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https://en.wikipedia.org/wiki/Bowel_Function

Bowel is one of the largest digestive organs, which is in the middle part in the abdomen. It is consisted of a series of complicated small organs which are categorized into two big sections, which is small intestines and big intestines. Bowel function is recognised as a crucial part of digestive system that included multiple small organs ‘movements and enzymatically transformation of food to support the vital function of the human body. The bowel is used to breakdown and absorb the food consumed, and then transport them to the liver. The small bowel and big bowel are both composed of various digestive tracts, most common diseases include Inflammatory bowel disease and irritable bowel syndrome. Symptoms are based on the severity of the disease, the most common symptoms include abdomen pain, diarrhoea, change of stool, bloating and others. Effective treatments are available for patients with these symptoms, these include drug and surgery treatments, dietary treatments and psychological treatments. Although the cause of these diseases are not entirely clear, the treatment options available are highly effective.

Anatomy
Bowel is positioned in the middle to lower part of abdomen, connecting the stomach and the lower part of the body. Small bowel (known as small intestines) and big bowel (big intestines) are the two main parts of bowel that determine bowel structure. Distinguished from the gut, the bowel is not including stomach, but cover all different intestines and colons.

Small Intestines
Small intestines origin from ‘pylorus’ and terminate at the ‘ileocecal valve’, while the overall position may change depending on the rotation of the stomach. The length of the small intestines varies from 5-6 m long. The Jejunum, duodenum and Ileum are the three major parts of small intestines, which starts from duodenojejunal and ends at the ileocecal junction. The jejunum is located in the centre of the abdomen followed by the ileum which occupies in the lower part.

Large Intestines
Large intestines, also defined as the large bowel, start from the lower-right side of the abdomen and stops at the cecum which contains various active bacteria and supports major digestion process of the human body. The caecum, appendix and colons form the structure of the large intestines, the average length is about 1.5 meters and 1.5 centre meter longitudinally. Large bowel is in a downward U shape and surrounding the small intestines.

Function
The overall bowel function is comprised of the function of small and big intestines, which oversees absorbing nutrients and process wastes inside the human body. Both small intestines and big intestines play different role to maintain the function of bowel system. The small intestines are the longest organs with large muscles surface inside the abdomen, which specialized in absorbing food nutrients, retaining water and balancing electrolyte inside the bowel environment. The larger mucosa surface area of the small intestines ensures maximum nutrient absorption.

Compared to the small intestines, the larger intestines are considered as a metabolically active organ that is responsible for absorbing ‘inorganic ions’ and water while dealing with wastes to maintain the balance of digestion system. Another significant function of large bowel is residue storage by forming feces and processing them by defecation.

Common Diseases
Irritable bowel syndrome and Inflammatory bowel diseases are the two main diseases that have been founded in human bowel system nowadays.

Inflammatory Bowel Diseases
Inflammatory disease is characterized as a long-lasting chronic inflammatory throughout the gastrointestinal tract. Crohn’s disease (CD) and ulcerative colitis(UC) are the two universal type of inflammatory bowel diseases that have been studied over a century, and they are closely related to different environmental risk factors, family genetic and people’s lifestyle such that smoking is considered highly associated with these diseases. Crohn’s disease is discovered to be related to immune disorders particularly. Different level of cumulative intestinal injuries can cause different complications, such as ‘fistulae, damage of bowel function and symptoms reoccur, disability’ etc. The patient group can vary from children to adults, the newest research revealed that immunodeficiency and monogenic are the causes of young patients with inflammatory bowel diseases. The onset rate keeps updating each year with dramatically increased number and the pathogen of the bowel disease are also complicated due to the complexity of the bowel organs, bowel diseases are diverse in terms of the small and big bowel.

Irritable Bowel Syndrome
Irritable bowel syndrome is diagnosed as an intestinal disorder with chronic abdominal pain and inconsistency form of stool, and it is a common bowel disease that can be easily diagnosed in modern society. The variety of incident rate is acceptable due to the different diagnostic criteria in different countries, the 18-34 age group is recognized as the high frequency incident group. The definite cause of the irritable bowel syndrome is remaining a mystery, however, it has been founded relating to multifactor, such as ‘alternation of mood and pressure, sleep disorders, food triggers, changing of dysbiosis and even sexual disfunction’. One third of irritable bowel syndrome patients has family history with the disease suggesting that genetic predisposition could be a significant cause for irritable bowel syndrome.

Inflammatory Bowel Diseases
Common symptoms for inflammatory bowel diseases differ by the infection level, sever abdomen pain, diarrhoea, fatigues and unexpected weight loss. Crohn’s disease, the chronic inflammatory bowel disease can lead to infection towards any parts of the digestive tract which including ileum to anus. Internal manifestations include diarrhoea, abdomen pain, fever, chronic anaemia etc. External manifestations include impact on skin, joints, eyes and liver. Significantly reduced ‘microbat diversity’ inside the gastrointestinal tract can also be observed. Ulcerative colitis mainly affects the function of the large bowel, and its incidence rate is three times larger than the Crohn’s disease. In terms of clinical features, over 90% of patients exhibited constant diarrhoea, ‘rectal bleeding, softer and mucus in the stool, tenesmus and abdomen pain’. At the same time, patient also reported to be having ‘arthralgia, episcleritis and erythema nodosum’. The symptoms can continue for around 6 weeks or even more than that.

Irritable Bowel Syndrome
Patients with irritable bowel syndrome are commonly experiencing abdomen pain, changing in stool form, reoccur abdomen bloating and gas, co-morbid disorders and alternation in bowel habits that caused diarrhea or constipation. However, anxiety and tension can also be detected, although patients with irritable bowel disease seems healthy. Apart from these typical symptoms, rectal bleeding, unexpected weight loss and increased inflammatory markers are required further medical examination and investigation.

Treatment
The inflammatory bowel diseases could be effectively treated by ‘pharmacotherapies’ to relieve and maintain the symptoms, which showed in ‘mucosal healing’ and symptoms elimination. However, an optimal therapy for curing both inflammatory diseases are still under research due to the heterogeneity in clinical feature. Although both UC and CD are sharing similar symptoms, the medical treatment of them are distinctively different. Dietary treatment can benefit for curing CD by increase the dietary zinc and fish intake, which is related to mucosal healing of the bowel. Treatments vary from drug treatment to surgery based on the active level of the CD. UC can also be relieved by using immunosuppressive therapy for mild to moderate disease level and application of biological agents for severe cases.

The treatment for irritable bowel diseases is multimodal. Dietary interference and pharmacotherapies both can relieve the symptoms to a certain degree. Avoiding trigger or allergy food group can be beneficial by reducing fermentation in the digestive tract and gas production, hence effectively alleviate abdominal pain and bloating. Drug interventions, such as laxatives, loperamide, and lubiprostone are applied to relieve intense symptoms including diarrhea, abdominal pain and constipation. Psychological treatment, dietary supplements and gut-focused hypnotherapy are recommended for targeting depression, mood disorders and sleep disturbance.