Lecture 21 Flashcards
Small intestine has 3 parts:
- duodenum
- jejunum
- ileum
Histology of the Small Intestine: structures that increase surface area
- plica circularis
- villi
- microvilli
plica circularis
- permanent ½ inch tall folds that contain part of submucosal layer
- not found in lower ileum
- can not stretch
villi:
- 1 Millimeter tall
- Core is lamina propria of mucosal layer
- Contains vascular capillaries and lacteals
microvilli:
cell surface feature known as brush border
Functions of Microvilli:
- Absorption and digestion
- Digestive enzymes found at cell surface on microvilli
- Significant cell division within intestinal glands
proteins
Roles of Intestinal Juice & Brush-Border Enzymes:
- Submucosal layer has duodenal glands
2. Mucosal layer contains intestinal glands
Submucosal layer secretes
alkaline mucus
Mucosal layer:
Crypts of Lieberkuhn
Crypts of Lieberkuhn :
- secretes intestinal juice
- brush border enzymes
- paneth cells secrete lysozyme kills bacteria
Digestion of Carbohydrates:
mouth
salivary amylase
Digestion of Carbohydrates:Esophagus & stomach—
nothing happens
Digestion of Carbohydrates: Duodenum—-
pancreatic amylase
Digestion of Carbohydrates: brush border enzymes
act on disaccharides
- produces monosaccharides
- lactose intolerance
Lactose Intolerance:
Mucosal cells of small intestine fail to produce lactase
Digestion of Proteins: stomach
- HCl denatures or unfolds proteins
2. pepsin turns proteins into peptides
Digestion of Proteins: pancreas
- digestive enzymes split peptide bonds between different amino acids
- brush border enzymes split off amino acid at amino end of molecule
Digestion of Lipids: mouth
lingual lipase
Digestion of Lipids: small intestine
- emulsification by bile
- pancreatic lipase splits into fatty acids & monoglyceride
- no enzymes in brush border
Digestion of Nucleic Acids:
- Pancreatic juice = RNA and DNA
- Nucleotides produced are digested by enzymes
- absorbed by active transport
Absorption of Monosaccharides:
into and out of epithelial cell
Absorption into epithelial cell
- glucose & galactose=sodium symporter
2. fructose=facilitated diffusion
Movement out of epithelial cell into bloodstream:
by facilitated diffusion
Absorption of small fatty acids:
enter cells & then blood by simple diffusion
Absorption of larger lipids:
exist only within micelles (bile salts coating)
Lipids enter cells by
simple diffusion leaving bile salts behind in gut
Bile salts reabsorbed into
blood & reformed into bile in the liver
Inside epithelial cells fats are
rebuilt and coated with protein to form chylomicrons
Chylomicrons leave intestinal cells by
exocytosis into a lacteal
- travel in lymphatic system
- removed from the blood by the liver and fat tissue
Sources of electrolytes:
GI secretions & ingested foods and liquids
Enter epithelial cells by
diffusion & secondary active transport
sodium & potassium move =
Na+/K+ pumps (active transport)
chloride, iodide and nitrate =
passively follow
iron, magnesium & phosphate ions =
active transport
Intestinal Ca+ absorption requires
vitamin D & parathyroid hormone
Fat-soluble vitamins:
travel in micelles & are absorbed by simple diffusion
Water-soluble vitamins
absorbed by diffusion
B12 combines with
intrinsic factor before it is transported into the cells
-receptor mediated endocytosis
Absorption of Water:
by osmosis
9 L each day
