An overview of the digestive system

mouthpharynxesophagus stomach small intestine large intestine rectum anus

Why GIT? What are the main steps in the digestive process?

• Ingestion – intake of food via the mouth

• Digestion – mechanically (Chyme= semifluid mass of food) or chemically breaking down food into their subunits (from mouth till Large Int. have digestive enzymes).

• Movement – food must be moved along the GI tract in order to fulfill all functions

• Absorption – movement of nutrients across the GI tract wall to be delivered to cell via the blood

• Elimination – removal of indigestible molecules (defecation).

Wall of the digestive tract?• Mucosa – innermost layer produces

mucus that protects the lining and also produces digestive enzymes,HCl (Diverticulosis).

• Submucosa – 2nd layer of loose connective tissue that contains blood vessels, lymphatic vessels and nodes (Peyer’s patches) and nerves ( IBD Inflammatory Bowel Disease)

10-25%

Crohn’s disease (IBD)Ulcerative colitis

The layers of the GI tract

Muscularis – 3rd layer made of 2 layers of smooth muscle that move food along the GI tract. Inner circular & outer longitudinal (IBSIrritable Bowel Syndrome)Serosa – outer lining that is part of the peritoneum. Appendix & Appendicitis & Peritonitis

The first part of the digestive tract includes the:

• Mouth• Pharynx• Esophagus

The mouth• 3 pairs of salivary glands secrete salivary amylase that begins

carbohydrate digestion

• Tonsils at the back of the mouth and other lymphatic tissues are important in fighting disease

• Contains teeth that begin the mechanical breakdown of food

• The tongue is covered in taste buds and also assists in the mechanical breakdown and movement of food

• The tongue forms a bolus (mass of chewed food) and moves it toward the pharynx

Anatomy of the mouth

Lips: Poorly keratinized skin.

Palate: Hard and Soft (with uvula)

Tonsils

Salivary Glands : Saliva= H2O+ mucus + Bicarbonate + Salivary Amylase ( mumps)

The pharynx and esophagus• Pharynx is a cavity between the mouth and

esophagus that serves as a passageway for food (and air)

• Esophagus is a long, muscular tube that carries food to the stomach. No digestion.(Peristalsis)

Lower gastroesophageal sphincter• Heart burn• Vomiting (contraction of abdominal muscles and

diaphragm)

How do we swallow food?• Voluntary phase:

– The beginning of food being swallowed from the mouth into the pharynx

• Involuntary phase:– Once the food is in the pharynx swallowing becomes

a reflex– The epiglottis covers the voice box to make sure food

is routed into the esophagus– Food moves down the esophagus through peristalsis

(rhythmic contraction)

How do we swallow food?

The stomach• Functions to store food (1 L capacity), start digestion of proteins and

controls movement of chyme into the small intestine

• J-shaped organ with a thick wall. No food absorption (only Alcohol).

• There are 3 layers of muscle in the muscularis layer of the stomach wall helping in mechanical digestion and allowing it to stretch (Circular, longitudinal, oblique)

• The mucosa layer has deep folds called rugae and gastric pits that lead into gastric glands that secretes gastric juice

• Gastric juice contains pepsin, an enzyme that breaks down proteins, and HCl and mucus

• HCl gives the stomach a pH of 2 which activates pepsin and breakdown the connective tissue of meat. It helps kill bacteria found in food

• A bacterium, Helicobacter pylori, lives in the mucus and can cause gastric ulcers

• The stomach empties chyme into the small intestine after 2-6 hrs.

Anatomy of the stomach

The small intestine• Averages 6m (18 ft) in length

• Enzymes secreted by the pancreas into the small intestine digest carbohydrates (amylase), proteins (Trypsin) and fats (Lipase)

• Bile is secreted by the gallbladder into the small intestine to emulsify fats

• Digested food is absorbed through large surface area created by numerous villi (finger-like projections) and microvilli

• Amino acids and sugars enter the capillaries while fatty acids and glycerol enter the lacteals (small lymph vessels).

How are nutrients digested and transported out of the small intestine?

What are the major digestive enzymes?

The three accessory organs

• Pancreas• Liver• Gallbladder

The pancreas

• Fished-shaped spongy organ behind the stomach• Functions of the pancreas:

1. Secretes enzymes into the small intestine• Trypsin digests proteins• Lipase digests fats• Pancreatic amylase digests carbohydrates

2. Secretes bicarbonate into the small intestine to neutralize stomach acids

3. Secretes insulin (and Glucagon) into the blood to keep blood sugar levels under control

The liver and gallbladder• Large metabolic organ that lies under the diaphragm and

is made of ~100,000 lobules• Filters blood from the GI tract thus acting to remove

poisons and detoxify the blood• Removes iron, vitamins A, D, E, K and B12 from the

blood and stores them• Stores glucose as glycogen and breaks it down to help

retain blood glucose levels• Makes plasma proteins and helps regulate cholesterol

levels by making bile salts• Makes bile (Bile salts+ H2O+ cholesterol+ Bicarbonate+

Bilirubin) that is then stored in the gallbladder to be secreted into the small intestine to emulsify fats

• Breaks down hemoglobin

How do hormones control digestive gland secretions?

Nervous (smell..ect)

Hormonal:

Gastrin (protein rich food in stomach) increase gastric glands secretion.

Secretin (inhibits HCl )

CCK (controls the partial digestion of proteins and fats) in Duedenum. Both secretin and CCK will go to the blood to reach and stimulate the pancreatic juice secretion, CCK also increased liver bile production, and gall bladder contraction

Liver functions• Detoxification & removal of poisonous substances.• Removes and stores iron and Vit. A, B12, D, E, and K• Plasma proteins synthesis (as albumin and fibrinogen).• Regulation of blood cholesterol level.(Converts some into bile salts).• Destruction of old RBC and conversion of Hb into bile.• Production of bile, and storage in gall bladder. Bile is composed of

Bilirubin (yellow to green) and bile salts (derived from cholesterol) which emulsify fat into droplets (big surface area so easy for digestion), water, and cholesterol.

• Maintains Constant Blood Glucose level (Insulin, Glucagon).• Storage of glucose as glycogen and breakdown of glycogen to

glucose. • Converts amino acids into glucose by deamination, and urea

formation.

The large intestine• Includes the cecum, colon, rectum and anal canal

• Larger in diameter (6.5 cm compared to 2.5 cm Intestine) but shorter than the small intestine

• The cecum has a projection known as the appendix that may play a role in fighting infections

• Ascending, transverse, and descending.

• Functions to:– Absorb water to prevent

dehydration– Absorbs vitamins (B complex and

K) produced by intestinal flora– Forms and rids the body of feces

through the anus (water ¾, ¼ fibers, bacteria ( gas, odor), (breakdown of bilirubin and oxidized iron give the color), & indigestable materials

– Inhabited by bacteria (not e coli as was thought)

Defecation reflex: Feces in rectum initiate the reflex to the spinal cord Relaxation of the sphincters. External sphincters is under voluntary control.