Calcium absorption occurs through two primary mechanisms within the small intestine: transcellular active transport and paracellular passive transport.
Transcellular Active Transport
This mechanism is prevalent in the duodenum and jejunum, the upper portions of the small intestine. It's an active process, meaning it requires energy. Here's a breakdown:
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Entry into Enterocytes: Calcium first enters the intestinal cells (enterocytes) through TRPV6 channels, which are located on the apical membrane (the side facing the intestinal lumen). This step is regulated by calcitriol (active vitamin D).
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Intracellular Transport: Once inside the enterocyte, calcium binds to calbindin-D9k, a calcium-binding protein. Calbindin facilitates the movement of calcium across the cell, preventing it from disrupting cellular processes.
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Exit from Enterocytes: Calcium is then actively transported out of the enterocyte across the basolateral membrane (the side facing the bloodstream) via:
- PMCA1b (Plasma Membrane Calcium ATPase 1b): An ATP-dependent pump that actively transports calcium out of the cell.
- NCX1 (Sodium-Calcium Exchanger 1): An antiporter that exchanges sodium ions for calcium ions across the membrane.
Paracellular Passive Transport
This mechanism occurs along the entire length of the small intestine. It's a passive process, meaning it doesn't require energy expenditure by the cells.
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Concentration Gradient: Paracellular transport relies on a high concentration of calcium in the intestinal lumen relative to the blood.
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Passage Between Cells: Calcium moves between the enterocytes, through the tight junctions that connect them. The permeability of these tight junctions to calcium is influenced by factors like vitamin D and the overall calcium concentration. This route is particularly important when calcium intake is high, saturating the transcellular pathway.
Factors Influencing Calcium Absorption
Several factors can influence the efficiency of these mechanisms:
- Vitamin D: Calcitriol (active vitamin D) significantly enhances transcellular calcium absorption by:
- Increasing the expression of TRPV6 channels.
- Increasing the synthesis of calbindin-D9k.
- Stimulating the activity of PMCA1b.
- Age: Calcium absorption tends to decrease with age.
- Dietary Factors: Certain dietary components (e.g., phytates, oxalates) can bind to calcium and reduce its absorption.
- Calcium Intake: At lower calcium intakes, the transcellular pathway becomes more important. At higher intakes, the paracellular pathway plays a larger role.
In summary, calcium absorption relies on a combination of active and passive transport mechanisms, primarily within the small intestine. The transcellular pathway is vitamin D-dependent and active, while the paracellular pathway is passive and concentration-dependent.
