A GUIDE TO DIGESTION AND ABSORPTION IN GASTROINTESTINAL TRACT

Introduction

The gastrointestinal tract (GIT) is responsible for the digestion and absorption of nutrients from the food we eat. The process of digestion involves the breakdown of food into smaller molecules that can be absorbed by the body, while absorption involves the movement of these molecules from the GIT into the bloodstream. Here is a brief overview of the process of digestion and absorption in the GIT:

• Mouth: The process of digestion begins in the mouth, where food is mechanically broken down by chewing and mixed with saliva. Saliva contains enzymes that begin to break down carbohydrates.
• Esophagus: The esophagus is a muscular tube that connects the mouth to the stomach. It uses contractions, known as peristalsis, to move the food towards the stomach.
• Stomach: In the stomach, food is further mechanically broken down by muscular contractions, known as mixing and churning. The stomach also secretes gastric juice, which contains hydrochloric acid and enzymes that break down proteins.
• Small Intestine: The small intestine is where most of the digestion and absorption takes place. It receives digestive enzymes from the pancreas and bile from the liver and gallbladder, which help to break down fats. The walls of the small intestine are lined with villi and microvilli, which increase the surface area for absorption. Nutrients are absorbed into the bloodstream through the walls of the small intestine and transported to the liver.
• Large Intestine: The large intestine absorbs water and electrolytes from the undigested food, forming solid feces. The feces are stored in the rectum and eliminated through the anus.

Overall, the process of digestion and absorption in the GIT is a complex process that involves the coordinated action of various organs and enzymes. Any disruptions to this process can lead to various digestive disorders.

 

Brunner’s glands are located in the submucosa layer of the duodenum, which is the first segment of the small intestine. These glands secrete an alkaline solution containing bicarbonate ions, which helps to neutralize the acidic chyme (partially digested food mixed with stomach acid) entering the duodenum from the stomach. This is important because the enzymes that digest food in the small intestine work best at a slightly alkaline pH. The secretion of Brunner’s glands is regulated by the enteric nervous system (a network of nerves that controls the digestive system) and the hormones secretin and cholecystokinin (CCK), which are released by the small intestine in response to the presence of acidic chyme.

Bile salts, on the other hand, play a crucial role in the digestion and absorption of fats in the small intestine. Bile is produced by the liver and stored in the gallbladder, and it is released into the small intestine when fat enters the duodenum. Bile salts emulsify the fat, breaking it down into smaller droplets and increasing its surface area, which makes it easier for lipases (enzymes that digest fats) to access and break down the fat molecules. Bile salts also help to solubilize the fat molecules, allowing them to be transported across the intestinal wall and into the bloodstream. The release of bile into the small intestine is regulated by the hormone CCK, which is released in response to the presence of fat and protein in the chyme, as well as by neural signals from the enteric nervous system.

 

The enterohepatic circulation of bile acids is a complex process that involves the secretion, transport, and reabsorption of bile acids between the liver and the small intestine. Bile acids are synthesized in the liver from cholesterol and are secreted into the bile ducts. From there, they are stored in the gallbladder until they are needed to aid in the digestion and absorption of dietary fats in the small intestine.

When fat enters the small intestine, it triggers the release of the hormone cholecystokinin (CCK), which causes the gallbladder to contract and release bile acids into the small intestine. The bile acids emulsify the fats, making them more accessible to digestive enzymes and allowing them to be absorbed into the bloodstream.

After the bile acids have done their job, they are reabsorbed into the bloodstream and returned to the liver through the portal vein. In the liver, they are taken up by hepatocytes (liver cells) and either excreted into the bile ducts or undergo further modification before being re-secreted into the bile. Some bile acids, however, escape the liver and make their way to the colon, where they are metabolized by gut bacteria and eventually excreted in feces.

The enterohepatic circulation of bile acids plays a critical role in maintaining the body’s lipid balance and is tightly regulated to ensure that the appropriate amount of bile acids are present in the body at any given time. Disruptions to this process can lead to conditions such as cholestasis (impaired bile flow) or bile acid malabsorption, which can result in diarrhea and malabsorption of fat-soluble vitamins.

 

The absorption of inorganic components from the diet involves different mechanisms, which vary depending on the type of nutrient and the segment of the digestive tract. Here are the mechanisms of absorption of the principal inorganic components of diets:

1. Calcium: Calcium absorption occurs mainly in the small intestine and involves two mechanisms: passive diffusion and active transport. Passive diffusion occurs when calcium moves down a concentration gradient, while active transport occurs when calcium is absorbed against its concentration gradient with the help of vitamin D and a calcium-binding protein called calbindin.
2. Iron: The absorption of iron occurs primarily in the duodenum and jejunum of the small intestine. Iron is absorbed through two mechanisms: heme and non-heme iron absorption. Heme iron is found in animal products and is more efficiently absorbed than non-heme iron, which is found in plant-based foods.
3. Zinc: Zinc absorption occurs mainly in the small intestine and involves both passive and active transport mechanisms. Zinc absorption is facilitated by proteins called metallothioneins, which bind to zinc and transport it across the intestinal wall.
4. Magnesium: Magnesium absorption occurs mainly in the small intestine and involves both active and passive transport mechanisms. Passive diffusion is the primary mechanism for magnesium absorption, while active transport is involved in times of magnesium deficiency.
5. Sodium and Chloride: Sodium and chloride are absorbed in the small intestine and colon through active transport mechanisms. The transport of sodium and chloride is facilitated by the sodium-potassium pump, which exchanges intracellular sodium for extracellular potassium.
6. Potassium: Potassium absorption occurs mainly in the small intestine and involves both passive and active transport mechanisms. Passive diffusion is the primary mechanism for potassium absorption, while active transport is involved in times of potassium deficiency.

Overall, the absorption of inorganic components from the diet is a complex process that involves multiple mechanisms and depends on several factors, including the type of nutrient, the segment of the digestive tract, and the presence of other nutrients or compounds that can affect absorption.

 

Membrane transport processes are critical for the movement of molecules, ions, and other particles across biological membranes, which are selectively permeable barriers that separate the inside of cells from their external environment. The molecular basis of membrane transport involves a variety of mechanisms that enable molecules to cross the membrane in either direction, depending on the concentration gradients and the specific properties of the membrane and the transported solutes. Some of the main types of membrane transport processes include passive diffusion, facilitated diffusion, active transport, and secondary active transport.

Passive diffusion involves the movement of solutes across the membrane along their concentration gradient, without the need for energy input or carrier proteins. This type of transport occurs when molecules are small enough to diffuse through the lipid bilayer, which is the main barrier to hydrophilic solutes. The rate of diffusion is proportional to the concentration gradient and the solubility of the solute in the lipid membrane.

Facilitated diffusion involves the movement of solutes across the membrane with the aid of carrier proteins, such as ion channels and transporters. These proteins can recognize specific molecules and facilitate their transport by changing their conformation to expose the binding site to either side of the membrane. Ion channels are typically selective for certain ions, while transporters can facilitate the movement of a wide variety of solutes.

Active transport involves the movement of solutes across the membrane against their concentration gradient, which requires the input of energy from ATP hydrolysis or other sources. This process is essential for maintaining concentration gradients across membranes, and is mediated by transporter proteins known as pumps, which can transport ions and other solutes against their electrochemical gradient.

Secondary active transport involves the movement of solutes across the membrane with the aid of a concentration gradient that has been established by primary active transport. This process can be either symport, where the solute and the ion move in the same direction, or antiport, where the solute and the ion move in opposite directions.

The molecular basis of membrane transport processes involves the specific interactions between solutes, carrier proteins, and the lipid bilayer of the membrane. These interactions determine the selectivity, affinity, and rate of transport for different solutes, and are influenced by various factors such as the pH, temperature, and lipid composition of the membrane. Understanding the molecular basis of membrane transport is essential for elucidating the mechanisms of various physiological processes, and for developing new drugs and therapies that target membrane transporters.

 

The absorption of molecules into the blood or lymphatic system is influenced by several factors, including the molecular size, polarity, solubility, and charge of the molecule.

1. Molecular size: Smaller molecules are generally more easily absorbed than larger ones. Large molecules such as proteins and nucleic acids are typically too large to be absorbed through the walls of blood or lymph vessels.
2. Polarity: Molecules with high polarity, such as those containing polar functional groups like hydroxyl (-OH) and carboxyl (-COOH), tend to be more water-soluble and are more likely to be absorbed into the blood than nonpolar molecules such as lipids and hydrocarbons.
3. Solubility: The solubility of a molecule in water and lipids is another important factor. Lipid-soluble molecules are more likely to be absorbed into the lymphatic system, while water-soluble molecules are more likely to be absorbed into the bloodstream.
4. Charge: The charge of a molecule also plays a role in absorption. Charged molecules such as ions and polar molecules can interact with the charged membranes of cells and may be more easily absorbed than uncharged molecules.
5. Route of administration: The route of administration can also affect whether a molecule is absorbed into the blood or lymph. For example, molecules that are ingested orally are typically absorbed through the gastrointestinal tract and enter the bloodstream, while molecules applied topically may be absorbed into the lymphatic system.

Overall, the specific properties of a molecule will determine its absorption characteristics and the route it takes through the body.

 

The process of absorbing nutrients from food starts in the mouth and continues through the gastrointestinal tract. The small intestine is the primary site of nutrient absorption. The mechanisms by which products of digestion of proteins, carbohydrates, and fats are absorbed into and through the cells lining the alimentary canal involve several steps, including digestion, transport, and absorption.

1. Protein Digestion and Absorption: Protein digestion begins in the stomach with the action of pepsin, which breaks down large protein molecules into smaller peptides. The small intestine continues protein digestion with the action of proteolytic enzymes, such as trypsin and chymotrypsin, which further break down peptides into individual amino acids. Amino acids are then transported across the brush border membrane of the small intestine cells by specialized transporters, where they enter the bloodstream and are transported to the liver for further processing.
2. Carbohydrate Digestion and Absorption: Carbohydrate digestion starts in the mouth with the action of salivary amylase, which breaks down starch into smaller sugars. In the small intestine, pancreatic amylase continues the breakdown of carbohydrates into monosaccharides. These monosaccharides, including glucose, fructose, and galactose, are then transported across the brush border membrane of the small intestine cells via specific transporters, such as the sodium-glucose cotransporter, and enter the bloodstream.
3. Fat Digestion and Absorption: Fat digestion begins in the small intestine with the action of bile, which emulsifies fats into smaller droplets, increasing their surface area for enzymatic action. Pancreatic lipase then breaks down the fats into monoglycerides and free fatty acids. These products of fat digestion combine with bile to form micelles, which can cross the brush border membrane of the small intestine cells. Inside the cells, these products are re-formed into triglycerides and are packaged into chylomicrons, which enter the lymphatic system and eventually reach the bloodstream.

In summary, the digestion and absorption of nutrients involve several complex mechanisms that require the coordinated action of various enzymes and transporters. The small intestine is the primary site of nutrient absorption, and the products of digestion of proteins, carbohydrates, and fats are transported across the brush border membrane of the small intestine cells and enter the bloodstream, where they are transported to the liver for further processing and utilization by the body.