NPB 101: Lecture 32

Small Intestine

Primary site of digestion and absorbtion

Sections of Small Intestine

3 sections that are collectively responsible for digestion and absorption of nutrients

• Duodenum (5% of length)

• Jejunum (35-40%)

• Ileum (55-60%)

Small Intestine - Segmentation

Segmentation

• Mixes and propels chyme. Propulsion occurs bc frequency of contractions gradually decreases along length of small intestine

  • 
    

Role of Segmentation

Mixes and propels chyme

• Mixing food with digestive juices

• Facilitate absorption

• Forward movement (slow and non-linear)

Small Intestine - Absorption

The small intestine has a large surface area to facilitate absorption

• Inner surface has circular folds → increase surface area 3x

• Surface of circular folds contain microscopic villi → 10x

• Surface of villi contain microvilli → 20x

All together, the folds, villi, and microvilli increase surface area by 600 times

Small Intestine- Digestion

In the intestinal lumen

• Carbohydrate / protein digestion accomplishied by pancreatic enzymes

• Fat digestion enhanced by bile secretions

3 types of enzymes on Brush Border

• Enteropeptidase

• The disaccharides: maltase, sucrase, and lactase which complete digestion of carbohydrates

• The aminopeptidases → complete digestion of proteins

Small Intestine- Carbohydrate Digestion

• Maltose, lactose, and sucrose are converted to monosaccharides (glucose, galactose, fructose) on brush border of intestinal epithelial cells by enzymes lactase, maltase, and sucrase

  • Glucose / Galactose absorbed into epithelial cell by active transport (SGLT)

  • Fructose enters epithelial cell by passive facilitated diffusion (GLUT-5)

• Glucose, galactose, and fructose exit cell into blood by passive facilitated diffusion (GLUT-2) to blood capillary

Small Intestine - Protein Digestion

• Proteins hydrolyzed into small peptide fragments and individual AA by pepsin and pancreatic proteolytic enzymes

  • Small peptides broken down into AA on brush border by peptidases and aminopeptidases

• AA absorbed into cell via Na⁺ and energy-dependent active transport → Enter blood capillaries down their conc. gradients

Small Intestine - Fat Digestion

• Fat emulsified by detergent action of bile salts / segmentation

• Lipases hydrolyze triglycerides into monoglycerides and free F.A.

• Water insoluble products move within interior of micelles to epithelial cell surface

• Monogylcerides and free F.A. diffuse into cell

• Monoglycerides and free F.A. resynthesize into triglycerides

• Triglycerides coated with lipoprotein → form chylomicrons that are exocytosed from cell

• Chylomicrons enter lympathic vessels

Small Intestine - Fat Digestion in hindsight

Fat (triglycerides) are emulsified by bile salts → lipase breaks into monoglyceride and free F.A. → move within water-soluble micelles → monoglycerides / free F.A. diffuse into cell → within cell, monoglycerides / free F.A. resynthesize into triglycerides → triglycerides coated with lipoprotein → form chylomicrons → chylomicrons are exocytosed from cell → enter lympathic vessels

One-way flow fromm small to large intestine

One-way flow of contents from ileum into cecum (1st part of large intestine)

• Pushes valve open and relaxes sphincter at ileum

• Pushes valve closed and contracts sphincter at cecum

Necessary to keep colonic bacteria from entering the ileum

Large Intestine Fxn’s

Primarily for drying and storage

Sections of Large Intestine

• Cecum (blind-ended pouch below ileocecal valve)

• Appendix (finger-like projection of lymphoid tissue)

• Colon (ascending, transverse, descending, and sigmoid)

• Rectum (“straight”, connected to anal canal)

Large Intestine - Anal Sphincter

Internal Anal Sphincter

• Smooth muscle → No voluntary control

External Anal Sphincter

• Skeletal muscle → Voluntary control → decide when is an appropriate time to deficate

Large Intestine- External Anal Sphincter

Skeletal muscle → Voluntary control → decide when is an appropriate time to deficate

Large Intestine - Haustral Contractions

Slowly shuffle contents of large intestine to aid absorption (primarily of water / salt)

• Gradually change location, forming haustra

Large Intestine - Mass Movement

Large contractions in colon that rapidly drive contents forward, typically after a meal. Presence of chyme in stomach triggers the gastrocolic reflex.

• Gastrocolic Reflex → Takes whatever contents have been building up in colon and a quick peristaltic wave start to push out, causing relaxation of internal sphinter and external sphincter if chosen → defecate

Gastrocolic Reflex

Takes whatever contents have been building up in the colon and a quick peristaltic wave starts to push out → causes relaxation of internal sphincter and if you chose to relax external sphincter → defecation

Large Intestine - Defecation Reflex

Mass movement of feces into rectum, which stimulates stretch receptors

• Causes internal anal sphincter (smooth muscle) to relax and rectrum / sigmoid colon to contract

• IF external anal sphincter (skeletal muscle) is also relaxed, defecation occurs

Constipation

Occurs if defecation is delayed too long and too much water is absorbed from the feces → become dry / hard

Appendicitis

Hardened feces lodged in appendix → obstruct normal circulation and mucus secretion