Gastrointestinal System Flashcards
Describe the 4 layers of the GI tract.
The innermost layer is the mucosa - an epithelial mucous membrane (secretes mucus to lubricate and protect the tract - type of epithelium depends on the location within the tract).
The submucosa is a layer of connective tissue containing blood vessels, lymph vessels, and nerves.
The muscularis is two layers of smooth muscle - a circular on the inside and longitundinal on the outside (for peristalsis).
The serosa is the outermost layer, and is make of connective tissue. It secretes fluid to lubricate the outside of the GI tract.
Describe the pathway that food takes from the mouth, through the digestive tract, and out the rectum.
Food enters via the mouth which is bordered by the lips, cheeks, palate, and tongue. The front 2/3 of the palate is hard, and the posterior 1/3 is muscle and connective tissue (soft palate). Food travels to the oesophagus via the oropharynx. The oesophagus is 25cm long, and 2cm wide. Food passes through the upper sphincter, through the thoracic cavity and into the abdominal cavity through an opening in the diaphragm called the hiatus, ending in the lower/cardiac sphincter, which opens into the stomach. After being partly digested in the stomach, food passes through the pyloric sphincter into the duodenum of the small intestine. It then passes through the duodenum, jejunum, and ileum, where over 90% of nutrients are absorbed.
The ileocecal valve/junction/sphincter opens into the large intestine from the small. The large intestine reabsorbs water from the food, compacting it into faeces (and absorbs remaining nutrients - mostly vitamins). Faeces travel up the ascending colon against gravity to the right hepatic flexure, then laterally to the left along the transverse colon to the left splenic plexure, then down the descending colon, and finally, the sigmoid colon, which leads to the rectum and anal canal.
Describe the histology of the pharynx, oropharynx, laryngopharynx, and oesophagus.
In the oropharynx and laryngopharynx, epithelium is stratified squamous (considered to be mucosa). Mucosa in the oesophagus is stratified squamous as well.
Describe the histology of the stomach.
The stomach wall contains many folds into the mucosa known as rugae, whose purpose is to decrease the surface tension during contraction of the heart - these contain microscopic invaginations known as gastric pits, which contain 5 types of surface cells:
1) Surface mucous cells - secrete alkaline mucous (neutralises lower stomach acid secretions)
2) Mucous neck cells - also secrete alkaline mucous
3) Parietal cells - secrete hydrochloric acid to break down food substrates
4) Chief cells secrete digestive enzymes
5) Endocrine cells secrete hormones (e.g. gastrin)
The stomach muscularis is composed of 3 layers rather than two (has an additional inner oblique layer) which helps the stomach churn and break down food.
Describe the histology of the small intenstine.
The small intestine can be divided into 3 regions - the duodenum, the jejunum, and the ileum.
The duodenum contains two entrance holes called major and minor duodenal papillae, where bile comes from the liver (as well as secretions from the pancreas). The submucosa of the duodenum contains extra alkaline mucus glands called Brunner glands.
The jejunum and ileum are both peritonised (covered by serosa). Permanent folds line the inside of the small intestine (stomach folds are more fluid than these) called the plicae circularis. Plicae circulari have villi projections from the mucosa to increase surface area, and these become less prominent as you travel distally down the small intestine, and the thickness of the wall also increases. These villi are covered by 4 types of simple columnar epithelium characterised by their excretions - absorptive (digestive enzymes), goblet (mucus), granular/Paneth (lysozymes), and endocrine (hormones), and all contain a blood capillary network connected to a lymphatic network - known as lacteals.
Describe the histology of the large intestine and rectum.
The mucosal layer is still simple columnar epithelium, but there are no villi or plicae here. The mucosal lining forms crypt-like structures, but the surface overall is smoother than in the small intestine. There are 3 types of cell here - absorptive, granular, and goblet. The muscularis layer is still comprised of two layers, however, it is not continuous - it forms 3 bands called taeniae coli, whose contractions gather the colon into pouches called haustra, allowing spaces for faecal matter to be stored. The large intestine is also peritonised, and the serosa contains fat-filled appendages called omental appendages.
The rectum is the terminal part of the large intestine, and it contains transverse rectal/mucosal folds - simple columnar epithelium with tubular glands. There is thick muscularis here. At the anal canal, muscularis forms two distinct sphincters (internal and external anal sphincters) - the internal is smooth muscle, and the external is skeletal.
Discuss the role of saliva in digestion, and how its production is mediated by the ANS.
Salivary glands secrete lubricating fluid which contains enzymes that break down carbohydrates. There are 3 pairs of major salivary glands (as well as hundreds of minor ones) called the sublingual, parotid, and submandibular glands. Saliva is composed of 99.4% water, as well as electrolytes (bicarbonate, sodium, chloride, phosphates, urea, etc), immunoglobulins and antibodies, mucins (macro-molecule proteins which help aggregate bacteria to prevent plaques), and enzymes (salivary amylase and lingual lipase).
Parasympathetic nerves control the volume of fluid secreted (via the glossopharyngeal nerve for the parotid; and facial nerve (VII) for the sublingual and submandibular glands ) via acetylcholinergic nerves.
Sympathetic nerves control the composition of the saliva (e.g. amylase of IgA secretions) via the preganglionic nerves (T1-3).
Discuss the secretions of the stomach and their role in digestion.
The stomach secretes gastric juice, which includes a variety of chemicals to aid chemical digestion and protect from infections. It is composed mainly of water, as well as hydrochloric acid, pepsinogen (breaks down proteins), mucus, and intrinsic factor. Intrinsic factor is secreted by parietal cells, and is required for the absorption of vitamin B12. Hydrochloric acid denatures proteins, kills microbes (pH1-3), and activates enzymes. Due to the alkaline mucus secreted in the gastric pits, neutralisation of the acids protects the stomach lining from damage of the hydrochloric acid, preventing the activity of enzymes which may otherwise digest the stomach wall. Chief cells secrete pepsinogen, which is converted to pepsin when pH<3, as well as gastric lipase which splits short-chain triglycerides into fatty acids and monoglycerides - this is the beginning of breakdown of fats and proteins.
Discuss the secretions of accessory organs and their role in digestion.
Accessory glands (pancreas and liver/gallbladder) secrete exocrine and endocrine chemicals which break down fats and proteins to their constituents. The pancreas produces many digestive enzymes, and supplies them through a duct which converges with the liver’s bile duct. These secretions combine and enter the duodenum of the small intestine.
In pancreatic glands, acinar cells produce digestive enzymes (pancreatic amylase to break down starches, pancreatic lipases to digest fats, ribonucleases to digest RNA, and deoxyribonucleases to digest DNA). In the ducts, ductal cells secrete bicarbonate to prevent digestive enzymes becoming active before reaching the small intestine. Amylases, lipases, and nucleases are secreted in active forms, but require ions or bile for optimal activity. Proteases are secreted in an inactive form, and are only activated in the duodenum, by enzymes which are present there.
The gallbladder stores bile secretions from the liver - bile is an alkaline digestive secretion which contains bile salts and blirubin (formed from breakdown of RBCs). It assimilates and eliminates lipids and neutralises gastric acid to provide optimum pH for pancreatic enzymes.
Discuss the secretions of the small intestine.
Secretions of the small intestine occur in Brunner’s glands and crypts of Lieberkuhn. Brunner’s glands secrete mucus which protects the small intestine mucosa from gastric acid. Secretions of Lieberkuhn crypts involve two main cell types - cryptic goblet cells (secrete mucous), and cryptic enterocytes (secrete up to 2L per day of fluid which adds to the intestinal chyme). The intestinal chyme has a similar electrolyte composition as the extracellular fluid for the passage of nutrients.
Give an overview of nervous and endocrine control of digestive secretions.
Digestive secretions are mediated by the ANS and endocrine system, and occur according to 3 phases:
1) The cephalic phase involves smell, sight, thought, and taste of food which activates the CNS, preparing the digestive tract before food enters by activating certain nerves such as the glossopharyngeal and vagus nerves, which stimulate exocrine secretions of the mouth and stomach.
2) The gastric phase involves food distending the stomach (stimulating stretch receptors or chemoreceptors), and stimulates peristalsis, churning, and gastric secretions - these functions are controlled by a negative feedback loop.
3) The intestinal phase begins when food enters the small intestine, and inhibits gastric motility via endocrine function (cholecystokinin CCK and secretin). It also induces contraction of the pyloric sphincter to prevent food passing back up into the stomach - this is a neural enterogastric reflex (nervous and endocrine mediated).
Explain the process of digestion of carbohydrates.
In the lumen, starches and glycogen are broken down by amylase into alpha-dextrins, maltotriose, and maltose. Glucoamylase, sucrase, and isomaltase are then used at the mucosa to break down these into glucose. Lactose is broken down into glucose or galactose by lactase, sucrose is broken down into glucose or fructose by sucrase, and trehalose is broken down into glucose by trehalase.
Water soluble monosaccharides can move into the cell via secondary active transport of facilitated diffusion. Sodium-glucose transporter 1 (SGLT1) is a symporter which takes up glucose and galactose against its concentration gradient by coupling a co-transporter with a sodium pump. They can then be held within the cytosol for the epithelium’s own metabolic needs, or can exit via the basolateral pole using GLUT2 transporter.
Fructose is taken across the apical membrane by GLUT5 transporter - uptake is relatively inefficient due to no energy coupling (therefore malabsorption is common).
Explain the process of digestion of proteins.
Proteins are broken down by gastric pepsin and pancreatic tryipsin, chymotrypsin, and carboxypeptidase (enzymes only become active in the duodenum). Specific peptidases in the small intestine microvilli break down these smaller polypeptide constituents into amino acids. Each amino acid class has a different cohort of transporters which take them in via facilitated diffusion. Di and tripeptides can be taken up by secondary active transport (Pept1 transporter), and then break down into constituent amino acids within the cell to pass through the basal side of the epithelium and into the bloodstream.
Explain the process of digestion of lipids.
Due to the nature of their chemistry, lipids are more soluble in organic solvents than in water. The predominant form of lipids in the human diet is triglycerides (oils and other fats), which comprised of 3 long chain fatty acids with a glyceride backbone. In the mouth, lingual lipase breaks <10% of lipids down, and then gastric lipase also breaks down some lipids in the stomach. However, pancreatic secretions are responsible for most of lipid breakdown. Lipases break down triglycerides into glycerol and fatty acids by hydrolysis, then lipids clump together in the watery environment of the GI tract, and float to the top. Churning of the stomach alters the suspension into fine droplets (increasing surface area) which are then emulsified by bile salts to form a micellar solution. Pancreatic lipase then digests fats into free fatty acids and monoglycerides which become coated in bile salt and are ferried to the intestinal mucosa.
When they reach the mucosa, fatty acids and monoglycerides leave micelles and diffuse into the epithelium where they are packaged with other free fatty acids and proteins to form chylomicrons. They then enter the lymphatic system via lacteals. They travel through the lymphatic system because lymph vessels have membranes which are more permeable to large particles, and lack basal membranes. They then enter the bloodstream in the thorax and through the blood to adipose tissue to be stored.
Explain the process of digestion of micronutrients.
Micronutrients are organic compounds which cannot be manufactured in the body but are vital for metabolism.
Fat-soluble vitamins (A, D, E, and K) are carried by micelles and then diffuse into absorptive cells.
Water-soluble vitamins (C and B) are absorbed by diffusion or by passive or active transport.
Vitamin B12 binds with intrinsic factor, and is absorbed by endocytosis.
In the large intestine, vitamins K and B are produced from bacterial metabolism and then absorbed.
