Digestive System

Physiology Exam 3 Material

four major layers of the digestive tract

• mucosa

• submucosa

• muscularis externa

• serosa

mucosa layer

nerves, blood and lymph vessels

submucosa layer

connective tissue, plexus

muscularis externa layer

two layers of smooth muscle, plexus

serosa layer

continuation of peritoneal membrane

portions of the stomach

• fundus

• body

• antrum

• pyloric valve

rugae

permits stomach expansion

relaxed rugae

mucosa forms numerous muscular ridges

stretched rugae

less prominent rugae

regions of the small intestine

• duodenum

• jejunum

• ileum

duodenum

10 inches long, receives digestive enzymes from the pancreas, bile from the liver and gall bladder

jejunum

8 feet long, most of the digestion and absorption occurs here

ileum

12 feet long

lining of the small intestine

surface area is increased by fingerlike villi and invaginations called crypts

structures within the villi

capillaries and lacteals

villi function

absorb the digested nutrients from the lumen of the small intestine into the capillaries

lacteal function

absorb material that cannot be absorbed by the capillaries

sections of the pancreas

endocrine and exocrine

endocrine cells

islet of langerhans, alpha cells (glucagon), beta cells (insulin)

exocrine ells

acting cells (zymogens), duct cells (bicarbonate)

gall bladder function

stores and concentrates bile

bile composition

• bile salts: facilitate enzymatic fat digestions

• bile pigments: bilirubin

• cholesterol

liver functions

• produces bile

• stores glycogen

• inactivates xenobiotics

• inactivates many hormones

how does the liver inactivate xenobiotics?

modifies their structures to make them water-soluble for excretion by the kidneys

liver microstructure

organized into lobules centered on central vein; within each lobule, ~70% of the surface area of each hepatocyte faces the sinusoids, maximizing the exchange between the blood and the cells

hepatic portal vein

absorbed nutrients from gut and Hb breakdown products from spleen

hepatic artery

oxygenated blood

what type of vascular supply does the liver have?

dual vascular supply

biological filter

absorbed materials go directly to the liver

liver as a storage organ

• stores fat-soluble vitamins A, and several months supply of E and K

• stores vitamin B12

• stores iron and copper

macronutrient metabolism of liver

• carbohydrate metabolism

• amino acid metabolism

• fatty acid metabolism

• protein metabolism

carbohydrate metabolism in liver

involved in glycogenesis, glycogenolysis, and gluconeogenesis

amino acid metabolism in liver

• catabolize amino acids or use them for gluconeogenesis

• synthesizes much of the body’s non-essential amino acids

fatty acid metabolism in liver

• produces triglycerides

• metabolizes fatty acids through beta-oxidation and makes ketones

• synthesized cholesterol

protein metabolism in liver

synthesizes the majority of plasma proteins

gluconeogenesis

synthesis of glucose from amino acids, lactate, glycerol

glycogenolysis

breakdown of glycogen into glucose

glycogenesis

formation of glycogen from glucose

four major classes of the circulating plasma lipoproteins

• chylomicrons: produced in small intestine

• very low-density lipoproteins (VLDLs): majority produced in the liver

• low-density/high-density lipoproteins (LDLs/HDLs): produced in plasma and the liver also produces a small amount

anatomy of the large intestine

cecum w/ appendix, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, anus

basic functions of GI tract

1. digestion

2. secretion

3. motility

4. absorption

digestion function

chemical and mechanism breakdown of foods into absorbable units

secretion function

transfer of water, ions and enzymes into lumen from ECF

motility function

moves food through GI tract

absorption function

movement of nutrients from lumen to ECF and blood

propulsion functions and locations

• swallowing: oropharynx

• peristalsis: esophagus, stomach, small intestine, large intestine

mechanical digestion functions and locations

• chewing (mouth)

• churning (stomach)

• segmentation (small intestine)

motility

contractions in the GI tract; migrating motor complex: “housekeeping”

motility contractions in the GI tract

• segmental contractions: mixing

• peristalsis: forward movement

secretions

1. ions and water: Na+, K+, Cl-, HCO3-, and H+

2. acids: parietal cells

3. bicarbonate: duct cells of pancreas, small amount of duodenal cells

4. digestive enzymes: zymogens (pancreas)

5. mucus: mucous cells (stomach) and goblet cells (intestines)

6. saliva

phases of processing food

1. cephalic

2. gastric

3. intestinal

cephalic processing

sight, smell, thoughts of food > brain activates feedforward response

gastric processing

foods combine with acid and enzymes to form chyme

intestinal processing

majority of digestion and absorption

what role does the mouth play in cephalic digestive processing?

site of chemical and mechanical digestion

how is saliva secretion controlled?

through the autonomic (parasympathetic) nervous system

saliva function

soften and lubricate food

chewing

mastication

enzymes involved in cephalic chemical digestion

salivary amylase and other lipases

types of saliva

• stimulated saliva

• unstimulated saliva

stimulated saliva

• 70-90% of saliva produced

• 99% water

• 1% proteins and minerals

unstimulated saliva

• background saliva

• viscous, elastic, sticky: mucins

• forms protective film on teeth

stomach functions in the gastric phase

1. storage

   • receptive relaxation of upper stomach

   • regulates entry into SI

2. digestion

   • acid, enzymes and signal molecules

   • formation of chyme

3. protection

   • destroys ingested bacteria/pathogens

   • protects itself with mucous-bicarbonate layer

secretory cells

• parietal cells

• chief cells

• g cells

parietal cell function

secrete intrinsic factor and gastric acid (HCl)

• intrinsic factor: facilitates the absorption of vitamin B12

• HCl: kills microorganisms and activates pepsinogen

chief cell functions

secrete pepsinogen, which is converted to pepsin via action of HCl

g cell function

produce hormone gastrin which causes the parietal and chief cells to release their products

key intestinal secretions for digestion

1. bicarbonate: neutralizes gastric acid

2. goblet cells: secrete mucus for protection and lubrication

3. isotonic NaCl: mixes with mucus for lubrication

4. bile: fat digestion

5. digestive enzymes: secreted by intestinal epithelium and pancreatic acinar cells

gallbladder functions

• stores and secretes bile

• expels bile into duodenum

• contraction stimulated by CCK (fats in meals)

lipid digestion in the mouth

• mechanical digestion

• mixing with saliva

• limited enzymatic digestion (lingual lipase)

lipid digestion in the stomach

• mixing/churning

• limited enzymatic digestion (gastric lipase)

lipid digestion in the small intestine

• emulsification (bile)

• enzymatic digestion (pancreatic lipases)

• micelles help with absorption

micelles

small disks with bile salts, phospholipids, fatty acids, cholesterol, and mono- and diglycerides

carbohydrate digestion in the mouth

enzyme salivary amylase begins breaking down starch into shorter polysaccharides

carbohydrate digestion in the stomach

salivary amylase is inactivated and no further carbohydrate digestion takes place

carbohydrate digestion in the small intestine

majority of starch digestion and breakdown of disaccharides occurs here, enzymatic pancreatic amylase breaks down starch into monosaccharides, disaccharides and oligosaccharides, digestion is completed by enzymes attached to the brush border of the villi

carbohydrate digestion in the large intestine

fiber and other indigestible carbohydrates are partially broken down by bacteria to form short chain fatty acids and gas, remaining fiber is excreted in the feces

carbohydrate digestion in the small intestine diagram

endopeptidase fucntion

digests terminal peptide bonds to release amino acids

endopeptidase examples

• pepsin (stomach)

• trypsin (small intestine)

• chymotrypsin (small intestine)

peptide absorption

• di- and tri-peptides: cotransport with H+

• amino acids: cotransport with Na+