The Human Digestive System

The Mouth

• Structure: see diagram • Function: Ingest and Digest• Ingestion - the teeth and tongue

(taste buds) take in the appropriate size and kind of food.

• Digestion - the teeth (incisors, canines and molars) cut, tear and grind food (mechanical digestion)

• Salivary glands add saliva (water, mucous, amylase) to further aid in mechanical breakdown

• Amylase starts the chemical digestion of starch (polysaccharide)

The Mouth continued:

• Saliva moistens food, breaking it down physically and also making it slippery

• The tongue rolls food around keeping it between teeth (mechanical digestion)

• Food eventually becomes ball of chewed food known as a bolus

• When swallowed, food is prevented from entering the nasal cavity by the uvula (pushed upwards) or the trachea by the epiglottis (cartilage flap closes downward)

Esophagus

• Esophagus - tube joining the pharynx to the stomach

• Lined with mucous and two smooth muscle layers (circular and longitudinal muscles)

• Pathway for bolus to reach the stomach

• Performs peristalsis (wave like muscle contractions that push and pull food to stomach)

Stomach

• Stomach Structure• J-shaped sac covered with

three muscle layers (c,l,oblique)

• Rugae are the inner folds with special gastric cells

• Gastric cells secrete mucous, hydrochloric acid and enzymes (pepsin)

• Two sphincters (cardiac, pyloric) seal stomach tightly

Stomach lining

• Nice folds!!

Stomach Function• The stomach completes mechanical digestion• Muscle layers slosh food around with hydrochloric acid

(corrosive) breaks food down into small pieces• Chyme = semi-liquid acidic food• Pepsin (enzyme) initiates protein digestion (chemical digestion)

breaking peptide bonds within complex proteins producing polypeptides (hydrolysis reactions)

• The acidic conditions of the stomach denatures amylase (stopping chemical digestion of starch) and kills most microorganisms ingested in your food

Stomach Cells (Gastric Pits)

Small Intestine

• Structure:• Long tube (6 m), narrow

lumen lined with villi and microvilli

• Includes rich blood supply• Three main regions:• Duodenum (25 cm)• Jejunum (3 m)• Ileum (3m)• Function:• Complete chemical digestion• Absorb nutrients

Duodenum and accessory organs

• The duodenum is the first part of the small intestine

• It is short (25 cm) and not as highly folded as the other regions of the s.int.

• The chyme from the stomach is very acidic (pH 2) so the duodenum is more mucous lined then the rest of the tube.

• It receives secretions from the pancreas and liver (gall bladder).

Liver and Gall Bladder

• Liver produces bile (gall bladder stores it)

• Carried to duodenum through the common bile duct

• Bile emulsifies fats (keeps them from joining together)

• Increasing surface area to improve the chemical digestion of fats.

Pancreas

• Pancreas adds two main secretions to the duodenum:

• A) Sodium bicaronate - baking soda to neutralize acidic chyme raising the pH to over 7.

• B) package of 28 digestive enzymes - protein, carbohydrate and lipid digesting enzymes to continue and help complete chemical digestion

Jejunum and Ileum

• The jejunum and the ileum are essentially the same continuous tube lined with enzyme producing cells to complete chemical digestion

• Carbohydrates - monosaccharides• Proteins - amino acids• Lipids - glycerol and fatty acids, cholesterol

Jejunum and Ileum

• Both are highly folded with villi and microvilli • These increase the surface area to facilitate absorption• Most nutrients absorbed by diffusion, some by facilitated diffusion and

active transport• Amino acids and monosaccharides, along with most vitamins and

minerals (element ions) enter the capillary networks while fatty acids and glycerol are picked up by the lacteals (lymph)

Large Intestine

• The remaining digested liquid food material enters the large intestine

• A wide tube lined with circular and longitudinal muscles

• The approximately 1-1.25 m long tube has three main parts: Ascending colon, Transverse colon and the Descending colon ending with the anus (rectum).

Large Intestine Function

• The primary function of the large intestine is to reabsorb water! Water has been added the entire length of the tube to help with moving the food along as well as digestion and now has to be returned to the body along with some electrolytes (minerals).

• The bacteria that live symbiotically in the large intestine also produce vitamins K, B (B12), riboflavin and niacin that are also very useful to us and are absorbed by the large intestine.

Human Digestive System

• Peristalsis moves food, chyme, liquid digestive food throughout system

• There can be issues with several (all) steps along the way that interfere with digestion/system.

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