1. The Digestive System Chapter 16

2. Function of the Digestive System
To break down food into a “usable” (absorbable) form
To supply our cells with the nutrients (carbohydrates, proteins, fats, vitamins, minerals, water) they need for energy, growth & repair

3. Organs of the Digestive System
Gastrointestinal tract (GIT) – continuous passageway which contains the food from the time it enters the body, until it leaves; organs include:
mouth (oral cavity), pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus
Accessory organs - participate in digestive processes; organs include:
teeth, tongue, salivary glands, liver, gall bladder, pancreas

4. Processes of Digestion
Ingestion
Movement along GIT
Voluntary – e.g. swallowing
Involuntary – e.g. peristalsis
3. Secretion – release of water, enzymes, acids, buffers, mucous, etc. into GIT for physical (mechanical) & chemical digestive processes

5. Processes of Digestion
Mechanical processing – physical breakdown of food; e.g. mastication, emulsification, mixing waves, segmentation
Chemical digestion – chemical breakdown of food; disassembling of organic molecules into their component parts; requires enzymes
carbohydrates (polysaccharides) disaccharides  monosaccharides
proteins  amino acids
lipids (triglycerides) fatty acids

6. Processes of Digestion
5. Absorption – movement of nutrients from GIT into blood capillaries (monosaccharides, amino acids, H2O, vitamins, minerals) or lymphatic capillaries (fatty acids)
6. Excretion (Defecation) – removal of waste products from GIT

7. Histology of the GIT 4 layers of tissue surround the lumen of the GIT
epithelium – stratified squamous or simple columnar
lamina propria – loose CT
muscularis mucosa – smooth muscle
Mucosa
4 layers of tissue surround the lumen of the GIT
Submucosa
CT with BV’s, nerves & lymphatics
Muscularis externa
Skeletal muscle at beginning & end of GIT, smooth muscle (inner circular; outer longitudinal layer) from lower esophagus to rectum
Serosa (a.k.a. visceral peritoneum)

8. Peritoneum & Mesenteries
The abdominal cavity is lined with parietal peritoneum & many of the organs within are covered with visceral peritoneum
Folds of peritoneum called “mesenteries” attach some organs to others
greater omentum
lesser omentum
mesentery proper

9. Mouth (oral cavity) Regions include the vestibule & oral cavity proper
Roof comprised of hard & soft palate; floor primarily comprised of tongue
Mucosa of stratified squamous epithelium (non-keratinized)

10. intrinsic & extrinsic muscles
Tongue –
intrinsic & extrinsic muscles
papillae
filiform
fungiform
circumvallate

11. Mouth (oral cavity)
Salivary glands – secrete saliva – made of H2O, salts & “salivary amylase”
Parotid gland
Sublingual gland
Submandibular gland

12. involved in “mastication”
Teeth –
involved in “mastication”

13. Pharynx Common passageway for air & food
oropharynx & laryngopharynx lined with stratified squamous epithelium (nasopharynx lined with PSCC)
uvula & epiglottis protect airway when swallowing (“deglutition”)
nasopharynx
uvula
oropharynx
epiglottis
laryngopharynx

14. Esophagus
muscular tube extending from pharynx, posterior to trachea, down thoracic cavity, through “esophageal hiatus” of diaphragm, to lower esophageal (a.k.a. cardiac) sphincter at junction of stomach
functions in “deglutition” through action of peristalsis
mucosa is stratified squamous epithelium
variations in muscularis externa – begins as skeletal muscle at upper 1/3, mixed skeletal & smooth muscle in middle, smooth muscle by lower 1/3

15. Stomach - Gross Anatomy
Lower esophageal (cardiac) sphincter
Pyloric sphincter

16. Stomach - Histology
Rugae – folds of mucosa & submucosa to allow for expansion of stomach
Mucosa of simple columnar epithelium with mucous cells
Gastric pit leading to gastric glands

17. Stomach – Histology (cont)
- Secrete mucus to protect epithelial cells from enzymes & acid
- Secrete HCl (for protein digestion)
- Secrete pepsinogen which gets converted to “pepsin” when mixed with HCl; for protein digestion
(Peptic)

18. Stomach
Modifications in stomach include 3 layers of smooth muscle in muscularis externa – outer longitudinal, middle circular, innermost oblique layer
polypeptides
pepsin
HCl
proteins
Functions of stomach include:
temporary storage of food
mechanical breakdown of food to “chyme” through powerful mixing waves
start of chemical digestion of proteins –

19. Small Intestine - Anatomy
connects stomach to large intestine; 15-20’ long; 1” diameter; held together in abdominal cavity by “mesentery proper”
site for completion of chemical digestion & absorption of nutrients
comprised of three regions:
Duodenum – 10” in length; receives chyme from stomach, secretions from liver, gallbladder & pancreas
Jejunum – 8’ long; most digestion & absorption occurs here
Ileum – 12’ long; connects to cecum of large intestine at iliocecal valve (sphincter)

20. Small Intestine
Modifications in mucosa & submucosa of intestinal wall designed to increase functional surface area:
Plicae circulares (circular folds) – large transverse ridges; most abundant in jejunum
Villi – small finger-like projections of mucosal folds across surface of intestine
Plicae circulares

21. Small Intestine
Villi
Villi lined with “absorptive cells” - mucosal epithelium of simple columnar epithelium with microvilli “brush border” . These cells also produce enzymes (disaccharidases & peptidases) for final digestion of carbs and proteins
Submucosa of each villus contains a capillary network & a “lacteal” (lymphatic capillary) for absorption of nutrients

22. Pancreas
Retroperitoneal elongated organ, deep to stomach, from duodenum to spleen
Both endocrine (pancreatic islets of Langerhans – secretes insulin & glucagon) & exocrine gland (pancreatic acini – secrete pancreatic juice aka pancreatin)
Stomach
Head
Body
Tail
Duodenum
Pancreatic
duct

23. Pancreas
Pancreatic juice – mixture of enzymes & buffers (sodium bicarbonate) secreted by acinar cells into pancreatic duct & released into duodenum
pancreatic amylase
Starch (polysaccharide) maltose (disaccharide)
lipase
Lipids (triglycerides) fatty acids + monoglycerol
proteases (trypsin, chymotrypsin, carboxypeptidase)
Proteins & polypeptides small tripeptides & dipeptides
nucleases – digest RNA & DNA
sodium bicarbonate – neutralizes acidic chyme because enzymes in small intestine need an alkaline pH

24. Liver - Anatomy Comprised of 4 lobes:
Largest organ within the body
Comprised of 4 lobes:
Large right & left lobes divided by falciform ligament; small caudate & quadrate lobes
Lobes of liver functionally divided into microscopic lobules

25. Liver - Histology
Lobules comprised of rows of Hepatocytes arranged radially around a central vein
Hepatocytes surround blood sinusoids (capillary structures) which are partially lined with phagocytic Kupffer (aka stellate reticuloendothelial) cells
hepatocytes
central vein
sinusoids

26. Liver
Hepatocytes produce bile, which gets secreted into bile canaliculi of lobule
Bile canaliculi merge to form bile ducts which eventually merge to create the right & left hepatic ducts

27. Liver & gall bladder
Right & left hepatic ducts unite to form common hepatic duct which merges with cystic duct of gall bladder to form common bile duct which enters the duodenum
Right hepatic duct
Left hepatic duct
Gall bladder – hollow muscular sac under right lobe of liver; stores & concentrates bile
Bile released into duodenum functions in emulsification of lipids, absorption of fats (due to presence of bile salts), & excretion of bilirubin

28. Liver - Functions The liver has over 200 functions including:
Bile production & excretion
Metabolic regulation –
storage of glycogen, fatty acids, fat-soluble vitamins & minerals
interconversion of nutrients (“gluconeogenesis”)
detoxification & removal of drugs, toxins & hormones
hematological regulation –
phagocytosis of worn-out RBCs, bacteria & other pathogens
synthesis of plasma proteins

29. Blood Supply to Liver
In order for the liver to perform all of its functions, it receives blood through 2 vessels:
Hepatic artery - delivers oxygenated blood into sinusoids of liver
Hepatic Portal vein – delivers de-oxygenated, nutrient-rich blood from digestive organs to sinusoids of liver
Liver uses O2 & nutrients within blood of sinusoids & then blood drains into central veins of lobule & eventually into IVC

30. Large Intestine cecum - blind pouch; has appendix attached
3 regions:
cecum - blind pouch; has appendix attached
colon – ascending, (hepatic flexure), transverse, (splenic flexure), descending, sigmoid
rectum – last 1” known as “anal canal”
Begins at the ilium & ends at the anus; 5’ long; 3” in diameter
ileum
Ileocecal sphincter
Cecum
Vermiform appendix
Ascending colon
Transverse colon
Descending colon
Sigmoid colon
Hepatic (rt. Colic) flexure
Splenic (lt. colic) flexure
Rectum
Anal canal
Internal anal sphincter
Rectum
Rectum
Anal canal
External anal sphincter
Anus

31. Large Intestine
main functions – H2O re-absorption; absorption of some vitamins & minerals; formation & temporary storage of fecal material
no chemical (enzymatic) digestion but some bacterial
Simple columnar epithelium in mucosa, except at anal canal (strat. Squam.)
No plicae circularis or villi
Modifications in muscularis externa & serosa :
longitudinal muscle layer forms bands called “taeniae coli” which create puckers known as “haustra”
serosa forms “epiploic appendages”
haustra
taeniae coli
epiploic appendages

32. Overview of digestion & absorption

33. The End!!!