digestive system

Digestive System

Introduction

At the top end of the endoderm (the oropharyngeal membrane), the ectoderm and mesoderm elaborate to protect the sensitive endoderm from direct contact with the outside world. Lips, teeth, and a large oral cavity grow as a kind of security post to guard the gut tube. They physically process what you acquire before you have to absorb it.From this point on or down,  as it were the endoderm conforms to a tube, and it absorbs both beneficial and harmful things alike. Some parts of the tube are more receptive to certain compounds, such as proteins, than to others, and some parts mostly reclaim water that the body has added to the mix to prevent dehydration. Entire organs develop from the tube to assist with the complex task of breaking down molecules before the long and winding intestinal road of absorption. The convoluted gut tube is suspended in the abdominal cavity in a sling of mesoderm, just as the early development of the embryo mandated. Like the lungs That derived from it, the gut tube presses against a closed sac in this case, the peritoneum. The parietal peritoneum is highly sensitive, and it often bears the consequence of gut tube disorders or at least traffics pain referred by them. The diagnosis and treatment of disease states related to the gut tube, however, rely primarily on blood chemistry data, which limit the applications of basic gross anatomy. The primary objectives for learning gut tube anatomy should be to understand the relative position of organs for physical diagnosis and clinical imaging and to master the position and informative qualities of the peritoneal sac against which the tube and its accessory organs grow.
Abdominal Cavity
When we last tracked the endoderm, it had rolled into a continuous tube and was virtually surrounded by a layer of mesoderm (now called visceral, or splanchnic, mesoderm). This arrangement is best appreciated in cross-section (Fig. 3.1). The endoderm develops primarily into the digestive system, which includes the tube itself plus the organs that bud off of it. The tube will expand, convolute, bud organs, and rotate as it becomes the adult gastrointestinal (GI) tract. Its contact with the visceral mesoderm leads to formation of a smooth muscle wall around the endoderm cells, which gives The gut tube a truly tubular appearance. It also enables a mechanical squeezing of the endodermal sleeve (peristalsis) that helps to move the processed food matter, or ingesta, along the system.




FIGURE 3.1 The endoderm folds into a tube.
This classic, cross-sectional view of the folding embryo shows how the endoderm layer of cells folds into a tube (A). As it becomes tubular, it maintains surface contact with the visceral layer of lateral plate mesoderm (B). The endodermal tube becomes suspended in a sling of visceral mesoderm. This sling is made up of two layers (one from each side of the body) and a potential space between them (C).

Note how the aorta is positioned such that it can send a branch to the gut tube between the two layers of the mesoderm that sling the gut tube. This construct of mesoderm is called a mesentery. Because the one shown here reflects off of the back wall of the body, it is called the dorsal mesentery. Remember that the endoderm gives rise only to the epithelium of the gut tube, not to the smooth muscle that acts on it. That smooth muscle is a derivative of mesoderm. When it contracts, the tube is squeezed, and this action facilitates the peristaltic effect of moving food particles along the production line. Part of the tube is found in the adult thorax, or thoracic cavity, and part is found in the abdomen, or abdominal cavity. These two cavities of the body are separated by the diaphragm, which, as you remember, was part of the transverse septum of mesoderm but became relocated during longitudinal folding. We should now finish the story of the cavities before we further examine the development of the tube itself. The lateral folding of the embryo creates a captured cavity, the intraembryonic coelom. This merged cavity space is continuous from top to bottom until the transverse septum cuts the single cavity in half during the longitudinal fold). The upper half becomes the thoracic cavity, and the piece of the coelom that remains there looks like a drooping arch. The heart grows against the bend of the arch, and the lungs grow against the limbs of the arch. Below the diaphragm, the merged limbs of the coelom become the peritoneal sac. The layer of mesoderm that completely lines and, thus, constitutes the peritoneal cavity is now called the peritoneum. Some of it coats the wall of the body (parietal peritoneum), and some of it coats the gut tube (visceral peritoneum). The highly elongated and organ-sprouting gut tube pushes against the peritoneum so much that very little cavity is left in the sac. Thus, fluid accumulation within the sac (ascites) quickly leads to discomfort and provokes medical attention.
Now that we have established how the peritoneal sac is formed, we can proceed to describe the gross anatomy of the digestive system. This relatively simple structural system of the body is incredibly complex physiologically. The clinical spectrum of complications in this system is vast, because its structure is in contact with the outside world and all its impurities. Major pathophysiologies, such as diabetes, cirrhosis, and colitis, result from dysfunctional behavior of this system consequent, in some cases, to consumption behavior. The clinical anatomy of these diseases is less apparent, so in studying the gross anatomy of the digestive system, the objectives are to master the names of its parts, to understand their nerve and blood supply, and to position the tube relative to the body wall that surrounds it.
Esophagus and Foregut The first part of the tube to consider is the section that connects the input hole (the mouth, or oral cavity) with the processing unit (the stomach, intestines, etc.). This part is called the esophagus, and it is located in the thorax. This section of the endodermal tube changes very little from its initial appearance (Fig. 3.2). It remains a flaccid tube surrounded by muscle. The muscle arises from the visceral mesoderm that coated the gut tube after lateral folding (see Fig. 1.16). When the muscles of the esophagus contract, they pulse whatever is inside the esophagus downward. This peristalsis is governed by parasympathetic fibers of the vagus nerve (cranial nerve X). Dysfunction of this process is increasingly common and can lead to gastroesophageal reflux disease (GERD).
The esophagus passes behind the diaphragm, but it projects forward just enough that the diaphragm collars it. The extent to which the diaphragm squeezes the transition between the esophagus and stomach (gastroesophageal junction) may lead to indigestion, reflux of food, and/or heartburn. Heartburn refers to the mistaken sense that the discomfort is in the nearby heart and not the esophagus, which, in turn, might lead the patient directly to the emergency room. The gastroesophageal junction also renders the diaphragm vulnerable to slackening, which could result in a herniation of the gut tube. A sliding hiatal hernia is one in which the entire junction and the upper part of the stomach slide up through the hiatus, creating an uncomfortable pinch of the stomach sac (Fig. 3.3).

FIGURE 3.2 The adult gut tube.
The esophagus is an unmodified tube, just a conduit between where food is initially processed (oral cavity) and where it is digested (stomach and beyond).

Anatomists describe the developing gut tube below the diaphragm as having three regions: a foregut, a midgut, and a hindgut. Each region draws a dedicated artery from the developing circulatory system, so this classification is somewhat logical. The foregut also is the part of the tube that buds off all the accessory organs, so the division of foregut and midgut is even more logical. The transition from midgut to hindgut is more arbitrary, in the sense that both have a similar function of absorption, their nerve supplies overlap, and the exact point at which the circulatory supply of one blends into the circulatory supply of the other is vague.

FIGURE 3.3 Hiatal hernia.
The relationship between the gut tube and the diaphragm is lax enough that the tube can herniate into the thorax, typically by sliding up the esophageal hiatus

The foregut region becomes the stomach, the accessory organs of digestion and the first part of the duodenal portion of the small intestine. All this makes sense considering what the digestive system must accomplish once the ingesta finally gets below the diaphragm. The foregut is the domain of the celiac trunk of arteries (Fig. 3.4), the first of the midline branches of the abdominal aorta. The foregut has one more distinguishing feature. When the septum transversum arrived to divide the thorax from the abdomen, it actually bridged the space from the foregut to the ventral body wall. As the cranial portion of the septum transversum developed into the diaphragm, the caudal portion thinned into a ventral mesentery. Only the foregut has a ventral mesentery (Fig. 3.5). This ventral mesentery, which is exactly similar in design to the dorsal mesentery that runs the entire length of the gut tube, is available to sandwich anything that might bud off from the foregut.


FIGURE 3.4 A dedicated branch of the aorta serves each gut tube region.

The celiac trunk serves the foregut region and the organs that bud from it. The superior mesenteric artery serves the midgut region, and the inferior mesenteric artery serves the hindgut region. Note that the term mesenteric is used here. This implies that the arteries are located within the mesentery between the body wall and the gut tube.

FIGURE 3.5 Formation of a ventral mesentery in the foregut region.
The accessory organs of digestion (liver, pancreas, and gallbladder) derive from the foregut only. Like the gut tube, they rest in a sling of mesoderm, but because the gut tube already occupies the dorsal mesentery, these organs need a mesentery of their own. The ventral mesentery appears to form from thinning of the overlying mesoderm of the septum transversum.

The first task of the foregut is to store the ingesta, and the first structure of the foregut is an inflated part of the tube called the stomach. Structurally, the stomach is simply an expansion of the gut tube to form a larger pouch. Functionally, the stomach secretes a variety of strong acids to reduce the ingesta even more. These acids work effectively on protein compounds.
The stomach is not centered in the middle of the body, which is where it starts out as part of the endodermal gut tube. Indeed, the stomach rotates as it forms (Fig. 3.6). The dorsal border of the foregut expands first, creating a greater curvature along that border and a lesser curvature along the ventral border. At the same time, the tube spins 90 on its own axis because of the rapid growth of the liver (see below). This positions the greater curvature facing the left side. Eventually, this expanded greater curvature sags down so that it points inferiorly, and this is the final position of the normal stomach, the dominant organ in the left upper quadrant of the abdomen (Fig. 3.7).


FIGURE 3.6 The dorsal mesentery of the stomach warps.
The stomach part of the gut tube balloons posteriorly but not anteriorly, resulting in a surface of greater curvature and a surface of lesser curvature. The stomach also rotates (A, B) to accommodate rapid growth of the neighboring liver (not shown). One result is a longer apron of dorsal mesentery, which is known as the greater omentum (C).

Remember that like all parts of the gut tube, the foregut suspends from the vertebral column within the sling of dorsal mesentery created by the visceral layer of mesoderm. The expansion, twisting, and sagging of the stomach region affects this mesentery as well. It follows the position of the greater curvature such that it greatly elongates and folds down like an apron by the end of growth. This apron of mesentery is called the greater omentum (see Fig. 3.7).
Each gut tube region is served by a dedicated artery. The blood supply to the stomach must be from the artery of the foregut, the celiac trunk. A larger point, however, is at play here. Note that the gut tube began as a midline structure running parallel to, but in front of, the vertebral column. The only structure between the two is the dorsal aorta. The shortest possible route for blood to reach the gut tube is as a direct branch of the aorta that runs between the two layers of mesoderm that droop off of the body wall to sling the gut tube (see Fig. 3.1). In the case of the stomach, the arteries are branches of the celiac trunk. However, because the dorsal mesentery of the stomach elongates so much as the foregut expands and rotates, it would not be economical for the blood supply to elongate and hang down like an apron as well. Instead, the blood supply to the stomach approaches from the top of the dorsal mesentery (to catch the very top of the greater and lesser curvatures), or it shuttles in at the bottom of the stomach expansion (to catch the bottom of the greater and lesser curvatures) .

FIGURE 3.7 The stomach and its mesenteries.
The dorsal mesentery persists as the remarkable greater omentum, a double-layer fold of fat-rich connective tissue (A). This unwieldy expanse of mesentery eventually incorporates the transverse part of the colon (B). It has been called the abdominal policeman because of its perceived role in defending the peritoneum by adhering to sites of inflammation, absorbing bacteria and other contaminants, and providing leukocytes for a local immune response. The ventral mesentery persists as the lesser omentum. It cordons a lesser part of the peritoneal sac posteriorly and ensheaths the ducts that connect the accessory organs back to the gut tube.

The portion of foregut distal to the stomach will form the proximal part of the duodenum (Fig. 3.8). This C-shaped tube marks the transition toward the absorbing portion of the gut tube; it also marks the end of the accessory organs that are attached to the tube (see below). Developmentally, the part of the duodenum that forms from the foregut region of the tube is indicated by the persistence of a ventral mesentery (see Fig. 3.7). All subsequent parts of the tube have only a dorsal mesentery.
The duodenum demonstrates a key principle of digestive system anatomy. The gut tube greatly elongates during growth, reaching a linear distance of approximately 20 feet. To package all of that in the small volume of the adult abdominal cavity, the tube must curl and ball up, much like trying to put a long hose in a small box. All this accommodation distorts the relationship of the tube to the dorsal mesentery. The possible outcomes are illustrated in Figure 3.9.


FIGURE 3.8 Regional anatomy of the duodenum.
The duodenum is the C-shaped continuation of the gut tube beyond the stomach (A). It rests in a key region of the abdomen, near each of the accessory organs of digestion and the kidneys, spleen, inferior vena cava, and aorta (B


FIGURE 3.9 The tube pushes against the peritoneum to varying degrees.
Because of cramped spacing in the abdominal cavity, the relationship of the gut tube to the dorsal mesentery distorts during growth. In some regions, the gut tube is pushed back against the body wall, effectively removing the dorsal mesentery. This condition is called retroperitoneal, and the gut tube is essentially fixed in space against the back of the abdomen. In other regions the dorsal mesentery expands and twists dramatically to give the gut tube maximum flexibility and mobility.


Remember that the mesentery is really just two layers of mesoderm with a space between them. Part of this space is occupied by the gut tube, and part of it is empty except for the blood vessels and nerves that must serve the tube and its coating. Sometimes, the tube pulls farther away from the vertebral column, thus stretching the dorsal mesentery. This gives the tube the property of being very bendable and movable in the abdominal cavity, because it is swinging more freely from the support post of the vertebral column. The jejunum and ileum of the small intestine are examples of this condition. Because it appears as though the tube is completely surrounded by the visceral mesoderm, this condition is called intraperitoneal. The tube is not inside the peritoneal sac, but you will appreciate this best if you follow the developmental possibilities (see Fig. 3.9). Parts of the tube are pushed back against the body wall by pressure from other organs. This condition is called retroperitoneal, because the whole tube appears to be behind the visceral mesoderm that forms the lining of the peritoneal sac. These parts of the tube are fixed in position, and they are only blanketed by the tangent peritoneal membrane. The duodenum has both an intraperitoneal part and a retroperitoneal part (Fig. 3.10). The first part of the duodenum, derived from the foregut, is intraperitoneal; the remaining two-thirds of the duodenum are retroperitoneal. The duodenum is really at the mercy of the developing stomach and the large liver. This means that as the stomach spins on its long axis and sags to the left, the duodenum is kicked up to the right, and in the end, the convexity of the C in the C-shaped duodenum lies to the right of the vertebral column (see Fig. 3.10). The position of the duodenum across the level of the first few lumbar vertebrae will prove to be a very busy area of the abdominal cavity.