Digestive system

Two main groups of organs of the Digestive system

Alimentary canal/ GI tract and Accessory digestive organs

Alimentary canal ( GI tract), What is it and the organs

Digest food into small fragments and absorb them through mucosal lining into blood

Mouth, Pharynx, Esophagus, Stomach, Small and Large intestine, anus

Accessory Digestive organs

Teeth, tongue, galbladder,

Salivary glands, liver, pancreas ( use ducts to secrete digestive fluids into GI tract

The six essential activities of the Digestive system

Ingestion, Propulsion, Mechanical Breakdown, Digestion, Absorption and Defecation

Ingestion

Eating

Propulsion

Movement of food through the alimentary canal includes

Swallowing (voluntary)

Peristalsis: Primary means of propulsion involves alternating ( involunatry) waves of contraction and relaxation

Mechanical breakdown

Increases surface area of nutrients, prepares food for chemical digestion by enzymes includes: Chewing, mixing food with saliva ( via toungue), churning food in the stomach, and segmentation

Segmentation

local contraction of SI (small intestine) that mix food with digestive fluids

Digestion

catabolism via enzymes, breaking down large food molecules into absorbable chemical building blocks

Absorption

Passage of nutrients from the lumen of the GI tract through the mucosal epithelium into the blood or lymph

Defecation

Elimination of indigestible substances in the form of feces

peritoneum

Serous membranes of abdominopelvic cavity that conssist of visceral and parietal peritoneum

Peritoneal cavity

space between two membranes contains serous fluid, which lubricates mobile organs as they change shape and move against each other

Mesentery

double layer of peritoneum, stores fats; provides a route for blood vessels, lymphatics, and nerves to reach organs

Most importantly hold digestive organs in place!!!

Intraperitoneal organs

surrounded by peritoneum anchored to body wall by mesentery

Retroperitoneal organs

lie posterior to parietal peritoneum, include Most of the pancreas and duodenum, and parts of large intestine

Peritonitis

inflammation of Peritoneum, usually caused by rupture of appendix

The four basic layers (tunics) of the alimentary canal

Mucosa, Submucosa, Muscularis externa, and serosa

Mucosa layer of the alimentary canal and its function

innermost layer in contact with luminal contents

function

secreting mucus, digestive enzymes and hormones

Absorb end products of digestion

Protect against infectious disease

has three sublayers

Three sublayers of the mucosa layer of the alimentary canal

Epithelium, -Simple columnar epithelium w/mucus-secreting cells of the tract ( secretes mucus to ease passage of food)

Secretes hormones and enzymes

Lamina propria - consists of areolar connective tissue

Capillaries nourish epithelium and absorb digested nutrients; defend against pathogens ( part of MALT); contains lymphoid follicles

Muscularis mucosae- thin layer of smooth muscle that produces local movements of mucosa

Startfied squamous epithelium is found in what organs of the alimentary canal

Mouth, Part of pharynx, esophagus and anus

Submucosa of the alimentary canal

consists of areolar connective tissue

Rich supply of blood and lymphatic vessels; lymphoid follicles and nerve fibers serve the surrounding tissue

• abundant elastic fibers , allowing organs to stretch as a large volume of food stuffs enters them and then recoil

Muscularis Externa

responsible for segmentation and peristalsis

Contains an inner circular layer and an outer longitudinal layer of smooth muscle

circular layer acts a a sphincter controlling the movement of food from organ to organ

Serosa

Visceral peritoneum

Outermost layer of intraperitoneal organs

Areolar connective tissue covered with endothelium ( simple squamous epithelium)

For organs that are part of the alimentary canal but outside of the abdominal-pelvic cavity, they have adventitia ( dense CT) rather than serosa Ex) esophagus

blood supply of the alimentary canal The splanchnic circulation

Arteries that branch from the aorta

Celiac trunk supplies blood to spleen, liver and stomach

Superior and inferior mesenteric arteries serves intestines

the superior serves lso the pancreas and appendix

the inferior mesenteric artery serves the rectum and anal canal

Hepatic portal circulation

delivers nutrient-rich venous blood from digestive organs to liver

The Gi tract nervous system is called

Enteric Nervous system

The Enteric Nervous System function and goal

It is a regulatory mechanism that controls the luminal conditions of the GI tract

to create optimal conditions in each organ for digestion and absorption

more neruons than spinal cord

Eneteric Nervous System structure

Two major intrinsic nerve plexuses that control GI tract motility

Submucosal nerve plexus and Myenteric nerve plexus

Enteric Enrvous System short reflexes arcs

mediated by intrinsic nerve plexuses in response to stimuli within GI tract

Enteric Nervous system participates in long reflex arcs

Medicated by CNS and extrinsic autonomic nerves in response to stimuli arising within or outside of the tract

Parasympathetic inputs stimulate digestive processes

Sympathetic inhibits them

Three key concepts regulate digestive activity

Neurons (intrinsic and extrinsic) and hormones control digestive activity

Various mechanical and chemical stimuli influence digestive activity

Effectors of digestive activity are smooth muscle and glands

regulation of digestive activity: Neurons (intrinsic and extrinsic) and hormones control digestive activity

Nervous system controls

Intrinsic controls: short reflexes ( enteric nervous system only)

Extrinsic controls: long reflexes (CNS and systemic nerves)

Hormonal controls

Hormones from the stomach and small intestine stimulate targets (muscles, glands) in the same or different organs to affect secretion or contraction

Regulator of digestive activity Mechanical and chemical stimuli influence digestive activity

Receptors ( mechanoreceptors and chemoreceptors) in the walls of GI tract organs respond to…

Stretch ( via foodstuff in the lumen), Changes in osmolarity and pH, Presence of substrate and end products of digestion

Regulation of digestive activity: effectors of digestive activity are smooth muscle and glands

When stimulated by intrinsic controls ( enteric nerve plexus) receptors intiate reflex that stimulate smooth muscle to mix and move lumen contents

Reflexes also activate or inhibit glands that secrete digestive juices or hormones

Mouth associated organs include

Tongue, Salivary gands, Teeth

What happens at the mouth?

The mouth is where food is chewed and mixed with enzyme-containing saliva that begins the process of digestion, also initiates propulsive process of swallowing

INGESTION OCCURS ONLY AT THE MOUTH

Mouth is aka

the oral cavity or buccal cavity

The mouth walls are lined with what type of epithelium?

Stratified squamous epithelium to protect against friction (

slightly keratinized) to provide protection on the gums, hard palate, and part of the tongue

Lips

composed of orbicularis oris muscle covered with skin

Cheeks

composed mostly of buccinator muscles

Oral vestibule

recess internal to lips and cheeks, external to teeth and gums

Oral cavity proper

area within teeth and gums

Labial frenulum

median attachment of each lip to the gum

The palate

Forms the roof of the mouth and consists of two parts

Hard palate: formed by palatine bones and the palatine process of the maxillae with a midline ridge called the raphe ( mucosa slightly on it to create friction against tongue

Soft palate: mobile fold formed mostly of skeletal muscle

paltine tonsil consist here ( aka fauces)

Uvula

fingerlike projection that points downward from free edge of soft palate

The tongue function

Grip, reposition, mix food ( with saliva) during chewing to form a bolus ( compact mass)

initiate swallowing (push bolus posteriorly into pharynx

speech and taste

Muscles of the tongue

Intrinsic muscles (within tongue): change shaped of tongue

Extrnsic muscles ( attached to bones and soft palate) alter tongue’s postion

Lingual frenulum

attaches tongue to floor of mouth, limits posterior movement

all papillae that house taste buds

Fungiform, Vallate papillae and Foliate papillae

Filiform papillae

Provide friction for gripping food ( no taste buds)

Smallest, most abundant; contain keratin ( give the tongue a whitish appearance)

Fungiform papillae

mushroom-shaped, scattered widely over the tongue,

gives. reddish hue

Vallate papillae

V shaped row on back tongue

Foliate papillae

pleat like, located on the lateral aspects of the posterior tongue

Terminal sulcus

groove located posterior to vallate papillae

divides body ( portion of tongue in oral cavity) and root ( portion in oropharynx)

lacks papillae

Saliva

secreted from the salivary gland

Cleanses mouth, dissolves food chemicals for taste, and moistens food to compact into a bolus, begins breaking down of starch via amylase

Minor or Intrinsic salivary glands

located throughout the oral mucosa help keep it moist

Most saliva is secreted by

Major or Extrinsic salivary glands, usually when we eat or anticipate eating

(Parotid gland, Submandibular gland, sublingual gland)

The salivary glands compose of two types of secretory cells

Serous cells produce watery secretion with enzymes, ions, and a tiny bit of mucin ( submandibular and parotid glands contain mostly serous cells)

Mucous cells: produce mucus ( sublingual glands contain mucous cells)

composition of saliva

Mostly water (97-99.5%), slightly acidic (ph 6.75 to 7)

Electrolytes Na+,K+,CI, PO4 (phosphate), HCO3-

Salivary amylase and lingual lipase

Proteins: mucin, lysozyme, and IgA, defnesins

Metabolic wastes urea and uric acid

Xerostomia

Uncomfortably dry mouth caused by too little saliva being made

Control of salivation

Salivation is controlled primarily by the parasympathetic division of the ANS

ingested food stimulates chemoreceptors and mechanoreceptors in mouth which signal the salivary nuclei in brain stem

This causes increase in parasympathetic impulses via facial (VII) and glosspharyngeal ( IX) nerves to the glands

Results in secretion of watery ( serous), enzyme-rich saliva

Stimuli of salivation

ingesting food, smell, sight, and thought of food , and irritation of lower Gi tract

Control of salivation ( sympathetic)

Sympathetic stimulation causes the release of thick, mucin-rich saliva

Strong sympathetic stimulation can inhibit salivation

Mastication

process that tears and grinds food into smaller fragments, partly voluntary and partly reflexive

teeth sockets

alveoli

Deciduous (dentiton)teeth

20 baby teeth and fall out around 6-12 years

Permanent dentition (teeth)

32 adult teeth

incisors teeth

chisel shaped for cutting or nipping off pieces of food

Canines teeth

fanglike teeth that tear or pierce

premolars/Molars teeth

best teeth grinders, rounded cusps to grind or crush

Tooth structure two major regions

crown and root

Crown fo teeth

exposed part above gingiva (gum)

covered by enamel the hardest substance in body ( mineralized by calcium salts, and hydroxyapatite crystals

Why is enamel unable to regenerate

Enamel producing cells degenerate when tooth erupts, so if there is a crack or decay there needs to be artificial filling

Root of tooth

portion embed in jaw-bone

Canines, incisors snd premolars have one root

first two lower molars have two roots

First upper molars have three roots

root is connected to the crown at the

Neck

Cementum ( cement)

calcified connective tissue, covers and attaches root to the periodontal ligament ( anchors tooth in bony socket)

Gingival sulcus

Groove where gingiva borders tooth

dentin

Bone-like material under enamel, maintained by odontoblasts of the pulp cavity

Bulk of the tooth that acts as a shock absorber,

contains pulp (blood vessels and nerve fibers) ( nourishes tooth, provides sensation)

Root canal

Narrow extension of the pulp cavity through the root

Contains an apical foramen; the proximal end of the root provides entry to blood vessels, nerves

Dental caries ( cavities)

demineralization of enamel and dentin from bacterial action

Dental plaque (film of sugar, bacteria and debris) Bacteria break down sugar, producing acid that dissolves calcium salts

Gingivitis

red, sore, swollen gums ( may bleed) from infection

plaque calcifies to form calculus ( tartar) disrupts seal of gingivae and teeth

Periodontits

Neglected gingivitis can escalate to disease, destroying the periodontal ligament (dissolving bone)

Risk factors: smoking, diabetes mellitus, oral piercings

The pharynx external muscle layers consists of

two skeletal muscle layers

inner layer cells run longitudinally

Outer layer cells encircle the wall, forming the pharyngeal constrictor muscles

food passes through the orthopharynx and laryngopharynx

The Esophagus

Muscular tube that runs from the laryngopharynx to the stomach

Collapses when not propelling food

Pierces the diaphragm at the esophageal hiatus to join the stomach at the cardiac orifice, which is surrounded by lower esophageal sphincter ( aka gastroesophagel)

aka for swallowing

deglutition

Lower esophageal sphincter ( swallowing)

Keeps the orifice closed when not swallowing, with assistance from diaphragm

Mucus cells on both sides of sphincter protect the esophagus from acid reflux

Esophagus has all four basic GI tract tunics

Mucosa: nonkeratinized stratified squamous epithelium; changes to simple

columnar at the stomach

– Submucosa: esophageal glands secrete mucus (lubricant) to aid passage of bolus

– Muscularis externa: skeletal muscle superiorly, smooth muscle inferiorly, mixed in

the middle

– Adventitia (instead of serosa)

Heartburn

burning, radiating substernal pain from stomach acid entering the esophagus

symptom fo GERD

hiatal hernia where stomach protrudes above diaphragm can cause HEartburn

Digestive process swallowing two phases

Buccal phase occurs voluntarily in the mouth,

ends when the food bolus leaves the mouth and stimulates receptors in the oropharynx

Pharyngeal-esophageal phase: involuntary, primarily via the vagus nerve, stimulates muscles of the pharynx and esophagus using the upper esophageal sphincter

Respiration pauses once food enters the pharynx

Controlled by swallowing center in the medulla and lower pons

Stomach

• temporary storage tank for ingested food

• breaks down food chemically and physcilally

• resuklting in paste-like chyme b and delivering it to small intestines

Rugae od the stomach

folds of stomach mucosa which allows stomach to expand

cardia

surrounds cardiac orifice of the stomach

Fundus

dome-shaped region beneath diaphragm, superior to cardia

Body

mid portion of the stomach

Pyloric part

Funnel-shaped region, continuation of the body

Contains the pyloric antrum, which narrows into pyloric canal and terminates at the pylorus

Pyloric sphincter ( valve) controls stomach emptying into the duodenum

Two curvatures of the stomach

Greater curvature: convex lateral surface of the stomach

Lesser curvature: concave medial surface of the stomach

Mesenteries ( aka omenta)

extend from curvatures and tether stomach to other digestive organs and body wall

Lesser omentum

runs from lesser curvature to liver

Lesser omentum anchors liver to lesser curvature of stomach

Greater omentum

drapes inferiorly from greater curvature over intestine, spleen and transverse colon

• contains fast deposists and lymph node