CH. 25 | Digestive System

Digestive is 30 pt quiz and Resp. 20pt

Digestion

• The nutrients in our food generally is not a form we can use

• First must be broken down into smaller components such as amino acids that are universal to all species

• The digestive system is basically a disassembly line, breaking down the food into nutrients that can be use, absorbing them so that they can be distributed in the body

• Five functions of digestions

  • Ingestion: selective intake of food

  • Digestion: mechanical and chemical breakdown of food

  • Absorption: uptake of nutrient molecules in our tissues

  • Compaction: absorbing water, compacting indigestible residue

  • Defecation: elimination of feces

General Anatomy

• GI system has two subdivisons; the digestive tract and accessory organs

• The digestive tract is open to the enviroment at both ends

• Includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine

• The stomach and intestines are considered part of the GI tract

• The teeth, tongue, salivary glands, liver, gallbladder, and pancreas are considered accessory organs

• In a sense, Defecated residual was never in the body

Structural plan of Digestive system

• Mucosa made of…

  • stratified squamous epithelium

  • lamina propria

  • muscularis mucosae

• Submucosa

  • esophageal gland

• Muscularis externa

  • inner circular layer

  • outer longitudinal layer

• Serosa

  • Areolar tissue

  • mesothelium

Tissue Layers

• Inner lining of the tract is called the mucosa

  • Consists of an epithelium, a loose connective tisue layer called the lamina propria, and a thin layer of smooth muscle called the muscularis mucosa

  • The epithelium is simple columnar is most of the tract, but stratified squamous from the oral cavity through the esophagus, and in the lower anal canal - areas most subject to abrasion

• The muscosa exhibits an abundance of lymphocytes and lymphoid nodules called the mucosa-associated lymphoid tissue (MALT)

• The submucosa is a thicker layer of loose connective tissue containing blood vessels, lymphatics, a nerve plexus, and glands that secrete mucous, and the MALT

• The muscularis externa consists usually of two layers of muscle near the outer surface, an inner layer that surrounds the lumen, and an outer layer that runs longitudinally

• The serosa is a think layer of areolar tissue topped by simple squamous epithelium

  • It begins near the end of the esophagus and ends just before the rectum

  • The pharynx and most of the esophagus, and the rectum have no serosa but are surrounded by a fibrous connective tissue called the adventita which blends in with the surrounding tissues

• The esophagus, stomach, and intestines have a nerous network called the enteric plexus which regulated motility, secretion, and blood flow. It can function independently from the CNS, but the CNS has significant influence on its actions. Has sensory neurons to sense stretch

  • Has two networks, the submucosal plexus in the submucosa, and the myenteric plexus of ganglia and nerve fibers btw the layers of the muscularis mucosa

  • The myenteric plexus controls peristalsis (series of movements that move substances through the body) and other contractions of the muscularis externa

  • Unlike the rest of the autonomic nervous system, the enteric plexus includes sensory neurons which monitor tension in the gut wall and conditions in the lumen

Relationship to the Peritoneum

• In processing food, the GI tract twists and turns, and needs freedom of movement

• The intestines are not tightly bound to the abdominal wall and is not connected to the abdominal wall but suspended by a sheet of connective tissues called the mesentery

  • the mesentery is a complexly folded serous membrane

  • holds the abdominal viscera in their proper relationship, provides for passage of blood vessels and nerves, and contain many lymph nodes

• The parietal peritoneum is a serous membrane that lines the abdominal wall. Along the posterior wall, it turns inward to form the posterior mesentery

• Upon reaching an organ, the two layer divide passing around the organ forming the serosa

• Two layers of the mesentery associated with the stomach form the omentum

  • the greater omentum hangs like an apron loosely covering the small intestine

  • The colon is supported by a portion of the mesentery called the mesocolon

  • When an organ is surrounded by the mesentery, it is referred to as intraperitoneal

  • If only covered on the anterior surface by the mesentery, said organ is referred to as retroperitoneal

• retroperitoneal - pancreas, kidneys, duodenum

The Mouth

• mouth lined with squamous epithelum

• The functions of the mouth include:

  • ingestion, taste and other sensory responses to food, mastication, chemical digestion, swallowing, speech, and respiration

• The checks and lips retain food and push it btw the teeth for chewing, speech, and suckling by infants

• The vestibule is the space btw the cheeks and teeth where we push a toothbrush

• Oral cavity

  • Vestibule

  • platoglossal

  • platopharyngeal

  • palatine tonsil

  • tongue

  • salivary duct orifices:

    • sublingual & submandibular

  • upper and lower lip

  • superior labial frenulum

  • hard palate and palatine rugae

  • uvula of soft palate

  • inferior labial frenulum

The tongue

• it muscular and bulky but agile, has sensory receptors for taste, texture, and temp

• it maneuvers food for chewing, secretes mucous & enzymes

• it initiates swallowing and is necessary for speech

• it has taste buds and is covered with keratinized squamous epithelium

• The anterior 2/3 is the body occupying the oral cavity, the posterior 1/3 is the root and occupies the oropharynx

• the extrinsic muscles produce the stronger tongue movement of food manipulation

• within the tongue are serous and mucous lingual glands which secrete a portion of saliva

The palate

• the palate separates the nasal from the oral cavity, making possible to breathe while eating

• The anterior part is the hard palate which is boney

• Posterior to it is the soft palate, soft and spongy but has no bone

• It has a concial medial extension, the uvula which helps retain food in the mouth until ready to swallow

• The palatine tonsils are located btw the posterior arches

Teeth

• teeth are called dentition

• break food down into smaller pieces

• Adults normally had 16 teeth in the mandible, and 16 in the maxilla

• From the midline to the rear there are 2 incisors, a canine, 2 premolars, and up to 3 molars on each side

  • incisors are for cutting food, the canines to puncture and shred it, but are much shorter than other mammals through evolution

  • the molars and premolars are broad and bumpy for grinding food

• Each tooth is embedded in a socket called the alveolus

  • the alveolus is lined by a periodontal ligament whose collagen fibers penetrate the bone on one side and the tooth on the other, anchoring the tooth but allows for slight movement while chewing

• The teeth must combine strong durability with sensitivity and neural control if they are to maintain the integrity over a lifetime of shredding and pulverizing a vast amount of food

• They connect to the brain and tell us when we’ve chewed enough to swallow the food

• the gums or gingiva covers the alveolar bone, The space btw the tooth and gum is the gingival sulcus, important in dental health

• Most of the teeth consists of dentin, covered with enamel in the crown and cement in the root. Dentin and root are living tissue which can regenerate, enamel cannot regenerate

• Internally, the tooth has a pulp cavity occupied by a pulp, containing blood, nerves, and lymphatic vessels

• The teeth develop beneath the gums and erupt in a predictable order

• 20 deciduous teeth erupt from the ages of 6 to 20 months beginning with the incisors

• Btw 6 and 25 years of age, these are replaced by up to 32 permanent teeth

• the thrid molars, wisdom teeth, erupt around 17 to 25. They are often below the gums and become impacted

Saliva and the Salivary Glands

• Saliva moistens and cleans the mouth, inhibits bacterial growth, dissolves molecules so they can stimulate the taste buds, digests a little starch and fat, and makes swallowing easier by penetrating the food and make into a bolus

• hypotonic, mostly water, with a pH of 6.8 to 7.0 and has the following solutes”

  • Mucus: binds and lubricates the food bolus

  • Electrolytes: Na+, K+, phosphate, & bicarb

  • Lysozymes: kills the bacteria

  • IgA: antibacterial antibody

  • Salivary amylase: begins starch digestion

  • Lingual lipase: begins the digestion of fat

• There are two kinds of salivary glands

  • the intrinsic salivary glands: small, located in the oral mucosa, and the distributed through the oral cavity. Secrete a think kind of salivary fluid containing lipase and lysozyme, at a constant rate whether we are eating or not

    • prevent bacteria from forming colonies on the teeth, promote healing of oral wounds, w/o them one can develop softening of the teeth and can’t chew properly

  • The extrinsic salivary glands are 3 pairs of more discrete organs located outside the oral mucosa composed of both mucous and serous cells

• The parotid glands just anterior to the ear lobes, its duct enters the oral cavity at the 2nd molar

• The submandibular glands, located halfway along the body of the mandible, their ducts empty into the mouth near the lower incisors

• The sublingual glands located in the floor of the mouth. They have multiple ducts that empty posterior to the submandibular gland ducts

Oral Cavity

• 1 to 1.5 liters of saliva a day

• salivation is controlled by the parasympathetic nerve fibers that come from the salivary nucleus located in the brainstem btw the pons and the medulla

• Sympathetic nerve fibers also innervate the salivary glands originating in the sympathetic chain in the neck, producing only a slight amount of saliva but may inhibit salivation during fear or nervousness (dry mouth)

• The salivary nuclei respond to food in the mouth, irritants, spicy foods and toxins, & stimulate salivation

• Salivary amylase begins to digest starch when chewed

• w/o mucous, one would have to drink much more liquid to swallow food

The Pharynx

• A muscular tube connecting the oral cavity to the esophagus and nasal cavity to the larynx

• It is the point where digestion and respiration intersect

• It has a deep layer of circular skeletal muscle arranged longitudinally, and a superficial muscle arranged circularly

• the circular muscle is divided into superior, middle, and inferior pharyngeal constrictors which force food downward when swallowing

• This constriction is the Upper esophageal sphincter, it is a physiologcal sphincter rather than an anatomical one

The Esophagus

• begins at C6 to mid thoracic

• the opening into the stomach is called the cardiac orifice. Food pauses momentarily at this point bc of a constriction called the lower esophageal sphincter

• The LES prevents food from regurgitating into the esophagus and protects the esophageal mucosa from gastric acid

• The wall of the esophagus is organized as described earlier. The submucosal contains esophageal glands which secretes a lubricating mucous

• The muscularis externa is composes of skeletal muscle in the upper 1/3 of the esophagus, a mixture of skeletal and sooth muscle in the middle third, and only smooth muscle in the lower third

Swallowing

• Swallowing is a complex action involving over 22 muscles in the mouth, pharynx, and esophagus coordinated with the swallowing center, a pair of nuclei, in the medulla

• The center communicates this action through the trigeminal, facial, glossopharyngeal and hypoglossal nerves. Swallowing involved three phases

• The oral phase, under voluntary control

  • food undergoes the process previously described, and goes around the epiglottis posteriorly into the larygnopharynx

  • To prevent chocking, breathing is suspended, and the vocal cords adduct to close the airway

• The esophageal phase is wave of involuntary contraction called peristalsis. The circular muscle constricts above and pushes the bolus down, the longitudinal muscles while the circular rests. This occurs regardless of position.

• Liquid takes 1 to 2 seconds, and food bolus 4 to 8 seconds. The LES relaxes and food enters the stomach

The Stomach

• The stomach is a muscular sac in the upper left of the abdominal cavity

• It has an internal volume of 50 ml when empty and 1.0 to 1.5 L after a meal

• When extremely full, it can hold up to 4L and extend nearly as far as the pelvis

• It mechanically breaks down food particles, liquefies food and begins the chemical digestion of proteins and fat

• This produces an acid, soupy or pasty mixture of semi digested food called chyme

• Most digestion occurs after the chyme passes into the small intestine

Gross Anatomy of Stomach

• the stomach is J shapes, relatively vertical in tall people and more horizontal in short people

• The stomach is divided into four regions

  • The cardiac part is a small area with in 3 cm of the cardiac orifice

  • the fundic region (fundus) is the dome superior to the esophageal attachment

  • The body is the greatest part distal to the cardiac orifice

  • The pyloric part is a narrower pouch at the inferior end; it is subdivided into a funnel like antrum and a narrower pylori canal

• The latter terminates at the pylorus a narrow passage into the duodendum

• The pylorus is surrounded by a thick ring of smooth muscle, the pyloric sphincter which regulates passage of the chyme into the duodendum

The Stomach - Innervation & Circulation

• the stomach receives parasympathetic fibers from the vagus and recivees sympathetic fibers from the celiac ganglia

• Is is also regulated by the extensive enteric pelxus described earlier

• The stomach receives blood form branches of the celiac trunk

• All blood drained from the stomach and intestines enters the hepatic portal circulation and filter through the liver before returning to the heart

The Stomach - Microscopic Anatomy

• The mucosa is covered by a simple columnar epithelium

• The apical region the cells are filled mucin - after secreted it fills with water and becomes mucous

• The lamina propria is almost entirely occupied by tubular glands

• The gastric mucosa is pocked with depressions called gastric pits, lined with the same columnar epithelium. 2 or 3 tubular glands open into the bottom of such puts

• In the cardiac and pyloric regions, they are called cardiac and pyloric glands, the rest of the stomach they are called gastric glands

The Gastric Glands

• Mucous cells - secrete mucous predominate in the cardiac and pyloric regions

• Regenerative (stem) cells: Found in the base of the pit and neck of the gland, divide rapidly to produce a continual supply of new cells which migrate up to the gastric surface

• Parietal Cells: found mostly in the upper half and secrete hydrochloric acid, intrinsic factor, and an appetite stimulating the hormone ghrelin. They are found mostly in the gastric glands.

• Chief cells: are the most numerous cells and secrete gastric lipase and pepsinogen. They dominate the lower half of the gastric glands but are absent form the glands in the cardiac and pyloric regions.

• Enteroendocrine cells: mostly in the lower end of the glands and secrete hormones and paracrine messengers that regulate digestions. There are 8 kinds of these cells each secretes a different chemical messenger.

• In general, the cardiac and pyloric glands secrete mainly mucous; acid and enzyme secretion mainly in the gastric glands and hormones secreted throughout the stomach.

Gastric Secretions

• The gastric Glands produce 2 to 3 L of gastric juice per day

• HCl acid-gastric juice has a pH as low as .8 due to the HCl

• Pepsin: secreted as inactive proteins called zymogens and are convered to active enzymes by removal of amino acids. Chief cells secrete a zymogen called pepsinogen which is converted to pepsin which digests dietary proteins

• Gastric lipase: secreted by chief cells, and digests10-15% of fat in the stomach; the rest is digested in the small intestine

• intristic factor: secreted by parietal cells, is essential for absorption of B12 in the small intestine. W/o IF, people can develop pernicious anemia as an autoimmune disease in which gastric mucosa is destroyed.

• Chemical messengers: by the enteroendocrine cells, some 20 hormones. These include substance P, VIP, secretin, GIP, cholecystokinin, & neuropetide Y

Hydrochloric Acid

• parietal cells pump H+ into the gastric lumen via a pump call the H+/K+ ATPase. This is an antiport that uses the energy of ATP to pump out the H+ and K+ into the cell

• The pH of the cell is not affected bc the H+ is pumped out as quickly as it is formed. The HCO3- is exchanged for Cl- from the blood plasma the chloride shift process mentioned before

• stomach acid has several functions:

  • activates the enzyme pepsin

  • it breaks up connective tissue and plant cells helping to liquefy the food

  • converts ingested ferric ions (Fe+++) into ferrous (Fe++) ions a form that can be absorbed and used for hemoglobin synthesis

  • it contributes to innate immunity be destroying most ingested pathogens

Gastric Motility

• when food is swallowed mechanoreceptors in the pharynx transmit signals to the medulla, which relays via the vagus to the stomach which results in the receptive-relaxation response which relaxes the stomach receive food

• soon the stomach show a rhythm of peristalic contraction set off by the enteric pacemaker cells in the muscularis externa

  • churns the food and mixes it with the gastric juice

• the antral pump breaks up the chyme into smaller pieces for the small intestine, only a small amount of chyme is allowed into the small intestine at a time

• A typical meal is emptied from the stomach in about 4 hours: it takes less time if the meal is mostly liquid and as long as 6 hours if the meal is mostly fat

Vomiting

Protection of the Stomach

• The stomach is protected in three ways

  • Mucous coat: the highly alkaline mucous resists the action of acid and enzymes. The mucous resists the action of acid and enzymes. The mucous also provides signals to dendritic cells that promotes tolerance to food and antigens and benefical bacteria

  • Tight junctions: The epithelial cells are joined by tight junctions that prevent seepage of gastric juice seeping btw them and digesting the connective tissue below

  • Epithelial cell replacement: The epithelial cells only survive 3-6 days and are sloughed off into the chyme and digested

• A breakdown of the protection can result in the inflammation or peptic ulcer formation

Regulation of Gastric Function

• Gastric activity is divided into 3 stages called the cephalic, gastric, and intestinal phases: the phases overlap and all occur simultaneously

  • The cephalic stage is when the stomach responsds to the mere sight, smell, taste, or thought of food, the sensory input goes to the thalamus, realys signals to the medulla which sends signals via the vagus to the stomach. This stimulates the parietal cells and acid output begins 40% of acid output occurs during this stage.

  • The gastric stage is when food and protein enter the stomach. Gastric secretion is stimulated by 3 chemical, ACTH, histamine, and gastrin. Gastrin is a hormone produced by eneroendrocrine G cells. All three stimulate the parietal cells to produce HCl. The amino acids produced by protein breakdown also stimulates the G cells to secrete even more gastrin. Finally, the acid level rises, the pH falls to 2 at which time the parietal cells are inhibits to produce more HCL

Liver

• two lobes

• round ligament

• gallbladder

• abt 3 lbs

• hepatic lobules

• heptaocytes

  • absorb for metabolism or storage

• hepatopancreatic ampulla

  • contains a sphincter

Hepatic circulation

• portal vein and hepatic arteries

• lipids go to lymphatic vessels instead of liver

• hepatic vein blood is venous blood

The Gall Bladder & Bile

• bag under liver

• simple columnar epithelilium

• acts as a reservoir

• bile is made up of

• lipid transport vesicles called micells

• urobilinogen

  • -ogen is a precursor enzyme

  • stercobilin

Gallstones

• mostly cholesterol

The Pancreas

• retroperitoneal

• ampulla

• why is pancreatic juice alkaline - chyme is neutralized by alkaline

• zymogens

  • alkaline activates the zymogens

Small Intestine

• need more surface area

• intestinal cytes

• paneth cells - and function

Intestinal Motitlity

Carbohydrates

• most digestable is starch

• cellulose is not digestable

• SGLT

• solvent drag?

• symport??

• transport videos