Digestive System: Physiology

What is the alimentary canal and what organs does it include?

-continuous muscular tube that winds through the body from the mouth to the anus

-include mouth, esophagus, pharynx, stomach, small intestine, and large intestine

What are accessory organs and what organs are included?

-assist with digestion

-teeth and tongue inside oral cavity

-others outside the alimentary canal and interact via secretions (salivary glands, liver, pancreas, gallbladder)

Functions of the digestive system

-Ingestion: eating

-Propulsion: swallowing (oropharynx), peristalsis (esophagus, stomach, small intestine, large intestine; squeezing of tube to propel forward)

-Mechanical breakdown: chewing (mouth), churning (stomach), segmentation (small intestine; food moved back and forth to mix food)

-Digestion: enzymes break down food molecules into chemical building blocks

-Absorption: passage of digested end products (vitamins, minerals, water) by active or passive transport into blood or lymph

-Defectation: eliminates indigestible substances from body via the anus

Peritoneum of the digestive system

-serous membrane of abdominal cavity

-visceral covers external surfaces of most digestive organs and parietal lines the body wall

-peritoneal cavity lies between the visceral and parietal peritonea

-Mesentery: specialized, double layer of peritoneum that anchors digestive organs to the body wall; provide routes for blood vessels, lymphatics, and nerves to reach the digestive viscera; hold organs in place; store fat

-most organs intraperitoneal and suspended from body wall by dorsal mesentery, but some may be suspended by ventral peritoneal or retroperitoneal

What is the histology of the alimentary canal?

Same basic tunics: mucosa, submucosa, muscularis externa, and serosa

Mucosa

• secrete mucus, digestive enzymes, hormones; absorb the end products of digestion into blood; protect against infectious disease

• epithelium (simple columnar), lamina propria (loose areolar connective tissue), muscular mucosae (smooth muscle that produce movements to enhance absorption/secretion)

Submucosa

• Areolar connective tissue with blood and lymphatic vessels, lymphoid follicles, and nerve fibers to supply surrounding tissues

• Elastic fibers enable stretch during big meal but recoil after

Muscularis externa

• Segmentation and peristalsis, circular layer for diameter control, longitudinal layer for length control

Serosa

• Areolar connective tissue covered with mesothelium (squamous epithelial) except in esophagus where it is covered with adventitia (dense connective tissue)

What is the splanchnic circulation?

Blood supply to the digestive tract

-includes arteries that branch off abdominal aorta to serve digestive organs and hepatic portal circulation

-arterial supply: receives 1/4 of cardiac output, but increases after meal

-Hepatic portal circulation: collects nutrient-rich venous blood draining from digestive viscera and delivers it to liver

In order to do absorption in digestion, we need nutrients from the blood

What is the enteric nervous system?

-in house nerve supply of alimentary canal (Gut-brain)

-staffed by enteric neurons that communicate to regulate digestive system activty

-Short reflexes: have internal stimuli (GI stretch, pH, nutrient/solute concentration) → chemoreceptors/osmoreceptors/mechanoreceptors → local intrinsic nerve plexus → secrete effectors (smooth muscle or glands) → response (change in contractile or secretory activity)

-Long reflexes: have internal stimuli (GI stretch, pH, nutrient/solute concentration) → chemoreceptors/osmoreceptors/mechanoreceptors or external stimuli → central nervous system activated through visceral afferents → CNS has external autonomic efferents that initiate local nerve plexus, effectors, and response

Ingestion of the mouth

-Boundaries: lips anteriorly, cheeks laterally, palate superiorly, tongue inferiorly, posteriorly continuous with oropharynx

-Lips and cheeks: help keep food between teeth, composed of skeletal muscle

-Palate: Hard palate forms rigid surface against which the tongue forces food during chewing, soft palate rises reflexively to close off nasopharynx when we swallow, palatoglossal arches form boundaries of the fauces

-Tongue: interlacing bundles of skeletal muscle fibers, grips food, mixes food with saliva (forms bolus), initiates swallowing; intrinsic muscles allow tongue to change shape while extrinsic extend from origin to bones and allow change in shape and position, papillae on tongue house taste buds

-Salivary glands: cleanse mouth, dissolves food chemicals, moistens food, contains enzyme amylase to begin digestion

What is the function of teeth?

-responsible for tearing and grinding food during chewing

-classified according to shape and function: incisors shaped like chisels for cutting or nipping off; canines shaped like fangs for piercing and tearing; premolars and molars contain cusps for grinding and crushing

-Two regions: crown and root, cement covers root and anchors to peridontal ligaments, made of dentin under enamel

Mastication (chewing)

-cooperative effort performed by structures of the mouth

-cheeks and lips hold food between teeth

-tongue mixes food with saliva to soften it

-teeth cut and grind food into smaller morsels

-partially voluntary and partially reflexive process: voluntarily put food into mouth and contract muscles that control our jaws, pattern and rhythm of continued jaw movement are regulated by stretch reflexes and pressure inputs from cheeks, gums, and tongue

Pharynx and esophagus

-food passes into oropharynx and into laryngopharynx during swallowing

-esophagus is a muscular tube running from laryngopharynx to stomach

• collapsed when not involved in food propulsion

• pierces diaphragm at esophageal hiatus

• joins stomach at cardiac orifice

Process of deglutition (swallowing)

1. Buccal phase: food still in mouth, tongue presses against hard palate forcing bolus into oropharynx

2. Pharyngeal-esophageal phase: tongue blocks the mouth, soft palate and uvula rise, larynx rises so epiglottis blocks trachea, food enters esophagus

3. Pharyngeal-esophageal phase continues: constrictor muscles of pharynx contract to force food into esophagus inferiorly, upper esophageal sphincter contracts after food enters

4. Peristalsis moves food through the esophagus to stomach

5. Gastroesophageal sphincter surrounding cardial orifice opens, sphincter closes after food enters stomach to prevent regurgitation

Gross anatomy of the stomach

-Cardia: surrounds cardial orifice through which food enters from esophagus

-Fundus: dome-shaped part, tucked beneath diaphragm

-Body: midportion, continuous with pyloric part

-Pyloric sphincter/valve: controls stomach emptying

-Greater curvature: attaches to greater omentum

-Lesser curvature: attaches to lesser omentum

-In muscularis externa has extra oblique layer that helps in mechanical destruction of food

Mesenteries of abdominal digestive organs

-Lesser omentum: runs from liver to lesser curvature, continuous with visceral peritoneum covering stomach

-Greater omentum: drapes inferiorly from greater curvature of stomach to cover coils of small intestine, wraps spleen and transverse portion of large intestine before blending with mesocolon

• Has fat deposits that give it appearance of lacy apron, contains lymph nodes (immune cells and macrophages)

Microscopic anatomy of stomach

-Gastric pits that lead into tubular gastric glands to produce stomach secretion called gastric juice

-Mucous neck cells: provide thin acidic mucus

-Parietal cells: secrete hydrochloric acid and intrinsic factor, bear dense microvilli to provide huge surface area for secretion in stomach lumen; help in activating pepsin

-Chief cells: produce pepsinogen to secrete the stomach enzymes for breakdown

-DNES cells: endocrine, secrete hormones (gastrin) and paracrine for signaling

-Enteroendocrine cells: deep in gastric glands, release chemical messengers into interstitial fluid of lamina propria; histamine and serotonin act as paracrines, somatostatin act as hormones and paracrines, gastrin responsible for regulating stomach secretion and motility

Regulating gastric secretion: Cephalic phase

Stimulatory

-sight and thought of food, stimulation of taste and smell receptors

-activates cerebral cortex→ hypothalamus and medulla oblongata→ vagus nerve→ stimulate stomach secretion activity

Inhibitory

-Loss of appetite, depression

-activates cerebral cortex→ decreases parasympathetic activity→ decrease in stomach secretion

Regulating gastrin secretion: Gastric phase

Stimulatory

-stomach distension activates stretch receptors→ long reflexes via medulla and vagus nerve or short reflexes→ increase stomach secretion

-food chemicals (especially peptides and caffeine) and rising pH activate chemoreceptors → G cells activated → Gastrin release in blood → increase stomach secretion

Inhibitory

-Excessive acidity of pH<2 in stomach → G cells activated→ decrease gastrin release to blood→ decrease stomach secretion

-Emotional stress → sympathetic nervous system activation overrides parasympathetic controls→ decrease stomach secretion

Regulating gastrin secretion: Intestinal phase

Stimulatory

-presence of partially digested foods in duodenum or distension of the duodenum when stomach begins to empty → intestinal (enteric) gastrin release to blood → brief effect → increase stomach secretion

Inhibitory

-distension of duodenum, presence of fatty/acidic/hypertonic chyme→ enterogastric reflex (involves both short and long reflexes) or release of enterogastrones (secretin, cholecystokinin)→ decrease stomach secretion

Mechanism of HCl secretion

1. H+ and HCO3- are generated from the dissociation of carbonic acid (H2CO3) produced from CO2 and H2O by carbonic anhydrase

2. H+-K+ ATPase pumps H+ into the lumen and K+ into the cell. K+ returns to the lumen through membrane channels

3. HCO3- leaves the cell (alkaline tide) to blood capillary in exchange for interstitial fluid Cl-

4. Cl- diffuses through membrane channels into the lumen

Gastric contractile activity

1. Propulsion: peristaltic waves move from the fundus toward the pylorus

2. Grinding: Most vigorous peristalsis and moving action occur close to the pylorus, pyloric end of stomach acts as a pump that delivers small amounts of chyme into the duodenum

3. Retropulsion: peristaltic wave closes the pyloric valve, forcing most of the contents of the pylorus backward into the stomach

Regulating gastric emptying

-stomach empties within 4 hours after a meal

-larger meals + more liquid = faster emptying

-rate of emptying also depends on contents of duodenum (proximal portion of small intestine)

• Presence of fatty/hypertonic/acidic chyme in duodenun

• Duodenal enteroendocrine cells secrete enterogastrones (secretin, cholecystokinin)→ decrease contractile force of stomach and rate of stomach emptying

• Chemoreceptors and stretch receptors → trigger enterogastric reflex → short reflex via enteric neurons and long reflex via CNS centers (increase sympathetic, decrease parasympathetic)