27.38
Absorption von Nährstoffen im Dünndarm
Unter Absorption versteht man die Aufnahme von Nährstoffen aus dem Darmlumen für den Transport durch den Körper. Nach der Verdauung im Dünndarm werden…
Bei der Verdauung im Dünndarm werden Makromoleküle – Kohlenhydrate, Proteine und Fette – in ihre einfachsten Formen zerlegt.
Diese vereinfachten Makronährstoffe und andere essentielle Substanzen wie Vitamine, Mineralien und Wasser werden für die Aufnahme in das Blut vorbereitet.
Die Schleimhaut der Dünndarmwände ist mit Enterozyten ausgekleidet – spezialisierten polaren Epithelzellen.
Diese Zellen haben Mikrovilli, winzige fingerartige Vorsprünge an ihren apikalen Enden, um die Oberfläche für die Absorption zu vergrößern.
Zusätzlich verhindern Tight Junctions an der apikalen Oberfläche den Durchgang von Substanzen zwischen den Enterozyten.
Infolgedessen gelangen Substanzen durch ihre apikale Membran aus dem Lumen in einen Enterozyten und treten durch die basolaterale Membran in die interstitielle Flüssigkeit auf der anderen Seite der Zelle aus.
Da die Plasmamembran der Zellen eine Lipiddoppelschicht ist, ermöglicht sie die passive Absorption von unpolaren Substanzen. Im Gegensatz dazu nutzen polare Substanzen aktive Transportprozesse, die direkt oder indirekt durch Energie angetrieben werden.
Einmal in der interstitiellen Flüssigkeit gelangt man in die Blutkapillaren.
View the full transcript and gain access to JoVE Core videos
Q1: What role do microvilli play in nutrient absorption?
Microvilli are tiny finger-like projections at the apical ends of enterocytes that dramatically increase the surface area available for nutrient absorption. This expanded surface allows more nutrients to be absorbed efficiently from the intestinal lumen into the specialized epithelial cells lining the small intestine.
Q2: How do tight junctions affect nutrient transport in the small intestine?
Tight junctions at the apical surface of enterocytes prevent substances from passing between cells. This forces all nutrients to enter through the apical membrane and exit through the basolateral membrane, ensuring controlled, directional transport of absorbed nutrients into the bloodstream.
Q3: What is the difference between passive and active absorption of nutrients?
Nonpolar substances are passively absorbed across the lipid bilayer plasma membrane without energy. Polar substances require active transport processes driven directly or indirectly by energy. Both pathways move nutrients from the intestinal lumen through enterocytes into the interstitial fluid for blood transport.
Q4: Why are enterocytes specialized for nutrient absorption?
Enterocytes are polar epithelial cells with distinct apical and basolateral membranes, allowing directional nutrient transport. Their microvilli increase absorption surface area, while tight junctions control the pathway nutrients take. This specialization ensures efficient movement of digested macronutrients and essential substances into the bloodstream.
Q5: What happens to absorbed nutrients after they enter the interstitial fluid?
After crossing the basolateral membrane of enterocytes, nutrients enter the interstitial fluid surrounding the small intestine. From there, they diffuse into blood capillaries for transport throughout the body, making the small intestine the primary site where digested carbohydrates, proteins, fats, vitamins, minerals, and water enter circulation.
Q6: How does the small intestine prepare macronutrients for absorption?
During mechanical and chemical digestion in the small intestine, carbohydrates, proteins, and fats are broken down into their simplest forms. These simplified macronutrients, along with vitamins, minerals, and water, are then positioned at the enterocyte surface where they can be absorbed through specialized transport mechanisms.
Q7: What structural features of the small intestine maximize nutrient absorption?
The small intestine's mucosa is lined with enterocytes possessing microvilli that increase surface area for absorption. Tight junctions control paracellular transport, while the polar nature of enterocytes enables directional movement of nutrients. These features work together to optimize the absorption of all essential nutrients into the bloodstream.