GI absorption week 2 Flashcards
What are the 3 types of digestion?
Digestion: chemical degradation of nutrient macromolecules by digestive enzymes
• Luminal digestion: enzymes secreted into the lumen of GI tract from salivary glands, stomach and pancreas
• Membrane or contact digestion: digestive enzymes synthesized by enterocytes and inserted into the brush border membranes. Integral part of the microvillar membrane in close vicinity of specific carrier proteins (= digestion-absorption coupling). Applies to carbohydrate and protein digestion/absorption
• Cytosol : peptidases break down absorbed di- and tripeptides to single amino acids
What is absorption?
What are the mechanisms by which absorption can occur?
Where do enterocytes release absorbed nutrients?
Absorption:
- Permeation of molecules from the lumen of the GI tract, through the epithelial cell (enterocyte) layer, into the blood and lymph vascular compartment.
- Occurs via diffusion, facilitated or active transport mechanisms across apical membrane.
- Enterocytes release absorbed nutrients into blood (e.g. amino acids, sugar) or lymphatic circulation (e.g. lipids) for delivery to sites of metabolism/utilization.
What special anatomical features increase the absorptive surface by about 600x?
Special anatomical features (circular folds, villi and microvilli) increase absorptive surface about 600 times.
What are the general differences btwn the small and large intestine as it pertains to fluid, electrolyte and nutrient absorption and secretion?
Both small intestine and large intestine absorb and secrete fluid and electrolytes, however there are regional differences in amounts, direction and mechanisms of fluid and electrolyte transport. Only the small intestine absorbs nutrients. There is substantial regional or segmental heterogeneity in nutrient absorption.
State where in the GI tract the following nutrients are absorbed. Indicate (if applicable) where the most absorption occurs.
carbohydrates
lipids
proteins
folate
calcium
iron
Vit B12
bile acids
What is the average daily loss of water in the GI tract? What is the average daily amount of GI secretion? How much water does the GI tract absorb daily?
Average daily intake: approximately 2 liters
Average daily loss through GI tract: 100 ml (only 5% of intake) through feces
Average daily GI secretion: 7 liters
Average daily water absorption by GI tract: ~9 liters (8.5 in small intestine alone)
Absorption of water is (passive/active) and is determined by differences in _____.
What is the standing osmotic gradient hypothesis?
Absorption of water is passive and is determined by differences in osmolarity.
Net transport of water between GI lumen and vascular system can occur in both directions. Water can be absorbed in the absence or even against (colon) an osmotic pressure gradient between GI lumen and blood by the mechanism of standing gradient osmosis.
Standing gradient osmosis:
- Active Na+ pumping (Na/K ATPase) into lateral intercellular space
- passive entry of Cl- into lateral intercellular space
- establish osmotic gradient in lateral space
- entry of water by osmosis into lateral space
- hydrostatic flow of water
Note that this costs energy! (ATP used by the Na+/K+ pump)
Tight junctions connect epithelial cells of the GI tract. Where in the GI tract are tight junctions the leakiest? For what 2 substances are they leaky?
What are the 2 types of transmucosal transport for these substances?
Tight junctions connect epithelial cells of the GI tract. Tight junctions are leaky (the most in the duodenum) for water and ions.
Transmucosal transport of water and ions can occur through tight junctions and lateral intercellular space (paracellular transport) or through epithelial cells (transcellular transport).
What is solvent drag?
Explain the absorption and secretion of water, Na+, K+, Cl-, and HCO3- in the small and large intestines.
Na absorption: electrogenic Na/K pump is responsible to maintain a large electrochemical gradient for Na to enter the cell. Therefore, the absorption of Na+ is often referred to as ‘active‘ because it is tightly linked to the activity of the ATP consuming Na/K pump.
Solvent drag: When large volumes of water are absorbed the solutes dissolved in the water are also carried along with the water (via paracellular route). For example: hypotonic meal –> large volumes of water are absorbed from lumen to blood together with small solutes (Na+, Cl-).
Summary of attached table:
- The small intestine is a net absorber of water, Na+, Cl-, and K+, but is a net secretor of HCO3-
- The large intetine is a net absorber of water, Na+, and Cl-, but is a net secretor of K+ and HCO3-.
Is the absorption of Ca2+ active or passive? Where does the most absorption of Ca2+ occur?
Calcium is absorbed actively in the duodenum and passively by all segments of smallintestine (highest absorption activity in duodenum and jejunum).
Explain the mechanism of active transport of Ca2+.
What hormone is the active transport of Ca2+ under the control of?
Explain the passive absorption of Ca2+.
Active Ca2+ absorption is transcellular and under the control of vitamin D.
Ca2+ passively enters enterocytes where it is bound by the calcium binding protein Calbindin or is sequestered in vesicles. This maintains a gradient for the diffusion of Ca2+ into enterocytes. The Ca2+/Caldindin complex diffuses through the enterocyte. Ca2+ is released into the blood in 3 ways:
- Ca2+ ATPase
- Na+/Ca2+ exchanger
- fusion of Ca2+ vesicles with the basolateral membrane (exocytosis)
Passive Ca2+ absorption occurs via paracellular pathway and is not controlled by vitamin D.
Where does active absorption of Mg2+ occur? Passive absorption?
Many enzymes and ATP-dependent processes require Mg2+ as cofactor. Active absorption occurs in the ileum. Mechanism is poorly described. Minor passive absorption occurs.
What are 3 disturbances of iron metabolism?
Disturbances of iron metabolism
- Iron depletion can lead to anemia.
- Iron overload (ingestion of large amounts of iron).
- Idiopathic hemochromatosis (excessive absorption of iron due to genetic defects of iron transporting proteins).
What are the 2 forms of dietary iron?
heme iron
non-heme iron
What are sources ofheme irion? What type of transport occurs for the absorption of heme iron? How do enterocytes process heme after absorption?
Heme iron (e.g. from hemoglobin, myoglobin and cytochromes) is absorbed by enterocytes. Absorption occurs by facilitated transport (heme transporter) or by endocytosis. Iron is split from heme intracellularly (heme oxygenase or heme oxidase reaction), releasing free Fe.