Notes from the Video

What You're Going to Learn from This Video on Small Intestinal Anatomy

How well do you understand small intestinal anatomy? 

Not just the gross anatomy but the development, the architecture and the histology?

If you want to understand surgical anatomy in the abdomen it is absolutely critical that you know small intestinal anatomy inside and out.

An Overview of the Small Intestine

The small intestine is a long, narrow tube that forms part of the digestive system. It is approximately 6 meters long in adults and is divided into three sections: the duodenum, the jejunum, and the ileum. Here is a detailed explanation of the gross anatomy of the small intestine:

What is the embryology of the small intestine?

The development of the small intestine begins during the fourth week of embryonic development and is completed by the tenth week. The process involves several complex steps that transform the primitive gut tube into the fully formed small intestine.

During the fourth week, the foregut undergoes rapid growth and elongation, forming a U-shaped tube that will eventually give rise to the entire gastrointestinal tract. At the same time, the dorsal mesentery, a double layer of peritoneum, begins to attach to the posterior abdominal wall, suspending the foregut within the peritoneal cavity.

Around the fifth week, the foregut undergoes further differentiation into three regions: the cranial part, which will form the pharynx and esophagus; the midgut, which will form the small intestine, cecum, appendix, ascending colon, and proximal two-thirds of the transverse colon; and the hindgut, which will form the distal third of the transverse colon, descending colon, sigmoid colon, rectum, and anal canal.

The midgut initially grows outside the abdominal cavity into the umbilical cord, forming the physiological herniation.

The midgut then returns to the abdominal cavity in a counterclockwise rotation around the superior mesenteric artery during the tenth week.

As the midgut returns, the cranial limb of the loop moves to the right and forms the duodenum, while the caudal limb moves to the left and forms the jejunum and ileum.

The midgut then undergoes extensive folding and elongation to form the mature small intestine.

The lumen of the small intestine initially forms as a solid cord of endodermal cells that subsequently recanalizes. The small intestine epithelium forms from the endoderm and gives rise to the crypts of Lieberkuhn, which contain the intestinal stem cells.

The small intestine also receives contributions from the surrounding mesoderm, which gives rise to the muscular layers and the blood vessels.

The smooth muscle cells in the wall of the small intestine develop from the mesodermal cells that surround the gut tube, while the blood vessels develop from the mesodermal cells that form the splanchnic layer of the lateral plate mesoderm.

What are the different parts of the small intestine?

Duodenum: The duodenum is the first part of the small intestine and is approximately 25 cm long. It is a C-shaped structure that begins at the pyloric sphincter of the stomach and ends at the jejunum. The duodenum receives digestive enzymes from the pancreas and bile from the liver and gallbladder. It also has specialized cells called Brunner's glands that secrete mucus to protect the duodenal lining from acidic stomach contents.

Jejunum: The jejunum is the middle section of the small intestine and is approximately 2.5 meters long. It is located in the upper left quadrant of the abdomen and is supported by the mesentery, a thin sheet of connective tissue that anchors the small intestine to the posterior abdominal wall. The jejunum is the primary site of nutrient absorption, and its walls are lined with finger-like projections called villi that increase the surface area for absorption.

Ileum: The ileum is the final section of the small intestine and is approximately 3.5 meters long. It is located in the lower right quadrant of the abdomen and terminates at the ileocecal valve, which separates it from the large intestine. The ileum is responsible for absorbing vitamin B12, bile salts, and any remaining nutrients that were not absorbed in the jejunum. The ileum also contains Peyer's patches, which are clusters of lymphoid tissue that help defend against pathogens.

How can a surgeon tell the difference between the jejunum and the ileum?

The jejunum and the ileum are two sections of the small intestine that are similar in appearance, and it can be challenging to distinguish them during surgery.

There are a few key differences that a surgeon can look for to differentiate the two:

Location: The jejunum is located in the upper left quadrant of the abdomen, while the ileum is located in the lower right quadrant. The ileum also extends closer to the midline of the body than the jejunum.

Thickness of the wall: The wall of the jejunum is thicker and more muscular than that of the ileum, which has a thinner wall and is more flaccid.

Length and number of plicae circulares: Plicae circulares are circular folds in the lining of the small intestine that help to increase its surface area. The jejunum has more and larger plicae circulares than the ileum, giving it a thicker and more irregular appearance.

Appearance of mesentery: The mesentery is the thin sheet of tissue that attaches the small intestine to the posterior abdominal wall. The mesentery of the jejunum is wider and more vascular than that of the ileum, which has a thinner and less vascularized mesentery.

What are Brunner's glands?

Brunner's glands, also known as duodenal glands, are compound tubular submucosal glands found in the submucosa of the duodenum. They are named after Johann Conrad Brunner, a Swiss anatomist who first described them in the 18th century.

Brunner's glands secrete an alkaline mucus-rich fluid that helps to neutralize acidic chyme (partially digested food) as it enters the duodenum from the stomach. This alkaline fluid also contains bicarbonate ions, which further helps to neutralize acid and create a suitable environment for digestive enzymes to work optimally.

The mucus secreted by these glands also lubricates and protects the intestinal wall from mechanical and chemical damage by the acidic chyme.

Brunner's glands are most numerous in the first part of the duodenum (the duodenal bulb), and their secretions are stimulated by the hormone secretin, which is released by the duodenum in response to acidic chyme. Dysfunction or damage to these glands can lead to disorders such as duodenitis (inflammation of the duodenum) and duodenal ulcers.

What is the histology of the small intestine and why is it important?

The histology of the small intestine refers to the microscopic structure and organization of its tissues, which includes the mucosa, submucosa, muscularis externa, and serosa layers.

These tissues play important roles in the absorption and digestion of nutrients from the food we eat.

The mucosa layer of the small intestine contains finger-like projections called villi, which are covered with microvilli. Together, the villi and microvilli greatly increase the surface area of the small intestine, allowing for efficient absorption of nutrients. The mucosa also contains goblet cells, which secrete mucus that helps protect the lining of the small intestine from digestive enzymes.

The submucosa layer of the small intestine contains blood vessels, lymphatic vessels, and nerve fibers that supply the small intestine with oxygen, nutrients, and information from the nervous system. The muscularis externa layer is responsible for moving food along the length of the small intestine through peristalsis, which is a coordinated contraction of smooth muscle fibers.

What are the intestinal crypts, what are the different cell types present and why are they important?

Intestinal crypts, also known as intestinal glands or crypts of Lieberkühn, are small tubular invaginations in the lining of the small and large intestine. These structures play an important role in the secretion and absorption of fluids and nutrients in the gut.

The intestinal crypts are composed of several specialized cell types, including:

Enterocytes: These are the most abundant cells in the intestinal crypts and they are responsible for absorbing nutrients from the lumen of the intestine.

Goblet cells: These cells produce and secrete mucus, which protects the lining of the intestine from the harsh environment of the gut.

Enteroendocrine cells: These cells produce and secrete hormones that help regulate various functions in the gut, such as digestion and absorption.

Paneth cells: These cells are found at the base of the crypts and produce and secrete antimicrobial peptides and other compounds that help protect the intestine from harmful bacteria.

The intestinal crypts are lined with a layer of stem cells that continually divide and differentiate into the various cell types mentioned above.

These stem cells are located near the base of the crypt and are responsible for the constant renewal of the intestinal epithelium.

In addition to their role in nutrient absorption and secretion, intestinal crypts also play a crucial role in maintaining the health of the gut microbiome.

The mucus produced by goblet cells and the antimicrobial peptides produced by Paneth cells help regulate the growth of beneficial bacteria in the gut, while preventing the overgrowth of harmful bacteria.

What is the vascular supply to the small intestine?

The small intestine is supplied by a rich network of arteries and veins that provide it with oxygen and nutrients and remove waste products.

The primary arterial supply to the small intestine is the superior mesenteric artery (SMA), which arises from the abdominal aorta just below the celiac artery. This occurs at the spine level of L1.

The superior mesenteric artery (SMA) gives off several branches that supply the small intestine and other adjacent structures.

These branches include:

Inferior pancreaticoduodenal artery: supplies the head of the pancreas and the duodenum.

Intestinal arteries: supply the small intestine and include the following branches:

Jejunal arteries: supply the jejunum (the proximal two-fifths of the small intestine).

Ileal arteries: supply the ileum (the distal three-fifths of the small intestine).

Ileocolic artery: supplies the terminal ileum, cecum, and ascending colon.

Right colic artery: supplies the ascending colon.

Middle colic artery: supplies the transverse colon.

Ileal branches: supply the terminal ileum.

Right colic artery: supplies the ascending colon.

Middle colic artery: supplies the transverse colon.

The venous drainage of the small intestine occurs via the superior mesenteric vein (SMV), which runs parallel to the SMA. The SMV drains blood from the small intestine into the portal vein, which carries it to the liver for processing.

The rich blood supply to the small intestine is essential for its function, as it allows for the efficient absorption of nutrients and fluids.

Disruption of blood flow to the small intestine, such as through occlusion of the SMA or SMV, can lead to ischemia (lack of oxygen) and necrosis (tissue death) of the affected area, which can be a life-threatening condition requiring urgent surgical intervention.

What is normal intestinal rotation and what are the consequences of abnormal rotation during embryological development?

Intestinal rotation is a complex process of embryonic development that occurs during the 5th to 10th week of gestation. It involves the rotation of the small intestine, the cecum, the ascending colon, and the transverse colon around the axis of the superior mesenteric artery (SMA) and the superior mesenteric vein (SMV).

The process of intestinal rotation is important for the proper development of the digestive system and its blood supply.

During the 5th week of embryonic development, the midgut region of the embryo grows rapidly and forms a U-shaped loop that protrudes into the umbilical cord.

At this stage, the SMA and SMV lie in the center of the U-shaped loop.

As the loop continues to grow, it rotates 90 degrees counterclockwise around the SMA and SMV, causing the cecum and ascending colon to move to the right side of the abdomen, and the transverse colon to move to the left side.

The rotation process continues until the intestine reaches its final position in the abdominal cavity for a total of 270 degrees of counterclockwise rotation. The cecum eventually becomes fixed in the right lower quadrant, while the ascending colon and the transverse colon rotate further until they assume their final position in the right upper quadrant and the left upper quadrant, respectively.

During this process, the small intestine also undergoes significant changes in shape and position.

As the intestinal loop rotates, it forms a series of coils, causing the small intestine to become elongated and to assume a characteristic "zig-zag" appearance. This elongation increases the surface area of the small intestine, which is important for the absorption of nutrients.

Failure of proper intestinal rotation can result in a number of congenital abnormalities, such as malrotation, volvulus, and intestinal atresia.

Malrotation occurs when the intestine does not rotate completely, resulting in a variety of abnormal positions of the intestine, including the possibility of the small bowel ending up on the right side of the abdomen.

Volvulus occurs when the intestine twists on its own axis, which can cause obstruction and ischemia.

Intestinal atresia occurs when part of the intestine fails to develop properly, resulting in a blockage in the digestive system.

What is a Meckel's Diverticulum?

A Meckel's diverticulum is a common congenital abnormality of the gastrointestinal tract that is caused by the incomplete closure of the vitelline duct.

It is named after Johann Friedrich Meckel, a German anatomist who first described it in 1809.

The rule of 2s is a mnemonic used to describe some of the characteristics of Meckel's diverticulum: it is present in approximately 2% of the population, is about 2 inches (5 cm) long, is located within 2 feet (60 cm) of the ileocecal valve, and is usually diagnosed by the age of 2 years.

On the other hand, a "Heckel's diverticulum" is a term sometimes used to describe a variant of Meckel's diverticulum that is located on the anti-mesenteric border of the intestine, rather than the usual mesenteric border. However, this term is not widely used or recognized, and the condition is usually still referred to as Meckel's diverticulum regardless of its location.

What is short gut syndrome?

Short gut syndrome, also known as short bowel syndrome, is a condition that occurs when a significant portion of the small intestine is surgically removed or doesn't function properly due to other medical conditions.

When a large portion of the small intestine is removed, the remaining intestine may not be able to adequately absorb enough nutrients and fluids, leading to malabsorption, malnutrition, dehydration, and other complications.

The severity of the condition depends on how much of the small intestine is removed and the overall health of the individual.

Common causes of short gut syndrome include surgical removal of the intestine due to conditions such as Crohn's disease, mesenteric ischemia, or trauma. Other conditions that may lead to short gut syndrome include radiation enteritis, congenital defects, or motility disorders.  Congenital problems that may lead to short gut syndrome include malrotation volvulus, necrotizing enterocolitis and intestinal atresia.

The symptoms of short gut syndrome may include diarrhea, abdominal cramps, bloating, malnutrition, weight loss, dehydration, and electrolyte imbalances.

Treatment may include a combination of nutritional support, medications to control symptoms, and sometimes surgery to lengthen the remaining small intestine. Nutritional support may include a combination of enteral feeds, parenteral nutrition (IV nutrition), and dietary modifications to optimize absorption.

How many centimeters of small intestine are required for survival?

The amount of small bowel needed for survival depends on several factors, such as the age and overall health of the individual, the degree of small bowel loss or dysfunction, and the ability of the remaining intestine to absorb nutrients and fluids.

Generally, it is estimated that a minimum of 100 cm of functional small bowel length is necessary for adequate absorption of nutrients and fluids to support life. However, this number can vary widely depending on the individual and their unique circumstances.

In cases of short gut syndrome the remaining intestine may need to adapt and compensate for the loss. This can occur through a process called intestinal adaptation, where the remaining intestine may undergo structural and functional changes over time to increase its ability to absorb nutrients and fluids.