DIGESTIVE SYSTEM Anatomy Physiology Journey of a Meal

DIGESTIVE SYSTEM Anatomy & Physiology

Journey of a Meal

You are what you eat? • You are what you can do with what you eat!? • You are the difference between what you eat and what you excrete!?

GI tract, Alimentary canal, Gut!!! • Remember anything inside this hollow, muscular tube is OUTSIDE US. • Part of our 1 st line of defence of our Immune System. • In fact 80% of our immunity happens within this tract!!! • Digestion is the process of both MECHANICALLY & CHEMICALLY breaking down our foods & fluids into their basic building blocks. These components are small enough to be absorbed through the lining of the gut into the blood.

4 layers of tissue in the GI tract that blend into each other as the cell and tissue types change. • MUCOSA…this is the innermost layer, which is actually made up of 3 layers! • 1. Inner ‘mucous epithelium’, where all absorption takes place. • 2. Loose connective tissue called ‘lamina propria’ for support, protection & insulation. • 3. Smooth muscle layer, the ‘muscularis mucosae’ which is highly folded, creating a corrugated appearance.

Submucosa • Thicker connective tissue which contains glands, blood vessels, nerves (part of autonomic nervous system). • Submucosal/Meissner plexus. These plexuses connect and run alongside the spine, constantly receiving information such as temperature, p. H of tissues, hydration etc…HOMEOSTASIS!

Muscularis • Thick layer of muscle tissue that wraps around the submucosa. It has an inner layer of circular and an outer layer of longitudinal smooth muscle. • Contains nerves organised into the myenteric plexus. • Innermost layer contracts, narrows the lumen, creating pressure on the contents, propelling forwards or backwards. • Outermost layer contracts to shorten the length of the lumen, again propelling forwards or backwards. • Vodka martini…shaken not stirred! PERISTALSIS.

Serosa • Outermost layer. Made up of connective tissue and peritoneum. This not only covers the outside of the GI tract, but also continues around the abdominal cavity to cover the other organs. Connective tissues are made of 2 layers. If you were to put a big straw on each side of the tissue, then peel the 2 layers slightly apart just enough to wrap up both straws, you would still have 1 tissue but covering 2 things!

Segmentation & Peristalsis. Food is churned and mixed as moves along

Where does digestion start? ? ?

Cephalic/Psychi c phase. The THOUGHT of food triggers our physiology…rememb er Bruce!!!

Medulla Oblongata in brain (control centre) • Via the Vagus nerve and parasympathetic fibres to the gastrin glands in the stomach. This is one way the gastric mucosa is stimulated to produce GASTRIN. In turn, this hormone accelerates the secretion of gastric juices to aid digestion.

Mouth, Teeth, Tongue What is the mouth for? ? ?

Chewing • Powerful buccinator muscles in cheeks. • Mucous glands squeezed, releasing mucous. • Keeps food and linings moist, reducing friction. • Quite incredible pressure! • The tongue shapes the chewed food, coats it with saliva and propels to the back of the throat for swallowing. DEGLUTITION.

• PAROTID…largest…in front of ears…secretes watery saliva rich in enzymes; SALIVARY AMYLASE & LINGUAL LIPASE. Salivary Glands • SUBMANDIBULAR…secretes both digestive enzymes and mucous. • SUBLINGUAL…under the tongue…produces only mucous saliva. • The combined secretions of these paired glands totals around 1 litre per day…grossly under active these days!!!!!

OESOPHAGUS… 25 cm long. Sits behind the windpipe/trachea, extending downwards through the thoracic cavity (chest), through the diaphragm into stomach.

Motilin is the hormone that is cyclically released by the enteroendocrine cells in the upper small intestine. Motilin stimulates gastric and small intestine motility, causing undigested food in these regions to move into the large intestine. Involved in the appropriate opening & closing of valves throughout whole tract.

STOMACH

Protein Digestion + Intrinsic factor • HCl secreted by parietal cells bombard the proteins mechanically and chemically. • HCl involved in activating enzymes. Main one being PEPSINOGEN + HCl makes PEPSIN. • Gastric juices contain enzymes. • Highly acidic conditions in stomach are part of the immune system. • Intrinsic factor, secreted by parietal cells chemically binds to B 12 to protect from acidity, then once out of stomach, helps it to be absorbed across S. I. wall.

Stomach also… • Acts as a reservoir, storing food until it can be moved into the duodenum. • The process of emptying the stomach takes about 2 -6 hours after a meal… • A few substances are absorbed directly from here, including water, some drugs, alcohol & some short chain fatty acids. • Pyloric sphincter at the base of the stomach. It’s one thing for the stomach to be ready to release the chyme into the duodenum, but another thing entirely for the intestine to be ready to receive this highly acidic & corrosive gastric content!

Chyme is ejected into the duodenum every 20 seconds or so… • Because the stomach is relatively large and the duodenum small, gastric emptying needs to be controlled…INTESTINAL phase. • This is done through both hormonal & nervous mechanisms. • ENTEROGASTRIC REFLEX. • GIP’s, SECRETIN, CCK.

PANCREAS This needs water from circulation to make its watery bicarbonate solution containing pancreatic enzymes…

LIVER & GALLBLADDER

BILE • Contains (amongst other things, LECITHIN & BILE SALTS, both of which emulsify lipids/fats. They break them into smaller fatty droplets, making them more easily digested. They wrap a hydrophilic shell around the droplets, making them water soluble, therefore able to move freely through the watery chyme. These droplets are called MICELLES. Lipases can now hydrolyse the fatty drops into fatty acids, monoglycerides & glycerol. • Bile also contains some sodium bicarbonate, to help with neutralisation.

SMALL INTESTINE DUODENUM, JEJUNUN & ILEUM Almost all absorption takes place here.

2. 5 cm in diameter… 20 -odd feet long!!! • Duodenum…means 12 fingerbreadths. Shaped like the letter C. Approx. 10 inches long. • Jejunum… 8 feet long. • Ileum…no clear demarcation from jejunum. Approx. 12 feet long. • Wall of S. I. is highly folded into PLICAE, which have many projections called VILLI, creating the BRUSH BORDER. • Intestinal digestive enzymes are produced in these brush border cells. • Each villi has an arteriole, a venule & a lymph vessel.

Intestinal Juices • Most intestinal cells make a water based solution of sodium, plus mucous, to buffer and lubricate. • Proteases digest proteins into amino acids. • Lipases digest fats into fatty acids & glycerol. • Amylase & Maltase digest polysaccharides into glucose.

Vermiform Appendix • Part of Immune System. • Breeding ground for bacteria to help with digestion. • Part of immune system due to being serviced by the lymph.

The Large Intestine/Bowel/Colon Water is absorbed into the body from indigestible faecal matter, solidifies, coated with mucous for easy passage and expelled through anus.

Gut Flora/Microbiata