Gastrointestinal tract

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They are located all over the body and enable an animal to monitor its state at any moment. It contains nearly all the protein of milk, as well as the carbohydrate, calcium and B vitamins. Soybeans have not become a popular food in Africa or Latin America, where there is little local knowledge of the best methods of preparing them. In addition, live probiotic bacteria improve the nutrient uptake of the shrimp and influence their digestive processes in a positive way. The seeds, which are of various colours, contain about 50 percent fat and 20 percent protein. However, in Katja Seipel and Volker Schmid suggested that cnidarians and ctenophores are simplified descendants of triploblastic animals, since ctenophores and the medusa stage of some cnidarians have striated muscle , which in bilaterians arises from the mesoderm.

Elimination

Physical Health

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Cancer, group of more than distinct diseases characterized by the uncontrolled growth of abnormal…. Light, electromagnetic radiation that can be detected by the human eye. View All Media 2 Images. If you prefer to suggest your own revision of the article, you can go to edit mode requires login. Thank you for your feedback. Facts matter and Britannica Insights makes it easier to find them. They rely on water or air currents or on the locomotion of their potential prey to bring food within reach.

Because food may come from any direction, many sessile animals evolve radial symmetry. Settlement may be permanent or temporary, but in all cases one stage of the life cycle is capable of moving actively or passively from its place of origin. The choice of attachment site can also be active or passive; passive choice is often associated with an ability to grow in such a way as to maximize feeding efficiency.

As with plants, passive settlers do well only with luck. The retention of locomotory capabilities requires energy and nutrients that can otherwise be diverted into growth or the production of offspring.

Sessile feeders need to move if feeding and resting sites differ. Sessile animals include filter feeders, predators, and even photosynthesizers; the latter include corals that house symbiotic algae. Internal parasites are usually sessile because they live within their lifetime food supply.

Mobile animals that pursue sedentary strategies for seeking prey include web-spinning spiders a terrestrial mode of filter feeding or deep-sea fishes with morphological adaptations that lure prey. Burrowing animals typically eat the rich organic substrates they move through. Others burrow for protection and either temporarily emerge and gather organic sediments at the top of their burrows or pump water with potential food through the burrow.

Instead of digging or finding burrows, some animals move into the centre of sponges, where they find protection and a renewing source of food. Active movement in search of food requires energy, but this expenditure is more than made up for by an ability to seek out areas of concentrated food.

This method of feeding applies to burrowing animals that eat the substrate through which they move, as well as to animals that move over solid surfaces, swim, or fly. Actively moving animals can feed on organisms that do not move, a rich variety coating virtually the entire solid surface of Earth , from the depths of the oceans to the peaks of many mountains. The main problem with this most productive avenue of food gathering is protection. Shells and poisons are the major types of defenses, although innovative detoxification metabolism and jaws of various kinds breach the defenses in part.

This is an escalating battle in which the defenses, as well as the weapons to penetrate them are continually improving. Nudibranchs, shell-less marine snails, incorporate the defensive stinging cells of prey cnidarians into their own skin. Poisonous plants are eaten by specialized insects that avoid or detoxify the poison. In fresh water, for reasons not known, the arms race has not proceeded as far as in the sea.

Cooperation of individuals enables social animals to obtain food in novel ways. Uncannily like humans, some ants farm and herd other organisms for food.

For example, some cultivate a fungus on leaves they cannot directly digest, while others herd aphids from which they milk nectar actually the phloem sap of plants. Some ants even raid the nests of other species and make slaves of them. Another form of cooperation is the mutualism between species that trade advantage for advantage.

Some fishes feed on parasites on the surfaces of other fishes, which benefits all but the parasites. In many animals, including termites and ruminants, microorganisms thrive in the gut and digest cellulose for them. Coherent movement results only when the muscles receive a sensible pattern of activating signals for example, antagonists must not be activated to contract simultaneously. Animals use specialized cells called neurons to coordinate their muscular activity; nerves are bundles of neurons or parts thereof.

Neurons communicate between cells by chemical messengers, but within a single cell often extremely long they can send high-speed signals through a wave of ionic polarization analogous to an electric current along their membranes, a property inherent in all cells but developed for speed in nerve cells by special modifications.

A system of communication requires three parts: In animals, sensory nerves and organs such as eyes collect the information; associative nerves usually concentrated into a brain integrate , evaluate, and decide its relevance; and effector or motor nerves convey decisions to the muscles or elsewhere.

Although all three parts of the nervous system have kept pace with increases in the size and complexity of animals, the simplest systems found among animals those of parazoans and coelenterates are nevertheless capable of intricate feats of coordination. All ends of a coelenterate bipolar neuron can both receive and transmit an impulse, whereas the unipolar neurons of more derived animals receive only at one end dendrite and transmit at the other axon.

A neuron can have multiple dendrites and axons. The earliest animals were probably radial in design, so that bipolar neurons arranged in a netlike pattern made sense. In such a design, a stimulus impinging at any point on the body can travel everywhere to alert a simple array of myofilaments to contract simultaneously. In the case of directed locomotion and relevant sensory input received at the head end of a bilateral animal, unidirectional transmission of nerve impulses to muscles becomes the only way to communicate effectively.

The location of the brain in the head also reflects efficiency and the speed of receipt of information, because this position minimizes the distance between sensory and associative neurons as well as concentrates these two functions in a small, protected part of the body.

In most animals nerve cells cannot be replaced if lost, although axons can be. Nerve cells tend to be concentrated centrally in ganglia or nerve cords, with long axons extending peripherally. Although certain animals may lose tails or limbs to predators or in accidents and then regenerate them, loss or damage to the central nervous system means death or paralysis.

The nervous system uses the transmission properties of neurons to communicate. To pass to the next cell at a synapse, where an axon meets a dendrite, a chemical transmitter is required. Although chemical transmission is considerably slower than the ionic wave, it is more flexible.

For example, learning involves in part increasing the sensitivity of a particular nerve pathway to a stimulus. The sensitivity of a synapse can be altered by increasing the amount of transmitter released from the axon per impulse received, increasing the number of receptors in the dendrite, or changing the sensitivity of the receptors. Bridging the synapse directly by the formation of membrane-bound gap junctions , which connect adjacent cells, enables an impulse to pass unimpeded to a connecting cell.

The increase in speed of transmission provided by a gap junction, however, is offset by a loss in flexibility; gap junctions essentially create a single neuron from several. The same result can be achieved more effectively by lengthening the axons or dendrites, making some nerve cells metres in length.

Situations arise where gap junctions become desirable, however. Gap junctions are found in vertebrate cardiac and smooth muscles, both of which transmit impulses along their cells to others.

This ability makes these muscles somewhat independent of nervous-system control. A body can thus be kept partly functioning for some time without the activity of a brain.

Nerve impulses travel faster along axons of greater diameter or along those with good insulation against ion leakage except at spaced nodes required for recharging. Vertebrates use their unique myelinated axons to increase the transmission rate of nerve impulses, whereas invertebrates are limited to using axons of greater diameter.

As a result, vertebrates can concentrate more small neurons into a body of a particular size, with the potential for greater complexity of behaviour. Memory is still a poorly understood aspect of the nervous system. As in learning, both short- and long-term memories seem to involve alterations in the ease with which subsequent impulses travel a particular pathway after it has been used.

Transfer of memory through direct ingestion of the brain has not been confirmed experimentally. Although the underlying mechanisms are only dimly understood, it is known that there is a correlation between learning and memory capacity.

The capacities for both increase with the number of associative neurons and the number of branches or interconnections formed. Since learning is a process of associating incoming cues with appropriate motor or internal response, greater memory capacity of a brain gives a more rapid learning process. Memory of inappropriate responses to an incoming set of cues can be used without motor repeat. The degree to which the neurons of a brain develop interconnections is correlated with the complexity of its environs while growing.

Basic, repeated behaviours are inherited or learned by the development of fixed pathways by which an environmental signal reaches the motor nerves rapidly with little or no variation reflex arcs. Nonreflex behaviour requires a decision to be made in the brain, with the resulting pathway to the motor nerves becoming more fixed habitual as one particular decision seems always to be correct.

Reflexes are faster than decisions, but their relative adaptiveness depends on context. Animals vary in the degree to which they use reflexes or make decisions, patterns that are strongly correlated to brain size.

Habitual actions are perhaps the most prevalent response, a compromise between the speed of a response and its appropriateness to context. Appropriate behaviour relies on receiving adequate information from the environment to alert an animal to the presence of food, mates, or danger. Although sensory nerves carry this information to the brain, they do not always directly perceive the external world.

Other modified cells intervene to convert light waves into vision, pressure waves in air or water into sound, chemicals into smell or taste, and simple contact into touch. Some animals have other senses, as for electric or magnetic fields. In vision , for example, a photosensitive molecule changes shape and thereby sets off a chain of reactions that ultimately depolarize the dendrite of a sensory nerve.

The associative neurons in the brain interpret the pattern of incoming impulses into a composite picture. The accuracy of what is seen increases with brain size and the complexity of the visual gathering system, or eyes. Animal eyes range from being able to discern only the presence or absence of light to being able to see objects in vivid colour and great detail. Some animals see in ranges beyond unaided human vision.

Pollinating insects in particular discern the colour of flowers differently than do humans; the ultraviolet reflection patterns of flowers do not always coincide with their coloured ones. Bees and birds perceive polarized light and can orient themselves by it. Some animals perceive long wavelengths, which are associated with heat infrared , and can locate the presence of warm-blooded prey by such a mechanism.

Chemoreceptors are usually little-modified sensory neurons, except for the taste receptors of vertebrates, which are frequently replaced cells in synaptic contact with permanent sensory neurons. Chemoreception is based on the recognition of molecules at receptor sites, lipid-protein complexes that are liberally scattered on the dendrites of a sensory neuron.

When the receptor recognizes one particular molecule by shape and sometimes chemical composition , it fires an impulse. The pattern of firings set off in the receptors of a certain molecule provides the information that the brain interprets as an odour or a taste. The details of how animals smell and taste are not as well understood as are the other senses. In many animals, chemoreceptors are not concentrated into obvious organs as they are in vertebrates, making even their location difficult to discern.

Sounds are waves of molecular disturbance that move through air, water, or solids, and their perception by animals simply uses sensitive mechanoreceptors.

Loud sounds can also be felt by the general touch receptors of the body and thereby influence its sense of well-being. Sound receptors are sensitive hair cells or membranes that depolarize a sensory neuron when bent by the passage of a sound wave.

Direct deformation of the dendritic membrane or release of transmitters by the hair cells fire the sensory neurons. Aside from a few insects, only vertebrates have organs with which to hear. Fishes and aquatic amphibians use a lateral-line system , and other vertebrates use ears; both organs use hair cells as phonoreceptors. Sound waves directly stimulate the hair cells of lateral-line systems, while sound waves only indirectly stimulate the hair cells of ears through an amplifying system of membranes and bones, which reaches a peak of complexity in mammals.

Sound is the preferred medium of communication between animals that hear. It can be used over longer distances than vision, and it can be used when vision is not possible. The signals decay more rapidly than do those of odours, and therefore the information can be more precise. Mechanoreceptors also respond to touch, pressure, stretching, and gravity. They are located all over the body and enable an animal to monitor its state at any moment. Much of this monitoring is subconscious but necessary for normal functioning.

Mechanoreceptors are often just sensory nerves, but other cells may be involved. Unlike other senses, that of touch is found in all animals, even sponges, where it reflects a general cellular trait of eukaryotes. Hormones are the chemical integrators of a multicellular existence, coordinating activities from daily maintenance to reproduction and development.

The neurotransmitters released by axons are one class of chemical communicators that act on an adjacent cell, usually a muscle cell or another neuron. Hormones are a mostly distinct class of chemical communicators secreted by nerves, ordinary tissue, or special glands; they act on cells far removed from the site of their release.

They can be proteins, single polypeptides, amines, or steroids or other lipids. Hormones travel to their place of action via the circulatory system and then match their particular configuration with a specific receptor molecule attached to a cell membrane or, more usually, located within the cell.

The nervous system coordinates the more rapid activities of animal life, such as movement, while the hormones integrate everything else. Only the larger, more complex animals, such as vertebrates and some arthropods, have special endocrine glands to produce hormones; other animals use nerve cells or tissues such as the gonads.

Endocrine glands are another example of a partitioning of functions into separate organs, a system that increases efficiency but that requires a relatively large size to maintain. Greater specialization is also associated with greater difficulties in regenerating lost parts or preventing breakdowns in functions. Although the list of hormones found in the mammalian body may seem large, the numbers are surprisingly low for the variety of functions they influence. Therefore, particle size reduction is often considered the key digestive difference between ecto- and endotherms that allows endotherms to rely on shorter digesta retention times without losing digestive efficiency, and hence facilitate the high level of food intake necessary to meet their increased metabolic requirements.

In contrast, adaptations for chewing intrinsically increase the weight of the head. The use of the gizzard system has the potential advantages that intake rate is not limited by chewing, that no investment in dental tissue is necessary, and that dental wear is not a determinant of senescence as observed in mammals.

The absence of age-dependent tooth wear might even be a contributing factor to the slower onset of senescence in birds as compared to mammals. On the other hand, the use of a gizzard requires the intake of suitable grit or stones—an action that represents, in the few studies where this has actually been quantified in birds, a relevant proportion of feeding time Fritz et al.

Gastrointestinal tracts of a carnivorous hawk, an omnivorous chicken, and 4 herbivorous birds. Note larger size of crop in omnivore and herbivores, and particularly in hoatzin. Ceca are small in hawks and relatively large in grouse.

Although ceca are relatively small in Hoatzins , Emus, and Ostriches, an expanded foregut Hoatzins , a much longer midgut Emus , or a much longer colon Ostriches compensates for this From: Stevens and Hume Over-reliance on the passive pathway provides metabolic advantages and ecological constraints.

It does provide birds with an absorptive process that can deal with rapid and large changes in intestinal sugar concentrations. The passive pathway is also energetically inexpensive to maintain and modulate. However, passive absorption through the paracellular pathway is dependent on concentration gradients.

In the absence of a transport system that selects which materials to absorb, this non-discriminatory pathway may also increase vulnerability to toxins, and thus constrain foraging behavior and limit the breadth of the dietary niche of the birds. Another problem is that when luminal sugar concentrations are lower than those in plasma, glucose may diffuse back into the lumen.

Cross-section of the intestine ileum of a Spotted Tinamou Nothura maculosa. Villi are lined with columnar epithelium EP , including goblet cells arrows that secrete mucus. The muscle layer includes longitudinal fibers MI on the perimeter, circular fibers Mc , and additional longitudinal fibers at the base of the villi muscularis muscosae; MM From: Chikilian and de Speroni Blue-headed Parrots at clay lick.

Meyer-Rochow and Gal determined that the pressures involved could be approximated if they knew the 1 distance the feces traveled, 2 density and viscosity of the material, and 3 shape, aperture, and height of the anus above ground. How penguins choose the direction of defecation, and how wind direction factors into that decision, remain unknown.

Avian Pancreas tissue Source: The Avian Digestive Tract. Avian geophagy and soil characteristics in southeastern Peru.

Luminal morphology of the avian lower intestine: Histological aspects of the stomach proventriculus and gizzard of the Red-capped Cardinal Paroaria gularis gularis. Comparative study of the digestive system of three species of tinamou. Crypturellus tataupa, Nothoprocta cinerascens , and Nothura maculosa Aves: Journal of Morphology Journal of Experimental Zoology Rictal bristle function in Willow Flycatcher.

Dysplastic koilin causing proventricular obstruction in an Eclectus Parrot Eclectus roratus. Journal of Avian Medicine and Surgery Anatomy and physiology of the digestive system in fowl. Pages in Proc. An histological and histochemical analysis of the inner lining and glandular epithelium of the chicken gizzard. American Journal of Anatomy An ecomorphological study of the raptorial digital tendon locking mechanism.

Dietary and developmental regulation of intestinal sugar transport. Digesta retention patterns in geese Anser anser and turkeys Meleagris gallopavo and deduced function of avian caeca. Comparative Biochemistry and Physiology A Histological and global gene expression analysis of the 'lactating' pigeon crop.

Vultures of the seas: Evolution of the structure and function of the vertebrate tongue. Journal of Anatomy Light and scanning electron microscopic study of the tongue in the cormorant Phalacrocorax carbo Phalacrocoracidae, Aves. Functional morphology of the tongue in the nutcracker Nucifraga caryocatactes. A tropical horde of counterfeit predator eyes. Instructed learning in the auditory localization pathway of the Barn Owl.

The morphology of the bill apparatus in the Steller's Sea Eagle. Wild Bird Society of Japan, Tokyo. Use of dung as a tool by burrowing owls. The integration of energy and nitrogen balance in the hummingbird Sephanoides sephaniodes.

Does gut function limit hummingbird food intake? Physiological and Biochemical Zoology Pressures produced when penguins pooh—calculations on avian defaecation. Scare tactics in a neotropical warbler: Gliding flight and soaring. Theoretical Ecology Series, vol. Modelling the flying bird C. Structure, form, and function of flight in engineering and the living world.

Phenotypic flexibility and the evolution of organismal design. Trends in Ecology and Evolution The hummingbird tongue is a fluid trap, not a capillary tube. Between air and water: Use of prey hotspots by an avian predator: Structure and mechanical behavior of a toucan beak. Movement and direction of movement of a simulated prey affect the success rate in Barn Owl Tyto alba attack. Musculoskeletal underpinnings to differences in killing behavior between North American accipiters Falconiformes: Accipitridae and falcons Falconidae.

Journal of Morphology, online early. Le Bohec, and Y. Adjustments of gastric pH, motility and temperature during long-term preservation of stomach contents in free-ranging incubating King Penguins. Journal of Experimental Biology A tough nut to crack. Adaptations to seed cracking in finches. Cost-benefit analysis of mollusc-eating in a shorebird. Optimizing gizzard size in the face of seasonal demands. How do woodpeckers extract grubs with their tongues? Why do woodpeckers resist head impact injury: Functional morphology of raptor hindlimbs: The turning- and linear-maneuvering performance of birds: Canadian Journal of Zoology Hummingbird jaw bends to aid insect capture.

A mechanical analysis of woodpecker drumming and its application to shock-absorbing systems. I - Introduction to Birds. VII - Circulatory System. Back to Avian Biology. Drawings of the digestive tracts of A a Greylag Goose and B a Wild Turkey and retention times of a solute, 2-mm particles, and 8-mm particles in the goose and turkey digestive systems Figure from Frei et al. The closed, air-filled spaces reduce overall weight without loss of rigidity. The capillary ratchet mechanism Surface tension transport of prey by feeding shorebirds: The serrated leading-edge feather of an owl Norberg Vortex generators on an airplane wing.

Fish-eating species like cormorants below - typically have small, undifferentiated tongue because fish are often swallowed whole. Representative caterpillar false eyes and faces. In some, like woodpeckers, the 'sticky' saliva aids in capturing prey. In others, like swifts, saliva is used in nest building see photo below. The muscular walls of the esophagus produce wave-like contractions peristalsis that help propel food from the oral cavity to the stomach. Anhinga swallowing a large fish.

HCL and pepsinogen are secreted by the deep glands see photomicrograph below. Pepsinogen is converted into pepsin a proteolytic, or protein-digesting, enzyme by the HCl.

The cuticle is secreted by simple tubular glands see photomicrograph below. Grinding action may, particularly in seed-eating birds, be assisted by grit and stones deliberately ingested. The avian gastrointestinal tract, unlike that of mammals, executes distinct reverse peristaltic movements that are critical to optimal digestive function Duke The gastric reflux allows material in the gizzard to reenter the proventriculus for additional treatment with acid and pepsin.

Villi are projections from the intestinal wall that increase the amount of surface area available for absorption. Further increasing the surface area are the numerous microvilli of the cells lining the surface of the villi. Inside each villus are blood vessels that absorb nutrients for transport throughout the body. Caeca are histologically similar to the small and large intestines and found in a wide variety of birds.

In these large ceca, food particles are acted upon by cecal secretions, bacteria, and fungi and nutrients can be absorbed. Lymphoid ceca are not important in digestion but contain lymphocytes white blood cells that produce antibodies Clench At various times and under various conditions, ceca are the site for 1 fermentation and further digestion of food especially for the breakdown of cellulose and absorption of nutrients, 2 production of antibodies, and 3 the use and absorption of water and nitrogenous components Clench The bursa is most prominent in young birds and serves as the area where B-lymphocytes the white blood cells that produce antibodies are generated T-lymphocytes are generated in the Thymus.

Bile emulsifies fats or, in other words, breaks fats down into tiny particles. Emulsification is important because it physically breaks down fats into particles than can then be more easily digested by enzymes lipase produced by intestinal cells and the pancreas.

This 'juice' contains a bicarbonate solution that helps neutralize the acids coming into the intestine from the stomach plus a variety of digestive enzymes. The enzymes help break down fats, proteins, and carbohydrates. The pancreas also produces the hormones insulin and glucagon which regulate blood sugar levels cells that produce these two hormones make up the 'islets of Langerhans', one of which is represented by the light-colored, circular structure in the photomicrograph below.

Hit 'Reload' or 'Refresh' to View Again! Particle retention time hr. Flamingos use a series of projections, or lamellae, to filter tiny food items from debris in the water. Wrens use their thin, probing bill to capture small insects. Curlews use their long bill to probe mudflats for small invertebrates.

Finches do not simply bite the seeds; instead; the lower mandible is moved toward the tip of the bill in a slicing motion. When most of the coat has been cracked or removed, the lower mandible is moved from side to side to remove the rest of the shell, thus releasing the kernel.

Some large finches also have raised hard surfaces in the upper palate that function as anvils so large seeds can be held firmly while the lower mandible slices and cracks the sides of the seed. As tricky as nutcracking sounds, most birds accomplish it rapidly, shelling small seeds in a few seconds and large finches can crack open and devour a large seed or nut in less than twenty seconds.

Big mouths get hummingbirds an in-flight meal - Hummingbirds have bendy lower beaks to help them catch insects Yanega and Rubega The flexibility allows long-beaked birds to open their mouths wide enough to hunt on the wing. Hummingbirds use their long, narrow beaks to probe flowers for nectar, but they also need insects for essential nutrients. It wasn't clear how they could catch them; birds that hunt flying insects usually have short beaks to help them open their mouths wide.

Pilcher, Nature Science Update. The force produced by talons may be related to time of activity. Owls hunt when light levels are low so if an attacking owl misses its prey, relocating it may be difficult.

Volcanoes: Volcanoes!