Digestive system (copy)

ANP1107B

Functions of the digestive system

• Take in food

• Break it down into nutrient molecules

• Absorb molecules into the bloodstream

• Rid body of any indigestible remains

• Nutrient production

  • synthesis of vitamins by bacteria that live in the intestine (ie. vitamin K(for clotting factors), Vitamin B, biotin)

• production of neurotransmitters, hormones, and hormone-like compounds

  • hormones like grehlin, cholecystokinin, etc

Alimentary canal

• mouth

• pharynx

• esophagus

• small intestine

• large intestine

accessory organs

• teeth

• tongue

• salivary glands

• liver

• gallbladder

• pancreas

3 salivary glands

• Parotid: contains mostly serous cells

• Sublingual: contains mostly mucous cells

• Submandibular: contains mostly serous cells

Gastrointestinal tract activites

• Ingestion

• churning due to action of smooth muscles

• Mechanical breakdown

• Propulsion

  • swallowing

  • peristalsis

• Chemical Digestion

• Absorption

  • catabolism

  • segmentation

• Compaction (job of the colon/large intestine)and Defecation

Peristalsis vs segmentation

• Peristalsis

  • involuntary

  • adjacent segments of the alimentary tract organs alternately contract and relax

  • some mixing may occur

  • food is moved forward (distally along the tract)

• Segmentation

  • Nonadjacent segments of alimentary tract organs alternately contract and relax

  • food mixing (pinching/contraction and relaxation at different points)and breakdown; slow propulsion occurs.

  • food is moved forward and backward

Peritoneum

serous membrane that lines the abdominal cavity that consists of visceral peritoneum and parietal peritoneum

Visceral peritoneum

Membrane on external surface of most digestive organs

Parietal Peritoneum

membrane that lines body wall

Parietal cavity

• Fluid-filled space within abdomen, lined by the peritoneum/ contains most abdominal organs

• fluid lubricates mobile organs

Intraperitoneal

organs located within the peritoneum

Retroperitoneal

• Located outside of/posterior to the peritoneum

• includes most of pancreas, duodenum, and parts of large intestine

Mesentery

• double layer of peritoneum fused together that extends to the organs from the body wall mostly posterior.

• provides support for organs/hold them in place

• provides support for vessels and nerves supplying organs

• stores fat

Alimentary canal

• extends from mouth to anus

• most of the length made up of small intestine

• consists of 4 tissue layers

• as food moves thru the canal, it is broken down chemically by a variety of juices secreted by the organs of the digestive system

4 basic layers/ tunics of digestive organs

Mucosa(innermost layer) →Submucosa→ Muscularis externa→ serosa

functions of the mucosa

• secretion: mucus, digestive enzymes, hormones

• absorption: end products of digestion

• protection: against infectious disease

3 sublayers of mucosa

• Epithelium(avascular)

  • simple columnar epithelium rich in mucus-secreting (goblet) cells

  • mucus (protects digestive organs from enzymes; eases food passage)

  • may secrete enzymes and hormones (e.g. in stomach and small intestine)

• Lamina Propria(below epithelium)

  • loose areolar CT; capillaries for nourishment/absorption

  • lymphoid follicles (part of MALT)-protection

• Muscularis mucosae

  • smooth muscle that produces local movements of mucosa

Submucosa

• areolar connective tissue

• blood and lymphatic vessels, lymphoid follicles

• submucosal nerve plexus

• abundant in elastic fibers (allows stomach to regain shape after large meal)

Muscularis externa

• segmentation and peristalsis

• inner circular and outer longitudinal layers

• sphincters in organ-to-organ junct

Serosa

• Visceral peritoneum-outermost protective layer

• areolar connective tissue covered with mesothelium

• replaced by the fibrous adventitia(area where there is no serosa, just collagen fibers) in the esophagus (adventitia: fibrous connective tissue that ‘binds’ the esophagus to surrounding tissues

What type of organs have both adventitia and serosa

• retroperitoneal organs have both adventitia and serosa

• serosa on the side facing the peritoneal cavity and an adventitia (fibrous sheath) on the side against the dorsal body wall

Splanchnic circulation

• Splanchnic=related to viscera

• portal system=back to back capillary beds

Arteries that branch off the abdominal aorta to serve the digestive organs

• Hepatic, splenic, and left gastric of the celiac trunk (serve the liver, spleen and stomach)

• Inferior and superior mesenteric (serve small and large intestine)

Venous return from much of the abdominopelvic region is via inferior vena cava.

Venous return from the digestive viscera is indirect via the hepatic portal circulation.

Why does venous return from digestive viscera via hepatic portal circulation

Enteric nervous system (ENS)

• Semi-autonomous

• Also called the gut brain; enteric neurons that communicate extensively with each other/major nerve supply to GI tract wall that controls motility

• Has 2 plexus

• linked to CNS via AFFERENT visceral fibers

  • motor fibers of the ANS

    • sympathetic impulses

Submucosal nerve plexus (neurons)

• contains sensory and meotor neurons/regulates glands an smooth muscles in mucosa

Myenteric nerve plexus

• located between circular and longitudinal muscles

• provides major nerve supply to GI tract

• controls GI motility (pacemaker (sets pace/rhythm) cells and local reflex arcs between enteric neurons)

Short (local)reflexes

• in response to stimuli inside the GI tract (internal)

  • control patterns of segmentation and peristalsis

  • NS controls digestive activity via intrinsic controls

Long reflexes

• in response to stimuli inside (internal) or outside(external) the GI tract

  • involve CNS centers and autonomic nerves

  • Because GI tract can’t act independently of the body, thus we need long reflexes

  • NS controls digestive activity via extrinsic controls

3 key concepts that regulate GI activity

1. Digestive activity provoked by mechanical/chemical stimuli

2. Effectors of digestive activity are SM and glands

3. Nervous system (intrinsic and extrinsic) and hormones control digestive activity

Digestive activity provoked by mechanical/chemical stimuli

• receptors in walls of GI tract organs respond to stretch, changes in osmolarity and pH, the presence of substrate and end products of digestion

Effectors of digestive activity are SM and glands

• Receptors initiate reflexes that stimulate SM to mix and move lumen contents

• reflexes can also activate or inhibit

Anatomy of digestive system

Oral (buccal) cavity

• basically your mouth

• bounded by lips, cheeks, palate and tongue

• oral orifice is the anterior opening

• walls lined with stratified squamous epithelia

  • Don’t contain keratin like epidermis

• beginning of digestion and initiation of swallowing

  • food is chewed and mixed with enzyme-containing saliva

• Associated organs include: tongue, salivary glands, teeth

Lips and cheeks

• lips (labia): composed of fleshy obicularis oris muscle

• oral vestibule(a space): recess internal to lips and cheeks, external to teeth and gums

• Cheeks : composed of buccinator muscles

• oral cavity proper: lies within teeth and gums

• lingual frenulum: attaches tongue to the floor of the mouth

• labial frenulum: median attachments of each lip to gum

• Hard palate

• soft palate

Tongue

• is a skeletal muscle

• has :

  • intrinsic muscles that change shape of tongue

  • extrinsic muscles alter the tongue’s position

functions of tongue

• repositioning and mixing of food during chewing

• formation of bolus

• initiation of swallowing, speech, taste

Ankyloglossia (tied tongue)

• lingual frenulum is really short

• congenital condition

Parts of the tongue

Terminal sulcus marks division between

• Body: anterior 2/3 residing in the oral cavity

• Root: posterior third residing in oropharynx

Surface papillae (projections of lamina propria covered with epithelium):

1. Foliate—on the lateral aspects of the posterior tongue

2. Vallate—V-shaped row in back of tongue

3. Filiform—whitish, give the tongue roughness and provide friction

4. Fungiform—reddish, scattered over the

tongue

*Vallate, foliate and fungiform papilla contain taste buds involved in detecting the elements of taste perception

• taste qualities are found in all areas of the tongue, some regions are more sensitive than others

Salivary glands

• secretes saliva which:

  • cleanses mouth

  • dissolves food chemicals for taste

  • moistens food; compacts for taste

  • begins breakdown of starch with enzyme amylase

Intrinsic (buccal) glands

scattered in oral mucosa

extrinsic (major glands)

• lie out the mouth and release their secretion into the mouth via ducts

Glands

• composed of 2 types of cells:

  • serous cells: produce watery secretion, enzymes, ions, bit of mucin

  • Mucous cells: produce mucus

Composition of saliva

Control of salivation

• 1500 ml/day can be produced

• Minor glands continuously keep mouth moist

When are major salivary glands activated by ANS

salivation is primarily controlled by parasympathetic division

when:

• Ingested food stimulates chemo- & mechanoreceptors in mouth, send signals to:

  • Salivatory nuclei in brain stem that stimulate parasympathetic impulses along fibers in cranial nerves VII and IX to glands

  • sympathetic fibers (T1-T3)=slow down prod. of saliva/produces thick mucin-rich saliva

  • parasympathetic=more prod. of saliva

• Other stimuli:

  • Swallowing irritating foods; nausea(protective reflex to protect the mouth/throat to neutralize stomach acid); smell/sight of food or upset GI can act as stimuli

Chemoreceptors are activated by…

• acidic substances

mechanoreceptors are activated by

any mechanical stimulus in the mouth

Teeth

• lies in sockets(dental alveoli) in gum-covered margins of mandible and maxilla

• 2 sets of teeth

  • deciduous(milk) teeth →24 mo.

  • 32 permanent teeth→ develop by 6 yrs

Mastication(must know)

• process of chewing that tears and grinds food into smaller fragments

• production of bolus (lump) easy to swallow

Mechanical mastication

• closed lips and cheeks

• teeth

• tongue

Chemical mastication: enzyme

• breakdown of starch by salivary amylase

• breaking of fats by lingual lipase (in the stomach but with the enzyme produced in the mouth)

incisors

• chisel shaped tooth for cutting

Canines

• fanglike tooth that tear or pierce

Premolars (bicuspids)

• posterior to canines

• broad crowns with rounded cusps used to grind and crush

Molars

• broad crowns, multiple rounded cusps: best grinders

2 major regions of a tooth

• crown: exposed part above gingiva (gum)

  • outermost part=enamel(contains hydroxyapatite minerals)

    • no cells/organic material

  • dentin: sensitive to stimuli

  • periodontal ligament: fibrous connective tissue

• Root: portion embedded in jawbone-connected to crown by neck

Digestive process of the mouth

1. ingestion

2. mechanical breakdown by chewing

3. initiates propulsion by swallowing

4. starts chemical digestion of polysaccharides

Pharynx(throat)

• allows passage of food, fluids, and air

• food passes from mouth into oropharynx and then into laryngopharynx

• stratified squamous epithelia lining with mucus (bc of goblet cells) producing glands

  • external muscle layers consists of 2 skeletal muscle layers

    • inner layer of muscles runs longitudinally

    • outer pharyngeal constrictors encircle wall of pharynx

Esophagus

• Flat muscular tube (~25 cm) that runs from laryngopharynx to stomach

• food moves via peristalsis

• collapsed when not involved in food propulsion

• pierces diaphragm at esophageal hiatus(opening in diaphragm)

• joins stomach at cardial orifice

• has all 4 alimentary canal layers unlike mouth and pharynx

lower esophageal sphincter

• AKA Gastroesophageal (cardiac) sphincter

• surrounds cardial orifice

• keeps orifice closed when food is no swallowed

• mucus cells on both sides of sphincter help protect esophagus from acid reflux

Hiatal hernia

• a structural abnormality in which the superior part of the stomach protrudes slightly above the diaphragm.

• Since the diaphragm no longer reinforces the sphincter, gastric juice may enter the esophagus, particularly when lying down.

4 alimentary canal layers of esophagus

• Esophageal mucosa contains SSE that changes to simple columnar epithelium at the stomach

• Esophageal glands in submucosa secrete mucus to aid in bolus movement

• Muscularis externa: skeletal muscle superiorly; mixed with skeletal and smooth in middle; smooth muscle inferiorly

  • due to swallowing of voluntary and involuntary phases

  • example of structure and function

• Has adventitia instead of serosa

Deglutition

• AKA swallowing

• Pharynx & esophagus: passage of food from mouth to stomach

• Major function of both organs is propulsion that starts with deglutition (swallowing)

• Involves the tongue, soft palate, pharynx, esophagus, 22 muscle groups & 2 phases

2 phases of deglutition

• Buccal phase

  • voluntary contraction of tongue

  • ends when bolus leaves the mouth and stimulates tactile receptors in posterior pharynx

• Pharyngeal-esophageal phase

  • involuntary

  • control swallowing center in medulla and lower ponds

How is passage of food regulated

• 2 sphincters: upper and lower esophageal sphincters

• peristalsis: (involuntary muscle movements controlled by oblongata) and facilitated by mucus produced by submucosal glands

Stomach

• 3 layers: longitudinal, circular, and oblique layer

• a temporary storage tank that starts chemical breakdown of protein digestion

  • Converts bolus of food to paste-like chyme (bolus + gastric juice)

  • Empty stomach has ~50 ml volume but can expand to 4L

  • When empty, stomach mucosa forms many folds called rugae

    • helps stomach expand for larger meals

Major regions of the stomach

• Cardial part (cardia): surrounds cardial orifice

• Fundus: dome-shaped region beneath diaphragm

• Body: midportion. major part for mixing food

• Pyloric part: wider and more superior portion of pyloric region, antrum, narrows into pyloric canal that terminates in pylorus

  • Pylorus is continuous with duodenum through pyloric valve (sphincter controlling stomach emptying)

• Greater curvature: convex lateral surface of stomach

• Lesser curvature: concave medial surface of stomach

Lesser omentum(curvature)

• runs from liver to lesser curvature of stomach, where it becomes continuous with visceral peritoneum

Greater omentum(curvature)

• drapes inferiorly from greater curvature over intestine, spleen, and transverse colon; blends with mesocolon (mesentery that anchors large intestine to abdominal wall)

• basically covers anterior part of abdominal cavity

• Contains fat deposits and lymph nodes

ANS supplies what part of stomach

• Sympathetic fibers via splanchnic nerves via the celiac plexus(ganglion)

• Parasympathetic fibers are supplied (resting and digesting) via vagus nerve(1 of 12 cranial nerves)

Blood supplies what part of stomach

• arterial supply of blood is provided celiac trunk system

• venous drainage: drain hepatic portal vein

Muscularis externa

• is modified

• Three layers of smooth muscle

• Circular, longitudinal & inner oblique layer allows stomach to churn, mix, move & physically break down food

Mucosa layer

• also modified

• lines the lumen

• Simple columnar epithelium entirely composed of mucous cells

  • Secrete 2-layer coat of alkaline mucus

    • Surface layer traps bicarbonate- rich fluid layer that is beneath it

• Dotted with gastric pits, which lead into gastric glands that produce gastric juice

Microscopic anatomy of stomach

Glandular cells of the stomach

• Mucous neck cells

• parietal cells

• chief cells

• enteroendocrine cells

Mucous neck cells

• Secrete thin, acidic, slightly soluble mucus of unknown function

  • helps lubricate and protect gastric glands

• different from mucus of the surface epithelium

• specialized stem cells

Parietal cells

• secrete Hydrochloric acid (HCl), which denatures protein, activates pepsin, breaks down plant cell walls, and kills many bacteria

• secretes Intrinsic factor: a glycoprotein req. for necessary absorption of vit. B12 in small intestine(ilium)

  • without it=pernicious anemia

Chief cells

• secrete Pepsinogen (activated to pepsin by HCl & by pepsin itself

  • +ve feedback mechanism that is limited by amount of pepsinogen present

  • -ogen: inactive form

• secretes gastric lipases (digests ~15% of lipids)

Enteroendocrine cells

• secrete Hormones:

  • gastrin (needed for HCL secretion; opening of pyloric sphincter, regulating stomach’s secretion and motility)

  • ghrelin (stimulates appetite, gastric motility and opening) & somatostatin

• Paracrines: histamine and serotonin

  • paracrine: acts on structures close by

HCL formation

Parietal cells pump H+ (from carbonic acid breakdown) into stomach lumen via H+/K+ ATPase (proton pumps)

– As H⁺ is pumped into stomach lumen, HCO₃^- is exported back to blood via Cl− and HCO₃^- antiporter

• Resulting increase of HCO₃^- in blood leaving stomach is referred to as alkaline tide

– Cl− is pumped out to lumen to join with H+, forming HCl

Carbonic anhydrase forms carbonic acid

1. H₂CO₃ (carbonic acid) → HCO₃^- + H+ (bicarbonate + hydrogen ions)

2. H⁺ + K⁺ ATPase(antiporter) pumps:

• H⁺→ the lumen

• K⁺→ the cell.

(K+ returns to the lumen through membrane channels)

3. Cl^- in the interstitial fluid is exchanged for intracellular HCO₃^-.

4. Cl^- diffuses through membrane channels into the lumen

Mucosal barrier of stomach

• Harsh digestive conditions require stomach to be protected

• Mucosal barrier protects stomach and is created by 3 factors

3 factors that create mucosal barrier

1. Thick layer of bicarbonate-rich mucus

2. Tight junctions between epithelial cells

• Prevent gastric juice from seeping underneath tissue

3. Damaged epithelial cells are quickly replaced by division of intestinal stem cells (ISC)

   • to avoid holes or sores in stomach

• Surface cells replaced every 3–6 days

Digestive processes of the stomach

• Holding area for food

  • Propulsion

  • Mechanical breakdown of food

  • Chemical Digestion

  • Absorption

  • Delivers chyme to small intestine

Propulsion activity of the stomach

• exhibits peristalsis

Mechanical breakdown of food in stomach

• caused by churning action by smooth muscle of stomach during peristalsis

Chemical digestion of food in stomach

• HCl denatures proteins by HCl in preparation for enzymatic digestion

• Pepsin: most important protein digesting enzyme is produced by gastric mucosa

  • In infants: milk protein (casein) is broken down by rennin(chymosin), secreted by stomach glands

  • Results in curdy substance

• gastric and lingual lipases acting in acidic pH of stomach aid in fat digestion

Absorption

• Not much absorption in the stomach

• 2 common lipid-soluble substances:

  • alcohol and aspirin are absorbed into blood via stomach mucosa

what is the only stomach function essential to life

• secretion of intrinsic factor for vitamin B12 absorption

  • B12 is needed for red blood cells to mature

  • Lack of intrinsic factor causes pernicious anemia

    • Treated with B12 injections

How is gastric secretion regulated

• Gastric mucosa secretes >3 L of gastric juice/day and are regulated by

  • Neural mechanisms:

  • Hormonal mechanisms

Neural mechanisms that controls gastric secretion

• Vagus nerve stimulation increases secretion

  • are parasympathetic

  • release ACh stimulates output of gastric juice

• sympathetic stimulation decreases secretion

Hormonal control of gastric secretion

• Gastrin stimulates enzyme and HCL secretion

• Gastrin antagonists are secreted by small intestines

3 phases of gastric secretions

1. Cephalic (reflex) phase

2. Gastric phase

3. intestinal phase

Cephalic (reflex) phase

• occurs before food enters the stomach

• reflexes triggered by sensory receptors in the head (sight, smell, taste), or by thought

  • triggers act via vagus nerve to stimulate gastric gland, preparing stomach to begin digestion

Gastric phase

• occurs once food enters stomach

• Lasts 3–4 hours & provides 2/3 of gastric juice

• Stimuli: distension, peptides and amino acids

  • Stretch (mechano) receptors – detect distention of stomach / initiate neural (both long & short) reflexes

  • Chemo receptors - chemical stimuli, e.g. peptides, caffeine, & low acidity activation of enteroendocrine (G) cells in lumen of stomach

    • detects chemical changes in composition of contents in stomach

role of gastrin in stimulation of gastric phase

• Buffering action of ingested proteins causes pH to rise, which activates more gastrin secretion

  • Prods parietal cells to secrete HCl by:

    • 1. Binding to receptors on parietal cells (directly)

    • 2. Stimulating enteroendocrine cells to release histamine (indirect)

Inhibition of gastric phase

• Low pH inhibits gastrin secretion

  • Occurs between meals

  • Occurs during digestion as negative feedback mechanism

    • The more protein, the more HCl acid is secreted, causing decline in pH, which inhibits gastrin secretion

Intestinal phase

• Begins with brief stimulatory component followed by inhibition

Stimulation of intestinal phase

• Partially digested food enters small intestine, causing a brief release of intestinal (enteric) gastrin

  • Encourages gastric glands of stomach to continue secretory activities

  • Stimulatory effect is brief and overridden by inhibitory stimuli as intestine fills

Inhibition by intestinal phase

• Four main factors in duodenum cause inhibition of gastric secretions:

  • Distension of duodenum due to entry of chyme

  • Presence of acidic / fatty / hypertonic chyme

• they prevent massive influx of chyme

• inhibition is achieved in 2 ways

  • enterogastric reflex and enterogastrones

Enterogastric reflex (neural)

• Duodenum inhibits acid secretion in stomach by: ENS short reflexes & long reflexes involving SNS and vagus nerve