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Digestive system
- The digestive system is responsible for the intake, processing, and absorption of ingested nutrients and water.
- It is critical to the provision of substrates, minerals, vitamins, and water and to the regulation and integration of metabolic processes throughout the body.
- Normal function of the digestive system is necessary for whole-body homeostasis as well as normal functioning of individual organ systems.
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General principles of physiology demonstrated
- 1. First, the endocrine, neural, and paracrine control of gastrointestinal function illustrates the general principle of physiology that information flow between cells, tissues, and organs is an essential feature of homeostasis and allows for integration of physiological processes.
- This is highlighted by the intimate relationship between the absorptive capacity of the gastrointestinal tract and the circulatory and lymphatic systems as pathways to deliver these nutrients to the tissues.
- 2. Second, many of the functions of the gastrointestinal tract illustrate the general principle of physiology that most physiological functions are controlled by multiple regulatory systems, often working in opposition.
- For example, the acidity of the contents of the stomach is increased or decreased by the influence of hormones released from the gastrointestinal tract as well as paracrine factors and neuronal inputs.
- 3. Third, the epithelium of the digestive tract regulates the transfer of materials from the environment to the blood, which exemplifies the general principle of physiology that controlled exchange of materials occurs between compartments and across cellular membranes.
- 4. Fourth, the very process of digestion depends on basic chemistry, reflecting yet another general principle of physiology, that physiological processes are dictated by the laws of chemistry and physics.
- 5. Finally, this chapter has many examples of how form and function are related at all levels of structure from cells to organs of the digestive system, which illustrates the general principle of physiology that structure is a determinant of—and has coevolved with—function.
- One of the most vivid examples is the large surface area for absorption of ingested materials made possible by the morphological specializations of the small intestine.
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15.1 Overview of the Digestive System
Digestive system
- The digestive system (Figure 15.1) includes the gastrointestinal (GI) tract (or alimentary canal), consisting of the mouth, pharynx, esophagus, stomach, small intestine, and large intestine;
- and the accessory organs and tissues, consisting of the salivary glands, liver, gallbladder, and exocrine pancreas.
- The accessory organs are not part of the tract but secrete substances into it via connecting ducts.
- The overall function of the digestive system is to process ingested foods into molecular forms that are then transferred, along with small molecules, ions, and water, to the body’s internal environment, where the circulatory system can distribute them to cells.
- The functions of the digestive system can be described in terms of these four major processes—secretion, digestion, absorption, and motility—and the mechanisms controlling them (Figure 15.3)
- Figure 15.1 Anatomy of the digestive system. The liver overlies the gallbladder and a portion of the stomach, and the stomach overlies part of the pancreas. The position of the trachea is shown for orientation; it is not part of the digestive system.
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Gastrointestinal tract
- The adult gastrointestinal tract is a tube approximately 9m (30 feet) in length, running through the body from mouth to anus.
- This is the approximate length in human cadavers; in life, the tract is shorter due to tonic contractions of the smooth muscle in its walls.
- The lumen of the tract is continuous with the external environment, which means that its contents are technically outside the body. This fact is relevant to understanding some of the tract’s properties.
- For example, the large intestine is colonized by billions of bacteria, most of which are harmless and even beneficial in this location. However, if the same bacteria enter the internal environment, as may happen, for example, if a portion of the large intestine is perforated, they may cause a severe infection
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Digestive system properties
- Most food enters the gastrointestinal tract as large particles containing macromolecules, which are unable to cross the intestinal epithelium.
- Before ingested food can be absorbed, it must be dissolved and broken down into small molecules.
- (Small nutrients such as vitamins and minerals do not need to be broken down and can cross the epithelium intact.)
- This dissolving and breakingdown process is called digestion and is accomplished by the action of a variety of digestive enzymes and other chemicals released by the system’s exocrine glands.
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Digestive enzymes
- Polysaccharide digestion is initiated by the enzyme amylase,
- Triglyceride digestion by the enzyme lipase, and
- Proteins are digested by a variety of proteases.
- These enzymes are released into the lumen of the GI tract through the process of secretion, in which the enzymes are released from exocrine cells by exocytosis into a duct that connects to the GI tract.
- In addition, some digestive enzymes are located on the apical membranes of the intestinal epithelium.
- Figure 15.2 Overview of the major macromolecules ingested, enzymes that digest them (in blue), and the resulting products of digestion. Details of enzyme locations and activity will be described later in the chapter.
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Absorption
- The molecules produced by digestion, along with water and small nutrients that do not require digestion, then move from the lumen of the gastrointestinal tract across a layer of epithelial cells and enter the interstitial fluid. This process is called absorption.
- Absorbed fats and fat-soluble nutrients are taken up by lymphatic vessels that ultimately empty into large veins near the heart (see Figure 12.50).
- All other absorbed nutrients enter directly into capillaries that drain into veins and eventually merge to form the hepatic portal vein.
- The hepatic portal vein drains into the liver, as will be described in more detail later in this chapter.
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Contractions of smooth muscles in the gastrointestinal tract wall
- While secretion, digestion, and absorption are taking place, contractions of smooth muscles in the gastrointestinal tract wall occur, where they serve two functions:
- 1. They mix the luminal contents with the various secretions, and
- 2. they move the contents through the tract from mouth to anus.
- These contractions are referred to as the motility of the gastrointestinal tract.
- In some cases, muscular movements travel in a wavelike fashion in one
- direction along the length of a part of the tract, a process called peristalsis, or peristaltic waves.
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Figure 15.3 The four major processes of the gastrointestinal tract:
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- Figure 15.3 The four major processes of the gastrointestinal tract: secretion, digestion, absorption, and motility.
- Inward-pointing (red) arrows represent the secretion of ions, enzymes, and bile salts into the GI tract.
- Outward-pointing (black) arrows indicate absorption of the products of digestion, water, minerals, and vitamins into the blood.
- The length and numbers of the arrows indicate the relative importance of each segment of the tract; the small intestine is where most digestion, absorption, and secretion occurs.
- The feces represent a fifth function of the GI tract: elimination.
- The wavy configuration of the small intestine represents muscular contractions (motility) throughout the tract.
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Digestion- mouth to stomach
- Digestion begins with chewing in the mouth, where large pieces of food are broken up into smaller particles, mixed with secretions of the salivary glands, and formed into a rounded mass referred to as a bolus (from Greek, meaning “lump”).
- The next segments of the tract, the pharynx and esophagus, do not contribute significantly to digestion, but the muscles in the walls of these
- segments participate in the swallowing reflex that moves ingested food and drink to the stomach.
- The stomach is a saclike organ that functions to store, dissolve, and partially digest the macromolecules in food, and to regulate the rate at which it empties its contents into the small intestine.
- Secretion of hydrochloric acid by cells in the wall of the stomach creates a highly acidic gastric (adjective for “stomach”) environment.
- The mixture of ingested food particles and gastric secretions is referred to as chyme.
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Digestion- small intestine
- The majority of the digestive process occurs in the next section of the tract, the small intestine.
- It is by far the longest section, and is divided into three regions.
- The duodenum receives chyme from the stomach, the jejunum is the middle section, and the ileum is the terminal region from which chyme enters the
- large intestine.
- The surface area of the small intestine is vastly increased by fingerlike projections called villi (singular villus), which will be described in detail later in the chapter.
- Aided by secretions that flow via ducts from the liver and pancreas, the
- small intestine digests all the categories of macromolecules into molecules small enough to be transported through the epithelial lining of the tract wall.
- These molecules, along with vitamins, minerals, and water, then enter blood and/or lymphatic vessels.
- Water-soluble substances absorbed into the blood travel first to the liver, where metabolic processing and storage of some nutrients occur. The liver also metabolizes and aids in the elimination of foreign and potentially toxic substances ingested with a meal.
- Water-insoluble substances, such as triglycerides, are absorbed into lymphatic vessels, which bypass the liver and enter the cardiovascular system in large veins near the heart (review Figure 12.50)
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Digestion: large intestine
- The final section of the tract, the large intestine, concentrates undigested material by absorbing most of the remaining ions and water.
- It also serves to temporarily store the undigested material until it is expelled from the body in the feces.
- The large intestine contains billions of beneficial bacteria that metabolize some of the substances not absorbed by the small intestine.
- Some products of bacterial metabolism are absorbed into the blood and provide important health benefits to other body systems.
- Most of the fecal mass consists of these bacteria; in this way, their populations are kept in balance.
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Absorption
- Within fairly wide limits, the digestive system will absorb as much of any particular substance that is ingested, with a few important exceptions (to be described later).
- Therefore, the digestive system does not regulate the total amount of most nutrients absorbed or their concentrations in the internal environment.
- The plasma concentration and distribution of the absorbed nutrients throughout the body are primarily controlled by hormones from a number of endocrine glands and by the kidneys.
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Elimination
- Small amounts of certain metabolic end products are excreted via the gastrointestinal tract, primarily by way of the liver.
- This represents a relatively minor function of the GI tract in healthy individuals—elimination.
- In fact, the lungs and kidneys are usually responsible for eliminating most of the body’s waste products, such as CO2 and urea.
- The feces leave the system via the anus at the end of the gastrointestinal tract.
- Feces consist almost entirely of bacteria and ingested material that was neither digested nor absorbed, and therefore was never actually within the internal environment.
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GI tract immune functions
- The GI tract also has a variety of immune functions, allowing it to produce antibodies and fight ingested microorganisms that are not destroyed by the acidity of the stomach.
- For example, the small intestine has regions of lymphatic nodules that contain immune cells; these cells secrete factors that alter intestinal motility
- and kill microorganisms.
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Average amounts of solids and fluid ingested, secreted, absorbed, and excreted from the gastrointestinal tract daily
- The average American adult consumes about 500–800g of food and 1200 mL of water per day, but this is only a fraction of the material entering the lumen of the gastrointestinal tract.
- An additional 7000 mL of fluid from salivary glands, gastric glands, pancreas,
- liver, and intestinal glands is secreted into the tract each day (Figure 15.4).
- Of the approximately 8 L of fluid entering the tract each day, as much as 99% is absorbed; only about 100–200 mL is normally lost in the feces.
- This small amount of fluid loss represents only 4% of the total fluids lost from the body each day.
- Most fluid loss is via the kidneys and respiratory system.
- Almost all the ions in the fluids that are secreted into the GI tract are also reabsorbed into the blood.
- Moreover, the secreted digestive enzymes are themselves digested, and the resulting amino acids are absorbed into the blood.
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Summary of the general functions of the digestive system
- 1. Ingestion of foods and liquids containing nutrients
- 2. Digestion of large molecules in ingested food into absorbable molecular forms
- 3. Absorption of nutrients from the gut into the internal environment
- 4. Metabolic transformation of fuel molecules and detoxification of foreign substances (liver)
- 5. Elimination of small amounts of metabolic end products secreted by the liver
- 6. Carrying out a variety of immune functions, including production of antibodies and fighting infectious microorganisms not destroyed by the acidity of the stomach
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