1、The digestive system is responsible for breaking down food,absorbing nutrients from the food,and ridding the body of waste products from the food.It is roughly nine meters long and begins at the mouth and ends at the anus.The digestive system consists of the:the alimentary tract mouth,pharynx,esopha
2、gus,stomach,small and large intestines,rectum and anus the accessory organs salivary glands,liver,gall bladder and pancreas The mouth,or oral cavity,is the first part of the digestive tract.It is adapted to receive food by ingestion,break it into small particles by mastication,and mix it with saliva
3、.The lips,cheeks,and palate form the boundaries.The oral cavity contains the teeth and tongue and receives the secretions from the salivary glands.Kids have 20 teeth,but by the time they are fully grown,they should have 32 teeth(some people do have less!).The esophagus is the second step to digestio
4、n and closely follows the oral cavity once swallowing has initiated.the Alimentary TractThe stomach is divided into the fundic,cardiac,body,and pyloric regions.The stomach has three layers of muscle:an outer longitudinal layer,a middle circular layer,and an inner oblique layer.The inner lining consi
5、sts of four layers:the serosa,the muscularis,the submucosa,and the mucosa.The mucosa is densely packed with gastric glands,which contain cells that produce digestive enzymes,hydrochloric acid,and mucus.The stomach is a multi-functional organ,it stores food,breaks it down and then empties it into the
6、 small intestine.The small intestine consists of three sections.duodenum,jejunum and ileum which attaches to the first portion of the large intestine.Functiondigestion,and absorption.The small intestine finishes the process of digestion,absorbs the nutrients,and passes the residue on to the large in
7、testine.The liver,gallbladder,and pancreas are accessory organs of the digestive system that are closely associated with the small intestine.The large intestine is made up of three portions:the ascending,transverse and descending colon.It is the portion of the digestive system most responsible for a
8、bsorption of water from the indigestible residue of food.The ileocecal valve of the ileum(small intestine)passes material into the large intestine at the cecum.Material passes through the ascending,transverse,descending and sigmoid portions of the colon,and finally into the rectum.From the rectum an
9、d anus,the waste is expelled from the body.Three pairs of major salivary glands(parotid,submandibular,and sublingual glands)Saliva contains water,mucus and enzyme amylase.Functions It has a cleaning action on the teeth.It moistens and lubricates food during mastication and swallowing.It dissolves ce
10、rtain molecules so that food can be tasted.It begins the chemical digestion of starches through the action of amylase,which breaks down polysaccharides into disaccharides.the Accessory Organs Liver is the largest gland in the body.On the surface,the liver is divided into two major lobes and two smal
11、ler lobes.It overlies and almost completely covers the stomach.Functions secretion synthesis of bile salts synthesis of plasma protein storage detoxification excretion carbohydrate metabolism lipid metabolism protein metabolism filtering The gallbladder is a pear-shaped sac that is attached to the v
12、isceral surface of the liver by the cystic duct.The principal function of the gallbladder is to serve as a storage reservoir for bile.Bile salts act as emulsifying agents in the digestion and absorption of fats.It is secreted from the intestine and specifically the duodenum.The pancreas is located i
13、n the back of the abdomen,behind the stomach.The pancreas makes pancreatic juices and hormones,including insulin.Pancreatic juices,also called enzymes,help digest food in the small intestine.Insulin controls the amount of sugar in the blood.Both enzymes and hormones are needed to keep the body working right.13 以上有不当之处，请大家给与批评指正，以上有不当之处，请大家给与批评指正，谢谢大家！谢谢大家！
