1. Chapter 24
The Digestive System
Part E

2. Microscopic Anatomy of the Liver
Hexagonal-shaped liver lobules are the structural and functional units of the liver
Composed of hepatocyte (liver cell) plates radiating outward from a central vein
Portal triads are found at each of the six corners of each liver lobule
Portal triads consist of a bile duct and
Hepatic artery – supplies oxygen-rich blood to the liver
Hepatic portal vein – carries venous blood with nutrients from digestive viscera

3. Microscopic Anatomy of the Liver
Figure 24.24c, d

4. Microscopic Anatomy of the Liver
Liver sinusoids – enlarged, leaky capillaries located between hepatic plates
Kupffer cells – hepatic macrophages found in liver sinusoids
Hepatocytes’ functions include:
Production of bile
Processing bloodborne nutrients
Storage of fat-soluble vitamins
Detoxification
Secreted bile flows between hepatocytes toward the bile ducts in the portal triads

5. Microscopic Anatomy of the Liver
Figure 24.24c, d

6. Homeostatic Imbalance
Hepatitis – inflammation of the liver often due to viral infection
Viruses causing hepatitis are catalogued has HVA through HVF
HVA and HVE are transmitted enterically and cause self-limiting infections
Hepatitis B is transmitted via blood transfusions, contaminated needles, and sexual contact, and increases the risk of liver cancer
Hepatitis C produces chronic liver infection
Nonviral hepatitis is caused by drug toxicity and wild mushroom poisoning

7. Homeostatic Imbalance
Cirrhosis – diffuse and progressive chronic inflammation of the liver
Typically results from chronic alcoholism or severe chronic hepatitis
The liver becomes fatty and fibrous and its activity is depressed
Scar tissue obstructs blood flow in the hepatic portal system causing portal hypertension

8. Composition of Bile
A yellow-green, alkaline solution containing bile salts, bile pigments, cholesterol, neutral fats, phospholipids, and electrolytes
Bile salts are cholesterol derivatives that:
Emulsify fat
Facilitate fat and cholesterol absorption
Help solubilize cholesterol
Enterohepatic circulation recycles bile salts
The chief bile pigment is bilirubin, a waste product of heme

9. The Gallbladder
Thin-walled, green muscular sac on the ventral surface of the liver
Stores and concentrates bile by absorbing its water and ions
Releases bile via the cystic duct which flows into the bile duct

10. Regulation of Bile Release
Acidic, fatty chyme causes the duodenum to release:
Cholecystokinin (CCK) and secretin into the bloodstream
Bile salts and secretin transported in blood stimulate the liver to produce bile
Vagal stimulation causes weak contractions of the gallbladder
Cholecystokinin causes:
The gallbladder to contract
The hepatopancreatic sphincter to relax
As a result, bile enters the duodenum

11. Regulation of Bile Release
Figure 24.25

12. Homeostatic Imbalance
Gallstones – crystallization of cholesterol which can obstruct the flow of bile
Current treatments include: dissolving the crystals with drugs, pulverizing them with ultrasound, vaporizing them with lasers, and surgical removal of the gallbladder
Obstructive jaundice – yellowish skin caused by bile pigments deposited in the skin
Due to blocked bile ducts

13. Pancreas Location Exocrine function
Lies deep to the greater curvature of the stomach
The head is encircled by the duodenum and the tail abuts the spleen
Exocrine function
Secretes pancreatic juice which breaks down all categories of foodstuff
Acini (clusters of secretory cells) contain zymogen granules with digestive enzymes
The pancreas also has an endocrine function – release of insulin and glucagon

14. Pancreas
Figure 24.26a

15. Composition and Function of Pancreatic Juice
Water solution of enzymes and electrolytes (primarily HCO3)
Neutralizes acid chyme
Provides optimal environment for pancreatic enzymes
Enzymes are released in inactive form and activated in the duodenum

16. Composition and Function of Pancreatic Juice
Examples include
Trypsinogen is activated to trypsin
Procarboxypeptidase is activated to carboxypeptidase
Active enzymes secreted
Amylase, lipases, and nucleases
These enzymes require ions or bile for optimal activity

17. Regulation of Pancreatic Secretion
Secretin and CCK are released when fatty or acidic chyme enters the duodenum
CCK and secretin enter the bloodstream
Upon reaching the pancreas:
CCK induces the secretion of enzyme-rich pancreatic juice
Secretin causes secretion of bicarbonate-rich pancreatic juice
Vagal stimulation also causes release of pancreatic juice

18. Regulation of Pancreatic Secretion
Figure 24.28

19. Digestion in the Small Intestine
As chyme enters the duodenum
Carbohydrates and proteins are only partially digested
No fat digestion has taken place
Digestion continues in the small intestine
Chyme is released slowly into the duodenum
Because it is hypertonic and has low pH, mixing is required for proper digestion
Required substances needed are supplied by the liver
Virtually all nutrient absorption takes place in the small intestine

20. Motility of the Small Intestine
The most common motion of the small intestine is segmentation
It is initiated by intrinsic pacemaker cells (Cajal cells)
Moves contents steadily toward the ileocecal valve
After nutrients have been absorbed:
Peristalsis begins with each wave starting distal to the previous
Meal remnants, bacteria, mucosal cells, and debris are moved into the large intestine

21. Control of Motility
Local enteric neurons of the GI tract coordinate intestinal motility
Cholinergic neurons cause:
Contraction and shortening of the circular muscle layer
Shortening of longitudinal muscle
Distension of the intestine
Other impulses relax the circular muscle
The gastroileal reflex and gastrin:
Relax the ileocecal sphincter
Allow chyme to pass into the large intestine

22. Large Intestine Has three unique features:
Teniae coli – three bands of longitudinal smooth muscle in its muscularis
Haustra – pocketlike sacs caused by the tone of the teniae coli
Epiploic appendages – fat-filled pouches of visceral peritoneum
Is subdivided into the cecum, appendix, colon, rectum, and anal canal
The saclike cecum:
Lies below the ileocecal valve in the right iliac fossa
Contains a wormlike vermiform appendix

23. Large Intestine
Figure 24.29a

24. Homeostatic Imbalance
Appendicitis – inflammation of the appendix resulting from blockage that traps infectious bacteria in its lumen
If the appendix ruptures, feces containing bacteria spray over the abdominal contents causing peritonitis
Treatment is surgical removal of the appendix