Anatomy Of Large And Small Intestine Biology Essay

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The small intestine is the only part of the gastrointestinal tract that is essential for life. It serves a neuroendocrine function, a digestive function (propulsion, mixing and absorption of food), and a secretory function. The liver, gallbladder and pancreas are accessory organs of the digestive system that is closely associated with the small intestine. The villous surface improves absorption by increasing the overall surface area. In addition to absorption nutrients, it is responsible for most water reabsorption in the GI tract (Edward, 1998).

The small intestine is made up of three segments, which form a passage from stomach (the opening between stomach and small intestine is called the pylorus) to large intestine. It is duodenum, jejunum and ileum. The duodenum is this short section is the part of the small intestine that takes in semi digested from stomach through the pylorus and continues the digestion process. The duodenum also uses bile from the gallbladder, liver and pancreas to help digest food. The jejunum is the middle section of the small intestine carries food through rapidly, with wave-like muscle contractions, towards the ileum. The last segment is ileum. The ileum is the longest part of small intestine. It is where most of the nutrients from food are absorbed before emptying into the large intestine (UPMC, 2010).

By the time food reaches small intestine, it has already been broken up and mashed into liquid by stomach. Each day, small intestine receives between one and three gallons (or six to twelve liters) of this liquid. The small intestine carries out most of the digestive process, absorbing almost all of the nutrients get from foods into bloodstream. The walls of the small intestine make digestive juices or enzymes that work together with enzymes from the liver and pancreas to do this. Although the small intestine is narrower than the large intestine, it is actually the longest section of the digestive tube, measuring about 22 feet (or seven meters) on average, or three-and-a-half times the length of body (UPMC, 2010).

1.2 Large Intestine

Figure 2: Large Intestine (A.D.A.M., 2012)

The large intestine is larger in diameter than the small intestine. It begins at the ileocecal junction, where the ileum enters the large intestine and ends at the anus. The large intestine consists of the colon, rectum and anal canal. Its primary function is to secrete mucin and form stool for excretion. It reabsorbs water and electrolytes but to a much lesser extent than the small intestine. It does not have sodium cotransport system (Edward, 1998). Unlike small intestine, the large intestine produces no digestive enzymes.

The wall of the large intestine has the same types of tissue that are found in other parts of the digestive tract but there are some distinguishing characteristics. The mucosa has a large number of goblet cells but does not have any villi. The longitudinal muscle layer, although present, is incomplete. The longitudinal muscle is limited to three district bands, called teniae coli that run the entire length of the colon (National Cancer Institute, 2012).

The rectum continues from the sigmoid colon to the anal canal and has a thick muscular layer. It follows the curvature of the sacrum and is firmly attached to it by connective tissue. The rectum and ends about 5 cm below the tip of the coccyx at the beginning of the anal canal. The last 2 to 3 cm of the digestive tract is the anal canal, which continues from the rectum and opens to the outside at the anus. The mucosa of the rectum is folded to form longitudinal anal columns. The smooth muscle layer is thick and forms the internal anal sphincter at the superior end of the anal canal (National Cancer Institute, 2012).

1.3 Intestinal Obstruction

Figure 3: Intestinal Obstruction (A.D.A.M., 2012)

Intestinal obstruction, also called ileus (from the Greek eilo, meaning to roll up) (Ivan, 1996). Intestinal obstruction is refers to a lack of movement of the intestinal contents through the intestine. Because of its smaller lumen, obstructions are more common and occur more rapidly in small intestine, but they can occur in large intestine as well. Depending on the cause and location, obstruction may manifest as an acute problem or a gradually developing situation. For example, twisting of the intestine could cause sudden total obstruction, whereas a tumor leads to progressive obstruction (Barbara, 2002).

Obstruction of the intestine causes the bowel to become vulnerable to ischemia. The intestinal mucosal barrier can be damaged, allowing intestinal bacteria to invade the intestinal wall and causing fluid exudation, which leads to hypovolemic and dehydration. About 7L of fluid per day is secreted into the small intestine and stomach and usually reabsorbed. During obstruction, fluid accumulates, causing abdominal distention and pressure on the mucosal wall, which can lead to peritonitis and perforation (Unbound Medicine, 2011).

Intestinal obstruction is most common on elderly individuals, due to the higher incidence of neoplasm and other causative diseases in this population. In neonates, intestinal obstruction may be caused by imperforate anus or other anatomic abnormalities. Obstruction may also be secondary to meconium ileus. In paediatric population, Hirschsprung disease can resemble intestinal obstruction (Christy, 2011).

Intestinal obstruction occurs in two forms. Mechanical obstructions are those resulting from tumor, adhesions, hernias or other tangible obstructions. Functional, or adynamic, obstructions result from neurologic impairment, such as spinal cord injury or lack of propulsion in the intestine and are often referred to as paralytic ileus. Adynamic or paralytic ileus usually related to inflammation or the disruption of innervation (Barbara, 2002).

2 CAUSES OF DISEASE

Intestinal obstruction can be caused by many different things. It occurs in two basic types which is mechanical obstruction and paralytic ileus. Functional obstruction or paralytic ileus is one of the major causes of the intestinal obstruction in infants and children. It is common in the following situations:

after abdominal surgery, in which the effects of the anesthetic combined with inflammation or ischemia in the operative area interfere with conduction of nerve impulses

in the initial stage of spinal cord injuries (spinal shock)

with inflammation related to severe ischemia

in pancreatitis, peritonitis, or infection in the abdominal cavity

with hypokalemia, mesenteric thrombosis or toxaemia

kidney or lung disease

Mechanical obstruction may result from the following:

Scar tissue in the belly, often called adhesions. This tissue can wrap around a piece of bowel. The contents of the bowel are prevented from moving normally through the intestine

Bowel that twits on itself or develops a bad kink. This is called volvulus

Fecal impaction or hard stool that cannot pass through the bowel

Cancer such as colon cancer or cancer of the pancreas

Hernias is protrusion of the abdominal contents through the abdominal wall

(Barbara, 2002)

3 INCIDENCE

4 MORTALITY/MORBILITY

5 PATHOPHYSIOLOGY

When mechanical obstruction of the flow of intestinal contents occurs, a sequence of events develops as follows. First, intestinal obstruction occurs when gases and fluids accumulate in the area proximal to the blockage, distending the intestine. Gases arise primarily from swallowed air but also from bacterial activity in the intestine. Second, increasingly strong contractions of the proximal intestine occur in an effort to move the contents onward. The increasing pressure in the lumen leads to more secretions entering the intestine and also compresses the veins in the wall, preventing absorption, as the intestinal wall becomes edematous. The intestinal distention leads to persistent vomiting with additional loss of fluid and electrolytes. With small intestinal obstruction, there is no opportunity to reabsorb fluid and electrolytes, and hypovolemia quickly results (Barbara, 2002).

If the obstruction is not removed, the intestinal wall becomes ischemic and necrotic as the arterial blood supply to the tissue is reduced by the pressure. If twisting of the intestine (e.g.: volvulus) has occurred or if immediate compression of arteries (e.g.: intussusception or strangulated hernia) results from the primary cause of obstruction, the intestinal wall becomes rapidly necrotic and gangrenous. Ischemia and necrosis of the intestinal wall eventually lead to decreased innervation and cessation of peristalsis. A decrease in bowel sounds indicates this change.

Usually, the obstruction promotes rapid reproduction of intestinal bacteria, some of which produce endotoxins. As the affected intestinal wall becomes necrotic and more permeable, intestinal bacteria or toxins can leak into peritoneal cavity (peritonitis) or into the blood supply (bacteremia and septicemia). In time, perforation of the necrotic segment may occur, leading to generalized peritonitis (Barbara, 2002).

For functional obstruction or paralytic ileus usually results from neurologic impairment. Peristalsis ceases and distention of the intestine occurs as fluids and electrolytes accumulate in the intestine. In this type of obstruction, reflex spasms of the intestinal muscle do not occur, but the remainder of the process is similar to that of mechanical obstruction (Barbara, 2002).

6 SIGN AND SYMPTOMS

With mechanical obstruction of the small intestine, severe colicky abdominal pain develops as peristalsis increase initially, nausea and constipation characterizing small bowel obstruction. It may also cause drowsiness intense thirst, malaise and aching may dry up oral mucous membranes and the tongue. Borborygmi are audible rumbling sounds caused by movement of gas in the intestine and intestinal rushes can be heard as the intestinal muscle forcefully contracts in an attempt to propel the contents forward. These are loud enough to be heard without stethoscope. Palpation elicits abdominal tenderness with moderate distention; rebound tenderness occurs when obstruction has caused strangulation with ischemia.

The signs of paralytic ileus differ significantly in that bowel sounds decrease or are absent and pain is steady. Vomiting and abdominal distension occur quickly with obstruction of the small intestine. Vomiting is recurrent and consists first gastric contents and then bile-stained duodenal contents. No stool or gas is passed. Restlessness and diaphoresis with tachycardia is present initial. As hypovolemia and electrolyte imbalance progress, signs of dehydration, weakness, confusion and shock are apparent (Barbara, 2002).

Signs and symptoms of large bowel obstruction develop more slowly because the colon can absorb fluid from its contents and distend well beyond its normal size. Constipation may be only clinical effect for days. Colicky abdominal pain may then appear suddenly, producing spasms that last less than 1 minute each and recur every few minutes. Continuous hypogastric pain and nausea may develop, but vomiting is usually absent at first. Large bowel obstruction can cause dramatic abdominal distention; loops of the large bowel may become visible on the abdomen. Eventually, complete large bowel obstruction may cause fecal vomiting, continuous pain or localized peritonitis. Patients with partial obstruction may display any of the above signs and symptoms in a milder form. However, leakage of liquid stool around the obstruction is common in partial obstruction (Lippincott Williams & Wilkins, 2007).

7 IMAGING MODALITIES

7.1 Laboratory Test

Laboratory evaluation of patients with suspected obstruction should include a complete blood count and metabolic panel. Hypokalemic, hypochlomeric metabolic alkalosis may be noted in patients with severe emesis. Elevated blood urea nitrogen levels are consistent with dehydration, and haemoglobin and hemotocrit levels may be increased. The white blood cell count may be elevated if intestinal bacteria translocate into the bloodstream. It can cause the systemic inflammatory response syndrome or sepsis. The development of metabolic acidosis especially in a patient with an increasing serum lactate level, may signal bowel ischemia (Patrick G. Jackson & Manish Raiji, 2011).

7.2 Plain Radiography

The initial imaging study of choice for confirming bowel obstruction with clinical signs and symptoms of intestinal obstruction should include pain upright abdominal radiography. This radiography can quickly determine if intestinal perforation has occurred. The free air can be seen above the liver in upright films or left lateral decubitus films. Radiography is 60-70% sensitivity for detection of small bowel obstruction and it performs better in high-grade obstruction. Radiography can be find the diagnosis of small bowel obstruction include distended loops of small bowel, collapsed colon, the “string of pearls sign resulting from small amount of residual air compared with the large amount of retained fluid and pseudotumour related to distended fluid filled loops.

In large bowel obstruction, it is important to note the degree of caecal distension on the plain abdominal films, since marked distension will point to the need for urgent decompression to prevent caecal perforation. In patients with small bowel obstruction, supine views show dilation of multiple loops of small bowel with a paucity of air in the large bowel (Figure 4). Those with large bowel obstruction may have dilation of the colon with decompressed small bowel in the setting of the competent ileocecal valve. Upright or lateral decubitus films may show laddering air fluid level (Figure 5). These findings show in conjunction with a lack of air and stool in the distal colon and rectum are highly suggestive of mechanical intestinal obstruction (Patrick G. Jackson & Manish Raiji, 2011).

Figure 4: Supine view of the abdomen in a patient with intestinal obstruction. Dilated loops of small bowel are visible (arrows) (Patrick G. Jackson & Manish Raiji, 2011).

Figure 5: Lateral decubitus view of the abdomen, showing air-fluid levels consistent with intestinal obstruction (arrows) (Patrick G. Jackson & Manish Raiji, 2011).

Limitations:

Fails to diagnose the cause of obstruction in most cases

In obstruction of ileocaecal region, it may be difficult to determine whether the level in the proximal large bowel or distal ileum

Cannot reliably detect the presence of ischaemic complication

7.3 Computed Tomography

7.3.1 Small bowel obstruction

Generally, it is considered the imaging modality of choice when plain abdominal radiography and the clinical features suggest an acute small bowel obstruction. CT is sensitive for detection of high-grade obstruction (90-96%). Although CT is highly sensitive and specific for high-grade obstruction, its value diminishes in patients with partial obstruction. In these patients, oral contrast material may be seen traversing the length of the intestine to the rectum with no discrete area of transition. It useful in:

Confirming or excluding small bowel obstruction (versus pseudo-obstruction)

Defining the degree and site of obstruction

Identifying the cause of small bowel obstruction (73-95% sensitivity)

Confirming or excluding the diagnosis of ischaemia (>90% sensitivity and specificity)

The advantages are:

Superior to enteroclysis in showing extraluminal masses, revealing abscesses, malignancy, anterior adhesions as well as features of strangulation.

It is ability to depict other causes of an acute abdomen

The limitations:

Lower sensitivity (approximately 50%) for the detection and location of low-grade small bowel obstruction

(Diagnostic Imaging Pathways, 2009)

7.3.2 Large Bowel Obstruction

It indicated as an alternative to contrast enema in evaluation of large bowel obstruction, particularly with elderly and immobile patients. CT findings in patients with intestinal obstruction include dilated loops of bowel proximal to the site of obstruction with distally decompressed bowel. The presence of discrete transition point helps guide operative planning (Figure 6). Absence of contrast material in the rectum is also an important sign of complete obstruction. For this reason, rectal administration of contrast material should be avoided (Patrick G. Jackson & Manish Raiji, 2011). The advantages are:

Does not require insertion of rectal tube and contrast and is therefore better tolerated than a contrast enema

In one study, successfully diagnosed colonic obstruction in 96% of patients and pseudo-obstruction in 93% of patients

The limitations for this are false negative and false positive results and limited diagnostic with partly obstructing lesions (Diagnostic Imaging Pathways, 2009).

Figure 6: Axial computed tomography scan showing dilated, contrast- filled loops of bowel on the patient’s left (yellow arrows), with decompressed distal small bowel on the patient’s right (red arrows). The cause of obstruction, an increased umbilical hernia, can also be seen (green arrows), with proximally dilated bowel entering the hernia and decompressed bowel exiting the hernia (Patrick G. Jackson & Manish Raiji, 2011).

7.4 Contrast Fluoroscopy

Contrast studies, such as small bowel follow-through, can be helpful in the diagnosis of a partial intestinal obstruction in patients with high clinical suspicion and in clinically stable patients in whom initial conservative management was not effective. The use of water-soluble contrast material is not only diagnostic, but may be can use for therapeutic in patients with partial small-bowel obstruction. Contrast fluoroscopy may also be useful in determining the area of intestine that need for surgery (Patrick G. Jackson & Manish Raiji, 2011).

There are several variations of contrast fluoroscopy. In the small bowel follow-through study, the patient drinks contrast material, then serial abdominal radiographs are taken to visualize the passage of contrast through the intestinal tract. The advantage of small bowel follow-through is does not require nasointestinal intubation, and compared to enteroclysis, it is easier to perform and does not require additional expertise. The limitations for this procedure are they take time for contrast to reach the obstruction and barium is diluted because of excess residual intraluminal fluid resulting in non-uniform small bowel filling (Diagnostic Imaging Pathways, 2009).

Enteroclysis involves naso- or aro-duodenal intubation, followed by the instillation of contrast material directly into the small bowel. Although this study has superior sensitivity compared with small bowel follow through, it is more labor-intensive and is rarely performed. The advantage enteroclysis are it has ability to gauge the severity of obstruction objectively. The limitations are it need for nasoenteric intubation and demonstration of extrinsic causes is sometimes difficult. Rectal fluoroscopy can be helpful in determining the site of a suspected large bowel obstruction (Patrick G. Jackson & Manish Raiji, 2011)

7.5 Magnetic Resonance Imaging (MRI)

Magnetic resonance imaging (MRI) may be more sensitive than CT in the evaluation of intestinal obstruction. MRI enteroclysis, which involves intubation of the duodenum and infusion of contrast material directly into small bowel. It can more reliably determine the location and cause of obstruction. However, MRI is not universally available and very expensive (Patrick G. Jackson & Manish Raiji, 2011).

7.6 Ultrasonography

In patients with high-grade obstruction, ultrasound evaluation of the abdomen has high sensitivity for intestinal obstruction, approaching 85 percent. However, because of the wide availability of CT, it has largely replaced ultrasonography as the first-line investigation in stable patients with suspected intestinal obstruction. Ultrasonography remains a valuable investigation for unstable patients with an ambiguous diagnosis and in patients that contraindication with radiation exposure, such as pregnant women (Patrick G. Jackson & Manish Raiji, 2011).

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