Digestive System Chapter 22 – Day 1 3/24/08

Digestive system Respiratory System Cardiovascular System Brings O2 to the body Cardiovascular System Brings O2 to the cells In tissues O2 is used for cellular respiration, BUT… O2 is only one of the ingredients – you also need Glucose Digestive System Brings glucose to the body Other sources of nutrients And other building blocks to make cells 3/24/08

Digestive system Food is broken down & processed Nutrients are absorbed Waste products are eliminated All of this happens in the digestive tract A.K.A. alimentary canal or GI tract The digestive tracts is a long, continuous muscular tube starting at the esophagus 3/24/08

Digestive system Fig. 21.11 3/24/08

Digestive system - Processes In order for nutrients to reach cells the digestive system carries out several processes Ingestion = food in the mouth Mechanical processing = physically breaking food into bits Digestion = enzymes and hormones Absorption = organic molecules enter interstitial fluid Excretion = elimination of unwanted materials 3/24/08

Digestive system – Cell/Tissue layers The digestive system has a distinct system of tissue layers There is an “open” surface inside the GI tract – epithelial tissue covering on outside 4 layers – from inside to outside These are there ALL ALONG the GI tract Mucosa innermost layer Submucosa Muscularis externa Serosa outermost layer Fig. 22.3 3/24/08

Cell/Tissue layers - MUCOSA Digestive epithelium Connective tissue – lamina propria (smooth) muscle tissue – muscularis mucosa Type of epithelium varies along the GI tract Pharynx & esophagus handle large masses of food = stratified epithelium In the stomach – food becomes liquid & passes to intestines A great deal of absorption happens in the intestines Simple columnar epithelium Secrete mucous Highly folded epithelial surface (small intestine) Lamina propria – areolar connective tissue right below basement membrane of epithelium Contains blood vessels, nerves, mucous glands (in the intestines this is where you find the Peyer’s patches) Muscularis mucosa = smooth muscle – helps movement in the GI tract 3/24/08

Cell/Tissue layers - SUBMUCOSA Dense connective tissue Nerve network Submucosal plexus Communication with the nervous system – helps muscle movement 3/24/08

Cell/Tissue layers – MUSCULARIS EXTERNA Smooth Muscle Tissue Nerve network Mysenteric plexus More forceful contractions for movement 2 layers of muscle Longitudinal Circular Allows 2 types of movement Rocking motion Forward movement 3/24/08

Cell/Tissue layers – SEROSA Fibrous tissue on outside Protective covering which helps in attachment to abdominal wall Fig. 22.3 3/24/08

Other important general features Membranes Perotineal cavity 2 membrane layers Encloses abdominal portion of digestive tract Serosa = visceral peritoneum (inner layer) Parietal peritoneum = lines abdominal wall (outer layer) Additional serous membrane connects serosa/visceral to parietal peritoneum The space between the 2 layers if filled with fluid Peritoneal fluid Secreted by cells of the peritoneum 3/24/08

Other important general features Mesenteries Sheets of serous membrane allow blood vessels, nerves, and lymph vessels to pass through & network Stabilize the position of the GI tract Prevents “flopping around” OMENTUS – associated with the stomach (Fig. 22-12) Lesser omentum & greater omentum Contains adipose tissue (fat around the belly) Mesentery proper Within small intestine, between loops Mesocolon Stabilizes large intestine Areas without mesentaries = pancreas & duodenum – which are attached to the abdominal wall for stability 3/24/08

Secretions Within the digestive tract – secretions are evident throughout Mucus To lubricate food & “walls” Digestive enzymes Break chemical bonds Electrolytes Buffers pH Cofactor for enzymes Parts of digestive system & direction of food… Explore important structures, secretions, absorption, digestion process, movement of food 3/24/08

Alvioli – Capillary Interface Fig. 21.11 3/24/08

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Movement of Bolus Forward movement Waves of contraction in muscularis externa Along length of tube Process of peristalsis Circular muscles contract behind bolus Longitudinal muscles contract at cardiac sphincter Wave of relaxation opens entrance to stomach 3/24/08

Movement of Food (bolus) Fig. 22.4 3/24/08

Movement of Bolus Forward movement Side to side movement Waves of contraction in muscularis externa Along length of tube Process of peristalsis Circular muscles contract behind bolus Longitudinal muscles contract at cardiac sphincter Wave of relaxation opens entrance to stomach Side to side movement No set direction Helps to mix bolus with mucus for more lubrication Contraction in muscles → segmentation Mostly in large & small intestine – helps to fragment bolus 3/24/08

Stomach Anatomy Shape Sphincters Folds = rugae Mucosa Cardiac Pyloric Deep muscular folds Mucosa Fig. 22.12 3/24/08

Stomach Anatomy Mucosa 4 types of secretory cells: Cell Secretion Gastric pits with gastric glands Secretory cells 4 types of secretory cells: Cell Secretion Chief cells → Pepsinogen Parietal cells → HCl (acid) Mucus cells → Mucus Enteroendocrine cells → Gastrin (hormone) 3/24/08

Stomach Processes What happens to food when it enters the stomach? Digestion & Secretion – almost no absorption 3 phases of secretion in the stomach Cephalic Begins at the sight of food Gastrin is secreted Stimulates HCl & pepsinogen Food enters the stomach 3/24/08

Phases of Gastric Secretion in Stomach Gastric Phase Secretion Mucus is secreted to protect stomach lining More gastrin, more pepsinogen Acidic environment – pH drops (pepsinogen → pepsin at low pH) Secretions stop when pH reaches 2.0 Digestion Proteins in food →pepsin →amino acids Milk proteins →gastric lipase → amino acids & renin 3/24/08

Phases of Gastric Secretion in Stomach Intestinal Phase Mixing Rugae become stretched – stomach is distended Muscular contractions mix food for several hours Food becomes watery mixture Chyme (acidic) After several hours of mixing waves of contractions (peristalsis) reach the lower end/base of the stomach – near the pyloric sphincter Sphincter opens & closes with each wave Squirts chyme into the duodenum The Duodenum secretes enteric gastrin starts next phase 3/24/08

Phases of Gastric Secretion in Stomach Gastric Phase Mixing Rugae become stretched – stomach is distended Muscular contractions mix food for several hours Food becomes watery mixture Chyme (acidic) After several hours of mixing waves of contractions (peristalsis) reach the lower end/base of the stomach – near the pyloric sphincter Sphincter opens & closes with each wave Squirts chyme into the duodenum The Duodenum secretes enteric gastrin starts next phase 3/24/08