The rest and digest system, also known as the digestive system, is a group of organs that work together to break down and absorb food. The main functions of the digestive system are ingestion, digestion, absorption, and defecation. When we eat, the digestive system takes over to break food down into nutrients that can be absorbed and used by the body for energy, growth, and cell repair.
What are the main parts of the digestive system?
The main parts of the digestive system are:
- Mouth
- Pharynx
- Esophagus
- Stomach
- Small intestine
- Large intestine
- Rectum
- Anus
Let’s look at each part in more detail:
Mouth
The mouth is the beginning of the digestive tract and serves two main functions:
- Ingestion – the mouth allows us to take in food.
- Mechanical digestion – the teeth grind up food into smaller pieces through chewing, while saliva begins to break down starches.
Pharynx
The pharynx, also known as the throat, connects the mouth to the esophagus. It plays an important role in swallowing food.
Esophagus
The esophagus is a muscular tube extending from the pharynx to the stomach. Using rhythmic waves of muscle contractions called peristalsis, the esophagus pushes food towards the stomach.
Stomach
The stomach is a J-shaped, hollow organ that temporarily stores and mixes food. It secretes gastric juices containing acids and enzymes that continue the digestion process by breaking down proteins.
Small intestine
The small intestine is a long, narrow tube about 20 feet in length where most digestion and nutrient absorption occurs. It has three sections:
- Duodenum – Initial digestion of carbohydrates, proteins, and fats occurs here. Bile and pancreatic juices are released to further break down food.
- Jejunum – Most nutrient absorption happens in this section.
- Ileum – Any remaining nutrients are absorbed here before food moves on to the large intestine.
Large intestine
The large intestine is about 5 feet long and has three main sections:
- Cecum – A pouch connected to the ileum where undigested food is temporarily stored.
- Colon – Absorbs water and electrolytes from indigestible matter. The colon has four regions – ascending, transverse, descending, and sigmoid colon.
- Rectum – Stores feces until defecation.
Anus
The anus forms the exit of the digestive tract and controls when feces leaves the body during defecation.
What happens during the digestive process?
The digestive process consists of five main stages:
- Ingestion
- Mechanical and chemical digestion
- Absorption
- Defecation
Let’s look at what happens during each stage:
1. Ingestion
Ingestion occurs in the mouth where food is taken in. Chewing breaks the food into smaller pieces while saliva moistens it for swallowing. The tongue helps push the food into the pharynx to be swallowed.
2. Mechanical and chemical digestion
Once food reaches the stomach, mechanical digestion continues through the churning action of the stomach’s smooth muscles. Gastric juices containing hydrochloric acid and pepsin break down proteins. The acidic environment also kills bacteria in the food.
In the small intestine, the liver produces bile which emulsifies fats. The pancreas releases enzymes like amylase, lipase, and trypsin to further break down nutrients into molecules that can be absorbed. The intestinal walls are lined with finger-like projections called villi which increase surface area for maximum absorption.
3. Absorption
The small intestine absorbs most nutrients from chyme, the liquid mixture of partially digested food and digestive juices:
- Amino acids and sugars are absorbed into the bloodstream.
- Fatty acids and glycerol are absorbed into the lymphatic system.
- Vitamins, minerals, and water are absorbed into the bloodstream.
4. Defecation
In the large intestine, excess water is absorbed and feces is formed. The rectum stores fecal matter until it can be expelled through the anus during defecation.
What hormones regulate digestion?
Several hormones play important regulatory roles in the digestive system:
Hormone | Source | Function |
---|---|---|
Gastrin | Stomach cells | Stimulates gastric acid secretion |
Secretin | Duodenum | Stimulates pancreatic bicarbonate release |
Cholecystokinin (CCK) | Duodenum | Stimulates release of pancreatic enzymes and bile |
Motilin | Duodenum and jejunum | Stimulates gastrointestinal motility |
These hormones are triggered when food enters different sections of the small intestine, stimulating the release of digestive juices and controlling gut motility to keep food moving through the system.
What is the enteric nervous system?
The enteric nervous system (ENS) is a division of the autonomic nervous system that directly controls the function of the gastrointestinal system. It has been nicknamed the “second brain” due to its complexity and ability to operate independently of the central nervous system.
The ENS contains over 100 million nerve cells that line the digestive tract from esophagus to anus. It communicates with the central nervous system through the vagus nerve and the sympathetic division. However, it can also process information and control digestion on its own.
Key roles of the ENS include:
- Regulating gastrointestinal motility
- Stimulating digestive secretions
- Controlling absorption of nutrients
- Mediating intestinal immune response
- Communicating with gut microbes
Diseases involving the ENS include irritable bowel syndrome, inflammatory bowel disease, and other functional GI disorders.
How do digestive enzymes work?
Digestive enzymes are proteins released by the digestive system that break down large nutrient molecules into smaller molecules that can be absorbed. The main enzymes involved in chemical digestion are:
- Amylase – Produced by the salivary glands and pancreas, amylase breaks down carbohydrates into simple sugars.
- Proteases – Pepsin in the stomach and trypsin/chymotrypsin from the pancreas split proteins into peptides and amino acids.
- Nucleases – Break down nucleic acids like DNA and RNA.
- Lipases – Lipases from the pancreas digest triglycerides into fatty acids and glycerol.
Each enzyme acts on a specific substrate and works optimally at a certain pH and temperature. By breaking larger molecules into smaller components, digestive enzymes increase the surface area available for absorption.
How are digestive enzymes regulated?
Enzyme secretion is carefully controlled to match the food entering the system:
- Enzyme production is stimulated by distension, vagus nerve signals, and hormonal secretions triggered by the presence of food.
- In the stomach, pH is optimized for pepsin activity. The small intestine neutralizes pH to favor pancreatic enzymes.
- Enzymes may be activated or deactivated depending on the environment. For example, trypsinogen is activated to trypsin in the alkaline environment of the small intestine.
- Enzyme inhibitors can regulate activity.
This tight regulation allows efficient digestion and prevents inappropriate enzyme activity and potential damage to the gastrointestinal tract.
What is the role of bile in digestion?
Bile is a greenish-yellow fluid produced by the liver and stored in the gallbladder. It contains bile salts, cholesterol, bilirubin, electrolytes, and water. Bile has two main digestive functions:
- Fat emulsification – Bile salts break up large fat droplets into tiny fat globules that are easier for lipases to break down.
- Fat absorption – Bile salts surround the emulsified fats to keep them dispersed in the watery environment of the small intestine, allowing absorption by intestinal cells.
Bile release is stimulated by cholecystokinin (CCK) when fatty acids and amino acids are detected in the duodenum. Bile flows from the gallbladder through the bile ducts into the duodenum. After use, about 90% of bile is reabsorbed and recycled back to the liver.
What is the small intestine’s role in nutrient absorption?
The small intestine, especially the jejunum and ileum, is the major site of nutrient absorption in the digestive system. Several structural and functional adaptations enable this:
- Folded surface containing finger-like villi and microvilli increases absorption area.
- Thin intestinal wall allows rapid transport across epithelium.
- Networks of blood and lymph vessels in the intestinal lining.
- Presence of transport proteins to actively or passively absorb specific nutrients.
- Alkaline environment and presence of digestive enzymes optimizes breakdown of nutrients.
Most nutrients are absorbed in the small intestine by one of these processes:
- Active transport – Using carrier proteins and cellular energy, nutrients are pumped against their concentration gradient.
- Passive diffusion – Nutrients move from high to low concentration down the concentration gradient.
- Facilitated diffusion – Transport proteins carry nutrients down their concentration gradient.
- Endocytosis – Cell membrane encloses nutrient molecules to bring them into the cell.
After absorption, nutrients enter blood or lymph vessels to be distributed around the body to provide energy and build tissue.
What is the role of gut flora in digestion?
The human gastrointestinal tract contains trillions of beneficial gut bacteria that play several roles in digestion:
- Ferment indigestible carbohydrates like fiber into short-chain fatty acids that provide an energy source for intestinal cells.
- Synthesize certain vitamins like vitamin K, folate, and biotin.
- Break down and absorb otherwise indigestible compounds.
- Protect against infection by competing with harmful bacteria.
- Influence gastrointestinal motility and sensitivity.
- Stimulate immunity by interacting with immune cells.
A healthy balance of gut microbes is necessary for proper digestion. Dysbiosis, or microbial imbalances, may contribute to digestive disorders. Probiotics and prebiotics help support the growth of beneficial bacteria.
What digestive disorders can occur?
Some common digestive disorders include:
Gastroesophageal Reflux Disease (GERD)
– Caused by stomach acid backing up into the esophagus due to weakness of the lower esophageal sphincter.
Peptic ulcers
– Open sores in the lining of the stomach, duodenum, or esophagus caused by H. pylori infection or NSAIDs.
Inflammatory Bowel Disease (IBD)
– Chronic inflammation of the gastrointestinal tract, including Crohn’s disease and ulcerative colitis.
Irritable Bowel Syndrome (IBS)
– Functional disorder causing abdominal pain, cramping, constipation, and/or diarrhea.
Celiac disease
– Autoimmune reaction to gluten causing intestinal damage and nutrient malabsorption.
Diverticulitis
– Infection or inflammation of intestinal pouches called diverticula.
Proper diagnosis and treatment of digestive conditions by a gastroenterologist is important for managing symptoms and improving digestive health.
Conclusion
In summary, the gastrointestinal system is responsible for the complex processes of ingestion, digestion, absorption, and defecation that allow us to utilize nutrients from food. Mechanical and chemical digestion break down macromolecules while specialized structures like villi absorb the nutrients into circulation. The entire system is regulated by neural and hormonal signals to match the absorptive and secretory processes to incoming nutrients. A healthy gut with adequate digestive function and a balanced microbiome is essential for overall health.