Small Intestine
The small intestine is where all the chemical digestion and absorption of nutrient take place.
The small instestine consists of three segments: the duodenum, jejunum, and ileum.
The cells of the intestinal lining themselves turn over rapidly, and the stool also contains residues of these dead cells that are shed from the lining after their function has been fulfilled.
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Common histologic features
Mucosa
Macroscopically, the lining of the small intestine shows a series of permanent circular or semilunar folds (plicae circulares), consisting of mucosa and submucosa (Figures 15–22a and 15–23), which are best developed in the jejunum.
Densely covering the entire mucosa of the small intestine are short (0.5-1.5 mm) mucosal outgrowths called villi that project into the lumen (Figure 15–22). These finger- or leaflike projections are covered by a simple columnar epithelium of absorptive cells called enterocytes, with many interspersed goblet cells. Each villus has a core of loose connective tissue that extends from the lamina propria and contains fibroblasts, smooth muscle fibers, lymphocytes and plasma cells, fenestrated capillaries, and a central lymphatic called a lacteal.
In the small intestine a single cell layer of columnar epithelial cells comprises the semipermeable barrier across which controlled uptake of nutrients takes place. Various glandular structures empty into the intestinal lumen at points along its length, providing for digestion of food components, signaling to distal segments, and regulation of the microbiota. There are also important motility functions that move the intestinal contents and resulting waste products along the length of the gut, and a rich innervation that regulates motility, secretion and nutrient uptake, in many cases in a manner that is independent of the central nervous system. There is also a large number of endocrine cells that release hormones that work together with neurotransmitters to coordinate overall regulation of the GI system.
Many cells are seen to have cytoplasmic projections visible with the electron microscope. Such extensions usually reflect cytoplasmic movements and activity of actin filaments and are both temporary and variable in their length, shape, and number.
However, in epithelial cells specialized for absorption, the apical surfaces present an array of projections called microvilli (L. villus, tuft). In cells such as those lining the small intestine, apical surfaces are densely covered with uniform microvilli, which are visible as a brush or striated border on these cells (Figure 4–8).
Absorptive cells lining the small intestine demonstrate the highly uniform microvilli of a striated or brush border particularly well. (a) A high-magnification light microscope shows many parallel microvilli and their connections to the terminal web (TW) in the underlying cytoplasm. X6500. (b) SEM of a sectioned epithelial cell shows both the internal and surface structure of individual microvilli and the association with actin filaments and intermediate filaments of the terminal web (TW). X7000. (c) TEM of microvilli sectioned longitudinally and transversely (inset) reveals the microfilament arrays that form the core of these projections. The terminal web (TW) of the cytoskeleton is also seen. The glycocalyx (G) extending from glycoproteins and glycolipids of the microvilli plasmalemma contains certain enzymes for late stages of macromolecule digestion. X15,000.
(d) The diagram shows a few microfilaments in a microvillus, with various actin-binding proteins important for F-actin assembly, capping, cross-linking, and movement. Like microfilaments in other regions of the cytoskeleton, those of microvilli are highly dynamic, with treadmilling and various myosin-based interactions. Myosin motors import various microvilli components along the actin filaments. (Figure 4–8b, with permission, from Dr John Heuser, Washington University School of Medicine, St. Louis, MO.)
The average microvillus is about 1 μm long and 0.1 μm wide, but with hundreds or thousands present on the end of each absorptive cell, the total surface area can be increased by 20- or 30-fold. Glycocalyx covering intestinal microvilli is thick and includes enzymes for digestion of certain macromolecules.
Each microvillus contains many bundles of actin filaments capped and cross-linked to each other and to the surrounding plasma membrane by many different actin-binding proteins (Figure 4–8). Although microvilli are often relatively stable, the microfilament arrays are dynamic and undergo various myosin-based movements, which help maintain optimal conditions for absorption via numerous channels, receptors, and other proteins in the plasmalemma. The actin filaments insert into the terminal web of similar filaments at the base of the microvilli.
Auerbach plexus, which is responsible for peristalsis, is located between the two muscle layers. Meissner plexus, which is responsible for sensation, is in the submucosa.
The intestine is composed of functional layers (See Figure Below).
Organization of the wall of the intestine into functional layers.
The function of the small intestine is to digest and absorb nutrients; regulate the secretion and absorption of water and electrolytes, the storage and subsequent transport of intraluminal contents aborally
Microvilli and circular folds (ie, valvulae conniventes, plicae circulares or valves of Kerkring) increase the surface area available for absorption and cause the intestinal contents to twist while flowing through the small intestine. These circular folds can be seen in radiographic studies. The small bowel is relatively free of microbes, whereas the large intestine is populated with commensal bacteria that aid digestion, synthesize a number of vitamins, and break down bilirubin.
The main motor functions are summarized in Table 55-1. Alterations in fluid and electrolyte handling contribute significantly to diarrhea. Alterations in motor and sensory functions of the colon result in highly prevalent syndromes such as irritable bowel syndrome (IBS), chronic diarrhea, and chronic constipation.
The main motor functions are summarized in Table 55-1. Alterations in fluid and electrolyte handling contribute significantly to diarrhea. Alterations in motor and sensory functions of the colon result in highly prevalent syndromes such as irritable bowel syndrome (IBS), chronic diarrhea, and chronic constipation.
The small intestine and colon have intrinsic and extrinsic innervation. The intrinsic innervation, also called the enteric nervous system, comprises myenteric, submucosal, and mucosal neuronal layers. The function of these layers is modulated by interneurons through the actions of neurotransmitter amines or peptides, including acetylcholine, vasoactive intestinal peptide (VIP), opioids, norepinephrine, serotonin, adenosine triphosphate (ATP), and nitric oxide (NO). The myenteric plexus regulates smooth-muscle function through intermediary pacemaker-like cells called the interstitial cells of Cajal, and the submucosal plexus affects secretion, absorption, and mucosal blood flow. The enteric nervous system receives input from the extrinsic nerves, but it is capable of independent control of these functions.
The final stage in the assimilation of a meal involves movement of digested nutrients out of the intestinal contents, across the intestinal lining, and into either the blood supply to the gut or the lymphatic system, for transfer to more distant sites in the body. Collectively, this directed movement of nutrients is referred to as absorption. The efficiency of absorption may vary widely for different molecules in the diet as well as those supplied via the oral route with therapeutic intent, such as drugs. The barriers to absorption encountered by a given nutrient will depend heavily on its physicochemical characteristics, and particularly on whether it is hydrophilic (such as the products of protein and carbohydrate digestion) or hydrophobic (such as dietary lipids). For the main substances vitally required by the body, the gastrointestinal tract does not rely solely on diffusion across the lining to provide for uptake, but rather has evolved active transport mechanisms that take up specific solutes with high efficiency.
In the small intestine a single cell layer of columnar epithelial cells comprises the semipermeable barrier across which controlled uptake of nutrients takes place. Various glandular structures empty into the intestinal lumen at points along its length, providing for digestion of food components, signaling to distal segments, and regulation of the microbiota. There are also important motility functions that move the intestinal contents and resulting waste products along the length of the gut, and a rich innervation that regulates motility, secretion and nutrient uptake, in many cases in a manner that is independent of the central nervous system. There is also a large number of endocrine cells that release hormones that work together with neurotransmitters to coordinate overall regulation of the GI system.
Many cells are seen to have cytoplasmic projections visible with the electron microscope. Such extensions usually reflect cytoplasmic movements and activity of actin filaments and are both temporary and variable in their length, shape, and number.
However, in epithelial cells specialized for absorption, the apical surfaces present an array of projections called microvilli (L. villus, tuft). In cells such as those lining the small intestine, apical surfaces are densely covered with uniform microvilli, which are visible as a brush or striated border on these cells (Figure 4–8).
Absorptive cells lining the small intestine demonstrate the highly uniform microvilli of a striated or brush border particularly well. (a) A high-magnification light microscope shows many parallel microvilli and their connections to the terminal web (TW) in the underlying cytoplasm. X6500. (b) SEM of a sectioned epithelial cell shows both the internal and surface structure of individual microvilli and the association with actin filaments and intermediate filaments of the terminal web (TW). X7000. (c) TEM of microvilli sectioned longitudinally and transversely (inset) reveals the microfilament arrays that form the core of these projections. The terminal web (TW) of the cytoskeleton is also seen. The glycocalyx (G) extending from glycoproteins and glycolipids of the microvilli plasmalemma contains certain enzymes for late stages of macromolecule digestion. X15,000.
(d) The diagram shows a few microfilaments in a microvillus, with various actin-binding proteins important for F-actin assembly, capping, cross-linking, and movement. Like microfilaments in other regions of the cytoskeleton, those of microvilli are highly dynamic, with treadmilling and various myosin-based interactions. Myosin motors import various microvilli components along the actin filaments. (Figure 4–8b, with permission, from Dr John Heuser, Washington University School of Medicine, St. Louis, MO.)
The average microvillus is about 1 μm long and 0.1 μm wide, but with hundreds or thousands present on the end of each absorptive cell, the total surface area can be increased by 20- or 30-fold. Glycocalyx covering intestinal microvilli is thick and includes enzymes for digestion of certain macromolecules.
Each microvillus contains many bundles of actin filaments capped and cross-linked to each other and to the surrounding plasma membrane by many different actin-binding proteins (Figure 4–8). Although microvilli are often relatively stable, the microfilament arrays are dynamic and undergo various myosin-based movements, which help maintain optimal conditions for absorption via numerous channels, receptors, and other proteins in the plasmalemma. The actin filaments insert into the terminal web of similar filaments at the base of the microvilli.
Auerbach plexus, which is responsible for peristalsis, is located between the two muscle layers. Meissner plexus, which is responsible for sensation, is in the submucosa.
The intestine is composed of functional layers (See Figure Below).Organization of the wall of the intestine into functional layers.
The function of the small intestine is to digest and absorb nutrients; regulate the secretion and absorption of water and electrolytes, the storage and subsequent transport of intraluminal contents aborally
Microvilli and circular folds (ie, valvulae conniventes, plicae circulares or valves of Kerkring) increase the surface area available for absorption and cause the intestinal contents to twist while flowing through the small intestine. These circular folds can be seen in radiographic studies. The small bowel is relatively free of microbes, whereas the large intestine is populated with commensal bacteria that aid digestion, synthesize a number of vitamins, and break down bilirubin.
The main motor functions are summarized in Table 55-1. Alterations in fluid and electrolyte handling contribute significantly to diarrhea. Alterations in motor and sensory functions of the colon result in highly prevalent syndromes such as irritable bowel syndrome (IBS), chronic diarrhea, and chronic constipation.
INTESTINAL MOTILITY
Allows propulsion of intestinal contents from stomach to anus and separation of components to facilitate nutrient absorption. Propulsion is controlled by neural, myogenic, and hormonal mechanisms; mediated by migrating motor complex, an organized wave of neuromuscular activity that originates in the distal stomach during fasting and migrates slowly down the small intestine. Colonic motility is mediated by local peristalsis to propel feces. Defecation is effected by relaxation of internal anal sphincter in response to rectal distention, with voluntary control by contraction of external anal sphincter.
The function of the small intestine is the digestion and assimilation of nutrients from food.
It also regulates the secretion and absorption of water and electrolytes,
the storage (?) and subsequent transport of intraluminal contents aborally.
- Proximal small intestine: iron, calcium, folate, fats (after hydrolysis of triglycerides to fatty acids by pancreatic lipase and colipase), proteins (after hydrolysis by pancreatic and intestinal peptidases), carbohydrates (after hydrolysis by amylases and disaccharidases); triglycerides absorbed as micelles after solubilization by bile salts; amino acids and dipeptides absorbed via specific carriers; sugars absorbed by active transport
- Distal small intestine: vitamin B12, bile salts, water
INTESTINAL MOTILITY
Allows propulsion of intestinal contents from stomach to anus and separation of components to facilitate nutrient absorption. Propulsion is controlled by neural, myogenic, and hormonal mechanisms; mediated by migrating motor complex, an organized wave of neuromuscular activity that originates in the distal stomach during fasting and migrates slowly down the small intestine. Colonic motility is mediated by local peristalsis to propel feces. Defecation is effected by relaxation of internal anal sphincter in response to rectal distention, with voluntary control by contraction of external anal sphincter.
Most fluid is absorbed in small bowel. Colonic absorption is normally 0.05–2 L/d, with capacity for 6 L/d if required. Intestinal water absorption passively follows active transport of Na+, Cl–, glucose, and bile salts. Additional transport mechanisms include Cl–/HCO3– exchange, Na+/H+ exchange, H+, K+, Cl–, and HCO3– secretion, Na+-glucose cotransport, and active Na+ transport across the basolateral membrane by Na+,K+-ATPase.
The final stage in the assimilation of a meal involves movement of digested nutrients out of the intestinal contents, across the intestinal lining, and into either the blood supply to the gut or the lymphatic system, for transfer to more distant sites in the body. Collectively, this directed movement of nutrients is referred to as absorption. The efficiency of absorption may vary widely for different molecules in the diet as well as those supplied via the oral route with therapeutic intent, such as drugs. The barriers to absorption encountered by a given nutrient will depend heavily on its physicochemical characteristics, and particularly on whether it is hydrophilic (such as the products of protein and carbohydrate digestion) or hydrophobic (such as dietary lipids). For the main substances vitally required by the body, the gastrointestinal tract does not rely solely on diffusion across the lining to provide for uptake, but rather has evolved active transport mechanisms that take up specific solutes with high efficiency.
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