Lecture 21 - Small Intestine

Small intestine is the main site for:

digestion & absorption

Functions of small intestine are:

• complete chemical digestion

• absorb nutrients

• absorb water

• move chyme toward large intestine

Subdivisions of small intestine:

1. Duodenum

2. Jejunum

3. Ileum

Duodenum

• 1st part

• receives:

  • chyme from stomach

  • pancreatic enzymes

  • bile

• contains duodenal glands (alkaline mucus)

• neutralizes acid

Jejunum

• major absorption region

• largest surface area activity

Ileum

• final absorption region

• ends at ileocecal valve

• absorbs

  • Vitamin B12

  • bile salts

pyloric sphincter

opening of stomach connecting to → duodenum

ileocecal valve

junction between small intestine → beginning of large intestine

Why is small intestine so long?

• to maximize absorption

• long length allows:

  • more contact time

  • more surface area

  • greater nutrient uptake

Structure for absorption

3 levels:

1. circular folds

2. villi

3. microvilli

Circular folds

• permanent folds

• slow movement of chyme

• force chyme to contact wall

villi

finger-like projections

• increase surface area for absorption

• inside each villus:

  • blood capillaries

  • lacteal (lymph vessel)

microvilli

(brush border)

• tiny projections on enterocytes

function:

• massive surface area increase

• contain brush border enzymes

important cells of small intestine

• enterocytes

  • main absorptive cells

• enteroendocrine cells

  • release hormones:

    • CCK

    • secretin

• goblet cells

  • secrete mucus

  • lubrication + protection

• paneth cells

  • antimicrobial defense

Intestinal crypts

• produce intestinal juice

• secrete water + mucus

intestinal juice characteristics

• slightly alkaline

• isotonic to plasma

• mostly water

• contains mucus

  • mucus is secreted by duodenal glands and goblet cells of mucosa

purpose of intestinal juice

• protect intestine

• lubricate chyme

• facilitate enzyme activity

Digestive enzymes come from:

Saliva

• salivary amylase

• lingual lipase

Stomach

• pepsin

• gastric lipase

Pancreas

• most IMPORTANT enzyme source

Brush border enzymes

• membrane-bound enzymes

how long does it take in the SI to absorb all nutrients and most water?

3-6 hours

chyme entering duodenum is usually:

hypertonic → chyme delivery has to be slow to prevent osmotic loss of water from blood

if too much enters:

• water leaves blood

• osmotic imbalance occurs

so entry must be slow!

hypertonic

high solute concentration

if too much enters:

• water leaves blood

• osmotic imbalance occurs

enterogastrones control…

movement of food into duodenum to prevent it from being overwhelmed

hormones that:

• slow gastric emptying

• protect duodenum

Motility of SI

• after a meal

  • segmentation → most common motion

  • mixes/moves contents toward ileocecal valve

  • moves slightly foward/backward

  • improves absorption

• between meals

  • peristalsis increases

  • initiated by rise in hormone motilin in late intestinal phase (every 90-120 mins)

  • migrating motor complex (MMC)

  • meal remnants, bacteria, and debris are moved toward large intestine

  • complete trip from duodenum to ileum takes ~2 hours

Segmentation function

• mixes chyme

• improves absorption

• moves slightly foward/backward

peristalsis function

• pushes food forward

Migrating motor complex (MMC)

occurs in between meals

function: intestinal cleaning wave

removes

• leftover food

• debris

• bacteria

triggered by motilin hormone

MMC is triggered by which hormone?

motilin hormone

ileocecal sphincter

• controls entry into large intestine

• opens when

  • gastroileal reflex activated

  • gastrin increases motility of ileum

Digestion begins in stomach when …

pepsinogen is converted to pepsin at pH 1.5 - 2.5

• becomes inactive in high pH of duodenum

Digestion pathway:

polysaccharides → disaccharides → monosaccharides

Final absorbable forms:

glucose, fructose, galactose

Electrolyte absorption

• most ions

• iron + calcium → absorbed in duodenum

• sodium → drives nutrient absorption

• Potassium → diffuses in response to osmotic gradients; lost if water absorption is poor

Water absorption

• 95% is absorbed in SI by osmosis

• most of the rest is absorbed in LI

• water uptake is coupled w solute uptake

Steps in lipid digestion in intestine

1. emulsification

• bile breaks fat into smaller droplets

• bile is NOT an enzyme

• bile = emulsifier

2. Digestion: pancreatic lipases

• breaks triglycerides into: fatty acids + monoglycerides

3. micelle formation

• micelles carry lipids to intestinal cells

4. diffusion of enterocyte

• lipids diffuse across membrane

5. Chylomicron formation

• lipid products are converted back into triglycerides and packaged w lecithin and lipoproteins, forming chylomicron

6. lacteal entry

• chylomicrons are exocytosed and enter lymphatic lacteal

Vitamin absorption

in small intestine:

• fat soluble vitamins → carried by micelles; diffuse into cells

• water soluble vitamins → absorbed by diffusion or by passive or active transporters

• vitamin B12 binds to intrinsic factor and is absorbed by endocytosis

Fat soluble vitamins

• A

• D

• E

• K

need:

• micelles

• fat absorption

water soluble vitamins

• B

• C

absorbed thru:

• diffusion

• transporters

Vitamin B12

• special case

• intrinsic factor

  • absorbed by endocytosis

Appendicitis

most common in adolescence when entrance to appendix is at widest (inflammation)

Appendicitis can cause _____

peritonitis (swelling/infection of abdomen or belly or peritoneum)

Lactose intolerance

• Any lactose eaten remains undigested and creates an osmotic gradient in intestine → prevents water from being absorbed

• pull water from interstitial space into intestinal lumen

Diarrhea

• watery stools

• causes

  • irritation of colon by bacteria or jostling of digestive viscera (marathon runners)

  • prolonged diarrhea may result in acidosis and loss of potassium

Constipation

• can cause low transit

• fiber helps by

  • adding bulk

  • stimulating motility

Malabsorption

caused by anything that interferes w delivery of bile or pancreatic juice

• damaged intestinal mucosa

celiac disease (gluten-sensitive enteropathy)

• common malabsorption disease

• immune reaction to gluten (damages villi and brush border)

• treatment: eliminate gluten from diet

GERD/heartburn

• stomach acid reflux into esophagus

• first symptom of gastroesophageal reflux disease (GERD)

• may cause:

  • esophagitis

  • ulcers

  • cancer risk

Gastritis

• inflammation caused by anything that breaches stomachs mucosal barrier

Peptic or gastric ulcers

erosions in stomach wall

• peritonitis and hemorrhage

• most ulcers caused by bacterium helicobacter pylori

• can also be caused by non-steroidal anti inflammatory drugs (NSAIDs) such as aspirin