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Physiology of the stomach

In the mouth, we chew and moisten solid food until it becomes a small mass known as a bolus. When we swallow each bolus, it then passes through the esophagus to the stomach where it is stored along with other boluses and liquids from the same meal.

The size of the stomach varies from person to person, but on average it can comfortably contain 1-2 liters of food and liquid during a meal. When stretched to its maximum capacity by a large meal or overeating, the stomach may hold up to 3-4 liters. Distention of the stomach to its maximum size makes digestion difficult, as the stomach cannot easily contract to mix food properly and leads to feelings of discomfort.

The stomach

The stomach is the main food storage tank of the body. If it were not for the stomach’s storage capacity, we would have to eat constantly instead of just a few times each day. The stomach also secretes a mixture of acid, mucus, and digestive enzymes that helps to digest and sanitize our food while it is being stored.

Anatomy of the stomach

Gross Anatomy
The stomach is a rounded, hollow organ located just inferior to the diaphragm in the left part of the abdominal cavity. Located between the esophagus and the duodenum, the stomach is a roughly crescent-shaped enlargement of the gastrointestinal tract. The inner layer of the stomach is full of wrinkles known as rugae (or gastric folds). Rugae both allow the stomach to stretch in order to accommodate large meals and help to grip and move food during digestion.

The esophagus

The esophagus is a long, thin, and muscular tube that connects the pharynx (throat) to the stomach. It forms an important piece of the gastrointestinal tract and functions as the conduit for food and liquids that have been swallowed into the pharynx to reach the stomach.

The esophagus is about 9-10 inches (25 centimeters) long and less than an inch (2 centimeters) in diameter when relaxed. It is located just posterior to the trachea in the neck and thoracic regions of the body and passes through the esophageal hiatus of the diaphragm on its way to the stomach.

At the superior end of the esophagus is the upper esophageal sphincter that keeps the esophagus closed where it meets the pharynx. The upper esophageal sphincter opens only during the process of swallowing to permit food to pass into the esophagus. At the inferior end of the esophagus, the lower esophageal sphincter opens for the purpose of permitting food to pass from the esophagus into the stomach. Stomach acid and chyme (partially digested food) is normally prevented from entering the esophagus, thanks to the lower esophageal sphincter. If this sphincter weakens, however, acidic chyme may return to the esophagus in a condition known as acid reflux. Acid reflux can cause damage to the esophageal lining and result in a burning sensation known as heartburn. If these symptoms occur with enough frequency, they are known as GERD (gastroesophageal reflux disease).

Physiology of the liver

Physiology of the liver

The liver plays an active role in the process of digestion through the production of bile. Bile is a mixture of water, bile salts, cholesterol, and the pigment bilirubin. Hepatocytes in the liver produce bile, which then passes through the bile ducts to be stored in the gallbladder. When food containing fats reaches the duodenum, the cells of the duodenum release the hormone cholecystokinin to stimulate the gallbladder to release bile. Bile travels through the bile ducts and is released into the duodenum where it emulsifies large masses of fat. The emulsification of fats by bile turns the large clumps of fat into smaller pieces that have more surface area and are therefore easier for the body to digest.

Bilirubin present in bile is a product of the liver’s digestion of worn out red blood cells. Kupffer cells in the liver catch and destroy old, worn out red blood cells and pass their components on to hepatocytes. Hepatocytes metabolize hemoglobin, the red oxygen-carrying pigment of red blood cells, into the components heme and globin. Globin protein is further broken down and used as an energy source for the body. The iron-containing heme group cannot be recycled by the body and is converted into the pigment bilirubin and added to bile to be excreted from the body. Bilirubin gives bile its distinctive greenish color. Intestinal bacteria further convert bilirubin into the brown pigment stercobilin, which gives feces their brown color.

The liver

Weighing in at around 3 pounds, the liver is the body’s second largest organ; only the skin is larger and heavier. The liver performs many essential functions related to digestion, metabolism, immunity, and the storage of nutrients within the body. These functions make the liver a vital organ without which the tissues of the body would quickly die from lack of energy and nutrients. Fortunately, the liver has an incredible capacity for regeneration of dead or damaged tissues; it is capable of growing as quickly as a cancerous tumor to restore its normal size and function.

The epiglottis

The epiglottis is a flexible flap at the superior end of the larynx in the throat. It acts as a switch between the larynx and the esophagus to permit air to enter the airway to the lungs and food to pass into the gastrointestinal tract. The epiglottis also protects the body from choking on food that would normally obstruct the airway.

The epiglottis is a thin, leaf-shaped structure at the superior border of the larynx, or voice box. In its relaxed position, the epiglottis projects into the pharynx, or throat, and rests just posterior to the tongue. Viewed from the posterior direction, it is shaped like a teardrop with a wide, rounded region at the superior end and a narrow tapered point at its inferior end. The epiglottis is also concave with the lateral edges pointing posteriorly. Two tiny ligaments - the thyroepiglottic and hyoepiglottic ligaments - hold the epiglottis in its resting position in the throat. The thin thyroepiglottic ligament connects the inferior point of the epiglottis to the thyroid cartilage of the larynx, while the hyoepiglottic ligament connects the anterior surface of the superior region to the hyoid bone.

The respiratory system of the head and neck

The respiratory system of the head and neck marks the starting point for where oxygen enters the body. The system begins at the nose and mouth where oxygen is inhaled. The areas of the respiratory in the head and neck allow air to flow in and out of the lungs.

The important parts of the respiratory system in the head and neck include the nasal cavity, which processes the airflow on its way through to the lungs. Connected to the nasal cavity is the pharynx that is actually a part of the respiratory and digestive systems. It allows for the passage of both food and air. It lies behind and to the sides of the larynx, or voice box, which forms part of a tube in the throat that carries air to and from the lungs and houses the epiglottis. At rest, the epiglottis is upright and allows air to pass through the larynx and into the rest of the respiratory system. During swallowing, it folds back to cover the entrance to the larynx, preventing food and drink from entering the windpipe. The trachea, or windpipe, allows the head and neck to twist and bend during the process of breathing.

All of these parts in the head and neck play a significant role in directing oxygen to the lungs so that the body can breathe in oxygen.

Physiology of the respiratory system

Pulmonary Ventilation
Pulmonary ventilation is the process of moving air into and out of the lungs to facilitate gas exchange. The respiratory system uses both a negative pressure system and the contraction of muscles to achieve pulmonary ventilation. The negative pressure system of the respiratory system involves the establishment of a negative pressure gradient between the alveoli and the external atmosphere. The pleural membrane seals the lungs and maintains the lungs at a pressure slightly below that of the atmosphere when the lungs are at rest. This results in air following the pressure gradient and passively filling the lungs at rest. As the lungs fill with air, the pressure within the lungs rises until it matches the atmospheric pressure. At this point, more air can be inhaled by the contraction of the diaphragm and the external intercostal muscles, increasing the volume of the thorax and reducing the pressure of the lungs below that of the atmosphere again.

To exhale air, the diaphragm and external intercostal muscles relax while the internal intercostal muscles contract to reduce the volume of the thorax and increase the pressure within the thoracic cavity. The pressure gradient is now reversed, resulting in the exhalation of air until the pressures inside the lungs and outside of the body are equal. At this point, the elastic nature of the lungs causes them to recoil back to their resting volume, restoring the negative pressure gradient present during inhalation.

The teeth

By the shape the teeth are divided into:
- incisors - have one root, wedge-shaped crowns, particularly upper incisors crown shape has the spatula, lower - bits;
- canines - have one root, crown conical shape;
- premolars: upper premolars have bifurcated root, on the horizontal cut their crown has oval shape; hills are almost identical; lower premolars have one root, on the horizontal cut their crown has round shape, vestibular hill is large, oral - smaller;
- molars: upper molars have three roots (two vestibular and one oral), diamond-shaped crown; lower molars have two roots, square shape of crown.

Adult human has 28-32 permanent teeth. Child has 20 milk teeth.
The teeth are located in the mouth.

The salivary glands

Salivary glands produce saliva.

Salivary glands are divided into small and large.
Small salivary glands located in the mucous membrane of the mouth:
- labial;
- buccal;
- palatine;
- lingual;
- molar.

Large salivary glands:
1) parotid gland - paired, is located near the ear, in the mandibular fold. Duct of parotid gland located on chewing muscle, passes through the buccal muscle and opens in the cheek mucosa opposite of 2nd upper molar. Parotid salivary gland - parenchymal organ, anatomical unit of which is the a particle;
2) submandibular gland - paired, located in submandibular triangle. Duct of submandibular gland opens in the proper oral cavity in the sublingual meet. Submandibular gland - parenchymal organ, anatomical unit of which is the a particle;
3) sublingual gland - paired, located in the sublingual crease. Large duct of sublingual gland opens in the sublingual meet, small duct opens directly in sublingual crease. Sublingual gland - parenchymal organ, anatomical unit of which is a particle.