THE RESPIRATORY SYSTEM
Respiration is the exchange of oxygen and carbon dioxide between the atmosphere and the cells of the body. There are two phases of respiration:
Oxygen and carbon dioxide exchange in equal volumes; however, certain physiological conditions may throw this balance off. For example, heavy smokers will find that the ability of their lungs to exchange gases is impaired, leading to shortness of breath and fatigue during even slight physical exertion. This debilitating situation is the direct result of their inability to draw a sufficient amount of oxygen into the body to replace the carbon dioxide build-up resulting in fatigue. Another example, hyperventilation brings too much oxygen into the body, overloading the system with oxygen, and depleting the carbon dioxide needed for balance.
ANATOMY OF THE RESPIRATORY SYSTEM
Air enters the nasal chambers and the mouth, then passes through the pharynx, larynx, trachea, and bronchi into the bronchioles. Each bronchiole is surrounded by a cluster of alveoli (Fig. 6-67).
Air enters the nasal cavity through the nostrils (nares). Lining the nasal passages are hairs (cilia), which, together with the mucous membrane, entrap and filter out dust and other minute particles that could irritate the lungs. Incoming air is warmed and moistened in the chambers of the nasal cavity to prevent damage to the lungs.
The sequence of air through the nose is anterior nares; vestibule; inferior, middle, and superior meatuses (simultaneously); and posterior nares (Fig-6-68). The nasal and oral cavities are separated by the palate. The anterior, rigid portion is called the hard palate, and the posterior fleshy part is called the soft palate. The mouth and nose serve as secondary respiratory structures.
The pharynx, or throat, serves both the respiratory and digestive systems and aids in speech. It has a mucous membrane lining that traps microscopic particles in the air and aids in adjusting temperature and humidifying inspired (inhaled) air. The pharynx connects with the mouth and nasal chambers posteriorly. According to its location, the pharynx is referred to as the nasopharynx (posterior to the nasal chambers), the oropharynx (posterior to the mouth), or the laryngopharynx (posterior to the pharynx).
The epiglottis is a lid-like, leaf-shaped cartilaginous structure that covers the entrance to the larynx and separates it from the pharynx. It acts as a trap door to deflect food particles and liquids from entering the larynx and trachea.
The larynx, or voice box, is a triangular cartilaginous structure located between the tongue and the trachea. It is protected anteriorly by the thyroid cartilage (called the Adam's apple) which is usually larger and more prominent in men than women. During the act of swallowing, it is pulled upward and forward toward the base of the tongue. The larynx is responsible for the production of vocal sound (voice). This sound production is accomplished by the passing of air over the vocal cords. The ensuing vibrations can be controlled to produce the sounds of speech or singing. The nose, mouth, throat, sinuses, and chest serve as resonating chambers to further refine and individualize the voice (Fig. 6-69).
The trachea, or windpipe, begins at the lower end of the larynx and terminates by dividing into the right and left bronchi. It is a long tube about 11 cm composed of 16 to 20 Cshaped cartilaginous rings, incomplete on the posterior surface, embedded in a fibrous membrane, that support its walls, preventing their collapse (Fig. 6-70).
The trachea has a ciliated mucous membrane lining that entraps dust and foreign material. It also propels secretions and exudates from the lungs to the pharynx, where they can be expectorated or swallowed.
The trachea splits into two primary bronchi, the right being larger and more vertical than the left; this is where they enter the lungs. This explains why more foreign objects get lodged on the right side. Once entering the lungs they immediately divide into smaller branches to carry air to each lung and further divide into the bronchioles.
The bronchioles are much smaller than the bronchi and lack supporting rings of cartilage. They terminate at the alveoli.
The alveoli are thin, microscopic air sacs within the lungs (Fig. 6-71). They are in direct contact with the pulmonary capillaries. It is here that oxygen exchanges with carbon dioxide by means of a diffusion process through the alveolar and capillary cell walls. The lungs are cone-shaped organs that lie in the thoracic cavity. Each lung contains thousands of alveoli with their capillaries. The right lung is larger than the left lung and is divided into superior, middle, and inferior lobes. The left lung has two lobes, the superior and the inferior (Fig. 6-72).
The pleurae are airtight membranes that cover the outer surface of the lungs and line the chest wall. They secrete a serous fluid that prevents friction during movements of respiration.
The mediastinum is the tissue and organs of the thoracic cavity that form a septum between the lungs. It extends from the sternum to the thoracic vertebrae and from the fascia of the neck to the diaphragm. The mediastinum contains the heart, great blood vessels, esophagus, a portion of the trachea, and the primary bronchi (Fig. 6-73).
The diaphragm is the primary muscle of respiration. It is a dome-shaped muscle and separates the thoracic and abdominal cavities. Contraction of this muscle flattens the dome and expands the vertical diameter of the chest cavity by descending into the abdominal cavity.
The intercostal muscles are situated between the ribs. Their contraction pulls the ribs upward and outward, resulting in an increase in the transverse diameter of the chest (chest expansion).
Inhalation is the direct result of the expansion caused by the action of the diaphragm and intercostal muscles. The increase in chest volume creates a negative (lower than atmospheric) pressure in the pleural cavity and lungs. Air rushes into the lungs through the mouth and nose to equalize the pressure. Exhalation results when the muscles of respiration relax. Pressure is exerted inwardly as muscles and bones return to their normal position, forcing air from the lungs.
THE PROCESS OF RESPIRATION
The rhythmical movements of breathing are controlled by the respiratory center in the brain. Nerves from the brain pass down through the neck to the chest wall and diaphragm. The nerve controlling the diaphragm is called the phrenic nerve; the nerve controlling the larynx is the vagus nerve; and the nerves controlling the muscles between the ribs are the intercostal nerves. The respiratory center is stimulated by chemical changes in the blood. When too much carbon dioxide accumulates in the blood stream, causing the blood to become acidic, the respiratory center signals the lungs to breathe faster to get rid of the carbon dioxide.
The respiratory center can be stimulated or depressed by a signal from the brain. For example, changes in one's emotional state can alter respiration through laughter, crying, emotional shock, or panic.
The muscles of respiration normally act automatically, with normal respiration being 14 to 18 cycles per minute. The lungs, when filled to capacity, hold about 6,200 ml of air, but only 500 ml of air is exchanged with each normal respiration. This exchanged air is called tidal air. The amount of air left in the lungs after forceful exhalation is about 1,200 ml and is known as residual air.
THE DIGESTIVE SYSTEM
The digestive system includes organs that digest and absorb food substances, and eliminate the unused residuals. The digestive system consists of the alimentary canal and several accessory organs. The accessory organs release secretions into the canal. These secretions assist in preparing food for absorption and use by body tissues. Table 6-8 illustrates principal digestive juices (secretions) produced by alimentary and accessory organs.
Digestion is both mechanical and chemical. Mechanical digestion occurs when food is chewed, swallowed, and propelled by a wavelike motion called peristalsis. When peristalsis occurs, a ring of reflex contraction appears in the walls of the alimentary canal. As the wave moves along, it pushes the canal's contents ahead of it (Fig. 6-74).
Chemical digestion consists of changing the various food substances, with the aid of digestive enzymes, into solutions and simple compounds. Complex carbohydrates (starches and sugars) change into simple sugars (glucose); fats change into fatty acids; and proteins change into amino acids. Once the food substances have been broken down into simple compounds, the cells of the body can absorb and use them.
THE ALIMENTARY CANAL
The alimentary canal (tract) is 9 meters in length, tubular, and includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.
The mouth, which is the first portion of the alimentary canal, is adapted to receive food and prepare it for digestion (Fig. 6-75). The mouth mechanically reduces the size of solid particles and mixes them with saliva; this process is called mastication. Saliva, produced by the salivary gland, moistens food making it easier to chew (Fig. 6-76). Saliva also lubricates the food mass to aid swallowing. The tongue assists with both mastication and swallowing.
Mastication and Deglutition
The mastication process includes the biting and tearing of food into manageable pieces. This usually involves using the incisors and cuspid teeth. The grinding of food is usually performed by the molars and premolars. During the mastication process, food is moistened and mixed with saliva.
Deglutition is the swallowing of food and involves a complex and coordinated process. It is divided into three phases; the first phase of swallowing is voluntarily; phases two and three are involuntary. Phase One, Oral Stage: the collection and swallowing of masticated food Phase Two, Pharyngeal Stage: Passage of food through the pharynx into the beginning of the esophagus Phase Three, Esophageal Stage: The passage of food into the stomach
The process of moving food from the pharynx into the esophagus requires that three openings must be blocked: the mouth, nasopharynx, and the larynx (Fig. 6-77).
Pharynx The pharynx is the passageway between the mouth and the esophagus and is shared with the respiratory tract. The epiglottis is a cartilaginous flap that closes the opening to the larynx when food is being swallowed down the pharynx. Food is deflected away from the trachea to prevent particle aspiration (inhalation).
The esophagus is a muscular tube about 25 cm (10 inches) long and pierces the diaphragm on its way to the stomach (Fig. 6-78). It is the passageway between the pharynx and the stomach. “Each end of the esophagus is encircled by muscular sphincters that act as valves to regulate passage of material. The upper esophageal sphincter in the cervical part of the esophagus helps prevent air entering the esophagus during respiration2;” it is also the valve that is relaxed when a person belches. The lower esophageal sphincter is at the junction with the stomach which help keeps food in, when this is damaged or does not work properly a patient gets heartburn or gastroesophageal reflux disease (GERD). By means of peristalsis, food is pushed along this tube to the stomach. When peristalsis is reversed, vomiting occurs.
The stomach acts as an initial storehouse for swallowed material and helps in the chemical breakdown of food substances. The stomach is a saccular enlargement of the gastrointestinal tube and lies in the left upper quadrant of the abdomen (Fig. 6-79). It connects the lower end of the esophagus with the first portion of the small intestine (the duodenum). The stomach is divided into the cardiac, fundus, body, and pyloric regions (Fig. 6-79). At each end of the stomach, muscular rings (or sphincters) form valves to close off the stomach. The sphincters prevent the stomach's contents from escaping in either direction while food substances are being mixed by peristaltic muscular contractions of the stomach wall. The sphincter at the esophageal end is the cardiac sphincter or lower esophageal sphincter; at the duodenal end it is the pyloric sphincter.
The chemical breakdown of food in the stomach is accomplished through the production of digestive juices (enzymes) by small (gastric) glands in the wall of the stomach. The principal digestive enzyme produced by the gastric glands is pepsinogen along with a secondary enzyme, hydrochloric acid. Hydrochloric acid activates pepsin from pepsinogen, kills bacteria that enter the stomach, inhibits the digestive action of amylase, and helps regulate the opening and closing of the pyloric sphincter. Pepsin is a protein-splitting enzyme capable of beginning the digestion of nearly all types of dietary protein.
Most food absorption takes place in the small intestine. In general, food is not absorbed in the stomach. An exception is alcohol, which is absorbed directly through the stomach wall. It is for this reason that intoxication occurs quickly when alcohol is taken on an empty stomach.
The stomach and intestines are enclosed in the abdominal cavity, the space between the diaphragm and the pelvis. This cavity is lined with a serous membrane called the peritoneum. The peritoneum covers the intestines and the organs; by secreting a serous fluid, it prevents friction between adjacent organs. The mesentery (double folds of peritoneum) extends from the cavity walls to the organs of the abdominal cavity, suspending them in position and carrying blood vessels to the organs.
The small intestine is a muscular, convoluted, coiled tube, about 7 meters (23 feet) long and attached to the posterior abdominal wall by its mesentery.
The small intestine is divided into three contiguous parts: the duodenum, jejunum, and ileum. It receives digestive juices from three accessory organs of digestion: the pancreas, liver, and gallbladder.
DUODENUM.—The duodenum is approximately 25 cm (10 inches) long and forms a C-shaped curve around the head of the pancreas, posterior to the liver. It has enzymes that start the breakdown of foods and receives enzymes from the pancreas that assist in digestion.
JEJUNUM.—The jejunum is the middle part of the small intestine; it is approximately 2.5 meters (8.2 feet) long. Its enzymes continue the digestive process.
ILEUM.—The ileum is the last and longest part of the small intestine; it is approximately 3.5 meters (12 feet) long.
Most of the absorption of food occurs in the small intestines, where fingerlike projections (villi) provide a large absorption surface. After ingestion, it takes 20 minutes to 2 hours for the first portion of the food to pass through the small intestine to the beginning of the large intestine.
The large intestine is so called because it is larger in diameter than the small intestine. It is considerably shorter, being about 1.5 meters (5 feet) long. It is divided into three parts: cecum, colon, and rectum.
CECUM AND COLON.—The unabsorbed food or waste material passes through the cecum into the ascending colon, across the transverse colon, and down the descending colon through the sigmoid colon to the rectum. Twelve hours after the meal, the waste material passes slowly through the colon, building in mass and reaching the rectum 24 hours after the food is ingested.
The appendix, a long narrow tube with a blind end, is a pouch-like structure of the cecum located near the junction of the ileum and the cecum. There is no known function of this structure. The appendix can become infected, causing inflammation to develop. This inflammation of the appendix is known as appendicitis (Fig. 6-80) and requires surgery to correct.
RECTUM.—The rectum is approximately 17-20 cm (7 or 8 inches) long and follows the contour of the sacrum and coccyx until it curves back into the short 2.5 cm (inch) anal canal. The anus is the external opening at the lower end of the digestive system. Except during bowel movement (defecation), it is kept closed by two sphincters. An internal one made of smooth muscle and external one made of striated muscle (Fig. 6-81).
ACCESSORY ORGANS OF DIGESTION
The accessory organs of digestion include the salivary glands, pancreas, liver, and gallbladder. As stated earlier, during the digestive process, the accessory organs produce secretions that assist the organs of the alimentary canal.
Salivary Glands The salivary glands are located in the mouth (Fig. 6-76). Within the salivary glands are two types of secretory cells, serous cells and mucous cells. The serous cells produce a watery fluid containing a digestive juice called amylase. Amylase splits starch and glycerol into complex sugars. The mucous cells secrete thick, sticky liquid called mucus. Mucus binds food particles together and acts to lubricate during swallowing. The fluids produced by the serous and mucous cells combine to form saliva. The salivary glands produce 1.7 liters of saliva daily, greatly aiding in the digestion process. Enzymes are present in saliva; they act on food, and start the breakdown process. In dentistry, location of the saliva glands and ducts (openings) is important in keeping the mouth dry during certain dental procedures. Table 6-5 lists the three major salivary glands.
The pancreas is a large, elongated gland lying posterior to the stomach (Fig. 6-82). As discussed earlier in "The Endocrine System," the pancreas has two functions: It serves both the endocrine system and the digestive system. The digestive portion of the pancreas produces digestive juices (amylase, proteinase, and lipase) that are secreted through the pancreatic duct to the duodenum. These digestive juices break down carbohydrates (amylase), proteins (proteinase), and fats (lipase) into simpler compounds.
The liver is the largest gland in the body. It is located in the upper abdomen on the right side, just under the diaphragm and superior to the duodenum and pylorus.
Of the liver's many functions, the following are important:
The gallbladder is a pear-shaped sac, stained dark green by the bile it contains. It is located in the hollow underside of the liver (Fig. 6-83). Its duct, the cystic duct, joins the hepatic duct from the liver to form the common bile duct, which enters the duodenum (Fig. 6-83). The gallbladder receives bile from the liver and then concentrates and stores it. It secretes bile when the small intestine is stimulated by the entrance of fats. Refer to Table 6-9 for a complete review of the digestive organs and processes.
Table 6-9.—The Big Picture
|David L. Heiserman, Editor||
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Revised: June 06, 2015