Table of Contents
The digestive system ingests and digests food, absorbs released nutrients, and releases out food components that are indigestible. The six activities involved in this process are ingestion, motility, mechanical digestion, chemical digestion, absorption, and defecation. These processes are controlled by neural and hormonal mechanisms. The digestive system uses mechanical and chemical activities to break food down into absorbable substances during its journey through the digestive system.
Digestive System – Organs and its Functions
Chews and mixes food
Begins chemical breakdown of carbohydrates
Moves food into the pharynx
Begins breakdown of lipids via lingual lipase
Moistens and dissolves food, allowing you to taste it
Cleans and lubricates the teeth and oral cavity
Has some antimicrobial activity
|Propels food from the oral cavity to the esophagus
Lubricates food and passageways
|Propels food to the stomach
Lubricates food and passageways
|Mixes and churns food with gastric juices to form chyme
Begins chemical breakdown of proteins
Releases food into the duodenum as chyme
Absorbs some fat-soluble substances (for example, alcohol, aspirin)
Possesses antimicrobial functions
Stimulates protein-digesting enzymes
Secretes intrinsic factor required for vitamin B12 absorption in small intestine
|Mixes chyme with digestive juices
Propels food at a rate slow enough for digestion and absorption
Absorbs breakdown products of carbohydrates, proteins, lipids, and nucleic acids, along with vitamins, minerals, and water
Performs physical digestion via segmentation
Provides optimal medium for enzymatic activity
|Liver: produces bile salts, which emulsify lipids, aiding their digestion and absorption
Gallbladder: stores, concentrates, and releases bile
Pancreas: produces digestive enzymes and bicarbonate
Bicarbonate-rich pancreatic juices help neutralize acidic chyme and provide optimal environment for enzymatic activity
|Further breaks down food residues
Absorbs most residual water, electrolytes, and vitamins produced by enteric bacteria
Propels feces toward rectum
Food residue is concentrated and temporarily stored prior to defecation
Mucus eases passage of feces through colon
Digestive System – Process
- The processes of digestion include six activities: ingestion, propulsion, mechanical or physical digestion, chemical digestion, absorption, and defecation
- The first of these processes, ingestion, refers to the entry of food into the alimentary canal through the mouth
- There, the food is chewed and mixed with saliva, which contains enzymes that begin breaking down the carbohydrates in the food plus some lipid digestion via lingual lipase
- Chewing increases the surface area of the food and allows an appropriately sized bolus to be produced.
- Food leaves the mouth when the tongue and pharyngeal muscles propel it into the esophagus
- The act of swallowing, the last voluntary act until defecation, is an example of propulsion, which refers to the movement of food through the digestive tract
- It includes both the voluntary process of swallowing and the involuntary process of peristalsis.
- Peristalsis consists of sequential, alternating waves of contraction and relaxation of alimentary wall smooth muscles, which act to propel food along
- These waves also play a role in mixing food with digestive juices
- Peristalsis is so powerful that foods and liquids you swallow enter your stomach even if you are standing on your head.
- These contractions isolate small sections of the intestine, moving their contents back and forth while continuously subdividing, breaking up, and mixing the contents.
- By moving food back and forth in the intestinal lumen, segmentation mixes food with digestive juices and facilitates absorption.
- In chemical digestion, starting in the mouth, digestive secretions break down complex food molecules into their chemical building blocks (for example, proteins into separate amino acids)
- These secretions vary in composition, but typically contain water, various enzymes, acids, and salts
- The process is completed in the small intestine.
- Food that has been broken down is of no value to the body unless it enters the bloodstream and its nutrients are put to work
- This occurs through the process of absorption, which takes place primarily within the small intestine
- There, most nutrients are absorbed from the lumen of the alimentary canal into the bloodstream through the epithelial cells that make up the mucosa
- Lipids are absorbed into lacteals and are transported via the lymphatic vessels to the bloodstream
Digestive System – Regulation
- Neural and endocrine regulatory mechanisms work to maintain the optimal conditions in the lumen needed for digestion and absorption
- These regulatory mechanisms, which stimulate digestive activity through mechanical and chemical activity, are controlled both extrinsically and intrinsically.
- The walls of the alimentary canal contain a variety of sensors that help regulate digestive functions.
- These include mechanoreceptors, chemoreceptors, and osmoreceptors, which are capable of detecting mechanical, chemical, and osmotic stimuli, respectively.
- For example, these receptors can sense when the presence of food has caused the stomach to expand, whether food particles have been sufficiently broken down, how much liquid is present, and the type of nutrients in the food (lipids, carbohydrates, and/or proteins).
- Stimulation of these receptors provokes an appropriate reflex that furthers the process of digestion. This may entail sending a message that activates the glands that secrete digestive juices into the lumen, or it may mean the stimulation of muscles within the alimentary canal, thereby activating peristalsis and segmentation that move food along the intestinal tract.
- The walls of the entire alimentary canal are embedded with nerve plexuses that interact with the central nervous system and other nerve plexuses—either within the same digestive organ or in different ones.
- These interactions prompt several types of reflexes. Extrinsic nerve plexuses orchestrate long reflexes, which involve the central and autonomic nervous systems and work in response to stimuli from outside the digestive system
- Short reflexes, on the other hand, are orchestrated by intrinsic nerve plexuses within the alimentary canal wall
- These two plexuses and their connections were introduced earlier as the enteric nervous system
- Short reflexes regulate activities in one area of the digestive tract and may coordinate local peristaltic movements and stimulate digestive secretions
- For example, the sight, smell, and taste of food initiate long reflexes that begin with a sensory neuron delivering a signal to the medulla oblongata
- The response to the signal is to stimulate cells in the stomach to begin secreting digestive juices in preparation for incoming food
- In contrast, food that distends the stomach initiates short reflexes that cause cells in the stomach wall to increase their secretion of digestive juices.
- A variety of hormones are involved in the digestive process.
- The main digestive hormone of the stomach is gastrin, which is secreted in response to the presence of food
- Gastrin stimulates the secretion of gastric acid by the parietal cells of the stomach mucosa. Other GI hormones are produced and act upon the gut and its accessory organs
- Hormones produced by the duodenum include secretin, which stimulates a watery secretion of bicarbonate by the pancreas; cholecystokinin (CCK), which stimulates the secretion of pancreatic enzymes and bile from the liver and release of bile from the gallbladder; and gastric inhibitory peptide, which inhibits gastric secretion and slows gastric emptying and motility
- These GI hormones are secreted by specialized epithelial cells, called endocrinocytes, located in the mucosal epithelium of the stomach and small intestine
- These hormones then enter the bloodstream, through which they can reach their target organs.
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