The Digestive System - Biology, The Science of Life
The Digestive System Lesson 01 Simple Digestive Tracts Digestive Tract in Vertebrates Very specialized. Digestion involves: a. Physical fragmentation
b. Chemical fragmentation The Human Digestive System Vertebrate Digestive Tract Mouth and Teeth: Many have teeth for mastication. Some have
gizzards where small pebbles grind the food. Mouth serves to ingest. Mouth
The tongue mixes the food with saliva. Saliva lubricates food.
The enzyme salivary amylase breaks polysaccharides. 3 pairs of salivary glands (humans). Mouth Swallowing: Voluntary action involuntary
control. Pressure against the palate, induces a swallowing reflex. Muscles contract and raise the larynx that pushes the glottis against the epiglottis (flap of tissue). The Esophagus
Swallowed food (bolus) enters the esophagus. The esophagus moves the bolus through smooth muscle contraction (peristalsis) When the bolus reaches the stomach,
the sphincter prevents the food to move back into the esophagus. The Stomach
Saclike structure. Inner surface is convoluted, expands when full. Churns food and mixes it with gastric juice. Its cells secrete HCl and pepsinogen.
The human stomach produces 2 L of HCl, an acidic solution. pH 2 in the stomach. Acidic solution denatures food proteins and keeps pepsin active. Chyme is the mixture of partially digested food and the gastric juice. Acidic solution kills bacteria.
Small Intestine Chyme leaves stomach through pyloric sphincter into the small intestine.
Function: terminal digestion and absorption Small Intestine
The duodenum: first part of the small intestine. The remainder is the jejunum and the ileum. The duodenum receives the chyme, digestive enzymes from the pancreas and bile from the liver and gallbladder. Digestion occurs in the duodenum
and jejunum. Small Intestine Villi and microvilli (brush border) cover the epithelial wall of the small intestine.
Function: produce enzymes such as lactase and absorption. Accessory Organs Pancreas:
Large exocrine and endocrine gland Secretes pancreatic fluid into the duodenum. Fluid contains trypsin and chymotrypsin, pancreatic amylase and lipase.
Accessory Organs Liver and gallbladder: Largest organ Secretes bile pigments and bile salts Bile salts prepare fats for enzymatic
digestion. Bile is stored in the gallbladder. Large Intestine
Colon Cecum and appendix (vestigial structures) Absorbs water No digestion, only absorption of fluids and elimination of waste material Digestive system generally allows
one-way flow of food. This is very important: Sequential steps of digestive events can be spatially organized. No mixing of food at different stages of digestion.
Morphology of Vertebrate Digestive Track Various secretions into different parts of the digestive tract and their daily quantities Progressive Dehydration of Carbohydrates
Fermentative breakdown in ruminants Progressive Dehydration of Proteins
Note that different enzymes carry out their action at different stages of protein digestion. Lipids are digested only in small intestine. Insoluble globules of fat in
the lumen of small intestine are attacked by lipase to produce oil droplets that are taken up by microvilli cells
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