General Anatomy and Digestive Processes

Digestive Functions and Processes

The digestive system has four functions: ingestion, digestion, absorption, and defecation.

The functions of the digestive tract are carried out through three principal processes: motility, secretion, and membrane transport.

Subdivisions of the Digestive System

The digestive system has two anatomical subdivisions: the digestive tract/alimentary canal/gastrodigestive (GI) tract and the accessory organs. The digestive tract is a tube extending from mouth to anus. It includes the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine. The accessory organs are the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

Relationship to the Peritoneum

Most of the digestive tract is within the peritoneal cavity, except for the duodenum, pancreas, and parts of the large intestine, which are retroperitoneal. Along the dorsal wall of the abdominal cavity, the parietal peritoneum turns inward and forms a sheet of tissue, the dorsal mesentery, extending to the digestive tract. The membrane then forms the outer covering, or serosa, of the stomach and most parts of the intestines. In some places it continues beyond as a sheet of tissue called the ventral mesentery, which may hang freely in the abdominal cavity.

Stages of Digestion

There are two stages of digestion: Mechanical digestion is achieved by the cutting and grinding action of the teeth and the churning contractions of the stomach and small intestine. Chemical digestion consists solely of hydrolysis reactions that break the dietary macromolecules into their monomers. It is carried out by digestive enzymes produced by the salivary glands, stomach, pancreas, and small intestine.

The gastrointestinal tract

General histology of the GI tract

The alimentary canal has 4 basic tunics (from the lumen outward):

1. Mucosa/mucous membrane – it is a wet epithelial membrane consists of the surface epithelium (usually simple columnar), the lamina propria (areolar connective tissue) and the muscularis mucosa (layer of smooth muscle fibers that enable movements of the mucosa). The major functions of the mucosa are secretion, absorption and protection.

2. Submucosa – is found adjacent to the mucosa. It is composed with dense connective tissue that contains blood and lymphatic vessels, lymph nodes and nerve fibers. Major functions are nutrition and protection.

3. Muscularis externa – bilayer of smooth muscle. The layer close to the submucosa is circular and the outer layer is longitudinal.

4. Serosa – is the visceral peritoneum. Consists of epithelium (mesothelium) and a thin layer of areolar connective tissue. Outside the abdominopelvic cavity the serosa is replaced by adventitia a layer of fibrous connective tissue that bids the organ to the surrounding tissues. The serosa reduces friction as the digestive system organs work and slide across one another and the cavity walls.

The GI organs

The Mouth

The mouth, also called the oral or buccal cavity, functions in the following: ingestion, taste and other sensory responses, mastication, chemical digestion, swallowing, speech, and respiration. The mouth is enclosed by the cheeks, lips, palate, and tongue and it is covered by mucous membrane with stratified sqouamous epithelium.

The Cheeks and Lips

The cheeks and lips retain food and push it between the teeth for mastication. They are essential for sucking and blowing actions. Externally, the lips are divided into a cutaneous area and a red area, the latter of which gets its color from the tall dermal papillae and the proximity of blood vessels to the surface. Each lip is attached to the gum behind it by the labial frenulum (inferior and superior). The vestibule is the space between the teeth and the cheeks or lips.

The Tongue

The tongue is a muscular organ that moves food for chewing and swallowing bears taste receptors, and aids in speech. The surface of the tongue is covered with lingual papillae, most of which have taste buds. The body of the tongue is attached to the floor of the mouth by the lingual frenulum. The mass of the tongue is composed mainly of two groups of lingual muscles made of skeletal muscle tissue. The tongue is supported by the mylohyoid muscle and it is attached to the hyoid bone, mandible, styloid process and pharynx.

The Palate

The palate, separating the oral cavity from the nasal cavity, makes it possible to breathe while chewing food. Its anterior portion, the hard palate, is supported by the palatine process of the maxilla and the palatine bones. Posterior to this is the soft palate. The soft palate has a conical, medial projection called the uvula.

The Pharynx

The pharynx has 3 parts: nasopharynx, oropharynx and laryngopharynx. The pharynx has a deep layer of longitudinally oriented skeletal muscle (levator muscles) and a superficial layer of circular skeletal muscle (constrictor muscles). The circular muscle is divided into superior, middle, and inferior pharyngeal constrictors, which force food downward during swallowing. The pharynx is lined by mucosa membrane contains stratified squamous epithelium.

The Esophagus

The esophagus is a straight muscular tube extending from the larynx to the stomach through the diaphragm to the gastroesophageal sphincter. The esophagus has no digestive or absorption functions. The wall of the esophagus consists of four layers: mucosa, submucosa, muscularis externa, adventitia (instead of serosa).

The walls of the superior end of the esophagus contain skeletal muscle. This is replaced by smooth muscle at the area close to the stomach. The gastroesophageal sphincter is a thickening of the smooth muscle layer at the esophagus-stomach junction and it controls the food passage to the stomach.

The Stomach

The stomach is a muscular sac in the upper left abdominal cavity immediately inferior to the diaphragm. It functions primarily as a food storage organ. When empty, is has a volume of 50 mL. When very full, it may hold up to 4L. The stomach mechanically breaks up food particles, liquefies the food, and begins the chemical digestion of proteins and a small amount of fat, producing a mixture of semidigested food called chyme.

The stomach is J-shaped, with a lesser curvature on its medial margin, and a greater curvature along its lateral margin. From each curvature extends a mesentery called omentum. The lesser omentum extends from the lesser curvature to the liver and the greater omentum extends from the greater curvature and downwards over the abdominal contents to cover them.

The stomach is divided into four regions: the cardiac region which is the area surrounding the cardiac orifice through which food enters the stomach from the esophagus; the fundic region that is the expended portion of the stomach and it is found superolateral 6to the cardiac region; the stomach body is the mid portion of the stomach; and the pyloric region. The pyloric region is subdivided into an atrium and a pyloric canal; the latter terminates at the pylorus a narrow passage leading to the duodenum that is surrounded by a muscular ring called the pyloric sphincter.

The Stomach Wall

The stomach wall has tissue layers similar to those of the esophagus, with some variations – it contains additional inner layer of oblique muscle fibers that help in mixing and breaking of the food. When the stomach is empty, the mucosa and submucosa form longitudinal wrinkles called rugae (ROO-ge). The gastric mucosa is pocked with depressions called gastric pits. Cells near the bottom of the pits divide repeatedly, providing a new source for epithelial cells.

At the bottom of the pits lie glands. In the cardiac and pyloric regions, these are called the cardiac and pyloric glands, respectively, and secrete mucus only.

In the rest of the stomach, the glands are called gastric glands and have a greater variety of cell types and secretions: Mucous cells secrete mucus; regenerative cells divide rapidly and produce a continual supply of new cells; parietal cells secrete hydrochloric acid and intrinsic factor. Gastric juice has a high concentration of hydrochloric acid (HCl) and a pH as low as 0.8.  Stomach acid has several functions: (1) it activates the enzymes pepsin and lingual lipase. (2) It breaks up connective tissues and plant cell walls. (3) It converts ferric ions to ferrous ions. (4) It contributes to nonspecific disease resistance by destroying ingested bacteria and other pathogens. Parietal cells also secrete a glycoprotein called intrinsic factor that is essential to the absorption of vitamin B₁₂ by the small intestine. The secretion of intrinsic factor is the only indispensable function of the stomach; chief cells secrete rennin and lipase in infancy and pepsinogen (inactive form of pepsin) throughout life. Several enzymes are secreted as inactive proteins called zymogens. Chief cells secrete the zymogen called pepsinogen. Hydrochloric acid removes some of the amino acids from pepsinogen and converts it to pepsin. The function of pepsin is to digest dietary proteins to shorter peptide chains. ; And enteroendocrine cells secrete hormones and paracrine messengers that regulate digestion. Gastric glands have various kinds of enteroendocrine cells that collectively produce as many as 20 secretions, most of which behave as hormones or paracrine secretions. Gastrin travels in the bloodstream and stimulates motility of the large intestine and it diffuses to nearby parietal and chief cells and stimulates the secretion of hydrochloric acid and enzymes.

Innervation and Circulation

The stomach receives parasympathetic and sympathetic stimulation. It is supplied with blood from the celiac artery. All blood leaving the stomach enters the hepatic portal circulation before returning to the heart.

The Small Intestine

Nearly all chemical digestion and nutrient absorption occur in the small intestine. The term "small" applies to its diameter, not its length. Circular folds, villi, and microvilli all serve to increase the surface area inside the small intestine.

The small intestine is divided into three regions. The duodenum constitutes the first 25 cm. It receives the stomach contents, pancreatic juice, and bile. Stomach acid is neutralized here, pepsin is inactivated by the elevated pH, and pancreatic enzymes take over the job of chemical digestion. The jejunum comprises the next 2.5 m and most of it is found in the umbilical region of the abdomen. The ileum forms the last 3.6 m and ends at the ileocecal junction at the large intestine. Most of it is found in the hypogastric region.

The largest folds of the intestinal wall are transverse to spiral ridges called circular folds/pelicae circulars. They occur from the duodenum to the middle of the ileum, where they cause the chyme to flow in a spiral path along the intestine. This slows its progress, allows more contact with the mucosa, and promotes more thorough mixing and nutrient absorption. The mucosa also possesses villi, which have fingerlike shapes. The largest villi are in the duodenum, and they become progressively smaller in more distal regions of the small intestine. A villus is covered with two kinds of epithelial cells— columnar absorptive cells and mucus-secreting goblet cells. The core of the villus is filled with areolar tissue containing a capillary network, an arteriole, a venule, and a lymphatic capillary. Each epithelial cell of a villus has a border of microvilli containing brush border enzymes (hydrolytic enzymes) that carry out contact digestion. On the floor of the small intestine, between the villi, are the intestinal crypts, which are similar in structure and function to the gastric glands. The duodenum has prominent duodenal (Brunner) glands in the submucosa. They secrete bicarbonate-rich mucus that neutralizes stomach acid while simultaneously protecting the mucosa. Throughout the small intestine, the lamina propria and submucosa have a large population of lymphocytes. Other lymphatic tissue, Peyers patches, are found in the ileum on one side of the intestinal wall.

The small intestine receives enzymes from the pancreas via the pancreatic duct and bile that formed in the liver via the bile duct. At the duodenum these 2 ducts join to form the hepatopancreatic ampulla. The entrance to the duodenum (duodenal papilla) is controlled by a muscular valve called the hepatopancreatic sphincter/sphincter of Oddi.

Intestinal Motility

Contractions of the small intestine serve three functions: (1) to mix chyme with intestinal juice, bile, and pancreatic juice; (2) to mix chyme and bring it in contact with the mucosa for digestion and absorption; and (3) to move residue toward the large intestine. Segmentation is the most common type of movement of the small intestine. Ringlike constrictions appear at several places along the intestine and then relax while constrictions occur elsewhere. When most nutrients have been absorbed and little remains but residue, segmentation slows and peristalsis begins. At the ileocecal junction, the muscularis of the ileum is thickened to form a sphincter, the ileocecal valve, which is usually closed. As the cecum fills with residue, the pressure pinches the valve shut, preventing the reflux of cecal contents into the ileum.

The Large Intestine

The large intestine begins with the cecum, a blind pouch inferior to the ileocecal valve. Attached to its lower end is the vermiform appendix. The appendix is densely populated with lymphocytes and is an important source of immune cells. The ascending colon begins at the ileocecal valve and passes up the right side of the abdominal cavity near the right lobe of the liver, where it becomes the transverse colon. At this point the right-angle turn is called the right colic (hepatic) flexure. The transverse colon passes horizontally across the upper abdominal cavity, turns 90^o downward, and becomes the descending colon. The left-angle turn is called the left colic (splenic) flexure. At the pelvic inlet, the colon turns medially and downward, forming a roughly S-shaped portion called the sigmoid colon. Within the pelvic cavity, the colon straightens and becomes the rectum. The rectum has three internal folds called rectal valves that enable it to retain feces while passing gas. The final 3 cm of large intestine is the anal canal, which passes through the levator ani muscle and terminates at the anus. Here the mucosa forms longitudinal ridges (anal columns) with depressions between them (anal sinuses).Large hemorrhoidal veins form superficial plexuses in the anal columns. The anus is regulated by two sphincters—an internal anal sphincter composed of smooth muscle, and an external anal sphincter composed of skeletal muscle.

The mucosa of the large intestine has a simple columnar epithelium in all regions except the anal canal, where it is stratified squamous. There are no circular folds or villi in the large intestine, but there are intestinal crypts, which are deeper than in the small intestine and have a greater density of goblet cells. Mucus is their only significant secretion.

In the large intestine, the longitudinal muscle layer is reduced to 3 longitudinal muscle bands called teniae coli. These muscles are shorter than the rest of the intestine wall and it causes the wall to look like a group of small pocketlike sacs called hustra.

The large intestine is densely populated with several species of bacteria referred to collectively as the bacterial flora. They ferment cellulose and other undigested carbohydrates and synthesize B vitamins and vitamin K, which are absorbed by the colon.

In the large intestine, food undergoes no further chemical digestion, but its volume is reduced as it passes through. The feces consist of 75% water and 25% solid matter, of which 30% is bacteria and 30% undigested fiber. In the large intestine a re absorption of water occurs.

Accessory organs of the GI tract

The Teeth

An adult normally has 32 teeth, 16 in the mandible and 16 in the maxilla. Collectively, the teeth are called the dentition. Teeth can be grouped into incisors, canines, premolars, and molars, based on their shape, location, and function. Each tooth is embedded in a socket called an alveolus that is lined by a periodontal ligament which holds the tooth in the alveolar socket and supply cushioning. The gum, or gingiva, covers the alveolar bone. The crown of the tooth is the part that extends above the gum line; the neck is the portion from the margin of the gum to the alveolar bone; and the root is the portion inserted into the alveolus. Most of a tooth consists of hard, yellowish tissue called dentin that is covered with enamel in the crown and neck and cementum in the root. Internally, a tooth has a dilated pulp cavity in the crown and a narrow root canal in the root. These spaces are occupied by pulp, which is composed of loose connective tissue, blood and lymphatic vessels, and nerves.

Mastication breaks food into pieces small enough to be swallowed and exposes more surfaces to the action of digestive enzymes. Mastication is the first step in mechanical digestion.

The Salivary Glands

The extrinsic salivary glands are situated outside the oral cavity but convey saliva to it through ducts. There are 3 pairs: the parotid glands that are located anterior to the ear and are ducting into the mouth over the second upper molar through the parotid duct; submandibular glands that are located along the medial aspect of the mandibular body and have a duct under the tongue at the base of the lingual frenulum; and sublingual glands that are located anteriorly in the floor of the mouth and empty their secretion under the tongue via many small ducts Saliva

Saliva moistens the mouth, digests a small amount of starch and fat, cleanses the teeth, inhibits bacterial growth, dissolves molecules so they can stimulate taste buds, and moistens food and binds particles together to aid in swallowing. Saliva has a pH of 6.8 to 7.0.

The Liver and gallbladder  

The liver is a reddish-brown gland located inferior to the diaphragm in the right hypochondriac and epigastric regions. It is the body's largest gland and performs a tremendous variety of functions, including the secretion of bile for digestive purposes.

The liver has four lobes, called the right, left, quadrate, and caudate lobes. The gallbladder adheres to a depression on the inferior surface of the liver between the right and quadrate lobes. The liver is attached to the diaphragm by the falciform ligament.

The liver parenchyma consists mostly of hepatocytes arranged in cylinders called hepatic lobules. Each lobule is about 1 mm in diameter and 2 mm long and has a central vein passing through its core. At each of the six corners of the lobules there is a portal tract/portal triad consists of portal arteriole, portal venule and a bile duct. At the lining of the sinusoids there are phagocytic cells – kupffer cells – that remove debris and bacteria from the blood. The parenchyma cells absorb oxygen and nutrients.

Glucose transported from the digestive system is stored in the liver in the form of glycogen. Amino acids are use to build plasma proteins.

Blood that passes through the sinusoids arrive to the central vein and from there it drains ultimately to the hepatic vein.

The liver secretes bile into narrow channels, called the bile canaliculi, between sheets of hepatocytes. Bile passes from there into the small bile ductules and then into the right and left hepatic ducts. These two ducts converge to form the common hepatic duct, which then joins the cystic duct coming from the gallbladder. The common bile duct descends through the lesser omentum and joins the duct of the pancreas, forming the hepatopancreatic ampulla. The major duodenal papilla on the duodenal wall contains the hepatopancreatic sphincter, which regulates the passage of bile and pancreatic secretion into the duodenum.

The Pancreas

The pancreas is a soft, spongy gland posterior to the greater curvature of the stomach and outside the peritoneal cavity. It has both endocrine and exocrine functions. Its endocrine part is the pancreatic islets, which secrete insulin and glucagon. Most of the pancreas is exocrine tissue that secretes 1,200–1,500 mL of pancreatic juice per day. This secretion ends up in the main pancreatic duct running lengthwise through the gland. It empties into the small intestine through the hepatopancreatic ampulla or by way of a smaller accessory pancreatic duct in some people.

Pancreatic juice is an alkaline mixture of water, enzymes, zymogens, sodium bicarbonate, and other electrolytes.