The Digestive System�Chapter 25

Function of the Digestive System

• To break down food into a “usable” (absorbable) form
• To supply our cells with the nutrients they need for energy, growth & repair

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

​

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

Processes of Digestion

4. 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 → disaccharides → monosaccharides
• proteins → amino acids
• lipids → fatty acids & monoglycerides

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

Histology of the GIT

4 layers of tissue surround the lumen of the GIT

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
• mesocolon

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)

• Joins to the oropharynx at the fauces

Tongue –

• stratified sqamous epith. over skeletal muscle
• intrinsic & extrinsic muscles
• papillae
• filiform
• fungiform
• circumvallate

taste buds

Parotid gland

Submandibular gland

Sublingual gland

• Salivary glands – secrete saliva – made of H₂O, salts & “salivary amylase”

Parotid duct

Submandibular duct

Teeth – involved in “mastication”

• 2 sets of teeth – deciduous & permanent
• 4 types of teeth – incisors, cuspids (canines), bicuspids (premolars), molars

​

Parts of a tooth –

• crown – dentin surrounded by enamel, has hollowed pulp cavity filled with CT pulp
• neck – at gingival border
• root – within mandible & maxilla, has hollowed root canal with BVs & nerves, root canal opens at apical foramen

Pharynx

nasopharynx

uvula

oropharynx

epiglottis

laryngopharynx

Common passageway for air & food

• oropharynx & laryngopharynx lined with stratified squamous epithelium (nasopharynx lined with PSCC)
• uvula & epiglottis protect airway when swallowing (“deglutition”)

Esophagus

• muscular tube running 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

Stomach - Gross Anatomy

Lower esophageal (cardiac) sphincter

Pyloric sphincter

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

Stomach – Histology (cont)

- Secrete mucus to protect epithelial cells from enzymes & acid

- Secrete HCl (for protein digestion) & intrinsic factor (for B12 absorption)

- Secrete pepsinogen which gets converted to “pepsin” when mixed with HCl; for protein digestion

- Secrete gastrin to regulate stomach emptying

Entero-

(peptic cells)

(G-cells)

Stomach

• Modifications in stomach include 3 layers of smooth muscle in muscularis externa – outer longitudinal, middle circular, innermost oblique layer

Functions of stomach include

• temporary storage of food
• mechanical breakdown of food to “chyme” through powerful mixing waves
• intrinsic factor for vitamin B12 absorption
• start of chemical digestion of proteins –

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)

Small Intestine

Modifications in mucosa & submucosa of intestinal wall designed to increase functional surface area:

Plicae circulares

• Plicae circulares (circular folds) – large transverse ridges; most abundant in jejunum
• Villi – small finger-like projections of mucosal folds across surface of intestine

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

Small Intestine

Villi

Pancreas

​

​

• Retroperitoneal elongated organ lying posterior to stomach, from duodenum to spleen
• Both endocrine (pancreatic islets of Langerhans – secretes insulin & glucagon) & exocrine gland (pancreatic acini – secretes pancreatic juice (aka pancreatin) through pancreatic duct(s) to duodenum

Pancreatic

duct

Duodenum

Pancreas

Pancreatic juice – mixture of enzymes & buffers (sodium bicarbonate) secreted by acinar cells into pancreatic duct & released into duodenum

• pancreatic amylase

Starch maltose

• lipase

Lipids fatty acids + monoglycerol

• proteases (trypsin, chymotrypsin, carboxypeptidase)

Proteins & polypeptides small peptides tri & dipeptides

• nucleases – digest RNA & DNA

• sodium bicarbonate – neutralizes acidic chyme because enzymes in small intestine need an alkaline pH

Liver - Anatomy

• Largest organ within the body
• Comprised of 4 lobes:
• Large right & left lobes divided by falciform ligament; small caudate (by IVC) & quadrate (by gall bladder ) lobes
• falciform ligament continues at inferior margin as ligamentum teres (round ligament) (remnant of umbilical vein)
• Lobes of liver functionally divided into microscopic lobules

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

Liver

• One function of hepatocytes is to produce bile, which gets secreted into bile canaliculi of lobule
• Bile canaliculi merge to form bile ducts, which are part of the portal triad seen at each corner of the lobules. Bile ducts merge to eventually create the right & left hepatic ducts

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 joins with pancreatic duct & enters the duodenum

• Gall bladder – hollow muscular sac under right lobe of liver; stores & concentrates bile; releases bile through cystic duct

• Bile released into duodenum functions in emulsification of lipids, absorption of fats (due to presence of bile salts), & excretion of bilirubin

Left hepatic duct

Right hepatic duct

Pancreatic & bile ducts�

Head

Pancreatic duct

Accessory pancreatic duct

Common bile duct

Liver - Functions

The liver has over 200 functions including (but not limited to):

• 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

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 which merge to form the hepatic veins, which drain into the IVC

Large Intestine

• Begins at the ilium & ends at the anus; 5’ long; 3” in diameter

Rectum

Anal canal

Rectum

Rectum

Internal anal sphincter

External anal sphincter

Anal canal

Anus

• 3 regions:

cecum - blind pouch; has appendix attached

colon – ascending, (hepatic flexure), transverse, (splenic flexure), descending, sigmoid

rectum – last 1” known as “anal canal”

Hepatic (rt. Colic) flexure

Splenic (lt. colic) flexure

Large Intestine

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

THE END (literally!)

• main functions – H2O re-absorption; absorption of some vitamins & minerals; formation & temporary storage of fecal material

no chemical (enzymatic) digestion but some bacterial