Intestinal Absorption Overview (Archive)

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Summary
Full Text

Summary

Absorption

• Uptake of digested nutrients and water from the lumen of the digestive tract into the bloodstream and lymphatic vessels.

Small intestine (specifically the duodenum)

• Major site of nutrient absorption.
• Three folded mucosal structures maximize surface area for absorption
  1 - Plicae circulares: wavy, folds on the inner walls of small intestine → form circular folds → increase surface area 3-fold.
  2 - Villi: finger-like projections that protrude from the plicae circulares → surface area by 10-fold. Arterioles, venules, and lymphatic vessels pass through the villi and uptake absorbed nutrients.
  3 - Microvilli (brush border): hair-like projections on columnar small intestine epithelial cells (face the lumen of the small intestine) → increase surface are 20-fold. Together, all folded layers = 600-fold surface area increase

Nutrients are absorbed in the small intestine

• Monosaccharides (digested carbohydrate products)
• Amino acids, di-peptides and tri-peptides (digested protein products)
• Intact proteins
• Short-chain fatty acids, long-chain fatty acids, and glycerol (which are digested lipid products)
• Vitamins
• Water and electrolytes

Nutrients cross the apical and basolateral surfaces of the intestinal epithelium for absorption into circulation or the lymphatic system.

• Apical surface: interfaces the intestinal lumen and epithelium.
• Basolateral surface: opposite to the apical surface, lines the inside of the villi.
• Capillaries and lacteals inside villi
  – Most nutrients cross the basolateral surface and pass directly into circulation.
• Fats, however, pass directly into lacteals (lymphatic system).

Key Transport Mechanisms

• Apical Surface
  – Secondary active transport: transporter moves an ion movement down its concentration gradient, which generates energy for it to move another ion (or molecule) against its concentration gradient.
  – Facilitated diffusion: Transporter passively moves an ion or molecule across the plasma membrane, down its concentration gradient.
  – Simple diffusion: in which non-charged, lipid, and hydrophobic molecules passively cross through the plasma membrane (without a transmembrane protein) down their concentration gradient.
  – Endocytosis: form of active, energy-requiring cellular ingestion, which transports large substances into the cell.

• Basolateral Surface
  – Facilitated diffusion.
  – Simple diffusion.
  – Exocytosis: the opposite of endocytosis – a vacuole actively fuses with the plasma membrane to release its contents into the extracellular environment.

• Water Absorption

Most relevant to the function of the large intestine but also occurs in the small intestine.

– Large intestine stores and concentrates fecal material before elimination.
– Mainly absorbs water and electrolytes to do so
– Also absorbs bacterial byproducts.

• Three General Steps of Water Absorption:

Step 1:

• Sodium-potassium pump on the basolateral surface
  – Pumps K+ into cell and Na+ out of cell (pump utilizes ATP to move sodium and potassium against their concentration gradients).
• Na+ passively enters via general sodium ion transporter on the apical surface (possible because of the active transport of sodium out of the cell)
• Net positive charge in the cell

Step 2:

• Chloride enters the cell through a general chloride ion transporter on the apical surfacen via facilitated diffusion down the electrical gradient.
• Higher solute concentration inside the cell relative to the lumen.

Step 3:

• Water crosses the apical surface via osmosis.

Full Text

• In this tutorial, we will introduce absorption in the small and large intestines.
  • We will discuss unique features of the small intestine, which maximize absorption, and key transport mechanisms common to both of them.

• First start a table. Begin with the 5 Key Functions of the digestive tract.
  • Motility
  • Secretion
  • Digestion
  • Absorption
  • Protection
  • Include the minor function: Elimination.

• Denote that absorption is the uptake of digested nutrients and water from the lumen of the digestive tract into the bloodstream and lymphatic vessels.

• Denote that the small intestine, specifically the duodenum, is the major site of nutrient absorption.

Let's discuss a few unique features of the small intestine that allow for efficient nutrient absorption.

• Denote that there are three folded mucosal structures that maximize surface area available for absorption.They are:
  • Plicae circulares
  • Villi
  • Microvilli (brush border)

• To begin, draw a longitudinal portion of the small intestine.

• Dissect away a segment of its wall.

• Draw plicae ciculares as wavy, folds on the inner walls of the tube.
  • Indicate that they form circular folds.
  • Write that they increase the surface area of the small intestine three-fold.

• Now, draw several magnified plicae circulares.
  • Label this layer as the mucosa.

• Then, draw the submucosal layer below, for context.

• Next, draw the villi, which are finger-like projections that protrude from the plicae circulares.
  • Write that they increase the small intestine surface area by 10-fold.
  • Arterioles, venules, and lymphatic vessels pass through the villi and uptake absorbed nutrients.

• Next, draw one large villus.

• Coat it with representative columnar epithelial cells.

• Indicate that they have microvilli, also called the brush border, which are hair-like projections that face the lumen of the small intestine.
  • Write that the microvilli increase the small intestine surface area by 20-fold.

• Indicate that, all together, these folded layers produce a 600-fold increase in the surface area of the small intestine!

Now, let's transition to the nutrients absorbed in the small intestine:

• Monosaccharaides, which are digested carbohydrate products
• Amino acids, di-peptides and tri-peptides, which are digested protein products
• Intact proteins
• Short-chain fatty acids, long-chain fatty acids, and glycerol, which are digested lipid products
• Vitamins
• Water and electrolytes
• All of these must cross both the apical and basolateral surfaces of the intestinal epithelium for absorption into circulation or the lymphatic system.

To understand these two surfaces, let's create a small diagram.

• First, draw a couple of representative intestinal epithelial cells.
  • Join them with tight junctions.

• Label apical for the wavy surface (from the microvilli). Label the lumen external to it.
  • The apical surface interfaces the intestinal lumen and epithelium.

• Label the basolateral surface opposite to the apical surface.
  • The basolateral surface lines the inside of the villi.

• Within the villus, draw a representative blood capillary.

• Then, a lacteal.

• Draw arrows from the intestinal lumen that cross the apical surface and extend into the intestinal epithelial cell to indicate nutrient absorption.

• Next, draw two arrows: one arrow from the epithelium that crosses the basolateral surface and extends into the blood capillary, and another that crosses the basolateral surface and extends into the lacteal.
  • Most nutrients cross the basolateral surface and pass directly into circulation.
  • Fats, however, pass directly into lacteals (lymphatic system).

Let's now address some key transport mechanisms for nutrient absorption.

• Indicate that the key transport mechanisms across the apical surface include:
  • Secondary active transport: Transporter moves an ion movement down its concentration gradient, which generates energy for it to move another ion (or molecule) against its concentration gradient.
  • Facilitated diffusion: Transporter passively moves an ion or molecule across the plasma membrane, down its concentration gradient.
  • Simple diffusion, in which non-charged, lipid, and hydrophobic molecules passively cross through the plasma membrane (without a transmembrane protein) down their concentration gradient.
  • Endocytosis: A form of active, energy-requiring cellular ingestion, which transports large substances into the cell.

• Indicate that the key transport mechanisms across the basolateral surface include:
  • Facilitated diffusion
  • Simple diffusion
  • Exocytosis, the opposite of endocytosis: A vacuole actively fuses with the plasma membrane to release its contents into the extracellular environment.

Lastly, let's focus on water absorption, which is most relevant to the function of the large intestine but also occurs in the small intestine.

• Denote that the main function of the large intestine is to store and concentrate fecal material before elimination.
  • Thus, denote that its mainly absorbs water and electrolytes to do so, but that it also absorbs bacterial byproducts.
    Let's discuss the transport of water. It involves three general steps.

• Draw an intestinal epithelial cell.
  • Label the apical and basolateral surfaces

• For Step 1, draw a sodium-potassium pump on the basolateral surface.
  • Show that potassium is pumped into the cell and sodium is pumped out of the cell.
  • This pump utilizes ATP to move sodium and potassium against their concentration gradients
  • Next, draw a general sodium ion transporter on the apical surface.
  • Show that sodium passively enters, which is possible because of the active transport of sodium out of the cell (in step 1).

• Write that now there is a net positive charge in the cell, which sets up step 2.

• For Step 2, draw a general chloride ion transporter on the apical surface.
  • Show that chloride enters the cell via facilitated diffusion down the electrical gradient.

• Write that now there is a higher solute concentration inside the cell relative to the lumen.

• For Step 3, show and write out that water crosses the apical surface via osmosis.