Download The Digestive System The Stages of Food Processing 1. Ingestion

The Digestive System
The Stages of Food Processing
1. Ingestion – the intake of food
2. Digestion – the mechanical and chemical breakdown of food into a form usable by the body
Anabolic reactions – constructing molecules from smaller units
Catabolic reaction – the breaking down of molecules into smaller units
3. Absorption – the uptake of nutrient molecules into the epithelial cells of the digestive tract
and then into the blood and/or lymph
4. Elimination – consolidating the indigestible residue into feces and eliminating from the body
Two Types of Digestion
Mechanical Digestion
- The physical breakdown of food into smaller molecules
- Cutting and grinding action of teeth
- Churning action of the stomach and small intestine
- Exposes more food surface area to the action of enzymes
Chemical Digestion
- A series of hydrolysis reactions that breaks dietary macromolecules into their
monomers
- Results in:
 Polysaccharides into monosaccharides
 Proteins into amino acids
 Fats into monoglycerides and fatty acids
 Nucleic acids into nucleotides
General Anatomy of the Digestive System
The digestive system has two anatomical subdivisions:
1. the digestive tract – Alimentary Canal
- 30 foot long muscular tube extending from mouth to anus
- mouth, pharynx, esophagus, stomach, small intestine, and large intestine
2. Accessory organs
- teeth, tongue, salivary glands, liver, gallbladder, pancreas
The Mouth
Oral Cavity:
- Saliva: mucin, buffers, antibacterial agents
- Digestion of carbohydrates initiated with salivary amylase
- Bolus – mass of food
Salivary Glands:
Parotid Glands: largest salivary gland, secretes clear, watery fluid with salivary amylase
Secretes into the Stenson’s Duct
Submandibular Gland: floor of the mouth, ducts (Wharton’s ducts) open near the
lingual frenulum, secretes serous and mucous fluids
Sublingual Gland: inferior to tongue, produce thick, stringy mucus, secretes into
Rhivinus’s duct
Pharynx:
- the throat, leads to the esophagus
- glottis and epiglottis
The Esophagus
- A straight muscular tube 25-30 cm long
- Extends from pharynx to cardiac orifice of stomach passing through the esophageal
hiatus In the diaphragm
- Lower esophageal sphincter
 Prevents stomach contents from regurgitating into the esophagus
 Protects esophageal mucosa from stomach acid
 Heartburn – burning sensation produces by acid reflux into the
esophagus
- Nonkeratinized stratified squamous epithelium
The Stomach
- Capacity of approximately 1 liter
Gross Anatomy of the Stomach
- Cardia – near esophageal sphincter
- Fundus – temporary storage
- Body – main part of stomach
- Pyloric Antrum – funnel shaped that narrows into pyloric canal
-
Greater Curvature
-
Lesser Curvature
-
Greater Omentum
-
Lesser Omentum
Gastric Secretions:
- Mucous Membrane – inner lining with many gastric glands
- 3 cell types:
1. Mucous cell: secretes mucous which prevents pepsin from digesting the
stomach
2. Chief cells: secrete digestive enzymes
Pepsinogen is the inactive form of Pepsin
HCL converts pepsinogen to pepsin
Pepsin also breaks down more pepsinogen to pepsin
3. Parietal cells: secrete HCl and intrinsic factor
Intrinsic is required for vitamin B12 absorption
Gastric Secretions:
1. Cephalic Phase (30% - 50%)
- Before food reaches the stomach
- sight, smell, and thought of food triggers gastric juice secretion through the
vagus nerve (cranial nerve X)
2. Gastric Phase (40% - 50%)
- food in stomach distends the stomach wall
- Gastrin released  gastric juice released
- Gastrin is a peptide hormone that increases gastric juice secretions
- pH approaches 1.5  gastrin secretions stops
- HCl is released
3. Intestinal Phase (5%)
- begins when food leaves the stomach
- Intestinal gastrin released  increases gastric juice secretion
Fats and proteins in intestine stimulate cholecystokinin  decreases
gastric motility
Fats also increases intestinal somatostatin  inhibits gastric juice
Alkaline Tide:
1. Stomach secretes HCl
2. Gets H+ from blood
3. Bicarbonate released into blood
4. Blood concentration of bicarbonate increases after meal
5 Urine excretes excess bicarbonate
Enterogastric Reflex:
Entero = small intestine
Gastric = stomach
- Food in duodenum stretches wall stimulating sensory receptors
- Reflex:
- decreased peristalsis in stomach
- intestinal filling slows
- This regulates the rate at which chime leaves the stomach
Small Intestine
- Major organ of digestion and absorption
- 20 feet in length (5.5 – 6.0- meters)
- 3 regions:
- Duodenum – shortest section (25 cm)
- Jejunum – 2/5 of intestine
- Ileum
- Mesentery – folds of the peritoneum that supports and acts as a conduit for blood vessels and
lymph vessels
- Greater Omentum – a drape over the stomach and small intestine
3 Main Functions of the Small Intestine:
1. Completes digestion – gets secretions from pancreas and liver
2. Absorbs digestive products
3. Transports remaining residue to large intestine
Structure of the Small Intestine:
- Intestinal Villi – most numerous in duodenum and proximal jejunum
- each villus:
- layer of simple columnar epithelium
- lacteal – a lymph capillary
- a blood capillary
-
Microvilli – brush border
Goblet cells and Brunner’s glands secrete mucous
Peyer’s Patches – lymph vessels found in abundance in the ileum Why?
Cell lining is replaces every 3-6 days through mitosis – Cellular Turnover
Feces is 25% dead intestinal cells
Plicae Circulares – circular folds of the mucosa - helps increase surface area
Intestinal Enzymes: Embedded in the membranes of the microvilli, they breakdown food
molecules prior to absorption
- Peptidases
- Sucrases, Maltase, Lactase
- Intestinal Lipase
- Enterokinase – converts trypsinogen into trypsin
Large Intestine
- 1.5 meters long
- Reabsorption of water
- 12 – 24 hours to travel length
- Rich flora of mostly harmless bacteria (E. coli)
- Ileocecal valve prevents movement back into the small intestine
- Terminal portion is the rectum – waste exits the anal sphincter through voluntary control
Parts of the Large Intestine:
1. Cecum – beginning, pouchlike structure, hangs inferior to the ileocecal valve
- the appendix – no digestive function
2. Colon – ascending, transverse, descending, sigmoid colon
3. Rectum
4. Anal Canal
- Walls of the large intestine lack villi and plicae circularis
- Teniae Coli – 3 distinct bands of muscle fibers which exert tension and create a series of
pouches – HAUSTRA
- No digestive function
- Has Goblet Cells – mucous secretion
- Absorption is limited to water and electrolytes
- Intestinal Flora – breakdown some cellulose and help produce vitamins such as K, B12,
thiamine, and riboflavin
- Feces – water, electrolytes, mucus, and bacteria. Bile gives its color
The Pancreas
Has both endocrine and exocrine functions
Exocrine:
- Pancreatic acinar cells form clusters called acini which produce pancreatic juice
- Pancreatic duct – extends the length of pancreas into duodenum
- Hepatopancreatic ampulla (ampulla of Vater) – pancreatic and bile ducts
- Hepatopancreatic sphincter (sphincter of Oddi) – band of smooth muscle that
surrounds the ampulla
Endocrine:
- Islets of Langerhans
- Account for approximately 1% of pancreatic cells
- Secrete Insulin and Glucagon in response to changes in blood glucose levels
Pancreatic Juice:
- Enzymes that digest carbs, fats, proteins, and nucleic acids
- Pancreatic amylase – almost identical to salivary amylase
- Pancreatic Lipase
- Trypsin
- Chymotrypsin
- Carboxypeptidase
- Nucleases
- A peptide hormone – SECRETIN- stimulates pancreas to secrete bicarbonate when acid
chime enters stomach. Secretin is released from duodenal mucous membrane
- Cholecystokinin stimulates pancreas to release pancreatic juice
* No single enzyme can split all possible Amino Acid combinations
The Liver
-
Largest internal organ
4 lobes
Liver cells - Hepatocytes
Falciform Ligament – separates right and left lobes
Lobes are separated into Hepatic Lobules – the functional unit of the liver