Human Digestive System: Anatomy and Function
Mouth
The mouth is divided into two parts: the oral vestibule, which is the space between the inside of the lips and the outer face of the teeth, and the oral cavity itself, which runs from the teeth internally to the entrance of the pharynx. The roof of the mouth is formed by the hard palate and soft palate, which is composed of muscle and covered by mucosa. In the midline of the soft palate, a small mass called the uvula projects downward. The mouth communicates with the pharynx through the fauces, which is in the back of the mouth. Bordering the fauces are four palatine folds or pillars departing from the uvula to the sides, forming two arches, between which are located the palatine tonsils. The floor of the mouth is formed by the tongue, which is formed by a mass of skeletal muscle. On the surface, there are some papillae called the taste buds, which are responsible for capturing different flavors. Two-thirds of the tongue are inside the mouth, and one-third is found in the pharynx. Between these areas, there is a kind of V that consists of taste buds larger than normal. On the underside of the tongue, we find the lingual frenulum, which is a fold that joins the tongue to the floor of the mouth. Salivary gland products lead to the salivary glands.
Salivary Glands
Parotid glands are the largest. They are located in front of the ear canal and out of the ramus of the mandible. The gland duct that opens into the mouth is in contrast with the outer face of the second molar (from inside the cheek). Inflammation of these glands results in mumps. The submandibular glands are located on the inside of the mandible near the mandibular angle. It also has tubes that empty into the floor of the mouth. The sublingual glands are under the tongue on each side of the frenulum.
Esophagus
The esophagus wall is smooth muscle that is normally closed and opens with the passage of food. It has a cervical portion that passes behind the trachea, then goes down the mediastinum behind the heart, and through the diaphragm via an opening called the esophageal hiatus to enter the abdomen to reach the stomach through the cardia. (esophagus cervical, thoracic, and abdominal)
Stomach
It is located below the diaphragm at the top left of the abdominal cavity, ahead of the pancreas. It is a dilated portion of the digestive tract, J-shaped, and it varies from person to person and according to the position. It has muscular walls with fibers that are arranged in multiple directions for added strength. The interior is lined with a mucous membrane with many folds. Its exterior is covered by a membrane called the peritoneum. The stomach has several parts:
- The cardia: is a sphincter (a ring of circular muscle fibers that are arranged around a hole) that connects the esophagus to the stomach and regulates the entry of food and prevents reflux in normal operation (that food goes back).
- The fundus is the upper stomach. This is where the accumulation of gases occurs, which can be seen in an X-ray standing (standing). The radiographic sign is known as a gas chamber.
- The body is the part that takes up most of the stomach.
- The antrum is a narrow zone that serves as a prelude to the pylorus.
- The pylorus and pyloric sphincter connect the stomach with the end of the first part of the small intestine, the duodenum.
The stomach has two curvatures, one directed towards the lower left and one pointing to the right.
Duodenum
It is the first part of the small intestine. It is composed of muscle fibers. C-shaped, its concave side fits the pancreas.
It has four parts: the first horizontal, the second down, inside which is the Ampulla of Vater which will lead to liver bile and pancreatic juice from the pancreas, the third horizontal, and the fourth upward. We performed a more complete description of the duodenum in studying the pancreas.
Pancreas
The pancreas is an elongated structure located in front of the spine and posterior to the stomach and liver. It is embedded in the duodenum and arranged horizontally at the upper area of the abdominal cavity.
The head of the pancreas is the part that fits into the duodenum. It has a downward extension known as the hook of the pancreas or uncinate process. In your head up, we called isthmus narrowing neck of the pancreas and then continue on the horizontal with the body of the pancreas to the left end with the tail of the pancreas.
There is a passage called the main pancreatic duct that runs throughout the pancreas to lead to the ampulla of Vater, located in the duodenum.
There is an accessory pancreatic duct that only covers the head of the pancreas and empties into the ampulla of Vater.
Both conduits pour pancreatic juice into the duodenum. The pancreatic juice contains enzymes involved in the digestion of fats.
Liver
The liver is the body’s largest organ, weighing over two kilos. It is situated below the diaphragm in the top right of the abdominal cavity and across the midline, placing ahead at this end of the stomach. Under normal conditions, it should not exceed the costal margin. (In case of pathologies, it can be felt below the costal margin).
The liver is divided into four lobes. The right lobe is the largest. The extension of the liver to the left is the left lobe. The other two lobes are on the bottom and are called the quadrate lobe, anterior inferior, and caudate lobe, posterior inferior.
The face, either past or diaphragmatic, has a smooth surface that fits perfectly to the diaphragm.
On the underside, you can see the liver hilum between the four lobes, where all the structures enter and leave: hepatic artery, portal vein, bile duct. Between the quadrate lobe and the right lobe is embedded the gallbladder, a sac-like structure that serves as a reservoir for the storage of bile formed in the liver. It is slightly above the edge of the liver.
On the back, we have the inferior vena cava, where it will lead to hepatic veins. On top of this side, we have the hilum suprahepatic or higher, where they exit from the hepatic veins to empty into the inferior vena cava.
The liver is formed by small anatomical units called hexagonal liver lobules. In the center of each is the central vein of the lobule, which flows into the hepatic veins. The lobules are formed by a group of cells called hepatocytes that are arranged around the central vein. In every corner of the hexagon, there is a set of structures that are branches of the hepatic artery, portal vein, and bile ducts.
The blood that comes from the hepatic artery oxygenates the liver cells. The blood that comes from the portal vein is metabolized by the liver to remove toxins. Both blood is directed from hepatocytes by the hepatic sinusoids (canalitos) to reach the central vein.
The bile canaliculi are very thin tubes that collect the bile secreted by hepatocytes. The canaliculi are united to form the right and left bile ducts that carry bile to the hepatic duct, continuous with the cystic duct from the gallbladder, and finally entered the gallbladder where it is stored.
At the time of digestion, the bile salts of the gallbladder pass through the cystic duct to the hepatic duct, joining the common bile duct, where it runs before emptying into the duodenum, the ampulla of Vater.