Digestive System

The System in Brief: Food enters via the mouth. dig system animation

Breaks down food, absorbs nutrients and eliminate wastes Breaks down food, absorbs nutrients and eliminate wastes Food must be broken down so that the pieces are small enough to pass through the walls of the intestinal villi Food must be broken down so that the pieces are small enough to pass through the walls of the intestinal villi Polymer Monomer

2 Forms of Breakdown: Mechanical and Chemical : 1. MECHANICAL in the mouth : teeth cut and grind the food into small pieces

Mechanical 2. in the stomach : strong muscles mix and churn the food

CHEMICAL : in the mouth : saliva is mixed with the food Saliva functions to: a) a) Lubricates esophagus b) b) Moisten food c) c) Contain enzyme – salivary amylase salivary amylase begins the chemical breakdown of starch into maltose (starch + H 2 O  maltose)

Swallowing l The tongue rolls food into a ball (bolus) and moves food to back of pharynx l The soft palate closes off the nasopharynx, and the epiglottis covers the opening to the trachea (glottis) Swallowing Animation Swallowing X-Ray

Peristalsis l Once the food enters esophagus, peristalsis begins. l muscular contraction that move bolus towards stomach. l an involuntary action throughout the alimentary canal Laryngoscopic Trachea vs Esophagus

l The cardiac sphincter is located between the esophagus and the stomach l Ring of muscle that acts as a valve, opening to allow the food bolus entry to the stomach, closing to prevent “back flow” of acidic chyme into the esophagus (causing burning sensation known as a heart burn) Esophagus Duodenum Stomach Cardiac Sphincter Pyloric Sphincter

The bolus enters the stomach through the cardiac sphincter.

Functions of Stomach 1) Store Food (chyme) 2) digestion of proteins, produces digestive enzymes 3) Kills bacteria in food with addition of HCl

Stomach l Muscular stomach wall churns food (peristalsis) l Lining of stomach contains “gastric pits” which lead to gastric glands (produces gastric juice containing enzyme pepsin, HCl and mucus) Gastric Pit Gastric Gland

HCl Causes highly acidic environment (pH of 2) Kills bacteria converts pepsinogen to pepsin Pepsinogen an inactive form of pepsin, (protein + H 2 O --> polypeptides) Gastric Juices

Gastrin (a hormone) stimulates the secretion of HCl at gastric glands Triggered by high protein meal Increase production of gastric juices

Mucus protects the stomach lining from digesting itself! (the lining is replaced every few days) Prevention of Ulcers

Helicobacter pylori (commonly referred to as h. pylori)is a bacterium that infects the mucus lining of the stomach. Many peptic ulcers and some types of gastritis are caused by H. pylori infection, although most humans who are infected will never develop symptoms. This bacterium lives in the human stomach exclusively and is the only known organism that can thrive in that highly acidic environment. It is helix-shaped (hence the name helicobacter) and can literally screw itself into the stomach lining to colonize. H. Pylori infections It is estimated that about two thirds of the world population are infected by helicobacter pylori. Actual infection rates vary - the Western/Developed nations have rates around 25% and less developed nations have much higher rates of infections. In the United States, infection is primarily in the older generations (about 50% for those over the age of 60 compared with 20% under 40 years) and the poorest. This is largely attributed to higher hygiene standards and widespread use of antibiotics. However, antibiotic resistance is appearing in H. pylori. There are already metronidazole resistant strains present in the UK population. There is also evidence of a higher incidence of symptoms in people who eat spicy foods, apparently because of the effect these foods have on the stomach lining. When the protective lining is reduced from certain foods, h. pylori is better able to spiral into the stomach wall. This is one of the reasons that it was (and still is) commonly thought that spicy foods caused stomach ulcers. In actual fact, it is the h pylori that causes the stomach ulcers, but the spicy foods creates the conditions for h. pylori to cause problems. In the absence of any treatments, H. pylori infection apparently persists for life; our immune systems are not able to eradicate it. Peptic ulcer