The Digestive System

I. Introduction

Digestion is the process of altering the physical state and chemical composition of food, so that it can be absorbed.

The digestive tract is also called the gastrointestinal tract or alimentary canal.

II. Digestive Organs Oral cavity (mouth)

hard palate and soft palate form roof Uvula – cone-shaped projection that keeps food

out of nasal cavity during swallowing Tongue –mixes food particles with saliva and

moves food toward pharynx; also aides in speech

Papillae – rough projections on surface of tongue that help handle food and contain taste buds (tip-salty/sweet; sides-sour; back-bitter)

Salivary glands – secrete saliva and are located near the ears and tongue; saliva helps moisten food particles and begins the chemical digestion of carbohydrates

II. Digestive Organs (cont) Teeth

break food into smaller pieces creating more surface area for digestion

32 total; covered with enamel, the hardest substance in the body

Begin physical digestion Pharynx (throat)

connects oral cavity to esophagus Esophagus

long tube (~25 cm) dorsal to trachea that delivers food to stomach

made of smooth muscle transports food by peristalsis (rippling contractions)

and secretes mucus

II. Digestive Organs (cont) Stomach

J-shaped organ with a capacity of 1 liter or more the stomach wall has folds called rugae Main site of physical digestion, which is almost

completed in the stomach by the peristalsis waves which churn the food

chemical digestion of proteins begins in the stomach and is completed in the small intestines

gastric juice – combination of pepsin, pepsinogen, hydrochloric acid, and mucus; pepsin begins the digestion of protein; pepsinogen reacts with hydrochloric acid and changes into pepsin

Little to no fat digestion takes place in the stomach. Food leaves the stomach as a liquid called CHYME

II. Digestive Organs (cont) Stomach

very limited amount of absorption occurs here; water, some drugs (ex. Aspirin), and alcohol

The cardiac sphincter (lower esophageal sphincter)– is located where the esophagus connects to the stomach. It is a muscular valve which prevents food and gastric enzymes from being pushed back into the esophagus.

The pyloric sphincter – is located where the stomach connects to the small intestine. It prevents backwash from the Small intestine into the stomach.

II. Digestive Organs (cont) Liver

weighs ~3lbs. and is located in the upper right of abdominal cavity

reddish-brown in color because it filters the blood,and is divided into right and left lobes

functions in protein, carbohydrate, and lipid metabolism

storage area for glycogen, iron, vitamins A, D, and B12

filters blood, destroying damaged RBCs and foreign antigens

removes toxic substances such as alcohol from blood

II. Digestive Organs (cont) Bile – is a yellowish-green enzyme that helps the

body digest fats by breaking the fats down physically.

Fats and oils stick together and move away from water this makes them difficult to digest because the enzymes can not get around the fats.

Bile causes EMULSIFICATION to happen – which means it separates the fats into tiny droplets which can now be chemically digested.

Bile is made in the liver, stored in the gall bladder (which is why it is green), and transported to the pancreas and then the duodenum by the bile duct.

II. Digestive Organs (cont) Pancreas

lies posterior to greater curvature of stomach secretes pancreatic juice into small intestine pancreatic juice contains enzymes that digest

carbs, fats, nucleic acids, and proteins

II. Digestive Organs (cont)

Small intestine between stomach and large intestine many loops and coils inner wall lined with villi – projections that increase

the surface area for absorption secretes enzymes that break down food and

receives secretions from the pancreas and liver completes digestion of nutrients in chyme absorbs 90% of products of digestion transports wastes into large intestine

II. Digestive Organs (cont)

Small intestine - Main site of chemical digestion. There are 3 sections to the small intestines: Duodenum which is the first section and is 25 cm

long. Pancreatic enzymes and bile are released into the small intestine in this section.

Jejunum is the second section and is about 2.5m (8 ft) long and passes imperceptibly into the

Ileum, which is the final section and is about 4m (12 ft).

II. Digestive Organs (cont)

large intestine also called colon reabsorbs water, vitamins and electrolytes last stages of chemical digestion occur here through

bacterial, rather than enzymatic action forms and stores feces (FYI - feces is ~75%water and

the odor results from bacteria products; feces is composed of water, salts, bacteria (such as E. coli), and undigested food)

feces formation/defecation: peristaltic waves occur only 2-3 times a day in the large intestine; the waves produce mass movements, usually after a meal

II. Digestive Organs (cont)

Large intestine is made up of 3 sections as well –

The first is the ascending colon which connects to the small intestine and has the appendix coming off of it.

The second is the transverse colon. The third is the descending colon.

Rectum – expandable organ for temporary storage of feces

Anus – final exit lined with keratinized epidermal tissue

Enzymes of the digestive systemENZYME SOURCE DIGESTIVE ACTION

Amylase Salivary glands,

pancreas

Begins digestion of carbs; polysacc to

disacc

pepsin stomach Begins digestion of protein

lipase

Small intestine

and pancreas

digestion of lipids; triglycerides to fatty acids and glycerol

chemical

Enzymes of the digestive system

proteolytic enzymes: ex.

trypsin

Pancreas Breaks down proteins into peptides

peptidasesSmall intestine

Breaks down peptides into amino acids

sucrase, maltase, lactase

Small intestine

Breaks down disacc into monosacc

bile Liver Physical digestion of fats

Digestive System Key Terms

Peristalsis: waves of rhythmic contractions that aid in digestive processes

Rugae: folds in the stomach Chyme: partially digested food mixed with

stomach acid (stomach to intestines) Sphincter: ring-like muscles that maintain

constriction ( like valves) Villi: projections of a membrane that

increases surface area

Digestive System Key Terms Bolus: chewed up food Mastication: the act of chewing Enzymes: proteins that act as catalysts in

the body Amylase: enzyme that breaks down

carbohydrates Pepsin: enzyme that breaks down protein Lipase: enzyme that breaks down fats Gallbladder: small organ that stores bile

Nutrition

Nutrition is the process by which the body takes in and utilizes necessary food sources (nutrients).

Metabolism

Cellular Metabolism: The chemical and physical reactions that take

place in the cell. Normally involves an enzyme to synthesize or

break down molecules (CHO, Lipid, PRO) for cell use.

Example: Mitochondria making energy for the body.

Nutrients

Carbohydrates: Organic Supply energy for cellular processes Sources: starch from grains, veggies; glycogen

from meat; disaccharides from cane sugar; monosaccharides from honey and fruits

Cellulose- sugar that cannot be digested, provides bulk (called fiber) which assists movement thru digestive system

RDI (Recommended Daily Intake) 125-175 grams

Nutrients

Lipids: Organic Supply energy for cellular processes and building

cellular structures Fats, oils Sources: Meats, eggs, milk, lard, seeds, nuts,

plant oils Saturated fats should account for no more than

10% of diet

Nutrients

Proteins: Organic Made of amino acids Imp. Components of enzymes, clotting factors,

keratin in skin and hair, collagen in connective tissue, muscle components (actin/myosin) hormones, antibodies

Sources: Meat, fish, poultry, cheese, nuts, milk, eggs, cereals, legumes

RDI is about 0.8 g per kilogram of body weight

Nutrients

Vitamins Organic Essential nutrients that must come from foods RDI varies for each vitamin See table 25-4, 25-5 in the Anatomy book Examples: A, B, C, K, Folic Acid

Nutrients

Minerals: Inorganic Essential for human metabolism Plants extract minerals from soil, we eat the plants,

therefore we get the minerals. Most concentrated in teeth and bones Play vital role in nerve impulse conduction, muscle fiber

contraction, blood coagulation (CA, NA, K, etc) Table 25-3 in Anatomy book for more info Examples: Zinc, Iron, Sodium, Potassium, Calcium,

Phosphorus

Metabolism

Metabolism - the series of processes by which food is converted into the energy and products needed to sustain life

On the cellular level, metabolism is the process of making ATP.

Why is ATP so important?

Metabolism

ATP- Adenosine triphosphate: The version of energy useful to cells ATP ADP (triphosphate to diphosphate)- loss

of the phosphate releases energy Lost phos. recharges (like a battery) and bonds

with an ADP to make an ATP: Energy must be available in the cells for this to occur

Cellular Respiration provides the energy to regenerate ATP, ATP provides the E for cellular activities.

Metabolism

Carbohydrate Pathways Carbohydrates are many sugars hooked together.

These pathways break them into individual glucose molecules

CHO used as energy or converted to glycogen or fat Glucose molecules absorbed from sm. Intestine into

blood stream and delivered to body cells Insulin helps transfer glucose across the cell

membrane 3 Steps of Cellular Respiration = 1 Glycolysis, 2

Kreb’s Cycle, and 3 Electron Transport Chain (ETC) combined

Metabolism

Glycolysis: Process that breaks down the glucose into pyruvic

acid Occurs in the cytoplasm (all cells) Product is pyruvic acid which is moved to the

mitochondria Net Yield 2 ATP

Metabolism

Kreb’s Cycle (Citric Acid Cycle): Occurs in membrane of the mitochondria, involves

the pyruvic acid made in glycolysis High E compounds made (8 NADH and 2 FADH2)

which donate electrons to the electron transport system (ETS)

Net Yield 2 ATP

Metabolism

Electron Transport System: ATP produced when NADH and FADH2 release

hydrogen atoms generating energy Net Yield around 32 ATP

**Note: A total of 36 ATP are generated from the energy in 1 molecule of Glucose.**

2 from glycolysis + 2 from Kreb’s cycle + 32 from ETS

Digestive System Disorders

Digestive System Disorders Crohn’s Disease - is an autoimmune disease

where the immune system attacks the gastrointestinal tract, producing inflammation.

Heartburn (acid reflux) – when stomach juices regurgitate back into the esophagus through the cardiac sphincter. This causes a burning sensation that has nothing to do with your heart.

Gallstones - form when bile stored in the gallbladder hardens into pieces of stone-like material.

The largest gallstone on record was removed from an 80-year old woman in 1952 and weighed 6.29 kg (13.84 pounds).

Digestive System Disorders Ulcers (peptic) – an erosion of the mucous or wall

of the stomach or small intestine. Extremely painful because of the acidic wearing down of the walls of your GI tract.

Hernia - is a protrusion of a tissue or part of an organ through the muscle tissue or the membrane by which it is normally contained.

Irritable bowel syndrome (IBS) - is a blanket term for a variety of diseases causing discomfort in the gastro-intestinal tract. It is also called spastic colon, characterized by chronic abdominal pain, discomfort, bloating, and alteration of bowel habits in the absence of any organic cause.

Digestive System Disorders

Cirrhosis – liver disease where the liver tissue is replaced by scar tissue. Typically caused by alcoholism or hepatitis.

Jaundice - is a yellowish discoloration of the skin typically caused by excess bilirubin or the breakdown of red blood cells.

Colon polyps - A colon polyp is a growth on the surface of the colon. Some colon polyps are benign (not cancerous), but some are cancerous.

Globally, cancer of the colonand rectum is the third leadingcause of cancer in males and the fourth leading cause of cancer in females.

Digestive System Disorders

Lactose intolerance - the inability to digest significant amounts of lactose, the major sugar found in milk. Lactose intolerance is caused by a shortage of the enzyme lactase.

Cystic Fibrosis – causes the blockage of the pancreatic duct due to the build up of hardened mucous.

Hemorrhoids - are abnormally swollen veins in the rectum and anus.