Functions of the digestive system

Ingestion- bringing in food/nutrients Mechanical processing- mechanically

breaking food down, chewing, etc. Digestion- chemically breaking food down Secretion- release of nutrients Absorption- movement of nutrients to the

blood stream Excretion- removal of waste

Organization of Digestive Tract

The digestive tract is lined w/ mucous membranes to protect the epithelium of the digestive tract, and to help food move through

Stratified in areas of high stress, like the mouth and simple in the small intestine where absorption takes place

Movement of Digestive Materials Peristalsis- waves of muscular

contractions that move a bolus, along the digestive tract

Segmentation- cycles of contraction that churn and fragment bolus

Control of Digestive Functions

Neural Mechanisms- sensory neurons in the digestive tract stimulate motor neurons to control the smooth muscle of the digestive tract

Hormonal control- a lot of digestion is controled by hormones, there are atleast 18 involved in digestion

Oral Cavity The tongue- mechanical processing of

food Salivary glands- 3 pairs, produce 1-1.5

liters of saliva a dayLubrication and moisteningSalivary amylase- digests complex

carbohydrates Teeth- mechanical digestion

Digestive Tract

Pharynx- Passageway from the mouth to the esophagus

Esophagus- hollow muscular tube, which moves food through using peristalsis

Stomach Four Regions

Cardia- superior portion that connects w/ the esophagus, contains lots of mucous glands to protect stomach lining

Fundus- upper curve of the stomach Body- largest region, gastric glands release

acid and enzymes, mixes food Pylorus- last section, connects to the

duodenum

Gastric Glands

Parietal cells- secrete HCl to keep stomach pH at 1.5-2.0. Acidic environment kills most microorganisms, breaks down proteins, breaks down plant cell walls, activates pepsin (protein digesting enzyme)

Chief cells- Secrete pepsin

Pyloric glands

Mostly produce mucous Produces hormone gastrin, which

stimulates secretion of the parietal and chief cells, as well as contractions of the stomach walls to mix and stir contents

Regulation of gastric activity Cephalic phase- prepares stomach for arrival of

food, stimulates mucous, acid, and enzyme activity

Gastric phase- begins w/ the arrival of food, 3-4 hours while ingested foods are processed

Intestinal phase- controls rate of chyme leaving stomach and entering the duodenum

Small Intestine

Absorption of nutrients Duodenum- “mixing bowl”, mixes chyme

and digestive secretions from pancreas and liver

Jejunum- most chemical digestion and absorption

Ileum- connects to the large intestine

Pancreas organ posterior to the stomach, secretes

enzymes and buffers into the duodenum Secretes insulin and glucagon to control blood

sugar levels Secretes pancreatic juice- consists of an

alkaline solution to neutralize acidity of the chyme and enzymes to complete digestion

Liver

Divided into two lobes, left and right Hepatocytes- adjust circulating levels

of nutrients Secretes bile, which helps break

down fats

Physiology of the liver Metabolic regulation- regulates

composition of blood before it is sent to the rest of the body Carbohydrate metabolism- if blood

glucose drops, hepatocytes break down glycogen into glucose. If blood glucose increases, hepatocytes remove glucose and store it as glycogen or store it as fat

Liver physiology cont. Lipid metabolism- regulate circulating

levels of fats Amino acid metabolism Removal of waste Vitamin storage- fat soluble vitamins

are stored (A, D, E, and K) Mineral storage and Drug inactivation

Liver Regulation

Remove debris and pathogens from the blood

Plasma proteins- regulate osmotic pressure of the blood

Removal of toxins Synthesis and secretion of bile- emulsifies

fats so they can be digested

Gallbladder

Stores bile, which can become more concentrated the longer it remains in the gall bladder

Large Intestine

Cecum- first section of large intestine, connects to the ileum

Colon Ascending Transverse Descending Sigmoid colon

Physiology of the large intestine

Reabsorption of water Absorption of vitamins- K (used by the

liver to make clotting factors, Biotin (glucose metabolism), Vitamin B5 (used for making steriod hormones) These are made by bacteria living in the

colon