Digestive System

Anatomy and Physiology

Functions of the Digestive System

• Mechanical and chemical breakdown of foods into forms that cell membranes can absorb.

• Ingestion – taking in the food• Digestion – breaking apart macromolecules found in

food– Mechanical digestion – break large pieces down and mix with saliva

(mastication); no chemical altering– Chemical digestion - break chemicals into simpler ones

• Absorption – passing the monomers into the blood stream

• Waste Elimination – removing the indigestible and unabsorbable foods

Parts (Alimentary Canal)

• Alimentary Canal – extends from mouth to anus (~8 meters long)– Mouth, pharynx, esophagus, stomach, small

intestine (5.5-6 m), large intestine (1.5 m), rectum, anus

Accessory Organs

• Accessory organs – release secretions into the canal– salivary glands,

tongue, liver, gallbladder, pancreas

Wall of Alimentary Canal- 4 distinct layers

– mucosa – deepest layer opening into lumen, developed to different degrees in different regions, glands secrete mucus and digestive enzymes, absorption

• Stomach – folds called rugae• Small intestines - folded with tiny projections called villi to increases

absorption that extend into lumen

– Submucosa – blood vessels embedded to nourish surrounding tissue, carry away absorbed materials

– muscular – two layers to provide movements of the tube (peristalsis – wave-like smooth muscle contractions to propel food)

– Serosa – outer covering of tube (mesentery), secretes serous fluid

“GI camera”

Just like snaking yourpipes in your house whenyou have plumbing issues

Path of Food Through the Body

• Oral Cavity of Mouth – mechanical digestion with teeth and chemical digestion with saliva– Saliva contains amylase which begins the breakdown of starch.

This is why chewing bread for a long time before swallowing or any complex carb begins to taste sweet, contains anti-bacterial agents, has a neutral pH to help protect teeth from acidic foods.

• Pharynx – common passage of food and air – splits to form the esophagus and trachea – has 2 layers of smooth muscle to cause peristalsis (wave of contraction and relaxation of a tubular muscular system that forces contents through it)

• Esophagus – Peristalsis continues to the stomach

Path of Food Through the Body

• Stomach (4-6 hrs.)– Mechanical digestion – smooth muscle contracts

which grinds the stomach and rugae (folds of mucosa) helps grind and expand the stomach

– Chemical Digestion (gastric juice)• Chief cells – make protease, pepsin which begins the

breakdown of proteins• Parietal cells – make hydrochloric acid (HCl) to activate

the pepsin by lowering the pH• Goblet cells – secrete mucus to protect the stomach lining

which replaces itself every day– Little to no absorption of food in the stomach

• Alcohol, which is not a nutrient, is absorbed in both the stomach and small intestine which is why you feel the intoxicating effects almost immediately after consumption as opposed to hours.

Stomach

Holds 1 gallon of food - turns it into chyme (pulpy acidic fluid consisting of gastric juices and partly digested food)

-Cardiac Sphincter – separates the esophagus from the stomach and keeps stomach acid from splashing back into the esophagus (heart burn or acid reflux)

-Pyloric Sphincter – holds the food (chyme) in the stomach until it’s ready to pass into the small intestine

Path of Food Through the Body

• Small Intestine (4-8 hrs.) Major organ of digestion and absorption of nutrients (arranged into villi for ↑absorption and cells have microvilli– Duodenum (10”) – food empties in from stomach

thru the pyloric sphincter• Pancreas empties sodium bicarbonate (base to neutralize

the stomach acid) into it• Pancreas empties almost all the digestive enyzmes into it (to

complete digestion of proteins and carbs, and fully digests lipids and nucleic acids)

• Liver empties bile into it to physically break apart fat so that enzymes have better access

• Gallbladder empties concentrated bile into it

– Jejunum to the Ileum (12’) – villi decrease as less is being absorbed

Gluten Free Food

• Not just a fad

• Malabsorption – small intestine digests food but does not absorb nutrients due to a reaction from gluten, a protein found in grains like wheat and rye.

• Celiac disease – microvilli are damaged or completely destroyed from gluten.

• Hence the need for “gluten free foods”

Path of Food Through the Body

• Large Intestine (12-24 hours) Mostly to remove excess water from undigested food, forms feces and moves it to the rectum– Cecum – undigested food enters thru the ileocecal valve– It moves thru the ascending, transverse, descending and

sigmoid colon– No villi, a lot of mucus producing cells and bacteria to digest the

undigested food (produce gas) and release some vitamins– If colon malfunctioned, would result in dehydration

• Rectum – storage until defecation• Anus – removal (anal sphincter – skeletal

muscle

Accessory Organs of Digestion

• Salivary glands - make mucus and saliva with amalayse (breaks down starch and kills bacteria)

• Pancreas – dumps sodium bicarbonate into sm. intestine to neutralize stomach acid and dumps many digestive enzymes into the duodenum for chemical digestion, non-digestive function – insulin – controls blood sugar levels

– Pancreatic amylase - further break down of carbs– Trypsin and chymotrypsin (proteases) – further

breakdown of proteins– Pancreatic lipase – begins break down of fats – Nucleases – begins break down of nucleic acids

Accessory Organs of Digestion

• Gallbladder – stores excess bile, concentrates it and dumps it into the duodenum to break down fat

• Liver – makes bile (physically breaks down fat and helps with vitamin absorption) & dumps it into the duodenum

If either organ malfunctioned, would result in fatty stool due to inability for body to break down the fat in food to be absorbed by the small intestines.

Accessory Organs of Digestion

• Liver functions continued (not all digestive)– Removes aa, glucose, f.a. from blood to make blood

proteins, cholesterol, and glycogen– Stores glycogen when too much glucose is in the

blood, breaks it back down into glucose and releases it into the blood when blood glucose is low

– Make LDL – carry cholesterol to cells and some deposits on the arteries

– Make HDL – cleans arteries and brings fats to the liver to be disposed of

– Makes albumin (blood homeostasis), and clotting proteins

– Detoxifies drugs and alcohol– Kills bacteria that enter blood stream from the

digestive tract

Summary of Digestion

• Carbohydrates – begin digestion in the mouth/finish in the small intestine

• Proteins – begin digestion in the stomach/finish in the small intestine

• Fats – total digestion in the small intestine

• Nucleic Acids (DNA & RNA) – total digestion in the small intestine

Uses of Monomers

• Carbs monosaccharides (glucose) is a reactant in cellular respiration, quick source of energy

• Proteins amino acids are building blocks for structural material (collagen, muscles, etc.) antibodies, some hormones

• Lipids long term energy, protection, create cell membranes

Vitamins - coenzymes

• Water Soluble• B – coenyzmes for cellular respiration• C – collagen synthesis, anti-oxidants

• Water Insoluble• A – visual pigments, anti-ox• E – prevent membrane damage, anti-ox• K – blood clotting• D – Calcium absorption

Minerals - Cofactors

• Ca, Na, Cl, K – muscle contraction

• Ca – harden bones

• I – make thyroid hormone

• Fe – iron is needed in hemoglobin to attach oxygen

• P – to make ATP and activate enzymes

• Mg, Zn, Cu, Mn, Se, Mb - cofactors