Humans have a a complete digestive tract, which begins withthe mouth and ends with the anus. The major structures of the human digestive tract are the :mouth,pharynx,esophagus,stomach,small intestine,large intestine,rectum andanus.
Why is digestionimportant?To convert large, often insolublemolecules of food into smallersoluble molecules capable ofbeing absorbed into the blood orlymph capillaries associated withthe digestive tract and used forvarious metabolic processes.
Ingestion: Food is placed intomouth. Teeth:- Incisors cut or bite the food.- Canines used to tear meat.- Molars and pre-molars grind the foodfine. Mechanical digestion
Secreted by the salivaryglands (parotid gland,sublingual gland and sub-maxillary gland) Mix with the food to form abolus. Saliva contains the enzymeamylase, that breaks downcooked starch into maltose. Chemical digestion.
Mixes food with saliva and pushes foodbetween teeth. Makes swallowing easier.
The bolus is forced down the esophagus whenthe muscular pharynx contract – swallowprocess. Peristalsis (contraction and relaxation ofantagonistic circular and longitudinal muscle)of the esophagus pushes the food downwardsinto the stomach, through the cardiac valve. No absorption takes place The epiglottis that covers thetrachea prevents food from goinginto the trachea when you swallow.
Food enters the stomach (fundus region) throughthe cardiac valve. Remains a half an hour still before the stomachmuscles (circular-, longitudinal- and obliquemuscles) starts contracting and relaxing(peristalsis). The food move with circular movements throughthe stomach (corpus and pyloric regions) andmixes with all the gastric juices. Mechanical digestion
Gastric juices are secreted afterthe hormone, gastrin stimulatesthe parietal cells in the fundusregion of the stomach. Gastric juices consist of HCl(Acidify stomach and neutralizesthe bolus, antisepticsolution, emulsifiesfats), digestive enzymes(pepsin, rennin andlipase), mucus (help protect theinner lining of the stomachagainst enzyme activity) andwater.
These gastric juices help with chemicaldigestion of food and the bolus is now calleda chym. Some absorption occurs in the stomach.Some water, glucose, salt and certain drugsand alcohol pass into the blood capillaries ofthe stomach wall.
As the chym enters the duodenum (first partof the small intestine) through the pyloricvalve it mixes with bile (excreted from theliver or gall bladder) and pancreatic juices.
Secretin is the hormone that stimulates thepancreas to secrete pancreatic juice into theduodenum. The pancreatic juice contain sodiumbicarbonate (neutralizes the chym, antiseptic)and digestive enzymes (Trypsin, amylase andlipase) Bile is produced in the liver and stored by thegall bladder. When fatty rich food enters the smallintestines, bile is secreted by the gall bladderand transported to the duodenum.
Neutralizes acid from stomachEmulsifies fats (increase surfacearea of fats for better digestion)Aid in the absorption of fats.Reduce the fluidity of the chymAntiseptic – prevent decompositionAssist in the absorption of the fat-soluble vitamins.
The chym then moves through the ileum(second part of small intestines) It mixes with intestinal juice (succusentericus) that contains the digestiveenzymes for the final digestion of food –Peptidase, lipase, lactase, maltase, sucrase Succus entericus is secreted by 2 glands:Crypt of Lieberkuhn and the glandsof Brunner (both situated in the lining of thesmall intestine).
The 2 muscle layer of the small intestine helpswith the process of peristalsis.Cross section through the small intestine
The liver produces and secretes bile. It stores glucose in the form of glycogen and is controlledby the hormone insulin. When the body requires glucose,glycogen is changed back into glucose.The liver alsoconverts glycerol, into glucose. It can convert excess carbohydrates into fatty acids whichcombine with glycerol to form fats. The liver stores blood temporarily, blood is also formed inthe liver of embryos. The liver manufactures plasma proteins e.g. albumen andfibrinogen. The liver forms heparin which prevents the clotting ofblood inside the blood vessels. The liver detoxifies the body.
•The liver breaks down surplus aminoacids through the process ofdeamination• Change amino acid into ammonia.•Two ammonia molecules combinewith one molecule of carbon dioxide toform urea and water.• Deaminated amino acids areconverted into glucose (glucose isconverted to glycogen and stored inthe liver)
The hepatic portal vein (transports digestedfood from the small intestine to the liver) andthe hepatic artery (transports oxygen andnutrients to the liver) enter the liver. Inside the liver the blood of these 2 bloodvessels mix and the products transported isexchanged between the blood and the livercells. The waste moves out of the liver cells and istransported away from the liver by means ofthe hepatic vein which joins up with theinferior vena cava.
Food is mainly absorbed from thesmall intestine, where all finaldigestive processes take place andthe end-products of digestion isformed.
The final products of digestion (glucose, aminoacids, fatty acids and glycerol) are formed in thesmall intestine. The absorptive surface area is increased by: The great length of the small intestine (7m) The circular folds of the mucosa lining of thesmall intestine. The millions of villi lining the folds. The chym is pushed along very slowly through thesmall intestine, allowing time for absorption totake place.
A villus consist of several capillary arteries,which supply the villus with OXYGEN andcapillary veins which carry food away from thevillus. Lacteals (lymph vessels) are found in the centreof each villus used for the absorption of fats. The vessels are surrounded by connective tissueand a layer of columnar epithelial cells in whichgoblet cells occur
The columnar epithelial cells play anactive part in the absorption and areable to allow substances to enter thevilli against the concentration gradient.Amino acids, glucose, mineral salts,water and vitamins are absorbeddirectly into the blood.
Fatty acids (insoluble) combine with bile salts To form fatty acid-bile salt complex which is soluble in water. This complex plus the glycerol component of fat is absorbed bythe columnar epithelial cells of the villi. Inside the villus the fatty acids are freed from the bile salts andrecombine with glycerol to form tiny fat globules. Some of the fat globules are absorbed directly into theblood capillaries. Most fat globules are absorbed by the lacteals. The lacteals unite to form small lymph vessels which enterthe main lymphatic system. This fat in the lacteal is now known as chyle, and it reachesthe bloodstream in the end.
No digestion takes place in the colon. Undigested food remains from the small intestine enterthe caecum through the ileocaecal valve. In the colon, water is absorbed so that the chym becomessemi-solid. Symbiotic bacteria present in the colon act upon the foodremains, decomposing them and turning them into faeces. The bacteria synthesize vitamins of the B group andvitamin K which is essential for the blood clotting process. Peristalsis in the colon is facilitated by mucus produced bynumerous mucous glands. Mucus assists the passage of faeces and protects the wallof the colon.
By the time the faeces reachthe rectum, they consist ofapproximately 70% water.Bacteria account for about 30%of the dry mass of faecesThe remainder is made up offood residue, mainly cellulose.
Defication is brought about bycontraction of the circular muscles ofthe rectum, and relaxation of themuscles which make up the analsphincter.