Digestive System

Part 2

Small IntestineFunctions to:

1. Complete digestion

2. Absorb nutrients

3. Produce regulating hormones

Duodenum

• The first section of the small intestine is called the duodenum

• Ducts from pancreases and liver enter the duodenum, with fluids that contain enzymes to complete digestion

Small Intestine

• Slightly basic (pH = 8.5)

• Pancreatic fluid contains sodium bicarbonate NaHCO3 to neutralize acidic chyme

Pancreatic Juices

• The PANCREAS secretes “pancreatic juice” which contains sodium bicarbonate (NaHCO3) (making it basic) and digestive enzymes:

Pancreatic Enzymesa. Pancreatic Amylase:• (starch + H2O --> maltose)

b. Trypsin:• (protein + H2O --> peptides)

c. Lipase

• (fat droplets + H2O glycerol + fatty acids)

BEFORE Lipase this step occurs:

Emulsification:

fat + bile salts (produced in liver) fat droplets

d. Nuclease• DNA/RNA + H2O --> nucleotides

Some Structures Up Close:

The Small Intestines: • small in diameter

but not in length! (approximately 6 meters long)

• Large surface area due to convoluted villi and microvilli lining the lumen (hole)

Villi Microvilli

Villi• Nutrients absorbed into

vessels within villi – blood capillaries

absorb water-soluble monomers such as glucose and amino acids, lacteals absorb fatty acids and glycerol into lymphatic system, and are eventually absorbed into the blood stream

Villi

Capillaries (aa and glucose enters)

Lacteal (fat and glycerol enters)Lymph drains lacteal

Duodenal cells

• Produce 2 hormones:

a) secretin – stimulated by acid in the chyme

b) CCK (cholecystokinin) – stimulated by partially digested protein and fat

Both target pancreases and liver, increasing pancreatic juice and bile production

In the intestines: the mucosa of the intestinal villi secrete 3 enzymes:

a. Peptidase

• (peptides + H2O amino acids)

b. Maltase

• finishes starch breakdown

• ( maltose + H2O glucose)

c. Nucleosidase

Nucleotides sugar + phosphate group + nitrogenous base

Large Intestine (colon)

Key Functions:

1. Absorption of water and salts

2. Contains bacteria that produce vitamins

3. Prepare feces for elimination

Cecum (blind end of intestine)

Appendix (no function; may help infection, subject to inflammation; appendicitis)

Ascending Colon

Transverse Colon

Descending Colon

Sigmoid Colon

Rectum

Large Intestine

• Incubates bacteria (E. coli) which break down indigestible material, produce vitamin K, gas, amino acids and growth factors

• Dead bacteria, mucus, water and undigested materials (cellulose/fibre) are moved to the rectum by peristalsis and stored in the rectum

• Stretching of the rectal wall stimulates the defecation reflex (peristalsis of rectum and relaxation of internal anal sphincter)

Interesting Fact

• contains bacteria (including E.coli.) that live on unabsorbed and undigested nutrients

• the bacteria ferment any remaining carbohydrates and release H+, CO2, and methane gas.

(flatus (flatulence)• by-products of the bacterial digestion account for

the color and the smell of the feces.

Anal Sphincter

• External anal sphincter is under voluntary control