The Digestive System
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What are the 9 parts of the digestive system?
- Mouth and Tongue (teeth)
- Small Intestine
- Large Intestine
- Salivary Glands
What are the 6 digestive system functions?
- 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)
What are the 4 layers of the GI tract?
- Muscularis Externa
- Mucosa (has 3 layers)
- (Muscularis Mucosa)
- (Lamina Propria)
- (Epithelium Lining)
What is the Serosa layer of the GI tract?
Outer tough connective tissue membrane for protection
What is the Muscularis Externa layer of the GU tract?
Longitudinal and circular muscle layers for contraction (moves food along)
What is the Submucosa layer of the GI tract?
Loose connective tissue, blood vessels and glands for secretion
What is the Mucosa layer of the GI tract?
- 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
The Enteric Nervous System (ENS) is part of the ANS used exclusively for digestion and is called the "Little Brain" of the GI tract and functions independently using what to control its functions?
- Sensory Neuron - Monitor GI tension, chemicals and hormone levels in the blood
- Motor Interneuronal Circuits - controls the GI muscle motility, blood flow and secretions
The Enteric Nervous System (ENS) consist of what 2 plexuses?
- Myenteric Plexus
- Submucosal Plexus
What is the Myenteric Plexus of the ENS?
- Linear chain of neruons located between longitudinal and circular muscle layers
- Controls GI muscles tone, contractions intensity, frequency and velocity
What is the Submucosal Plexus of the ENS?
- 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)
What are digestive enzymes?
- 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
What part of digestion happens in the mouth?
- 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
What are the 3 salivary glands?
- Parotid glands
- Sublingual glands
- Submandibular glands
Where are the Parotid Salivary Glands and what do they secrete?
- Inferior to the zygomatic arch
- serous (watery) secretions + amylase (salivary)
Where are the Sublingual Salivary Glands and what do they secrete?
- Under the base of the tongue
- Mucous secretions (mucins) ONLY
Where are the Submandibular Salivary Glands and what do they secrete?
- On inner surface of mandible
- All three types of secretions
- Serous (watery secretions) + mucous (mucins)+ amylase (salivary)
What are the 3 phases of swallowing?
- Buccal Phase
- Pharyngeal Phase
- Esophageal Phase
What happens in the Buccal Phase of swallowing?
Tongue pushes bolus of food from oral cavity into oropharynx and is voluntary
What happens in the Pharyngeal Phase of swallowing?
- The soft palate closes nasopharynx and epiglottis closes larynx
- choking can occur if food bolus gets stuck in the laryngopharynx
What happens in the Esophageal phase of swallowing?
- Upper esophageal sphincter opens
- Peristalsis (muscle contractions) propels bolus down esophagus toward stomach
- Cardiac (lower esophageal) sphincter opens and bolus enters stomach
What are the 4 functions of the stomach?
- 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
What are the 4 gastric glands?
- Mucous neck cells
- Parietal cells
- Chief cells
- G cells
What do Mucous neck cells secrete?
Protective mucous (mucins)
What do Parietal cells secrete?
Hydrochloric acid (which lays on the mucins to protect from lower secretions) and intrinsic factor
What do Chief cells secrete?
Pepsinogen and gastric lipase
What do G cells secrete?
- the hormone gastrin which stimulates gastric secretions.
- Gastrin is a hormone
Describe chemical digestion?
- Pepsinogen converted to active enzyme pepsin in stomach lumen by hydrochloric acid (HCL)
- Pepsin digests proteins to smaller polypeptides
- Gastric lipase starts lipid digestion
What are the layers of the small intestine?
Serosa, Muscularis Externa, Submucosa and Mucosa
Describe the histology of the small intestine?
- 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
What are the 4 cells of the villus?
- Absorptive cells - digestion and absorption
- Goblet cells - secretes mucous
- Endocrine cells - secrete intestinal hormones
- Paneth cells - Macrophages "pac man cells"
What are the three functions of the small intestines?
- Mechanical digestion
- Chemical digestion
Describe the mechanical digestion of the small intestine?
- Peristalsis (muscle contractions) propels chyme along the intestine
- segmentation move chyme back and forth to mix it thoroughly with digestive enzymes
Describe the Chemical digestion of the small intestines?
- 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
What does the small intestine absorb?
Absorption of most substances occurs in the small intestines
Describe the pancreas?
- 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
Most of the cells in the pancreas are exocrine cells producing what?
Where are the endocrine cells in the pancreas?
Endocrine cells are in pancreatic islets and produce hormones
What are the 2 cells of the pancreas?
What are the Acinar Cells of the pancreas?
Exocrine cells that secrete the digestive enzymes into ducts
What are the Ductal Cells of the pancreas?
Secrete a Bicarbonate Solution that neutralizes the acidic chyme from the stomach and protects the mucosa of the duodenum
What are the 3 digestions done by pancreatic enzymes?
- Protein digestion
- Starch digestion
- Fats digestion
Describe Proteins Digestion by pancreatic enzymes?
- 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
Describe Starch Digestion by pancreatic enzymes?
- Remaining starch is digested in the intestine by pancreatic amylase enzyme to disaccharides
- Digestion is the same as in the mouth
Describe Fats Digestion by the pancreatic enzymes?
- Triglyerides digested in the small intestine by pancreatic lipase enzyme
- Digestion of each triglyceride yields a monoglyceride molecule and two fatty acid molecules
What are the roles of Bile?
- 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
In the liver the lobes contain miroscopic liver lobules, these lobules have rows consisting of what?
- 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
What do the livers hepatocytes do?
They absorb solutes from the blood and secrete materials into the blood
What is the name of the Pac Man cells of the liver?
- The livers macrophages are called Kupffer cells in the sinusoid phagocytes microorganisms
- Stay in the sinusoids
What is the route of Bile?
- 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
What are the 8 functions of the liver?
- 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
What are the three digestional enzymes?
- Intestinal Lipase
What does the digestive enzyme Peptidases digest?
Digests tripeptides and dipeptides into single amino acids
What does the digestive enzyme Intestinal Lipase digest?
Digests fats into glycerol and free fatty acids
What does the digestive enzyme disaccharidases digest?
- digests disaccharides to individual monosaccharides
- Sucrose + sucrase = glucose + fructose
- Maltose + maltase = glucose + glucose
- Lactose + lactase = glucose + galactose
What is absorption in the small intestine?
- 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
What is absorbed in the lacteals of 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
What is absorbed in the blood of the small intestine?
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
What are the layers of the large intestines?
Serose, muscularis externa, submusosa and mucosa
What layer of the large intestines has intestinal glands that secrete lots of mucus?
Submucosa had deep crypts with intestinal glands
What layer of the large intestine has no villi and covered by simple columnar epithelium?
What are the 5 functions of the large intestines?
- 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
When do you feel the desire to poop?
- 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
What controls the internal anal sphincter?
- It is made of smooth muscle (involuntary)
- Relaxed by parasympathetic Pelvis Nerve stimulation (ANS)
- Constricted by sympathetic hypogastric nerve stimulation (ANS)
What controls the external anal sphincter?
- Is made of skeletal muscle (voluntary)
- innervated by the pudendal nerve - SNS
- Under voluntary conscious control
What are the 3 defecation relfexes?
- Intrinsic reflex
- Parasympathetic reflex
- Conscious reflex
What is the Intrinsic reflex of defecation?
- 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
What is the Parasympathetic reflex of defecation?
- Rectum distension transmits signals to sacral segments of the spinal cord via pelvic nerve
- Intensify peristalsis and increases relaxation of the internal anal sphincter
What is the conscious control of defecation?
- Relaxation of the external anal sphincter
- Contraction of abdominal muscles
- Emptying of descending colon, sigmoid colon and rectum
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