Notes from the Video

What You're Going to Learn from This Video on Small Intestinal Physiology

How does the small bowel work?  Why is it important?

Do you have a solid understanding of small intestinal physiology?

How do we digest and absorb carbohydrates, proteins and fats?

How about water, electrolytes and vitamins?

What are brush border enzymes and how about enterohepatic circulation?

Digestion and absorption gets complicated.  Let's make it easy!

A brief overview of the physiology of the small intestine?

Remember from the Small Intestinal Anatomy talk, the small intestine is a long, highly convoluted tube that is about 6 meters long in adults, with three distinct regions: duodenum, jejunum, and ileum.

The small intestine has TWO MAJOR roles: digestion and absorption.

How about under the microscope?

If we look at the microscopic anatomy, the walls of the small intestine have four main layers: mucosa, submucosa, muscularis, and serosa.

The mucosa layer is the innermost layer and contains tiny finger-like projections called villi, which increase the surface area of the intestine, allowing for more efficient absorption.

Due to millions of villi and microvilli, the surface area of the small intestine reaches the size of a tennis court!  So you can see how powerful the multiplier villi and microvilli represent.

The villi are covered by a layer of epithelial cells, which produce digestive enzymes, mucin and absorb nutrients.

What is secreted into the small intestine?

The small intestine receives digestive secretions from the pancreas and liver via the common bile duct, which enters the duodenum at the ampulla of Vater and aids in the digestion and absorption of fats.

The pancreas secretes pancreatic juice, which contains enzymes that break down carbohydrates, proteins, and fats.

How are carbohydrates absorbed by the small intestine?

Carbohydrates are primarily absorbed in the duodenum and jejunum, the first two sections of the small intestine.

The absorption of carbohydrates occurs through the following mechanisms:

Passive diffusion: Monosaccharides such as glucose, galactose, and fructose passively diffuse across the intestinal wall and into the bloodstream. This process does not require energy and occurs down a concentration gradient.

Facilitated diffusion: Some monosaccharides such as fructose and galactose are transported across the intestinal wall with the help of transport proteins called GLUT5 and SGLT1, respectively. These transporters allow the monosaccharides to move across the membrane against their concentration gradient but do not require energy.

Active transport: Glucose is primarily absorbed through active transport using the SGLT1 protein. This process requires energy in the form of ATP and involves the transport of glucose against its concentration gradient.

Once the monosaccharides are absorbed into the bloodstream, they are transported to the liver through the portal vein. The liver then processes the sugars, storing excess glucose as glycogen or releasing it into the bloodstream to be used as energy by the body's cells.

How are proteins absorbed by the small intestine?

Proteins are broken down into smaller peptides and amino acids through the action of digestive enzymes, primarily secreted by the pancreas and the small intestine itself. Once the proteins are broken down into their constituent parts, they are absorbed by the small intestine and transported to the liver through the bloodstream.

Protein absorption occurs primarily in the jejunum and ileum, the middle and last sections of the small intestine.

The absorption of proteins occurs through the following mechanisms:

Active transport: Amino acids are absorbed by active transport across the brush border membrane of the small intestine. This process requires energy and involves the use of carrier proteins that transport the amino acids against their concentration gradient.

Peptide transport: Small peptides, consisting of 2-3 amino acids, are absorbed through the PepT1 transporter in the brush border membrane. This process is also energy-dependent, with the transporter moving the peptides into the cell against their concentration gradient.

Endocytosis: Larger protein molecules are absorbed by endocytosis. In this process, the protein molecules are taken up by the cell membrane and enclosed in a vesicle that then moves into the cell's cytoplasm.

The enzymes responsible for protein digestion include pepsin, secreted by the stomach, and pancreatic proteases, such as trypsin and chymotrypsin. These enzymes break down the protein molecules into smaller peptides and amino acids, allowing for absorption in the small intestine.

Once the amino acids and small peptides are absorbed into the bloodstream, they are transported to the liver, where they are processed and utilized in various metabolic processes. Some amino acids may also be transported to other tissues in the body for protein synthesis.

How are fats absorbed by the small intestine?

Dietary fats are first broken down into smaller molecules called fatty acids and monoglycerides by the action of digestive enzymes in the mouth and stomach.

These smaller molecules are then further broken down in the small intestine and absorbed into the bloodstream for transport to the liver and other tissues in the body.

Fat absorption primarily occurs in the jejunum and ileum, the middle and last sections of the small intestine. The absorption of fats occurs through the following mechanisms:

Emulsification: Before fats can be absorbed, they must first be broken down into smaller droplets to increase their surface area. This process occurs in the small intestine through the action of bile, a fluid produced by the liver and stored in the gallbladder. Bile emulsifies the fats, breaking them down into smaller droplets called micelles.

Digestion: Once the fats are emulsified, pancreatic enzymes called lipases break them down into smaller molecules such as fatty acids, monoglycerides, and glycerol. These smaller molecules can then be absorbed by the small intestine.

Absorption: The fatty acids and monoglycerides are absorbed by the small intestine through passive diffusion.

However, they are insoluble in water and must be packaged into lipoproteins called chylomicrons for transport in the bloodstream. Chylomicrons are transported to the liver via the lymphatic system.

The enzymes responsible for fat digestion include lipases, produced by the pancreas, and bile salts produced by the liver. Lipases are responsible for breaking down the fats into smaller molecules, while bile salts emulsify the fats, making them more accessible to the lipases.

What is enterohepatic circulation and why is it important?

Enterohepatic circulation is a complex process in which substances such as bile acids, bilirubin, and drugs are transported between the liver and small intestine multiple times before being eliminated from the body.

This process is important for the efficient recycling and elimination of substances, as well as the maintenance of bile acid homeostasis in the body.

The enterohepatic circulation of bile acids is one of the most extensively studied examples of this process.

Bile acids are produced in the liver and stored in the gallbladder until they are needed to aid in the digestion and absorption of fats in the small intestine. Once in the small intestine, the bile acids emulsify the fats, allowing them to be more easily digested and absorbed.

After the bile acids have fulfilled their digestive role, they are reabsorbed by the small intestine and transported back to the liver through the portal vein.

Once in the liver, the bile acids are either recycled and used again or converted into other substances such as secondary bile acids.  Secondary bile acids are formed through the actions of gut bacteria on primary bile acids that have been reabsorbed into the intestine. These secondary bile acids have been shown to play a role in regulating glucose and lipid metabolism, as well as modulating the composition of the gut microbiome.

One example of a secondary bile acid is deoxycholic acid, which is produced from the primary bile acid cholic acid.

Deoxycholic acid has been implicated in the development of colon cancer, as it can promote inflammation and damage DNA in the colon.

This continuous cycle of secretion, reabsorption, and recycling of bile acids is referred to as enterohepatic circulation.

The importance of enterohepatic circulation lies in its ability to efficiently recycle and conserve bile acids, which are essential for the digestion and absorption of fats in the small intestine. Without enterohepatic circulation, the body would need to continuously produce new bile acids, which would be inefficient and potentially harmful.

Additionally, enterohepatic circulation plays a critical role in the elimination of drugs and other toxic substances from the body. Many drugs and toxic substances are metabolized in the liver and excreted into the bile, where they are transported to the small intestine. Once in the small intestine, they can be reabsorbed into the bloodstream and transported back to the liver, where they can be metabolized again and eliminated from the body.