Physiology of the Gastrointestinal System

Physiology of the Gastrointestinal System

Organs of the Gastrointestinal System: An Overview

Each of the body cell needs a constant supply of nutrients to provide energy and building blocks for the manufacture of body substances. Food as we take it in, however, is too large to enter the cells. It must first be broken down into particles small enough to pass through the cell membrane. This process is known as digestion. After digestion, food must be carried to the cells in every part of the body by the circulation. The transfer of food into the circulation is called absorption.

The Gastrointestinal system consists a group of organs designed to take in foods, initially process foods, digest the foods, and eliminate unused of materials of food items. It is a hollow tubular system from one end of the body to the other end. The major organs of the gastrointestinal tract include; the mouth or oral complex, pharynx, esophagus, stomach, small intestine and associated glands, large intestines, rectum and anal canal and anus. In addition, the liver, gall bladder and pancreas are considered as accessory organ of digestion, which are necessary for the digestive process but are not a direct part of the digestive tract. They release substances into the digestive tract through ducts.

The Pharynx, Deglutition and the Esophagus

The Throat (Pharynx): The throat is the common portion of the respiratory and digestive tracts adjoining the nasal and oral cavities. It is a tube, about 12 cm long, attached to the base of the skull. The nasal cavity opens into the upper part (nasopharynx), the oral cavity into the middle part (oropharynx), and the larynx and esophagus into the lower part (hypopharynx). The respiratory and digestive passages cross in the oropharynx. The pharyngeal wall consists of mucosa, striated muscle, and a connective tissue fascia. The pharyngeal muscles include the muscles taking part in swallowing, namely, the pharyngeal constrictors and the pharyngeal levators. The pharyngeal constrictors are strong muscles that can narrow the pharynx and lift the larynx and hyoid bone.

The act of swallowing prevents the food from reaching the trachea. It includes a voluntary and an involuntary (reflex) phase. To begin the act of swallowing, the floor of the mouth is contracted voluntarily and the bolus is pressed against the soft palate. This action initiates the swallowing reflex, which includes the involuntary sealing of the respiratory passage. As the soft palate is lifted against the posterior pharyngeal the upper respiratory passage is sealed from the digestive tract. The hyoid bone and the larynx are raised by the contraction of the floor of the mouth (pharyngeal constrictors and muscles of the floor of the mouth) and the epiglottis approaches the entrance to the larynx. During this action the glottis is closed and the breath is held. In this way the lower respiratory passages are also separated from the digestive tract. After the act of swallowing is complete, the infrahyoid muscles pull the larynx down again and so open the respiratory passages. This important and complex reflex is regulated by the swallowing center (deglutition center) in the medulla (medulla oblongata) in the brain.

The esophagus transports the bolus from the pharynx into the stomach. This transport is accomplished by waves of circular muscle contractions (peristalsis) that are normally directed toward the stomach. The esophagus is also subject to a longitudinal tension (fixed by the larynx above and the diaphragm below) that stabilizes its course and favors the passage of the bolus of food during swallowing. In the adult, the esophagus is about 25-30 cm long. It runs through the thorax behind the trachea and in front of the spine. Below, the esophagus penetrates the diaphragm through the esophageal hiatus to empty directly into the stomach.

In certain places the esophagus is narrowed by esophageal constrictions. The highest constriction is at the cricoid cartilage; it is the narrowest with a diameter of about 14mm. In the middle the closely related aortic arch causes a constriction. The lowest constriction corresponds to the diaphragmatic hiatus and is the site of a complex closing mechanism. By alternating the contractions of the circular and longitudinal muscles segment by segment (peristalsis), this arrangement of the muscles facilitates the transport of food toward the stomach. After passing downward through the mediastinum, the esophagus penetrates the diaphragm at an opening called the esophageal hiatus, continues another 3 to 4 cm, and meets the stomach at an opening called the cardiac orifice (named for its proximity to the heart). The inferior end of the esophagus is more constricted than the rest, forming the lower esophageal sphincter. This is not an anatomical feature but is a physiological constriction that helps close the cardiac orifice. Gastrosophageal reflux, the backflow of stomach contents into the esophagus, is normally prevented partly by the tonus of this sphincter but more importantly by constriction of the diaphragm around the lower esophagus. "Heartburn" has nothing to do with the heart, but is the burning sensation produced by acid reflux into the esophagus.

The Stomach

The stomach is J-shaped relatively vertical in tall people, and more horizontal in short people

The lesser curvature of the stomach extends the short distance from esophagus to duodenum along the medial to superior aspect, while the greater curvature extends the longer distance from esophagus to duodenum on the lateral to inferior aspect. The stomach is divided into four regions: The cardiac region (cardia) is a small area immediately inside the cardiac orifice. The fundic region (fundus) is the dome shaped portion superior to the esophageal attachment. The body (corpus) makes up the greatest part of the stomach inferior to the cardiac orifice. The pyloric region is a slightly narrower pouch at the inferior end; it is subdivided into a funnel-like antrum and a narrower pyloric canal. The latter terminates at the pylorus, a narrow passage into the duodenum. The pylorus is surrounded by a thick ring of smooth muscle, the pyloric (gastroduodenal) sphincter, which regulates the passage of chyme into the duodenum.

Gastric secretion: The gastric glands produce 2 to 3 L of gastric juice per day (pH 1.5-2 /as low as 0.8/), consisting essentially of water, mucus, hydrochloric acid, and protein-splitting enzymes (pepsin), the stomach macerates and liquefies food chemically. The food forms a paste (chyme) that is moved back and forth and, after a delay of variable length (1-5 hours), transported into the small intestine in batches. Secretion of gastric juices occurs in three phases:

1. A cephalic (reflex) phase
2. A local (gastric) phase
3. An intestinal phase

Major Secretions of Gastric Glands and Their Function

The circular, longitudinal and oblique arrangement of muscle in the stomach wall is more complex than in other parts of the GI tract. Three types of motility patterns can be detected in the stomach: receptive, relaxation and contraction, peristaltic propulsion and mixing, and the migrating motor complex.

During feeding, smooth muscle in the wall of the proximal stomach relaxes to accommodate the food with little increase in intragastric pressure (receptive relaxation). This is followed by contractions of low amplitude and long duration which reduces the size of the stomach as gastric emptying occurs. In the proximal stomach, food may remain undisturbed for up to an hour, enabling carbohydrate digestion to take place by salivary amylase. In the mid and distal parts of the stomach, peristalsis is more vigorous. Contractions are generated in the mid-stomach and pass distally towards the pylorus at a rate of three per minute. Propagation speed increases as the pylorus is approached when the peristaltic wave overtakes the gastric contents. This causes forward propulsion of some contents through the pylorus and backwards movement of the remainder into the body of the stomach. Vagal stimulation and gastrin increase gastric peristalsis, whereas sympathetic stimulation, secretin and somatostatin depress activity.

The speed at which substances empty from the stomach depends on their physical state and chemical composition. Liquids empty more rapidly than solids. Nutrients in the duodenum activate chemoreceptors which reflexes inhibit gastric emptying, allowing time for further digestion and absorption in the small intestine. Hypertonicity, fatty acids and hydrogen ions activate the secretion of cholecystokinin, secretin and GIF, which inhibit gastric emptying.

Factors affecting the rate of gastric emptying:-Several physiological, pathological and pharmacological factors occurring in the perioperative period may delay gastric emptying and increase the risk of inhalation of gastric contents.

Perioperative Causes of Delayed of Gastric Emptying

The Pancreas, Liver & Gall Bladder

The pancreas is the most important digestive gland. It is an exocrine gland and secretes about 2 liters of pancreatic juice a day. The endocrine islet apparatus secretes hormones that are instrumental in the regulation of the blood sugar level. Pancreatic juice is alkaline, being remarkable for its high content of bicarbonate (HCO3-ions), which neutralizes the acid milieu of the duodenum. Pancreatic secretions contain numerous enzymes that digest fat (lipases; e. g., phospholipase, proteins (proteases; e. g., trypsin, chymotrypsin), and carbohydrates (amylases). The enzymes reach the duodenum in the form of inactive precursors (e. g., trypsinogen) and are then activated. The secretion and composition of pancreatic juice are regulated partly by the vagus, and partly especially by two mucosal hormones of the duodenum (secretin and cholecystokinin). Their secretion is triggered by fats and the low pH value of the chyme coming from the stomach. The pancreas lies behind the stomach at the level of L2. The pancreatic duct, about 2mm thick, runs through the whole length of the gland and opens, often jointly with the common bile duct, at the duodenal papilla (sphincter of Oddi) into the descending part of the duodenum.

The liver lies in the right upper abdominal quadrant, directly under the diaphragm. It weighs 1500-2000 g and so is the largest gland in the human body. Because it secretes bile, it is an exocrine gland. The main components of bile are bile acids, which enable the absorption of fats in the intestine by emulsifying them. Bile pigments (e. g., bilirubin) are end products of hemoglobin formed during the breakdown of dead red blood cells. Numerous other substances (cholesterol, minerals) are excreted in the bile. The liver is the largest metabolic organ and fulfills important functions in the metabolism of carbohydrates, proteins, and fats, as well as playing a part in detoxification. For this reason about 1.5 liters of blood flows into the liver through the hepatic artery proper every minute. Moreover, the nutrients absorbed in the intestine reach the portal vein and from there the liver through the portal circulation.

Within the liver, carbohydrates are stored as glycogen and are released again when needed. Fats and proteins are constantly transformed and broken down (e. g., fatty acid synthesis, amino acid breakdown, urea synthesis), and foreign substances such as medications or poisons are inactivated. The liver also takes part in the synthesis of numerous blood components (e. g., albumin, clotting factors).

Drainage of the bile occurs in a direction opposite to the flow of blood, through the cholangioles (bile capillaries), which are dilated intercellular spaces between adjoining hepatocytes. They have no walls themselves and drain into the small biliary ductules of the portal canals. These join into larger ducts that transport the bile by way of the common hepatic duct into the cystic duct and the gallbladder.

The gallbladder is a thin-walled pear-shaped sac with a capacity of about 30-35ml. Its blood supply is the cystic artery, a branch of the hepatic artery proper. It lies on the visceral side of the liver and can be regarded as a reservoir for the bile. Bile is concentrated there (gallbladder bile) and when required released by way of the cystic duct into the common bile duct. The common bile duct formed by the junction of the cystic duct and the common hepatic duct is also called the choledochous duct. The common bile duct is about 6-8 cm long and about the thickness of a pencil. It runs behind the duodenum in the direction of the head of the pancreas. In about 77% of cases it joins the pancreatic duct and the ducts drain jointly into the major papilla of the duodenum.

The muscle of the bile passages is smooth muscle. Just before its junction with the duodenum, the common duct has a circular sphincter that remains contracted during digestive quiescence, damming the bile back through the common duct into the gall- bladder. The mouth of the bile duct opens shortly after food intake. Smooth muscle at the junction of the pancreatic duct as a rule prevents reflux of the bile into the pancreatic duct.

The gastrointestinal tract has a nervous system all its own called the enteric nervous system. It lies entirely in the wall of the gut, beginning in the esophagus and extending all the way to the anus. This highly developed enteric nervous system is especially important in controlling gastrointestinal movements and secretion. The enteric nervous system is composed mainly of two plexuses, an outer plexus lying between the longitudinal and circular muscle layers, called the myenteric plexus or Auerbach's plexus, and an inner plexus, called the submucosal plexus or Meissner's plexus, that lies in the submucosa. The myenteric plexus controls mainly the gastrointestinal movements, and the submucosal plexus controls mainly gastrointestinal secretion and local blood flow.

Although the enteric nervous system can function on its own, independently of these extrinsic nerves, stimulation by the parasympathetic and sympathetic systems can greatly enhance or inhibit gastrointestinal functions. The sympathetic fibers to the gastrointestinal tract originate in the spinal cord between segments T-5 and L-2. In general, stimulation of the sympathetic nervous system inhibits activity of the gastrointestinal tract, causing many effects opposite to those of the parasympathetic system. The vagus nerves provide extensive innervations to the esophagus, stomach, and pancreas and somewhat less to the intestines down through the first half of the large intestine. The sacral parasympathetics originate in the second, third, and fourth sacral segments of the spinal cord and pass through the pelvic nerves to the distal half of the large intestine and all the way to the anus. Stimulation of these parasympathetic nerves causes general increase in activity of the entire enteric nervous system. This in turn enhances activity of most gastrointestinal functions.

The blood vessels of the gastrointestinal system are part of a more extensive system called the splanchnic circulation. It includes the blood flow through the gut itself plus blood flows through the spleen, pancreas, and liver. The design of this system is such that all the blood that courses through the gut, spleen, and pancreas then flows immediately into the liver by way of the portal vein. In the liver, the blood passes through millions of minute liver sinusoids and finally leaves the liver by way of hepatic veins that empty into the vena cava of the general circulation. This flow of blood through the liver, before it empties into the vena cava, allows the reticuloendothelial cells that line the liver sinusoids to remove bacteria and other particulate matter that might enter the blood from the gastrointestinal tract, thus preventing direct transport of potentially harmful agents into the remainder of the body.