Ten Flashcards
Describe the 4 layers of the intestinal tract.
- Mucosa
A single layer of epithelial cells linked together by tight junctions.
- Exocrine cells secrete mucus into the lumen.
- Endocrine cells release hormones into the blood.
Lamina propria (connective tissue)
Muscularis mucosa (smooth muscle)
- Submucosa
A network of nerve cells (submucosal plexus) and blood and lymphatic vessels
- Muscularis externa (propria)
Thick inner layer of circular muscle. Contraction produces narrowing of the lumen.
Thinner outer layer of longitudinal muscle.
Contraction shortens the tube.
A network of nerve cells (myenteric plexus) is between the two muscle layers - Serosa
Surrounds the outer surface and connects to the abdominal wall via thin sheets of connective tissue
Describe various characteristics of the SI as they pertain to digestion and absorption.
There is extensive folding of the intestinal surface.
Extending from the surface are finger-like projections
known as villi.
- The surface of each villus is covered with a single
layer of epithelial cells whose surface membranes
form small projections called micro-villi (collectively = brush border).
The combination of folded mucosa, villi and microvilli increases the small intestine’s surface area available for absorption about 600-fold over that of a flat-surfaced tube having the same length and diameter.
About 17 billion epithelial cells are replaced each day, and the entire epithelium of the small intestine is replaced entirely every 5 days.
- The center of each intestinal villus is occupied both by a single blind-ended lymphatic vessel termed a lacteal and by a capillary network.
- Most of the fat absorbed in the small intestine enters the lacteals, while other absorbed nutrients enter the blood capillaries.
- The venous drainage from the intestinal villi passes via the hepatic portal vein into the liver. Thus, material absorbed into the intestinal capillaries, in contrast to the lacteals, can be processed by the liver before entering the general circulation.
Describe monosaccharides. Disaccharides. Oligosaccharides. Polysaccharides. Describe starches and how they are digested. Cellulose too.
- Monosaccharides: simple sugars. Can be efficiently absorbed across the wall and transported into blood.
- Disaccharides: two monosaccarides linked together by a glycosidic bond
- Oligosaccharides: short chains of monosaccharides; intermediates in the breakdown of
polypsaccharides to monosaccharides
- Polysaccharides:
• Starch (plant)
– Occurs in two forms:
– Links are alpha (1->4) glycosidic bonds, which are hydrolyzed by amylases
Alpha-amylose: glucoses linked together in straight chains
Amylopectin: glucose chains highly branched
in the digestive tract of mammals. alpha1,6
• Cellulose (plant)
– Unbranched, linear chains of D-glucose molecules
– Links are beta (1->4) glycosidic bonds, which cannot be enzymatically digested by any vertebrate
– Herbivores subsist largely on cellulose because their digestive tracts teem with microbes that produce cellulases that hydrolyze cellulose.
• Glycogen (animal)
Glucose molecules linked together by alpha(1->4) glycosidic bonds.
Describe the human consumption pattern of carbs
- Intake per day about 250-800 g in a typical American diet.
- About two-thirds is the plant polysaccharide starch, and most of the remainder consists of the disaccharides sucrose (table sugar) and lactose (milk sugar).
- Only small amounts of monosaccharides are normally present in the diet.
- Cellulose found in vegetable matter cannot be broken down by the enzymes (e.g.fiber) and is passed on to the large intestine, where it gets partially metabolized by bacteria.
Describe how the mouth and pharynx, esophagus, stomach, pancreas, liver, and gallbladder all play a part in digesting material prior to it entering into the SI.
Mouth and Pharynx
Chewing
Lubrication
Polysaccharide-digestion (amylase)
Esophagus Transit to stomach
Stomach
Mix and dissolve
Kill microbes (HCl)
Protein digestion (pepsin)
Pancreas
Digest carbs, fats, proteins and nucleic acids. Bicarb neutralizes acid.
Liver
Bile salts solubilize water-insoluble fat
Elimination of organic waste products and metals in feces
Gall bladder Storing of bile
Describe the process of starch digestion down to monosacharides. Which enzymes are involved in each step and where does each step occur?
- Starch digestion by salivary amylase begins in the mouth and continues in the upper part of the stomach before amylase is destroyed by gastric acid.
- Starch digestion is completed in the small intestine by pancreatic amylase.
The products produced by both amylases are the di-saccharide maltose and a mixture of short, branched chains of glucose molecules.
• Maltose, along with ingested sucrose and lactose, are then broken down by enzymes located on the brush-border membranes of the small-intestine epithelial cells.
– maltase cleaves maltose into two molecules of glucose
– lactase cleaves lactose into a glucose and a galactose
– sucrase cleaves sucrose into a glucose and a fructose
Describe how the various monosaccharides are transported into and out of enterocytes. Which transporters are involved?
• These monosaccharides are now ready to be transported across the intestinal epithelium into the blood.
• Glucose and galactose are taken into the enterocyte by
cotransport with sodium using the same transporter (secondary active transport): SGLT-1
• Fructose enters the enterocyte cell via facilitated
diffusion through another transporter: GLUT-5
• Once the monosaccharides are inside the enterocyte,
GLUT-2 mediates their efflux across the basolateral
membrane into the intestinal space.
Give 4 general reasons of what would cause malabsorption of carbs along with specific examples.
- Severe pancreatic insufficiency
- Selective deficiencies of brush-border disaccharidases (e.g. lactase deficiency)
• Generalized impairment of brush-border and enterocyte function
o celiac disease, tropical sprue, gastroenteritis
• Loss of mucosal surface area - e.g., the short bowel syndrome.
Describe the digestion of proteins down to free amino acids. Which enzymes perform each step and where does each step occur?
- Proteins are broken down to peptide fragments:
- in the stomach by pepsin
- in the small intestine by pancreatic trypsin and chymotrypsin
• Peptide fragments are further digested to free amino acids by enzymes located on the brush border:
- pancreatic carboxy-peptidase
- intestinal amino-peptidase
Describe the process of absorption of proteins across an enterocyte. Which transporters and enzymes are involved? What can cause failure of absorption?
• The enterocyte directly absorbs some of the small
oligopeptides via the action of the oligopeptide cotransporter: PepT1
• Small peptides are digested to amino acids by
peptidases in the cytoplasm of the enterocyte.
• Several Na independent amino acid transporters move amino acids out of the cell across the basolateral membrane.
• Failure of absorption of certain amino acids across
the apical membrane is seen in some hereditary
conditions (Hartnup disease and Cystinuria).
How are when does absorption of intact proteins occur?
In special circumstances (neonates/ antibodies) a
small amounts of intact proteins are able to cross
the intestinal epithelium and gain access to the
interstitial fluid by a combination of endocytosis
and exocytosis.
What happens to most vitamins upon being consumed?What happens with fat soluble vitamins (which ones are they?)? What happens with water soluble vitamins? How are most water soluble vitamins absorbed? What is one exception? Why?
- Most vitamins undergo little enzymatic modification.
- Digestion of food particles releases vitamins in a soluble form so they can be absorbed.
- Fat-soluble vitamins ADEK follow the fat pathway thus any interference with the secretion of bile or the action of the bile salts in the intestines decreases their absorption.
- Most water-soluble vitamins are absorbed by diffusion or carrier-mediated transport.
However, one—vitamin B12—is a very large, charged molecule
Describe the process of Cobolamin (B12) Absorption. Give 4 examples of when cobolamin deficiency can be seen?
B12 bound to proteins in food.
Acid pH and pepsin release B12 from proteins.
Gastric glands release haptocorrin which binds to B12. Gastric parietal cells release IF.
The pancreas releases HCO3 and proteases.
B12 is released after degradation of haptocorrin.
IF-B12 complex forms.
Complex absorbed by ileal enterocytes.
• Cobolamin (B12) deficiency is seen:
o Strict vegetarian diet
o Atrophy of parietal cells (decreased IF and gastric acid)
o Bacterial overgrowth in small bowels (bacteria consumes cobolamin)
o Crohn’s disease and loss of ileum (absence of receptors for IF-cobolamin complex)
How much ingested and secreted water enter the SI per day? How much moves on to the LI? How does net movement of water across cell membranes occur? Describe the process of sodium absorption? What is it dependent on? What happens once it enters the cell? How does this affected water absorption?
• Approximately 9 L of ingested and secreted fluid
enters the small intestine
• Only 1.5L is passed on to the large intestine
• Net movement of water across cell membranes
always occurs by osmosis:
• There is a tight coupling between water and solute absorption.
- Sodium is absorbed into the cell mainly by
cotransport with glucose and amino acids –
• Efficient sodium absorption is dependent on absorption of these organic solutes. - Absorbed sodium is rapidly exported from the cell
via sodium pumps - Water diffuses in response to the osmotic gradient
established by sodium - Water, as well as sodium, then diffuses into
capillary blood within the villus.
How much of daily ingested iron is absorbed into the blood? How is iron absorption typical of most trace elements? Describe the process of iron absorption. What happens to iron once in the blood? What are 3 things that can lead to iron overload?
• Only 10% of daily ingestion is absorbed into the blood.
• The absorption of iron is typical of that of most trace metals in several respects:
– Cellular storage proteins and plasma carrier proteins are involved
– The control of absorption, rather than urinary excretion, is the major mechanism for the hemeostatic control of the body’s content of the trace metal
Iron binds to transferrin. Complex then enters the cell through transferrin receptor.
pH causes iron to unbind. Transferrin recycled.
Iron also cotransports with H+ via DCT
It also enters with Heme. Heme oxygenase releases iron
Iron is transferred to mobiliferrin
Iron leaves the cell and binds to another transferrin
- Ferric iron (Fe +++) in the duodenal lumen is reduced to its ferrous (Fe++) form through the action of a brush border ferrireductase.
- Iron ions are actively transported into intestinal epithelial cells in the proximal duodenum, and most of them are incorporated into ferritin (protein-iron complex functioning as an intracellular iron store).
- Once iron has entered the blood, the body has very little means of excreting it and it will accumulate in tissues.
Iron abundance/overload
• Massive oral intake
• Multiple blood transfusions
• Hemochromatosis: autosomal recessive disorder in which HFE gene mutations cause increased iron absorption
