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Most rats are still able to breathe through this true choking is rare in rats , and work the food out themselves in time, but serious cases may require veterinary asssitance. Poisonous plants may be classified according to the chemical nature of their toxic constituents , their phylogenetic relationship, or their botanical characteristics. Further inquiry into the exact mechanisms of vomiting in different species would cast more light on whether vomiting has evolved multiple times. Atmospheric pressures fall at higher altitudes, but the composition of the atmosphere remains unchanged. The next tissue affected is the gastrointestinal tract. Rats can't vomit for several related reasons: Such structures have the advantage of a protected internal location, but this requires some sort of pumping mechanism to move the external gas-containing medium in and out.
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A fish experiencing both rising water and body temperatures is under a double handicap: The amount of oxygen available in natural waters is also limited by the amount of dissolved salts. This factor is a determinant of oxygen availability in transitional zones between sea and fresh water. Bodies of water may have oxygen-poor zones. Such zones are especially evident in swamps and at the lower levels of deep lakes.
Many animals are excluded from such zones; others have become remarkably adapted to living in them. It is composed of a mixture of gases held in an envelope around the globe by gravitational attraction.
The atmosphere exerts a pressure proportional to the weight of a column of air above the surface of the Earth extending to the limit of the atmosphere: Dry air is composed chiefly of nitrogen and inert gases These percentages are relatively constant to about At sea level and a barometric pressure of millimetres of mercury, the partial pressure of nitrogen is The existence of water vapour in a gas mixture reduces the partial pressures of the other component gases but does not alter the total pressure of the mixture.
To calculate the partial pressures of the respiratory gases, this value must be subtracted from the atmospheric pressure. Atmospheric pressures fall at higher altitudes, but the composition of the atmosphere remains unchanged. At 7, metres 25, feet the atmospheric pressure is millimetres of mercury and the partial pressure of oxygen is about 59 millimetres of mercury. Oxygen continues to constitute only The rarefaction of the air at high altitudes not only limits the availability of oxygen for the air breather, it also limits its availability for aquatic forms, since the amount of dissolved gas in water decreases in parallel with the decline in atmospheric pressure.
The variations in the characteristics of air and water suggest the many problems with which the respiratory systems of animals must cope in procuring enough oxygen to sustain life.
Respiratory structures are tailored to the need for oxygen. Minute life-forms, such as protozoans, exchange oxygen and carbon dioxide across their entire surfaces. Multicellular organisms, in which diffusion distances are longer, generally resort to other strategies. Aquatic worms, for example, lengthen and flatten their bodies to refresh the external medium at their surfaces.
Sessile sponges rely on the ebb and flow of ambient water. By contrast, the jellyfish, which can be quite large, has a low oxygen need because its content of organic matter is less than 1 percent and its metabolizing cells are located just beneath the surface, so that diffusing distances are small. Organisms too large to satisfy their oxygen needs from the environment by diffusion are equipped with special respiratory structures in the form of gills, lungs, specialized areas of the intestine or pharynx in certain fishes , or tracheae air tubes penetrating the body wall, as in insects.
Respiratory structures typically have an attenuated shape and a semipermeable surface that is large in relation to the volume of the structure.
Within them there is usually a circulation of body fluids blood through the lungs, for example. Two sorts of pumping mechanisms are frequently encountered: In air-breathing vertebrates, alternately contracting sets of muscles create the pressure differences needed to expand or deflate the lungs, while the heart pumps blood through the respiratory surfaces within the lungs.
Oxygenated blood returning to the heart is then pumped through the vascular system to the various tissues where the oxygen is consumed. Two common respiratory organs of invertebrates are trachea and gills. Diffusion lungs, as contrasted with ventilation lungs of vertebrates, are confined to small animals, such as pulmonate snails and scorpions. This respiratory organ is a hallmark of insects.
It is made up of a system of branching tubes that deliver oxygen to, and remove carbon dioxide from, the tissues, thereby obviating the need for a circulatory system to transport the respiratory gases although the circulatory system does serve other vital functions, such as the delivery of energy-containing molecules derived from food. The pores to the outside, called spiracles , are typically paired structures, two in the thorax and eight in the abdomen.
Periodic opening and closing of the spiracles prevents water loss by evaporation, a serious threat to insects that live in dry environments. Muscular pumping motions of the abdomen, especially in large animals, may promote ventilation of the tracheal system.
Although tracheal systems are primarily designed for life in air, in some insects modifications enable the tracheae to serve for gas exchange under water. Of special interest are the insects that might be termed bubble breathers, which, as in the case of the water beetle Dytiscus , take on a gas supply in the form of an air bubble under their wing surfaces next to the spiracles before they submerge.
Tracheal gas exchange continues after the beetle submerges and anchors beneath the surface. As oxygen is consumed from the bubble, the partial pressure of oxygen within the bubble falls below that in the water; consequently oxygen diffuses from the water into the bubble to replace that consumed. The carbon dioxide produced by the insect diffuses through the tracheal system into the bubble and thence into the water. The bubble thus behaves like a gill.
There is one major limitation to this adaptation: As oxygen is removed from the bubble, the partial pressure of the nitrogen rises, and this gas then diffuses outward into the water. The consequence of outward nitrogen diffusion is that the bubble shrinks and its oxygen content must be replenished by another trip to the surface. A partial solution to the problem of bubble renewal has been found by small aquatic beetles of the family Elmidae e. Several species of aquatic beetles also augment gas exchange by stirring the surrounding water with their posterior legs.
An elegant solution to the problem of bubble exhaustion during submergence has been found by certain beetles that have a high density of cuticular hair over much of the surface of the abdomen and thorax. The hair pile is so dense that it resists wetting, and an air space forms below it, creating a plastron , or air shell, into which the tracheae open.
As respiration proceeds, the outward diffusion of nitrogen and consequent shrinkage of the gas space are prevented by the surface tension —a condition manifested by properties that resemble those of an elastic skin under tension—between the closely packed hairs and the water. Since the plastron hairs tend to resist deformation, the beetles can live at considerable depths without compression of the plastron gas. One extraordinary strategy used by the hemipteran insects Buenoa and Anisops is an internal oxygen store that enables them to lurk for minutes without resurfacing while awaiting food in relatively predator-free but oxygen-poor mid-water zones.
The internal oxygen store is in the form of hemoglobin-filled cells that constitute the first line of oxygen delivery to actively metabolizing cells, sparing the small air mass in the tracheal system while the hemoglobin store is being depleted.
The book lungs contain blood vessels that bring the blood into close contact with the surface exposed to the air and where gas exchange between blood and air occurs. In addition to these structures, there may also be abdominal spiracles and a tracheal system like that of insects.
Since spiders are air breathers, they are mostly restricted to terrestrial situations, although some of them regularly hunt aquatic creatures at stream or pond edges and may actually travel about on the surface film as easily as on land. The water spider or diving bell spider , Argyroneta aquatica —known for its underwater silk web , which resembles a kind of diving bell—is the only species of spider that spends its entire life underwater.
Research has shown that the inflated web serves as a sort of gill, extracting dissolved oxygen from the water when oxygen concentrations inside the web become sufficiently low to draw oxygen in from the water. As the spider consumes the oxygen, nitrogen concentrations in the inflated web rise, causing it to slowly collapse. Most of the life cycle of the water spider, including courtship and breeding, prey capture and feeding, and the development of eggs and embryos, occurs below the water surface.
Many immature insects have special adaptations for an aquatic existence. Thin-walled protrusions of the integument , containing tracheal networks, form a series of gills tracheal gills that bring water into close contact with the closed tracheal tubes.
The nymphs of mayflies and dragonflies have external tracheal gills attached to their abdominal segments, and certain of the gill plates may move in a way that sets up water currents over the exchange surfaces.
Dragonfly nymphs possess a series of tracheal gills enclosed within the rectum. Periodic pumping of the rectal chamber serves to renew water flow over the gills. Removing the gills or plugging the rectum results in lower oxygen consumption. Considerable gas exchange also occurs across the general body surface in immature aquatic insects. The insect tracheal system has inherent limitations. Gases diffuse slowly in long narrow tubes, and effective gas transport can occur only if the tubes do not exceed a certain length.
It is generally thought that this has imposed a size limit upon insects. Gills are evaginations of the body surface. Some open directly to the environment; others, as in fishes , are enclosed in a cavity.
In contrast, lungs represent invaginations of the body surface. Many invertebrates use gills as a major means of gas exchange; a few, such as the pulmonate land snail , use lungs.
Almost any thin-walled extension of the body surface that comes in contact with the environmental medium and across which gas exchange occurs can be viewed as a gill. Gills usually have a large surface area in relation to their mass; pumping devices are often employed to renew the external medium. Although gills are generally used for water breathing and lungs for air breathing, this association is not invariable, as exemplified by the water lungs of sea cucumbers.
The marine polychaete worms use not only the general body surface for gas exchange but also a variety of gill-like structures: The tufts, used to create both feeding and respiratory currents, offer a large surface area for gas exchange. In echinoderms starfish, sea urchins, brittle stars , most of the respiratory exchange occurs across tube feet a series of suction-cup extensions used for locomotion.
The gills of mollusks have a relatively elaborate blood supply, although respiration also occurs across the mantle, or general epidermis. Vomiting is an active process: In contrast, regurgitation is the passive, effortless flow of undigested stomach contents back into the esophagus.
Regurgitation happens without any forceful abdominal contractions. There is at least one report of rats choking on regurgitated stomach contents Will et al. Upon necropsy, the regurgitated stomach contents regurgitant were found to be thick and pasty. They were packed into the rats' pharynx, larynx and esophagus. The action of the tongue had packed the regurgitant into a plug, causing choking. The rats' tongues were also lacerated or bruised from attempts to remove the material by chewing or clawing.
Regurgitation was more common in rats fed bulky diets than those fed on standard diets, and more common in females than in males. Rats may have trouble swallowing a food item. A rat who has trouble swallowing a food item may strain intently, pull his chin down toward his throat and flatten his ears. He may drool saliva, paw at his mouth, and rub his mouth on nearby surfaces.
Most rats are still able to breathe through this true choking is rare in rats , and work the food out themselves in time, but serious cases may require veterinary asssitance. Difficulty swallowing may superficially resemble vomiting because partly processed food may come back out of the mouth, but it is not vomiting, which is the forceful, rapid, coordinated, reflexive explusion of stomach contents.
This foam is not made of stomach contents, but of mucus brought up from the lungs that has been whipped up into a froth. This foam is a symptom of a respiratory problem, not regurgitation or vomiting pers comm B.
Diagram of the rat's stomach. Adapted from Moore Diagram of a rat stomach opened along the greater curvature of the stomach. Adapted from Robert Diagram of the crural sling and the muscle bundles of the esophageal sphincter, which make up the gastroesophageal barrier and are responsible for closing the esophagus. Adapted from Montedonico et al. The rat's esophagus has two layers of striated muscle outer longitudinal and inner circular , which become smooth near the attachment point with the stomach.
The esophagus is closed off from the stomach by the gastroesophageal barrier , which consists of the crural sling , the lower esophageal sphincter , and the several centimeters of intraabdominal esophagus that lie between them Soto et al. Humans also have a crural sling and an esophageal sphincter, but ours are placed right on top of one another Mittal In rats, they are separated by several centimeters of intraabdominal esophagus Soto et al.
The crural sling is part of the diaphragm its outer contour is continuous with the diaphragm. It is a U-shaped bundle of fibers that wraps around the esophagus and attaches to the vertebrae. When the crural sling contracts it pinches the esophagus closed.
The esophageal sphincter is a circular muscle that surrounds the base of the esophagus. At its lower edge, it has muscle fibers that insert into the limiting ridge Fig 4. So when the sphincter contracts, it not only constricts the walls of the esophagus, it also pulls the sides of the limiting ridge's "U" together, thus hiding and tightly closing the esophageal opening Montedonico et al.
Diagram of the limiting ridge and the esophageal opening in the rat's stomach when the esophageal spincter is a open and b closed. Anatomical textbooks on rats usually mention in passing that rats can't vomit. They tend to implicate the limiting ridge or the lack of striated muscle in the rat's esophagus, and sometimes both Fox et al. Looking deeper into the scientific literature, I found a complex story about why a rat is unable to vomit:.
Rats have a powerful and effective gastroesophageal barrier , consisting of the crural sling, the esophageal sphincter, and the centimeters of intraabdominal esophagus see above.
The pressure at the two ends of this barrier is much higher than the pressure found in the thorax or abdomen during any phase of the the breathing cycle Montedonico et al. The strength and pressure of this barrier make reflux in rats nearly impossible under normal conditions Montedonico et al. In order to vomit, the rat would have to overcome this powerful barrier. Evidence suggests that rats cannot do this, because 1 they can't open the crural sling at the right time, and 2 they can't wrench open the esophageal sphincter.
The skin of the aquatic turtles is more permeable for allowing them to respire while the cloaca is modified in various species to increase the gas exchange area. Despite these adaptations, lungs remain a very important part of their respiratory system. The main Reptile groups accomplish lung ventilation in different procedures. Squamates are known to ventilate the lugs mainly by their axial musculature. Certain lizard species are capable of buccal pumping apart from the normal axial breathing.
The proto-diaphragm in Tegu lizards separates their pulmonary cavity from visceral cavity, helping with their respiration by allowing greater lung inflation. The muscular structure of the diaphragm in the Crocodilians species resembles that of various mammals. However, there are some differences in their diaphragmatic setup. They also have two aortas playing a major role in their systemic circulation.
The oxygenated and deoxygenated blood may get mixed with each other in their three-chambered heart with the level of mixing depending on the species and the physiological state of the animal. Their circulatory system is capable of shunting back the deoxygenated blood to the body and the oxygenated blood to the lungs if necessary.
Unlike other Reptiles, animals in the crocodilian subgroup have four-chambered hearts. But, their two systemic aortas are only capable of bypassing their pulmonary circulation. On the other hand, the three-chambered hearts in various lizard and snake species can function as the four-chambered ones during contraction. Majority of these animals have short digestive tracts because their diet mainly consists of meat, which is very simple to digest. Their digestion process is slower than that in mammals due to their inability of mastication and their low metabolism rate while resting.
The energy requirements for their poikilotherm metabolism are very low which allows large animals from this class such as various constrictors and crocodiles to survive for months from one large meal, digesting it slowly. Herbivorous reptiles are also unable to masticate their food, which slows down the digestive process.
Some species are known to swallow pebbles and rocks that help in grinding up plant matters within the stomach, assisting their digestion. The basic nervous system in the Reptiles is similar to that in the Amphibians. But, Reptiles have slightly larger cerebrum and cerebellum. Most of the important sensory organs are properly developed in these creatures.
However, there are certain exceptions such as the absence of external ears in snakes they have the inner and middle ears. Reptiles have twelve cranial nerve pairs. They have to use electrical tuning for expanding the range of their audible frequencies because they have short cochlea. These animals are believed to be less intelligent compared to mammals and birds because the relative size of their brain and body is much smaller than that of the latter.
However, the brain development can be more complex in some larger Reptiles. Modern species also have pineal glands in their brains. Most of these animals are tetrapods, meaning they have four legs. Snakes are examples of legless Reptiles. Their skeletal system is similar to other tetrapods with a spinal column supporting their bodies. Their excretory system consists of two small kidneys. The diapsid species excrete uric acid as the principal nitrogenous waste product. But, turtles excrete mainly urea.
Some of these species use their colons for reabsorbing water, while some are able to absorb the water stored in their bladders. Certain Reptiles excrete the excess salts in their bodies through the lingual and nasal salt glands.
Reptiles have certain characteristic features that help in distinguishing them from Amphibians, Mammals and Aves:. They are capable of adapting to almost all kinds of habitats and environmental conditions, except for extremely cold regions. These animals can inhabit dry deserts, forests, grasslands, wet meadows, shrub lands and even marine habitats. Reptiles are capable of adapting to extremely high temperatures because they are cold blooded. Various snakes including the Rattle Snakes and King Snakes as well as different lizards like the Gila Monsters live in desert habitats.
Grassland is another common type of habitat for various snakes and lizards e. Garter Snakes, Fox Snakes. The vegetation in this habitat attracts many insects and rodents, making it easier for the Reptiles to catch prey.
Swamps and large water bodies are inhabited by different Reptiles such as crocodiles, alligators, certain turtles and snakes. Animals like the Saltwater Crocodile and Marine Iguana inhabit seaside, travelling in and out of ocean as necessary. Some species, such as the Sea Snakes and Sea Turtles, live in the ocean. They leave the waters only during the breeding season for laying eggs. These animals typically practice sexual reproduction with some specific species using asexual reproduction.
Majority of these animals are amniotes, laying eggs covered with calcareous or leathery shells. The eggs are generally laid in underground burrows dug by the females. The viviparity and ovoviviparity modes of reproduction are used by many species such as all boas and many vipers. However, the level of viviparity may vary with some species retaining their eggs until shortly before hatching while others nourish the eggs for supplementing the yolks.
In some Reptile species, the eggs do not have any yolk with the adults providing all the necessary nourishment through a structure resembling the mammalian placenta. Six lizard families and one snake family from the Squamata sub-group are known to be capable of agamogenesis or asexual reproduction. In some squamate species, the females are capable of giving birth to unisexual diploid clones of themselves. This type of asexual reproduction is known as parthenogenesis, occurring in various teiid lizards, geckos and lacertid lizards.
Komodo dragons have reproduced through parthenogeny in captivitiy. Like many mammals, birds and Amphibians, their embryonic life consists of an amnion, chorion, as well as an allantois. The incubation period may vary depending on the species and other factors like the temperature of the surroundings.
Usually, hatchlings are able to take care of themselves almost immediately after coming out of the eggs. But, the females of some species are known to protect their eggs and hatchlings.