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Small intestine
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Small Intestine
Diagram showing the small intestine and surrounding structures
Details
Artery
Superior mesenteric artery
Vein
Hepatic portal vein
Nerve
Celiac ganglia, vagus[1]
Lymph
Intestinal lymph trunk
Identifiers
Latin
Intestinum tenue
MeSH
A03.556.124.684
Code
4453467
Dorlands
/Elsevier
Small intestine r
TA
A05.6.01.001
FMA
7200
Anatomical terminology
[edit on Wikidata]
The small intestine or small bowel is the part of the gastrointestinal tract between the stomach and the large intestine, and is where most of the end absorption of food takes place. The small intestine has three distinct regions – the duodenum, jejunum, and ileum. The duodenum is the shortest part of the small intestine and is where preparation for absorption begins. It also receives bile and pancreatic juice through the pancreatic duct, controlled by the sphincter of Oddi. The primary function of the small intestine is the absorption of nutrients and minerals from food, using small finger-like protrusions called villi.[2]
Contents
[hide]
1 Structure
1.1 Size
1.2 Parts
1.3 Blood supply
1.4 Histology
1.5 Development
2 Function
2.1 Digestion
2.2 Absorption
2.3 Immunological
3 Gene and protein expression
4 Clinical significance
5 Other animals
6 Society and culture
7 Additional images
8 See also
9 References
10 Bibliography
11 External links
Structure[edit]
Size[edit]
The length of the small intestine can vary greatly, from as short as 2.75 m (9.0 ft) to as long as 10.49 m (34.4 ft).[3] The average length in a living person is 3m-5m.[4][5] The length depends both on how tall the person is and how the length is measured.[3] Taller people generally have a longer small intestine and measurements are generally longer after death and when the bowel is empty.[3]
It is approximately 1.5 cm in diameter in newborns after 35 weeks of gestational age,[6] and 2.5–3 cm (1 inch) in diameter in adults. On abdominal X-rays, the small intestine is considered to be abnormally dilated when the diameter exceeds 3 cm.[7][8] On CT scans, a diameter of over 2.5 cm is considered abnormally dilated.[7][9] The surface area of the human small intestinal mucosa, due to enlargement caused by folds, villi and microvilli, averages 30 square meters.[10]
Parts[edit]
The small intestine is divided into three structural parts.
The duodenum is a short structure ranging from 20 cm (7.9 inches) to 25 cm (9.8 inches) in length, and shaped like a "C".[11] It surrounds the head of the pancreas. It receives gastric chyme from the stomach, together with digestive juices from the pancreas (digestive enzymes) and the liver (bile). The digestive enzymes break down proteins and bile and emulsify fats into micelles. The duodenum contains Brunner's glands, which produce a mucus-rich alkaline secretion containing bicarbonate. These secretions, in combination with bicarbonate from the pancreas, neutralize the stomach acids contained in gastric chyme.
The jejunum is the midsection of the small intestine, connecting the duodenum to the ileum. It is about 2.5 m long, and contains the plicae circulares, and villi that increase its surface area. Products of digestion (sugars, amino acids, and fatty acids) are absorbed into the bloodstream here. The suspensory muscle of duodenum marks the division between the duodenum and the jejunum.
The ileum: The final section of the small intestine. It is about 3 m long, and contains villi similar to the jejunum. It absorbs mainly vitamin B12 and bile acids, as well as any other remaining nutrients. The ileum joins to the cecum of the large intestine at the ileocecal junction.
The jejunum and ileum are suspended in the abdominal cavity by mesentery. The mesentery is part of the peritoneum. Arteries, veins, lymph vessels and nerves travel within the mesentery.[12]
Blood supply[edit]
The small intestine receives a blood supply from the coeliac trunk and the superior mesenteric artery. These are both branches of the aorta. The duodenum receives blood from the coeliac trunk via the superior pancreaticoduodenal artery and from the superior mesenteric artery via the inferior pancreaticoduodenal artery. These two arteries both have anterior and posterior branches that meet in the midline and anastomose. The jejunum and ileum receive blood from the superior mesenteric artery.[13] Branches of the superior mesenteric artery form a series of arches within the mesentery known as arterial arcades, which may be several layers deep. Straight blood vessels known as vasa recta travel from the arcades closest to the ileum and jejunum to the organs themselves.[13]
Histology[edit]
Main article: Gastrointestinal wall
Micrograph of the small intestine mucosa showing the intestinal villi and crypts of Lieberkühn.
The three sections of the small intestine look similar to each other at a microscopic level, but there are some important differences. The parts of the intestine are as follows:
Layer
Duodenum
Jejunum
Ileum
Serosa 1st part serosa, 2nd–4th adventitia Normal Normal
Muscularis externa Longitudinal and circular layers, with Auerbach's (myenteric) plexus in between Same as duodenum Same as duodenum
Submucosa Brunner's glands and Meissner's (submucosal) plexus No BG No BG
Mucosa: muscularis mucosae Normal Normal Normal
Mucosa: lamina propria No PP No PP Peyer's patches
Mucosa: intestinal epithelium Simple columnar. Contains goblet cells, Paneth cells Similar to duodenum ?
Development[edit]
See also: Development of the digestive system
The small intestine develops from the midgut of the primitive gut tube.[14] By the fifth week of embryological life, the ileum begins to grow longer at a very fast rate, forming a U-shaped fold called the primary intestinal loop. The loop grows so fast in length that it outgrows the abdomen and protrudes through the umbilicus. By week 10, the loop retracts back into the abdomen. Between weeks six and ten the small intestine rotates anticlockwise, as viewed from the front of the embryo. It rotates a further 180 degrees after it has moved back into the abdomen. This process creates the twisted shape of the large intestine.[14]
Function[edit]
Food from the stomach is allowed into the duodenum through the pylorus by a muscle called the pyloric sphincter.
Digestion[edit]
The small intestine is where most chemical digestion takes place. Many of the digestive enzymes that act in the small intestine are secreted by the pancreas and liver and enter the small intestine via the pancreatic duct. Pancreatic enzymes and bile from the gallbladder enter the small intestine in response to the hormone cholecystokinin, which is produced in the small intestine in response to the presence of nutrients. Secretin, another hormone produced in the small intestine, causes additional effects on the pancreas, where it promotes the release of bicarbonate into the duodenum in order to neutralize the potentially harmful acid coming from the stomach.
The three major classes of nutrients that undergo digestion are proteins, lipids (fats) and carbohydrates:
Proteins are degraded into small peptides and amino acids before absorption.[15] Chemical breakdown begins in the stomach and continues in the small intestine. Proteolytic enzymes, including trypsin and chymotrypsin, are secreted by the pancreas and cleave proteins into smaller peptides. Carboxypeptidase, which is a pancreatic brush border enzyme, splits one amino acid at a time. Aminopeptidase and dipeptidase free the end amino acid products.
Lipids (fats) are degraded into fatty acids and glycerol. Pancreatic lipase breaks down triglycerides into free fatty acids and monoglycerides. Pancreatic lipase works with the help of the salts from the bile secreted by the liver and stored in the gall bladder. Bile salts attach to triglycerides to help emulsify them, which aids access by pancreatic lipase. This occurs because the lipase is water-soluble but the fatty triglycerides are hydrophobic and tend to orient towards each other and away from the watery intestinal surroundings. The bile salts emulsify the triglycerides in the watery surroundings until the lipase can break them into the smaller components that are able to enter the villi for absorption.
Some carbohydrates are degraded into simple sugars, or monosaccharides (e.g., glucose). Pancreatic amylase breaks down some carbohydrates (notably starch) into oligosaccharides. Other carbohydrates pass undigested into the large intestine and further handling by intestinal bacteria. Brush border enzymes take over from there. The most important brush border enzymes are dextrinase and glucoamylase, which further break down oligosaccharides. Other brush border enzymes are maltase, sucrase and lactase. Lactase is absent in some adult humans and, for them, lactose, like most poly-saccharides, is not digested in the small intestine. Some carbohydrates, such as cellulose, are not digested at all, despite being made of multiple glucose units. This is because the cellulose is made out of beta-glucose, making the inter-monosaccharidal bindings different from the ones present in starch, which consists of alpha-glucose. Humans lack the enzyme for splitting the beta-glucose-bonds, something reserved for herbivores and bacteria from the large intestine.
Absorption[edit]
Digested food is now able to pass into the blood vessels in the wall of the intestine through either diffusion or active transport. The small intestine is the site where most of the nutrients from ingested food are absorbed. The inner wall, or mucosa, of the small intestine is lined with simple columnar epithelial tissue. Structurally, the mucosa is covered in wrinkles or folds called plicae circulares, which are considered permanent features in the wall of the organ. They are distinct from rugae which are considered non-permanent or temporary allowing for distention and contraction. From the plicae circulares project microscopic finger-like pieces of tissue called villi (Latin for "shaggy hair"). The individual epithelial cells also have finger-like projections known as microvilli. The functions of the plicae circulares, the villi, and the microvilli are to increase the amount of surface area available for the absorption of nutrients, and to limit the loss of said nutrients to intestinal fauna.
Each villus has a network of capillaries and fine lymphatic vessels called lacteals close to its surface. The epithelial cells of the villi transport nutrients from the lumen of the intestine into these capillaries (amino acids and carbohydrates) and lacteals (lipids). The absorbed substances are transported via the blood vessels to different organs of the body where they are used to build complex substances such as the proteins required by our body. The material that remains undigested and unabsorbed passes into the large intestine.
Absorption of the majority of nutrients takes place in the jejunum, with the following notable exceptions:
Iron is absorbed in the duodenum.
Vitamin B12 and bile salts are absorbed in the terminal ileum.
Water and lipids are absorbed by passive diffusion throughout the small intestine.
Sodium bicarbonate is absorbed by active transport and glucose and amino acid co-transport.
Fructose is absorbed by facilitated diffusion.
Immunological[edit]
The small intestine supports the body's immune system.[16] The presence of gut flora appears to contribute positively to the host's immune system. Peyer's patches, located within the ileum of the small intestine, are an important part of the digestive tract's local immune system. They are part of the lymphatic system, and provide a site for antigens from potentially harmful bacteria or other microorganisms in the digestive tract to be sampled, and subsequently presented to the immune system.[17]
Gene and protein expression[edit]
Further information: Bioinformatics § Gene and protein expression
About 20,000 protein coding genes are expressed in human cells and 70 % of these genes are expressed in the normal duodenum.[18][19] Some 300 of these genes are more specifically expressed in the duodenum with very few genes expressed only in the small intestine. The corresponding specific proteins are expressed in glandular cells of the mucosa, such as fatty acid binding protein FABP6. Most of the more specifically expressed genes in the small intestine are also expressed in the duodenum, for example FABP2 and the DEFA6 protein expressed in secretory granules of Paneth cells.[20]
Clinical significance[edit]
See also: Gastrointestinal disease
The small intestine is a complex organ, and as such, there are a very large number of possible conditions that may affect the function of the small bowel. A few of them are listed below, some of which are common, with up to 10% of people being affected at some time in their lives, while others are vanishingly rare.
Small intestine obstruction or obstructive disorders
Paralytic ileus
Volvulus
Hernia
Intussusception
Adhesions
Obstruction from external pressure
Obstruction by masses in the lumen (foreign bodies, bezoar, gallstones)
Infectious diseases
Giardiasis
Ascariasis
Tropical sprue
Tape worm (Diphyllobothrium latum, Taenia solium, Hymenolepsis nana)
Hookworm (e.g. Necator americanus, Ancylostoma duodenale)
Nematodes (e.g. Ascaris lumbricoides)
Other Protozoa (e.g. Cryptosporidium parvum, Cyclospora, Microsporidia, Entamoeba histolytica)
Bacterial infections
Enterotoxigenic Escherichia coli
Salmonella enterica
Campylobacter
Shigella
Yersinia
Clostridium difficile (antibiotic-associated colitis, Pseudomembranous colitis)
Mycobacterium (Mycobacterium avium paratuberculosis, disseminated Mycobacterium tuberculosis)
Whipple's disease
Vibrio (cholera)
Enteric (typhoid) fever (Salmonella enterica var. typhii) and paratyphoid fever
Bacillus cereus
Clostridium perfringens (gas gangrene)
Viral infections
Rotavirus
Norovirus
Astrovirus
Adenovirus
Calicivirus
Neoplasms (cancers)
Adenocarcinoma
Carcinoid
Gastrointestinal stromal tumor (GIST)
Lymphoma
Sarcoma
Leiomyoma
Metastatic tumors, especially SCLC or melanoma
Developmental, congenital or genetic conditions
Duodenal (intestinal) atresia
Hirschsprung's disease
Meckel's diverticulum
Pyloric stenosis
Pancreas divisum
Ectopic pancreas
Enteric duplication cyst
Situs inversus
Cystic fibrosis
Malrotation
Persistent urachus
Omphalocele
Gastroschisis
Disaccharidase (lactase) deficiencies
Primary bile acid malabsorption
Gardner syndrome
Familial adenomatous polyposis syndrome (FAP)
Other conditions
Crohn's disease, and the more general inflammatory bowel disease
Typhlitis (neutropenic colitis in the immunosuppressed
Coeliac disease (sprue or non-tropical sprue)
Mesenteric ischemia
Embolus or thrombus of the superior mesenteric artery or the superior mesenteric vein
Arteriovenous malformation
Gastric dumping syndrome
Irritable bowel syndrome
Duodenal (peptic) ulcers
Gastrointestinal perforation
Hyperthyroidism
Diverticulitis
Radiation enterocolitis
Mesenteric cysts
Peritoneal Infection
Sclerosing retroperitonitis
Small intestinal bacterial overgrowth
Other animals[edit]
The small intestine is found in all tetrapods and also in teleosts, although its form and length vary enormously between species. In teleosts, it is relatively short, typically around one and a half times the length of the fish's body. It commonly has a number of pyloric caeca, small pouch-like structures along its length that help to increase the overall surface area of the organ for digesting food. There is no ileocaecal valve in teleosts, with the boundary between the small intestine and the rectum being marked only by the end of the digestive epitheliu [21]
In tetrapods, the ileocaecal valve is always present, opening into the colon. The length of the small intestine is typically longer in tetrapods than in teleosts, but is especially so in herbivores, as well as in mammals and birds, which have a higher metabolic rate than amphibians or reptiles. The lining of the small intestine includes microscopic folds to increase its surface area in all vertebrates, but only in mammals do these develop into true villi.[21]
The boundaries between the duodenum, jejunum, and ileum are somewhat vague even in humans, and such distinctions are either ignored when discussing the anatomy of other animals, or are essentially arbitrary.[21]
There is no small intestine as such in non-teleost fish, such as sharks, sturgeons, and lungfish. Instead, the digestive part of the gut forms a spiral intestine, connecting the stomach to the rectum. In this type of gut, the intestine itself is relatively straight, but has a long fold running along the inner surface in a spiral fashion, sometimes for dozens of turns. This valve greatly increases both the surface area and the effective length of the intestine. The lining of the spiral intestine is similar to that of the small intestine in teleosts and non-mammalian tetrapods.[21]
In lampreys, the spiral valve is extremely small, possibly because their diet requires little digestion. Hagfish have no spiral valve at all, with digestion occurring for almost the entire length of the intestine, which is not subdivided into different regions.[21]
Society and culture[edit]
In traditional Chinese medicine, the small intestine is a yang organ.[22]
Additional images[edit]
Small intestine in situ, greater omentum folded upwards.
Third state of the development of the intestinal canal and peritoneum, seen from in front (diagrammatic). The mode of preparation is the same as in Fig 400
Second stage of development of the intestinal canal and peritoneum, seen from in front (diagrammatic). The liver has been removed and the two layers of the ventral mesogastrium (lesser omentum) have been cut. The vessels are represented in black and the peritoneum in the reddish tint.
First stage of the development of the intestinal canal and the peritoneum, seen from the side (diagrammatic). From colon 1 the ascending and transverse colon will be formed and from colon 2 the descending and sigmoid colons and the rectum.
See also[edit]
Wikimedia Commons has media related to Small intestine.
Large intestine
Stomach
References[edit]
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Jump up ^ Standring, Susan (2016). Gray's Anatomy. UK: Elsevier. p. 1124. ISBN 978-0-7020-5230-9. ..and has a mean length of 5 metres (3 - 8.5 metres) when measured intraoperatively in the living adult (Tietelbaum et al 2013).
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Jump up ^ "Intestinal immune cells play an unexpected role in immune surveillance of the bloodstream". Massachusetts General Hospital. 13 December 2012.
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Jump up ^ "The human proteome in small intestine - The Human Protein Atlas". www.proteinatlas.org. Retrieved 2017-09-26.
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Bibliography[edit]
Sherwood, Lauralee (2006). Fundamentals of physiology: a human perspective (Third ed.). Florence, KY: Cengage Learning. p. 768. ISBN 0-534-46697-4.
Solomon et al. (2002) Biology Sixth Edition, Brooks-Cole/Thomson Learning ISBN 0-03-033503-5
Townsend et al. (2004) Sabiston Textbook of Surgery, Elsevier ISBN 0-7216-0409-9
Thomson A, Drozdowski L, Iordache C, Thomson B, Vermeire S, Clandinin M, Wild G (2003). "Small bowel review: Normal physiology, part 1". Dig Dis Sci. 48 (8): 1546–64. doi:10.1023/A:1024719925058. PMID 12924651.
Thomson A, Drozdowski L, Iordache C, Thomson B, Vermeire S, Clandinin M, Wild G (2003). "Small bowel review: Normal physiology, part 2". Dig Dis Sci. 48 (8): 1565–81. doi:10.1023/A:1024724109128. PMID 12924652.
External links[edit]
Small intestine at the Human Protein Atlas
[hide]
v
t
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Anatomy of the gastrointestinal tract, excluding the mouth
Upper
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peripharyngeal
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text:
Geography of North America
From Wikipedia, the free encyclopedia
Jump to navigation Jump to search
Global view centered on North America
North America is the third largest continent, and is also a portion of the second largest supercontinent if North and South America are combined into the Americas and Africa, Europe, and Asia are considered to be part of one supercontinent called Afro-Eurasia.
With an estimated population of 460 million and an area of 24,346,000 km² (9,400,000 mi²), the northernmost of the two continents of the Western Hemisphere[1] is bounded by the Pacific Ocean on the west; the Atlantic Ocean on the east; the Caribbean Sea, Atlantic and Pacific oceans, and South America on the south; and the Arctic Ocean on the north.
The northern half of North America is sparsely populated and covered mostly by Canada, except for the northeastern portion, which is occupied by Greenland, and the northwestern portion, which is occupied by Alaska, the largest state of the U.S. The central and southern portions of the continent are represented by the United States, Mexico, and numerous smaller states primarily in Central America and in the Caribbean.
The continent is delimited on the southeast by most geographers at the Darién watershed along the Colombia-Panama border, placing all of Panama within North America.[2][3][4] Alternatively, a less common view would end North America at the man-made Panama Canal. Islands generally associated with North America include Greenland, the world's largest island, and archipelagos and islands in the Caribbean. The terminology of the Americas is complex, but "Anglo-America" can describe Canada and the U.S., while "Latin America" comprises Mexico and the countries of Central America and the Caribbean, as well as the entire continent of South America.
Natural features of North America include the northern portion of the American Cordillera, represented by the geologically new Rocky Mountains in the west; and the considerably older Appalachian Mountains to the east. The north hosts an abundance of glacial lakes formed during the last glacial period, including the Great Lakes. North America's major continental divide is the Great Divide, which runs north and south down through Rocky Mountains. The major watersheds all drain to the east: The Mississippi/Missouri and Rio Grande into the Gulf of Mexico, and St. Lawrence into the Atlantic.
Climate is determined to a large extent by the latitude, ranging from Arctic cold in the north to tropical heat in the south. The western half of North America tends to have milder and wetter climate than other areas with equivalent latitude, although there are steppes (known as "prairies") in the central and western portions, and deserts in the Southwestern United States of Arizona, Colorado, California, Nevada, New Mexico, Utah, Oklahoma, and Texas; along with the Mexican states of Baja California, Baja California Sur, Sonora, Chihuahua, Coahuila, Nuevo Leon and Tamaulipas.
The Blue Marble, NASA; east more rain than west.
Contents
1 Paleogeography
2 Physiography
3 Surface and climate
3.1 North America's greatest snowfalls
3.2 Hydrography and Deserts
3.2.1 River systems
4 Deserts
4.1 Climate and vegetation
5 Zoology
6 Mining and petroleum
6.1 Rocky Mountains
7 Agriculture and forestry
8 See also
9 References
10 Notes
11 External links
12 Bibliography
Paleogeography[edit]
The paleogeological origins of the "basement" rocks underlying North America.
Age of the bedrock underlying North America, from red (oldest) to blue, green, yellow (newest).
Seventy percent of North America is underlain by the Laurentia craton,[5] which is exposed as the Canadian Shield in much of central and eastern Canada around the Hudson Bay, and as far south as the U.S. states of Michigan, Wisconsin, and Minnesota. The continental crust started to form 4 billion years ago (Ga), and six of the microcontinents collided to form the craton about 2 Ga. This core has been enlarged by plate tectonics, most notably between 1.8 and 1.65 Ga when a piece currently stretching from Arizona to Missouri fused with the south and west portion of the craton. The craton started to rift about 1.1 Ga, and the fissure (now the Midcontinent Rift System) ran between Kansas and Lake Superior before stopping, perhaps due to the Grenville collision in the east. Otherwise the craton has remained relatively stable, with some rocks dating from 2.5 to 4 Ga, including what may be the world's oldest known rock: Specimens from the Nuvvuagittuq greenstone belt on the coast of the Hudson Bay have been dated to 4.38 Ga,[6][7] though the dating methods are disputed.[8] Periodic flooding by inland seas, most recently the Western Interior Seaway during the Cretaceous, caused the layer of sedimentary rock over the remainder of the craton. The Laurentia craton is the center of the Proterozoic supercontinent Rodinia in most models,[9] and was also part of the later Laurussia, Pangea, and Laurasia supercontinents.
Roughly 3 million years ago (Ma), the volcanic Isthmus of Panama formed between the North and South American continents creating a bridge over what was the Central American Seaway and allowing the migration of flora and fauna between the two landmasses in the Great American Interchange. Starting 2.58 Ma, the Quaternary glaciation covered much of the continent with ice, centered west of Hudson Bay, the floor of which is slowly rebounding after being depressed by the great weight of the ice. Glaciers descended the slopes of the Rocky Mountains and those of the Pacific Margin. Extensive glacial lakes, such as Glacial Lake Missoula, Bonneville, Lahontan, Agassiz, and Algonquin, formed by glacial melt water. "Remnants of them are still visible in the Great Basin and along the edge of the Canadian Shield in the form of the Great Salt Lake, the Great Lakes, and the large lakes of west central Canada."[10] The last glacial period of the current ice age caused a lowering of the sea level, exposing the Bering Land Bridge between Alaska and Siberia, which resulted in human migration from Asia to the Americas between 40,000 and 15,000 years ago.[11]
North American watersheds (Atlantic, Arctic, Great Basin, & Pacific)
North America can also be divided into four great regions:[citation needed]
Great Plains: stretching from the Gulf of Mexico to the Canadian Arctic;
the geologically young, mountainous west: including the Rocky Mountains, the Great Basin, California and Alaska;
the raised but relatively flat plateau of the Canadian Shield in the northeast;
the varied eastern region: including the Appalachian Mountains, the coastal plain of the Atlantic Seaboard, and the Florida peninsula.[12]
Mexico and its long plateaus and cordilleras fall largely in the western region, although the eastern coastal plain does extend south along the Gulf.
North American Plate (center top)
Caribbean Plate (center)
Physiography[edit]
North America may be divided into at least five major physiographic regions:[citation needed]
Canadian Shield
This is a geologically stable area of rock dating between 2.5 and 4 Gya that occupies most of the northeastern quadrant, including Greenland.
Appalachian Mountains
The Appalachians are an old and eroded system that formed about 300 Ma and extends from the Gaspé Peninsula to Alabama.
Atlantic Coastal Plain
The plain is a belt of lowlands widening to the south that extends from south New England to Mexico.
Interior Lowlands
The lowlands extend down the middle of the continent from the Mackenzie Valley to the Atlantic Coastal Plain, and include the Great Plains on the west and the agriculturally productive Interior Plains on the east.
North American Cordillera
The cordillera is a complex belt of mountains and associated plateaus and basins some of which were formed as recently as 100–65 Ma, during the Cretaceous. The cordillera extend from Alaska into Mexico and includes two orogenic belts — the Pacific Margin on the west and the Rocky Mountains on the east — separated by a system of intermontane plateaus and basins.[10]
The Coastal Plain and the main belts of the North American Cordillera continue in the south in Mexico (where the Mexican Plateau, bordered by the Sierra Madre Oriental and the Sierra Madre Occidental, is considered a continuation of the intermontane system) to connect the Transverse Volcanic Range, a zone of high and active volcanic peaks south of Mexico City.
The vast majority of North America is located on the North American Plate, centered on the Laurentia craton. Parts of California and western Mexico form the partial edge of the Pacific Plate; the two plates meet along the San Andreas Fault. The southern portion of the Caribbean and parts of Central America compose the much smaller Caribbean Plate.
The western mountains have split in the middle, into the main range of the Rockies and the Coast Ranges in California, Oregon, Washington, and British Columbia with the Great Basin (a lower area containing smaller ranges and low-lying deserts) in between. The highest peak is Mount McKinley/Denali in Alaska. The geographic center of North America is approximately "6 miles west of Balta, Pierce County, North Dakota" at 48⁰ 10′north, 100⁰ 10′west,[13] and a 15-foot (4.5 m) field stone obelisk in Rugby, North Dakota (~15 miles or 25 km away) claims to mark the center.[citation needed]
Three countries (Canada, the United States, and Mexico) make up most of North America's land mass; they share the continent with 34 other island countries in the Caribbean and south of Mexico.
Surface and climate[edit]
The Great Plains
The Great Plains is the broad expanse of prairie and steppe which lies east of the Rocky Mountains in the United States and Canada. The narrow plains in the Mexican coast and the savannas of the Mississippi are analogous to, respectively, the Patagonian steppes and the pampas of the Piranha, Paraguay, and Rio de la Plata. Thus the Appalachians and the mountain chains of Brazil are regarded as creating similar interruptions to the plains community.[14]
North America extends to within 10° of latitude of both the equator and the North Pole. It embraces every climatic zone, from tropical rain forest and savanna on the lowlands of Central America to areas of permanent ice cap in central Greenland.[14] Subarctic and tundra climates prevail in north Canada and north Alaska, and desert and semiarid conditions are found in interior regions cut off by high mountains from rain-bearing westerly winds.[12] However, most of the continent has temperate climates very favorable to settlement and agriculture. Prairies, or vast grasslands cover a huge amount in mountain ranges.[12]
North America's greatest snowfalls[edit]
Greatest Snowfalls
Places
Date
Inches
Centimeters
24 hours
Silver Lake, Colorado
April 14–15, 1921
76
195.6
1 month
Tamarack, California
January 1911
390
991
One storm
Mt. Shasta Ski Bowl, Calif.
February 13–19, 1959
189
480
One season
Mount Baker, WA
1998–1999
1, 140
2, 895.6
[15]
Hydrography and Deserts[edit]
See also: hydrography, evapotranspiration, surface runoff, and percolation
The average rainfall in North America is 76 cm/year, which produces some 18 petaliters of water.[16]
River systems[edit]
North American rivers include:
Saint Lawrence River on the New York–Ontario border
The Upper Rio Grande by Creede, Colorado
Approaching the Arctic Ocean
Albany
Churchill
Mackenzie
Nelson
Severn
Approaching the Atlantic Ocean
Arkansas
Churchill
Connecticut
Delaware
Grijalva
Hudson
James
Mississippi
Missouri
Ohio
Potomac
Rio Grande
Saint John
St. Johns
St. Lawrence
Savannah
Susquehanna
Tennessee
Usumacinta
The Columbia River from Canada to the Pacific
Approaching the Pacific Ocean
Balsas River
Colorado
Columbia
Fraser
Fuerte River
Lerma River
Sacramento
San Joaquin
Suchiate River
Yukon
Within the Great Basin
Bear River
Humboldt River
Sevier River
Deserts[edit]
North America map of Köppen climate classification.
The Sierra Nevada and Cascade mountain ranges run along the entire Pacific Coast, acting as a barrier to the humid winds that sweep in from the ocean. The rising topography forces this air upwards, causing moisture to condense and fall in the form of rain on the western slopes of the mountains, with some areas receiving more than 70 inches (1.8 m) of rainfall per year. As a result, the air has lost much of its moisture and becomes hot and dry when it reaches the areas east of the coastal mountain ranges.[12] These arid conditions are, in some instances, exacerbated in regions of extremely low altitude (some near or below sea level) by higher air pressure, resulting in drier conditions and adiabatic heating effects, some of these pocket deserts exist in valleys well north of the Canada–US border in interior British Columbia. What precipitation does fall generally does not last long, lost primarily to evaporation, as well as rapid runoff and efficient water uptake and storage by native vegetation.
Climate and vegetation[edit]
There are various plant life distributions in North America. Plant life in the Arctic includes grasses, mosses, and Arctic willows. Coniferous trees, including spruces, pines, hemlocks, and firs, are indigenous to the Canadian and Western U.S. mountain ranges as far south as San Francisco. Among these are giant sequoias, redwoods, great firs, and sugar pines. Sugar pines are generally confined to the northwestern area of the United States. The central region of the country has hardwoods. Southern states grow extensive yellow pines. In addition, mahogany, logwood, and lignumvitae - all tropical in nature - are grown. The southwest has desert plants, including yucci and cacti. The cultivated native plants of North America are tobacco, maize, potato, vanilla, melons, cacao, gourds, indigo plant, and bean.
Major habitat types of the United States and Canada
Ecoregions map of Canada, United States and Mexico
Zoology[edit]
North America is home to many native mammal species. Several species of deer, including elk, caribou, moose, mule deer, and the abundant white-tailed deer are found throughout various regions, along with the bison and musk ox in the central and northern plains, respectively. Three species of bear, several subspecies of wolf, and various other carnivores such as raccoons, skunks, and cats including cougars and lynxes are widely distributed. The family Mustelidae is well represented, including badgers, otters, ferrets, and wolverines. Numerous species of squirrels and other rodents, such as beavers and muskrats, can be found in virtually every region of the continent. Central America has adapted sloths, anteaters, and armadillos. Other animals includes the condor, among the heights of the Andes, the parrots and the monkeys of Tropical forests, the humming bird, rattlesnake, alligator, and Cayman of the banks of the streams, and swarms of mosquitoes on the wide plains.
Mining and petroleum[edit]
The mining and petroleum industries are important in Canada, the United States and Mexico. These natural resources make the region one of the richest on the earth.[1][1][17]
Rocky Mountains[edit]
Picture of Rocky Mountains
The Rocky Mountain region is known for vast resources and rich mineral deposits including copper, lead, gold, silver, tungsten, uranium, and zinc. Coal, petroleum and natural gas are mineral fuels found.[1][17] Old mine tailings are present in the Rocky Mountain landscape.
Agriculture and forestry[edit]
Agriculture and forestry are two major industries. Agriculture includes arid land and irrigated farming and livestock grazing.[1] Livestock are often moved between high-elevation summer pastures and low-elevation and winter pastures.
See also[edit]
Geography portal
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Geography of North America
Geography of Canada
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Geography of El Salvador
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Geography of Panama
Geography of Greenland
Geography of the Lucayan Archipelago
Geography of the Bahamas
Geography of the Turks and Caicos Islands
Geography of Mexico
Geography of the United States
Geology of North America
List of mountain peaks of North America
Transportation in North America
References[edit]
"Deserts of America". The Golden Treasury of Knowledge. 4, book 13. Fratelli Fabbri. 1961. pp. 1008–1091. 61-10594. |access-date= requires |url= (help)
"Geography of North America". Universal World Reference Encyclopedia. 11, book 1. V.S. Thatcher. 1964. pp. 231–233. 64-12955. |access-date= requires |url= (help)
Notes[edit]
^ Jump up to: a b c d e "Geographic Guide - Images of North America". Retrieved 2006-10-11.
Jump up ^ "Americas" Standard Country and Area Codes Classifications (M49), United Nations Statistics Division
Jump up ^ "North America" Archived October 21, 2006, at the Wayback Machine. Atlas of Canada
Jump up ^ North America Atlas National Geographic
Jump up ^ "United Plates of America". Smithsonian Museum of Natural History. Retrieved on 2009-01-31.
Jump up ^ Thompson, Andrea (2008-09-25). "Oldest rocks on Earth found". MSNBC. Retrieved 2009-02-01.
Jump up ^ O'Neil, Jonathan; Carlson, Richard W.; Francis, Don; Stevenson, Ross K. (2008-09-26). "Neodymium-142 Evidence for Hadean Mafic Crust". Science 321 (5897): 1828–1831. doi:10.1126/science.1161925.
Jump up ^ Brahic, Catherine (2008-09-26). "Discovery of world's oldest rocks challenged". NewScientist. Retrieved 2009-02-01.
Jump up ^ "Rodinia" Archived 2009-02-18 at the Wayback Machine. (2005). Palæos. Retrieved on 2009-02-01.
^ Jump up to: a b Jones, Steve. "North America's Geology and Geography". USA Today. Retrieved 2006-08-19.
Jump up ^ "Historical & Cultural Significance" (1995). Bering Land Bridge Natural Preserve.
^ Jump up to: a b c d "Encyclozine - North America". Retrieved 2006-10-11.
Jump up ^ "Elevations and Distances in the United States". 1995. Archived from the original (Online Edition) on July 22, 2012. Retrieved 2009-02-05. No marked or monumented point has been established by any government agency as the geographic center of the 50 States, the conterminous United States, or the North American continent.
^ Jump up to: a b "North America climate". Retrieved 2006-08-19.
Jump up ^ "Private Tutor". Infoplease.com. 2004-04-11. Retrieved 2010-06-30.
Jump up ^ North America water resources.
^ Jump up to: a b "Fact Monster - North America: Resources and Economy". Retrieved 2006-10-11.
External links[edit]
The Future for Water in North America
Bibliography[edit]
Map and aerial photos
North America map
Physical map
Map of the Great Lakes
North American map
North America Political map
Oldest Human Remains in North America Found
T. H. Clark and C. W. Stearn, The Geological Evolution of North America (1968)
W. P. Cumming et al., The Discovery of North America (1972)
R. C. West et al., Middle America: Its Lands and Peoples (3d ed. 1989)
T. L. McKnight, Regional Geography of the United States and Canada (1992)
S. Birdsall, Regional Landscapes of the United States and Canada (4th rev. ed. 1992)
T. Flannery, The Eternal Frontier: An Ecological History of North America and Its Peoples (2001)
A. Taylor, American Colonies (2001).
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