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Anatomy & Physiology Lesson 10 THE RESPIRATORY SYSTEM The body’s cells are constantly using O2 in the metabolic processes that release energy from nutrient molecules and produce ATP. These reactions also produce CO2, which must be eliminated quickly, since too much CO2 produces acid that is toxic to cells. The cardiovascular system and the respiratory system cooperate equally in the process of supplying O2 to the cells of the body and eliminating CO2 waste. FUNCTIONS OF THE RESPIRATORY SYSTEM Provides for gas exchange—intake of O2 for delivery to body cells and elimination of CO2 produced by body cells. Contains receptors for the sense of smell, filters inspired air, produces sounds, and helps eliminate waste. RESPIRATION Respiration is the process during which gases are exchanged between the atmosphere, blood, and body cells. It takes place in three basic steps: Pulmonary ventilation (or breathing)—The inspiration (inflow) and expiration (outflow) of air between the atmosphere and the lungs. External (pulmonary) respiration—The exchange of gases between the air spaces of the lungs and blood in the pulmonary capillaries. The blood gains O2 and loses CO2. Internal (tissue) respiration—The exchange of gases between blood and tissue cells. The blood loses O2 and gains CO2. Within the cells, the metabolic reactions that consume O2 and release CO2 during ATP production are called cellular respiration. STRUCTURES OF THE RESPIRATORY SYSTEM The respiratory system consists of the nose, pharynx (throat), larynx (voice box), trachea (windpipe), bronchi, and lungs. Structurally, the respiratory system consists of two parts: The upper respiratory system—nose, pharynx, and associated structures. The lower respiratory system—larynx, trachea, bronchi, and lungs. Functionally, the respiratory system also consists of two parts: The conducting portion—a series of interconnecting cavities and tubes that conduct air into the lungs (nose, pharynx, larynx, trachea, bronchi, bronchioles, and terminal bronchioles). The respiratory portion—the portions of the respiratory system where the exchange of gases occurs (respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli). STRUCTURES OF THE RESPIRATORY SYSTEM NOSE The nose has an external portion and an internal portion. The external portion is composed of a bone and hyaline cartilage framework covered with muscle and skin and lined by mucous membrane. The openings to the nose are called external nares or nostrils. The interior structures of the nose are specialized for three functions: • Incoming air is warmed, moistened, and filtered. • Olfactory stimuli are received. • Large, hollow resonating chambers modify speech sounds. The space inside the internal nose is the nasal cavity, which is divided into right and left halves by the bone and cartilage nasal septum. Three delicate boney projections called conchae subdivide each side of the nasal cavity into the superior, middle, and inferior meatuses, which act as turbines for warming, moisturizing, and filtering air as it enters the body. PHARYNX The pharynx (throat) is a funnel-shaped tube that measures about 13 cm (5 in) long and extends from the internal nares to the level of the cricoid cartilage. The pharynx functions as a passageway for air and food, provides a resonating chamber for speech sounds, and houses the tonsils. The pharynx is further subdivided into the nasopharynx, oropharynx, and laryngopharynx. NASOPHARYNX The nasopharynx lies posterior to the nasal cavity and extends to the level of the soft palate. It has five openings in its walls: the two internal nares, the two openings into the Eustachian canals, and the opening into the oropharynx. It is lined with pseudostratified ciliated columnar epithelium, which moves dust-laden mucus received from the nasal cavity downwards to be eliminated by swallowing or spitting. It also exchanges small amounts of air with the auditory (Eustachian) tubes, to equalize air pressure between the pharynx and middle ear. OROPHARYNX The oropharynx extends from the soft palate to the level of the hyoid bone and opens into the mouth. Since it forms a common passageway for food, drink, and air, it is both respiratory and digestive in function. It is lined with nonkeratinized stratified squamous epithelium to protect it from trauma from abrasive food particles. It contains the two palatine and two lingual tonsils. LARYNGOPHARYNX The laryngopharynx or hypopharynx begins at the level of the hyoid bone and connects the esophagus with the larynx. The laryngopharynx is also both respiratory and digestive in function and is lined with nonkeratinized stratified squamous epithelium. LARYNX The larynx is a short passageway, composed of nine pieces of cartilage, that connects the laryngopharynx to the trachea. At its superior end, the larynx is protected by a flap of epithelial-covered elastic cartilage called the epiglottis. During swallowing, the larynx moves upward so the epiglottis closes over its opening, diverting liquids and foods into the esophagus and away from the airways. The glottis is in the superior portion of the larynx. It consists of a pair of mucous membrane folds called the vocal folds or true vocal cords and the space between them called the rima glottidis. Deep to the vocal folds are bands of elastic ligaments stretched between pieces of rigid cartilage and controlled by skeletal muscles of the larynx. As these ligaments are stretched tight and air passes by the membrane, sound is produced. The vocal folds, in conjunction with other structures of the head and neck, allow for the production of speech. THE UPPER RESPIRATORY SYSTEM TRACHEA The trachea is a tubular air passageway that is about 12 cm (5 in) long by 2 ½ cm (1 in) in diameter. It is located anterior to the esophagus and extends from the larynx to the carina, where it divides into right and left primary bronchi. The trachea is composed of four layers. From deep to superficial, they are: A mucosa Submucosa Hyaline cartilage Adventitia (areolar CT) The epithelium of the mucosa provides the same protection against dust as the membrane lining the nasal cavity and the larynx. The cartilage consists of 16-20 “C” shaped pieces arranged one above the other. The open side of the “C” is adjacent to the esophagus, allowing flexibility for swallowing. The cartilage rings protect the front and sides of the trachea and keep it from collapsing while breathing. PRIMARY BRONCHI the superior border of the 5th thoracic vertebra, the trachea divides into a right primary bronchus which goes into the right lung and a left primary bronchus which goes into the left lung. The right primary bronchus is more vertical, shorter, and wider than the left, so is more likely to trap an aspirated object. Like the trachea, the primary bronchi contain incomplete cartilage rings and are lined with pseudostratified ciliated columnar epithelium. At THE BRONCHIAL TREE Upon entering the lungs, the primary bronchi branch to form the smaller secondary (lobar) bronchi—one for each lobe of the lung (three on the right, two on the left). The secondary bronchi again branch to form tertiary (segmental) bronchi, which branch again to become bronchioles. The bronchioles continue to further divide until they eventually become the smallest terminal bronchioles. All of these branching bronchial structures are commonly referred to as the bronchial tree. As the branching of the bronchial tree progresses, several structural changes can be noted: The epithelium changes from pseudostratified ciliated columnar epithelium to nonciliated simple cuboidal epithelium. The incomplete rings of cartilage are gradually replaced by plates of cartilage, which eventually disappear all together. As the cartilage decreases, smooth muscle increases, encircling the lumen in spiral bands. The smooth muscle can be affected by both the ANS and various chemicals. LUNGS The lungs, separated by the mediastinum, occupy most of the thoracic cavity. Each lung is surrounded by a double layer of serous membrane called the pleural membrane. The superficial layer, the parietal pleura, lines the wall of the thoracic cavity. The deep layer, the visceral pleura, covers the lungs. Between the parietal and visceral pleurae is the pleural cavity, which contains lubricating fluid. Each lung is divided into lobes by deep fissures. The right lung has three lobes, while the left lung only has two. Each lobe has its own secondary bronchus. The secondary bronchi branch into ten tertiary bronchi in each lung. Each tertiary bronchus serves a specific segment of lung tissue called a bronchopulmonary segment. LUNGS Each bronchopulmonary segment of a lung contains many small compartments called lobules. Each lobule is wrapped in elastic CT, contains a lymphatic vessel, a venule, and a branch from a terminal bronchiole. Terminal bronchioles subdivide into microscopic respiratory bronchioles. Respiratory bronchioles again subdivide into alveolar ducts. Around the circumference of each alveolar duct are many alveoli and alveolar sacs. An alveolus is a tiny sac of simple squamous epithelium supported by a thin elastic basement membrane. It is through the walls of the alveoli that gas exchange occurs with capillaries from the circulatory system. LOBULE OF THE LUNG PULMONARY VENTILATION Pulmonary ventilation is the process in which gases are exchanged between the atmosphere and the alveoli of the lungs. Pulmonary ventilation consists of inspiration and expiration and occurs as the result of a pressure gradient. Inspiration occurs when alveolar pressure falls below atmospheric pressure. When the diaphragm and external intercostal muscles contract, the size of the thorax increases, causing the lungs to expand. The expansion of the lungs decreases alveolar pressure, moving air along the pressure gradient from the atmosphere into the lungs. Expiration occurs when alveolar pressure is higher than atmospheric pressure. As the diaphragm and external intercostal muscles relax, the chest wall and lungs recoil, causing lung volume to decrease. This increases the pressure within the alveoli, forcing air to move from the lungs into the atmosphere. CONTROL OF RESPIRATION The respiratory center of the brain is composed of neurons in the medullary rhythmicity center of the medulla oblongata and the pneumotaxic and apneustic areas of the pons. The inspiratory area of the medullary rhythmicity area establishes the basic rhythm of breathing. The expiratory area of the medullary rhythmicity area only causes contractions of the external intercostal and abdominal muscles during forceful expiration. The pneumotaxic area sends inhibitory impulses to the inspiratory area, limiting inspiration and establishing expiration. The apneustic area sends stimulatory impulses to the inspiratory area to prolong inspiration when the pneumotaxic area is inactive. Respirations may also be modified by a number of other factors, including: cortical influences, the inflation reflex, chemical stimuli (O2, CO2, and H+ levels), neural changes due to movement, blood pressure changes, the limbic system, temperature, pain, and irritation to the airways. THE DIGESTIVE SYSTEM Digestion is the breaking down of larger food particles into molecules small enough to enter body cells. Absorption is the passage of these smaller molecules into blood and lymph. The organs that work together to perform digestion and absorption are called the digestive system. The organs of the digestive system can be broken down into two main groups: The gastrointestinal (GI) tract or alimentary canal is a continuous tube that extends from the mouth to the anus. It consists of the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The accessory structures form the second group of organs. They are the teeth, tongue, salivary glands, liver, gallbladder, and pancreas. THE DIGESTIVE SYSTEM FUNCTIONS OF THE DIGESTIVE SYSTEM Ingestion—Taking food into the mouth. Secretion—Liberation of water, acid, buffers, and enzymes into the GI tract. Mixing and propulsion—Churning and passage of food through the GI tract. Digestion—Mechanical and chemical breakdown of food. Absorption—Passage of food from the GI tract into the blood and lymph. Defecation—The elimination of indigestible substances from the GI tract. LAYERS OF THE GI TRACT The wall of the GI tract, from the stomach to the anal canal, has the same basic arrangement of tissues: The innermost mucosa—a mucous membrane. The submucosa—highly vascularized areolar CT that binds the mucosa to the muscularis. The muscularis—a double layer of smooth muscle fibers that is involuntary in action and propels food through the GI tract. (Muscles in the mouth, pharynx, superior part of the esophagus, and external anal sphincter are skeletal and under voluntary control). The serosa—the superficial serous membrane layer that surrounds the organs suspended in the abdominopelvic cavity and forms part of the peritoneum. THE PERITONEUM The peritoneum is the largest serous membrane of the body. The parietal peritoneum lines the wall of the abdominopelvic cavity and the visceral peritoneum covers some of the organs in the cavity. Between the parietal and visceral layers of the peritoneum is the serous fluid-filled peritoneal cavity. Some organs (such as the kidneys and pancreas) lay posterior to the abdominal wall, so are said to be retroperitoneal. The pericardeum contains large folds that weave between the viscera, bind the organs together, and contain blood, lymph, and nerve supplies for the abdominal organs. THE PERITONEUM The peritoneum has several extensions that perform different functions: The mesentary binds the small intestine to the posterior abdominal wall, while still allowing for significant movement. The mesocolon binds the large intestine to the posterior abdominal wall, while still allowing for movement during digestion. The falciform ligament attaches the liver to the anterior abodominal wall and diaphragm. The lesser omentum suspends the stomach and duodenum from the liver. The greater omentum drapes over and protects the transverse colon and small intestines. MOUTH The mouth or oral (buccal) cavity is formed by the cheeks, hard and soft palates, and tongue. Digestion begins in the mouth, where the teeth mechanically break food into smaller pieces and salivary enzymes begin to chemically break down food. TONGUE The tongue, with its associated muscles, forms the floor of the mouth. It is composed of skeletal muscle covered with mucous membrane. The dorsum (top surface) and lateral borders of the tongue are covered with projections called papillae, some of which contain taste buds. The tongue has serous ducts at its posterior border and glands on its dorsum that secrete lingual lipase, an enzyme which begins digesting triglycerides into fatty acids and monoglycerides. SALIVARY GLANDS The salivary glands secrete saliva, a fluid which moistens the mucous membranes of the mouth and pharynx, cleanses the mouth and teeth, and lubricates, dissolves, and begins the chemical breakdown of food. Although there are many small accessory glands located throughout the mouth, most saliva is produced by three main pairs of salivary glands: The parotid glands are located inferior and anterior to the ears, just deep to the skin. The parotid ducts empty into the mouth near the upper molars. The submandibular glands are located beneath the base of the tongue in the posterior floor of the mouth. The submandibular ducts enter the mouth on either side of the lingual frenulum. The sublingual glands are located superior to the submandibular glands. Their ducts open onto the floor of the mouth, on either side of the lingual frenulum. SALIVA Chemically, saliva is 99 ½% water and ½% solutes. Solutes in saliva include: sodium, potassium, chloride, bicarbonate, and phosphate ions, some dissolved gases, urea, uric acid, serum albumin and globulin, mucus, lysozyme (an enzyme that destroys bacteria), and salivary amylase (an enzyme which begins the digestion of carbohydrates). Each type of salivary gland produces saliva with slightly different characteristics: The parotid glands secrete a watery saliva that is rich in salivary amylase. The submandibular glands secrete a thicker fluid with more mucous, but still containing a fair amount of enzyme. The sublingual glands secrete a much thicker mucous fluid with only a small amount of amylase. The various solutes in saliva serve to activate salivary amylase, act as a buffer against acidic foods, help to remove waste, and work to destroy bacteria. TEETH Teeth are used for mastication (chewing)— mechanically grinding food and (with the tongue) mixing it with saliva to form a soft, flexible bolus that is easily swallowed. Humans have two sets of teeth during their lifetime: The primary dentition consists of 20 deciduous teeth, which begin to erupt at about 6 mos. of age and are usually completely replaced by the time a child is 12 or 13 years old. The permanent dentition consists of 32 teeth that usually appear between the ages of 6 and adulthood. TEETH The normal adult dentition consists of: 12 molars—posterior teeth used for crushing and grinding food. 8 premolars or bicuspids—intermediate teeth that assist the molars in crushing and grinding. 4 canines or cuspids—pointed teeth that tear and shred food. 8 incisors—sharp anterior teeth that cut into or bite food. PRIMARY & PERMANENT DENTITIONS TEETH Each tooth is composed of a crown (the part normally above the gums or gingiva) and a root (the part normally below the gingiva and embedded in alveolar bone). At the center of a tooth is the pulp cavity, which contains the tooth’s nerve and blood supply. It is the “living” part of the tooth. The bulk of the tooth is composed of a hard, calcified CT layer called dentin. Dentin is porous and, if exposed, allows for some communication between the exterior of the tooth and the pulp cavity. The crown of the tooth is covered with enamel, the hardest substance in the human body. Enamel protects a tooth from the wear of chewing and damage from acids. The root of the tooth is covered with cementum, which is hard, but softer than either enamel or dentin. The periodontal ligament attaches the tooth to the alveolar bone, via the cementum. If root becomes exposed, the cementum can easily be worn away, leaving the underlying dentin unprotected. TOOTH STRUCTURE DENTAL CARIES Dental caries is the disease condition of teeth commonly referred to as “cavities.” Carious lesions result from bacterial infections of the structures of teeth. Oral bacteria metabolize carbohydrates (sugars and starches) to produce acid. When acid is allowed to remain in contact with calcified tooth structures, it begins to dissolve these structures. Although enamel is very hard, it will eventually be destroyed by acid attack. Once enamel has been breeched, unless the decay is removed, it can spread quickly through the softer dentin until it reaches the pulp cavity, causing acute pain, inflammation, and death to the tooth. ESOPHAGUS The esophagus is a collapsible muscular tube located posterior to the trachea. It is about 25 cm (10 in) long, passes through the mediastinum and diaphragm, and connects the laryngopharynx to the stomach. The esophagus secretes mucus, which provides lubrication for the smooth passage of a food bolus. Muscular contractions of the esophagus, called peristalsis, force the food bolus downwards toward the stomach. The upper esophageal sphincter is located at the entrance to the esophagus. It relaxes and opens during swallowing. The lower esophageal (gastroesophageal) sphincter is a narrowing in the esophagus just superior to the diaphragm. It also opens during swallowing to allow food to enter the stomach. If it does not close adequately, it allows stomach acid to enter the esophagus, resulting in painful heartburn and indegestion. STOMACH The stomach is a “J” shaped enlargement of the GI tract inferior to the diaphragm. It connects the esophagus to the duodenum of the small intestine. The stomach has four main areas: The cardia surrounds the superior opening to the stomach. The fundus is the rounded portion superior and to the left of the cardia. The body is the large central portion of the stomach. The pylorus is the region of the stomach that connects to the duodenum. The pyloric sphincter separates the stomach from the duodenum. When the stomach is relaxed, the mucosa forms large folds called rugae. STOMACH The stomach participates in both mechanical and chemical digestion. After food enters the stomach, gentle, rippling mixing waves pass over the stomach. These waves mechanically macerate food, mix it with secretions of the gastric glands, and reduce it to a thin liquid called chyme. As food moves through the stomach, the mixing waves become more vigorous. When food approaches the pyloric sphincter, small amounts of chyme are forced through the sphincter into the duodenum with each mixing wave. The rest of the food is forced back into the body of the stomach, where it is subjected to further mixing. STOMACH When foods entering the stomach are mixed with acidic gastric juice, the enzymes lingual lipase and salivary amylase are inactivated (this may take an hour or more). Mucous cells in the stomach secrete a thick alkaline mucus layer that coats the mucosal layer of the stomach with a 1-3 mm barrier. This protects the stomach lining from destruction by either hydrochloric acid or pepsin. Mucosal cells also secrete a variety of other products to aid in digestion: Pepsinogen is converted to pepsin—breaks certain peptide bonds in proteins. Gastric lipase—splits short-chain triglycerides into fatty acids and monoglycerides. Hydrochloric acid—kills microbes in foods, denatures (unfolds) proteins, converts pepsinogen into pepsin, inhibits secretion of gastrin, stimulates secretion of the hormones that promote bile flow and pancreatic juice. Intrinsic factor—necessary to absorb vitamin B12. Gastrin—stimulates secretion of HCl acid and pepsinogen, contracts lower esophageal sphincter, increases stomach motility, relaxes pyloric sphincter. PANCREAS The pancreas is a retroperitoneal gland, about 12-15 cm (5-6 in) long by 2 ½ cm (1 in) thick, and is connected to the duodenum by two ducts. Clusters of glandular epithelial cells called acini constitute the exocrine portion of the pancreas. (The endocrine portion was discussed earlier with the endocrine system.) Cells within the acini secrete a mixture of fluid and digestive enzymes called pancreatic juice. Pancreatic juice contains: Pancreatic amylase—a carbohydrate-digesting enzyme. Trypsin, chymotrypsin, carboxypeptidase, and elastase— protein-digesting enzymes. Pancreatic lipase—the principal triglyceride-digesting enzyme in an adult. Ribonuclease and deoxyribonuclease—nucleic acid-digesting enzymes. LIVER The liver is the second largest organ of the body (after the skin) and the heaviest gland, weighing about 1.4 kg (3 lb). It is located on the right side of the abdominopelvic cavity, inferior to the diaphragm. The liver has two primary lobes—the large right lobe and the smaller left lobe. Each lobe of the liver is divided into many functional units called lobules. A lobule consists of specialized epithelial cells called hepatocytes arranged around a central vein. The liver has endothelial-lined spaces called sinusoids through which blood passes, instead of veins. The sinusoids are also partially lined with stellate reticulo-endothelial (Kupffer’s) cells, which are phagocytes that destroy worn-out WBCs and RBCs, bacteria, and other foreign matter in the blood draining from the GI tract. BILE Each day, hepatocytes secrete about 800-1000 ml (1 qt) of bile, which passes through a network of bile ducts, eventually exiting the liver through the common hepatic duct. The common hepatic duct joins with the cystic duct from the gallbladder to form the common bile duct. Bile is a yellow, brownish, or olive-green liquid with a pH of 7.6-8.6. It is composed mostly of water and bile acids, bile salts, cholesterol, a phospholipid called lecithin, bile pigments, and several ions. Bile is partially an excretory product and partially a digestive excretion. Bile salts help with emulsification—the breakdown of large lipid globules into a suspension of droplets, which helps pancreatic lipase digest lipids more rapidly. Bile salts and lecithin also make cholesterol soluble. The primary pigment in bile is bilirubin. LIVER The liver performs many vital functions. Among these are: Carbohydrate metabolism—The liver is very important in maintaining blood glucose levels. When the blood glucose level is low, the liver can break down glycogen into glucose, convert certain amino acids and lactic acid to glucose, or convert other sugars into glucose. When the blood glucose level is high, the liver converts glucose into glycogen and triglycerides for storage. Lipid metabolism—The liver stores some tryglycerides and breaks down others. It synthesizes lipoproteins, which transport fatty acids, triglycerides, and cholesterol to and from body cells. Hepatocytes synthesize cholesterol and use cholesterol to make bile salts. Protein metabolism—Without the liver’s participation in protein metabolism, death would occur in a few days. Hepatocytes synthesize most plasma proteins (alpha and beta globulins, albumin, prothrombin, and fibrinogen). Liver enzymes can convert one amino acid to another (transamination). It also removes the amino group from amino acids so they can be used for ATP production or converted to carbohydrates or fats. The liver converts toxic ammonia into less toxic urea for excretion in urine. LIVER More functions of the liver: Removal of drugs and hormones—The liver can detoxify some substances (alcohol), excrete drugs (penicillin, erythromycin, sulfonamides) into bile, and chemically alter or excrete thyroid and steroid hormones (estrogens, aldosterone). Excretion of bilirubin—Bilirubin, derived from the heme of worn-out blood cells, is absorbed by the liver and secreted into bile. Most bilirubin in bile is metabolized by intestinal bacteria and eliminated in feces. Synthesis of bile salts—Bile salts are used in the small intestine for the emulsification and absorption of lipids, cholesterol, phospholipids, and lipoproteins. Storage—The liver stores glycogen, vitamins (A, B12, D, E, K) and mineral (iron, copper) for use elsewhere in the body, when needed. Phagocytosis—The stellate reticuloendothelial (Kupffer’s) cells of the liver break down worn-out RBCs, WBCs, and some bacteria. Activation of vitamin D—The liver participates with the skin and kidneys in activating vitamin D. GALLBLADDER The gallbladder is a pear-shaped sac about 7-10 cm (34 in) long, located in a depression on the posterior surface of the liver. The wall of the gallbladder is composed of only three layers (it lacks a submucosa). The mucosa is composed of simple columnar epithelium arranged in rugae similar to those of the stomach. The muscular coat is composed of smooth muscle fibers that eject the contents of the gallbladder (bile) into the cystic duct when they contract. The visceral peritoneum is the outer layer. The gallbladder functions to store and concentrate (up to tenfold) bile until it is needed by the small intestine. In the concentration process, water and ions are absorbed by the mucosa. SMALL INTESTINE The small intestine is a long tube where the major events of digestion and absorption occur. (90% of absorption occurs in the small intestine, while 10% occurs in the stomach and large intestine.) It extends from the pyloric sphincter of the stomach, coils through the abdominal cavity, and eventually terminates at the large intestine. It averages 2 ½ cm (1 in) in diameter and 3 m (10 ft) in length. The small intestine is divided into three segments: The duodenum is the shortest segment, retroperitoneal, starts at the pyloric sphincter, and extends about 25 cm (10 in). The jejunum is about 1 m (3 ft) long and extends from the duodenum to the ileum. The ileum is about 2 m (6 ft) long and joins the large intestine at the ileocecal sphincter. SMALL INTESTINE Several special features of the small intestine increase its surface area to enhance absorption. Circular folds or plicae cirulares are permanent ridges in the mucosa that are about 1 cm high. Some extend all the way around the circumference of the intestine, while other only extend part way. The circular folds begin in the duodenum and end midway through the ileum. In addition to increasing surface area, they also cause the chyme to spiral as it moves through the small intestine. Villi are small fingerlike projections of the mucosa. Each villus is ½-1 mm long and contains an arteriole, a venule, a capillary network, and a lacteal (lymphatic capillary). Nutrients absorbed by the epithelial cells of the villi enter blood or lymph through the capillaries or lacteals. Microscopic microvilli line the surfaces of absorptive cells of the mucosa. These microvilli form a fuzzy surface next to the lumen of the small intestine called the brush border. The brush border increases surface area to allow for greater absorption and contains several digestive enzymes. SMALL INTESTINE The small intestine participates in both mechanical and chemical digestion. Mechanical digestion movements are divided into two types: Segmentation is the major movement of the small intestine. It is a localized contraction occurring only in areas containing food. It mixes chyme with the digestive juices and brings food particles into contact with the mucosa for absorption. This movement squeezes segments of chyme back and forth along a length of small intestine about 12-16 times, thoroughly mixing its contents. Peristalsis propels chyme forward through the intestinal tract. Peristatic contraction in the small intestine are weak compared with those in the esophagus or stomach, so chyme remains in the small intestine for 3-5 hours. Both segmentation and peristalsis are controlled by the ANS. SMALL INTESTINE Every day the small intestine produces 1-2 L of intestinal juice, a yellow fluid consisting of water and mucus, with a pH of 7.6. Together, pancreatic and intestinal juices provide a vehicle for the absorption of substances from chyme as they come in contact with the microvilli of the small intestine. The absorptive epithelial cells also synthesize several digestive enzymes called brush border enzymes. SMALL INTESTINE All chemical and mechanical digestion occurring between the mouth and the small intestine has the purpose of changing food into forms that can pass through the epithelial cells lining the mucous membrane into the underlying blood and lymphatic vessels. These forms are monosaccharides (glucose, fructose, and galactose) from carbohydrates, single amino acids, dipeptides, and tripeptides from proteins, and monoglycerides from triglycerides. The passage of these nutrients from the GI tract into the blood or lymph is called absorption. The small intestine participates in digesting and absorbing all of the different food categories (carbohydrates, proteins, and lipids), as well as nucleic acids, water, electrolytes, and vitamins. LARGE INTESTINE The large intestine is about 1 ½ m (5 feet) long and 6 ½ cm (2 ½ in) in diameter. It extends from the ileum to the anus. The large intestine is divided into four principal regions: The cecum is a blind pouch that hangs inferior to the ileocecal valve. The appendix or vermiform appendix is a twisted, coiled tube, measuring about 8 cm (3 in) in length attached to the cecum. The colon extends from the cecum to the rectum and is divided into ascending, transverse, descending, and sigmoid portions. The rectum, the last 20 cm (8 in) of the GI tract, lies anterior to the sacrum and coccyx. The final 2-3 cm (1 in) of the rectum is called the anal canal. The external opening of the anal canal is the anus, which is guarded by an internal sphincter of smooth muscle (involuntary) and an external sphincter of skeletal muscle (voluntary). LARGE INTESTINE The wall of the large intestine does not contain any villi or permanent circular mucosal folds (unlike the small intestine). The epithelium consists of mostly absorptive and goblet cells. The absorptive cells are primarily involved in water absorption. The goblet cells produce mucus that lubricates the colonic contents in their passage. Unlike other parts of the GI tract, the muscularis layer possesses three thickened longitudinal muscle bands called taeniae coli, which run the full length of the large intestine. Tonic contractions of the taeniae coli gather the colon into a series of pouches called haustra, which give the colon a puckered appearance. LARGE INTESTINE Mechanical digestion occurs in the large intestine through several types of movements: Haustral churning is characteristic of the large intestine. The haustra remain relaxed and distended as they fill up. When the distention reaches a certain point, the walls contract, squeezing the contents into the next haustrum. Peristalsis occurs at a slower rate than in other parts of the GI tract (3-12 contractions per minute). Mass peristalsis is a strong peristaltic wave that begins at about the middle of the transverse colon and quickly drives colonic contents into the rectum. Food in the stomach initiates this gastrocolic reflex, so mass peristalsis usually occurs 3-4 times per day, during or immediately after a meal. LARGE INTESTINE The final stage of digestion occurs in the colon through bacterial activity. (No enzymes are secreted by the glands of the large intestine.) Bacteria in the large intestine prepare chyme for elimination. They ferment any remaining carbohydrates (releasing hydrogen, carbon dioxide, and methane gas), convert remaining proteins into amino acids and break down amino acids into simpler substances, and decompose bilirubin into simpler pigments (giving feces their brown color). Colonic bacteria also produce several vitamins, including some B vitamins and vitamin K. LARGE INTESTINE In addition to any excess water remaining in the chyme, the large intestine absorbs electrolytes (including sodium and chloride) and some vitamins. By the time chyme has remained in the large intestine for 3-10 hours, it has become solid or semisolid and is known as feces. Feces consists of water, inorganic salts, sloughed-off epithelial cells from the mucosa of the GI tract, bacteria, products of bacterial decomposition, and undigested food parts. LARGE INTESTINE Mass peristalsis pushes fecal matter from the sigmoid colon into the rectum. Distention of the rectal wall stimulates stretch receptors, which initiate the defecation reflex to empty the rectum. Since the external anal sphincter is under voluntary control, if it is voluntarily relaxed, defecation through the anus occurs. If it is voluntarily constricted, defecation can be postponed. Voluntary contractions of the diaphragm and abdominal muscles also aid defecation by increasing the pressure inside the abdomen.