Human Digestive System

General features

Comparative Digestive Physiology
Detritus, which typically contains a lot of refractory material although it has not been analyzed in a strictly comparable fashion to the other food types, is included as a food type because ecologists have found that it may support over half the animal production in some ecosystems The pinocytotic uptake capacity declines at weaning, although molecular details of this have not been elucidated. Articles from Britannica Encyclopedias for elementary and high school students. Whenever possible allowing access to natural sunlight is encouraged in an enclosure that can ventilate well and not get overheated. Notably, the neutral amino acid transporter in Drosophila DmNAT6 can mediate the transport of most amino acids apart from lysine, arginine, aspartate, and glutamate; and, remarkably, it can also take up D-isomers of several amino acids

Introduction

Reptile Nutrition: From Mouth to Vent

Recent research on the diversity of the microbiota in the GI tract has been dominated by molecular analyses of bacterial diversity in the feces of humans and model rodent species, based on the assumption that fecal diversity is representative of the microbial community in situ. The bacterial complement in mammals is dominated by two phyla, the Bacteroidetes and Firmicutes, each of which is represented by tens-to-hundreds of taxa, as identified by 16S rRNA gene sequence data Among humans, the composition varies widely among individuals, and is influenced by age 87 , , diet , and medical condition , including history of orally administered antibiotic treatment , Fecal analyses of a range of mammals reveal diet as an important determinant of taxonomic composition and genetic capacity for metabolism , such that the microbiota of mammals cluster according to whether the host is a carnivore, omnivore or herbivore, largely independent of the phylogenetic position of the mammal Fig.

Interesting outliers in this dataset are the pandas which, although folivores, have a microbiota that clusters with carnivores. In humans and other mammals, all regions of the GI tract are colonized, including the highly acidic stomach, which bears a diverse community of bacteria and some fungi Variation in bacterial communities of mammals with diet, analyzed by principal components analysis. The analysis was conducted on individuals of 60 species from 13 orders of mammals.

The three herbivores circled are individuals of red and giant panda, which are members of the order Carnivora. The species richness of the microbiota in the GI tract of many invertebrate animals is apparently an order of magnitude lower than in mammals, commonly with just 10 to 20 taxa per individual 7 , 22 , , , , , Nevertheless, the global diversity of microorganisms associated with the GI tract of invertebrates is substantial with different dominant species, phyla or even kingdoms in different animal taxa.

For example, the bacteria in the GI tract of Drosophila fruit-flies with a natural diet of rotting fruit are dominated by Acetobacter and Lactobacillus species 98 , , while the related tephritid Med fly, Ceratitis capitata , feeding on unripe fruits is colonized principally by Enterobacteriaceae, including Klebsiella , Pantoea , and Enterobacter species Analysis of the gut microbiota in Drosophila has revealed considerable variation in the dominant bacterial taxon with developmental age, even under uniform rearing conditions Fig.

The incidence of eukaryotic microorganisms e. Composition of bacterial species at different life stages of Drosophila melanogaster. Microorganisms in the GI tract of many animals have a great diversity of glucohydrolases active against complex plant polysaccharides. Resident bacteria in the GI tract of humans also have considerable capacity to utilize carbohydrates, including complex plant polysaccharides.

The genome of one common human gut symbiont Bacteroides thetaiotaomicron contains a total of glycoside hydrolases and polysaccharide lyases A metagenome analysis of fecal samples from 18 human individuals revealed a very diverse array of bacterial genes active against carbohydrates, collectively accounting for 2.

The relationship between the degradative capabilities of the bacteria in the GI tract and diet is further vividly illustrated by the discovery of genes for porphyranases and agarases in the gut bacterium Bacteroides plebeius isolated from Japanese but not North American individuals These enzymes are active against the sulfated polysaccharides in Porphyra seaweeds that form a regular part of the typical Japanese, but not North American, diet.

Furthermore, there is phylogenetic evidence that the genes for these glucohydrolase activities have been transferred horizontally from marine bacteria associated with Porphyra to the gut bacteria of humans. The GI tracts of animals, including herbivorous mammals and wood-feeding insects, are recognized as cellulose-rich environments that are currently being targeted in gene discovery projects for biofuels development and other industrial purposes Microbial breakdown of complex carbohydrates can be nutritionally significant to the animal host, where the gut habitat is oxygen deficient, such that the microbial metabolism is strictly fermentative, and not aerobic.

Specifically, the complex polysaccharides are hydrolyzed to simple sugars, and then subjected to bacterial fermentation, with the net release of fermentation waste products, typically SCFAs, including acetate, butyrate, and propionate These final products diffuse across the animal gut wall, and are used as substrates for aerobic respiration, gluconeogenesis, and lipogenesis in the animal.

The suite of reactions responsible for the transformation of complex carbohydrates to SCFAs is mediated by consortia of multiple bacteria with complementary capabilities , with cross-feeding of intermediate metabolites among bacteria with different capabilities Fig.

For example, in the human colon, Bacteroides species degrade complex polysaccharides to sugars; the sugars are respired by Bifidobacterium and other anaerobic bacteria to lactate; and the lactate is fermented by bacteria such as Eubacterium hallii and Roseburia hominis , producing butyrate Fig.

Butyrate, which is a waste product of the microbial community metabolism, is the principal respiratory substrate used by the gut epithelial cells Fermentative degradation of complex carbohydrates by consortia of bacteria in the human colon.

A Functional groups of bacteria SRBs, sulfate-reducing bacteria. B Major bacterial taxa responsible for degradation of starch and fructan-carbohydrates. Multiple factors beyond the biochemical capabilities of the microbiota determine the nutritional significance of microbial fermentation for an animal. Of particular importance are: All vertebrates apparently lack the capacity to degrade cellulose and related complex polysaccharides of plant cell walls.

Consequently, the amount of breakdown in the vertebrate GI tract is dictated by the scale of microbial fermentation, which varies from trivial, for example, in pandas Ailurus fulgens , A. Some invertebrate animals have enzymes capable of degrading plant cell-wall components. The phylogenetic distribution of intrinsic cellulases is not fully understood, but genome analyses indicate that members of at least five phyla have cellulases of glucose hydrolase family 9: The relative importance of intrinsic and microbial cellulolysis has been investigated, especially in insects , revealing considerable variation.

The capacity of some insects to degrade plant cell-wall components is further illustrated by the identification of enzymes from eight enzyme families capable to degrading plant cell-wall polysaccharides in a recent sequence analysis of seven species of phytophagous beetles Turning to the relationship between diet and microbial fermentation, various studies suggest that the taxonomic composition and metabolic traits of the gut microbiota can be influenced by diet, potentially with effects on the digestive function of the GI tract.

Indications that the microbial changes can be very rapid come from an analysis of laboratory mice with GI tract colonized by the microbiota from human fecal samples. Remarkably, the composition of the microbiota and gene expression profile was altered within a single day of transferring the mice from a low-fat diet with high plant polysaccharide content to a high-fat, high-sugar diet Although the entire length of the GI tract is colonized by microorganisms in most animals, the highest microbial densities and abundance tend to be in postgastric regions, for example, the large intestine of mammals, hind gut of insects, and this is the usual site of microbial fermentation chambers.

From the perspective of the animal, the key benefit of a postgastric fermentation chamber is that the substrates available to the microorganisms are those that are intractable to digestive action in the gastric region.

This design minimizes the competition between animal and resident microorganisms for ingested nutrients that can be processed readily by the animal. Pregastric fermentation chambers have evolved rarely, and are apparently restricted principally to mammals, with five independent evolutionary origins [in the Artiodactyls in the ruminants, camels, and hippos , in the colobine monkeys, and the Macropodidae kangaroos ]; the remarkable S American bird, the hoatzin, also has a pregastric fermentation chamber , The relative merits of pre- and postgastric fermentation have been discussed extensively , The key disadvantage of pregastric fermentation for the animal is that ingested food is available for microbial metabolism before digestion by the animal.

This can result in reduced nutritional gain from high-quality foods. For example, an animal derives more energy from simple sugars by gastric digestion and assimilation than by microbial fermentation; and more nitrogen from protein by gastric processing than microbial metabolism. The adaptive advantage of pregastric fermentation for very efficient breakdown of the plant polysaccharides is enhanced by rumination i.

It has been argued that pregastric fermentation chambers may have evolved in relation to functions other than cellulose degradation, for example to facilitate microbial detoxification of allelochemicals in ingested plant foods, and only subsequently became important in digestion of plant material Some animals possess a substantial fermentative microbiota that produces SCFAs without a morphologically distinct fermentation chamber.

This is particularly evident among herbivorous fish, including various tropical perciforms In one detailed analysis of three temperate fish species feeding on seaweed, the rate of production of one SCFA, acetate, was similar to those in the guts of herbivorous reptiles and mammals, even though the fish lacked coherent fermentation chambers Further research is required to determine the mechanisms underlying fermentation in these fish, and the nutritional significance of the SCFAs produced.

Absorption refers to the transfer of compounds from the gut lumen across the gut wall to the body tissues, including the lymph or blood of vertebrates and hemolymph of arthropods.

At the cellular level, organic compounds can be absorbed from the gut lumen by paracellular and transcellular routes. Paracellular transport refers to movement between cells of the gut epithelium, while the transcellular route involves transport across the apical cell membrane of gut epithelial cells, transit across the cell for some molecules with metabolic transformations in the cell , and then export at the basolateral membrane.

This section considers absorption of organic compounds, particularly products of digestion: With the exception of SCFAs, these products are absorbed principally distal to the gastric region of the alimentary tract, for example, small intestine of vertebrates and midgut of insects. The absorptive cells are columnar epithelial cells called enterocytes. Exceptionally, SCFAs produced by the microbiota in the hindgut e. In this section, two aspects of nutrient absorption are addressed: Most organic compounds absorbed across animal guts are polar, and their transport is predominantly or exclusively carrier-mediated, that is, mediated by membrane-bound transporters and displaying the twin characteristics of saturation kinetics and competitive inhibition.

Two forms of carrier-mediated transport are recognized: Simple diffusion, that is, down the concentration gradient and involving neither a carrier nor cellular energy, is an additional mode of absorption that is especially important for small, nonpolar molecules. Monosaccharides cross the apical and basolateral membranes of gut epithelial cells by carrier-mediated mechanisms. Fructose is transported principally via the facilitative transporter GLUT5 These transporters are expressed predominantly in the small intestine.

The expression of SGLT1 in the intestine is restricted to the apical membrane of enterocytes. Once in the cell, the glucose is widely accepted to be transported down its concentration gradient across the basolateral membrane into the circulation by GLUT2.

Under conditions of high luminal glucose content, however, GLUT2 in rodents is inserted into the apical membrane, where it mediates the high flux of glucose into the enterocyte Some data suggest that sugar-induced translocation of GLUT2 may not occur universally in mammals 18 , , and further research is required to establish the distribution of this effect with respect to phylogeny and diet. The mechanism by which GLUT2 is inserted into the apical enterocyte membrane is understood in outline This process occurs very rapidly.

In the mouse, the responsiveness of GLUT2 insertion to luminal sugars varies among sugars, being triggered much less efficiently by glucose and complex sugars than by fructose, sucrose, and a mixture of glucose and fructose ; mice fed on a high-fructose diet have been reported to bear GLUT2 permanently on the apical membrane of enterocytes Artificial sweeteners, such as sucralose, dramatically increase GLUT2 insertion and the resultant uptake of glucose, such that the sugar is absorbed efficiently from lower concentrations in the presence of the artificial sweetener than in its absence The implications of these rodent studies for human nutrition are not yet fully resolved.

Phylogenetic analysis assigns the mammalian GLUT2 to a clade that includes three further mammalian GLUTs GLUT1, 3, and 4 and invertebrate, but no nonmetazoan, GLUTs, suggesting that this group of transporters may have evolved in the basal metazoans or immediate ancestors of animals There is also evidence that SGLT1 and GLUT transporters contribute to intestinal glucose absorption in nonmammalian vertebrates, including fish 72 , The molecular basis of sugar uptake across the gut wall has not, however, been investigated widely in the invertebrates.

Among insects, glucose transport across the midgut of the hymenopteran parasite Aphidius ervi is mediated by a SGLT1-like transporter on the apical membrane, together with a GLUT2-like transporter on both the apical and basolateral membranes of the enterocytes; and a second passive transporter similar to GLUT-5 is implicated in fructose uptake This condition is not, however, universal among insects.

These included an abundantly expressed gene ApSt3 , a hexose uniporter with specificity for glucose and fructose in the distal midgut. Aphids may not, however, be typical of insects because their diet of plant phloem sap is sugar rich, and a concentration gradient from gut lumen to epithelial cell and hemocoel is maintained by the excess sugar in the gut lumen The products of protein digestion taken up by enterocytes of the mammalian intestine are free amino acids, dipeptides, and tripeptides.

Free amino acids are taken up from the small intestine of mammals by multiple carriers with overlapping specificities, with the result that most individual amino acids are transported by more than one transporter. By contrast, peptides are taken up by a single transporter with very low selectivity, as considered at the end of this section. The amino acid transporters are classified by their activity specificity and kinetics into multiple systems, and by sequence homology into solute carrier SLC families.

The principal transporters mediating amino acid transport in the human intestine are summarized in Table 3. Studies on human, rodent and rabbit suggest that the amino acid transporters in the mammalian small intestine can be assigned to four groups, mediating the transport of neutral, cationic, anionic, and imino acids, respectively Uptake across the apical membrane is mediated by: The rich classical literature on the kinetics of amino acid transport across the intestinal epithelium of various nonmammalian vertebrates and invertebrates is summarized by and , and there is increasing interest in analysis from a molecular perspective [e.

As in mammals, multiple transporters are expressed, with overlapping specificities for amino acids. Some are very specific, for example, NAT6 and NAT8 in the distal midgut of mosquito Anopheles gambiae transport just aromatic amino acids , Other SLC6 transporters have a very broad range.

Notably, the neutral amino acid transporter in Drosophila DmNAT6 can mediate the transport of most amino acids apart from lysine, arginine, aspartate, and glutamate; and, remarkably, it can also take up D-isomers of several amino acids This capability can be linked to the abundance of D-amino acids in the cell walls of bacteria, which are an important component of the natural diet of Drosophila species.

DmNAT6 is an active transporter, capable of mediating uptake against the concentration gradient. Multiple transporters are involved with a range of specificities, including two neutral amino acid transporters in Manduca sexta KAAT1 and CAATCH1 , both members of the SL6 family 71 , with distinctive amino acid selectivities Amino acid transporters are also expressed in the apical membrane of the insect hindgut epithelium, where they mediate the uptake of amino acids in the primary urine produced in the Malpighian tubules.

Proline is also taken up, and is a major respiratory substrate of rectal cells It can transport thousands of di- and tripeptides with low affinity and high capacity, but neither free amino acids nor tetrapeptides This property is intelligible from the structural features of the binding pocket of the protein, which can accommodate compounds with oppositely charged head groups carboxyl and amino groups separated by a carbon backbone of 0.

Neutral and most cationic peptides are cotransported with one proton, while anionic peptides require two protons Peptides taken up into the enterocyte are hydrolyzed by a diversity of cytoplasmic peptidases Fig. The peptides are hydrolyzed by multiple cytosolic hydrolases, and the resultant amino acids are exported via the basolateral membrane by multiple transporters see Table 3.

The efflux of unhydrolyzed peptides across the basolateral membrane is mediated by peptide transporters that have not been identified at molecular level.

The peptide transporter family to which the mammalian PEPT1 protein belongs is ancient, with the defining peptide transporter motif PTR motif evident in proteins of bacteria, fungi, plants, and animals Analysis of basal animal groups is required to establish the evolutionary origin s of gut-borne peptide transporter s in metazoans.

Of central importance is the relative importance of peptide and amino acid uptake in the protein nutrition of the animal. The significance of PEPT1 for the protein nutrition of other animals remains to be established. In vertebrates, the absorption of lipid hydrolysis products and sterols is dependent on their incorporation into micelles formed in the lumen of the small intestine. Micelles are 4 to 8 nm diameter aggregations of the hydrophobic lipid products with bile acids, which act as amphipathic detergents and mediate the passage of the lipid products across the aqueous boundary layer to the apical membrane of intestinal enterocytes.

A proportion of the micelle-associated molecules pass across the apical membrane by simple diffusion, according to the concentration and permeability coefficient of each compound, but carrier-mediated transport is also involved. The dominant lipids in most diets are triacylglycerols TAGs , accompanied by small amounts of various polar and nonpolar lipids, including phospholipids, sterols, and the fat-soluble vitamins A and E.

The products of lipid digestion include free FAs, glycerol, monoglycerides, and lysophospholipids. Following uptake by diffusion and via transporters, these products are transported to the endoplasmic reticulum, where they are used to synthesize diacylglycerols DAGs , TAGs, phospholipids, cholesterol esters, etc.

They are then packaged with lipoproteins to form chylomicrons, which are passed through the Golgi apparatus for exocytosis. In mammals, the chylomicrons are delivered to the lymphatic vessels. The mechanism of chylomicron assembly is reviewed by reference Of particular note are the transporters mediating sterol flux across the apical membrane of enterocytes.

In mammals, a steep diffusion gradient across the apical membrane is generated by acyl-CoA: There is now overwhelming physiological and molecular evidence for carrier-mediated uptake and also efflux across the apical membrane Fig.

The key transporter mediating cholesterol uptake is Niemann Pick C1-like 1 NPC1L1 protein, identified initially as the transporter sensitive to ezetimibe, a highly specific and potent inhibitor of intestinal cholesterol absorption 6 , , However, overexpression of NPC1L1 in nonenterocyte cells has not yielded cholesterol transport activity, suggesting that additional proteins may be required to reconstitute a fully functional cholesterol transporter.

ABC transporters generally have 12 transmembrane domains, but each of ABCG5 and ABCG8 has just six transmembrane domains; transport activity is mediated by the heterodimer, comprising a transmembrane protein complex Cholesterol molecules that are not esterified in the endoplasmic reticulum are eliminated from the enterocyte to the intestinal lumen and voided via the feces. Absorption of cholesterol in mammalian intestine. These esterified products are incorporated into apolipoprotein apo Bcontaining chylomicrons in a microsomal triglyceride transport protein-dependent manner.

After further processing, the chylomicrons are released from the basolateral membrane by exocytosis. Mammals feeding on fungal or plant material need to process the dominant sterols in these foods: These sterols have the tetracyclic ring structure and side chain at C17, as in cholesterol, but the side chain in phytosterols is alkylated at C e.

In healthy individuals, dietary phytosterols reduce serum cholesterol levels, probably through their more efficient incorporation than cholesterol into micelles, resulting in reduced cholesterol uptake ; this is why sitosterol is sold as a functional food.

A dietary supply of cholesterol is not required by mammals, which can synthesize sterols de novo. Among invertebrates, most research on lipid absorption has concerned insects. The products of insect lipid digestion are absorbed principally across the midgut epithelium, although absorption in the foregut, e.

Lipid absorption in insects differs from vertebrates in several important respects. Unlike chylomicrons, lipophorin is not synthesized in enterocytes; it is localized in the hemolymph blood , where it acts as a shuttle delivering lipids to the fat body and other organs. Lipophorin has been implicated in the transport of hydrocarbons, carotenoids, sterols, and phosopholipids, as well as DAGs.

The products of lipid digestion in the gut of the spider Polybetes phythagoricus are taken up by cells of the midgut diverticulum, where they are processed to TAGs and phospholipids and exported via two distinct carriers: This class of lipid-related molecules is distinctive from other lipids in two important respects.

First, they have lower hydrophobicity than long-chain fatty acids. Consequently, SCFAs permeate membranes more slowly by simple diffusion, and cellular transport mechanisms are especially important for SCFA absorption.

For example, in humans, acetate, propionate, and butyrate are produced in the ratio 3: Topics not considered here are the role of SCFAs in the regulation of fluid and electrolyte movement of the vertebrate gut, reviewed by reference 32 , and importance of butyrate in the regulation of colonic cell proliferation and differentiation [see review of reference ]. SCFAs are transported across the colon wall of mammals by a combination of simple diffusion and carrier-mediated processes.

The SCFA transporter s have yet to be identified definitively. The fate of SCFAs in the gut epithelium has been studied particularly in the rumen. A proportion of the SCFAs taken up is metabolized to lactate and ketonic acids including acetoacetate and 3-hydroxybutyrate ; these products are transported from the basolateral membrane of epithelial cells, probably via MCT1, to the blood.

The intraepithelial metabolism of SCFAs contributes to the high-energy demands of these cells. Additional advantages are the maintenance of the concentration gradient between the lumen of the rumen and epithelial cell contents, so promoting sustained SCFA uptake, and the greater solubility of the products lactate etc.

Paracellular transport across the gut is constrained by tight junctions at the apical end of the lateral membrane of all cells in the epithelium. Tight junctions have selective permeability, discriminating among solutes by charge and size. Two pathways across the tight junction have been identified in various epithelial cell types, including gut epithelia: Caco-2 cells display a third pathway that allows the passage of molecules up to 0. Although the contribution of the various tight junction proteins to the restriction of movement between epithelial cells is not fully understood, there is growing evidence that: For an excellent review on the molecular determinants of the function and plasticity of tight junctions, the reader is referred to For humans and biomedical rodent models, the paracellular pathway makes a negligible contribution to absorption of many solutes.

Despite the growing evidence for dynamic selective permeability of tight junctions, the predominance of transcellular transport has been attributed to the superior selectivity of transcellular transport via carrier-mediated transporters on the apical membrane of enterocytes, thereby protecting the animal from many toxins or otherwise deleterious compounds breaching the gut wall.

Nevertheless, there is substantial evidence for extensive paracellular transport of solutes in flying birds and fruit bats. Particular insight into the mode of sugar transport comes from parallel analysis of absorption of L-glucose the stereoisomer that does not interact with the glucose transporters and is transported exclusively by paracellular route , and D-glucose or 3-O-methyl-d-glucose 3OMD-glucose , a nonmetabolizable analogue of D-glucose that can be transported into cells.

Karasov and colleagues measured total absorption mediated and passive of D-glucose or 3OMD-glucose and passive absorption of L-glucose in intact animals by a standard pharmacokinetic methodology, for example, references 78 , , , In analogous studies in rats , dogs , and humans L-glucose, and hence passive absorption, is quantitatively much less important, confirming the likely phylogenetic difference between birds and mammals in the importance of paracellular transport.

Paracellular absorption of glucose in the American robin Turdus migratorius investigated by pharmacokinetic methodology, using D-glucose, L-glucose the glucose stereoisomer that is not be transported across the intestinal membrane , and 3-O-methyl- d -glucose 3OMD-glucose, a nonmetabolizable but actively transported analogue of D-glucose. A The dose-corrected plasma concentration of [ 3 H]L-glucose as a function of time since American robins were injected unfilled symbols or gavaged filled symbols with the probe solution containing L-glucose.

Intestinal paracellular absorption in nonflying mammals and birds appears to be qualitatively similar in regards to molecular size selectivity, as characterized using a series of nonelectrolyte water-soluble probes that differ in molecular dimension 80 , and in charge selectivity as characterized using relatively inert charged peptides 81 , A Fractional absorption of water soluble carbohydrates by intact birds triangles, solid line and nonflying eutherian mammals circles, dashed line.

Arabinose, rhamnose, cellobiose, and lactulose are inert, nonactively transported compounds whereas 3-O-methyl- d -glucose is not metabolized but is transported actively as well as passively absorbed. Fractional absorption of the passively absorbed probes declined with increasing molecule size and differed significantly between the two taxa, although the difference diminished with increasing molecule size. In contrast, absorption of 3-Omethyl- d -glucose did not differ significantly between the taxa.

The interpretation is that species in both groups absorb most glucose, but that birds relied more on the passive, paracellular route. Figure 4A adapted, with permission, from reference B Small intestine nominal smoothbore tube surface area in omnivorous birds and mammals same symbols and lines as in A.

When the lines were fit to the common slope of 0. Figure 4B adapted from reference The difference in paracellular absorption between birds and nonflying mammals is not simply explained by mediated absorption in birds of the carbohydrate probes that are presumed to be absorbed passively.

Nor is the difference in paracellular absorption between birds and nonflying mammals explained by longer retention of digesta in the gut of the former relative to the latter. Avian species typically have shorter mean retention time of digesta than do similar sized nonflying mammalian species Because birds typically achieve higher paracellular absorption with less intestinal length and surface area than do similar sized nonflying mammals, there apparently are differences in intestinal permeability per unit intestinal tissue.

This was confirmed in a comparison of pigeons and laboratory rats. The difference in paracellular solute absorption between mammals and birds cannot be linked to differences in solvent drag because it is so difficult to distinguish between water absorbed by the paracellular route versus aquaporins, which occur in intestine of both mammals and birds Enhanced paracellular absorption may have evolved as a compensation for smaller intestinal size in birds compared with nonflying mammals Fig.

The difference in intestinal surface area between birds and nonflying mammals did not depend on diet in the analysis. Diet did have a significant effect on gut size, but the effect was on cecal and large intestine size.

Another advantage of paracellular absorption is that it is an energetically cheap way to match absorption rate to substrate concentration in the diet and lumen. If there has indeed been natural selection for smaller intestinal size in fliers, and increased paracellular absorption as a compensation, then one might expect to find the same patterns found in flying birds versus nonflying mammals in a comparison within mammals between fliers i.

Preliminary evidence suggests that this is the case 75 , but more extensive sampling is necessary. Generally, in vertebrates, the more carnivorous the species, the lower its rate of intestinal mediated glucose absorption This pattern, first described in a survey of more than 40 species drawn from the major vertebrate classes , is apparent also in comparative studies within fish 51 and birds Based on phlorizin-binding studies in a limited number of species, it appeared that species differences in tissue-specific glucose uptake may largely reflect species differences in the number of copies of the main apical membrane glucose transporter SGLT1, although it is possible that differences in turnover time of the transporter can also contribute There was no marked pattern of higher intestinal transport activity for amino acids among the more carnivorous vertebrate species , This is perhaps expected because all animals, regardless of diet, need protein and so there should not be strong selection for very low protein processing capability in animals.

In addition, it has been argued that it would be advantageous for herbivores with relatively rapid gut throughput to have compensatorily higher biochemical capacity to process proteins and recover them rather than excrete them. There is overwhelming evidence that the digestive and absorptive function of the GI tract of animals can vary with diet composition. This flexibility is exhibited at two levels: The biochemical flexibility is generally considered to maximize the acquisition of carbon for energy production and essential nutrients for maintenance and growth, while protecting against the acquisition of excessive, potentially toxic, amounts of certain dietary constituents e.

Any nutritional imbalance that might arise from this strategy is widely considered to be corrected postabsorption, so that the retention and use of certain nutrients are optimized, while surplus metabolites can be eliminated , In this section, the relationship between diet composition and digestive enzyme activity is addressed first, followed by consideration of transporters in the GI tract.

Many studies on vertebrates have demonstrated that the production of digestive enzymes increases with availability of substrate in the gut lumen. For example, this effect has been confirmed in rodents for all of the major pancreatic enzymes amylase, lipase, and proteases and enzymes of the intestinal brush border sucrase-isomaltase, maltase-glucoamylase, and aminopeptidase-N Other data relate to a variety of mammals, birds, reptiles and fish, as well as a number of invertebrates [reviewed in reference ].

This mode of regulation both maximizes the digestibility of substrates and minimizes the cost of synthesizing excess enzyme when the substrate is at low levels. The mechanistic basis of the impact of diet on digestive enzyme activity has not been investigated in most species but, where studied, there is persuasive evidence that differential enzyme activity is underpinned by changes in gene expression.

For example, the elevated expression of intestinal sucrase-isomaltase gene in the intestine of rats and mice fed on high-carbohydrate diets is controlled by the transcription factors Cdx-2 and HNF-1 36 ; and the recruitment of these transcription factors to the promoter region is correlated with the acetylation of histones H3 and H4 associated with this gene Adaptive variation in digestive enzyme activity with diet composition is crucial to the lifestyle of many animals.

For example, female Aedes aegypti mosquitoes feed on both sugar-rich nectar and protein-rich vertebrate blood. The gut protease activity is undetectable in individuals feeding on a sugar meal but, within hours of taking a bloodmeal, the digestive protease activity in the midgut increases rapidly, reaching a maximum after about 2 days.

Snakes have no movable eyelids, their eyes being covered by a fixed transparent scale. Visual acuity varies greatly among living reptiles, being poorest in the burrowing lizards and snakes which often have very small eyes and greatest in active diurnal species which usually have large eyes.

Judging by the size of the skull opening in which the eye is situated, similar variation existed among the extinct reptiles. Extinct forms, such as the ichthyosaurs, that hunted active prey had large eyes and presumably excellent vision; many herbivorous types, such as the horned dinosaur Triceratops , had relatively small eyes and weak vision.

Colour vision has been demonstrated in few living reptiles. We welcome suggested improvements to any of our articles. You can make it easier for us to review and, hopefully, publish your contribution by keeping a few points in mind. Your contribution may be further edited by our staff, and its publication is subject to our final approval. Unfortunately, our editorial approach may not be able to accommodate all contributions.

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Digestive and urogenital systems The digestive system of modern reptiles is similar in general plan to that of all higher vertebrates. Sense organs Sight In general construction the eyes of reptiles are like those of other vertebrates.

Previous page Circulatory system. Page 9 of Learn More in these related Britannica articles: Reptile s, of which there are few endemic families, have mainly Old World affinities. Those most likely to be seen include lizards of the agamid family, skinks a family of lizards characterized by smooth overlapping scales , crocodiles, and tortoises.

Endemic reptiles include girdle-tailed…. Although amphibian gastrulation is considerably modified in comparison with that in animals with oligolecithal eggs e.

Such is not the case, however, in the higher vertebrates that possess eggs with…. The living reptiles belong to four orders: The reptile ear has many different forms, especially within the suborder Sauria lizards , and variations occur in….

Unlike lungfishes and amphibians, reptiles depend entirely on their lungs for respiration. Gills and skin do not provide additional sources of oxygen. Because of the development of a neck…. Reptile s are the first vertebrates that, in an evolutionary sense, have evolved an egg that is truly independent of water. Indeed, many snakes and lizards have even gone beyond this stage and have attained complete viviparity. It is difficult to generalize about reproductive behaviour…. More About Reptile 41 references found in Britannica articles Assorted References annotated classification In vertebrate: Annotated classification characteristics of chordates In vertebrate: The tetrapods paleontological record In animal: Neither you, nor the coeditors you shared it with will be able to recover it again.

Comments 0 Please log in to add your comment. Their digestion is slower than in mammals. Some of them can live a long period of time with out eating. For example the snakes eat a big pray; they spend 1 week digesting itand they can pass 2 more weeks with out eating again, that makes 1 month without eating. Stomach It is a dilatation of the digestive tube, in there the food is stored while it is passed into the intestine.

The stomach througs some especial liquids that help to digest the food. The Pharinx and the Esophagus The pharinx is a muscular tube that comunicates the digestive system with the respiratoy system. The esophagus is also a muscle and helps food or ''cud'' to move into the stomach of the reptile. But we want to focus on their digestive system. These are some parts of it: Large intestine and small intestine Into the intestines there are 4 different functions:

Human Digestive System