4.3: Other Body Systems Involved

What happens after molecules are absorbed?

When the digestive system has broken down food into its nutrient components, they are ready to be absorbed. Absorption is the movement of the nutrient molecules from the digestive tract into the blood or lymph. If the absorbed nutrient is water soluble, it will be absorbed into the blood, and if the nutrient is fat soluble, it will be packaged into a special transport package called a chylomicron and absorbed into the lymph. For water-soluble nutrients, the first stop is the liver. The liver can store some nutrients, send some to other parts of the body, and/or change some into other molecules the body needs! Fat-soluble nutrients, because they are absorbed into the lymph and only later spill into the blood, travel first to the heart, but eventually, they too will end up in the liver.

The liver is the only organ in the human body capable of exporting nutrients for energy production to other tissues. Therefore, when a person is between meals (fasted state), the liver exports nutrients, and when a person has just eaten (fed state), the liver itself stores nutrients. Nutrient levels and the hormones that respond to their levels in the blood provide the input so that the liver can distinguish between the fasted and fed states and distribute nutrients appropriately.

The Liver

Figure \(\PageIndex{1}\): The liver. Water-soluble nutrients in the blood liver (shown in dark red) is the first place nutrients are stored, changed, and distributed in the body after they are absorbed. Source: Betts J, Young K, Wise J, et al. Anatomy and Physiology. OpenStax.  https://openstax.org/details/books/anatomy-and-physiology-2e

Other Organ Systems Involved in Digestion

All eleven organ systems in the human body require nutrient input to perform their specific biological functions. No energy in means no work output. Nutrients fuel both overall health and the ability to carry out all of life’s basic processes. Without them, organ systems would fail, humans would not reproduce, and the race would disappear. In this section, we will discuss some of the critical nutrients that support specific organ system functions.

The Circulatory System (Cardiovascular System)

The circulatory system, part of the cardiovascular system, is one of the 11 organ systems of the human body. Its main function is to transport nutrients to cells and wastes from cells (Figure \(\PageIndex{2}\)). This system consists of the heart, blood, and blood vessels. The heart pumps the blood, and the blood is the transportation fluid. The transportation route to all tissues is a highly intricate blood vessel network comprising arteries, veins, and capillaries. Nutrients absorbed in the small intestine travel mainly to the liver through the hepatic portal vein. From the liver, nutrients travel upward through the inferior vena cava blood vessel to the heart. The heart forcefully pumps the nutrient-rich blood first to the lungs to pick up some oxygen and then to all other cells in the body. Arteries become smaller and smaller on their way to cells so that by the time blood reaches a cell, the artery’s diameter is extremely small, and the vessel is now called a capillary. The reduced diameter of the blood vessel substantially slows the speed of blood flow. This dramatic reduction in blood flow gives cells time to harvest the nutrients in the blood and exchange metabolic wastes.

The Circulatory System Moves Nutrients

Figure \(\PageIndex{2}\): The circulatory system transports nutrients to all cells and carries wastes out. The heart pumps blood through blood vessels (shown above in red and blue). Source: Fowler, S., Roush, R., & Wise, J. (2022). Concepts of Biology. OpenStax. https://openstax.org/books/concepts-...1-introduction

 

The Nervous System

The human brain (which weighs only about 3 pounds, or 1,300 kilograms) is estimated to contain over 100 billion specialized cells called neurons. Neurons form the core of the nervous system, which consists of the brain, spinal cord, and other nerve bundles in the body. The main function of the nervous system is to sense changes in the external environment and create a reaction to them. For instance, if your finger comes into contact with a thorn on a rose bush, a sensory neuron transmits a signal from your finger up through the spinal cord and into the brain. Another neuron in the brain sends a signal that travels back to the muscles in your hand and stimulates muscles to contract, causing you to jerk your finger away. All of this happens within a tenth of a second.

The enteric nervous system is the part of the nervous system that controls gastrointestinal function. Nerves in the enteric nervous system are embedded around the gastrointestinal tract, from the esophagus to the anus. These nerves control key parts of the digestive process, including peristalsis, segmentation, and the secretion of digestive enzymes. The enteric nervous system can operate autonomously, which means it can operate without input from the brain and spinal cord. The brain and spinal cord make up the central nervous system, which plays a role in sensing hunger and regulating appetite.

Every day, the brain uses over 20% of the energy obtained from nutrients. Its main fuel is glucose, and only in extreme starvation does it use anything else. For acute mental alertness and clear thinking, glucose must be systematically delivered to your brain. This does not mean that sucking down a can of sugary soda before your next exam is a good thing. Just as too much glucose is bad for other organs, such as the kidneys and pancreas, it also produces negative effects on the brain. Excessive glucose levels in the blood can cause a loss of cognitive function, and chronically high blood-glucose levels can damage brain cells. The brain’s cognitive functions include language processing, learning, perceiving, and thinking.

All nerve impulses travel by the movement of charged sodium, potassium, calcium, and chloride atoms. These are some of the essential minerals in our diets—essential because they are absolutely required for central nervous system function. Nerves communicate with each other via chemicals built from amino acids called neurotransmitters. Eating adequate protein from a variety of sources will ensure the body gets all of the different amino acids that are so important for central nervous system function.

The Nervous System Aids Digestion

Figure \(\PageIndex{3}\): The nervous system aids digestion by regulating key digestive processes like peristalsis, segmentation, and the secretion of digestive enzymes. Source: Betts J, Young K, Wise J, et al. Anatomy and Physiology. OpenStax. https://openstax.org/details/books/anatomy-and-physiology-2e

Nutritional Research: The MIND Diet and Cognitive Health

Over 55 million people live with dementia, which is a broad term that describes disorders affecting memory, thinking, behavior, and emotions.¹ Alzheimer's disease is the most common form of dementia. There is currently no cure for dementia, but recent research has shown that nutrition and diet can have an impact on cognitive function and dementia. The MIND diet, which recommends whole grains, vegetables, and fruits while limiting red meat, cheese, and processed foods, has been shown to slow the decline in cognitive function associated with age. In a study of 960 participants over the course of nearly 5 years, people who adhered to the MIND diet demonstrated cognitive function that was equivalent to being 7.5 years younger than people who did not adhere to the MIND diet.²

The Muscular System

The muscular system allows the body to move voluntarily, but it also controls involuntary movements of other organ systems, such as heartbeat in the circulatory system and peristaltic waves in the digestive system. Muscle cells receive signals from the nervous system to expand and contract, which is responsible for voluntary and involuntary movements. The muscular system consists of over six hundred skeletal muscles, as well as the heart muscle, the smooth muscles that surround your entire alimentary canal, and all your arterial blood vessels.

Muscle contraction relies on energy delivery to the muscle. Each movement uses up cellular energy, and without an adequate energy supply, muscle function suffers. Muscle, like the liver, can store the energy from glucose in the form of glycogen, a slow-releasing carbohydrate. But unlike the liver, muscles use up all of their own stored energy and do not export it to other organs in the body. When muscle energy stores are diminished, muscle contraction weakens. However, muscle is not as susceptible to low levels of blood glucose as the brain because it will readily use alternate fuels, such as fatty acids and protein, to produce cellular energy.

“Hitting the Wall” or “Bonking”

If you are familiar with endurance sports, you may have heard of “hitting the wall” or “bonking.” These colloquial terms refer to the extreme fatigue that sets in after about 120 minutes of performing an endurance sport, such as marathon running or long-distance cycling. The physiology underlying “hitting the wall” means that muscles have used up all their stored glycogen and are, therefore, dependent on other nutrients to support their energy needs. Fatty acids are transported from fat-storing cells to the muscle to rectify the nutrient deficit. However, fatty acids take more time to convert to energy than glucose, thus decreasing performance levels. To avoid “hitting the wall” or “bonking,” endurance athletes load up on carbohydrates a few days before the event. This will maximize an athlete’s glycogen stored in their liver and muscle tissues. It is important not to assume that carbohydrate loading works for everyone. Without accompanied endurance training, you will not increase the amount of stored glucose. If you plan on running a five-mile race for fun with your friend and decide to eat a large amount of carbohydrates in the form of a big spaghetti dinner the night before, the excess carbohydrates will be stored fat. Another way for athletes to avoid “hitting the wall” is to consume carbohydrate-containing drinks and foods during an endurance event. In fact, throughout the Tour de France—a twenty-two-day, twenty-four-hundred-mile race—the average cyclist consumes greater than 60 grams of carbohydrates per hour.

To avoid “hitting the wall,” athletes consume large amounts of carbohydrates before and during events to ensure enough glucose is available for optimal performance, especially during endurance activities such as cycling. (TourDeFrance 2005 07 09 by Vzach has been released into the Public Domain).

The Endocrine System

The functions of the endocrine system are intricately connected to the body’s nutrition. This organ system is responsible for regulating appetite, nutrient absorption, nutrient storage, and nutrient usage, in addition to other functions, such as reproduction. The glands in the endocrine system are the pituitary, thyroid, parathyroid, adrenals, thymus, pineal, pancreas, ovaries, and testes. The glands secrete hormones, biological molecules that regulate cellular processes in other target tissues, so they require transportation by the circulatory system. These hormones can have a major impact on nutrient status. For example, the adrenal gland can release corticosteroid hormones, such as cortisol, that can increase the amount of glucose in the bloodstream by promoting the release of stored sugars in the liver. The pancreas also regulates blood glucose levels by releasing the hormone insulin (see Connection box below). Adequate nutrition is critical for the functioning of all the glands in the endocrine system. A protein deficiency impairs the release of hormones from reproductive tissues, thereby preventing reproduction. Children who are malnourished usually do not produce enough growth hormone and may exhibit stunted growth.

The Endocrine System

Figure \(\PageIndex{4}\): The Endocrine System. The endocrine system regulates appetite and the absorption, storage, and usage of nutrients. Source: Betts J, Young K, Wise J, et al. Anatomy and Physiology. OpenStax. https://openstax.org/details/books/anatomy-and-physiology-2e³

Connection: Diabetes and Obesity

Diabetes is a condition in which blood glucose levels are not regulated correctly, resulting in high levels of glucose in the blood. This damages the heart and blood vessels, nerves, kidneys, and many other organs in the body. In Type 2 diabetes, which often occurs in older adults, the pancreas fails to produce enough insulin, and the body's other cells do not respond to insulin correctly. The Centers for Disease Control and Prevention (CDC) estimates that 37.3 million Americans have Type 2 diabetes, which accounts for 11.3% of the US population.⁴

People diagnosed with diabetes may see an endocrinologist, a doctor who specializes in the endocrine system. One of the greatest risk factors for type 2 diabetes is being overweight and inactive. Although it is not clear how increased fat tissue causes diabetes, research suggests that several factors are responsible for the connection between obesity and diabetes. Increased fat tissue is initially associated with increased insulin release from the pancreas. Over time, the pancreas begins to fail to release appropriate amounts of insulin, which leads to insulin resistance in other organs. Increases in inflammation and circulating fatty acids in the blood may also play a role in the progression to diabetes.⁵

Video \(\PageIndex{1}\): Understanding Type 2 Diabetes.⁶

The Immune System

The immune system is comprised of several types of white blood cells that circulate in the blood and lymph. Their jobs are to seek, recruit, attack, and destroy foreign invaders, such as bacteria and viruses. Other less realized components of the immune system are the skin (which acts as a barricade), mucus (which traps and entangles microorganisms), and even the bacteria in the large intestine (which prevent the colonization of bad bacteria in the gut).

Immune system functions are completely dependent on dietary nutrients. In fact, malnutrition is the leading cause of immune system deficiency worldwide. When immune system functions are inadequate, there is a marked increase in the chance of getting an infection. Children in many poor, developing countries have protein- and/or energy-deficient diets that can cause two different syndromes, kwashiokor and marasmus. These children often die from infections that their bodies could normally have fought off, but because their protein and/or energy intake is so low, the immune system cannot perform its functions. Other nutrients, such as zinc, selenium, copper, folate, and vitamins A, B₆, C, D, and E, all provide benefits to immune system function. Deficiencies in these nutrients can increase the risk of infection and death. Zinc deficiency results in suppression of the immune system’s barrier functions by damaging skin cells; it is also associated with a decrease in the number of circulating white blood cells.

Just as undernutrition compromises immune system health, so does overnutrition. People who are obese are at increased risk for developing immune system disorders such as asthma, rheumatoid arthritis, and some cancers. Both the quality and quantity of fat affect immune system function. High intakes of saturated and trans fats negatively affect the immune system, whereas increasing your intake of omega-3 fatty acids, found in salmon and other oily fish, decreases inflammatory responses. High intakes of omega-3 fatty acids are linked to a reduction in the risk of developing certain autoimmune disorders, such as rheumatoid arthritis, and are used as part of a comprehensive treatment for rheumatoid arthritis.

Connection: The Immune System and the Microbiome

Our bodies have trillions of microbes, such as bacteria, living inside our gut that help us digest our foods. This collection of microbes that live in and on us is called our microbiome, and some scientists consider the microbiome another organ system because it is so important in our health. Most of the microbes are beneficial, which means they help us in some way. In the large intestines, microbes help break down complex sugars by releasing digestive enzymes. A healthy gut microbiome is critical for regulating the immune system in all stages of life. For example, infants with less diverse microbiomes were more likely to develop allergic reactions such as eczema.⁷ Diets high in fiber, or prebiotic foods, can protect against infections by harmful bacteria, such as Clostridium difficile, by making the colon more acidic. In fact, patients with severe C. difficile infections can sometimes be treated with fecal transplants, in which the stool of a healthy donor is administered to a patient to restore a healthy microbiome.⁸ Probiotics, or live cultures of beneficial bacteria, can also be taken as supplements or found in fermented foods such as yogurt to promote a healthy microbiome. Probiotics have been associated with several health outcomes, including reduced risks of infection.⁹

Figure \(\PageIndex{5}\): Yogurt contains live cultures of beneficial bacteria. Source: "Yogurt 364" by ljguitar is licensed under CC BY 2.0.

Attributions

• Zimmerman, "An Introduction to Nutrition (Zimmerman)", CC BY-NC-SA 3.0. Figures were updated and the flow of information was slightly changed.

References

1. Alzheimer's Disease International. Dementia facts & figures. Accessed July 21, 2023. https://www.alzint.org/about/dementia-facts-figures/.
2. Morris MC, Tangney CC, Wang Y, et al. MIND diet slows cognitive decline with aging. Alzheimers Dement. 2015;11(9):1015-1022. doi:10.1016/j.jalz.2015.04.011.
3. Betts J, Young K, Wise J, et al. Anatomy and Physiology. Open Stax; 2013. Accessed July 25, 2023. https://openstax.org/books/anatomy-and-physiology-2e/pages/17-1-an-  overview-of-the-endocrine-system?query=endocrine%20system&target=%7B%22index%22%3A0%2C%22type%22%3A%22search%22%7D#fs-id2766959.
4. Centers for Disease Control and Prevention. National Diabetes Statistics Report. cdc.gov. Updated May 15, 2024. Accessed July 22, 2023.             https://www.cdc.gov/diabetes/php/data-research/?CDC_AAref_Val=https://www.cdc.gov/diabetes/data/statistics-report/index.html.
5. Klein S, Gastaldelli A, Yki-Jarvinen H, Scherer PE. Why does obesity cause diabetes? Cell Metab. 2022;34(1):11-20. doi:10.1016/j.cmet.2021.12.012.
6. Animated Diabetes Patient. Understanding Type 2 Diabetes [Video]. YouTube. https://youtu.be/JAjZv41iUJU?si=MkFiskzdBTfws3oB. Published June 9, 2014. Accessed July 19, 2023.
7. Wopereis H, Sim K, Shaw A, Warner JO, Kroll JS. Intestinal microbiota in infants at high risk for allergy: Effects of prebiotic and role in eczema development. J Allergy Clin Immunol. 2018;141(4):1334-1342. https://www.jacionline.org/article/S0091-6749(17)31343-X/pdf. Accessed July 26, 2023.
8. Johns Hopkins Medicine. Fecal Transplant. Accessed July 26, 2023. https://www.hopkinsmedicine.org/health/treatment-tests-and-therapies/fecal-transplant.
9. Hori T, Matsuda K, Oishi K. Probiotics: A dietary factor to modulate the gut microbiome, host immune sytem, and gut-brain interaction. Microorganisms. 2020;8(9):1401. doi:10.3390/microorganisms8091401.