The gases in the environment
Remember that Davis et al. Alpha particles , beta particles, neutrons, and positrons are examples of particulate ionizing radiation. Benzoyl peroxide and parabens applied to the skin may be toxic. Both ozone and nitrogen oxides are oxidizing pollutants. They tend to implicate the limiting ridge or the lack of striated muscle in the rat's esophagus, and sometimes both Fox et al.
What Is a Reptile?
As a result, intestinal bleeding occurs, which can lead to the development of shock. Among tricyclic antidepressants, amitriptyline and imipramine account for most of the fatal cases of poisoning. These drugs have a number of effects, including blockage of the parasympathetic system and damage to the central nervous system, the latter producing symptoms such as fatigue, weakness, lowered body temperature, seizures, and respiratory depression Table 3. Death is usually caused by damage to the heart.
Lithium salts, used to treat manic depression, have a relatively low therapeutic index. Mind-altering drugs commonly abused include amphetamines, cocaine , phencyclidine, heroin , and methaqualone. These drugs are primarily toxic to the central nervous system; amphetamine and cocaine cause stimulation of the system hallucinations and delirium , and heroin causes the depression of the system depressed respiration and coma.
In contrast, phencyclidine and methaqualone are biphasic, producing first depression drowsiness and then stimulation of the central nervous system delirium and seizures. Amphetamines also affect the gastrointestinal tract anorexia, nausea, vomiting, diarrhea and stimulate the cardiovascular system increased blood pressure and heart rate, palpitations, and abnormal heart rhythm. In addition to hallucinations and delirium, cocaine causes euphoria, sexual arousal, confusion, and sympathetic stimulation.
Phencyclidine is also known to cause aggression and psychotic behaviour, while methaqualone produces excessive dreaming and amnesia. Digitalis overdose usually begins with gastrointestinal symptoms, such as anorexia, nausea, and vomiting, followed by sensory symptoms, such as pain and visual disturbances Table 3.
There are also effects on the central nervous system, characterized by delirium and hallucinations. The major toxicities of beta blockers e. Sympathetic stimulation relaxes smooth muscles in the tracheobronchial wall and makes the heart beat faster and more forcefully. Blockage produced by propranolol or metoprolol can cause bronchoconstriction and heart failure Table 3. Drugs for treating asthma, such as theophylline and aminophylline, are structurally similar to caffeine.
Like caffeine, which is a stimulant, theophylline and aminophylline also stimulate the central nervous system. Therefore, excitement, delirium, rapid breathing, increased heart rate, and seizures occur with an overdose. With excessive stimulation of the heart, palpitations and irregular heart rhythm arrhythmia can result, leading to sudden death. Biotoxins can be conveniently grouped into three major categories: The geographic distribution of poisonous organisms varies greatly; poison-producing microorganisms tend to be ubiquitous in their distribution.
Poisonous plants and animals are found in greatest abundance and varieties in warm-temperate and tropical regions. Relatively few toxic organisms of any kind are found in polar latitudes. Knowledge of the evolutionary significance and development of most biotoxins is largely speculative and poorly understood.
In some instances they may have developed during the evolution of certain animal species as part of the food procurement mechanism e. Biotoxins may also function as defensive mechanisms, as in some snakes, fishes, arthropods e.
The defense may be quite complex—as in the protection of territorial rights for reproductive purposes—and inhibitory or antibiotic substances may be produced that result in the exclusion of competitive animal or plant species.
Certain marine organisms and terrestrial plants may release into the water, air, or soil inhibitory substances that discourage the growth of other organisms; well-known examples include the production of antibiotic substances by microorganisms. Similar chemical-warfare mechanisms are used in battles for territorial rights among the inhabitants of a coral reef , a field, or a forest.
Thus biotoxins play important roles in the regulation of natural populations. Of increasing interest has been the discovery that certain substances, which may be toxic to one group of organisms, may serve a vital function in the life processes of the source organism.
Venom-producing animals and stinging and dermatogenic i. Biotoxic agents may produce their injurious effects by becoming involved in the food supply; ingestion of a poisonous microbial organism, plant, or marine animal or one of their toxic by-products may cause intoxication. An example is that of the shore fishes of many tropical islands; otherwise valuable food fishes are frequently contaminated by a poison called ciguatoxin.
The poison, a potent neurotoxin nerve poison , is accidentally ingested by the fishes in their food; such fish can no longer be used for either human or animal consumption. Some of the effects produced by biotoxins on humans are of an acute nature, and the injuries they cause are readily discernible.
The effects of some of the mycotoxins poisons produced by fungi and poisons produced by plants, however, are long-term and chronic; they result in the development of cancerous growths and other chronic degenerative changes that are sometimes difficult to detect.
Microbial poisons are produced by the Monera bacteria and blue-green algae and Protista algae, protozoa, and others , and the Fungi. Various classifications have been proposed for the microbial poisons, but none is entirely satisfactory. The problems encountered when dealing with these organisms result from a lack of precise knowledge concerning their biological nature and their phylogenetic relationships; in addition, their poisons show great diversity and chemical complexity.
The following outline, however, is useful in dealing with this subject. The main differences in these toxins lie in their chemical structure. Poisonous proteins from bacteria are sometimes referred to as bacterial exotoxins. The exotoxins are generally produced by gram-positive organisms i.
The exotoxins usually do not contain any nonprotein substances, and most are antigenic ; i. The exotoxins may appear in the culture medium in which the bacteria are growing during the declining phases of growth; in some cases they are released at the time of normal destruction of the cells after death autolysis. The exotoxins are less stable to heat than are the endotoxins, and they may be detoxified by agents that do not affect endotoxins.
They are more toxic than endotoxins, and each exotoxin exerts specific effects which are collectively known as pharmacological properties. Exotoxins are neutralized by homologous antibodies— i. Endotoxins are antigens composed of complexes of proteins, polysaccharides large molecules built up of numerous sugars , and lipids fats.
The protein part determines the antigenicity, or quality of being reacted against as a foreign substance in a living organism. The polysaccharide part determines the immunological specificity, or limitations on the types of antibodies that can react with the endotoxin molecule and neutralize it the immunological reaction. Some of the lipids possibly determine the toxicity.
Endotoxins are derived from the bacterial cell wall and, when cells are grown in culture , are released only on autolysis. Endotoxins are not neutralized by homologous antibodies and are relatively stable to heat; all of them have the same pharmacological properties. The Cyanobacteria, or blue-green algae , are among the most primitive and widely distributed of all organisms.
They have extreme temperature tolerances. Some strains of a species are toxic; other strains of the same species are not. Water blooms of blue-green algae have been responsible for the death of fishes, waterfowl, cattle, horses, swine, and other animals.
Blue-green algae have also been implicated as causes of human intoxications. Fungi are plantlike members of the kingdom Fungi Mycota that do not contain chlorophyll.
A significant number are known to produce poisons of various types. Toxic fungi can be roughly divided into two main categories on the basis of their size: The toxic microfungi are members of one of two classes: Ascomycetes , or the sac fungi, and the Deuteromycetes , or the imperfect fungi i.
The large toxic mushrooms, or toadstools, are mostly members of the class Basidiomycetes , although some Ascomycetes, such as the poisonous false morel Gyromitra esculenta , may attain a size as large as some of the mushrooms. The ability of certain fungi, such as ergot Claviceps purpurea and some mushrooms, to produce intoxication has long been known.
During the 19th century it was recognized that molds are responsible for such diseases as yellow-rice toxicoses in Japan and alimentary toxic aleukia in Russia. The eruption of so-called turkey X disease in England in and the resulting discovery of the substance known as aflatoxin see Table 4 stimulated study of the subject of mycotoxicology.
Poisonous mushrooms , or toadstools as they are commonly called, are the widely distributed members of the class Basidiomycetes, although only a few are known to be poisonous when eaten see Table 5 ; some of the poisons, however, are deadly.
Most deaths attributed to mushroom poisoning result from eating members of the genus Amanita. Wild mushrooms should be eaten only if they have been accurately identified by an experienced person; the safest procedure is to eat only cultivated species. The problem of toxicity in mushrooms is complex; no single rule or test method exists by which the toxicity of a mushroom can be determined. The most poisonous species closely resemble some of the most prized edible species; in addition, toxicity within a given wild species may vary from one set of ecological conditions or from one geographical locality to the next.
Moreover, although some mushrooms that are poisonous when fresh are edible when cooked, dried, salted, or preserved in some other way, others remain poisonous in spite of all preparation procedures.
It has also been observed that some people may become poisoned by eating mushrooms that apparently do not affect others. As with microfungi, the mushroom poisons vary in their chemical and biological properties from species to species. The dinoflagellates , important producers of the primary food supply of the sea, are microscopic one-celled organisms that are dependent upon various inorganic nutrients in the water and upon radiant energy for photosynthesis, the process by which they produce their own food supplies.
Although dinoflagellates inhabit both marine waters and freshwaters, most species are marine. Dinoflagellates are most often found in cool or temperate waters. During periods of planktonic blooms times of high concentrations of microscopic organisms in the water dinoflagellates multiply in large numbers. These planktonic blooms, sometimes referred to as red tide because they discolour the water, are often associated with weather disturbances that may bring about changes in water masses or upwellings.
During periods of bloom large numbers of toxic dinoflagellates may be ingested by shellfish; the poisons accumulate in their digestive glands.
Animals and humans may in turn be poisoned by eating poisoned shellfish. Certain species of dinoflagellates are capable of producing some of the most toxic substances known. The two species of dinoflagellates most commonly involved in human intoxications have been Gonyaulax catenella along the Pacific coast of North America and G.
Intoxications from these organisms are known as paralytic shellfish poisoning. The symptoms, which begin with a tingling or burning sensation, then numbness of the lips, gums, tongue, and face, gradually spread.
Gastrointestinal upset may be present. Other symptoms include weakness, joint aches, and muscular paralysis; death may result.
There is no specific treatment or antidote. The poison, variously called paralytic shellfish poison, mussel poison, and saxitoxin , is a complex nonprotein nitrogen-containing compound. Paralytic shellfish poisoning is best avoided by following local public-health quarantine regulations.
Respiratory irritation may result from the inhalation of toxic products in the windblown spray from red-tide areas containing the toxic dinoflagellate Gymnodinium breve , which is found in the Gulf of Mexico and Florida; the nature of the poison is unknown. Deaths of large numbers of brackish-water pond fishes because of Prymnesium parvum have been reported in Israel; the poison is known as prymnesin. The study of plant poisons is known as phytotoxicology.
Most of the poisonous higher plants are angiosperms , or flowering plants, but only a small percentage are recognized as poisonous. Several systems have been devised for the classification of poisonous plants, none of which is completely satisfactory. Poisonous plants may be classified according to the chemical nature of their toxic constituents , their phylogenetic relationship, or their botanical characteristics.
The following classification, which is based on their toxic effects, has been found to be useful: Plant poisons, or phytotoxins, comprise a vast range of biologically active chemical substances, such as alkaloids , polypeptides, amines, glycosides, oxalates, resins, toxalbumins, and a large group of miscellaneous compounds whose chemical structure has not yet been determined. Alkaloids, most of which are found in plants, are characterized by the presence of nitrogen and their ability to combine with acids to form salts.
They are usually bitter in taste. It has been estimated that about 10 percent of the plant species contain some type of alkaloid. Only a few of the 5, alkaloids characterized thus far do not produce any biological activity; most cause a strong physiological reaction when administered to an animal. Amines are organic compounds containing nitrogen. A polypeptide is a string of three or more amino acids. A few polypeptides and amines are toxic to animals. Some glycosides , which are compounds that yield one or more sugars and one or more other compounds— aglycones nonsugars —when hydrolyzed chemically degraded by the introduction of water molecules between adjacent subunits , are extremely toxic to animals.
Toxicity resides in the aglycone component or a part of it. Oxalates are salts of oxalic acid, which under natural conditions is not toxic but becomes so because of the oxalate ion. Resins , a heterogeneous assemblage of complex compounds, differ widely in chemical properties but have certain similar physical properties.
Some resins are physiologically very active, causing irritation to nervous and muscle tissue. Toxalbumins are highly toxic protein molecules that are produced by only a small number of plants. Ricin , a toxalbumin from the castor bean Ricinus communis , is one of the most toxic substances known. Under certain ecological conditions plants may become poisonous as a result of the accumulation of toxic inorganic minerals such as copper, lead, cadmium, fluorine, manganese, nitrates, or selenium.
Photosensitization, an unusual toxic reaction resulting from the ingestion of certain plants, may be of two types. The toxic substance may be obtained directly from the plant, which thereupon acts on the skin primary photosensitivity , or the toxicity may result from liver damage caused by the metabolism of a toxic plant and failure of the breakdown products to be eliminated by the liver hepatic photosensitivity.
In either case the animal reacts by becoming restless; in addition, the skin reddens, and a severe sloughing of the skin develops. A large number of poisonous plants occur throughout the world; a few representative species and their poisons are listed in Table 6. Poisonous animals are widely distributed throughout the animal kingdom; the only major group that seems to be exempt is the birds.
Zootoxins can be divided into several categories: Oral zootoxins see Table 7 are generally thought to be small molecules; most venoms Table 8 are believed to be large molecules, usually a protein or a substance in close association with one. Venoms, which are produced by specialized poison glands, are injected by means of a mechanical device that is able to penetrate the flesh of the victim.
Little is known about the biological or chemical properties of most crinotoxins Table 9. The term poisonous may be used in the generic sense to refer to all three categories of zootoxins. Some of the most complex relationships in biotoxicology are found in the marine environment. Certain marine biotoxins , such as ciguatera fish poison, apparently originate in marine plants, are ingested by herbivores and then passed on to carnivores and eventually to humans. The extremely complex mechanism by which this is accomplished is not clear.
With the buildup of toxic industrial chemical pollutants in the marine environment, the problems of toxicity in marine organisms are becoming increasingly more serious. There is evidence that under certain conditions chemical pollutants may trigger biotoxicity cycles in marine organisms. The outbreaks in Japan of Minamata disease were the result of such a cycle: The relationships of representative poisonous animals and their position in the total framework of the animal kingdom can best be appreciated by categorizing them according to the group in which they belong.
They are further grouped as to whether they are poisonous to eat, venomous, or crinotoxic in Tables 7, 8, and 9. Radiation is a flow of energy through space or matter. It takes the form of particles e. Radiation can be classified as either ionizing or nonionizing depending on its ability to produce ions in the matter it interacts with. Ionizing radiation is more toxic than nonionizing radiation.
Radioactivity is the emission of radiation caused by the disintegration of unstable nuclei of radioisotopes. After disintegration, a radioisotope may become a radioisotope of another element, which will further disintegrate.
The disintegration series continues until a stable isotope is formed. Ionizing radiation is radiation that produces ions in matter during interaction with atoms in the matter. The toxic effect of ionizing radiation is related to the ionization. Because radicals are very reactive chemically, biological damage, such as attacks on DNA and proteins, results. There are two classes of ionizing radiation: Alpha particles , beta particles, neutrons, and positrons are examples of particulate ionizing radiation.
Gamma rays and X rays are electromagnetic ionizing radiation. Among particulate ionizing radiation, alpha and beta particles are the forms most commonly encountered in the environment and are biologically the most significant.
Although they do not penetrate tissue very well, alpha particles turn many atoms in their short paths into ions, producing intense tissue ionization. In contrast to alpha particles, beta particles are electrons of little mass carrying only one negative charge. They penetrate up to several millimetres in soft tissues. Their low mass and low charge mean that only moderate ionization is produced in tissues when beta particles collide with atoms in its path.
Gamma rays and X rays are electromagnetic radiation of similar properties, with gamma rays having higher energy than X rays. Gamma rays usually accompany the formation of alpha or beta particles. Neither gamma rays nor X rays carry a charge, and neither have mass; consequently, they can penetrate tissues easily, creating moderate ionization along their paths. Biological damage is related to the degree of tissue ionization produced by radiation. Thus, a physical dose of alpha particles does not produce the same amount of damage as that produced by the same dose of beta particles, gamma rays, or X rays.
Radiation is either natural or man-made. Natural radiation includes cosmic radiation, terrestrial radiation, radioisotopes inside human bodies, and radon gas. Cosmic radiation consists of charged particles from outer space, and terrestrial radiation of gamma rays from radionuclides in the Earth.
Radioisotopes in human bodies come from the food, water, and air consumed. Cosmic and terrestrial radiation, together with radioisotopes inside human bodies, contribute only one-third of the total natural radiation dose. The remaining two-thirds can be attributed to radon , a radioactive gas released from soil that may reach a high level inside buildings with poor ventilation. Man-made radiation consists of radiation from medical and dental diagnostic procedures, atmospheric tests of atomic bombs, emissions from nuclear plants, certain occupational activities, and some consumer products.
The largest nonoccupational radiation sources are tobacco smoke for smokers and indoor radon gas for the nonsmoking population. Emissions from nuclear power plants contribute only a very small portion of the total yearly radiation received. The low dose reflects the negligible amount of radionuclides released during normal operation, although the amount released can be much higher after a nuclear reactor accident. Not every reactor accident is a disaster, however. The nuclear reactor accident at Chernobyl in the Soviet Union , in , however, was much more devastating, leading to more than 30 deaths and the evacuation of thousands of nearby residents.
Ionizing radiation quickly kills rapidly dividing cells. In general, immature blood cells in bone marrow, cells lining the mucosa of the gastrointestinal tract, and cells in the lower layers of the epidermis and in hair follicles are the most rapidly dividing cells in the body. As a result, radiation leads to the decreased production of blood cells, nausea, vomiting, diarrhea, malabsorption by the intestine, skin burns, and hair loss. Because of its relatively selective lethal effect on rapidly dividing cells, however, ionizing radiation is used in the treatment of certain cancers.
Some cells in the embryo and fetus also divide rapidly, and thus ionizing radiation can cause malformations and even fetal death. Ionizing radiation can also produce mutations by altering the DNA, and it can result in cancer.
X rays and gamma rays irradiate the body uniformly and acutely affect all of the tissues discussed above. At sufficiently high doses, this type of radiation can lead to a condition known as acute radiation syndrome. The most sensitive tissue is the bone marrow, where blood cells are generated.
The next tissue affected is the gastrointestinal tract. If the dose is high, the central nervous system is affected and the person becomes uncoordinated and disoriented and experiences tremors, convulsions, and coma.
At even higher doses, the skin, eyes, and ovaries and testes are affected. Death may follow from 2 to 35 days after exposure. Exposure to radiation can also result in cancers of the bone marrow leading to leukemia , lungs, kidneys, bladder, esophagus, stomach, colon, thyroid, or breasts. Radioisotopes that are absorbed and distributed evenly throughout the body also can result in whole-body irradiation.
Examples are tritium and cesium , both of which release beta particles that can lead to bone marrow toxicities and even, in the case of cesium, to death. The toxicity of tritium is less severe than that of cesium because the beta particles generated by tritium are less energetic and because cesium also releases gamma rays. Unlike tritium and cesium, the isotopes strontium , iodine, and cerium emit beta particles that are not distributed evenly in the body.
Strontium releases only beta particles, while iodine and cerium release both beta particles and gamma rays, but their toxicities are primarily caused by the beta particles.
These radioisotopes produce toxicities in the tissues where they are stored or concentrated. Strontium and cerium chemically resemble calcium and as a result are stored in bone. Therefore, these two radioisotopes produce bone cancer and leukemia, which is a result of the irradiation of bone marrow.
Iodine is concentrated in the thyroid and produces thyroid damage and tumours. There are radioisotopes that emit primarily alpha particles , together with some gamma rays. Because the destructive effect on tissues of alpha particles is far greater than that of gamma rays, the toxicities of these radioisotopes are contributed mainly by the alpha particles.
Because of the limited penetrability of alpha particles, only tissues in the near vicinity of the isotopic molecules are affected. These radioisotopes typically produce tumours at the storage site. Most of the common alpha-particle emitters belong to the uranium series , which consists of radioisotopes that form one after another, via a nuclear decay reaction, and release mainly alpha particles.
The series starts with uranium The nuclear disintegration of uranium forms radium which disintegrates to form radon gas radon Radon decays to form a series of daughter nuclides, most of which are alpha-particle-releasing isotopes, such as polonium The radioisotopes in the uranium series are important because uranium is the starting fuel for many nuclear reactors and because daughter nuclides in this series are commonly found in the environment.
The toxicity of uranium depends on the water solubility of the uranium compound. Water-soluble forms mainly cause kidney injury, while the insoluble forms produce fibrosis and cancer of the lung. Because of its similarity to calcium, radium is stored mainly in the bone, and it produces abnormal changes in the bone marrow, including anemia and leukemia, cancers of the bone, and paranasal sinuses.
The next radioisotope in the uranium series is radon, radon Although radon is radioactive, its toxicity is not due to retention of the gas by the lungs but rather to the series of radioactive daughter nuclides in particulate form.
These particulate daughter nuclides are deposited on the respiratory tract when inhaled, the respiratory tract is irradiated by the alpha particles released, and lung cancer can result. Other radioisotopes do not belong to the uranium series. For example, radium, which is deposited mainly on bone surfaces, has been used in Europe to treat ankylosing spondylitis. Because of its short half-life 3. Its major toxicity is the production of bone cancer.
Like uranium, plutonium, which is used in some nuclear reactors and in nuclear bombs, primarily releases alpha particles.
Although there are no human data, animal studies indicate that the toxicity of plutonium is similar to that of insoluble uranium in causing fibrosis and cancer of the lung. Nonionizing radiation includes ultraviolet light, infrared radiation, microwaves, and radio frequencies, all of which are electromagnetic waves. The toxicity of radio frequencies is rather low. Rats are considered a non-vomiting species also called nonemetic Hatcher Rats do not vomit in response to cues that cause vomiting in other animals, like emetic drugs, poison, motion-sickness, and radiation e.
Rats also don't belch and experience hardly any reflux heartburn. Rats cannot vomit, but they do regurgitate occasionally. Regurgitation is different from vomiting. Vomiting is the forceful expulsion of stomach contents from the mouth. 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.
In addition, 3 rats lack the necessary neural connections to coordinate the muscles involved in vomiting. The diaphragm is has two muscles: The esophagus passes through the crural sling, so when the crural diaphragm contracts the esophagus is pinched closed. During the expulsive phase of vomiting in humans, the activity of these two diaphragm muscles diverges. The costal section contracts, putting pressure on the stomach, while the crural section relaxes, allowing stomach contents to pass through the esophagus reviewed in Pickering and Jones Rats, however, do not dissociate the activity of these two parts of their diaphragm: Instead, both muscles contract or relax together Pollard et al.
The rat's inability to separately and selectively control its two diaphragmatic muscles therefore plays an important role in its inability to vomit: In humans, the esophageal sphincter is opened during vomiting with the help of the longitudinal muscle of the esophagus Lang and Sarna This allows the expulsion of stomach contents during vomiting. Rats, however, have only a thin, weak longitudinal muscle which is unstriated where it joins the stomach.
It is too weak to wrench open the sphincter and permit the evacuation of stomach contents Steinnon Animal species that vomit have a "vomiting center" in the brainstem, consisting of several interconnected nuclei that coordinate all the many muscles involved in vomiting see Borison and Wang Animals that don't vomit, like rats and rabbits, have the brainstem nuclei and the muscle systems used in vomiting, but they don't have the complex connections between the nuclei or between the brainstem and the viscera that are required for such a coordinated behavior King As of yet, no empirical research has been done on whether the inability to vomit benefits the rat in some way.