The Gastrointestinal System Flashcards
(18 cards)
Introduction of the gastrointestinal tract(GIT)
The GIT is one long muscular tube from the mouth to the anus.
The digestive system breaks down food and drink through the process of digestion into nutrients, specifically carbohydrates, proteins and fats. These nutrients are absorbed into the blood stream and transported to cells throughout the body. Once in the cells they provide fuel for cell function and the building blocks required for cell growth and repair.
Gross anatomy of the digestive system
It consists of : Mouth Oropharynx Oesophagus Stomach Small intestine(duodenum, jejunum and ileum) Large intestine(caecum, colon ascending,transverse,descending, sigmoid) Rectum and Anus
Digestive system accessory organs
Salivary glands : parotid, submandibular, sublingual Tongue Teeth Liver Gallbladder Pancreas Appendix
Layers of the GI tract
Mucosa: the innermost layer composed of mucus membrane and lines the tract from the mouth to the anus. It protects the tract, secretes substances and absorbs the products of digestion.
Submucosa: Outside the mucosa is the sub -mucosa which contains blood vessels lymphatic vessels and nerve fibres. This is surrounded by muscle tissue.
Serosa: the outermost layer of the GI tract and consists of connective tissue and peritoneum. The peritoneum lines the abdomen and is differentiated into visceral and parietal
Functions of the digestive system
Ingestion : taking food and drink
Digestion: the mechanical and chemical processes that break down the food into small molecules
Absorption: the entrance of products of digestion into the epithelial cells lining the lumen. The absorbed substances pass into blood or lymph and circulate to cells throughout the body
Elimination: wastes and indigestible materials that were not absorbed are excreted as waste( defecation)
Process of digestion
An enzymatic process which converts complex foodstuffs into smaller absorbable units. The primary process used is hydrolysis ( breakdown by addition of water)
Some components such as water, mineral salts, vitamins and glucose are already in a suitable form for absorption
Digestion in oropharynx
The food bolus is moved through the pharynx and oesophagus by the action of swallowing which is under voluntary control.
As the food passes down it is prevented from entering the trachea by the epiglottis
Digestion in the oesophagus
Once in the oesophagus the muscular action becomes involuntary and is called peristalsis
Digestion in the stomach
In the stomach food is stored and churned into chyme. A ring of muscle at the entrance- cardiac sphincter prevents reflux of stomach contents. Hydrochloride acid is produced in the stomach: breaks up food substances, destroys microorganisms and activates enzymes.
Enzyme pepsin digests proteins. Some absorption takes place in the stomach
Digestion in the small intestine
Chyme passes through the pyloric sphincter into the duodenum, here enzymes from the gallbladder and pancreas are added.
Bile from the gall bladder contains substances which breakdown fats
The pancreas secretes many enzymes including trypsin, chymotrypsin, amylase and lipase into the duodenum via pancreatic juice these aid the digestion of starches proteins and fats.
The job of the jejunum and ileum is to absorb nutrients. The lining of the small intestine is covered in villi hair like projections one cell thick with a very good blood supply. Villi also produce mucous which protects the gut wall
Digestion in the large intestine( colon)
The principle function of the large intestine or colon is to reabsorb water and store faeces. The appendix once believed to be redundant is now thought to play a part in human immunity. Bacteria living in the colon assist in the breakdown of waste matter and produce some vitamins B group and vitamin K
Carbohydrates
This includes sugars and starches. Only simple sugars can be absorbed into the bloodstream and utilised as sources of energy
Carbohydrates digestion
The enzymes involved are amylase: which acts on starch to give maltose. Invertase : which acts on sucrose to give fructose and glucose. Lactase: which acts on lactose giving glucose and galactose and Maltase: which acts on maltose giving glucose.
Absorption of sugars
Glucose, galactose and fructose are then absorbed in the small intestine. Glucose and galactose enter the blood stream by sodium dependent active transport whereas fructose is transported by passive facilitated diffusion.
Digestion of carbohydrate
Carbohydrate digestion begins in the mouth and continues in the body of the stomach. The majority of hydrolysis is performed in the small intestine after the chyme is mixed with pancreatic juice( from the pancreas) and bile (from the liver)
Proteins
Dietary protein is required to supply the amino acids for formation of body protein. Any surplus is used as a source of heat and energy.
There are 20 different amino acids found in our proteins. Some of these(12) can be synthesised from other amino acids whereas others (8) have to be provided in adequate quantity from the diet.
Hydrolysis of proteins starts in the stomach under the influence of pepsin, a proteolytic enzyme which converts proteins into peptide fragments in the acid environment of the stomach.
Hydrolysis continues after addition of pancreatic juice ( which contains trypsin, chymotrypsin and carboxypeptidase) in the duodenum.
These further break down the peptides into smaller fragments which can be absorbed by the small intestine and hydrolysed by aminopeptidases to give amino acids which can be actively transported into the blood stream
Fats
The main useful components of ingested fats are the triglycerides or tricylglycerols. Because fats are not soluble in water, digestion of fats is complicated.
Chemicals produced in the liver- the bile salts- emulsify the fat and render it susceptible to the action of lipases secreted by the pancreas which hydrolyse it to form monoglycerides and free fatty acids.
Nerve supply
ANS( autonomic nervous system) through opposite action of parasympathetic and sympathetic.
The parasympathetic is responsible for digestion. A pair of cranial nerve(vagus) supply most part of the GIT and accessory organs. Sacral nerves supply the most distal part of the tract. It’s stimulation increase muscular activities (peristalsis) and glandular secretion.
The sympathetic supply is by various nerves that emerge from the spinal cord in the thoracic and lumbar region to form plexus. The effects decrease muscular activities and glandular secretion.
