insulin and glucagon that regulate metabolic activities of the absorptive and postabsorptive states respectively. The major digestive function of the liver is the production of bile. Bile is produced continuously. When the hepatopancreatic sphincter is closed, bile goes to the gallbladder where it is stored and concentrated. Bile has two components that are regulated separately: organic compounds that are essential to emulsify fat and a bicarbonate solution. Recycled bile salts in portal blood regulate secretion of bile, and secretin stimulates secretion of bicarbonate solution. Bile is important for: digestion and absorption of fat and protection. The organic portion of bile is a mixture that includes: bile salts, lecithin, cholesterol, and bilirubin. Bile does not digest fat. Both bile salts, which are derived from cholesterol, and lecithin emulsify fat. They keep fat droplets from coalescing thus promoting enzymatic breakdown and absorption of fat. We will study digestion and absorption of fat in detail in the next topic. Cholesterol and bilirubin (a breakdown product of heme) are eliminated. Bile salts are recycled. Bile enters the duodenum during a meal, and travels through the small intestine. Bile salts are then actively reabsorbed in the terminal ileum, and return to the liver in the hepatic portal blood. In the liver, bile salts stimulate secretion of bile. This pathway is called the enterohepatic circulation. Bile salts may circulate through this pathway several times during digestion of a meal. Cells that line the bile ducts secrete a watery solution containing bicarbonate. This solution is identical to pancreatic bicarbonate solution and neutralizes acidic chyme in the duodenum. Secretin promotes secretion of both. The small intestine secretes a watery mucus solution and hormones that control GI activities. Small intestine secretions are important for: protection, digestion, and control. The intestinal mucosa is specialized for absorption. Mucus-secreting goblet cells are plentiful. Mucus protects the intestinal mucosa from digestion by acid and proteases, and lubricates the contents for ease of movement. The duodenum is at greatest risk for acid damage. The submucosa of the duodenum contains mucus-secreting duodenal glands that secrete alkaline mucus. Cells of the crypts secrete the water and electrolytes that combine with mucus to form intestinal juice. Water provides an environment for digestive reactions and adjusts osmolarity. Cells of the small intestine do not secrete any digestive enzymes into the lumen, but they do produce digestive enzymes. These enzymes, called brush border enzymes, are bound to microvilli cell membranes. We have already encountered the brush border enzyme enterokinase, which activates trypsinogen. In general, secretin and CCK promote intestinal activities and inhibit activities of the stomach. Contents in chyme stimulate secretion of the intestinal hormones, and both hormones have trophic effects on the pancreas. Acid in the duodenum stimulates secretion of secretin. Secretin stimulates duct cells of the pancreas and the liver to release bicarbonate solution. Because of its actions, secretin is called “nature’s antacid.” Fat (and to a lesser degree peptides) in the duodenum stimulates secretion of CCK into the bloodstream. CCK stimulates pancreatic acinar cells to release digestive enzymes. CCK is named for its second major function. Chole means bile; cysto means bladder; and kinin means mover. Therefore, CCK is the “bile bladder mover.” It causes gallbladder contraction and relaxation of the hepatopancreatic sphincter. Notice that CCK causes both enzymes and bile to enter the small intestine, thus providing all the essential compounds needed for digestion. In summary, let’s look at a table that lists the functions of all of the secretions that enter the small intestine. Mucus is a protection secretion that prevents acid and protease damage to the intestine Alkaline solutions function as both protection and digestion secretions. They neutralize acidic chyme. Neutrality reduces the possibility of acid damage to the intestinal wall and is essential for maximal digestive and absorptive activities. Secretin and CCK are important in both control and digestion. Secretin stimulates secretion of bicarbonate solutions from the pancreas and liver. CCK stimulates secretion of enzymes from the pancreas. It also stimulates contraction of the gallbladder and relaxation of the hepatopancreatic sphincter. Both CCK and Secretin inhibit gastric secretion and motility. And finally, in their role as digestion secretions, CCK and Secretin have trophic effects on the pancreas. Bile is important for both absorption and digestion. Its job is to emulsify fat. Emulsification is necessary for both digestion and absorption of fat. Other secretions that are important to digestion are: Enzymes which break down all major foodstuffs. Trypsin which activates chymotrypsin and procarboxypeptidase. And water which provides an environment for digestion and adjusts osmolarity. The large intestine secretes an alkaline mucus solution containing bicarbonate and potassium. Its sole function is protection. The alkaline mucus solution protects the intestine wall from damage by acids released by resident bacteria. Mucus eases the passage of feces and protects the wall from mechanical damage. Acid and mechanical stimulation, mediated by both long and short reflexes, increase the secretion of alkaline mucus.
Table of contents
- 1. Introduction to Anatomy & Physiology5h 40m
- What is Anatomy & Physiology?20m
- Levels of Organization13m
- Variation in Anatomy & Physiology12m
- Introduction to Organ Systems27m
- Homeostasis9m
- Feedback Loops11m
- Feedback Loops: Negative Feedback19m
- Feedback Loops: Positive Feedback11m
- Anatomical Position7m
- Introduction to Directional Terms3m
- Directional Terms: Up and Down9m
- Directional Terms: Front and Back6m
- Directional Terms: Body Sides12m
- Directional Terms: Limbs6m
- Directional Terms: Depth Within the Body4m
- Introduction to Anatomical Terms for Body Regions3m
- Anatomical Terms for the Head and Neck8m
- Anatomical Terms for the Front of the Trunk8m
- Anatomical Terms for the Back9m
- Anatomical Terms for the Arm and Hand9m
- Anatomical Terms for the Leg and Foot15m
- Review- Using Anatomical Terms and Directions12m
- Abdominopelvic Quadrants and Regions19m
- Anatomical Planes & Sections17m
- Organization of the Body: Body Cavities13m
- Organization of the Body: Serous Membranes14m
- Organization of the Body: Serous Membrane Locations8m
- Organization of the Body: Thoracic Cavity8m
- Organization of the Body: Abdominopelvic Cavity12m
- 2. Cell Chemistry & Cell Components12h 37m
- Atoms- Smallest Unit of Matter57m
- Isotopes39m
- Introduction to Chemical Bonding19m
- Covalent Bonds40m
- Noncovalent Bonds5m
- Ionic Bonding37m
- Hydrogen Bonding19m
- Introduction to Water7m
- Properties of Water- Cohesion and Adhesion7m
- Properties of Water- Density8m
- Properties of Water- Thermal14m
- Properties of Water- The Universal Solvent17m
- Acids and Bases12m
- pH Scale21m
- Carbon8m
- Functional Groups9m
- Introduction to Biomolecules2m
- Monomers & Polymers11m
- Carbohydrates23m
- Proteins25m
- Nucleic Acids34m
- Lipids28m
- Microscopes10m
- Prokaryotic & Eukaryotic Cells26m
- Introduction to Eukaryotic Organelles16m
- Endomembrane System: Protein Secretion34m
- Endomembrane System: Digestive Organelles15m
- Mitochondria & Chloroplasts21m
- Endosymbiotic Theory10m
- Introduction to the Cytoskeleton10m
- Cell Junctions8m
- Biological Membranes10m
- Types of Membrane Proteins7m
- Concentration Gradients and Diffusion9m
- Introduction to Membrane Transport14m
- Passive vs. Active Transport13m
- Osmosis33m
- Simple and Facilitated Diffusion17m
- Active Transport30m
- Endocytosis and Exocytosis15m
- 3. Energy & Cell Processes10h 7m
- Introduction to Energy15m
- Laws of Thermodynamics15m
- Chemical Reactions9m
- ATP20m
- Enzymes14m
- Enzyme Activation Energy9m
- Enzyme Binding Factors9m
- Enzyme Inhibition10m
- Introduction to Metabolism8m
- Redox Reactions15m
- Introduction to Cellular Respiration22m
- Types of Phosphorylation11m
- Glycolysis19m
- Pyruvate Oxidation8m
- Krebs Cycle16m
- Electron Transport Chain14m
- Chemiosmosis7m
- Review of Aerobic Cellular Respiration19m
- Fermentation & Anaerobic Respiration23m
- Introduction to Cell Division22m
- Organization of DNA in the Cell17m
- Introduction to the Cell Cycle7m
- Interphase18m
- Phases of Mitosis48m
- Cytokinesis16m
- Cell Cycle Regulation18m
- Review of the Cell Cycle7m
- Cancer13m
- Introduction to DNA Replication22m
- DNA Repair7m
- Central Dogma7m
- Introduction to Transcription20m
- Steps of Transcription19m
- Genetic Code25m
- Introduction to Translation30m
- Steps of Translation23m
- Post-Translational Modification6m
- 4. Tissues & Histology10h 3m
- Introduction to Tissues & Histology16m
- Introduction to Epithelial Tissue24m
- Characteristics of Epithelial Tissue37m
- Structural Naming of Epithelial Tissue19m
- Simple Epithelial Tissues1h 2m
- Stratified Epithelial Tissues55m
- Identifying Types of Epithelial Tissue32m
- Glandular Epithelial Tissue26m
- Introduction to Connective Tissue36m
- Classes of Connective Tissue8m
- Introduction to Connective Tissue Proper40m
- Connective Tissue Proper: Loose Connective Tissue56m
- Connective Tissue Proper: Dense Connective Tissue49m
- Specialized Connective Tissue: Cartilage44m
- Specialized Connective Tissue: Bone12m
- Specialized Connective Tissue: Blood9m
- Introduction to Muscle Tissue7m
- Types of Muscle Tissue45m
- Introduction to Nervous Tissue8m
- Nervous Tissue: The Neuron8m
- 5. Integumentary System2h 20m
- 6. Bones & Skeletal Tissue2h 16m
- An Introduction to Bone and Skeletal Tissue18m
- Gross Anatomy of Bone: Compact and Spongy Bone7m
- Gross Anatomy of Bone: Periosteum and Endosteum11m
- Gross Anatomy of Bone: Bone Marrow8m
- Gross Anatomy of Bone: Short, Flat, and Irregular Bones5m
- Gross Anatomy of Bones - Structure of a Long Bone23m
- Microscopic Anatomy of Bones - Bone Matrix9m
- Microscopic Anatomy of Bones - Bone Cells25m
- Microscopic Anatomy of Bones - The Osteon17m
- Microscopic Anatomy of Bones - Trabeculae9m
- 7. The Skeletal System2h 35m
- 8. Joints2h 17m
- 9. Muscle Tissue2h 33m
- 10. Muscles1h 11m
- 11. Nervous Tissue and Nervous System1h 35m
- 12. The Central Nervous System1h 6m
- 13. The Peripheral Nervous System1h 26m
- Introduction to the Peripheral Nervous System5m
- Organization of Sensory Pathways16m
- Introduction to Sensory Receptors5m
- Sensory Receptor Classification by Modality6m
- Sensory Receptor Classification by Location8m
- Proprioceptors7m
- Adaptation of Sensory Receptors8m
- Introduction to Reflex Arcs13m
- Reflex Arcs15m
- 14. The Autonomic Nervous System1h 38m
- 15. The Special Senses2h 41m
- 16. The Endocrine System2h 48m
- 17. The Blood1h 22m
- 18. The Heart1h 42m
- 19. The Blood Vessels3h 35m
- 20. The Lymphatic System3h 16m
- 21. The Immune System14h 37m
- Introduction to the Immune System10m
- Introduction to Innate Immunity17m
- Introduction to First-Line Defenses5m
- Physical Barriers in First-Line Defenses: Skin13m
- Physical Barriers in First-Line Defenses: Mucous Membrane9m
- First-Line Defenses: Chemical Barriers24m
- First-Line Defenses: Normal Microbiota7m
- Introduction to Cells of the Immune System15m
- Cells of the Immune System: Granulocytes28m
- Cells of the Immune System: Agranulocytes26m
- Introduction to Cell Communication5m
- Cell Communication: Surface Receptors & Adhesion Molecules16m
- Cell Communication: Cytokines27m
- Pattern Recognition Receptors (PRRs)48m
- Introduction to the Complement System24m
- Activation Pathways of the Complement System23m
- Effects of the Complement System23m
- Review of the Complement System13m
- Phagocytosis17m
- Introduction to Inflammation18m
- Steps of the Inflammatory Response28m
- Fever8m
- Interferon Response25m
- Review Map of Innate Immunity
- Introduction to Adaptive Immunity32m
- Antigens12m
- Introduction to T Lymphocytes38m
- Major Histocompatibility Complex Molecules20m
- Activation of T Lymphocytes21m
- Functions of T Lymphocytes25m
- Review of Cytotoxic vs Helper T Cells13m
- Introduction to B Lymphocytes27m
- Antibodies14m
- Classes of Antibodies35m
- Outcomes of Antibody Binding to Antigen15m
- T Dependent & T Independent Antigens21m
- Clonal Selection20m
- Antibody Class Switching17m
- Affinity Maturation14m
- Primary and Secondary Response of Adaptive Immunity21m
- Immune Tolerance28m
- Regulatory T Cells10m
- Natural Killer Cells16m
- Review of Adaptive Immunity25m
- 22. The Respiratory System3h 20m
- 23. The Digestive System2h 5m
- 24. Metabolism and Nutrition4h 0m
- Essential Amino Acids5m
- Lipid Vitamins19m
- Cellular Respiration: Redox Reactions15m
- Introduction to Cellular Respiration22m
- Cellular Respiration: Types of Phosphorylation14m
- Cellular Respiration: Glycolysis19m
- Cellular Respiration: Pyruvate Oxidation8m
- Cellular Respiration: Krebs Cycle16m
- Cellular Respiration: Electron Transport Chain14m
- Cellular Respiration: Chemiosmosis7m
- Review of Aerobic Cellular Respiration18m
- Fermentation & Anaerobic Respiration23m
- Gluconeogenesis16m
- Fatty Acid Oxidation20m
- Amino Acid Oxidation17m
- 25. The Urinary System2h 39m
- 26. Fluid and Electrolyte Balance, Acid Base Balance Coming soon
- 27. The Reproductive System2h 5m
- 28. Human Development1h 21m
- 29. Heredity Coming soon
23. The Digestive System
The Stomach
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