The small intestine Flashcards
What gets delivered to the SI?
The SI receives chyme from the stomach and secretions from pancreas and liver. Chyme is acidic so need to ensure SI is protected -> mucus, thin layer. Goblet cells found along whole of SI. In the duodenum there are mucus secreting glands in the submucosa (lots of mucus to ensure tissue is not damaged). Bicarbonate secretion from duct cells of pancreas to neutralise pH setting right environment for processes in SI
What are the three main regions in the SI?
Duodenum ~ 25cm , Jejunum ~ 2.5m , Ileum ~ 3.6m
Duodenum is the first part, then jejunum and then ileum
Total length of the SI is 6m
Functions of the three main regions
The duodenum receives chyme from stomach, enzymes and bicarbonate from pancreas and bile from liver
Jejunum and ileum function for digestion and absorption - need lots of SA for this to occur hence why SI is so long
What is the mesentery?
The initial segment (duodenum) of the SI is retroperitoneal, the SI is held in place by the mesentery which allows movement but prevents tangling. Mesentery is a double layer of visceral peritoneum which wraps around an organ. Contains arteries, veins, nerves and lymphatics
Mesenteric blood vessels and lymphatic drainage
As the blood drains away from SI it is nutrient rich and deoxygenated as it enters the mesenteric veins. These veins drain to the hepatic portal vein. Lymph lacteals ultimately drain into the cisterna chyli , through acid duct, then left subclavian vein. Blood from SI travels into superior and inferior mesenteric vein and through hepatic portal vein delivering blood to liver where blood is detoxified
4 key ways digestion and absorption carried out in SI
The SI needs to carry out further digestion and absorption hence need a huge surface area. This is achieved in 4 key ways, length, folding of intestinal wall (plicae circulares), extensions of mucosa (villi) and extensions of epithelial cells (microvilli)
What are plicae circulares?
Plicae circulares are permanent large folds of small intestine, have a core of submucosa with overlying mucosa
What are villi?
Numerous villi found on the surface of the plicae circulares which are made of mucosa. Muscularis mucosae allows villi to wiggle back and forth, wiggling means more contact, more digestion, more absorption
What is lamina propria function?
Core of each villus is made of lamina propria (connective tissue) which contain lymph lacteal (products of fat digestion) and capillary network which is dense and wraps around the lacteal (products of protein and carbohydrate digestion)
What is the epithelia of the SI?
Epithelium of SI is simple columnar, the absorptive cells (enterocytes) have microvilli on apical surfaces. Best cells for absorption as they are very tall thin cells with a centralised nucleus. Predominant cells in the SI, required for absorption and contact digestion
What is the glycocalyx?
Brush boarder formed when we have many microvilli. The glycocalyx is a mesh work of glycoproteins which form a branching network and anchor things in place, localised enzymes on surface of cell. Contact digestion involves enzymes - attached to brush boarder
What are the junctions between epithelial cells in the SI?
The plasma membrane is a barrier that is selectively permeable which has protein channels and transporters. Tight junctions are very tight and only allow small molecules to diffuse through, they hold the cell together. This allows us to control what we want to absorb
What are the epithelium of the SI?
Enterocytes - absorptive cells, involved in absorption with microvilli
Goblet cells - secrete mucous for protection, layer across cell surface found on surface of villi
Paneth cells - granules, antibacterial enzymes, protect us from anything that could do damage, found at bottom of intestinal crypt
Endocrine cells - secrete hormones
Stem cells - make all cell types
What is the ileocecal valave?
Connects small and large intestine and regulates the passage of material into the large intestine
