Aims
To describe the development and function of the small bowel.
Anatomy
The length of the small intestine varies from 10 to 33 feet (3–10 metres). The average length is considered to be approximately 22 feet (6.5 metres). A considerable length of small bowel can be excised and yet this may be compatible with a normal life. In some cases up to only 18 inches (45 cm) of small bowel has been preserved and the patient has survived satisfac- torily.
The mesentery of the small intestine has a 6 inch (15 cm) origin from the posterior abdom- inal wall and commences at the duodenal- jejunal junction, just to the left of the second lumbar vertebra. The mesentery passes down- wards towards the right sacral-iliac joint. The mesentery contains the superior mesenteric vessels along with lymphatics and lymph nodes.
These drain the small intestine. There are a number of autonamic nerve fibres within the mesentery.
The small bowel is divided into three sec- tions. The first section is the duodenum, which is approximately 1 foot in length (25 cm) and extends from the pylorus to the duodenal-
jejunal flexure; this point is marked by the liga- ment of Treitz. The duodenum is anatomically divided into four parts and curves in the shape of the letter C around the head of the pancreas.
At its origin the duodenum is covered with peri- toneum for about an inch (2.5 cm) after which it becomes a retroperitoneal organ.
The upper half of the small intestine is termed the jejunum and the remainder is the ileum.
There is no obvious distinction between the two parts and the division is one of convention only.
However, the character of the small intestine does change as it is followed distally towards the caecum.
The jejunum has a thicker wall as the circu- lar folds of mucosa (valvulae conniventes) are larger and thicker. The proximal small bowel is of greater diameter than the distal small bowel.
In addition, the jejunum tends to lie towards the umbilical region of the abdomen and the ileum to the hypogastrium and pelvis.
Mesenteric vessels tend to form fewer arcades in the jejunum with long and relatively infre- quent terminal branches passing to the intesti- nal wall. However, the ileum tends to be supplied by shorter and more numerous vessels which arise from a number of complete arcades.
Blood Supply to the Small Intestine
The small intestine develops from the midgut and this extends from the mid-duodenum to the 5
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The Anatomy and Physiology of the Small Bowel
David Gourevitch
distal transverse colon and is supplied by the superior mesenteric artery, which arises from the aorta at the level of L1. The branches of superior mesenteric artery include:
1. The inferior pancreaticoduodenal artery, which supplies the pancreas and duode- num.
2. Jejunal and ileal branches of the superior mesenteric artery; these give the blood supply to the bulk of the small intestine.
3. The ileal-colic artery, which supplies the terminal ileum, the caecum and the proximal part of the ascending colon.
This also goes off an appendicular branch to the appendix.
4. The right colic artery, which supplies the ascending colon.
5. The middle colic artery, which supplies the transverse colon to approximately two-thirds along its length. This vessel creates a watershed between the superior mesenteric artery and the inferior mesenteric artery.
The small intestine drains via the superior mesenteric vein and forms a confluence with the splenic vein to form the portal vein. This runs through the free edge of the lesser omentum and forms part of the superior border to the gastroepiploic foramen, before the portal vein continues to the liver.
Histology of the Intestinal Wall
The mucosa of the small bowel is thrown into a series of folds by the plicate are the valvulae conniventes. This greatly increases the surface area available for absorption within the small bowel.
The mucosa of the small bowel contains intestinal villi which are covered by simple colu- mnar epithelium and broken into microvilli.
The wall of the small intestine is divided into the lamina propria, and this is divided from the submucosa by the muscularis mucosae. Within the lamina propria there is an extensive net- work of capillaries which transports respiratory gases and absorb material to the hepatoportal circulation. In addition, there are capillaries and nerve endings within the lamina propria and each villi also contains a terminal lymphatic called a lacteal.
The name lacteal refers to the cloudy appear- ance of the lymph contained within these channels. The lacteals themselves transport materials that fail to enter the local capillaries because they are unable to cross the capillary wall. Examples would be of fatty acids and proteins which are too large to diffuse into the bloodstream. These lipoproteins form small partials called chylomicrons which pass through the lymphatic system and account for the milky appearance within the lacteal.
Intestinal Crypts
Within the columnar epithelium there are goblet cells which produce mucus onto the intestinal surfaces. At the base of the villi there are also found entrances to the intestinal crypts.
These extend deep into the underlying lamina propria. Within the intestinal crypts there are a number of different cell populations including stem-cell divisions which continue to produce new generations of columnar and goblet cells.
These new cells are continuously displaced towards the intestinal surface and within a few days will reach the tip of the villi, where they will be shed or exfoliated into the intestinal lumen.
It is this process of exfoliation of the intestinal cells which ensures that the epithelial surface continues to be renewed. This also adds intra- cellular enzymes to the intestinal contents. One of these enzymes would be enterokinase, which although it does not does not directly partici- pate in the digestion of food, is important because it activates proenzymes secreted by the pancreas.
Cells within the intestinal crypts also contain enteroendocrine cells. These are responsible for the production of several intestinal hormones including cholecystokinin and secretin.
Physiology
The Duodenum
The duodenum has very little absorptive function and acts mainly to neutralise the acidic contents delivered to it by the stomach.
The duodenum receives the chyme from the stomach and its essential function is to buffer the gastric acid and enzymes before delivering the contents to the jejunum.
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The histological characterisation of the duo- denum reveals abundant presence of mucus- secreting glands. These submucosal glands, known as Brunner’s glands, assist in the pro- duction of copious amounts of mucus. The secretion of this mucus is to protect the duode- nal mucosa and also to neutralise the acid pH of the chyme. The submucosal glands are most abundant in the proximal duodenum and decrease in number towards the jejunum. The pH of the duodenal contents rises from a pH of 1–2 to 7–8 by the time it is delivered to the jejunum. In addition, the chyme is diluted by mixing with the intestinal, pancreatic and hepatic secretions.
The duodenal ampulla lies within the wall of the second part of the duodenum and allows for the delivery of bile and pancreatic enzymes to initiate the digestion and breakdown of the chyme. Absorption may occur; it is more effec- tive under these conditions and the increase in the surface area of the duodenum in its third and fourth parts supports this increased absorptive capacity.
The Jejunum and Ileum
The anatomy of the jejunum is a testament to the absorptive capacity. The valvulae con- niventes and villi remain most prominent over the proximal half of the jejunum and it is over this area that most absorption occurs. As the ileum is approached the valvulae conniventes and villi become smaller and less numerous;
they continue to diminish in size along the length of the small bowel. Under normal cir- cumstances all absorption occurs before the chyme reaches the terminal ileum. It is squirted into the caecum where the mucosal villi are less numerous and reduced in height.
One of the major differences between the large and small intestine is the presence of bacterial activity. Bacterial presence in the large bowel is normal. However, the epithelium and underly- ing cells of the immune system protect the small bowel from the bacteria migrating from the large bowel. Within the ileum there are concentra- tions of lymphatic tissue in the submucosa called Peyer’s patches. These represent aggrega- tions of lymphatic tissues and they may extend for an inch or more in length. The patches are most abundant in the terminal ileum although they are seen occasionally in the duodenum.
These aggregating lymphatic follicles are placed lengthways in the intestinal wall and are usually found towards the ante-mesenteric border. Each Peyer’s patch is formed by a group of solitary lymphatic follicles covered with columnar epithelium, but as a rule, the patches do not possess villi on their free surfaces. There is an abundance of lymphoid vessels around these patches. Ulceration of these lymphatic fol- licles may occur in typhoid fever where oval ulcers are formed in the long axis of the bowel.
Perforation may occur following infection with Salmonella Typhi.
Small Bowel Peristalsis
The absorptive capacity of the small bowel is enhanced by the surface area and the peristaltic movements within the small bowel. The indi- vidual villi and microvilli are manipulated by the underlying muscularis mucosae and this increases the environment around the small bowel mucosa, thereby maximising the oppor- tunity for absorption throughout its length. At least 80% of absorption takes place within the small intestine and only a small amount within the stomach and the large intestine.
Absorption occurs as the chyme is moved along the length of the small bowel by peristaltic contractions. These contractions are myenteric reflexes associated with the splanchnic nerves and are not under the control of the central nervous system. The effect of this nerve plexus is limited to a short length of the small bowel at the site of the stimulus for peristalsis. However, there are more elaborate and coordinated reflexes and two examples are the gastroenteric reflex and the gastroileal reflex.
Gastroenteric and Gastroileal Reflexes
The gastroenteric reflex is initiated by disten- sion of the stomach and this stimulates an increase in the rate of glandular secretion and peristalsis activity in the duodenum and the small bowel. This increased peristalsis allows for chyme to move through the duodenum and into the small bowel.
The gastroileal reflex is a combination of a neural mediated gastroenteric reflex and a response to circulating levels of the hormone 111
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gastrin. The entry of food into the stomach trig- gers the release of gastrin and this relaxes the sphincter at the ileocaecal junction allowing for increased ileal peristalsis and the passage of chyme into the caecum. This reflex is often asso- ciated with the experience of dull, lower abdom- inal discomfort and a person’s desire to open their bowels.
Intestinal Secretions
There is a large flow of secretions cross the small bowel mucosa which is estimated to be between 1.5 and 2.5 litres of fluid within a 24-hour period. The fluids pass into the intestinal lumen and the vast majority will be re-absorbed before the contents enter the caecum. This movement of fluid across the intestinal mucosa is called succus entericus. Some of this fluid enters the intestinal lumen through osmosis because of the relatively concentrated chyme. The remain- ing fluid is produced by the intestinal glands and stimulated by activity of touch and stretch receptors within the wall of the intestine.
The intestinal juice assists in buffering the acids and dissolves the digestive enzymes and the products of digestion.
Regulation of the secretory output of the larger digestive and accessory glands is via the central nervous system and hormonal control. These mechanisms tend to occur in the region of the duodenum where acids are neu- tralised and enzymes released. The submucosal glands within the duodenum protect the duo- denum from the gastric acid output and the local secretion of enzymes.
There are local reflexes mechanisms which stimulate the secretory activity of the duode- num and this activity is enhanced by the parasympathetic (vagal) stimulation. It is recog- nised that duodenal glands begin secreting before chyme reaches the pylorus. This is con- sidered the cephalic phase of gastric secretion.
Stimulation of the sympathetic chain inhibits secretory activity leaving the duodenal mucosa unprepared for receiving chyme, and this may have some part to play in the production of duodenal ulcers.
Intestinal Hormones
The enteroendocrine cells within the duode- num produce hormones which coordinate the
secretory activity of the stomach, duodenum, liver and pancreas. Enterocrinin is a hormone which is released by the duodenal mucosa when the acid chyme from the stomach enters the small intestine. There are many other hor- mones secreted which have both primary and secondary effects, and which act in a comple- mentary fashion. The three most important hor- mones involved in the regulatory activity of the small intestine are secretin, cholecystokinin and the glucose-dependent insulinotropic peptide.
Secretin
Secretin is produced in response to the presence of acid within the duodenum. The primary effect of secretin is to increase the production of water and buffers by the pancreas and liver. It also has an effect on stimulating the duodenal submucosal glands.
Cholecystokinin (CCK)
The duodenal mucosa is stimulated to produce cholecystokinin when chyme arrives within the lumen of the duodenum and particularly when it contains lipids and partially digested proteins. This hormone has a target effect both on the pancreas and on the liver. The pancreas is stimulated to produce and secrete digestive enzymes, and the hormone also increases the passage of bile by stimulating the gall bladder to contract.
The net effect of cholecystokinin is to increase the secretion of pancreatic enzymes and stimu- late the production of bile. However, in high concentration both secretin and cholecys- tokinin have the additional effect of producing gastric motility and secretions.
Glucose-dependent Insulinotropic Peptide (GIP)
This peptide is released by the duodenal mucosa in response to fats and glucose entering the duo- denum. This peptide stimulates the release of insulin from the pancreatic islet cells, although at high concentration it can also inhibit gastric activity. (Originally this was named the gastric inhibitory peptide.)
Vasoactive Intestinal Peptide (VIP)
Several other hormones are produced in small quantities in response to chyme entering the
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duodenum. For example, relatively large amounts of undigested proteins will stimulate the release of gastrin by the duodenal cells.
Vasoactive intestinal peptide or VIP is also pro- duced and it stimulates the secretion of the intestinal glands whilst inhibiting acid produc- tion within the stomach. Previously, it was con- sidered that the enzyme called enterogastrin was responsible for inhibiting gastric activity.
However, it is now considered that this inhibi- tion of gastric motility is the product of GIP and VIP.
The number and diversity of the hormones produced by the small bowel are well recog- nised, but poorly understood. Many of the hor- mones have a similar chemical structure and it is difficult to differentiate the primary effects of these various hormones. Analysis has led to an increased number of hormones being identified, although their specific functions are poorly understood.
The Embryology of the Small Intestine (Midgut)
In the adult the midgut starts immediately distal to the point where the bile duct enters the duo- denum and it terminates at the junction of the proximal two-thirds of the transverse colon with a distal third. The superior mesenteric artery supplies the entire length of the midgut.
Within the 5-week-old embryo the midgut is suspended by a short mesentery from the pos- terior abdominal wall and it communicates with the yolk sac by way of the vitello-intestinal duct. At the apex of the midgut loop there is a connection with the yolk sac via the vitelline duct. The proximal or cephalic limb of the loop becomes the distal part of the duodenum, the jejunum and part of the ileum, and the cordal or distal portion of the loop becomes the ileum, caecum, appendix, ascending and proximal transverse colon.
With the rapid growth and expansion of the liver and the elongation of the midgut, the abdominal cavity becomes too small to contain the intestinal loops. For a period during the sixth week of development, the intestinal loops enter an extra-embryonic cavity within the umbilical cord; this is considered to be a phys- iological umbilical herniation.
By the tenth week the herniated intestinal loops are returning to the abdominal cavity.
The precise factors responsible for this are not known although as the mesonephric kidney regresses and there is a reduced growth of the liver with some expansion of the abdominal cavity, space becomes available to allow for the return of the midgut to the abdomen. As the midgut retracts into the abdomen it also rotates and with the expansion of the caecal bud, which appears around the sixth week, the characteris- tic placement of the midgut within the abdom- inal cavity occurs. The distal midgut expands and there is some separation into the small and large intestine. A small narrow diverticulum is formed from the caecal bud which develops into the appendix.
The mesenteries of the intestinal loops are produced during the changes and rotation of the midgut around the superior mesentery vessels.
With fusion of the mesenteric layers the small intestine retains a long and mobile mesentery;
however, the caecum and ascending colon become fused with the posterior abdominal wall.
Associated with the embryological develop- ment of the small intestine a number of abnor- malities can occur. Abnormal rotation of the intestinal loop may occur and this results in a volvulus where the blood supply to the loop is compromised, particularly when the base of the small bowel mesentery is shortened. On occa- sions there can be reverse rotation of the intesti- nal loop and the small intestine is found towards the right side of the abdomen, with the caecum and the large intestine to the left. Further abnor- malities may include duplication of the intesti- nal loop with cysts. These cysts are most frequently found within the region of the ileum and they may vary from a long segment to a short one with a small diverticulum.
Other abnormalities of the small intestine may be associated with defects within the abdominal wall. An omphalocele (exomphalos) involves the herniation of the abdominal viscera through a defect within the umbilical ring. This defect often contains small bowel, and liver, stomach, spleen and gall bladder may also be included. The defect is thought to be caused by failure of the bowel to return to the body cavity following its physiological herniation between the sixth and tenth week of development. This defect may occur in up to 2.5 per 10 000 births and it is associated with a high mortality.
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A further abnormality which can be confused with the omphalocele is gastroschisis. This defect is characterised by an abnormality within the abdominal wall and occurs lateral to the umbilicus. The viscera are found within the aminotic cavity and are not covered with the peritoneum. It is thought that this abnormality occurs in approximately 1 in 10 000 births and there has been some increase in its frequency among babies born to young women who are using cocaine. Unlike the omphalocele, in which 50% of infants have a chromosome abnormal- ity, gastroschisis is not usually associated with any specific chromosomal abnormality.
More commonly vitello-intestinal duct abnormalities occur in 2–4% of people. These abnormalities may consist of a small out-pouch- ing of the ileum with the development of a Meckel’s diverticulum, but other abnormalities may occur and include a vitelline ligament or a cyst within the ligament, and occasionally a fistula is formed which connects the small bowel with the umbilicus. With this last defect there is usually a small faecal fistula present at the time of birth. These abnormalities are usually easy to correct surgically, although the patient can be quite ill and present with an intestinal obstruc- tion, often at a young age.
Other common abnormalities within the small bowel may include atresias or stenoses.
These can occur at any point along the intestine, although they occur most frequently within the duodenum. These abnormalities are likely to be the result of “vascular accidents”. They may occur as a result of malrotation, volvulus, and can be associated with an omphalocele or gas- troschisis, when the blood supply to the bowel is compromised and this results in fibrosis and narrowing of the bowel. On occasions, a small segment of bowel can be lost and replaced with a fibrous cord, or more simply, there can be a length of narrowing within the small bowel, and children present with subacute intestinal obstruction.
Questions
1. Describe the anatomy of the midgut.
2. Name the important hormones of the small bowel and describe their function.
3. What abnormalities can occur in devel- opment?
Further Reading
Sadler TW. Langman’s medical embryology, 9th edition.
Philadelphia: Lippincott Williams & Wilkins, 2003.
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