Digestion 1 Dr Viv Rolfe Alternative formats and large print versions of these handouts are available upon request.

Lecture Contents Functions of the digestive system. Role of key hormones in digestive processes. Role of bile. The basics of nutrient absorption.

Main Function of the Gut Food is taken into the body via the gut in a process called ……………….... The mechanical and chemical (enzymes) break down food into small units is called …………….... Nutrients are transported across the gut wall into the blood stream by …………….. Waste products are …………….... The GastroIntestinal tract is important for ……………………….. (barrier, stomach acid, lymphoid tissue) and plays a role in body ……………………... ingestion digestion absorption eliminated natural defences water and electrolyte homeostasis

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Structure of the Gut Wall Single layer of epithelial cells Networks of blood capillaries and lacteals Immune cells Smooth muscle (2 layers)

Gut Functions (from top to bottom)

What goes in? What are the major food groups? Fat Protein Carbohydrate Vitamins Minerals Water

The Oral Cavity: 1.Oral cavity – buccal cavity – mouth. 2.Teeth chew the food to reduce particle size. 3.Tongue moves food in the mouth and forms a soft BOLUS. 4. Saliva acts as a lubricant, and contains the enzyme SALIVARY AMYLASE which breaks STARCH to DISACCHARIDE.

Polysaccharides (sugars & starches) Disaccharides Salivary Amylase Enzymatic Activity

About 25 cm long in adults. Transports the bolus to the stomach by PERISTALSIS. The Oesophagus

A “ring” of smooth muscle contraction that propels food through the intestine. Peristalsis

The Stomach An expandable receptacle made of 3 muscle layers.

Contractions of the muscle mash and mix the contents with gastric secretions to form CHYME. Two main type of gastric gland line the stomach wall: 1) PEPTIC GLANDS secrete PEPSINOGEN. 2) PARIETAL CELLS secrete HYDROCHLORIC ACID. The passage of food into the duodenum is controlled by GASTRIC EMPTYING. Stomach

Protein polypeptides Peptides HCl (pH 2) converts pepsinogen to pepsin Enzymatic Activity

25-30 feet long and divided into three regions – Duodenum, Jejunum & Ileum It has a huge surface area for digestion and absorption to take place: 1) The gut walls are folded. 2) The surface comprises of VILLI (finger-like) which project into the lumen. The Small Intestine

The Duodenum Alkaline mucus is secreted to NEUTRALIZE stomach acids in the chyme. The duodenum receives secretions from the PANCREAS and GALL BLADDER.

Food in the duodenum triggers the secretion of a hormone - CHOLECYSTOKININ (CCK). CCK contracts the gall bladder and BILE empties into the duodenum. Bile EMULSIFIES fat enabling lipid to be digested. Gall Bladder

The pancreas synthesises and then secretes enzymes that digest all 3 major food groups (carbohydrates, protein and fat). The presence of food in the stomach and duodenum causes CCK and SECRETIN to be released. 1) SECRETIN stimulates the secretion of alkaline juice from the pancreas. 2) CCK stimulates the release of enzymes. Pancreas

Polysaccharides (sugars and starches) Disaccharides (e.g. maltose) Pancreatic Amylase Carbohydrate digestion

Protein polypeptides Di-peptides PancreaticTrypsin Protein digestion

Bile Fats Emulsification Fat digestion

Triglycerides Fatty acids and glycerol Pancreatic Lipase Fat digestion Fatty Acid Glycerol

Chyme enters the jejunum and ileum where the enzymatic breakdown of food is completed. Digested food is absorbed across the gut wall. Jejunum and ileum

e.g.Maltose Glucose Maltase Carbohydrate digestion Disaccharides monosaccharides Enzymes

Di-peptides Amino acids Peptidase Protein digestion

Glucose & fructoseSucroseSucrase GlucoseMaltoseMaltase TryglyceridesFatsBile salts Fatty acids and monoglycerides TriglyceridesPancreatic lipase Glucose & galactoseLactoseLactase Jejunum/ileu m Duodenum PeptidesPolypeptidesPancreatic Trypsin Amino acidsPeptidesPeptidases DisaccharidesPolysaccharidesPancreatic amylase PeptidesProtein (Polypeptides) PepsinStomach Disaccharides(Carbohydrate) Polysaccharides Salivary amylase Mouth Into…Breaks down…EnzymeRegion

Nutrient Absorption Amino acids Fatty acids Glycerol Monosaccharides

Absorption… Mono- saccharides Absorbed across gut wall Enter blood capillaries Taken via the hepatic portal vein to the LIVER Amino acidsAbsorbed across gut wall Enter blood capillaries Taken via the hepatic portal vein to the LIVER

ENERGY

Fats are hydro-phobic (don’t dissolve in water) so can’t freely absorb across the gut wall. The fatty acids are enclosed in bile salts to form MICELLES which are absorbed across the gut wall into LYMPH VESSELS. Fat absorption

Fat joins the Blood at the left subclavian vein Glucose & amino acids enter the liver for metabolism and storage Hepatic Portal Vein

Nutrients are stored and metabolised in the liver (see Digestion 2 in DN8).

Now digestion and absorption are complete but gut functions do not end here. About 4 hours after eating, chyme enters the large intestine (colon) via the ileo-caecal valve.

Colonic Functions The colon absorbs WATER and SALTS from the CHYME thus dehydrating them to form FAECES. This is important for the balance of fluid and electrolytes in the body.

Colonic Functions The colon contains vast numbers of COMMENSAL bacteria (microflora). Bacteria ferment digestive left overs (mainly fibre) to salvage energy in the form of short-chain fatty acids. Gas is a waste product.

Defecation The rectum stores (and further dehydrates) the stools. Defecation reflexes (See Digestion 2) control the elimination of faeces from the body.