Digestion is the mechanical and chemical breakdown of food, a process required for survival. Without the capability to break food down into molecules small enough to enter the blood stream, an organisms energy levels would not support life. In humans and animals, blood cells must maintain a constant supply of energy for growth, repair, and reproduction. This energy is derived through ingested food that is broken down to a usable form through digestion. The digestive system is a contributor to the internal stability maintained for health and overall survival (Ch. 3: Human Organization, n.d).
There are five stages of digestion that the human body goes through. Movement is the first stage; it is necessary to, first, propel food through the digestive organs so the following stages can occur. Stage two is glandular secretions that are released as a response to specific stimuli. These digestive juices assist in the chemical break down of food. In stage three, digestion breaks down food to a small enough form that will enable the molecular components to cross plasma membranes in cells, which allows stage four to take coarse. In stage four, absorption occurs as the molecules are passed into the bodies interior, or passed throughout the body if the molecules are unnecessary for the health of the organism. Finally, stage five, the process of the passing of the unneeded molecules discovered in stage four. The body must eliminate waste and undigested food in this final stage. Each stage involves appointed organs to do specific jobs along the process of digestion (Audersirk, Audersirk, & Byers, 2008).
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Picture how a well oiled machine works, or a fine tuned orchestra; each instrumentalist working as an individual to produce one intricate melody. The individual string players produce one sound within the melody, the percussion produce another contributing sound, the winds, another, and so on. The equilibrium of such a scenario is very similar to how the human body performs homeostasis. Each different organ system maintains stability within the human body by contributing its own specific function through individual organs. These functions involve coordinated responses to abnormal stimuli or situations, and then work together to alleviate the abnormality (Audersirk, et al., 2008).
There are five main functions that are involved in human homeostasis. The protective skin; featuring hair, nails, sweat and oil glands is known as the integumentary system. Support and movement of the body is provided by the muscular and skeletal systems. Survival, integration and control of the human body are supported by the nervous system, which senses stimuli and allows the body to control itself through thought; musculoskeletal system, where the muscle and bone functions combine to produce reflexes and strength, glandular system, producing chemicals for cellular stability; and endocrine system, releasing hormones. Continuance of the species relies on the reproductive organ systems, gametes such as sperm and eggs, and endocrine systems to release hormones. The overall maintenance of the body relies on several organ systems that are cardiovascular, respiratory, lymphatic, and urinary and last, but certainly not least, the digestive system (Ch. 3: Human Organization, n.d).
The digestive system keeps the conditions of the body normal, contributing to the tissue fluids that bathe cells with needed nutrients. Blood and tissue need to maintain a constant composition for cells to function and support the organism life. Although the endocrine and nervous system act as the composers of the hypothetical symphony that is homeostasis by coordinating organ system activities, the digestive system is a key member. This organ system is important because it helps maintain levels of energy in the cells through the absorption of nutrients, fats and glucose. Those nutrients, fats and glucose provide support for other systems and their cells as well as the overall health of the being. The digestive system is a contributor to the dynamic equilibrium that homeostasis is (Ch. 3: Human Organization, n.d.).
HOW THE DIGESTIVE SYSTEM INTERACT WITH OTHER ORGAN SYSTEMS?
The digestive system works in parallel with the kidneys and urinary (excretory system). While the digestive system collects and removes solids not digested, the excretory system filters compounds from the blood stream and collects them in urine. These systems work together to control the amount of water in your body. (Dowshen M.D., 2007)
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The circulatory system interacts with the digestive system to distribute the absorbed vitamins and nutrients through the body. To control the speed of digestion, the circulatory system carries chemical signals from the endocrine system (Dowshen M.D., 2007). The Endocrine system directs all the systems in the body to complete their intended functions. One way in which this occurs is through the release of hormones. Three major hormones, Gastrin, Secretin, and Cholecystokinin, control the digestion process. Produced and released through the mucosa of the small intestine, they travel through the heart, and arteries back to the digestive system where they stimulate digestive juices and organ movement. Other hormones in the digestive system that regulate the appetite are Ghrelin and Peptide YY. They work on the brain to help regulate the intake of food for energy (American Dietetic Association, 2008).
The nervous system is also involved and supports the digestive system. Two nerves help control the action of the digestive system. The Extrinsic (outside) nerves come to the digestive organs from the brain/spinal cord. Acetylcholine, which causes the muscle layers in the digestive organs to squeeze with more force increasing digestive juices and the push of food through the digestive tract travels through these nerves. Adrenaline has the opposite effect, it relaxes the stomach and intestine, and decreases blood flow to the organs, slowing or stopping digestion (American Dietetic Association, 2008).
The second is the Intrinsic (inside) nerve; it makes up a dense network embedded in the walls of the esophagus, stomach, small intestine, and colon. This nerve is triggered when the walls of the hollow organs are stretched by food. It releases many different substances, which will speed up or delay the movement of food and production of juices by the digestive system. (American Dietetic Association, 2008).
A perfectly functioning digestive system relies on many different systems to reach peak performance. Without the support and contribution of these systems the digestive system would be compromised affecting an individuals health.
ORGANS IN THE DIGESTIVE SYTEM
There are many organs in the digestive system that contribute to food digestion and the overall health of an individual. They include the oral cavity, salivary glands (Parotid, Submandibular, and Sublinqual), pharynx, epiglottis, liver, gallbladder, stomach, pancreas, small intestine, large intestine and rectum (Audersirk, et al., 2008).
The Oral Cavity (mouth) is where food begins its digestive journey. As food is taken into the mouth it is chewed into small bits and mixed with saliva from the salivary glands: Paratoid, Submandibular, and Sublingual. Saliva aids digestion through the contribution of Amylase which is a digestive enzyme used to form sugar from starch, helping to prevent bacterial infections, and making it easier to swallow. As food passes from the mouth to the Pharynx, swallowing occurs which activates the Epiglottis (muscle flap) to cover the opening to a person's airway. The Epiglottis is a director keeping food out of areas it doesn't belong and sending it into the esophagus (food highway). The Esophagus is a passageway that travels from the pharynx to the stomach. Food moves through the esophagus towards the stomach with the aid of peristalses (muscle contractions). As food moves through the esophagus mucus is released to coat the passageway and protect if from being damaged as food travels to the stomach (Audersirk, et al, 2008).
As the food reaches the Stomach it passes through the Lower Esophageal Sphincter which acts as a lid to prevent stomach contents from entering the esophagus. The stomach is a muscular plant that functions to regulate the flow of food into the small intestine create smaller pieces of food through muscular contractions, and breakdown food through chemical processes. Gastric glands located in the stomach lining add Pepsinogen (inactive digestive enzyme), hydrochloric acid, and mucus into the stomach cavity to aid in the digestive process. The Pepsinogen becomes Pepsin (the active enzyme) when it is introduced to the hydrochloric acid. This separation of substances prevents the stomach from eating itself. The pepsin then starts protein digestion while the food is within the stomach. The mixing and churning of the stomach created by peristalsis, and the digestive enzymes and substances released into the stomach produces Chyme (partially digested food and digestive secretions ( Audersirk, et al., pg. 698 2008]). Chyme is the substance released at a regulated rate through the pyloric sphincter into the
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small intestine. This portion of digestion can occur within 2 to 6 hours (Audersirk, et al., 2008).
The Small Intestine is where the majority of digestion and absorption occurs. As chyme passes into the small intestine several enzymes and digestive secretions are released. The Liver contributes bile which is comprised of bile salts, water, other salts, and cholesterol. (Audersirk, et al., pg. 698, 2008) The salts contained within bile aid with the digestion and diffusion of fats within the chyme. The Pancreas disburses pancreatic juice (water, sodium bicarbonate, and digestive enzymes (Audersirk, et al., pg. 699, 2008]) into the small intestine which lowers the acidic levels of the chyme and aids in digestion. Digestive enzymes within pancreatic juice include pancreatic Amylase (effective carbohydrate enzyme), lipases (breaks down fats, lipids), and proteases (finishes protein breakdown). Once the useful substances contained in chyme (carbohydrates, protein, nutrients, lipids) are broken down, absorption occurs through the walls of the small intestine into the bloodstream, except for glycerol and fatty acids. These must travel a different absorption route. Chylomicrons (groups of fat particles and protein) travel through the lacteal into the lymphatic system and end up in a vein close to the heart. The leftover waste is then passed into the large intestine (Audersirk, et al., 2008).
The Large Intestine consists of the colon (ascending, transverse, and descending), and the rectum. The main function of the large intestine is to produce vitamins necessary for the body such as vitamin B12 and vitamin K. These vitamins are synthesized (produced chemically or biologically [MSN.Encarta Dictionary, 2008]) by bacteria within the large intestine and are then absorbed.
This absorption triggers muscles (peristalsis) that contract the remaining waste into feces to be expelled from the body through the rectum (Audersirk, et al., 2008).
All the organs in the digestive system are formed specifically for the purpose of their function. Take the esophagus for example. The esophagus main purpose is get food from the mouth to the stomach, which makes its composition critical. Beginning at the base of the pharynx, the esophagus opens through the Upper Esophageal Sphincter (UES) and ends at the stomach with the Lower Esophageal Sphincter (LES). The UES directs food into the Esophagus while the LES keeps food that has already left the esophagus and stomach acid from getting into it. Approximately 10 in length it is comprised of various muscular tissues. The upper third of the esophagus is striated muscle which is triggered by voluntary impulses. Once past the upper third of the esophagus the muscles turn smooth becoming the smoothest in the bottom portion of the esophagus.
Smooth muscle is operated involuntarily by the nervous system. These muscles are in two layers. The outer layer of muscle fibers of the esophagus run vertically, while the inner layer of muscle fibers surrounds the esophagus in rings. These muscles are what move the food down the esophagus to the stomach. Prior to food entering, the esophagus remains collapsed, but as food enters it a signal is sent to the nervous system that triggers peristaltic contractions. These muscular contractions are the power behind the movement of food (Rosenberger M.D., n.d; Pope, C.E., 2008).
The muscular layers of the esophagus are joined to an internal mucous layer via the Areolar or sub-mucous layer. This mucous membrane covers the interior of the esophagus and is lined with stratified squamous epithelium consisting of a basal layer of two to three layers of cells Located within the sub-mucous layer are the mucous type esophageal glands. The mucous membrane protects the esophagus from damage and aids the movement of food toward the stomach (Coutsoukis, n.d.).
The esophagus receives blood from the inferior thyroid artery, tracheobronchial arteries, and the left gastric artery. Its venous pathways (route blood leaves the esophagus) are the superior vena cava, azygous system, and gastric veins. The structure of the esophagus and the support of the circulatory and nervous system all combine to make the esophagus a highly complex organ capable of completing its function as a highly developed transportation system (Rosenberger M.D., n.d; Pope, C.E., 2008).
DIGESTIVE SYSTEM DISEASES
Several diseases and conditions can develop in the digestive system that can seriously affect an individuals overall health and quality of life. A person receives the nutrients that are necessary to sustain a healthy body through the digestive system, which makes its proper function critical to sustain life. Some of the diseases associated with this system develop due to a nutrient or vitamin deficiency in an individuals diet, consumption of excessive alcoholic beverages, or conditions that arise from a lack of development prior to birth. Two of these types of diseases are preventable through proper diet and good lifestyle choices, while the third is beyond the control of an individual to influence. In an effort to illustrate the differences, a disease from each type was chosen and discussed in the following paragraphs. Hirschsprungs is a disease caused by a birth defect, while Cirrhosis is caused from the consumption of excessive alcoholic beverages or exposure to Hepatitis A, B, or C (Audersirk, et al., 2008).
Hirschsprungs disease (HD) is a condition that humans are born with due to lack of development in the large intestine or colon. This disease usually manifests in children and causes constipation or the complete inability to have a bowel movement. This occurs because the large intestine lacks the necessary equipment to do the job. Special nerve cells in the intestine, called ganglion cells make the muscles push. (Hirschsprung's disease, n.d.).
These muscular contractions push the feces into the rectum so that it can exit the body. A child born with Hirschsprungs disease doesn't have these cells in parts of their intestine. As the stool enters one of these sections it stops creating a blockage. The stool then backs up into the small intestine and can cause a serious infection known as Enterocolitis. If left untreated, it can cause the colon to rupture creating the possibility of death.
HD is common in children with Downs syndrome. Currently, surgery is the only available treatment and requires removing the section of diseased intestine. A colostomy bag is attached to the exterior of the body to receive bowel movements until the intestine heals. After it heals a second surgery is needed to reconnect the intestine with the anus. Children that receive the surgery can go on to lead normal lifestyles. The bodies inability to remove waste created from digestion created a life threatening situation (Hirschsprung's disease, n.d.).
Cirrhosis of the liver is an example of a disease in the digestive system that is preventable. Cirrhosis is from damaged or scarred liver tissue. The scar tissue prevents the liver from functioning to its full capacity of making protein, fighting infections, cleaning the blood, assistance in digesting food and storing energy. Normal blood flow through the liver is blocked by the scar tissue. Excessive consumption of alcoholic beverages is the main cause for Cirrhosis. However, it can occur in humans if hepatitis A, B or C is contracted.
Some causes of the disease may not be identifiable even after blood tests and an examination by a physician. It can be life threatening, but is usually treatable if detected early. People with Cirrhosis have to be extremely careful about medications and alcohol intake since the liver acts as a catalyst in breaking down chemicals and nutrients which may cause the liver to stop functioning completely. Regular exercise and a nutritious diet can prevent contracting the disease. Vaccinations are also available for Hepatitis A and B, but not for C (Audersirk, et al., 2008).
A digestive system must be whole in order to function appropriately. A person's best bet for digestive health is to eat a healthy well rounded diet full of fiber, fruit, and the vitamins and nutrients necessary to promote a healthy body. Taking preventative sensible measures won't guarantee a person's health but it will greatly increase the odds in their favor.