Digestive

Digestive system 

Functions, in order:

  1. Ingestion= take in food/drink 

  2. Digestion= breakdown food/drink to nutrient molecules 

  3. Absorption= move nutrient molecules into blood (molecules cross membrane and enter body)

  4. Defecation= eliminate indigestible wastes

2 groups of digestive system organs

  1. Alimentary canal= gastrointestinal tract 

  • food goes through hollow GI tract organs 

  • Where ingestion,digestion absorption and defection occurs 

  • 2. Accessory organs

  • Food doesn’t go through them 

  • Assist in Gi tract functions 

  • Secrets digestive enzymes into GI tract



GI tract organs 

All hollow organs have a lumen 

Lumen is fluid space inside organ not empty 



In order 

  1. Oral cavity 

    1. mucosa = inner= mucous membrane = open to outside = prone to infection 

    2. Palatine tonsils (2) lateral to uvula

    3. Lingual tonsils on posterior of tongue 

    4. Palate separates nasal cavity and oral cavity

      1. Anterior, bony hard palate 

      2. Posterior soft palate

      3. Uvula = tip of soft palate in posterior of mouth 

    5. Mouth Function

      1. Mastication (chewing)

      2. Tongue: skeletal muscles attached to mandible (bone)

      3. Voluntary = controlled by somatic motor division 

      4. Mixes chewed food with saliva from salivary glands

      5. Starts swallowing process

      6. Has taste buds = special sense organs for taste 

  2. Pharynx

  3. Esophagus 

  4. Stomach 

  5. Small intestine

  6. Large intestine



Pharynx and Esophagus 

  • Food moves posteriorly from the oral cavity into…

  • Oropharynx, then inferiorly into…

  • Laryngopharynx (posterior/superior to larynx), then into…

  • Esophagus = food tube going inferiorly from laryngopharynx through diaphragm to stomach 

Peristalsis = slow, directional contraction…

  • Of involuntary, smooth muscles…

  • In walls of hollow Gi tract organs 

  • Propels (squeezes) food through Gi tract

Muscosa 

  • Inner Lining of Lumen 

    • Lumen = fluid space inside = where food is

    • Mucous membrane = open to outside (food)

    • Contains: inner layer = epithelium (E

    • Lamina propria = thin CT layer dep to Et

    • Thin smooth muscle layer deep to lamina propria 

  • Lining of mouth, pharynx, esophagus, anus

    • Stratified squamous ET 

    • Function = protection from abrasion 

  • Lining of stomach, small intestine, large intestine 

    • Simple columnar epithelium tissue 

    • Functions = secretion and absorption

Submucosa

  • Soft connective tissue with 

  • Blood vessel (vascular) 

  • Nerve endings 

  • Lymphoid tissue  

Musclarsis 

  • involuntary , smooth muscles 

  • Inner circular layer

  • Outer longitudinal layer



Serosa

  • Thin outer covering - simple squamous epithelium

  • Serous membrane - not open to outside 

  • Fruition - secretes serous fluid for lubrication

  • Visceral peritoneum - on walls of gi tract organs 

  • Parietal peritoneum- surrounds perimeter of abdominopelvic (peritoneal) cavity and lines this cavity with mesenteries 

  • Parietal = perimeter 



Cardia is entrance from esophagus to stomach

Pylorus is the exit of stomach to Duodenum

Stomach has three layers of Muscularis 

  • Longitudinal layer

  • Circular Layer 

  • Oblique Layer 

Stomach

  • Food enters stomach from esophagus thru cardiac sphincter 

  • Food exits stomach into small intestine thru pyloric sphincter 

  • Sphincter is a ring of smooth muscle around passageway from 1 organ to the next 

  • Sphincter = gatekeeper = opens/closes to control flow thru passageway 

  • Involuntary = controlled by autonomic motor div.

Gastric Glands

  • Mucous neck cells produce alkaline mucus 

  • Enteroendocrine cells produce gastrin (hormone)

  • Parietal cells produce HCI

  • Chief cells produce pepsinogen:

    • Converted to protein-digesting enzyme pepsin

    • When mixed with HCI in hot environment (=in body)

Small Intestine 

  • Longer, narrower vs Large intestine 

  • From pyloric sphincter to ileo cecal valve 

  • Most nutrient absorption occurs here 

  • Mesenteries suspend the small intestine from pos. Abdominal wall (see figure)

  • Keeps intestines in place, prevents tangling

  • Stomach > duodenum> jejunum> \ileum>cecum 

  • Cecum = first part of large intestine

  • Ileum connects to cecum at ileocecal valve



Chemical Digestion in Small intestine

  • Intestinal mucosa (inner lining) cells produce and secrete digestive enzymes

  • Pancreas and Liver

    • Also produce digestive enzymes

    • Are exocrine organs= secrete digestive enzymes through ducts into duodeum 



Absorption in Small intestine

  • Three structures increase surface area for more absorption of nutrient molecules:

  1. Circular Folds = folds of mucosa and submucosa 

    1. “Speed bumps” = slow down flow to increase absorption

  2. Villi = finger like projections of mucosa

    1. Have capillaries = absorb nutrients into blood

  3. Microvilli = tiny projections of plasma membranes of epithelial cells on surface of villi 

Large Intestine

  • Shorter, wider vs small intestine

  • From ileocecal valve to anal canal

  • Expels indigestible food residues from body

  • Subdivisions 

  1. Cecum

  2. Colon

  3. Rectum 

  4. Anal canal 

Colon 

  1. Ascending colon = “food residue” SHIT travels up right side of abdominal cavity 

  2. Transverse colon = travels across abdomen 

  3. Descending colon = travels down left side 

  4. Sigmoid Colon = S-shaped part, enters pelvic cavity

    1. Sigmoid colon, rectum, anal canal all in pelvic cavity 

Anal Canal

  • Internal anal sphincter

    • Smooth muscle 

    • Involuntary = controlled by autonomic motor div

  • External anal sphincter 

    • Skeletal muscle, attached to pelvis (bone)

    • Voluntary = controlled by somatic motor division 

  • Anus = final orifice (opening) of Gi tract 



Accessory Organs

  • Teeth and tongue

  • Salivary glands 

  • Pancreas

  • Liver

  • Gallbladder 

Teeth

  • 32 permanent teeth = 8 in each of 4 quadrants in oral cavity 

  • Ant to Pos

    • 2 incisors 

    • 1 canine 

    • 2 premolars

    • 3 molars

      • 3rd molars = wisdom teeth

  • Regions of a tooth 

  1. Crown = superior to gingiva (gum), 3 layers

  • Enamel = outer layer of crown = hardest material 

  • Dentin = middle layer, surrounds pulp

  • Pulp = inner layer = CT,blood vessels, nerves 

    • Root canal = pulp cavity into root



Three Pairs of salivary glands = 6 total 

  • 2 Paraloid glands = anterior to right and left ears

  • 2 submandibular glands = interior to 

  • 2 sublingual glands = interior to 



Salivary Glands

Produce and secrete

  • Moistens and binds food into a bolus (ball)

  • Contains salivary amylase = enzyme that begins starch digestion in mouth 

  • Dissolves chemicals in food so taste buds can taste them 



Pancreas

  • Endocrine and digestive organ

  • Digestive (exocrine) function

    • Produces digestive enzymes to chemically digest 

      • Proteins 

      • Carbs

      • Fats 

  • Secrete enzymes into duodenum through pancreatic duct 

  • Endocrine function:

    • Produce and secrete hormones directly into blood to regulate blood glucose:

      • Insulin 

      • Glucagon 

Liver 

  • Largest gland 

  • Right side, interior to diaphragm 

Bile

  • Produced and secreted by liver 

  • Stored in gallbladder 

  • Contains bile salts = enzymes that break up (emulsify) fats

  • Bile exits liver thru common hepatic duct

  • Cystic duct from gallbladder joins to form common bile duct

  • Bile enters duodenum through common bile duct

Gallbladder

  • Fats in duodenum to be digested 

    • Bile secreted from gallbladder thru cystic duct, then thru common bile duct, to duo\odenum

    • No fats in duodenum ?

      • Bile backs up into cystic duct for storage in gallbladder

    • Gallstones = crystalized cholesterol

      • Can block cystic duct = pain 

Swallowing and Other functions 

Three stages of swallowing 

  1. Voluntary Phase

    1. In oral cavity 

    2. Tongue shapes food into bolus

      1. Tongue = voluntary, skeletal muscle 

    3. Uvula down

    4. Epiglottis up 

    5. Superior esophageal sphincter contracted = closed

  2. Pharyngeal phase (involuntary)

    1. Bolus of food moves into oropharynx 

    2. Uvula up: blocks entrance to nasopharynx 

    3. Epiglottis down: blocks entrance to larynx/ trachea 

      1. Thyroid cartilage of larynx up

    4. Superior esophageal sphincter relaxed = open 



  1. Esophageal Phase (involuntary)

  1. Larynx, epiglottis and uvula return to pre-swallowing positions 

  2. Superior esophageal sphincter contracted = closed

  3. Peristalsis = involuntary smooth muscles contract and push bolus down esophagus to stomach  

  4. Cardiac sphincter of stomach relaxes/ opens as bolus presses on it  




Digestive System 



Stomach functions

Stomach secretes the hormone gastrin when:

  • Food present or

  • pH rises = alkaline 

Gastrin causes gastric glands to secrete these:

  1. Pepsinogen = precursor of pepsin 

  2. HCI to lower pH (acidic) 

    1. HCI converts pepsinogen to protein digesting enzyme pepsin 

    2. HCI stops microorganism growth 

Small intestine functions 

  • chemical digestion/ absorption

  • Chyme enters duodenum, causes secretion of hormones: cholecystokinin (CCK) and secretin 

  • Causes pancreas to secrete pancreatic juice thru duct to

  • Cause liver and gallbladder to secrete bile thru duct to

  • This is a ___ stimulus 

Large Intestine 

Intestinal flora = 700+ species of bacteria:

  • Digest remaining nutrients 

  • Produce vitamins K and B12

  • Release gas 

H2O absorbed to create feces 

Vitamins K and B12 absorbed 

Remaining solid waste (feces) defecated  

Diarrhea = not enough H2O absorbed 



Digestion and Absorption 

Three Phases of digestion 

  1. Propulsion 

  2. Mechanical digestion

  3. Chemical digestion 



Propulsion of food thru GI tract:

Peristalsis = smooth muscle contract in waves 

  • squeeze and push food through GI tract

Mixing = back and forth motion

  • Alternating smooth muscle contractions 

  • Mixes food with enzymes in small intestine



Mechanical digestion

  • Food broken into small bits:

    • Teeth and tongue in mouth chew it 

    • 3 smooth muscle layers in stomach smash it 

    • 2 smooth muscle layers in small intestine mix it with digestive enzymes

Chemical Digestion

  • Enzymes break down large molecules into their components by breaking chemical bonds 

  • Components absorbed into bloodstream 

  • Different enzymes for each food type 

    • Carbs, fats, proteins mostly carbs