Digestive System: Small and Large Intestine
Structure of the small intestine
The wall of the small intestine is arranged in large circular folds (plicae circulares) to maximize surface area in contact with nutrients.
The entire inner surface of the small intestine is covered with finger-like projections called villi; singular: villus.
Each villus contains a capillary and a lacteal (small lymph vessel).
Nutrients must cross only the single cell layer of the mucosa to reach the blood or lymph for delivery to tissues of the body.
A large circular fold is shown, followed by an image illustrating the lumen from top to bottom with microvilli, mucosal cells, lacteal, capillary, artery, vein, and lymph vessel.
Some images show the microvilli via real-life pictures.
Each villus is covered with tiny projections of the mucosal cell membrane called microvilli; singular: microvillus; brush border.
Some digestive enzymes produced by the small intestine are located in the microvilli membrane, and some are located inside mucosal cells.
Three structural features of the small intestine increase its surface area: (1) large circular folds (plicae circulares), (2) villi, and (3) microvilli (brush border).
Digestive enzymes and digestion in the small intestine
Pancreatic amylase continues the job of breaking down starches into sugars that began in the mouth with salivary amylase.
Pancreatic proteases (protein-digesting enzymes such as trypsin and chymotrypsin) break proteins into shorter and shorter chains of amino acids.
Pancreatic lipases break fats into fatty acids.
Proteases from the pancreas are released in an inactive form to prevent them from digesting the glands that produce them (zymogen activation).
Intestinal digestive enzymes found in the cell membrane or inside the mucosal cells aid the digestion of disaccharides (double sugars) into monosaccharides, and digestion of short amino acid chains into single amino acids.
The sugars from carbohydrate digestion and the amino acids from protein digestion pass into the blood and are delivered to the liver.
The gallbladder stores and secretes bile, a fluid containing bile acids and cholesterol, produced in the liver and necessary for fat digestion and absorption.
Bile that is secreted into the small intestine mixes with fat.
Bile acids help divide large lipid droplets into small globules, allowing lipases to access and digest fat molecules more efficiently.
The bile acids and digested fats form small droplets that facilitate the absorption of fat into mucosal cells.
Inside the mucosal cells, the products of fat digestion are incorporated into transport particles.
These transport particles are absorbed into the lymph before passing into the bloodstream.
A process diagram (Figure 3.10) outlines the pathways for carbohydrates, proteins, and fats:
Carbohydrates: sugar, starches, and fibers; disaccharides to monosaccharides and hepatic delivery via blood.
Proteins: long amino acid chains to amino acids and short amino acid chains, then absorption into mucosal cells and into blood.
Fats: large lipid droplets and short-chain fatty acids; long-chain fatty acids form droplets with bile; absorption into mucosal cells; transport via lymph to the bloodstream.
Summary of the breakdown and absorption steps (as described in the transcript):
Pancreatic amylases digest starch to disaccharides and short glucose chains.
Fiber cannot be digested by human enzymes and passes to the large intestine.
Enzymes in the microvilli digest disaccharides into monosaccharides, which are absorbed into the blood.
Pancreatic proteases along with proteases in the microvilli digest long amino acid chains into amino acids and short amino acid chains.
Amino acids and short amino acid chains are absorbed into the mucosal cells where they are digested into single amino acids, which pass into the blood.
Bile helps divide large fat globules; pancreatic lipases digest fat molecules into fatty acids.
Short-chain fatty acids are absorbed into the mucosal cells and then pass directly into the blood.
Long-chain fatty acids and other lipids combine with bile to form small droplets that aid absorption of fatty acids and other fat-soluble substances into the mucosal cell.
Absorbed lipids are incorporated into transport particles that pass into the lymph and then enter the bloodstream, without first passing through the liver.
Figure 3.10: Digestion and absorption in the small intestine
Most digestion and absorption occurs in the small intestine.
Bile is a digestive fluid containing bile acids and cholesterol made in the liver and stored in the gallbladder; released into the small intestine to aid fat digestion and absorption.
Absorption: the small intestine is the main site for absorption of nutrients.
For absorption, nutrients must pass from the lumen of the GI tract into the mucosal cells lining the tract and then into either the blood or the lymph.
Absorption mechanisms in the small intestine (Figure 3.11)
A variety of mechanisms transport nutrients from the lumen into mucosal cells:
Simple diffusion: unassisted diffusion across a cell membrane; substance moves from high to low concentration; no energy required.
Osmosis: diffusion of water across a cell membrane; water moves into or out of the lumen depending on solute concentrations.
Facilitated diffusion: requires a carrier molecule; still a passive process; example: fructose uses a carrier.
Active transport: requires energy and a carrier molecule; can move substances from low to high concentration (against gradient); allows absorption even when lumen concentrations are lower.
Examples from the figure:
Fatty acids: simple diffusion into the mucosal cell.
Water: osmosis into the mucosal cell.
Fructose: facilitated diffusion into the mucosal cell.
Amino acids: active transport into the mucosal cell.
The figure labels A (simple diffusion), B (osmosis), C (facilitated diffusion), D (active transport).
The Large Intestine (Colon and Rectum)
The large intestine begins with a sphincter between the small and large intestines that prevents material from reentering the small intestine.
Length and subdivisions: about five feet long; divided into the colon (majority) and the rectum (last eight inches).
The large intestine opens to the exterior of the body at the anus.
Although most nutrient absorption occurs in the small intestine, water and some vitamins and minerals are absorbed in the colon.
Peristalsis occurs more slowly in the large intestine than in the small intestine.
Materials may remain in the large intestine for about twenty-four hours, compared to three to five hours in the small intestine.
The slow movement favors the growth of microorganisms, collectively referred to as the intestinal microbiota.
The microbiota produce nutrients and have other effects that impact the health of the host.
One product of fiber breakdown by the microbiota is gas, which causes flatulence. In normal adult humans, between 202,000 ml of intestinal gas is produced per day.
The large intestine diagram (Figure 3.12) breaks down nonabsorbed materials by intestinal microbiota, absorption processes, and storage/transport structures:
Blue: absorption
Purple: microbiota activity
Green: storage and transport
Yellow: storage and elimination
Functions of the large intestine
Some water and vitamins/minerals are absorbed in the initial sections of the large intestine.
Much of the material not absorbed is metabolized by the intestinal microbiota, producing gas and other products.
What remains moves to the later sections of the large intestine and, with the rectum, transports waste to eliminate it as feces.
Feces are a mixture of undigested/unabsorbed matter, dead cells, secretions from the GI tract, water, and bacteria.
The amount of bacteria can make up more than half of the weight of feces.
Water content in feces is affected by fiber and fluid intake; adequate fiber and fluids result in feces with higher water content and easier passage.
Practical and biological connections
Diet and fiber intake influence stool consistency and transit time via water content and microbial activity.
Bile function is essential for fat digestion; disruptions can affect fat absorption and overall nutrition.
Absorption mechanisms (diffusion, osmosis, facilitated diffusion, active transport) underlie how nutrients enter mucosal cells and reach systemic circulation or the lymphatic system.
The intestinal microbiota play a major role in processing nonabsorbed materials, producing vitamins and gases, and impacting host health beyond digestion.
Summary (key takeaways)
The small intestine specializes in maximizing absorption through circular folds, villi, and microvilli.
Carbohydrates are digested to monosaccharides and absorbed into the blood; proteins to amino acids absorbed into the blood; fats are emulsified by bile, digested into fatty acids, absorbed into mucosal cells, and often transported via the lymph.
Absorption mechanisms include simple diffusion, osmosis, facilitated diffusion, and active transport.
The large intestine reclaims water and some nutrients, houses microbiota that digest what remains, and forms and stores feces for elimination.
Fiber and hydration influence stool consistency, microbiota activity, gas production, and overall digestive health.