The digestive system is the body system responsible for the digestion and absorption of food and nutrients. This complex system breaks down the food we eat, extracts nutrients, and eliminates waste products. The main organs involved in digestion include the mouth, esophagus, stomach, small intestine, large intestine, pancreas, liver and gallbladder.
Digestion starts in the mouth, both mechanically and chemically. When we chew, we mechanically break down food into smaller pieces with our teeth. This increases the surface area for chemical digestion by enzymes. Salivary glands in the mouth secrete saliva containing digestive enzymes like salivary amylase which begins the breakdown of starches and carbohydrates while the food is still in the mouth. The saliva also lubricates the food into a bolus for swallowing. The tongue helps mix food with saliva and also helps push the food into the throat to be swallowed. Once swallowed, the food travels down the esophagus by peristaltic movements into the stomach.
The stomach is a J-shaped, hollow, muscular organ that temporarily stores and mechanically churns food into a liquid mixture called chyme. Chyme results from gastric juices which are digestive enzymes and hydrochloric acid secreted by cells in the stomach lining. The enzyme pepsin begins protein digestion, breaking proteins into smaller polypeptides. Hydrochloric acid creates the optimal acidic environment for pepsin to work and also kills many bacteria in food. The stomach muscles contract in rhythmic churning motions to mix food with gastric juices until chyme is formed. Pyloric sphincter valves at the bottom of the stomach regulate the emptying of chyme into the small intestine.
The Small Intestine
The small intestine is where the majority of chemical digestion and nutrient absorption occurs. It is divided into three parts:
The first section of the small intestine is called the duodenum. This is where partially digested chyme from the stomach mixes with digestive juices from the pancreas, liver and gallbladder. Pancreatic juices contain enzymes like trypsin and chymotrypsin to further break down proteins, amylase to break down starches and lipase to break down fats. Bile produced in the liver and stored in the gallbladder emulsifies fats into smaller droplets for the lipase to act on. The duodenum’s alkaline environment produced by bicarbonate from the pancreas also neutralizes the acidity of chyme entering from the stomach.
The middle section of the small intestine is called the jejunum. Here, hydrolysis of nutrients is completed with the help of brush border enzymes lining the intestinal wall. The brush border has surface area in the shape of microvilli to maximize absorption. Carbohydrates are broken down into simple sugars like glucose, proteins into amino acids, and fats into fatty acids and glycerol. The broken down nutrients are then absorbed through the intestinal wall into the bloodstream to be transported around the body.
The final section of the small intestine is called the ileum. Any remaining nutrients are absorbed here, along with vitamin B12 and bile salts. Water, electrolytes and minerals are also absorbed in the ileum. The undigested food particles, water and waste products are passed into the large intestine in the form of semi-solid matter.
The Large Intestine
The large intestine consists of the cecum, colon, rectum and anal canal. It absorbs water and vitamins from indigestible food matter creating feces. Contractions of smooth muscle in the colon propel and mix the contents towards the rectum. Here water is absorbed turning feces into a solid form. Fecal matter is stored until voluntary defecation through the anal canal disposes it from the body. Bacteria living in the large intestine can digest some of the remaining carbohydrates and make certain B vitamins and vitamin K. These are absorbed before defecation.
Accessory Digestive Organs
Besides the main digestive tract, other organs like the pancreas, liver and gallbladder provide essential secretions, storage and regulation functions:
The pancreas produces pancreatic juices containing important digestive enzymes and bicarbonate. Enzymes are secreted into ducts leading to the duodenum so they can mix with chyme for digestion. Bicarbonate neutralizes the acidity of chyme to provide the right pH for the enzymes to work.
The liver produces bile needed for fat digestion. Bile emulsifies large fat globules into smaller fat droplets the enzyme lipase can act on. Bile is stored and concentrated in the gallbladder when eating is not occurring.
The gallbladder stores, concentrates and releases bile into the duodenum through the biliary ducts. Bile helps digest fats and also enables the absorption of fat-soluble vitamins A, D, E and K.
Different digestive enzymes are secreted along the gastrointestinal tract to break down macromolecules:
|Substrate Acted On
|Brush border enzymes
|Small intestine lining
|Carbs, proteins, fats
Hormones Involved in Digestion
Several hormones are secreted to control and regulate the digestive process:
Gastrin is released by G cells in the stomach in response to the presence of food. This stimulates stomach acid secretion and increases gastrointestinal motility.
Secretin is secreted by the duodenum in response to the acidity of chyme entering from the stomach. Secretin stimulates the pancreas to produce alkaline bicarbonate which neutralizes chyme.
CCK is released by duodenal cells in response to fat and protein digestion. This stimulates the release of pancreatic enzymes and bile from the gallbladder.
Ghrelin is the “hunger hormone” produced by empty stomach cells. It stimulates appetite by sending signals to the brain.
Peptide YY is secreted by ileum and colon cells after eating. This slows digestion to increase nutrient absorption time and suppresses appetite signals to the brain.
Absorption in the Small Intestine
The small intestine is specially adapted to maximize the absorption of nutrients:
Transport Across the Epithelial Cells
The mucosa layer of the small intestine is folded into millions of microscopic finger-like projections called villi. This increases the surface area for absorption. Each villi has many microvilli giving even greater area via the brush border enzymes. Nutrients like amino acids, monosaccharides and electrolytes can pass directly into the epithelium cells through protein channels. From there they enter the blood capillaries.
Lacteals for Fat Absorption
After fat digestion, the resulting fatty acids and monoglycerides are coated with bile salts forming micelles. The micelles can fuse with and pass through the epithelial cell membrane. Inside the cell they reform into triglyceride droplets that get coated with proteins forming chylomicrons. Chylomicrons are too big to pass into blood capillaries. Instead they enter the lymphatic system via small lacteals in each villus. This forms the lipid-rich lymph that travels through lymphatic vessels and is eventually drained into the blood circulation.
Water and Electrolyte Absorption
About 1.5-2 liters of water arrives in the small intestine daily as part of digestion. Most of this is absorbed by osmosis. Sodium, potassium and other electrolytes are also actively transported across the epithelium to be absorbed.
Large Intestine Absorption
The large intestine absorbs any residual water, electrolytes and vitamins. The colon can absorb sodium, chloride and potassium creating an isotonic fluid. This helps form solid feces for elimination. Bacteria in the large intestine produce certain vitamins like vitamin K, B1, B2, B6 and B12 which are absorbed through the colon epithelium.
Transport of Absorbed Nutrients
The nutrients absorbed from the small intestine have different transport mechanisms to reach the liver first before systemic circulation:
Simple Sugars and Amino Acids
Glucose, other monosaccharides, and amino acids directly enter the blood capillaries of the villi. They are carried in the hepatic portal vein to the liver for processing.
Fatty Acids and Glycerol
The end products of fat digestion enter the lacteals and form chylomicrons to enter the lymphatic system. The lymph travels through the thoracic duct to ultimately reach venous circulation. Chylomicrons transport lipids to cells throughout the body.
Vitamins like vitamin C are absorbed into the bloodstream through the small intestine. These travel to the liver via the portal vein.
Fat-soluble Vitamins and Lipids
The fat-soluble vitamins A, D, E, K are absorbed along with dietary lipids into the lymph. These travel through the lymphatic system before being released into the bloodstream.
The liver processes absorbed nutrients before they reach the rest of the body:
Glucose and galactose are converted to glycogen for storage. Fructose is converted to triglycerides or metabolized for energy.
Amino acids are processed and converted to various proteins like albumin for bodily needs. Excess amino acids are deaminated to be used for energy or converted to carbohydrates or fats.
Fatty acids can be used for energy or reformed into triglycerides. These are incorporated along with cholesterol to form lipoproteins for transport to cells and tissues.
Water-soluble vitamins entering the liver are either stored or converted to active coenzyme forms. Any excess is excreted out in urine. Fat-soluble vitamins are processed and transported to tissues or stored in fatty tissues like liver or adipose tissue.
The liver helps detoxify drugs, ammonia and other compounds. These may be metabolized, stored or secreted into bile.
In summary, the digestive system provides the crucial functions of ingesting food, digesting macromolecules, absorbing nutrients, processing metabolites and eliminating waste. Mechanical and chemical digestion starts in the mouth. After swallowing, chyme is formed in the stomach and further digested by enzymes from the stomach, pancreas, liver and small intestine. The majority of absorption occurs across the epithelial lining of the small intestine. The nutrients are transported via the blood to the liver for processing and distributed to bodily tissues. Any undigested matter is passed as feces from the large intestine out of the body. The digestive system works closely with other organs and hormone signals to efficently extract nutrients and energy from food consumed.