Anatomy & Physiology Lecture Outline: Week #1

• The Digestive System

Thibodeau Chapters 25, 26

Functions of the digestive system

1. Prepares food for absorption
2. Prepares food for cellular use – most foods can not pass through the intestinal mucosa
3. Eliminates of unabsorbed food

 

Chapter 25 Anatomy of the Digestive System

Organs of the digestive system - see (Fig. 25-1; Table 25-1)

Basic tubular structure (GIT, alimentary canal), with accessory organs

Structure of the GIT - see (Fig. 25-2)

• mucosa – innermost layer facing the lumen (open space of the tube)
• submucosa – connective tissue – highly vascularized
• muscularis – inner (circular) and outer (longitudinal) layer of smooth muscle
• serosa – connective tissue layer, peritoneum

Specialization of basic structure for function - see (Table 25-2) review for Thursday, pay attention to structure modification and functional relationship

 

Mouth

• lips – filtrum, oral fissure
• cheeks
• hard and soft palates
• tongue
  • intrinsic muscle
  • extrinsic muscle
  • papillae, tastebuds
  • lingual frenulum
• Salivary glands – Fig 25-6, compound tubulo-alveolar, exocrine
  • parotid: secrete enzymes, no mucus
  • submandibular: secrete enzymes and mucus
  • sublingual: secrete mucus only
• teeth – organs of mastication
  • crown
  • neck
  • root
  • pulp cavity

    dental formula (Fig 25-8; Table 25-3)

    • deciduous teeth (20)
    • permanent teeth (32)

Pharynx

• see oropharynx on p 684

Esophagus

• see Fig 25-9, compare with the four layers of the generalized GIT (fibrous outer layer, abrasion-resistant inner lining, striated muscle in upper third, mixed striated and smooth in middle third, smooth muscle in lower third)

Stomach

• position (Fig. 25-10)
• anatomy
  • fundus
  • body
  • pylorus
  • lesser curvature
  • greater curvature
  • sphincters (cardiac, pyloric)
• structure: see (Figs. 25-10, 25-11, 25-12), compare with the four generalized layers
  • gastric mucosa
    • Rugae
    • Gastric pits
    • Mucus glands
    • Gastric glands
      • Chief cells – secrete gastric enzymes
      • Parietal cells – secrete hydrochloric acid and intrinsic factor
  • gastric muscle – three distinct layers (Fig. 25-10)
  • gastric serosa – fold of peritonem which anchors it in place

   

• functions
  • reservoir
  • secretes acid and enzymes
  • mechanical breakdown of food
  • secretes intrinsic factor (protects vitamin B12 from acid and needed for absorption)
  • small amount of absorption (limited to certain drugs, some water, alcohol, and some fatty acids)
  • secretes gastrin - local hormone which regulates digestion
  • non-specific immune function - destruction of bacteria

Small intestine

• components
  • duodenum - short C shaped section at outflow of stomach, 10 inches long
  • jejunum - 8 feet
  • ileum - 12 feet

• structure of wall - see Fig. 25-14, 25-15, 25-17
  • serosa - peritoneal fold (mesentery)
  • muscular layers
  • mucosa
    • circular plicae (folds)
    • villi
    • microvilli
    • goblet cells - produce mucus
    • crypts between the villi - site of enzyme production

show that small intestine is site of digestion AND absorption

Large intestine

• components
  • cecum – blind pouch with vermiform appendix attached
  • colon - ascending, transverse, descending, sigmoid - Fig 25-17
  • rectum, anal canal see Fig.25-20
• structure of wall - see Fig. 25-14, 25-15, 25-17
  • serosa - peritoneal fold (mesocolon)
  • muscularis layer - longitudinal layer of muscle divided into tenia coli
  • mucosa - mainly mucus producing

Peritoneum

• parietal layer - lines the abdominal cavity
• visceral layer- covers and suspends the abdominal organs - see Fig. 25-22, 25-23

Liver

• position (Fig. 25-10)
• anatomy
  • R and L lobes
  • greater curvature
  • sphincters (cardiac, pyloric)
• structure
  microscopic structure – hepatocytes arranged in strings into hexagonal lobules (Fig. 25-25)
• 2 types of cells
  • hepatocytes – perform metabolic function
  • Kupffer cells – non-specific immunity - phagocytosis
• Blood enters and flows together from the portal vein (drains intestines)and hepatic artery. Blood drains through strings of hepatocytes, where metabolism takes place, and is collected by hepatic vein -
• hepatocytes make bile, remove and store some vitamins, detoxify poisons
• bile flow through liver (biliary tree Fig. 25-27)
  • Hepatic duct (one right and one left, which join and form one hepatic duct)
  • Cystic duct (merges with the hepatic duct from the gallbladder)
  • Common bile duct (formed by the merger of the hepatic duct and cystic duct)

     

• functions
  • detoxification of amino acids - convert N₂ to urea
  • breakdown of drugs
  • breakdown of hormones
  • production of bile
  • storage of glycogen , vitamins (ADEKB₁₂), iron
  • production of proteins - clotting factors, and all other plasma proteins except antibodies
  • site of fetal hematopoesis

   

Gall bladder

• storage and concentration of bile

Pancreas

• exocrine and endocrine gland
• head (in C of duodenum), body and tail
• main (pancreatic duct) and accessory ducts - Fig. 25-27,25-29 enters duodenum with common bile duct
• exocrine tissue - 98% of gland simple cuboidal, compound acinar - secretes enzymes
• endocrine - Islets of Langerhans - b cells secrete insulin and a cells secrete glucagon

 

 

Chapter 26 Physiology of the Digestive System

Table 26-1 Food is:

• ingested
• digested by mechanical and enzymatic breakdown
• moved along by motility of GIT
• absorption of nutrients out of GIT into intestinal cells and bloodstream takes place
• waste is eliminated

Mechanical Digestion (Table 26-2)

Chemical Digestion (Table 26-3)

Enzyme action

• proteins, some require vitamins to work as cofactors
• Fig. 26-4 - enzyme aids in reaction but is not changed itself
• enzymes are specific for function - act on one substrate only (chemical "fit")
• greatly speed chemical reaction - usually reversible, according to conditions
• each enzyme works best at optimal pH amd temp - Fig. 26-5
• named according to type of reaction each catalyses - hydrolases, lipases etc

Digestion in mouth

• mechanical breakdown of food by teeth (mastication)
• secretion of saliva - contains amylase which catalyses breakdown of starch to sugar
• saliva contains mucus which lubricates food as it is swallowed (deglutition Fig. 26-1)

Movement of food through esophagus

• peristalsis - Fig. 26-2

Digestion in stomach

• further breakdown of food by - hydrochloric acid, pepsin, proteolytic enzymes
• mixing or segmentation contractions - Fig.26-3
• stomach emptied slowly after 1-8 hours by peristaltic contractions

Control of events in stomach

• stomach acid production, enzyme production, and motility are controlled by the vagus (parasympathetic nerve) and by the hormone gastrin, produced in the stomach
• Both are stimulated by:
  • thought of food
  • presence of food in stomach, esp, alcohol, caffeine, soup
• Both are inhibited by:
  • presence of food in duodenum
  • other local hormones active in duodenum

Digestion in duodenum

• in duodenum pancreatic juice has alkaline pH
• pancreatic enzymes - proteases, lipases, amylases
• bile emulsifies fats for lipase action
• segmental and peristaltic contractions take place

Digestion in ileum

• final enzymatic digestion in brush border of intestine
• absorption takes place through villi – Fig. 26-14, 26-15
• proteins absorbed as amino acids, carbohydrates as monosaccharides; both absorbed by active transport through the intestinal cell into the blood vessel of the villus
• Fat digestion - glycerol and fatty acids are wrapped in bile salts which make them soluble and absorbed into the intestinal cell wall. They are repackaged as chylomicra, and absorbed into the lacteal (lymph system) of the villus
• motility continues - mixing and propulsive waves

Digestion in large intestine

• no further enzymatic action
• water reabsorbed
• storage and compaction function
• many mucous glands produce mucus for lubrication
• motility - mixing and peristalsis continue
• also "mass action" large propulsive waves sweep through entire colon

Defecation

• mass action takes place 1-2 X daily
• presence of stool in rectum causes relaxation of internal anal sphincter (involuntary)
• external sphincter relaxed voluntarily

See The Big Picture p 780 (Review Figures: 26-17 absorption; 28-18 functions)

 

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