The Digestive System

• Mouth and Tongue (teeth)
• Pharynx
• Esophagus
• Stomach
• Small Intestine
• Large Intestine
• Salivary Glands
• Liver
• Pancreas

• Ingestion (taking in the food)
• Mechanical Digestion (grinding of stomach)
• Chemical Digestion (acids, enzymes, hormones)
• Secretion (by digestive system cells)
• Absorption (smallest elements, taking goodies)
• Excretion ("crap" left behind)

• Serosa
• Muscularis Externa
• Submucosa
• Mucosa (has 3 layers)
• (Muscularis Mucosa)
• (Lamina Propria)
• (Epithelium Lining)

Outer tough connective tissue membrane for protection

Longitudinal and circular muscle layers for contraction (moves food along)

Loose connective tissue, blood vessels and glands for secretion

• Mucosa is made of 3 layers
• Muscularis Mucosa or Interna
• Lamina Propria (connective tissue)
• Epithelium Lining (inner most lining)
• Functions for digestion and absorption or nutrients

• Sensory Neuron - Monitor GI tension, chemicals and hormone levels in the blood
• Motor Interneuronal Circuits - controls the GI muscle motility, blood flow and secretions

• Myenteric Plexus
• Submucosal Plexus

• Linear chain of neruons located between longitudinal and circular muscle layers
• Controls GI muscles tone, contractions intensity, frequency and velocity

• Nonlinear neurons scattered in the submucosa (loose connective tissue)
• Controls intestinal secretions, digestion and absorption (submucosa and mucosa layers)
• Parasympathetic (PNS) influences activation of ENS (GI tract function trigger)
• Sympathetic (SNS) influences inhibition of the ENS (GI tract function brake)

• Proteins Catalyists that speeds up the digestion chemical reaction at body temperature
• They are not altered during the reaction but are sensitive to changes in temp and pH (stops them)
• Are hydrolytic enzymes and use water to split food molecules

• Mastication breaks and saliva lubricates food for easy swallowing plus increasing the surface area for enzymes actions. (smaller pieces more enzyme)
• Complex starch digestion by salivary amylase to disaccharides

• Parotid glands
• Sublingual glands
• Submandibular glands

• Inferior to the zygomatic arch
• serous (watery) secretions + amylase (salivary)

• Under the base of the tongue
• Mucous secretions (mucins) ONLY

• On inner surface of mandible
• All three types of secretions
• Serous (watery secretions) + mucous (mucins)+ amylase (salivary)

• Buccal Phase
• Pharyngeal Phase
• Esophageal Phase

Tongue pushes bolus of food from oral cavity into oropharynx and is voluntary

• The soft palate closes nasopharynx and epiglottis closes larynx
• choking can occur if food bolus gets stuck in the laryngopharynx
• Involuntary

• Upper esophageal sphincter opens
• Peristalsis (muscle contractions) propels bolus down esophagus toward stomach
• Cardiac (lower esophageal) sphincter opens and bolus enters stomach
• Involuntary

• Storage - Holding of food during feeding
• Mechanical Digestion - Grinding waves makes liquid chyme and pushes it onto the small intestine
• Chemical Digestion - (starts digestion) Pepsin begins proteins digestion (pepsinogen -to- pepsin)
• Absorption - Limited to water, alcohol and some drugs like nitro and aspirin

• Mucous neck cells
• Parietal cells
• Chief cells
• G cells

Protective mucous (mucins)

Hydrochloric acid (which lays on the mucins to protect from lower secretions) and intrinsic factor

Pepsinogen and gastric lipase

• the hormone gastrin which stimulates gastric secretions.
• Gastrin is a hormone

• Pepsinogen converted to active enzyme pepsin in stomach lumen by hydrochloric acid (HCL)
• Pepsin digests proteins to smaller polypeptides
• Gastric lipase starts lipid digestion

Serosa, Muscularis Externa, Submucosa and Mucosa

• Musosa has 4 to 5 million villi
• Villus covered with columnar epithelium and goblet cells
• blood capillaries inside villus for nutrients absorption
• Lymphatic capillary (lacteal) inside villus for lipid digestion products absorption

• Absorptive cells - digestion and absorption
• Goblet cells - secretes mucous
• Endocrine cells - secrete intestinal hormones
• Paneth cells - Macrophages "pac man cells"

• Mechanical digestion
• Chemical digestion
• Absorption

• Peristalsis (muscle contractions) propels chyme along the intestine
• segmentation move chyme back and forth to mix it thoroughly with digestive enzymes

• Enzymes from pancreas and small intestine complete digestion of protein, starch (to sugar), disaccharide sugars (to monosaccharide sugars) and fat (to fatty acids)
• Gallbaldder empties bile into small intestine to aid in fat digestion

Absorption of most substances occurs in the small intestines

• 5 inches long extending from duodenum to spleen
• Consists of 4 parts: Head, neck, body & tail
• Most cells are exocrine producing digestive enzymes
• Endocrine cells in the pancreatic islets produce hormones

digestive enzymes

Endocrine cells are in pancreatic islets and produce hormones

• Acinar Cells
• Ductal Cells

Exocrine cells that secrete the digestive enzymes into ducts

Secrete a Bicarbonate Solution that neutralizes the acidic chyme from the stomach and protects the mucosa of the duodenum

• Protein digestion
• Starch digestion
• Fats digestion

• By Proteolytic Enzymes
• 4 enzymes secreted as inactive proenzymes
• Proenzymes sequentially activated in the duodenum to form the active enzymes
• These enzymes digest proteins & polypeptides to tripeptides, diepetides and single amino acids

• Remaining starch is digested in the intestine by pancreatic amylase enzyme to disaccharides
• Digestion is the same as in the mouth

• Triglyerides digested in the small intestine by pancreatic lipase enzyme
• Digestion of each triglyceride yields a monoglyceride molecule and two fatty acid molecules

• Bile from the liver is required so that the pancreatic lipase can digest fats more efficiently
• Bile flows from gallbladder down bile ducts into duodenum to mix with and emulsify the fats
• Emulsification is breaking fats drops into very small droplets for efficient enzyme action
• Bile aids in the absorption of digested fats
• Bile is a medium for billirubin and cholesterol excretion by the liver

• Hepatocytes and a rich blood supply.
• Blood is supplied by branches of the hepatic artery and portal vein at the six corners of each lobule.
• Blood is carried to the central canal by the liver sinusoids and is surrounded by hepatocytes

They absorb solutes from the blood and secrete materials into the blood

• The livers macrophages are called Kupffer cells in the sinusoid phagocytes microorganisms
• Stay in the sinusoids

• Liver synthesize bile
• Bile flows from liver thru hepatic ducts into gall bladder
• Gallbladder stores and concentrates bile
• Common hepatic duct and cystic duct from gallbladder unite to form the common bile duct
• Common bile duct unites with pancreatic duct
• Bile and pancreatic juices enter the duodenum

• 1. Carbohydrate, lipid & protein metabolism
• 2. Storage of glycogen, vitamins & iron
• 3. Removal of hormones & antibodies
• 4. Removal of metabolic waste products
• 5. Detoxification of drugs and toxins
• 6. Phagocytosis by Kupffer cells (major cleaners)
• 7. Plasma protein synthesis
• 8. Bile systhesis & secretion

• Peptidases
• Intestinal Lipase
• Disaccharidases

Digests tripeptides and dipeptides into single amino acids

Digests fats into glycerol and free fatty acids

• digests disaccharides to individual monosaccharides
• Sucrose + sucrase = glucose + fructose
• Maltose + maltase = glucose + glucose
• Lactose + lactase = glucose + galactose

• Absorption in the transfer of substances into absorptive cells then into the blood or lymph capillaries
• Villi and microvilli of absorptive cells provide a very large surface are for absorption
• Most digested foods, water, electrolytes and vitamins are absorbed in the small intestine

• In lacteals: Fats and fat soluable vitamins in chylomicrons
• For the lymph (cholesterol, phospholipids, triglycerides form balls) and enter the lacteals into the left subclavian vein

In the blood: Monosacchrides (starches), Amino acids (proteins), electrolytes, short chain fatty acids, water and water soluable vitamins are sucked into the aveolus capillaries and dumped into the hepatic portal vein

Serose, muscularis externa, submusosa and mucosa

Submucosa had deep crypts with intestinal glands

Mucosa

• 1. Digestion of undigested food by bacteria
• 2. Formation of vitamin B & K by bacteria
• 3. Absorption of water, electrolytes, vitamins and bile salts
• 4. Feces formation by bacterial action
• 5. Propelling of feces by mass movement contractions

• The desire to poop is initiated when mass movement contractions in the colon push poop into the rectum
• Tonic contractions of the internal and external anal sphincters prevents actual pooping except in babies

• It is made of smooth muscle (involuntary)
• Relaxed by parasympathetic Pelvis Nerve stimulation (ANS)
• Constricted by sympathetic hypogastric nerve stimulation (ANS)

• Is made of skeletal muscle (voluntary)
• innervated by the pudendal nerve - SNS
• Under voluntary conscious control

• Intrinsic reflex
• Parasympathetic reflex
• Conscious reflex

• A myenteric plexus reflex initiated by distension of rectum with fecal matter
• Trigger peristalsis in colon & rectum plus relaxation of the internal anal sphincter
• Too week to cause defication on its own

• Rectum distension transmits signals to sacral segments of the spinal cord via pelvic nerve
• Intensify peristalsis and increases relaxation of the internal anal sphincter

• Relaxation of the external anal sphincter
• Contraction of abdominal muscles
• Emptying of descending colon, sigmoid colon and rectum