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Following this question regarding absorption in human oral cavity (sadly not yet fully answered), I'm curious if any nutrient absorption occurs during the descent of food through the esophagus.
And if so, what requirements must a nutrient meet for that.
For several reasons, I seriously doubt that this has been studied extensively.
There is a good reason to study absorption through oral mucosa because a drug or glucose can be held there by people who needing rapid absorption of a drug (e.g. sublingual nitroglycerine) or by people who may be unable to swallow (e.g. a hypoglycemic individual who has lost consciousness). Where there is a reason to know, there is a reason to study something.
The esophagus, on the other hand, is normally an area of rapid transport (a pill, or a fluid, for instance, is in the esophagus less than 10 seconds). While it's assumed that some pregastric absorption through the esophagus occurs and might be beneficial to avoid a "first pass" metabolism of the drug by the liver, there is no good method to keep a drug in the esophagus. With that in mind, absorption from the sublingual mucosa (where a pill might be held) is more valuable than what might get absorbed in as little as 6 seconds in transit to the stomach. From that point, it will spend significant amounts of time in the stomach, the various sections of the small intestine, and the large intestine.
Absorption of drugs from the rectum has been studied, because it can be used, once again, as a method of drug delivery when another route for whatever reason cannot,
Esophageal motility disorders occur, so studies of esophageal function are common. But as the esophagus is never normally the place for absorption of nutrients, there is no reason to study it except for the sake of satisfying pure curiosity. Studies take time, involve volunteers (or paid recipients), risk, and money. Pure curiosity is a reason to study something. But to get the all the previously mentioned factors to come together and fund a study, the reason usually outweighs pure curiosity.
Esophageal scintigraphy: reproducibility and normal ranges
Oesophageal transit of a radionuclide solid bolus in normals
DRUG ABSORPTION BY SUBLINGUAL AND RECTAL ROUTES
28 3.6 The Digestive System
All living organisms need nutrients to survive. While plants can obtain nutrients from their roots and the energy molecules required for cellular function through the process of photosynthesis, animals obtain their nutrients by the consumption of other organisms. At the cellular level, the biological molecules necessary for animal function are amino acids, lipid molecules, nucleotides, and simple sugars. However, the food consumed consists of protein, fat, and complex carbohydrates. Animals must convert these macromolecules into the simple molecules required for maintaining cellular function. The conversion of the food consumed to the nutrients required is a multistep process involving digestion and absorption. During digestion, food particles are broken down to smaller components, which are later absorbed by the body. This happens by both physical means, such as chewing, and by chemical means.
One of the challenges in human nutrition is maintaining a balance between food intake, storage, and energy expenditure. Taking in more food energy than is used in activity leads to storage of the excess in the form of fat deposits. The rise in obesity and the resulting diseases like type 2 diabetes makes understanding the role of diet and nutrition in maintaining good health all the more important.
From the Mouth to the Stomach
There are four steps in the digestion process (Figure 3.5 “The Human Digestive System”). The first step is ingestion, which is the intake of food into the digestive tract. It may seem a simple process, but ingestion involves smelling food, thinking about food, and the involuntary release of saliva in the mouth to prepare for food entry. In the mouth, where the second step of digestion starts, the mechanical and chemical breakdown of food begins. The chemical breakdown of food involves enzymes, such as salivary amylase that starts the breakdown of large starch molecules into smaller components.
Mechanical breakdown starts with mastication (chewing) in the mouth. Teeth crush and grind large food particles, while saliva provides lubrication and enables food movement downward. The slippery mass of partially broken-down food is called a bolus, which moves down the digestive tract as you swallow. Swallowing may seem voluntary at first because it requires conscious effort to push the food with the tongue back toward the throat, but after this, swallowing proceeds involuntarily, meaning it cannot be stopped once it begins. As you swallow, the bolus is pushed from the mouth through the pharynx and into a muscular tube called the esophagus. As the bolus travels through the pharynx, a small flap called the epiglottis closes to prevent choking by keeping food from going into the trachea. Peristaltic contractions also known as peristalsis in the esophagus propel the food bolus down to the stomach (Figure 3.6 “Peristalsis in the Esophagus”). At the junction between the esophagus and stomach there is a sphincter muscle that remains closed until the food bolus approaches. The pressure of the food bolus stimulates the lower esophageal sphincter to relax and open and food then moves from the esophagus into the stomach. The mechanical breakdown of food is accentuated by the muscular contractions of the stomach and small intestine that mash, mix, slosh, and propel food down the alimentary canal. Solid food takes between four and eight seconds to travel down the esophagus, and liquids take about one second.
Figure 3.6 Peristalsis in the Esophagus
Image by Allison Calabrese / CC BY 4.0
Following Food from Mouth to Anus
To understand how our food is digested in the digestive system, it might be very useful to follow our food along its normal path, starting from the mouth.
Imagine for just a second that you’re hungry and your eyes gaze upon a nice home cooked thanksgiving turkey dinner. Your mouth starts to water. The salivary glands in your mouth are triggered to start producing saliva, a compound that will aid in the digestion of the meal.
As food enters your mouth, your teeth begin mechanically breaking down the food into small and smaller pieces. The saliva starts to chemically break it down as well. Soon, your conscious mind says, “lets swallow this food.” You swallow it and take another bite.
While you’re thinking about your next bite of food, your nervous system is helping to move the bolus (the food package you swallowed), down throat. A small flap of skin called your epiglottis makes sure your food goes down your esophagus. Movements of the smooth muscles, known as peristalsis help move that bolus down your esophagus. When it reaches your stomach, a sphincter opens and dumps the food in.
Inside the stomach, cells start to secrete different acids that help increase acidity to a pH of 2. This strong acidic environment kills most bacteria and starts to chemically break apart the food. Movements of the smooth muscles in the stomach, known as peristalsis mix and churn the food up more. After the food has been well mixed and has a consistency of oatmeal, it is ready to move to the small intestine. At this stage it is known as chyme.
To move into the small intestine, chyme must pass through the pyloric sphincter. From here it enters the duodenum, the first part of the small intestine. The liver mixes in bile, which helps break down fats in the food. The pancreas also secretes digestive enzymes that aid in digestion.
Most of the nutrients are absorbed from the small intestine and moved into the blood stream via a system of small folds, called vili.
After the food moves through the small intestine it enters the large intestine. The large intestine is named for the diameter of the cavity and not for the length. It is actually much shorter than the small intestine. The role of the large intestine is to remove any extra water from the digested material before it is finally excreted.
So there you have it – a basic rundown of what happens to the food we eat from the time we eat it, to when we excrete it. While we didn’t go into detail on all the steps, we would encourage you to explore as much as you can about each step of the process. Review our links below for more detailed explanations of the entire process.
From the Small Intestine to the Large Intestine
The process of digestion is fairly efficient. Any food that is still incompletely broken down (usually less than ten percent of food consumed) and the food&rsquos indigestible fiber content move from the small intestine to the large intestine (colon) through a connecting valve. A main task of the large intestine is to absorb much of the remaining water. Remember, water is present not only in solid foods and beverages, but also the stomach releases a few hundred milliliters of gastric juice, and the pancreas adds approximately 500 milliliters during the digestion of the meal. For the body to conserve water, it is important that excessive water is not lost in fecal matter. In the large intestine, no further chemical or mechanical breakdown of food takes place unless it is accomplished by the bacteria that inhabit this portion of the intestinal tract. The number of bacteria residing in the large intestine is estimated to be greater than 1014, which is more than the total number of cells in the human body (1013). This may seem rather unpleasant, but the great majority of bacteria in the large intestine are harmless and many are even beneficial.
Immune Function of the GI Tract
The GI tract plays an important role in protecting the body from pathogen s . The surface area of the GI tract is estimated to be about 32 square metres (105 square feet), or about half the area of a badminton court. This is more than three times the area of the exposed skin of the body, and it provides a lot of area for pathogens to invade the tissues of the body. The innermost mucosal layer of the walls of the GI tract provides a barrier to pathogens so they are less likely to enter the blood or lymph circulations. The mucus produced by the mucosal layer, for example, contains antibodies that mark many pathogenic microorganisms for destruction. Enzyme s in some of the secretions of the GI tract also destroy pathogens. In addition, stomach acids have a very low pH that is fatal for many microorganisms that enter the stomach.
From the Stomach to the Small Intestine
When food enters the stomach, a highly muscular organ, powerful peristaltic contractions help mash, pulverize, and churn food into chyme . Chyme is a semiliquid mass of partially digested food that also contains gastric juices secreted by cells in the stomach. These gastric juices contain hydrochloric acid and the enzyme pepsin , that chemically start breakdown of the protein components of food.
The length of time food spends in the stomach varies by the macronutrient composition of the meal. A high-fat or high-protein meal takes longer to break down than one rich in carbohydrates. It usually takes a few hours after a meal to empty the stomach contents completely into the small intestine.
The small intestine is divided into three structural parts: the duodenum, the jejunum, and the ileum. Once the chyme enters the duodenum (the first segment of the small intestine), the pancreas and gallbladder are stimulated and release juices that aid in digestion. The pancreas secretes up to 1.5 liters (.4 US gallons) of pancreatic juice through a duct into the duodenum per day. This fluid consists mostly of water, but it also contains bicarbonate ions that neutralize the acidity of the stomach-derived chyme and enzymes that further break down proteins, carbohydrates, and lipids. The gallbladder secretes a much smaller amount of a fluid called bile that helps to digest fats. Bile passes through a duct that joins the pancreatic ducts and is released into the duodenum. Bile is made in the liver and stored in the gall bladder. Bile’s components act like detergents by surrounding fats similar to the way dish soap removes grease from a frying pan. This allows for the movement of fats in the watery environment of the small intestine. Two different types of muscular contractions, called peristalsis and segmentation, control the movement and mixing of the food in various stages of digestion through the small intestine.
Similar to what occurs in the esophagus and stomach, peristalsis is circular waves of smooth muscle contraction that propel food forward. Segmentation from circular muscle contraction slows movement in the small intestine by forming temporary “sausage link” type of segments that allows chyme to slosh food back and forth in both directions to promote mixing of the chyme and enhance absorption of nutrients (Figure 2.7 “Segmentation”). Almost all the components of food are completely broken down to their simplest units within the first 25 centimeters of the small intestine. Instead of proteins, carbohydrates, and lipids, the chyme now consists of amino acids , monosaccharides, and emulsified components of triglycerides.
“Segmentation” by OpenStax College / CC BY 3.0
The third step of digestion (nutrient absorption) takes place mainly in the remaining length of the small intestine, or ileum (> 5 meters). The way the small intestine is structured gives it a huge surface area to maximize nutrient absorption. The surface area is increased by folds, villi, and microvilli. Digested nutrients are absorbed into either capillaries or lymphatic vessels contained within each microvillus.
The small intestine is perfectly structured for maximizing nutrient absorption. Its surface area is greater than 200 square meters, which is about the size of a tennis court. The large surface area is due to the multiple levels of folding. The internal tissue of the small intestine is covered in villi, which are tiny finger-like projections that are covered with even smaller projections, called microvilli (Figure 2.8 “Structure of the Small Intestine”). The digested nutrients pass through the absorptive cells of the intestine via diffusion or special transport proteins. Amino acids, short fatty acids, and monosaccharides (sugars) are transported from the intestinal cells into capillaries, but the larger fatty acids, fat-soluble vitamins, and other lipids are transported first through lymphatic vessels, which soon meet up with blood vessels.
Figure 2.8 Structure of the Small Intestine
“Histology Small Intestines” by OpenStax College / CC BY 3.0
Regulation of Digestion
Our nervous system and hormones control digestion. The nervous system consists of the central nervous system, and the peripheral nervous system. Our brain and spinal cord make up the central nervous system while the peripheral system lies outside the skull and vertebral column. There are two components to the peripheral system: the somatic system that supplies the skin and muscle, and the autonomic system which supplies smooth muscle, cardiac muscle, and glands. The autonomic system has two divisions: the parasympathetic (PSNS or PNS) and sympathetic system (SNS). The PSNS supplies signals to maintain normal function and conserve body processes. The SNS provides signals to accelerate the process. Our gastrointestinal tract receives signals from the central and autonomic systems as well as sends signals to these systems.
Hormones are also involved in regulating digestion. Your digestive tract secretes hormones to control the release of digestive enzymes and juices. Here is a table of some hormones.
|Gastrin||Stomach||Food, especially proteins, caffeine, spices, alcohol||Stimulates stomach acids and enzymes|
|Cholecystokinin (CCK)||Small Intestine||Fat and protein||Stimulates pancreas and liver secretions (enzymes and bile) for protein and fat digestion|
|Secretin||Small Intestine||Acid (from stomach) in small intestine||Secretes bicarbonate to neutralize acid|
|Gastric inhibitory protein (GIP)||Small Intestine||Fat and protein||Inhibits gastric motility and secretion of gastric juices|
Our appetite and hunger are controlled by a complex process that involves many signals. Here is a brief overview of that process.
- The breakdown of complex macromolecules in foods to simple absorbable components is accomplished by the digestive system. These components are processed by cells throughout the body into energy or are used as building blocks.
- The digestive system is composed of the mouth, pharynx, esophagus, stomach, small intestine, large intestine (or colon), rectum, and anus. There are four steps in the digestion process: ingestion, the mechanical and chemical breakdown of food, nutrient absorption, and elimination of indigestible food.
- The mechanical breakdown of food occurs via muscular contractions called peristalsis and segmentation. Enzymes secreted by the salivary glands, stomach, pancreas, and small intestine accomplishes the chemical breakdown of food. Additionally, bile emulsifies fats.
Decide whether you want to consume pre- and probiotic foods to benefit your health. Visit the websites below to help in your decision-making process. Defend your decision scientifically.
From the Mouth to the Stomach
There are four steps in the digestion process (Figure (PageIndex<2>)). The first step is ingestion , which is the collection of food into the digestive tract. It may seem a simple process, but ingestion involves smelling food, thinking about food, and the involuntary release of saliva in the mouth to prepare for food entry. In the mouth, where the second step of digestion occurs, the mechanical and chemical breakdown of food begins. The chemical breakdown of food involves enzymes, which break apart the components in food. Theses enzymes are secreted by the salivary glands, stomach, pancreas, and small intestine. Mechanical breakdown starts with mastication (chewing) in the mouth. Teeth crush and grind large food particles, while saliva initiates the chemical breakdown of food and enables its movement downward. The slippery mass of partially broken-down food is called bolus, which moves down the digestive tract as you swallow. Swallowing may seem voluntary at first because it requires conscious effort to push the food with the tongue back toward the throat, but after this, swallowing proceeds involuntarily, meaning it cannot be stopped once it begins.Figure (PageIndex<2>): Components of the Human Digestive System. All digestive organs play integral roles in the life-sustaining process of digestion. (CC BY 3.0 OpenStax).
As you swallow, the bolus is pushed from the mouth through the pharynx and into a muscular tube called the esophagus. As it travels through the pharynx, a small flap called the epiglottis closes, to prevent choking by keeping food from going into the trachea. Peristaltic contractions in the esophagus propel the food down to the stomach. At the junction between the esophagus and stomach there is a sphincter muscle that remains closed until the food bolus approaches. The pressure of the food bolus stimulates the lower esophageal sphincter to relax and open and food then moves from the esophagus into the stomach. The mechanical breakdown of food is accentuated by the muscular contractions of the stomach and small intestine that mash, mix, slosh, and propel food down the alimentary canal. Solid food takes between four and eight seconds to travel down the esophagus, and liquids take about one second.
BIO 140 - Human Biology I - Textbook
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Digestive System Processes and Regulation
OpenStax , Digestive System Processes and Regulation. OpenStax CNX. Jun 28, 2013 http://cnx.org/contents/[email protected] © Jun 28, 2013 OpenStax . Textbook content produced by OpenStax is licensed under a Creative Commons Attribution License 3.0 license.
- Discuss six fundamental activities of the digestive system, giving an example of each
- Compare and contrast the neural and hormonal controls involved in digestion
The digestive system uses mechanical and chemical activities to break food down into absorbable substances during its journey through the digestive system. Table 1 provides an overview of the basic functions of the digestive organs.
Table 1: Functions of the Digestive Organs
- Ingests food
- 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
- Eliminates feces
- Food residue is concentrated and temporarily stored prior to defecation
- Mucus eases passage of feces through colon
Watch the video linked to below for an overview of digestion of food in different regions of the digestive tract. Note the route of non-fat nutrients from the small intestine to their release as nutrients to the body.
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. This 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 (Figure 1). 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.
Figure 1: Peristalsis moves food through the digestive tract with alternating waves of muscle contraction and relaxation.
Digestion includes both mechanical and chemical processes. Mechanical digestion is a purely physical process that does not change the chemical nature of the food. Instead, it makes the food smaller to increase both surface area and mobility. It includes mastication , or chewing, as well as tongue movements that help break food into smaller bits and mix food with saliva. Although there may be a tendency to think that mechanical digestion is limited to the first steps of the digestive process, it occurs after the food leaves the mouth, as well. The mechanical churning of food in the stomach serves to further break it apart and expose more of its surface area to digestive juices, creating an acidic &ldquosoup&rdquo called chyme . Segmentation , which occurs mainly in the small intestine, consists of localized contractions of circular muscle of the muscularis layer of the alimentary canal. 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 (the subclavian veins near the heart). The details of these processes will be discussed later.
In defecation , the final step in digestion, undigested materials are removed from the body as feces.
Aging and the&hellip
Digestive System : From Appetite Suppression to Constipation
Age-related changes in the digestive system begin in the mouth and can affect virtually every aspect of the digestive system. Taste buds become less sensitive, so food isn&rsquot as appetizing as it once was. A slice of pizza is a challenge, not a treat, when you have lost teeth, your gums are diseased, and your salivary glands aren&rsquot producing enough saliva. Swallowing can be difficult, and ingested food moves slowly through the alimentary canal because of reduced strength and tone of muscular tissue. Neurosensory feedback is also dampened, slowing the transmission of messages that stimulate the release of enzymes and hormones.
Pathologies that affect the digestive organs&mdashsuch as hiatal hernia, gastritis, and peptic ulcer disease&mdashcan occur at greater frequencies as you age. Problems in the small intestine may include duodenal ulcers, maldigestion, and malabsorption. Problems in the large intestine include hemorrhoids, diverticular disease, and constipation. Conditions that affect the function of accessory organs&mdashand their abilities to deliver pancreatic enzymes and bile to the small intestine&mdashinclude jaundice, acute pancreatitis, cirrhosis, and gallstones.
In some cases, a single organ is in charge of a digestive process. For example, ingestion occurs only in the mouth and defecation only in the anus. However, most digestive processes involve the interaction of several organs and occur gradually as food moves through the alimentary canal (Figure 2).
Figure 2: The digestive processes are ingestion, propulsion, mechanical digestion, chemical digestion, absorption, and defecation.
Some chemical digestion occurs in the mouth. Some absorption can occur in the mouth and stomach, for example, alcohol and aspirin.
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&mdasheither 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.
The digestive system ingests and digests food, absorbs released nutrients, and excretes 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 regulated by neural and hormonal mechanisms.