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12.4: Digestion and the Hormonal Control of Metabolism

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    137595
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    Learning Objectives
    • Distinguish between digestion and metabolism.
    • Described the process of digestion, noting the roles of the mouth, stomach, intestines (small and large), and accessory organs.
    • Describe how hormones regulate metabolism.
    • Describe the three phases of digestion.

    Overview

    While we know that eating is motivated by the drive state of hunger, what is being regulated? What is the system variable - or variables - that are monitored?

    Food:

    • Provides energy.
    • Provides the building blocks that we use to construct and maintain our bodies - we don't just need calories (energy, fuel), we need to eat certain things - ideally we are monitoring not only the quantity (how much) of food we eat, but the quality (what we eat). In other words, an ideal system for the control of our eating would be one that triggers a hunger that is specific - ensuring that we both get the quantity we need to meet our energy needs, but also the right types of foods to meet our nutritional needs.

    The process by which we obtain the substances that the body needs begins with the consumption of food and is achieved when the nutrients provided by food are absorbed. This is the process of digestion, the process of ingesting, breaking down food, and absorbing the nutrients.

    Ultimately, food is delivered to the body in three forms:

    • Lipids (fats),
    • amino acids (proteins), and
    • glucose (carbohydrates).

    These fuels enter the body by way of the digestive tract, but the digestive tract is often empty. Because of this, and the importance of having fuel available, we have mechanisms that allow us to store energy to support us. This stored energy is primarily fats, but also glycogen and proteins. Before discussing how we store energy (and access stored energy), we need to understand the process of digestion.

    The Human Digestive System

    The process of digestion begins in the mouth with the intake of food (Figure \(\PageIndex{1}\)). The teeth play an important role in masticating (chewing) or physically breaking food into smaller particles. The enzymes present in saliva also begin to chemically break down food. The food is then swallowed and enters the esophagus - a long tube that connects the mouth to the stomach. Using peristalsis, or wave-like smooth-muscle contractions, the muscles of the esophagus push the food toward the stomach. The stomach contents are extremely acidic. This acidity kills microorganisms, breaks down food tissues, and activates digestive enzymes. Further breakdown of food takes place in the small intestine where bile produced by the liver, and enzymes produced by the small intestine and the pancreas, continue the process of digestion. The smaller molecules are absorbed into the blood stream through the epithelial cells lining the walls of the small intestine. The waste material travels on to the large intestine where water is absorbed and the drier waste material is compacted into feces; it is stored until it is excreted through the anus.

    The basic components of the human digestive system begins at the mouth. Food is swallowed through the esophagus and into the kidney-shaped stomach. The liver is located on top of the stomach, and the pancreas is underneath. Food passes from the stomach to the long, winding small intestine. From there it enters the wide large intestine before passing out the anus. At the junction of the small and large intestine is a pouch called the cecum.
    Figure \(\PageIndex{1}\): The components of the human digestive system are shown.

    Oral Cavity

    Both physical and chemical digestion begin in the mouth or oral cavity, which is the point of entry of food into the digestive system. The food is broken into smaller particles by mastication, the chewing action of the teeth. Almost all mammals have teeth and can chew their food to begin the process of physically breaking it down into smaller particles. (Mammals without teeth have very limited diets - ants or plankton - and include pangolins, anteaters and some species of whales.)

    The chemical process of digestion begins during chewing as food mixes with saliva, produced by the salivary glands. Saliva contains mucus that moistens food and buffers the pH of the food. Saliva also contains enzymes that begin the process of breaking down some foods. The chewing and wetting action provided by the teeth and saliva prepare the food into a mass called the bolus for swallowing. The tongue helps in swallowing - moving the bolus from the mouth into the pharynx. The pharynx opens to two passageways: the esophagus and the trachea. The esophagus leads to the stomach and the trachea leads to the lungs. The epiglottis is a flap of tissue that covers the tracheal opening during swallowing to prevent food from entering the lungs.

    Esophagus

    The esophagus is a tubular organ that connects the mouth to the stomach. The chewed and softened food passes through the esophagus after being swallowed. The smooth muscles of the esophagus undergo peristalsis that pushes the food toward the stomach. The peristaltic wave moves food from the mouth to the stomach, and reverse movement is not possible, except in the case of vomiting (initiated by the gag reflex). The peristaltic movement of the esophagus is an involuntary reflex; it takes place in response to the act of swallowing.

    Ring-like muscles called sphincters form valves in the digestive system. The gastro-esophageal sphincter (or cardiac sphincter) is located at the stomach end of the esophagus. In response to swallowing and the pressure exerted by the bolus of food, this sphincter opens, and the bolus enters the stomach. When there is no swallowing action, this sphincter is shut and prevents the contents of the stomach from traveling up the esophagus. Acid reflux or “heartburn” occurs when the acidic digestive juices escape into the esophagus.

    Stomach

    A large part of protein digestion occurs in the stomach. The stomach is a saclike organ that secretes gastric digestive juices.

    Protein digestion is carried out by an enzyme called pepsin in the stomach chamber. The highly acidic environment kills many microorganisms in the food and, combined with the action of the enzyme pepsin, results in the catabolism (break down) of protein in the food. Chemical digestion is facilitated by the churning action of the stomach caused by contraction and relaxation of smooth muscles. The partially digested food and gastric juice mixture is called chyme. Gastric emptying occurs within two to six hours after a meal. Only a small amount of chyme is released into the small intestine at a time. The movement of chyme from the stomach into the small intestine is regulated by hormones, stomach distension, and muscular reflexes that influence the pyloric sphincter.

    The stomach lining is unaffected by pepsin and the acidity because pepsin is released in an inactive form and the stomach has a thick mucus lining that protects the underlying tissue.

    Small Intestine

    Chyme moves from the stomach to the small intestine. The small intestine is the organ where the digestion of protein, fats, and carbohydrates is completed. The small intestine is a long tube-like organ with a highly folded surface containing finger-like projections called the villi. The top surface of each villus has many microscopic projections called microvilli. The epithelial cells of these structures absorb nutrients from the digested food and release them to the bloodstream on the other side. The villi and microvilli, with their many folds, increase the surface area of the small intestine and increase absorption efficiency of the nutrients.

    The human small intestine is divided into three parts: the duodenum, the jejunum, and the ileum. The duodenum is separated from the stomach by the pyloric sphincter. The chyme is mixed with pancreatic juices, an alkaline solution rich in bicarbonate that neutralizes the acidity of chyme from the stomach. Pancreatic juices contain several digestive enzymes that break down starches, disaccharides, proteins, and fats. Bile is produced in the liver and stored and concentrated in the gallbladder; it enters the duodenum through the bile duct. Bile contains bile salts, which make lipids accessible to the water-soluble enzymes. The monosaccharides, amino acids, bile salts, vitamins, and other nutrients are absorbed by the cells of the intestinal lining.

    The undigested components of the food are sent to the colon from the ileum via peristaltic movements. The ileum ends and the large intestine begins at the ileocecal valve. The vermiform, “worm-like,” appendix is located at the ileocecal valve.

    Large Intestine

    The large intestine reabsorbs the water from indigestible food material and processes the waste material (Figure \(\PageIndex{2}\)). Compared to the small intestine, the human large intestine is much smaller in length but larger in diameter. It has three parts: the cecum, the colon, and the rectum. The cecum joins the ileum to the colon and is the receiving pouch for the waste matter. The colon is home to many bacteria or “intestinal flora” that aid in the digestive processes. The colon has four regions, the ascending colon, the transverse colon, the descending colon and the sigmoid colon. The main functions of the colon are to extract the water and mineral salts from undigested food components, and to store waste material.

    Illustration shows the structure of the large intestine, which begins with the ascending colon. Below the ascending colon is the cecum. The vermiform appendix is a small projection at the bottom of the cecum. The ascending colon travels up the right side of the body, then turns into the transverse colon. On the left side of the body the large intestine turns again, into the descending colon. At the bottom, the descending colon curves up; this part of the intestine is called the sigmoid colon. The sigmoid colon empties into the rectum. The rectum travels straight down, to the anus.
    Figure \(\PageIndex{2}\): The large intestine reabsorbs water from undigested food and stores waste until it is eliminated. (credit: modification of work by Mariana Ruiz Villareal)

    The rectum (Figure \(\PageIndex{2}\)) stores feces until defecation. The feces are propelled using peristaltic movements during elimination. The anus is an opening at the far-end of the digestive tract and is the exit point for the waste material. Two sphincters regulate the exit of feces, the inner sphincter is involuntary and the outer sphincter is voluntary.

    Accessory Organs

    The organs discussed above are the organs of the digestive tract through which food passes. Accessory organs add secretions and enzymes that break down food into nutrients. Accessory organs include the salivary glands, the liver, the pancreas, and the gall bladder (Figure \(\PageIndex{3}\)). The secretions of the liver, pancreas, and gallbladder are regulated by hormones in response to food consumption.

    The liver is the largest internal organ in humans and it plays an important role in digestion of fats and detoxifying blood. The liver produces bile, a digestive juice that is required for the breakdown of fats in the duodenum. The liver also processes the absorbed vitamins and fatty acids and synthesizes many plasma proteins. The gallbladder is a small organ that aids the liver by storing bile and concentrating bile salts.

    The pancreas secretes bicarbonate that neutralizes the acidic chyme and a variety of enzymes for the digestion of protein and carbohydrates.

    Illustration shows the human lower digestive system, which begins with the stomach, a sac that lies above the large intestine. The stomach empties into the small intestine, which is a long, highly folded tube. The beginning of the small intestine is called the duodenum, the long middle part is called the jejunum, and the end is called the ileum. The ileum empties into the large intestine on the right side of the body. Beneath the junction of the small and large intestine is a small pouch called the cecum. The appendix is at the bottom end of the cecum. The large intestine travels up the left side of the body, across the top of the small intestine, then down the right side of the body. These parts of the large intestine are called the ascending colon, the transverse colon and the descending colon, respectively. The large intestine empties into the rectum, which is connected to the anus. The pancreas is sandwiched between the stomach and large intestine. The liver is a triangular organ that sits above and slightly to the right of the stomach. The gallbladder is a small bulb between the liver and stomach.
    Figure \(\PageIndex{3}\): The stomach has an extremely acidic environment where most of the protein gets digested. (credit: modification of work by Mariana Ruiz Villareal)

    Nutrition

    The human diet should be well-balanced to provide nutrients required for bodily function and the minerals and vitamins required for maintaining structure and regulation necessary for good health and reproductive capability. The organic molecules required for building cellular material and tissues must come from food.

    During digestion, digestible carbohydrates are ultimately broken down into glucose and used to provide energy to the cells of the body and the brain. Complex carbohydrates can be broken down into glucose through biochemical modification; however, humans do not produce the enzyme necessary to digest fiber. The intestinal bacteria in the human gut are able to extract some nutrition from these plant fibers. These plant fibers are known as dietary fiber and are an important component of the diet. The excess sugars in the body are converted into glycogen and stored for later use in the liver and muscle tissue. Glycogen stores are used to fuel prolonged exertions, such as long-distance running, and to provide energy during food shortage. Fats are stored under the skin of mammals for insulation and energy reserves and provide cushioning and protection for many organs.

    Proteins in food are broken down during digestion and the resulting amino acids are absorbed. All of the proteins in the body must be formed from these amino acid constituents; no proteins are obtained directly from food.

    Fats add flavor to food and promote a sense of satiety or fullness. Fatty foods are also significant sources of energy, and fatty acids are required for the construction of lipid membranes. Fats are also required in the diet to aid the absorption of fat-soluble vitamins and the production of fat-soluble hormones.

    While the human body can synthesize many of the molecules required for function from precursors, there are some nutrients that must be obtained from food. These nutrients are termed essential nutrients, meaning they must be eaten, because the body cannot produce them.

    Metabolism - An Overview

    Metabolism is the process of making energy available for use. If your metabolism is high, you are breaking stored fuel down at a high rate - you are burning more calories. Note that the different forms of stored energy are found in different places in the body and the flow of energy into and out of storage is under the control of hormones. What hormones are involved directly in this process? Glycogen is the form in which carbohydrate is stored in cells of the liver and muscles. Ingested carbohydrates, in the form of glucose, are converted into glycogen by insulin and then stored. Insulin both promotes the storage of glucose as glycogen and allows body cells to use glucose. In other words, the cells of the body do not have access to glucose if insulin is not present. While insulin promotes the storage and use of glucose, glucagon (a hormone created in the pancreas) promotes the conversion of stored fuels to a form that can be readily used. In other words, if glucose is gone, glucagon makes more fuel available.

    Regulation of Blood Glucose Levels by Insulin and Glucagon

    Blood glucose levels vary widely over the course of a day as periods of food consumption alternate with periods of fasting. Insulin and glucagon are the two hormones primarily responsible for maintaining homeostasis of blood glucose levels. Additional regulation is mediated by the thyroid hormones.

    Cells of the body require nutrients in order to function, and these nutrients are obtained through feeding. Excess intake is converted to stores and removed when needed. Hormones moderate our energy stores. Insulin is produced by the beta cells of the pancreas, which are stimulated to release insulin as blood glucose levels rise (for example, after a meal is consumed). Insulin lowers blood glucose levels by enhancing the rate of glucose uptake and use by cells. Insulin also stimulates the liver to convert glucose to glycogen, which is then stored by cells for later use. Some cells, including those in the kidneys and brain, can access glucose without the use of insulin. Insulin also stimulates the conversion of glucose to fat in adipocytes and the synthesis of proteins. These actions mediated by insulin cause blood glucose concentrations to fall, called a hypoglycemic “low sugar” effect, which inhibits further insulin release from beta cells through a negative feedback loop.

    Impaired insulin function can lead to a condition called diabetes mellitus, the main symptoms of which are illustrated in Figure \(\PageIndex{4}\). This can be caused by low levels of insulin production by the beta cells of the pancreas, or by reduced sensitivity of tissue cells to insulin. This prevents glucose from being absorbed by cells, causing high levels of blood glucose, or hyperglycemia (high sugar). High blood glucose levels make it difficult for the kidneys to recover all the glucose from the urine, resulting in glucose being lost in urine. High glucose levels also result in less water being reabsorbed by the kidneys, causing high amounts of urine to be produced; this may result in dehydration. Over time, high blood glucose levels can cause nerve damage to the eyes and peripheral body tissues, as well as damage to the kidneys and cardiovascular system. Oversecretion of insulin can cause hypoglycemia, low blood glucose levels. This causes insufficient glucose availability to cells, often leading to muscle weakness, and can sometimes cause unconsciousness or death if left untreated.

    Symptoms of diabetes include excessive thirst, excessive hunger, lethargy and stupor, blurred vision, weight loss, breath that smells like acetone, hyperventilation, nausea, vomiting, abdominal pain, frequent urination, and glucose in the urine.
    Figure \(\PageIndex{4}\): The main symptoms of diabetes are shown. (credit: modification of work by Mikael Häggström)

    When blood glucose levels decline below normal levels, for example between meals or when glucose is utilized rapidly during exercise, the hormone glucagon is released from the alpha cells of the pancreas. Glucagon raises blood glucose levels, causing what is called a hyperglycemic effect, by stimulating the breakdown of glycogen to glucose in skeletal muscle cells and liver cells in a process called glycogenolysis. Glucose can then be utilized as energy by muscle cells and released into circulation by the liver cells. Glucagon also stimulates absorption of amino acids from the blood by the liver, which then converts them to glucose. This process of glucose synthesis is called gluconeogenesis. Glucagon also stimulates adipose (fat) cells to release fatty acids into the blood. These actions mediated by glucagon result in an increase in blood glucose levels to normal homeostatic levels. Rising blood glucose levels inhibit further glucagon release by the pancreas via a negative feedback mechanism. In this way, insulin and glucagon work together to maintain homeostatic glucose levels, as shown in Figure \(\PageIndex{5}\).

    When blood glucose levels fall, the pancreas secretes the hormone glucagon. Glucagon causes the liver to break down glycogen, releasing glucose into the blood. As a result, blood glucose levels rise. In response to high glucose levels, the pancreas releases insulin. In response to insulin, target cells take up glucose, and the liver converts glucose to glycogen. As a result, blood glucose levels fall.
    Figure \(\PageIndex{5}\): Insulin and glucagon regulate blood glucose levels. (credit: modification of OpenStax image)

    Long-Term Energy Stores

    "Fat" is composed of triglycerides, glycerol and fatty acids. Fat, or adipose, tissue is found beneath the skin and in various areas around the abdominal cavity. The cells in this tissue are able to absorb nutrients from the blood and are quite versatile - changing dramatically in size as the levels of stored triglycerides change. Once the short-term carbohydrate reserve is depleted, triglycerides begin to be converted to a form that cells can use and are released. Free fatty acids can be used for energy by all cells, except those of the central nervous system. Where does the brain get energy in this case? While the cells in the body are using free fatty acids, the liver is taking up the glycerol and converting it to glucose. As discussed earlier, when glucose is gone, we get the release and action of glucagon - a hormone that converts stored fuels to those that can be readily used, like glucose.

    What happens when glucagon is high and insulin is low, when stored fuels are being made available? The available free fatty acids (previously stored in adipose tissue) are converted to ketones which can be used by muscles. The breakdown of fat for energy results in a high level of ketones, a phenomenon that can also occur when one is starving. Normally, ketones are excreted in the urine. When the body is not able to process excess ketones, ketoacidosis occurs. Ketoacidosis literally leads to an increase in the acidity of the body which creates a whole host of problems - even death. Dehydration occurs as the body tries to adjust. What you have is also the movement of water out of cells - which will make more sense once we cover thirst.

    The Three Phases of Digestion

    When we look at the process of digestion, we can recognize three different phases. Each has its own unique hormonal profile.

    • Cephalic - During the cephalic or "head" phase, the body is preparing to eat. The expectation of food, the anticipation of food, leads to the release of insulin. If the smell or sight of food leads to a hunger pang, this just might be a consequence of insulin causing any available glucose to be used or stored. This phase ends when those ingested nutrients are absorbed into the blood stream. So the cephalic phase begins when the body prepares for food ingestion and ends when that ingested food impacts fuel levels in the bloodstream.
    • Absorptive - During the absorptive phase, insulin is actively promoting the use - and storage - of ingested fuels. This phase ends when the unstored energy from the meal has been used.
    • Fasting Phase - When the fuel from the meal is gone, glucagon levels rise and energy stores are converted into usable fuels.

    When there is no fuel available in the blood, when glucose levels drop, energy must be mobilized (removed) from energy stores. What part of the nervous system makes energy available? The sympathetic nervous system, the fight or flight component of the autonomic branch of the peripheral nervous system, causes the secretion of glucagon when there is a lack of fuel in the blood.

    Summary

    The process of digestion begins in the mouth where food begins to be broken down both chemically and physically into the lipids, amino acids, and glucose that are used by the body. Digestion begins in the mouth and ends when fuels are absorbed into the blood stream. Digestion consists of three phases: cephalic, absorptive, and fasting.

    Metabolism, a process controlled by the hormones insulin and glucagon, makes energy available for use. Insulin is produced by the pancreas in response to rising blood glucose levels and allows cells to use the available blood glucose and store excess glucose as glycogen for later use. Diabetes mellitus is caused by reduced insulin activity and causes high blood glucose levels, or hyperglycemia. Glucagon is released by the pancreas in response to low blood glucose levels and stimulates the breakdown of glycogen into glucose, which can be used by the body.

    Attributions (listed in order of appearance)

    "The Human Digestive System" and "Nutrition" adapted from Samantha Fowler, S., Roush, R. & Wise, J, (2013). 16.2 Digestive System - Concepts of Biology. OpenStax. Retrieved April 24, 2022, from https://openstax.org/books/concepts-biology/pages/16-2-digestive-system (CC BY)

    "Hunger, Satiety, and the Brain" adapted from Bhatia, S. & Loewenstein, G. (2022). Drive states. In R. Biswas-Diener & E. Diener (Eds), Noba textbook series: Psychology. Champaign, IL: DEF publishers. Retrieved from http://noba.to/pjwkbt5h (CC BY-NC-SA)

    "Regulation of Blood Glucose Levels by Insulin and Glucagon" adapted from Rye, C., Wise, R., Jurukovski, V., DeSaix, J., Choi, J., & Avissar , Y. (2016, October 21). 37.3 Regulation of Body Processes - Biology. OpenStax. Retrieved February 6, 2022, from https://openstax.org/books/biology/p...body-processes (CC BY)


    This page titled 12.4: Digestion and the Hormonal Control of Metabolism is shared under a mixed license and was authored, remixed, and/or curated by Multiple Authors (ASCCC Open Educational Resources Initiative (OERI)) .