The Digestive System

Functions of Digestive System

food

Intestines

Absorbablenutrients

lumen

blood

wallwall

digestionfood

Intestines

Absorbablenutrients

lumen

blood

wallwall

digestionfood

Absorbablenutrients

blood

absorption

Forms of Nutrients

In Food Absorbable by Intestine

Protein Amino acidsCarbohydrates Monosaccharides (glucose)Fat Fatty acids, glycerol DNA, RNA Bases + monosaccharides Vit B12 B12+intrinsic factor Other vitamins Original formCholesterol Original formElectrolytes Original form Water Original form

digestionfood

blood

absorption

Amino acids

Glucose

Fatty acids,

Glycerol

Bases +

monosaccharides

vitamins

Cholesterol

Electrolytes,

Water

The Organs of the Digestive System

The Mouth

The Salivary Glands

The Esophagus

The Stomach

The Liver

The Gallbladder

The Pancreas

The Small intestines

The Large intestines

The Mouth

has the following functions :

1) breaking food

2) sense of taste

3) swallowing

4) speech

Saliva and the Salivary Glands

1) moisten food and the mouth

2) clean the teeth, inhibit bacterial growth,

3) dissolve molecules so they can stimulate taste buds

4) digest a small amount of starch and fat

Esophagus

Esophagus

The Stomach

The Stomach

Functions

- food storage- host defense- minor role in digestion and

absorption of nutrients

- The above functions are performed via gastric secretion and motility

Gastric pits

G cells: produce hormones that regulate digestion

chief cells: produce pepsinogen

parietal cells: produce HCl and intrinsic factor

mucous cells: produce mucusThe bottom part of

the pits is the gastric gland.

The gastric glands

produce 2-3 L of gastric juice

daily, containing:

pepsinogen

HCl

intrinsic factor

Pepsinogen

Pepsinogen

- is the inactive precursor of the active enzyme pepsin.

- activated by HCl or pepsin.

Pepsin: chop proteins into small fragments

  Hydrochloric Acid ( HCl )

- reduces pH of

gastric juice to as low

as pH 0.8.

HCl

HCl

Functions of HCl:

(1) activates pepsinogen

into pepsin.

(2) breaks up connective

tissues and plant cell

walls.

(3) converts ferric ions to

ferrous ions.

(4) destroys ingested

pathogens.

1) a highly alkaline mucous coat.

2) rapid replacement of epithelial cells (3-6 d)

3) tight junctions between epithelial cells

HClSelf protection of the stomach from HCl and pepsin by

 Intrinsic Factor

- is essential to the absorption of vitamin B12

- is the only indispensable function of the stomach.

Intrinsic Factor

Gastrinsecreted by G cells in gastric gland not into gastric juice but blood

- is a hormone - stimulates

1) the secretion of HCl and pepsinogen

2) motility of the large intestine

- relax during swallowing

- stretch further when food enters the stomach (stress-relaxation response)

Gastric Motility

- Next, pacemaker cells in the greater curvature initiate peristaltic contractions.

- Each peristaltic wave pushs ~ 3 mL of chyme into the duodenum.

Regulation of Gastric Function

Gastric secretion and motility is divided into three stages:

1) Cephalic

2) Gastric

3) Intestinal phases

The cephalic phase is stimulated by the sight, smell, taste, or mere thought of food.

1) The Cephalic Phase

- is stimulated by food in the

stomach

- accounts for two-thirds of

gastric secretion.

2) The Gastric Phase

a) the enterogastric reflex.

b) local hormones

Secretin

Cholecystokinin

gastric inhibitory peptide

3) The Intestinal Phase

After entering

small intestines, chyme inhibit

gastric secretion and mobility

via:

The small intestine receives not

only chyme from the stomach but

also secretions from the liver and

pancreas.

The Liver

has tremendous

variety of functions,

including the

secretion of bile

for digestive

purposes.

Hepatocytes secret bile into the bile canaliculi and bile ductules.

bile ductule

Bile ductules hepatic ducts common hepatic duct

cystic duct

common hepatic duct

gallbladder

common bile duct

hepatopancreatic sphincter

pancreas

duodenum

When no chyme is in the small intestine, the hepatopancreatic sphincter is closed. Bile flows into gallbladder.

cystic duct

common hepatic duct

gallbladder

common bile duct

hepatopancreatic sphincter

pancreas

duodenum

Function of Gallbladder - absorbs water and stores the bile for

later use.

gallbladder

hepatopancreatic sphincter

duodenum

- is constantly produced by the liver (500-1,000 mL of bile per day).

- is a yellow-green fluid containing :

minerals

bile pigments

bile acids

phospholipids

cholesterol

neutral fats

Bile

facilitate fat digestion and absorption

- reabsorbed in the ileum and returned to the liver via enterohepatic circulation.

Bile acidsphospholipids

Recycle of Bile acids and Phospholipids

The Pancreas

Endocrine:

- secretes insulin, glucagon, somatostatin into the blood.

The Pancreas

Exocrine: - secretes pancreatic

juice into the lumen of the pancreatic duct

Exocrine

secretes 1,200-1,500 mL of pancreatic juice per day into the main pancreatic duct.

It empties into duodenum when hepato-pancreatic sphincter opens.

1) sodium bicarbonate:

neutralize HCl

2) inactive digestive enzymes and zymogens which are activated after secreted into duodenum.

The activated enzymes digest carbohydrates, lipids, RNA, DNA, and proteins.

Pancreatic juice is an alkaline mixture containing:

- secreted in response to similar stimuli.

-Cephalic and Gastric phases

The vagus nerves stimulate pancreatic secretion.

Regulation of Bile and Pancreatic Secretion

1) relaxation of the hepatopancreatic sphincter,

2) the contraction of the gallbladder

3) secretion of pancreatic juice and enzymes.

The Intestinal Phase

- Chyme with acid and fat, stimulate the duodenal mucosa to secrete cholecystokinin (CCK).

secretion of bicarbonate

by both the hepatic and pancreatic ducts

The Intestinal Phase

- Acidic chyme also stimulates the duodenum to release secretin.

The Small Intestine

Overview

- Nearly all chemical

digestion and nutrient

absorption occur in the

small intestines.

1) The duodenum

- ~ 25 cm

2) The jejunum

- comprises the next 2.5 m.

3) The ileum

- forms the last 3.6 m.

- The surface area inside the small intestine is greatly increased by circular folds, villi, and microvilli.

villi

Microvilli (brush border)

brush border enzymes

brush border enzymes

- activates zymogens- complete digestion of carbohydrates and proteins

- 1-2 L of intestinal juice per day.

- The duodenum endocrine cells secret cholecystokinin (CCK) and secretin. (Both are hormones.)

Secretion by the small intestines

Chemical Digestion and Absorption of Nutrients

Chemical Digestion and Absorption of Nutrients

Carbohydrates

Proteins

Lipids

Nucleic Acids

Vitamins

Minerals

Water

Digestion and Absorption of Carbohydrate

- Most digestible dietary carbohydrate is starch.

- The starch digestion begins in the mouth by salivary amylase.

- But fully digestion of starch occurs in the small intestines.

- Starch is digested to oligosaccharides (3-8 glucose residues), disaccharide maltose, and glucose.

starcholigosaccharides glucose

pancreatic amylaseBrush borderenzymes

Intestinal lumen Intestinalepithelialcells

blood

glucose glucose

Glucose is absorbed by: - sodium-dependent glucose transporter (SGLT).

- solvent drag

Digestion and Absorption of Proteins

- Proteins are digested by proteases and peptidases.

- Protein digestion starts in the stomach.

Protein digestion continues in the small intestine by pancreatic enzymes trypsin and chymotrypsin.

Protein digestion is completed in the small intestine by brush border enzymes carboxypeptidase, aminopeptidase, and dipeptidase.

Amino acid absorption is similar to that of monosaccharides, via several sodium-dependent amino acid cotransporters.

proteins short peptides amino acids

Gastric pepsin &pancreatic proteases

Brush borderenzymes

Intestinal lumen Intestinalepithelialcells

blood

Digestion and Absorption of Lipids

- Lipids are digested by enzymes called lipases.

- Most fat digestion occurs in the small intestine via several steps.

1) Fats are first broken up into smaller emulsification droplets by lecithin and bile salts (acids) in the bile.

2) When lipase digests fats, the products are two fatty acids (FFAs) and a monoglyceride.

3) Bile salts coat these and other lipids and form droplets called micelles.

Micelles release their lipids, which diffuse freely across the plasma membrane.

Resynthesis of triglycerides. Coating with protein, forming droplets called chylomicrons.

Intestinal absorptive cell

Chylomicrons are too large to enter blood capillaries and must be first transported in the lymphatic lacteal.

Fat globule

emulsification droplets micelles

chylomicrons

Intestinal lumen Intestinalepithelialcells

lacteal blood

Bilesalts

lipase &bile salts

Digestion and Absorption of Nucleic Acids

- The pancreatic nucleases hydrolyze nucleic acids to their component nucleotides.

- The brush border nucleosidases and phosphatases further break them down, and the products are transported across the intestinal epithelium by membrane carriers.

Absorption of Vitamins

- Vitamins are absorbed without digestion.

- The fat-soluble vitamins are absorbed with other lipids.

- Water soluble vitamins are absorbed by simple diffusion, with the exception of vitamin B12.

- Vitamin B12 is an unusually large molecule that

can only be absorbed with the help of intrinsic factor.

Absorption of Minerals

- Minerals (electrolytes) are absorbed without digestion.

- Iron and calcium are unusual in that they are absorbed in proportion to the body's need.

- Other minerals are absorbed at fairly constant rates regardless of need.

Absorption of Water

- The digestive tract receives about 9 L of water per day

0.7 L in food1.6 L in drink6.7 L in gastrointestinal secretions

- ~ 8 L absorbed by the small intestine via osmosis;

Intestinal Motility

serve three functions:

1) to mix chyme with intestinal juice, bile, and pancreatic juice (segmentation)

2) to churn chyme and bring it in contact with the brush border for digestion and absorption;(segmentation)

3) to move residue toward the large intestine. (peristalsis)

- Segmentation is the most common type of movement of the small intestine.

- When nutrients have been absorbed, segmentation slows and peristalsis begins.

- The intensity of the contractions is modified by nervous and hormonal influences.

The Large Intestine

- No further chemical digestion

- Water (~ 1L) is further absorbed.

- The feces consists of: 75% water 25% solid matter, of which 30% is bacteria, and 30% undigested fiber.

Bacterial Flora

- refer to several species of useful bacteria in large intestine.

- ferment cellulose and other undigested carbohydrates

- synthesize B vitamins and vitamin K, which are absorbed by the colon.

Strong contractions

called mass

movements occur

one to three times a

day, last about 15

minutes each, and

occur especially an

hour after breakfast.

Neural Control of Defecation

1. Filling of the rectum

2. Reflex contraction of rectum & relaxation of internal anal sphincter

3. Voluntary relaxation of external sphincter

Diarrhea

too little water absorbed

Constipation

too much water absorbed, causing difficulty in defecation

In the absence of bile, a fat-rich diet causes diarrhea.

Accumulation of fat molecules

osmolarity of intestinal content

retain water

diarrhea

People lacking lactase have diarrhea after drinking milk.

Lactose (a dimer)

glucose galactose

lactase

People lacking lactase have diarrhea after drinking milk.

Accumulation of Lactose

osmolarity of intestinal content

retain water

diarrhea

The Stomach

The Liver

The Gallbladder

The Pancreas

The Small intestine

The Large intestine

Digestion and Absorption of Nutrients

Carbohydrates

Proteins

Lipids

Nucleic Acids

Vitamins

Minerals

Water

The Organs

SUMMARY