6 Digestive System
6.1 Overview of the Digestive System
Introduction
Function, localization, and terms: the digestive organs break down ingested solids and fluids, absorbs their nutrients, and excrete the remaining substances that the body can‘t use (digestion). The organs form a continuous tube from the head to the lesser pelvis, and thus traverse the body cavities in the thorax, abdomen, and pelvis. The entire system is also referred to as the “digestive apparatus,” and the portions that are contained in the body cavities are called the “gastrointestinal tract.” In addition to the usual terms of location and direction, the terms “oral and “aboral” are used when discussing the digestive system. They refer to direction along the longitudinal axis of the tract: “oral” = “toward the mouth” (os = mouth), “aboral” = “away from the mouth.”
Structure of the digestive apparatus and processing of food: the digestive system is made up of a continuous series of tube-like organs that transports a bolus of food in an oral to aboral direction. In the first portion of this tube system (oral cavity to stomach), food is broken up into small pieces. The next, and longest, portion (small intestine to colon) is responsible for absorption of nutrients and water. The terminal portion (rectum and anal canal) is responsible for temporary storage and controlled excretion (defecation) of feces. In the digestive system:
• Solid food is broken up, mixed with water and converted into a bolus (chyme). Enzymes in the stomach and small intestine digest the food into absorbable components. Most of the nutrients are absorbed through the epithelial cells lining the small intestine and into blood capillaries. They are then transported by the portal vein to the liver where they are further metabolized. Fats, however, are absorbed directly into lymphatic vessels, and they bypass the portal venous system and liver metabolism.
• Water is mostly absorbed by the intestinal wall and into blood or lymph capillaries. As part of the regulation of the osmotic pressure of blood, the kidneys also control water excretion and resorption (see urinary organs, p. 50).
Additional factors that aid digestion: The stomach and portions of the intestinal tract are in constant motion in order to churn the bolus and propel it along the digestive tract. The movement of the bolus in the aboral direction toward the rectum is called peristalsis. The enteric nervous system, which is the gastrointestinal tract’s intrinsic nervous system, controls peristalsis. Glands, which are either attached to the tube system or located directly in the system’s walls, secrete hydrochloric acid, enzymes, and other substances that mix with the bolus and water to aid digestion. Parts of the lymphatic system (tonsils and lymph follicles in the intestinal wall) are also located in the gastrointestinal tract and play an important role in the body’s immune system.
A Regional organization of the digestive organs
Digestive organs and associated structures are are located in the following regions:
In the head and upper part of the neck:
• Oral cavity with fauces at the transition between mouth and pharynx
In the middle and lower part of the neck, and in the thorax:
• Oropharynx and laryngopharynx
• Esophagus with cervical and thoracic parts
In the abdomen:
• Abdominal part of esophagus
• Stomach
• Small intestine with duodenum, jejunum and ileum
• Large intestine with cecum, appendix, and colon (ascending, transverse, descending, and sigmoid).
In the pelvis:
• Large intestine with rectum and anal canal.
Glands involved in digestion and their locations:
• Salivary glands (submandibular, sublingual, and parotid as well as small salivary glands in the oral cavity)
• Pancreas in the abdomen
• Liver with gall bladder in the abdomen
Numerous small glands are present in the walls of the digestive organs from the esophagus to the rectum.
B Oral cavity, fauces, pharynx, esophagus, and stomach
In the oral cavity the teeth, tongue, and salivary glands chop food into small pieces and moisten it with saliva. The three large paired salivary glands, sublingual, submandibular, and parotid, secrete saliva into the oral cavity through their ducts.
Fauces and pharynx: The oral cavity connects to the pharynx through the fauces. The pharynx, which is also a part of the respiratory system, is divided into three sections. The lower portion of the pharynx, the laryngopharynx, connects to the esophagus. In some textbooks, the entire pharynx is considered part of the neck.
Esophagus and stomach: The pharynx is continuous with the esophagus, which traverses the thorax and diaphragm and ends in the stomach. The function of the esophagus is to transport fluids and food to the stomach, where the food bolus is further broken down by the stomach’s churning. The bolus is mixed with acids to help denature proteins, and enzymatically digested. Over time the bolus is parceled out into the small intestine through the pyloric orifice.
C Small intestine, large intestine, and abdominal glands (liver, gallbladder, and pancreas)
Small and large intestine: the upper portion of the small intestine, the duodenum, is a C-shaped structure located behind and beneath the liver. The subsequent parts of the small intestine, the jejunum and ileum, are difficult to distinguish from one another. Their multiple loops are located behind the anterior abdominal wall and are surrounded by the large intestine. While nutrients are absorbed along the entire length of the small intestine, the large intestine absorbs primarily water and electrolytes. Stool is evacuated from the rectum.
The liver is located in the right upper quadrant of the abdomen (a). The liver metabolizes the nutrients and other compounds that are brought to it from the small intestine through the venous portal system (see p. 13). The liver produces bile, which it delivers to the duodenum via the bile duct. Bile, which emulsifies fats to ease their absorption, is stored in the gallbladder located beneath the liver. The pancreas (b) is located in the craniodorsal part of the abdomen close to the duodenum and consists of two glands:
• An exocrine gland, which discharges a watery, enzyme-rich secretion into the duodenum through the pancreatic duct. These enzymes aid in the digestion of substrates.
• An endocrine gland (the “islet cells”), which produce several compounds including the hormones insulin and glucagon which regulate blood sugar levels.
D Schematic cross-section illustrating the histology of the gastrointestinal tract
All segments of the gastrointestinal tract are divided into four layers:
• Mucosa: an epithelial layer surrounding the lumen.
• Submucosa: a layer of connective tissue surrounding the mucosa; contains blood and lymphatic vessels and autonomic nerves.
• Muscularis: the layer surrounding the submucosa; consists of an inner circular and an outer longitudinal layer of smooth muscle.
• Adventitia or serosa (depending on its location in the gastrointestinal tract): the outermost layer that attaches the gastrointestinal tract to its surroundings.
6.2 Development and Differentiation of the Gastrointestinal Tract
Introduction
The digestive organs are located in the head, neck, and major body cavities. Their complex development influences the structure of the body cavities, and thus their development will be discussed in relation to that of the body cavities. When development is complete there is a continuous tube extending from the oral cavity (“entrance”) to the anus (“exit”). The liver and gallbladder, and the pancreas, discharge their secretions into this tube in the abdomen.
A Development of the gastrointestinal tract: Overview (after Sadler)
a Overview; b Midsagittal section of an embryo at the beginning of the 5th week.
The primitive gut tube is derived from the dorsal part of the yolk sac, which is incorporated into the body of the embryo. The formation of two intestinal portals divides the primitive gut tube into three sections:
• The cranially located foregut
• The midgut (the longest portion of the gut)
• The caudally located hindgut
Cranially and caudally the primitive gut tube ends blindly. The cranial end of the foregut is closed by the oropharyngeal membrane and the caudal end of the hindgut by the cloacal membrane. The two membranes lie in contact with two ectodermal depressions. The depression at the cranial end is called the stomodeum and at the caudal end it is called the proctodeum. At first, the initially very short midgut is in direct communication with the yolk sac along its entire length. During embryonic folding additional portions of the yolk sac become incorporated into the developing midgut. The midgut is a continuation of the foregut at the anterior intestinal portal, and the hindgut is a continuation of the midgut at the posterior intestinal portal. The hindgut is connected to the allantois, which is an outpouching of the caudal wall of the yolk sac in the early embryo (see b).
B Development of the gastrointestinal tract from the three germ layers
The organs of the gastrointestinal tract are derived from the three germ layers.
C Differentiation of the gastrointestinal tract
The primitive gut tube gives rise to all parts of the digestive tract. The epithelium of the gut tube is derived from endoderm (see B). The oropharyngeal membrane and cloacal membrane, which are lined on the outside with ectoderm, break down later in development to allow the gut tube open access to the external environment (see p. 47).
• The foregut divides into a cranial portion, which gives rise to the pharynx, and a caudal portion, which gives rise to the esophagus, stomach, and the superior part of the duodenum (see p. 42). The respiratory diverticulum, which will develop into the trachea and lungs, marks the boundary between the two portions of the foregut (see p. 35).
• The midgut gives rise to the remainder of the small intestine as well as the ascending colon and oral ⅔ of the transverse colon.
• The hindgut gives rise to the remainder of the colon and the rectum. The expanded caudal end of the hindgut is called the cloaca. Both the rectum and part of the urogenital system develop from the cloaca.
The anterior intestinal portal marks the border between the foregut and midgut. It is located along the duodenum and it gives rise to the liver, gallbladder, and pancreas. The junction between the midgut and hindgut is called the posterior intestinal portal and it lies between the oral ⅔ and aboral ⅓ of the transverse colon. This region, also referred to as the Cannon-Boehm-point, plays a significant role in the arrangement of the colon’s autonomic innervation. The stomodeum develops into the oral cavity and the proctodeum gives rise to the anal canal. The epithelial lining of both structures is derived from ectoderm and is continuous with the outer lining of the oropharyngeal and cloacal membranes. At these two sites, the endodermal and ectodermal epithelium are adjacent to each other. The obliteration of the oropharyngeal and cloacal membranes results in the primitive gut tube becoming open to the embryo‘s external environment.
6.3 Mesenteries and Primordia of the Digestive Organs in the Caudal Foregut Region; Stomach Rotation
Introduction
Two processes are crucial for the embryological development of the digestive organs:
• The rotation of the stomach in the caudal foregut (see p. 44)
• The rotation of the intestinal loop (the loop-shaped fetal gut tube, see p. 46) in the midgut and hindgut.
