In addition to eating The small and the large intestines perform all of the essential duties of the digestion system. This is where the real business of digestion occurs. The intestines occupy the majority of the space within the abdominal cavity. They comprise the largest part of the intestinal (GI) tract with regard to weight and length. The fourth part of this series of six about the GI tract describes the anatomy and role of the small intestinal tract. Part 5 explains the anatomy and function of the large intestine and also the most common conditions that affect both the large and small intestines.
Anatomie of the large intestinal tract
The large intestine measures approximately 1.5m long and includes the colon, caecum anal canal, rectum and anus (Fig 1.). The anatomy of the large intestinal tract is quite like that of the smaller intestine, however, the mucosa of this intestine is free of villi.
Caecum and appendix
Chyme that is not taken in when it leaves the small intestine, passes by the ileocaecal valve before it is absorbed into the large intestine via the caecum. After receiving food items in the ileum the caecum continues to absorb salts and water.
The caecum is approximately 6cm in length and extends into the appendix. It is a tubular sac that is winding and contains lymphoid tissue. The appendix is believed to be the remnant of a redundant organ. its shape, which is narrow and twisted creates a favourable location for the growth and multiplication of intestinal bacteria.
At the opposite end, the caecum connects to the colon, which is the longest segment in the large intestine (Fig 1.). Food residues begin to move upwards through the colon, which is located on the left side of your abdomen. The colon ascends, which is bent near the liver’s right colic-flexure (or the hepatic flexure) and transforms into the transverse colon. It travels across the side that is left of your abdomen. Near the spleen, at the right colic-flexure (or splenic the flexure) and the transverse colon transforms into the colon that descends across the left side of the abdomen. After the next bend, the colon that is descending changes into the sigmoid colon.
The colon is divided into appearance. The segments that are caused by the sacculation process, are referred to as haustra. The colon that ascends, the rectum, and the descending colon are found inside the retroperitoneum (outside the peritoneal cavity). The sigmoid and transverse colons are joined to the abdominal wall of the posterior via the mesocolon.
Rectum, canal anal and anus
The large intestine is opened into the rectum. It is then joined by the canal anal. The rectum is the last twenty centimeters of GI tract. It runs parallel to the sigmoid colon. It connects with the canal of the anal and the anus (Fig 2 page 52). The rectum extends to an extended section, known as the rectal ampulla. It is the place where faeces are kept prior to being released. The rectum is normally empty as there is no need to store faeces in the rectum for very long.
The canal that runs through the perineum (outside the abdominopelvic cavity) can be 3.8-5cm long and connects towards the outside of the body near the anus (Fig 2.). There are two sphincters in it:
- The internal anal sphincter is controlled by voluntary muscles;
- External anal Sphincter, which is composed of skeletal muscle and is controlled by voluntary effort.
Except during defecation, anal sphincters remain shut.
The function of the large functions of the large
The food particles travel into the large and small intestines after 8 to 9 hours following the intake. The small intestine has absorb around 95% of ingestion water. The large intestine absorbs the bulk part of the remaining liquid which converts the liquid chyme residue to semi-solid stools, or the faeces. The large intestine performs three primary purposes:
- The absorption of electrolytes and water;
- The formation and transportation of faeces
- Chemical digestion by gut microbes.
Absorption of electrolytes and water
Food residues within the colon trigger haustral contractures which happen approximately every 30 minutes, and last for about one minute. Each haustrum contracts and expands and pushes the food residues through the next one. The contractions also mix food particles, which aids in the absorption process of the water.
The large intestine is also a source of electrolytes. In the intestine, sodium ions are taken up by the sodium/potassium pump. This causes potassium and sodium ions to move to opposite sides of cell membranes which promotes the absorption of sodium and loss of potassium by the release of the hormone aldosterone.
Antiperistaltic contractions push food residues back toward the ileocaecal valve, slowing it down and providing the larger intestine to absorb water as well as electrolytes.
Transport and formation of Faeces
For every 500ml of food waste that is absorbed into the caecum daily approximately 150ml of it is Faeces. They contain a lot of bacteria, epithelial cells that have died of the intestinal mucosa organic waste as well as food matter that is not digested and fibre, and water that helps it move effortlessly through the digestive tract. They also contain small amounts of protein and fats. Their distinctive brown colour result from the presence of stercobilin as well as Urobilin, which are haemoglobin breakdown products that are derived made from old red blood cells.
Since chyme residue stays inside the large intestinal tract for between 12 and 24 hours, the majority part of 1.5L of fluid flowing through the large intestine each day is absorbed and less than 100ml out through the faeces. This little amount of fluid is what gives faeces their semi-solid texture. Faeces are also softened due to dietary fibre. Mucus produced by goblet cells in the entire colon, aids to bind chyme that has been dehydrated and helps in lubricating the flow of the faeces.
The colon’s movement is slow. It takes three days to eliminate 70 per cent of the food and the complete elimination of any remnants could take as long as a week while transit times are faster for males than females (Degen and Phillips 1996). The normal bowel emptying pattern varies significantly between individuals, ranging generally ranging from 3 times per day up to three times each week (Walter and colleagues (2010)).
Colonic mass movement
Peristalsis in the ileum pushes the chyme to enter the caecum. Caecum distention causes the gastric colic response and colonic mass movements commence. Instigated by stomach dilation and irritation of the colon, mass movements are typically seen every three to four times per day, usually during or shortly after meals. These intense waves that can last for up to 30 minutes start halfway through the colon’s transverse part. Aided by haustral contractions they push the dry contents of the colon towards the rectum. The intake of fibre increases the force of colonic contractions that propel the faeces toward the anus.
Colonic mass movements fill the rectum giving rise to the urge to vomit. It is crucial to respond to this urge because when the motions have stopped the urge will also stop. If the urge to vomit is not addressed for a long amount of time, the rectum fills up and the large intestine absorbs more water, and the faeces get harder and dry. This could cause constipation.
The physiology of defecation
When faeces begin filling the rectum as they fill the rectum, the rectal wall expands and sends an electrical signal to the nerve centres in the spinal cord, triggering the reflex of defecation in the spine. This leads to the relaxation of the anal sphincter in the body and allows a small number of faeces through the anus. The anus can tell if the substance is gaseous or solid and reacts in a manner that is consistent with the situation. If the substance is solid then the external anal sphincter expands and defecation can take place. But, the external anal sphincter can be controlled via voluntary muscle which means it is able to be controlled to hold off defecation until a better time. The majority of children have learned this behaviour before the age of 2 or 3 years old. Dementia sufferers may not know how to perform this.
Faeces are typically cleared out through the rectal muscles, assisted by a voluntary process known as Valsalva’s manoeuvre. The process involves contacting the abdominal wall and diaphragm muscles, which raises abdominal pressure and pulls faeces out of the rectum.
If the nerves connecting the external anal sphincter as well as the centre for defecation in the medulla become damaged as could be the case following stroke, during the case of multiple sclerosis or after a spinal injury – the capacity to stop defecation could be lost, which could lead to the condition of faecal incontinence. As we age the ability of the anus to recognize the presence of gas or faeces can be diminished and faecal matter could be considered gas, leading to the condition of faecal incontinence.
Chemical digestion by gut microbes
The large intestine is not able to release the digestive enzymes it produces. within this section of the GI tract, chemical digestion is only possible by the actions of colonic bacteria in the millions. By fermentation, these bacteria break down some of the remaining carbohydrates. This releases hydrogen, methane and carbon dioxide that make flatus (gas). Colonic bacteria also shield the intestine from harmful bacteria from the outside and synthesize certain vitamins. The role of these bacteria will be deeply explored in part 6 of the series.
Lactose intolerance sufferers are unable to digest lactose from food sources. The undigested lactose is fermented inside the large intestinal tract and causes gas and abdominal cramps. It also causes bloating, abdominal cramps and diarrhoea. The symptoms vary from mild discomfort up to severe discomfort. The gas created through the bacterial fermentation process of lactose within the colon is hydrogen. Those who suffer from lactose intolerance breathe hydrogen. The breath test for hydrogen can be used to determine the problem (Argnani and colleagues (2008)).
Coeliac disorder is an intolerance of gluten, an ingredient that is present in Wheat, Barley and Rye. When people suffering from coeliac disease consume gluten, the intestinal immune cells (T cells) release inflammatory mediators that result in a flattening of the mucosal lining, affecting the capacity to absorb and digest foods. The symptoms vary from mild to serious and include diarrhoea and abdominal pain, bloating, flatulence, constipation and indigestion In severe cases, the condition could cause malnutrition.
Diarrhoea (loose and watery stool) is usually caused by norovirus, gastroenteritis, or food poisoning. However, it could be caused by allergies or food intolerances or irritable bowel syndrome. coeliac disease and diverticular diseases.
If the intestines are not able to absorb fluids, your body could lose some litres of fluid every day, resulting in the consequences of dehydration, the loss of electrolytes (potassium and sodium Ions) and a higher chance for blood clotting. The loss of large amounts of potassium ions like could cause cardiac arrest. The sole absorption mechanism that isn’t affected by diarrhoea is the glucose/sodium co-transport. This means that people suffering from diarrhoea are more likely to increase their absorption of sodium essential and water, even in conditions of high glucose.
If someone suffers from diarrhoea, it’s important to replenish electrolytes and fluids by administering a solution that has the right proportion of electrolytes and glucose (for instance, Dioralyte). Drinks like lemonade and squash may not have the proper balance.
If diarrhoea results in hypernatraemia (serum sodium levels 135mmol/L) This needs to be treated immediately. Treatment could include the administration of hypertonic sodium saline. However, it is important to make sure that blood sodium levels aren’t allowed to rise too fast in order to trigger an abrupt shift in the number of the brain’s cells to water, which can lead to fatal central pontine myelinolysis complication (Rusoke-Dierich 2018, 2018).
Constipation is the irregular and painful, painful and slow elimination of faeces because of slow movements of dry, hard faeces. It can cause abdominal pain and discomfort and, if not treated the faecal tract may become impure and cause GI obstruction. The problem could result from a lack of constipation and a diet deficient in fibre, or immobility. Certain medicines such as eating disorders, or the excessive use of laxatives can result in – or worsen – constipation. Incorporating 20-60g of fibre per day in your diet and drinking a couple of glasses of fluids at every meal can help avoid constipation. Nurses must keep at all times that constipation may signal a serious physiological issue or illness, like diverticulitis, obstruction caused by a tumour or paralytic ileus.
Adhesions, tumours, adhesions to the intestinal wall foreign bodies, or affected faeces could cause the intestines to become partly or totally blocked and cause the contents of the intestinal tract to be able to get back in. This can cause abdominal pain, swelling nausea, cramps and extreme constipation or diarrhoea. Another reason for intestinal obstruction is called paralytic Ileus which causes a drastic slowing down of the normal peristaltic motion within the digestive tract. The condition can be caused by fungal or bacterial diseases, ischaemia mesenteric appendicitis and abdominal surgery, as well as certain drugs.
The pea-sized pouches known as diverticula can appear on weak areas of the intestinal wall due to increased pressure, for instance when you strain when defecating. They are more frequent on the inside of the sigmoid colon however, the exact location and frequency differ depending on age. For example, in the US there is a report that the rate of the condition was 35 per cent for individuals younger than 50 and 40% for those aged 50-59 years and 58% among people over 60 (Peery and co (2016)). Diverticula sufferers with have no or minimal symptoms are believed to have the benign condition called diverticulosis. The condition can be a source of complications in around 20% of those suffering from diverticulosis. They may develop diverticulitis, which is an infection and inflammation of the diverticula. It is usually caused by bacteria accumulating in the diverticula, which are blocked by debris. The bleeding can be caused by diverticular and can cause persistent injury to the tiny blood vessels located next to the colonic obstruction and the diverticula.
Inflammatory Bowel disease
The condition known as Inflammatory Bowel Disease (IBD) is an uncontrolled inflammation and bowel injury in the large intestine that results in extreme discomfort, with symptoms that include constipation, abdominal pain gas, liquid motions, and diarrhoea. There is often a strong urge to flush and may also be bleeding or anal/rectal discharge. A severe IBD can cause appetite loss, losing weight, as well as iron deficiency anaemia.
The two major IBD types include Crohn’s disease as well as ulcerative colitis. While ulcerative colitis typically manifests as a continuous inflammation and is typically treated by eliminating the affected regions, Crohn’s disease tends to create a patchy pattern of inflamed ulcerations that could affect any area within the GI tract, but more typically the terminal ileum or the colon. This makes the treatment and surgical procedure more challenging.
In the past, Crohn’s disease was believed to be an immune condition where your immune system destroyed the body’s gut lining. Nowadays, the evidence seems to indicate an immune system overreacts and targets a microbial antigen in the lining of the gut (Torres and colleagues 2017). A majority of patients with Crohn’s disease get diagnosed before they reach the age of 30. Intestinal obstruction, which is a frequent condition, is caused by the formation of scar tissue. It can also cause swelling of the bowel wall and a narrowed passageway for the intestinal tract (strictures). The bowel’s perforation can occur due to the fistula or abscess.
Ulcerative colitis is characterized by ulceration and inflammation in the colon’s lining as well as the rectum, as well as rectal urgency which can cause diarrhoea with bloody pain up to 20 times daily. The symptoms can be intermittent, but 5-10% of sufferers have persistent symptoms. Perforation can be a possible problem, as persistent inflammation or ulceration can cause the intestinal wall to be weakened to the point that a hole can be created. It’s usually linked to toxic megacolon. It is an emergency condition wherein the colon ceases to contract function and gas accumulate. The ensuing perforation could cause life-threatening peritonitis.
Malabsorption syndrome is a variety of conditions that affect the small intestine. cannot absorb enough nutrients (proteins and fats, minerals or vitamins, as well as carbohydrates) along with fluids leading to the loss of nutrients, malnutrition, and loss. If a patient has suffered from over 50% of their small intestine is removed from the body, nutrient absorption can be seriously impaired.
When the appendix gets blocked, it can become inflamed, leading to appendicitis. Obstruction can cause pressure build-up which can cause a compression of and engorge the wall of the stomach leading to ischaemic injury as well as an infection caused by bacteria. The typical sign is pain that starts at the umbilicus before expanding into the right iliac fossa. The symptoms include nausea, vomiting, and fever could follow. If the condition isn’t treated and the appendix ruptures, it could be fatal leading to dangerous peritonitis and the bacterial infection can rapidly propagate throughout the peritoneal cavity, possibly leading to death in the shortest time possible. Appendicitis is among the most frequent causes of abdominal pain that are acute.
Colorectal cancer is the 2nd most frequent cause of cancer death within the UK and can be detected by diarrhoea or constipation and abdominal pain, cramps, and rectal bleeding. These can be visible or concealed within the stool (occult). Alcohol consumption, smoking and a diet that is high in animal fats and proteins are all linked to the risk of being more susceptible to colorectal cancer. A few reports suggest an increase in your dietary fibre intake may lower cancer’s chance of developing the disease (Yang and Yu (2018)).
Because the majority of colon cancers result from benign mucosal growths, also known as polyps, the prevention strategy is centred around finding the polyps. The screening for occult blood from faeces is offered every two years to those aged 60-74 across the majority of the UK however in Scotland it’s offered to start at age 50. Public England last year announced plans to expand the screening program over time that will cover people aged 50 to 74 years old age (PHE 2018, 2018).