2 - Small Intestine and Pancreatic Secretions

Question 1

Objectives: Describe different types of contractions in the small intestines

Segmentation

Peristaltic

Migrating myoelectric complex

Answer 1

• Segmentation Contraction - Most common
  
  • Smooth muscle of isolated segments contract; propel food toward colon
  • Proximal Intestine Frequency > Distal Intestine Frequency
  • Mixes chyme with digestive juices
• Peristaltic
  
  • Propels chyme down small intestine; occurs only over short distances
• Migrating Myoelectric Complex (like stomach)
  
  • ~ 90 min
  • Clears remaining chyme

Question 2

Objectives: Describe how flow rate affects the ionic composition of pancreatic juice

Na

Cl

K

HCO₃

Answer 2

• Na - Stays the same
• Cl - Decreases
• K - Stays the same
• HCO₃ - Increases

Question 3

Objectives: Explain how the aqueous component of pancreatic juice is formed

Rates of secretion tonicity compared to plasma?

HCO3? Cl? K?

Rate dependent changes?

Answer 3

• At all rates of secretion, juice isotonic with plasma
• [HCO₃-] higher than plasma
• [Cl-] lower than in plasma
• [K+] ~ plasma
• At lowest flow rates: Juice = Na / Cl
• At highest flow rates: Juice = Na / HCO₃

AS FLOW RATE INCREASES, Na, K STAY THE SAME, HCO3 INCREAES, AND Cl DECREASES

Question 4

Objectives: Explain how the aqueous component of pancreatic juice is formed

Ductile / Centroacinar Modifications

H+

HCO3

Cl

Na

Water

Answer 4

• Acinus produces small volume of initial pancreatic juice, mainly Na, Cl
• Ductile and Centroacinar cells modify via:
  
  • Secrete HCO₃^-
  • Absorb Cl^-
• H+ transported out of cell by NHE-1 antiporter
  
  • Venous blood around pancrease has lower pH
• HCO3 co-transported into cell by NBC
  
  • Exchanged for Cl at lumen
  • Rate of secretion depends on Cl in lumen
• Cl enters lumen through apical channels
  
  • CFTR channels mutates in cystic fibrosis
• Na enters cell in exchange for H+
  
  • Tx out of cell by Na K ATPase
    
    • Keeps [Na] low
    • K exits through chnnel
• Water moves into lumen along osmotic gradient as pancreatic ducts are water-permeable

Question 5

Objective: Explain how pancreatic secretion is regulated

Regulation is phase dependent:

Cephalic and Gastric

Intestinal Phase (majority)

Potentiation

Answer 5

• Cephalic and Gastric Phase
  
  • ACh acts on acinar and ductile cells
  • Vagovagal reflex, stimulates secretion
    
    • Stomach distension
  • CCK releasing peptide and monitor peptide released in response to nerve input on I-cells
• Intestinal Phase (majority)
  
  • Stimulated by acid/fat/protein in duodenum
  • Acid releases secretin from S-cells
    
    • Secretin acts on ductule cells to increase HCO3 secretion which neutralizes acid
  • CCK - Increase enzyme secretion
    
    • ​Released from I cells in duodenum
  • CCK and ACh potentiate action of secretin on ductal cell secretion

Question 6

Objectives: Explain how Cystic Fibrosis and Pancreatitis affect pancreatic function

Cystic Fibrosis

Answer 6

• Cause:
  
  • Defect in Cl- channels
• Effect:
  
  • Reduces secretion of aqueous and enymatic components; lack of aqueous secretion leads to concentration of pancreatic juice, which blocks secretion of enzymation component
• Result:
  
  • Malabsorption and Steatorrhea

Question 7

Objectives: Explain how Cystic Fibrosis and Pancreatitis affect pancreatic function

Pancreatitis

Answer 7

• Forms:
  
  • Chronic
  • Acute
• Causes:
  
  • Chronic Alcoholism
  • Gallstones
  • High TAGs
  • Smoking
• Acute Symptoms:
  
  • Abdominal pain, swollen/tender abdomen
  • Nausea, Vomiting, Diarrhea, Fever
• Result:
  
  • ​Activated enzymes digest pancreatic tissue
  • Serum Amylase / Lipase ELEVATED
• Chronic:
  
  • Alcohol Abuse
  • High protein concentration in pancreatic juice
    
    • ​Reduced secretion of water, bicarbonate

Question 8

What are the different cell types of the small intestine?

Enterocytes

Goblet Cells

Crypt Cells

Answer 8

• Enterocytes
  
  • Epithelial; microvilli protrude from surface
  • Digestion, Absorption, Secretion
• Goblet Cells
• Crypt Cells
  
  • Proliferative Cells - form enterocytes / goblet cells
  • Secrete fluids and electrolytes

Question 9

Objectives: Describe different types of contractions in the small intestines

Control:

Peristaltic Reflex/Rush

Intestinointestinal Reflex

Gastroileal Reflex

Gastrocolic Reflex

Ileus

Answer 9

• Peristaltic Reflex/Rush
  
  • Initiated by chyme in intestine; causing distension/irritation
  • Sever infectious diarrhea
• Intestinointestinal Reflex
  
  • Over distension of one segment inhibits contractile activity in rest of intestine
  • Prevents further adding contents to a full space
• Gastroileal Reflex
  
  • Gastric secretion and emptying triggers increases peristalsis in ileum
  • Relaxes ileocecal sphincter and movement of ileal contents into colon
• Gastrocolic Reflex
  
  • When you eat you have to poop.
• Ileus
  
  • Loss (reduction) in contractile activity of intestines in absence of blockage
  • Irritation of peritoneum
  • Cause: Surgery, illness, electrolyte imbalance

Question 10

Describe the different pancreatic cell types

Acinar - what do they produce?

Centroacinare and Duct cells - purpose?

Answer 10

• Acinar cells produce:
  
  • Peptidases - digest protein
  • Lipases - digest fat
  • Amylase - digest carbs
• Centroacinare and Duct cells secrete pancreatic juice
  
  • ​High HCO₃- neutralizes gastric acid in duodenum
  • Raises pH for optimal enzymatic digestion of nutrients

Question 11

How are the enzymatic components of pancreatic secretions stored and released?

Lipase

Amylase

Proteases

Answer 11

• Lipase / Amylase secreted in active form
  
  • ​Fats, Carbs
• Proteases (Trypsin, Chymotrypsin) secreted in inactive form
  
  • ​Proteins
  • Activated in small intestine
  • Trypsin inhibitor secreted to protect pancreas from autodigestion