Human Anatomy and Physiology Unit 5, Part 2: The Digestive System

Human Anatomy and Physiology Unit 5, Part 2: The Digestive System

Organization Gastrointestinal tract (alimentary canal)
Tube for transit of food during processing. Functional segments include: Mouth, esophagus, stomach, S.I, L.I Accessory structures Contribute to the food processing. Teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

Figure 24.1 Organs of the Digestive System
Important Diagram Accessory organs are highlighted in pink

Six Basic Digestive Processes
Ingestion Secretion Mixing and propulsion Digestion Mechanical digestion Chemical digestion Absorption Defecation

LAYERS OF THE GI TRACT

Four Layers of the GI tract
Mucosa - innermost Submucosa Muscularis Serosa - outermost

Mucosa Composed of 3 layers: Epithelium Lamina Propria
Muscularis mucosa Important role in digestion & absorption Muscularis mucosae Mucosa-associated lymphatic tissue (MALT) MUCOSA: Epithelium Lamina propria

Mucosa Epithelium Mucus and enteroendocrine cells
Nonkeratinized stratified squamous epithelium Simple columnar epithelium Mouth Pharynx Esophagus Anal canal FNC: Protection Stomach Intestines FNC: Secretion and Absorption Mucosa Epithelium Mucus and enteroendocrine cells

Mucosa 2. Lamina Propria Supports epithelium
Binds to muscularis mucosa Areolar connective tissue Blood & lymphatic vessels MALT Prominently tonsils, S.I., appendix, L.I. Nerves and sensors

Mucosa 3. Muscularis mucosa Smooth muscle
Creates folds in stomach and S.I Increases surface area

Submucosa Composed of: Areolar connective tissue (highly vascular)
Submucosal plexus (ENS) Binds mucosa to muscularis Regulates chemical digestion

Muscularis Skeletal muscle in mouth, pharynx, superior esophagus, and external anal sphincter Rest of GI tract is composed of smooth muscle: Inner circular muscle Outer longitudinal muscle Contains myenteric plexus between (ENS) Important role in mixing and propulsion

Serosa Serous membrane
Areolar connective tissue Simple squamous epithelium Esophagus lacks serosa – replaced with adventitia (single layer of loose areolar CT) Inf to diaphragm = Visceral peritoneum Secretes serous fluid

Figure 24.2 Layers of the GI tract
Important Diagram

NEURAL INNERVATION OF THE GI TRACT

Enteric Nervous System
Second brain  “Brain of the gut.” Consists of 100 million motor neurons, interneurons, and sensory neurons that extend from the esophagus to the anus. Located in sheaths of tissue lining the esophagus, stomach, S.I and colon Neurons arranged into the myenteric plexus and the submucosal plexus.

Enteric Nervous System
Submucosal neurons control the secretory cells. Myenteric neurons control gastric motility. Can function independently of the CNS.

Figure 24.4 Organization of the Enteric Nervous System

Autonomic Nervous System
Vagus nerve (X)  parasympathetic fibers  stimulate ENS neurons. Stimulate GI secretion and motility Sympathetic nerves (thoracic & upper lumbar regions of SC) inhibit synapse of ENS neurons. Anger, fear and anxiety slow digestion

Gastrointestinal Reflex Pathways
Regulate secretions and motility in response to stimuli present in the lumen. Reflexes begin with receptors associated with sensory neurons of the ENS. ENS, CNS, and ANS can modulate reflexes.

Sensory Receptors in G.I Tract
Chemoreceptors Mechanoreceptors Controls of the digestive activity are both extrinsic and intrinsic (nervous and hormonal)

Gastrointestinal Reflex Pathways
Long reflexes to the digestive system involve a sensory neuron that sends external or internal digestive information to the brain. This type of reflex includes reactions to food, emotion, or danger. Short reflexes to the digestive system provide shortcuts for the enteric nervous system (ENS) to act quickly and effectively, and form a sort of digestive brain. It reacts to digestive movement and chemical changes. The enterogastric reflex is stimulated by the senses. This reflex releases acid in the duodenum or in the stomach, and suppresses the release of digestive proteins. The gastrocolic reflex increases movement in the gastrointestinal tract, and reacts to stretches in the stomach walls as well as in the colon. It is responsible for the urge to defecate, the movement of digested material in the small intestine, and it makes room for more food within the stomach. The gastroileal reflex works with the gastrocolic reflex to stimulate the urge to defecate. It does so by opening the ileocecal valve and moving the digested contents from the ileum of the small intestine into the colon for compaction.

PERITONEUM

Key Terms Serous Membrane - Smooth, transparent, two-layered membrane that lines internal cavities of the body, lubricated by a fluid derived from serum. Mesothelium. Peritoneum – body’s largest serous membrane Parietal peritoneum – lines abdominal wall Visceral peritoneum - covers most abdominal organs (serosa); some organs (kidneys, pancreas) are retroperitoneal Peritoneal cavity - potential space between parietal and visceral peritoneum; contains serous fluid.

Figure Relationship of the peritoneal folds to one another and to organs of the digestive system

Main extensions of the peritoneum
Greater omentum Falciform ligament Lesser omentum Mesentery Mesocolon

Greater Omentum Adipose tissue Arises from stomach and duodenum
Covers S.I Attaches to transverse colon.

Falciform Ligament Attaches liver to abdominal wall and diaphragm.

Lesser Omentum Suspends stomach and small intestine from liver.

Mesentery Largest fold.
Binds small intestine to posterior abdominal wall.

Mesocolon Binds large intestine to posterior abdominal wall.
Works with mesentery to keep the intestines in place.

Retroperitoneal Only covered on anterior surface
Kidneys, suprarenal glands, pancreas, ascending and descending colon, duodenum

Mouth (oral or buccal cavity)
Cheeks, hard & soft palate, lips, tongue. Covered externally by skin. Covered internally by mucous membrane.

Mouth (oral or buccal cavity)
Divided into: Vestibule - bounded externally by cheeks and lips and internally by gums and teeth. Oral cavity proper - extends from gums and teeth to fauces

Cheeks – contain buccinator muscles
Lips (labia) – contain orbicularis oris muscles; attached to gum by labial frenulum Lips and cheeks keep food between teeth and assist in speech. Uvula – muscular structure hanging at the back of the throat; blocks nasopharyngeal opening during swallowing

Palate Hard palate – anterior portion of mouth’s roof; formed by maxillae and palatine bones Soft palate – posterior portion of mouth's roof; formed by two muscular arches: Palatoglossal arch - extends from palate to tongue anterior to palatine tonsils Palatopharyngeal arch - extends from palate to pharyngeal wall posterior to palatine tonsil

Figure 24.6 Structures of the mouth (oral cavity)

Salivary Glands General Saliva Functions:
Keeps the oral and pharyngeal mucous membranes moist. Lubricates and dissolves food. Starts carbohydrate chemical digestion. Most saliva is secreted by salivary glands. Some saliva comes from buccal glands in the mucous membrane that lines the mouth. Minor glands = labial, buccal, lingual and palatal.

Salivary Glands Outside the mouth  ducts  oral cavity
Parotid glands - below ear, over masseter; parotid ducts open into vestibule beside 2nd maxillary molar. Submandibular glands - under mandible; submandibular ducts open lateral to lingual frenulum Sublingual glands - floor of mouth deep to tongue; lesser sublingual ducts open into floor of mouth

Figure 24.7 The three major salivary glands – parotid, sublingual, and submandibular

Salivary Glands Salivary gland cells are organized into acini.
Small saclike clusters surrounded by secretory cells. Serous acini - secrete a watery fluid Mucous acini - secrete a slimy, mucus secretion Parotid glands – serous acini only Submandibular glands – mostly serous acini and a few mucous acini Sublingual glands - mostly mucous acini and a few serous acini

Figure 24.7B Histology of the submandibular gland

Composition and Functions of Saliva
99.5% water 0.5% solutes (salts, gases, mucous, ions, enzymes, lysozyme, IgA) Functions: Lubricate food Dissolve food for tasting Moistens mucous membranes Bicarbonate ions buffer acidic foods Waste removal – urea + uric acid Salivary amylase begins chemical digestion of starch (maltose) Lysozyme  antibacterial IgA  1st line of defense

Salivation Secretion Moisten mucous membranes Speech Reabsorption
Dehydration ANS control

Salivation Increase salivation Sight, smell, sounds, thoughts/memory.
Tongue stimulation Salivatory nuclei increase salivation Parasympathetic fibers in CN VII & CN IX Parasympathetic nerve impulses stimulate saliva Facial (VII) and glossopharyngeal (IX) nerves. Stop salivation Sympathetic nerves inhibit salivation

Tongue Papillae cover upper surface (dorsum) and sides.
Projections of lamina propria Some contain taste buds Lingual frenulum - attaches tongue to mouth’s floor

Teeth Project into the mouth
Alveolar processes of mandible and maxilla Covered by gingivae

Teeth Adapted for mechanical digestion. A typical tooth consists of:

Teeth Composed primarily of dentin Dentin of crown covered in enamel
Encloses the crown’s pulp cavity the roots’ root canals. Dentin of crown covered in enamel Root’s dentin is covered by cementum Cementum attaches root to periodontal ligament Fibrous connective tissue Anchor Shock absorber

Figure 24.8 A typical tooth and surrounding structures

Dentition Figure 24.9 in textbook
There are two sets of teeth in an individual’s lifetime: Deciduous (primary) teeth 20 teeth that start erupting at 6 months Permanent (secondary) teeth 32 teeth that erupt between 6 and 12 years of age Figure 24.9 in textbook

Tooth Types Incisors – chisel shaped; cutting
Cuspids (canines) – pointed; tearing Premolars – bicuspid; crushing and grinding Molars – four cusps; crushing and grinding

Digestion in the Mouth Mechanical digestion (mastication or chewing)
Mixes with saliva  bolus Chemical digestion Amylase Begins starch digestion at pH of 6.5 or 7.0 When bolus & enzymes hit the pH of 2.5 (gastric juices), hydrolysis ceases Lingual lipase Secreted by glands in tongue Begins breakdown of triglycerides once it reaches the acidic pH in the stomach

Look at Table 24.1

PHARYNX

Pharynx Figures 23.3 / 24.11 Funnel-shaped tube
Internal nares  esophagus and larynx Skeletal muscle + mucous membrane 3 parts: Nasopharynx – respiratory Oropharynx – digestive & respiratory Laryngopharynx – digestive & respiratory

Figure The pharynx

Pharynx Deglutition is facilitated by saliva and mucus
Bolus pushed into the oropharynx Sensory nerves send signals to deglutition center in brainstem Soft palate lifts  closes nasopharynx Larynx is lifted as epiglottis is bent to cover glottis Ingested food  mouth  oropharynx  laryngopharynx  muscular contractions  esophagus  stomach

ESOPHAGUS

Esophagus Collapsible muscular tube Anterior to vertebrae
Figure 24.1 Esophagus Collapsible muscular tube Anterior to vertebrae Posterior to trachea and heart Connects pharynx to stomach Secretes mucus Transport food to the stomach. Bound by sphincters Pierces the diaphragm at hiatus

Histology of the Esophagus
Wall has 3 layers: Mucosa  Protection Normal 3 layers (NKSSE, LP, MM) Mucous glands Submucosa  ACT, BV and large mucous glands Muscularis  Upper 1/3 is skeletal Middle is mixed Lower 1/3 is smooth Upper & lower esophageal sphincters are prominent circular muscle Skeletal and smooth Figure 24.10

Histology of the Esophagus
Adventitia Outermost layer of CT Blends with surrounding connective tissue No peritoneum Differences from serosa Serosa  lubrication  secretions Adventitia  binds structures  no secretions

Physiology of the Esophagus - Deglutition
Figure 24.11 Physiology of the Esophagus - Deglutition Moves bolus from the mouth to the stomach. Mouth, tongue, pharynx, esophagus Saliva and mucus. 3 stages: Voluntary stage Pharyngeal stage Esophageal stage

Figure 24.11 Physiology of the Esophagus - Deglutition Voluntary stage Food: Tongue  back of oral cavity  oropharynx Pharyngeal stage Moves food from pharynx to esophagus Oropharynx receptors stimulate deglutition center in M.O. Signals stimulate closing of airways (breathing stops) Uvula and soft palate lift to close off nasopharynx Vocal cords close Epiglottis closes over larynx Upper esophageal sphincter relaxes

Figure 24.11 Deglutition (swallowing)

Figure 24.11 Physiology of the Esophagus - Deglutition 3. Esophageal stage Bolus enters esophagus Upper sphincter relaxes when larynx is lifted Peristalsis begins Circular fibers contract behind bolus Longitudinal fibers shorten the distance Lower sphincter relaxes as food approaches Travel time: Solids: 4-8 seconds Liquids: 1 second

Deglutition Summary Swallowed food Mouth Oropharynx Laryngopharynx
Upper esophageal sphincter Esophagus (peristalsis) Lower esophageal sphincter Stomach

STOMACH

Stomach J shaped enlargement of GI tract
Figure 24.12 Stomach J shaped enlargement of GI tract Mixing chamber and holding reservoir Does very little “digestion” Proteins Triglyceride + starch Bolus  chyme Some absorption 2 curves

Stomach Four Main Regions: Cardia Fundus Body Pyloris
Figure 24.12b Stomach Four Main Regions: Cardia Fundus Body Pyloris Three regions: Pyloric antrum Pyloric canal Pylorus Pyloric sphincter Rugae – folds in mucosa

Histology of the Stomach
Figure 24.13 Histology of the Stomach Surface of mucosa – simple columnar cells = surface mucous cell Form gastric pits & gastric glands

Figure 24.13b Histology of the Stomach 3 types of exocrine gastric glands: Mucous neck cells Zymogenic (chief) cells Parietal (oxyntic) cells Gastric glands also contain entero-endocrine cells (G cells) Gastrin Stimulate gastric activity

Figure 24.13c Histology of the stomach

Figure 24.13 Histology of the Stomach Submucosa Areolar connective tissue

Figure 24.13 Histology of the Stomach Muscularis 3 layers of smooth muscle Longitudinal Circular Inner oblique

Figure 24.12 Histology of the Stomach Serosa Lesser curvature, visceral peritoneum = lesser omentum Greater curvature, visceral peritoneum = greater omentum

Digestion in the Stomach
Figure 24.12 Digestion in the Stomach Mechanical Peristalsis Propulsion Retropulsion Gastric emptying*

Chemical Digestion in the Stomach
Salivary amylase Lingual lipase Parietal cells HCl  H+ Cl- Proton pumps Carbonic anhydrase Antiporters HCO3-

Figure 24.14 Secretion of HCl by parietal cells in the stomach

Activation of HCl secretion

Chemical Digestion in the Stomach
Pepsinogen  pepsin Chief (zymogenic cells) pH of 2 activates Proteins  peptides Gastric lipase Triglycerides (milk)  fatty acids + monoglycerides Limited in adult stomach pH 5-6

Protection of the Stomach
Figure 24.13b Protection of the Stomach Zymogenic cells Pepsinogen  pepsin Prevents digestion of protein in organ Protection Surface and neck cells 1-3 mm alkaline mucous Limited absorption Certain drugs (aspirin, alcohol), electrolytes, some water

Digestion in the Stomach
Mechanical Gastric emptying* 2-6 hours after ingestion 1st carbs, 2nd protein, last fats

Look at Table 24.3

PANCREAS

Pancreas Retroperitoneal gland Head, body and tail
Figure 24.16 Pancreas Retroperitoneal gland Head, body and tail Connects to duodenum via 2 ducts Secretes pancreatic juices Hepatopancreatic ampulla

Figure 24.16b Details of the Hepatopancreatic Ampulla

Figure 24.16c

Histology of the Pancreas
Small clusters of glandular epithelial cells Exocrine  Acini – 99% of clusters Secrete pancreatic juice Endocrine  Pancreatic islets – 1% of clusters Secrete hormones

Pancreatic Juice mL Clear liquid: Water, salt, NaHCO3, enzymes Starch – pancreatic amylase Proteins – trypsin, chymotrypsin, carboxypeptidase Fats – pancreatic lipase Nucleic acids – ribonuclease, deoxyribonuclease Sodium bicarbonate – brings pH to Inactivates pepsin Stimulates pancreatic enzymes

Pancreatic Hormones Glucagon Insulin Somatostatin
Promotes glycogen  glucose in the liver. Insulin Regulates blood glucose Somatostatin AKA - Growth hormone-inhibiting hormone (GHIH) Inhibits glucagon, insulin, gastrin, secretin and histamine Slows digestive processes Pancreatic polypeptide Released after a high protein meal, exercise, or fasting Regulates pancreatic hormone secretions

Human Anatomy and Physiology Unit 5, Part 2: The Digestive System

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Human Anatomy and Physiology Unit 5, Part 2: The Digestive System

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