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Lecture

BIOB32H3 Lecture Notes - Yolk Sac, Appendicitis, Starch


Department
Biological Sciences
Course Code
BIOB32H3
Professor
Kenneth Welch

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The Digestive System
Digestive System: Overview
The alimentary canal or gastrointestinal (GI) tract digests and absorbs food
Alimentary canal mouth, pharynx, esophagus, stomach, small intestine, and large intestine
Accessory digestive organs teeth, tongue, gallbladder, salivary glands, liver, and pancreas
Digestive Process
The GI tract is a “disassembly” line
Nutrients become more available to the body in each step
There are six essential activities:
ingestion, propulsion, and mechanical digestion
chemical digestion, absorption, and defecation
Essential Activities of Digestion
Ingestion taking food into the digestive tract
Propulsion swallowing and peristalsis
Peristalsis waves of contraction and relaxation of muscles in the organ walls
Mechanical digestion chewing, mixing, and churning food
Chemical digestion catabolic breakdown of food
Absorption movement of nutrients from the GI tract to the blood or lymph
Defecation elimination of indigestible solid wastes
GI Tract
External environment for the digestive process
Regulation of digestion involves:
Mechanical and chemical stimuli stretch receptors, osmolarity, and presence of
substrate in the lumen
Extrinsic control by CNS centers
Intrinsic control by local centers
Receptors of the GI Tract
Mechano- and chemoreceptors respond to:
Stretch, osmolarity, and pH
Presence of substrate, and end products of digestion
They initiate reflexes that:
Activate or inhibit digestive glands
Mix lumen contents and move them along
Nervous Control of the GI Tract
Intrinsic controls
Nerve plexuses near the GI tract initiate short reflexes
Short reflexes are mediated by local enteric plexuses (gut brain)
Extrinsic controls
Long reflexes arising within or outside the GI tract
Involve CNS centers and extrinsic autonomic nerves
Digestive System Organs and Peritoneum
Peritoneum serous membrane of the abdominal cavity
Visceral covers external surface of most digestive organs
Parietal lines the body wall
Peritoneal cavity
Lubricates digestive organs

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Allows them to slide across one another
Mesentery double layer of peritoneum that provides:
Vascular and nerve supplies to the viscera
A means to hold digestive organs in place and store fat
Retroperitoneal organs organs outside the peritoneum
Peritoneal organs (intraperitoneal) organs surrounded by peritoneum
Homeostatic Imbalance
Peritonitis inflammation of the peritoneum caused by a piercing wound, perforating ulcer,
or burst appendix
Often, infected membranes tend to stick together localizing the infection
Generalized or widespread peritonitis is dangerous and often lethal
Treatment includes removing infectious debris and massive doses of antibiotics
Blood Supply: Splanchnic Circulation
Arteries and the organs they serve include
The hepatic, splenic, and left gastric: spleen, liver, and stomach
Inferior and superior mesenteric: small and large intestines
Hepatic portal circulation:
Collects nutrient-rich venous blood from the digestive viscera
Delivers it to the liver for metabolic processing and storage
Histology of the Alimentary Canal
From esophagus to the anal canal the walls of the GI tract have the same four tunics
From the lumen outward they are the mucosa, submucosa, muscularis externa, and serosa
Each tunic has a predominant tissue type and specific digestive function
Mucosa
Moist epithelial layer that lines the lumen of the alimentary canal
Its three major functions are:
Secretion of mucus
Absorption of the end products of digestion
Protection against infectious disease
Consists of three layers: a lining epithelium, lamina propria, and muscularis mucosae
Mucosa: Epithelial Lining
Consists of simple columnar epithelium and mucus-secreting goblet cells
The mucus secretions:
Protect digestive organs from digesting themselves
Ease food along the tract
Stomach and small intestine mucosa contain:
Enzyme-secreting cells and
Hormone-secreting cells (making them endocrine and digestive organs)
Mucosa: Lamina Propria and Muscularis Mucosae
Lamina Propria
Loose areolar and reticular connective tissue
Nourish the epithelium and absorb nutrients
Contains lymph nodes (part of MALT) important in defense against bacteria
Muscularis mucosae smooth muscle cells that produce local movements of mucosa
Mucosa: Other Sublayers

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Submucosa dense connective tissue containing elastic fibers, blood and lymphatic vessels,
lymph nodes, and nerves
Muscularis externa responsible for segmentation and peristalsis
Serosa the protective visceral peritoneum
Replaced by the fibrous adventitia in the esophagus
Retroperitoneal organs have both an adventitia and serosa
Enteric Nervous System
Composed of two major intrinsic nerve plexuses
Submucosal nerve plexus regulates glands and smooth muscle in the mucosa
Myenteric nerve plexus:
Major nerve supply that controls GI tract mobility
Segmentation and peristalsis are largely automatic involving local reflex arcs
Linked to the CNS via long autonomic reflex arc
Mouth
Oral or buccal cavity:
Is bounded by lips, cheeks, palate, and tongue
Has the oral orifice as its anterior opening
Is continuous with the oropharynx posteriorly
To withstand abrasions:
The mouth is lined with stratified squamous epithelium
The gums, hard palate, and dorsum of the tongue are slightly keratinized
Lips and Cheeks
Have a core of skeletal muscles
Lips: orbicularis oris
Cheeks: buccinators
Vestibule bounded by the lips and cheeks externally and teeth and gums internally
Oral cavity proper area that lies within the teeth and gums
Labial frenulum median fold that joins the internal aspect of each lip to the gum
Palate
Hard palate underlain by palatine bones and palatine processes of the maxillae
Assists the tongue in chewing
Slightly corrugated on either side of the raphe (midline ridge)
Soft palate mobile fold formed mostly of skeletal muscle
Closes off the nasopharynx during swallowing
Uvula projects downward from its free edge
Palatoglossal and palatopharyngeal arches form the borders of the fauces
Tongue
Occupies the floor of the mouth and fills the oral cavity when mouth is closed
Functions include:
Gripping and repositioning food during chewing
Mixing food with saliva and forming the bolus
Initiation of swallowing, and speech
Intrinsic muscles change the shape of the tongue
Extrinsic muscles alter the tongue’s position
Lingual frenulum secures the tongue to the floor of the mouth
Homeostatic Imbalance
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