Essay on Digestive System
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100 Words Essay on Digestive System
Introduction to digestive system.
The digestive system is a group of organs that work together to change the food we eat into energy for our bodies. It’s like a food processing factory. It includes the mouth, esophagus, stomach, small intestine, large intestine, rectum, and anus.
Process of Digestion
Digestion starts in the mouth when we chew food. It then travels down the esophagus to the stomach. In the stomach, food is mixed with stomach acids to break it down into a liquid. This liquid then moves to the small intestine.
Role of Small Intestine
The small intestine plays a major role in digestion. Here, nutrients from the liquid food are absorbed into the bloodstream. The blood then carries these nutrients to all parts of the body. The leftover food, which the body can’t use, moves to the large intestine.
Role of Large Intestine
The large intestine is the last part of the digestive process. It absorbs water from the leftover food and turns it into waste. This waste then leaves the body through the rectum and anus. This whole process is known as digestion.
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250 Words Essay on Digestive System
What is the digestive system.
The digestive system is a group of organs that work together to change the food you eat into energy and basic nutrients to power your body. It is like a food processing plant that takes in raw materials (food) and turns them into something the body can use.
Parts of the Digestive System
The digestive system is made up of several parts. It starts with the mouth, where you chew and swallow your food. Then there’s the esophagus, a tube that carries food to your stomach. The stomach is like a mixer, churning and breaking down food into a liquid.
How Food Travels
From the stomach, the liquid food then goes into the small intestine. Here, it is broken down even more so your body can absorb the nutrients. Finally, what’s left goes into the large intestine, and then out of your body as waste.
The Role of the Liver and Pancreas
The liver and the pancreas also play important roles in digestion. The liver makes a juice called bile that helps to break down fats. The pancreas makes juices that help to break down carbohydrates, fats, and proteins.
Importance of the Digestive System
The digestive system is very important. Without it, our bodies wouldn’t get the nutrients they need. It keeps us healthy and gives us energy. So, remember to eat a balanced diet to keep your digestive system happy and healthy.
500 Words Essay on Digestive System
The digestive system: an introduction.
The digestive system is a group of organs that work together to change the food we eat into energy our bodies can use. It’s like a food processing factory inside our body. It includes the mouth, esophagus, stomach, small intestine, large intestine, rectum, and anus. The liver and pancreas also play a big role in digestion.
Starting Point: The Mouth
Digestion begins in the mouth. When we eat, our teeth break down the food into smaller pieces. Our saliva, a liquid made by the salivary glands, mixes with these pieces, making them easier to swallow. Saliva also starts the process of breaking down the food chemically.
The Esophagus: The Food Pipe
The esophagus is a long tube that connects the mouth to the stomach. It uses a process called peristalsis to move food. This process is like a wave of muscle contractions that pushes the food down into the stomach.
The Stomach: The Mixing Pot
The stomach is like a mixing pot. Here, the food is mixed with stomach acid and enzymes, which break it down into a liquid. This liquid is then sent to the small intestine.
The Small Intestine: The Nutrient Absorber
The small intestine is where most of the digestion happens. It is a long, coiled tube where nutrients from the food are absorbed into the bloodstream. The liver and pancreas help in this process. The liver makes bile, a substance that helps break down fats. The pancreas makes enzymes, which assist in breaking down proteins and carbohydrates.
The Large Intestine: The Water Saver
The large intestine, also known as the colon, is the final part of the digestive system. Its job is to absorb water from the remaining indigestible food matter, and then to pass useless waste material from the body.
The End of the Journey: The Rectum and Anus
The rectum and anus are the last parts of the digestive system. The rectum stores the waste until it’s ready to leave the body. Then, it passes through the anus and out of the body as feces.
Conclusion: The Importance of the Digestive System
The digestive system is vital for our survival. It turns the food we eat into nutrients that our body needs for energy, growth, and cell repair. Without it, we wouldn’t be able to live. So, next time when you are eating your favorite food, remember the amazing journey it takes through your body!
Remember, eating a balanced diet and drinking plenty of water can help keep your digestive system healthy and working well. Regular exercise is also important as it helps keep food moving through the digestive system, reducing the risk of constipation.
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10.8: Case Study Conclusion: Pressure and Chapter Summary
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- Suzanne Wakim & Mandeep Grewal
- Butte College
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Case Study Conclusion: Under Pressure
As you learned in this chapter, the human body consists of many complex systems that normally work together efficiently like a well-oiled machine to carry out life’s functions. For example, Figure \(\PageIndex{1}\) illustrates how the brain and spinal cord are protected by layers of membrane called meninges and fluid that flows between the meninges and in spaces called ventricles inside the brain. This fluid is called cerebrospinal fluid (CSF) and as you have learned, one of its important functions is to cushion and protect the brain and spinal cord, which make up most of the central nervous system (CNS). CSF additionally circulates nutrients and removes waste products from the CNS. CSF is produced continually in the ventricles, circulates throughout the CNS, and then is reabsorbed by the bloodstream. If too much CSF is produced, its flow blocked, or if not enough is reabsorbed, the system becomes out of balance, and CSF can build up in the ventricles. This causes an enlargement of the ventricles called hydrocephalus that can put pressure on the brain, resulting in the types of neurological problems that former professional football player, Dayo, described at the beginning of this chapter, is suffering from.
Recall that Dayo’s symptoms included loss of bladder control, memory loss, and difficulty walking. The cause of their symptoms was not immediately clear, although their doctors suspected that it related to the nervous system since the nervous system acts as the control center of the body, controlling and regulating many other organ systems. Dayo’s memory loss directly implicated the involvement of the brain, since that is the site of thoughts and memory. The urinary system is also controlled in part by the nervous system, and the inability to hold urine appropriately can be a sign of a neurological issue. Dayo’s trouble walking involved the muscular system, which works alongside the skeletal system to enable movement of the limbs. In turn, the contraction of muscles is regulated by the nervous system. You can see why a problem in the nervous system can cause a variety of different symptoms by affecting multiple organ systems in the human body.
To try to find the exact cause of Dayo’s symptoms, their doctors performed a lumbar puncture, or spinal tap, which is the removal of some CSF through a needle inserted into the lower part of the spinal canal. Doctors then analyzed Dayo’s CSF for the presence of pathogens such as bacteria to determine whether an infection was the cause of their neurological symptoms. When no evidence of infection was found, Doctors used an MRI to observe the structures of Dayo's brain. This is when Doctors discovered Dayo's enlarged ventricles, which are a hallmark of hydrocephalus.
To treat Dayo’s hydrocephalus, a surgeon implanted a device called a shunt in Dayo's brain to remove the excess fluid (Figure \(\PageIndex{2}\)). One side of the shunt consists of a small tube, called a catheter , which was inserted into Dayo’s ventricles. Excess CSF is then drained through a one-way valve to the other end of the shunt, which was threaded under their skin to their abdominal cavity, where the CSF is released and can be reabsorbed by the bloodstream.
Implantation of a shunt is the most common way to treat hydrocephalus, and for some people, it can allow them to recover almost completely. However, there can be complications associated with a brain shunt. The shunt can have mechanical problems or cause an infection. Also, the rate of draining must be carefully monitored and adjusted to balance the rate of removal of CSF with the rate of its production. If it is drained too fast, it is called overdraining, and if it is drained too slowly, it is called underdraining . In the case of underdraining, the pressure on the brain and associated neurological symptoms will persist. In the case of overdraining, the ventricles can collapse, which can cause serious problems such as the tearing of blood vessels and hemorrhaging. To avoid these problems, some shunts have an adjustable pressure valve where the rate of draining can be adjusted by placing a special magnet over the scalp. You can see how the proper balance between CSF production and removal is so critical – both in the causes of hydrocephalus and in its treatment.
In what other ways does your body regulate balance, or maintain a state of homeostasis? In this chapter, you learned about the feedback loops that keep body temperature and blood glucose within normal ranges. Other important examples of homeostasis in the human body are the regulation of the pH in the blood and the balance of water in the body. You will learn more about homeostasis in different body systems in the coming chapters.
Thanks to Dayo’s shunt, their symptoms are starting to improve, but they have not fully recovered. Time may tell whether the removal of the excess CSF from their ventricles will eventually allow them to recover normal functioning or whether permanent damage to their nervous system has already been done. The flow of CSF might seem simple but when it gets out of balance, it can easily wreak havoc on multiple organ systems because of the intricate interconnectedness of the systems within the human “machine."
Chapter Summary
This chapter provided an overview of the organization and functioning of the human body. You learned that:
- The human body consists of multiple parts that function together to maintain life. The biology of the human body incorporates the body’s structure, or anatomy, and the body’s functioning, or physiology.
- The organization of the human body is a hierarchy of increasing size and complexity, starting at the level of atoms and molecules and ending at the level of the entire organism.
- Variations in cell function are generally reflected in variations in cell structure.
- Some cells are unattached to other cells and can move freely; others are attached to each other and cannot move freely. Some cells can divide readily and form new cells; others can divide only under exceptional circumstances. Many cells are specialized to produce and secrete particular substances.
- All the different cell types within an individual have the same genes. Cells can vary because different genes are expressed depending on the cell type.
- Many common types of human cells consist of several subtypes of cells, each of which has a special structure and function. For example, subtypes of bone cells include osteocytes, osteoblasts, osteogenic cells, and osteoclasts.
- Connective tissues, such as bone and blood, are made up of cells that are separated by non-living material, called the extracellular matrix.
- Epithelial tissues, such as skin and mucous membranes, protect the body and its internal organs and secrete or absorb substances.
- Muscle tissues are made up of cells that have the unique ability to contract. They include skeletal, smooth, and cardiac muscle tissues.
- Nervous tissues are made up of neurons, which transmit electrical messages, and glial cells of various types, which play supporting roles. Types of nervous tissues include gray matter, white matter, nerves, and ganglia.
- Many organs are composed of a major tissue that performs the organ’s main function, as well as other tissues that play supporting roles.
- The human body contains five organs that are considered vital for survival. They are the heart, brain, kidneys, liver, and lungs. If any of these five organs stops functioning, the death of the organism is imminent without medical intervention.
- There are 11 major organ systems in the human organism. They are the integumentary, skeletal, muscular, nervous, endocrine, cardiovascular, lymphatic, respiratory, digestive, urinary, and reproductive systems. Only the reproductive system varies significantly between males and females.
- The ventral cavity is at the anterior, or front, of the trunk. It is subdivided into the thoracic cavity and abdominopelvic cavity.
- The dorsal cavity is at the posterior, or back, of the body, and includes the head and the back of the trunk. It is subdivided into the cranial cavity and spinal cavity.
- Cellular respiration is a good example of organ system interactions because it is a basic life process that occurs in all living cells. It is the intracellular process that breaks down glucose with oxygen to produce carbon dioxide and energy. Cellular respiration requires the interaction of the digestive, cardiovascular, and respiratory systems.
- The fight-or-flight response is a good example of how the nervous and endocrine systems control other organ system responses. It is triggered by a message from the brain to the endocrine system and prepares the body for flight or a fight. Many organ systems are stimulated to respond, including the cardiovascular, respiratory, and digestive systems.
- Digesting food requires teamwork between the digestive system and several other organ systems, including the nervous, cardiovascular, and muscular systems.
- Playing softball or doing other voluntary physical activities may involve the interaction of nervous, muscular, skeletal, respiratory, and cardiovascular systems.
- For any given variable, such as body temperature, there is a particular set point that is the physiological optimum value. The spread of values around the setpoint that is considered insignificant is called the normal range.
- Homeostasis is generally maintained by a negative feedback loop that includes a stimulus, sensor, control center, and effector. Negative feedback serves to reduce an excessive response and to keep a variable within the normal range. Negative feedback loops control body temperature and blood glucose level.
- Sometimes homeostatic mechanisms fail, resulting in homeostatic imbalance. Diabetes is an example of a disease caused by homeostatic imbalance. Aging can bring about a reduction in the efficiency of the body’s control system, making the elderly more susceptible to disease.
- Positive feedback loops are not common in biological systems. Positive feedback serves to intensify a response until an endpoint is reached. Positive feedback loops control blood clotting and childbirth.
The severe and broad impact of hydrocephalus on the body’s systems highlights the importance of the nervous system and its role as the master control system of the body. In the next chapter, you will learn much more about the structures and functioning of this fascinating and important system.
Chapter Summary Review
- Compare and contrast tissues and organs.
- Osteocyte cells are part of which type of tissue and organ system?
- mucous membranes
- gray matter
- Which type of tissue lines the inner and outer surfaces of the body?
- True or False. The extracellular matrix that surrounds cells is always solid.
- True or False. Skin is an organ.
- What is a vital organ? What happens if a vital organ stops working?
- Name three organ systems that transport or remove wastes from the body.
- Name two types of tissue in the digestive system.
- Processes sensory information
- Secretes hormones
- Releases carbon dioxide from the body to the outside world
- Produces gametes
- Controls water balance in the body
- Integumentary
- Describe one way in which the integumentary and cardiovascular systems work together to regulate homeostasis in the human body.
- Name the two largest body cavities in humans and describe their general locations.
- What are the names given to the three body cavity divisions where the reproductive organs are located?
- True or False. There are two pleural cavities.
- True or False. Body cavities are filled with air.
- The pituitary gland is in which organ system? Describe how the pituitary gland increases metabolism.
- When the level of thyroid hormone in the body gets too high, it acts on other cells to reduce the production of more thyroid hormone. What type of feedback loop does this represent?
- What is the stimulus in this feedback loop?
- If the level of B1 falls significantly below the setpoint, what do you think happens to the production of A1? Why?
- What is the effector in this feedback loop?
- If organs A and B are part of the endocrine system, what type of molecules do you think A1 and B1 are likely to be?
- What are the two main systems that allow various organ systems to communicate with each other?
- spinal cord
- thoracic cavity
- What are two functions of the hypothalamus that you learned about in this chapter?
Attributions
- Brain and Nearby Structures by NIH Image Gallery , public domain via Flickr
- Diagram showing a brain shunt by Cancer Research UK , CC BY 4.0 via Wikimedia Commons
- Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0
The Digestive System in the Human Body Essay
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Introduction
The stomach, large intestines, small intestines, works cited.
Human biology is a field in biology that deals with nutrition, anthropology, and medicine. The body of a human being contains a digestive system that deals with the digestion and absorption of food nutrients into the body. The stomach, small intestines, and large intestines are part of the digestive system. All of them are located at different positions in the body and have different functions and structures.
The stomach is an organ of digestion located between the esophagus and the duodenum on the left upper part of the abdominal cavity. It is a muscular, elastic pear-shaped bag lying just below the diaphragm. It is made up of five layers. The innermost layer is referred to as mucosa. The four sections of the stomach are the fundus, cardia, body, and pylorus. It is involved in the second phase of digestion after chewing. It secretes enzymes that help in the digestion of proteins. It also secretes some acids that help in the digestion of food. The major secretions are hydrochloric acid and protease enzymes. The work of hydrochloric acid is to kill any bacteria that may be in the food and provide an acidic pH that allows proteases to work effectively (Chiras 95).
The large intestines of the human body are located between the ileocecal junction and the rectum. They are about one and a half meters long and two and a half centimeters wide. The major parts of the large intestines are the caecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal (Tortora and Grabowski 46). The large intestines receive food contents and fluids from the small intestines. The large intestines help in the re-absorption of water into the body system. The water absorbed is used to maintain the fluid balance of the body. Some vitamins that are created by the bacteria in the large intestines are also absorbed. They also help in the transport of waste products in form of stool out of the body.
The small intestines are located between the stomach and the large intestines. Specifically, they start at the pyloric sphincter and end at the ileocaecal valve. They are narrow and long to allow digestion and absorption of food. The standard length of the small intestines in an adult human being is five meters but they may be shorter or longer. The three parts of the small intestines are the duodenum, jejunum, and ileum (Tortora and Grabowski 49). The majority of digestion and absorption of food takes place in the small intestines. Chemical digestion takes place in the small intestines where many enzymes that facilitate digestion are secreted. In the small intestines, proteins and peptides are digested into amino acids. Lipase enzymes are used to degrade fats into glycerol and fatty acids. Some carbohydrates are also digested in the stomach to form simple sugars. The process of absorption takes place where food nutrients are diffused into the bloodstream. The small intestines have many villi that have networks of capillaries to help indigestion. Monosaccharides, amino acids, water, iron, and many others are absorbed here.
The digestive system in the human body mainly deals with digestion and absorption of food nutrients and fluids. The stomach is located between the esophagus and the duodenum and helps in the digestion of partially chewed food. The large intestines are located between the ileocecal junction and the rectum. They help in the absorption of water, some nutrients, and the excretion of wastes out of the body. The small intestines are located between the pyloric sphincter and the ileocaecal valve. They help in the digestion of food and absorption of food nutrients into the bloodstream.
Chiras, Daniel. Human Biology. Sixth Edition. London: Jones and Bartlett Punishers, 2009. Print.
Tortora, Gerald and Grabowski, Sandra. Introduction to the human body: the essentials of anatomy and physiology. Volume 1. New York: John Wiley & Sons, 2004. Print.
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Kvietys PR. The Gastrointestinal Circulation. San Rafael (CA): Morgan & Claypool Life Sciences; 2010.
The Gastrointestinal Circulation.
Chapter 12 summary and conclusions.
In animals and man, assimilation of nutrients from the external environment is the responsibility of the gastrointestinal tract. The gastrointestinal tract is a tubular structure, which, in a strict sense, is open at both ends. Samples of the external environment are ingested orally, digested to absorbable constituents within the tract, assimilated and delivered to other organs of the body by the circulation, and the residue expelled from the distal end. The circulation of the gastrointestinal tract is critical for the overall function of this organ system, as exemplified by the fact that it receives one-fourth of the cardiac output.
The gastrointestinal circulation is responsible for supporting the enhanced functional activities associated with assimilation of nutrients, i.e., absorption, secretion, and motility. Intrinsic regulatory mechanisms modulate both resistance vessels (blood flow) and precapillary sphincters (capillary density) to ensure the adequate delivery of O 2 to meet the increased O 2 demand associated with enhanced postprandial functional activity. These regulatory mechanisms also account for the localization of hyperemia to only those regions of the small intestine through which chyme is passing. Hydrolytic products of food digestion in the lumen elicit the postprandial intestinal hyperemia, particularly long chain fatty acids (e.g., oleic acid) and monosaccharides (e.g., glucose).
The fenestrated capillaries of the gastrointestinal mucosa allow for vast amounts of transcapillary movement of solutes and fluid. Intestinal capillaries are highly permeable to small solutes, yet relatively impermeable to macromolecules (e.g., proteins). This permselectivity allows for the maintenance of a fairly constant interstitial volume during absorption by restricting proteins to the intravascular compartment, yet allowing absorbed nutrients and solutes fairly unrestricted transcapillary movement for efficient assimilation. During fluid absorption, appropriate alterations in capillary and interstitial forces allows for the movement of fluid into the capillaries and lymphatics, thereby preventing excessive hydration of the interstitium. Although the capillaries are the major conduits for fluid transport out of the interstitium, the lymphatic contribution is greater during oleic acid absorption than during glucose absorption. Although less well characterized, solute-coupled secretion is believed to also result in appropriate alterations in transcapillary forces governing fluid exchange to prevent interstitial dehydration. The water removed from the interstitium during secretion is derived solely from the capillaries at a rate sufficient to support the secretory process, thereby preventing interstitial dehydration.
The gastrointestinal circulation also contributes to the mucosal defense mechanisms whose function is to protect the mucosa from potential injury induced by noxious substances either ingested (e.g., oleic acid) or produced endogenously (e.g., acid). During an enhanced acid load in either the stomach or upper small intestine, interstitial nociceptor fibers elicit an intense mucosal hyperemia, which serves to dilute/remove the excess protons from the interstitium. A similar reflex appears to be involved in eliciting the hyperemia associated with an enhanced lipid load, albeit not as well characterized. Although the gastrointestinal circulation plays an important defensive role against noxious luminal stresses, it can also be a significant detrimental contributor to mucosal pathology elicited by stresses within the body, such as I/R and PH. The gastrointestinal microcirculation contributes to the inflammatory response elicited by I/R and the vascular congestion and edema induced by PH.
A predominant part of the database for current concepts regarding the physiology and pathophysiology of the gastrointestinal circulation is derived from studies in animals and mathematical modeling approaches. Recent technological advances (e.g., confocal endomicroscopy) should extend this database to humans and provide the necessary information for the development of rational therapeutic approaches aimed at gastrointestinal disorders involving the circulation.
- Cite this Page Kvietys PR. The Gastrointestinal Circulation. San Rafael (CA): Morgan & Claypool Life Sciences; 2010. Chapter 12, Summary and Conclusions.
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21.3: Digestive System Processes and Regulation
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- Whitney Menefee, Julie Jenks, Chiara Mazzasette, & Kim-Leiloni Nguyen
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By the end of the section, you will be able to:
- Discuss seven fundamental activities of the digestive system, giving an example of each
- Describes the functions of each digestive organs
- Describe the difference between mechanical digestion and chemical digestion
- Describe the difference between peristalsis and segmentation
The digestive system uses mechanical and chemical activities to break food down into absorbable substances during its journey through the digestive system. Table \(\PageIndex{1}\) provides an overview of the basic functions of the digestive organs.
Digestive Processes
The processes of digestion include seven activities: ingestion, propulsion, mechanical or physical digestion, chemical digestion, secretion, 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 secreted by salivary glands, 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 (chunk) 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 of circular and longitudinal layers of the muscularis externa (alimentary wall smooth muscles), which act to propel food along (Figure \(\PageIndex{1}\)). 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.
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 “soup” 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.
Chemical digestion is aided by secretion of enzymes. 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...
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’t 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’t 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—such as hiatal hernia, gastritis, and peptic ulcer disease—can 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—and their abilities to deliver pancreatic enzymes and bile to the small intestine—include 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 from the anus. However, most digestive processes involve the interaction of several organs and occur gradually as food moves through the alimentary canal (Figure \(\PageIndex{2}\)). Figure 21.3.2 shows the digestive tract with the locations of propulsion, chemical digestion, mechanical digestion, and absorption in different organs.
While most chemical digestion occurs in the small intestine, some occurs in the mouth (carbohydrates and lipids) and stomach (proteins). Absorption, also largely carried out by the small intestine, some can occur in the mouth, stomach, and large intestine. For example, alcohol and aspirin are absorbed by the stomach and water and many ions are absorbed by the large intestine.
Regulatory Mechanisms
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.
Neural Controls
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 (enteric nervous system, submucosal and myenteric 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.
Hormonal Controls
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 enteroendocrine cells, 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.
Concept Review
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 (mouth), motility (GI tract), mechanical digestion (mouth, stomach, small intestine), chemical digestion (mouth, stomach, small intestine), absorption (mouth, stomach, small and large intestines), and defecation (anus). Contractions of smooth muscles (muscularis externa) result in peristalsis to push contents along in the GI tract and segmentation to mix the content with enzymes. These processes are regulated by neural and hormonal mechanisms.
Review Questions
Q. Which of these processes occurs in the mouth?
A. ingestion
B. mechanical digestion
C. chemical digestion
D. all of the above
Q. Which of these processes occurs throughout most of the alimentary canal?
B. propulsion
C. segmentation
D. absorption
Q. Which of the following occur(s) in the mouth?
A. mechanical digestion
B. chemical digestion
C. mastication
Q. Which of these statements about the colon is false?
A. Chemical digestion occurs in the colon.
B. Absorption occurs in the colon.
C. Peristalsis occurs in the colon.
D. Diverticular disease occurs in the colon.
Critical Thinking Questions
Q. Offer a theory to explain why segmentation occurs and peristalsis slows in the small intestine.
A. The majority of digestion and absorption occurs in the small intestine. By slowing the transit of chyme, segmentation and a reduced rate of peristalsis allow time for these processes to occur.
Q. Which organ is mostly responsible for diarrhea and constipation and why?
A. The colon absorbs water. If it absorbs too much water, then the remaining contents (stool) may be hard and constipation may result. If it absorbs very little water or even secretes water, then the remaining contents will be loose and watery, resulting in diarrhea.
Contributors and Attributions
OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology
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Digestive System And Function
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