Quiz Human Digestive System
The expression of various transporter genes is regulated in anticipation of food. Zoo and Wild Animal Medicine. Introduction General features Importance Size range Distribution and ecology North temperate zone Central and South America Asia Australia Africa Natural history Life cycle and life history Courtship and fertilization Embryonic development and parental care Growth and longevity Behaviour Defense Avoidance and noise Body form and posturing Display of colour Striking and biting Spitting Use of the tail Balling Odours Feeding habits Locomotion Walking and crawling Clinging and climbing Swimming Flying Form and function External covering Internal features Skeletal system Skull and dentition Nervous system Circulatory system Respiratory system Digestive and urogenital systems Sense organs Sight Hearing Chemoreception Thermal relationships Evolution and paleontology Historical development Fossil distribution Classification Distinguishing taxonomic features Annotated classification Critical appraisal. 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. Reptiles, of which there are few endemic families, have mainly Old World affinities.
In the mouth, food is moistened by saliva, a sticky fluid that binds food particles together into a soft mass. Three pairs of salivary glands —parotid, submaxillary, and sublingual—secrete saliva into the mouth.
The saliva contains an enzyme called amylase, which digests starch molecules into smaller molecules of the disaccharide maltose. During chewing, the tongue moves food about and manipulates it into a mass called a bolus.
The bolus is pushed back into the pharynx throat and is forced through the opening to the esophagus. The esophagus is a thick-walled muscular tube located behind the windpipe that extends through the neck and chest to the stomach.
The bolus of food moves through the esophagus by peristalsis: The contractions are assisted by the pull of gravity. The esophagus joins the stomach at a point just below the diaphragm.
A valvelike ring of muscle called the cardiac sphincter surrounds the opening to the stomach. The sphincter relaxes as the bolus passes through and then quickly closes. The stomach is an expandable pouch located high in the abdominal cavity. Layers of stomach muscle contract and churn the bolus of food with gastric juices to form a soupy liquid called chyme. The stomach stores food and prepares it for further digestion.
In addition, the stomach plays a role in protein digestion. Gastric glands called chief cells secrete pepsinogen. Pepsinogen is converted to the enzyme pepsin in the presence of hydrochloric acid. Hydrochloric acid is secreted by parietal cells in the stomach lining.
The pepsin then digests large proteins into smaller proteins called peptides. To protect the stomach lining from the acid, a third type of cell secretes mucus that lines the stomach cavity.
An overabundance of acid due to mucus failure may lead to an ulcer. The soupy mixture called chyme spurts from the stomach through a sphincter into the small intestine. For millions of years representatives of these two groups were superficially similar.
However, slowly lifestyles diverged, and from the synapsid line came hairy mammals that possessed an endothermic warm-blooded physiology and mammary glands for feeding their young. All birds and some groups of extinct reptiles, such as selected groups of dinosaurs, also evolved an endothermic physiology. However, the majority of modern reptiles possess an ectothermic cold-blooded physiology. Today only the leatherback sea turtle Dermochelys coriacea has a near-endothermic physiology.
So far no reptile, living or extinct, has developed specialized skin glands for feeding its young. Reptiles, 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 reptile s include girdle-tailed…. Most reptiles have a continuous external covering of epidermal scales.
Reptile scales contain a unique type of keratin called beta keratin; the scales and interscalar skin also contain alpha keratin, which is a trait shared with other vertebrates. Keratin is the main component of reptilian scales. Scales may be very small, as in the microscopic tubercular scales of dwarf geckos Sphaerodactylus , or relatively large, as in the body scales of many groups of lizards and snakes.
The largest scales are the scutes covering the shell of a turtle or the plates of a crocodile. Other features also define the class Reptilia. The occipital condyle a protuberance where the skull attaches to the first vertebra is single. The cervical vertebrae in reptiles have midventral keels, and the intercentrum of the second cervical vertebra fuses to the axis in adults. Taxa with well-developed limbs have two or more sacral vertebrae.
The lower jaw of reptiles is made up of several bones but lacks an anterior coronoid bone. In the ear a single auditory bone, the stapes, transmits sound vibrations from the eardrum tympanum to the inner ear.
Sexual reproduction is internal, and sperm may be deposited by copulation or through the apposition of cloacae. Asexual reproduction by parthenogenesis also occurs in some groups. However, development in most species is external, with embryos enclosed in shelled eggs. In all cases each embryo is encased in an amnion , a membranous fluid-filled sac. In the agriculture industry as a whole, reptiles do not have a great commercial value compared with fowl and hoofed mammals ; nonetheless, they have a significant economic value for food and ecological services such as insect control at the local level, and they are valued nationally and internationally for food, medicinal products, leather goods, and the pet trade.
Reptiles have their greatest economic impact in some temperate and many tropical areas, although this impact is often overlooked because their contribution is entirely local. A monetary value is often not assigned to any vertebrate that provides pest control. Nonetheless, many lizards control insect pests in homes and gardens; snakes are major predators of rodents , and the importance of rodent control has been demonstrated repeatedly when populations of rodent-eating snakes are decimated by snake harvesting for the leather trade.
The absence of such snakes allows rodent populations to explode. Similarly, turtles, crocodiles, snakes, and lizards are regularly harvested as food for local consumption in many tropical areas. When this harvesting becomes commercial, the demands on local reptile populations commonly exceed the ability of species to replace themselves by normal reproductive means.
Harvesting is often concentrated on the larger individuals of most species, and these individuals are often the adult females and males in the population; their removal greatly reduces the breeding stock and leads to a precipitous population decline. The overharvesting of crocodiles for the leather industry in the s and s caused the widespread extirpation, or localized extinction , of many crocodilian species. In addition, surviving populations experienced a near-worldwide drop in numbers.
Ambush species like pythons that sit and wait for long periods of time actually can shut down their GI tract altogether to conserve energy for months at a time. Species that hibernate or go through periods of estivation also can slow their metabolic rates to extreme levels to conserve energy and rely on fat stored in the liver. When the ambush species finally eats or the hibernation ends a metabolic rate can increase times to allow for digestion or food seeking behavior.
Herbivore species gain a lot less energy from their food sources, but spend considerably less energy acquiring their nutritional demands. An advantage to being an ectothermic species is they do not waste energy maintaining their body temperature. If we were to compare the caloric demands of two species, take an insectivore species of birds for instance, the caloric intake for the bird in one day would last a reptile of similar size 35 days!
This conservatory and efficient use of calories has allowed reptiles to survive in particularly harsh environments successfully for millions of years. The gastrointestinal tract in reptiles, like other animals, starts at the mouth. Turtle and tortoise species have a beak, compared to snakes and lizards that have teeth. Apprehension and mastication of food is variable among species since forelimbs are rigid or non-existent in some reptile species. The tongue in certain lizards can be quite muscular and used for food apprehension.
Mastication is minimal, with large chunks of food being swallowed after aggressive shaking or tearing. Snakes, on the other hand, swallow their prey whole. To safely swallow such large portions, reptiles have specially evolved oral secretory glands that function to lubricate the food moving down the esophagus. Some snake species have even more evolved secretory glands that inject venom into their prey to immobilize them before swallowing.
This is crucial to prevent internal trauma from prey that can cause damage while passing down the esophagus. Reptiles have a fairly kinetic jaw that can open wide for food consumption.
Snakes, however, have a particularly kinetic jaw that allows it to essentially walk along the prey when being swallowed. The esophagus in most reptiles can act as a storage unit for large prey until it is able to pass completely into the stomach.
The stomach, as in mammals, contains hydrochloric acid, which not only aids in the breakdown of the food, but also kills live prey. Hydrochloric acid can also help decalcify bony material. Herbivore species have an exceptionally large cecum and have a shorter GI tract compared to carnivorous species. They also have a longer digestion rate as they utilize hindgut fermentation to breakdown the cellulose into usable fatty acids after more digestible proteins are absorbed in the stomach and small GI tract.
The GI activity will essentially stop during cool nights and nearly all the moisture content, as well as every possible nutrient, is extracted before elimination.
Speaking of elimination…the reptile GI tract ends with the rectum transitioning into a cloaca. The cloaca is a pouch-like anatomical feature that is crucial for last-ditch efforts in water absorption from the waste before excretion.