Lymph is the fluid that flows through the lymphatic system, a system composed of lymph vessels and intervening lymph nodes whose function, like the venous system, is to return fluid from the tissues to the central circulation. Interstitial fluid - the fluid, between the cells in all body tissues - enters the lymph capillaries; this lymphatic fluid is transported via progressively larger lymphatic vessels through lymph nodes, where substances are removed by tissue lymphocytes and circulating lymphocytes are added to the fluid, before emptying into the right or the left subclavian vein, where it mixes with central venous blood. Since the lymph is derived from the interstitial fluid, its composition continually changes as the blood and the surrounding cells continually exchange substances with the interstitial fluid, it is similar to blood plasma, the fluid component of blood. Lymph returns excess interstitial fluid to the bloodstream. Lymph transports fats from the digestive system to the blood via chylomicrons.
Bacteria may be transported to lymph nodes, where they are destroyed. Metastatic cancer cells can be transported via lymph; the word lymph is derived from the name of the ancient Roman deity of Lympha. Lymph has a not identical to that of blood plasma. Lymph that leaves a lymph node is richer in lymphocytes; the lymph formed in the human digestive system called chyle is rich in triglycerides, looks milky white because of its lipid content. Blood supplies nutrients and important metabolites to the cells of a tissue and collects back the waste products they produce, which requires exchange of respective constituents between the blood and tissue cells; this exchange is not direct, but instead occurs through an intermediary called interstitial fluid, which occupies the spaces between cells. As the blood and the surrounding cells continually add and remove substances from the interstitial fluid, its composition continually changes. Water and solutes can pass between the interstitial fluid and blood via diffusion across gaps in capillary walls called intercellular clefts.
Interstitial fluid forms at the arterial end of capillaries because of the higher pressure of blood compared to veins, most of it returns to its venous ends and venules. Thus, lymph when formed is a watery clear liquid with the same composition as the interstitial fluid. However, as it flows through the lymph nodes it comes in contact with blood, tends to accumulate more cells and proteins. Lymph returns excess interstitial fluid to the bloodstream. Lymph may bring them to lymph nodes, where they are destroyed. Metastatic cancer cells can be transported via lymph. Lymph transports fats from the digestive system to the blood via chylomicrons. Tubular vessels transport lymph back to the blood replacing the volume lost during the formation of the interstitial fluid; these channels are the lymphatic channels, or lymphatics. Unlike the cardiovascular system, the lymphatic system is not closed and has no central pump, or lymph heart. Lymph transport, therefore, is sporadic. Despite low pressure, lymph movement occurs due to peristalsis and compression during contraction of adjacent skeletal muscle and arterial pulsation.
Lymph that enters the lymph vessels from the interstitial spaces does not flow backwards along the vessels because of the presence of valves. If excessive hydrostatic pressure develops within the lymph vessels, some fluid can leak back into the interstitial spaces and contribute to formation of oedema. Flow of the lymph in the thoracic duct in an average resting person approximates 100ml per hour. Accompanied by another ~25ml per hour in other lymph vessels, total lymph flow in the body is about 4 to 5 liters per day; this can be elevated several fold while exercising. It is estimated. In 1907 the zoologist Ross Granville Harrison demonstrated the growth of frog nerve cell processes in a medium of clotted lymph, it is made up of lymph vessels. In 1913, E. Steinhardt, C. Israeli, R. A. Lambert grew vaccinia virus in fragments of tissue culture from guinea pig corneal grown in lymph
The placenta is a temporary organ that connects the developing fetus via the umbilical cord to the uterine wall to allow nutrient uptake, thermo-regulation, waste elimination, gas exchange via the mother's blood supply. Placentas are a defining characteristic of placental mammals, but are found in marsupials and some non-mammals with varying levels of development; the placenta functions as a fetomaternal organ with two components: the fetal placenta, which develops from the same blastocyst that forms the fetus, the maternal placenta, which develops from the maternal uterine tissue. It metabolizes a number of substances and can release metabolic products into maternal or fetal circulations; the placenta is expelled from the body upon birth of the fetus. The word placenta comes from the Latin word for a type of cake, from Greek πλακόεντα/πλακοῦντα plakóenta/plakoúnta, accusative of πλακόεις/πλακούς plakóeis/plakoús, "flat, slab-like", in reference to its round, flat appearance in humans; the classical plural is placentae, but the form placentas is common in modern English and has the wider currency at present.
Placental mammals, such as humans, have a chorioallantoic placenta that forms from the chorion and allantois. In humans, the placenta averages 22 cm in length and 2–2.5 cm in thickness, with the center being the thickest, the edges being the thinnest. It weighs 500 grams, it has crimson color. It connects to the fetus by an umbilical cord of 55–60 cm in length, which contains two umbilical arteries and one umbilical vein; the umbilical cord inserts into the chorionic plate. Vessels branch out over the surface of the placenta and further divide to form a network covered by a thin layer of cells; this results in the formation of villous tree structures. On the maternal side, these villous tree structures are grouped into lobules called cotyledons. In humans, the placenta has a disc shape, but size varies vastly between different mammalian species; the placenta takes a form in which it comprises several distinct parts connected by blood vessels. The parts, called lobes, may number two, four, or more.
Such placentas are described as bilobed/bilobular/bipartite, trilobed/trilobular/tripartite, so on. If there is a discernible main lobe and auxiliary lobe, the latter is called a succenturiate placenta. Sometimes the blood vessels connecting the lobes get in the way of fetal presentation during labor, called vasa previa. About 20,000 protein coding genes are expressed in human cells and 70% of these genes are expressed in the normal mature placenta; some 350 of these genes are more expressed in the placenta and fewer than 100 genes are placenta specific. The corresponding specific proteins are expressed in trophoblasts and have functions related to female pregnancy. Examples of proteins with elevated expression in placenta compared to other organs and tissues are PEG10 and the cancer testis antigen PAGE4 expressed in cytotrophoblasts, CSH1and KISS1 expressed in syncytiotrophoblasts, PAPPA2 and PRG2 expressed in extravillous trophoblasts; the placenta begins to develop upon implantation of the blastocyst into the maternal endometrium.
The outer layer of the blastocyst becomes the trophoblast, which forms the outer layer of the placenta. This outer layer is divided into two further layers: the underlying cytotrophoblast layer and the overlying syncytiotrophoblast layer; the syncytiotrophoblast is a multinucleated continuous cell layer that covers the surface of the placenta. It forms as a result of differentiation and fusion of the underlying cytotrophoblast cells, a process that continues throughout placental development; the syncytiotrophoblast, thereby contributes to the barrier function of the placenta. The placenta grows throughout pregnancy. Development of the maternal blood supply to the placenta is complete by the end of the first trimester of pregnancy week 14. In preparation for implantation of the blastocyst, the endometrium undergoes decidualization. Spiral arteries in the decidua are remodeled so that they become less convoluted and their diameter is increased; the increased diameter and straighter flow path both act to increase maternal blood flow to the placenta.
There is high pressure as the maternal blood fills intervillous space through these spiral arteries which bathe the fetal villi in blood, allowing an exchange of gases to take place. In humans and other hemochorial placentals, the maternal blood comes into direct contact with the fetal chorion, though no fluid is exchanged; as the pressure decreases between pulses, the deoxygenated blood flows back through the endometrial veins. Maternal blood flow is 600–700 ml/min at term; this begins at day 5 - day 12 Deoxygenated fetal blood passes through umbilical arteries to the placenta. At the junction of umbilical cord and placenta, the umbilical arteries branch radially to form chorionic arteries. Chorionic arteries, in turn, branch into cotyledon arteries. In the villi, these vessels branch to form an extensive arterio-capillary-venous system, bringing the fetal blood close to the maternal blood. Endothelin and prostanoids cause vasoconstriction in placental arteries, while nitric oxide causes vasodilation.
On the other hand, there is no neural vascular regulation, catecholamines have only little effect. The fetoplacental circulation is vulnerable to persistent hypoxia or intermittent hypoxia and
The reproductive system or genital system is a system of sex organs within an organism which work together for the purpose of sexual reproduction. Many non-living substances such as fluids and pheromones are important accessories to the reproductive system. Unlike most organ systems, the sexes of differentiated species have significant differences; these differences allow for a combination of genetic material between two individuals, which allows for the possibility of greater genetic fitness of the offspring. In mammals, the major organs of the reproductive system include the external genitalia as well as a number of internal organs, including the gamete-producing gonads. Diseases of the human reproductive system are common and widespread communicable sexually transmitted diseases. Most other vertebrate animals have similar reproductive systems consisting of gonads and openings. However, there is a great diversity of physical adaptations as well as reproductive strategies in every group of vertebrates.
Vertebrate animals all share key elements of their reproductive systems. They all have gamete-producing gonads. In females, these gonads are connected by oviducts to an opening to the outside of the body the cloaca, but sometimes to a unique pore such as a vagina or intromittent organ; the human reproductive system involves internal fertilization by sexual intercourse. During this process, the male inserts his erect penis into the female's vagina and ejaculates semen, which contains sperm; the sperm travels through the vagina and cervix into the uterus or fallopian tubes for fertilization of the ovum. Upon successful fertilization and implantation, gestation of the fetus occurs within the female's uterus for nine months, this process is known as pregnancy in humans. Gestation ends with birth, the process of birth is known as labor. Labor consists of the muscles of the uterus contracting, the cervix dilating, the baby passing out the vagina. Human's babies and children are nearly helpless and require high levels of parental care for many years.
One important type of parental care is the use of the mammary glands in the female breasts to nurse the baby. The female reproductive system has two functions: The first is to produce egg cells, the second is to protect and nourish the offspring until birth; the male reproductive system has one function, it is to produce and deposit sperm. Humans have a high level of sexual differentiation. In addition to differences in nearly every reproductive organ, numerous differences occur in secondary sexual characteristics; the male reproductive system is a series of organs located outside of the body and around the pelvic region of a male that contribute towards the reproduction process. The primary direct function of the male reproductive system is to provide the male sperm for fertilization of the ovum; the major reproductive organs of the male can be grouped into three categories. The first category is sperm storage. Production takes place in the testes which are housed in the temperature regulating scrotum, immature sperm travel to the epididymis for development and storage.
The second category are the ejaculatory fluid producing glands which include the seminal vesicles and the vas deferens. The final category are those used for copulation, deposition of the spermatozoa within the male, these include the penis, vas deferens, Cowper's gland. Major secondary sexual characteristics includes: larger, more muscular stature, deepened voice and body hair, broad shoulders, development of an adam's apple. An important sexual hormone of males is androgen, testosterone; the testes release a hormone. This hormone is responsible for the development of physical characteristics in men such as facial hair and a deep voice; the human female reproductive system is a series of organs located inside of the body and around the pelvic region of a female that contribute towards the reproductive process. The human female reproductive system contains three main parts: the vulva, which leads to the vagina, the vaginal opening, to the uterus; the breasts are involved during the parenting stage of reproduction, but in most classifications they are not considered to be part of the female reproductive system.
The vagina meets the outside at the vulva, which includes the labia and urethra. The vagina is attached to the uterus through the cervix, while the uterus is attached to the ovaries via the fallopian tubes; each ovary contains hundreds of ova. Every 28 days, the pituitary gland releases a hormone that stimulates some of the ova to develop and grow. One ovum is released and it passes through the fallopian tube into the uterus. Hormones produced by the ovaries prepare the uterus to receive the ovum, it awaits the sperm for fertilization to occur. When this does not occur i.e. no sperm for fertilization, the lining of the uterus, called the endometrium, unfertilized ova are shed each cycle through the process of menstruation. If the ovum is fertilized by sperm, it attaches to the endometrium and the fetus develops. Most mammal reproductive systems are similar, there are some notable differences between the non-human mammals and humans. For instance, most male mammals have a penis, stored internally until erect, most have a penis bone or baculum.
Red blood cell
Red blood cells known as RBCs, red cells, red blood corpuscles, erythroid cells or erythrocytes, are the most common type of blood cell and the vertebrate's principal means of delivering oxygen to the body tissues—via blood flow through the circulatory system. RBCs take up oxygen in the lungs, or gills of fish, release it into tissues while squeezing through the body's capillaries; the cytoplasm of erythrocytes is rich in hemoglobin, an iron-containing biomolecule that can bind oxygen and is responsible for the red color of the cells and the blood. The cell membrane is composed of proteins and lipids, this structure provides properties essential for physiological cell function such as deformability and stability while traversing the circulatory system and the capillary network. In humans, mature red blood cells are oval biconcave disks, they lack most organelles, in order to accommodate maximum space for hemoglobin. 2.4 million new erythrocytes are produced per second in human adults. The cells develop in the bone marrow and circulate for about 100–120 days in the body before their components are recycled by macrophages.
Each circulation takes about 60 seconds. A quarter of the cells in the human body are red blood cells. Nearly half of the blood's volume is red blood cells. Packed red blood cells are red blood cells that have been donated and stored in a blood bank for blood transfusion. All vertebrates, including all mammals and humans, have red blood cells. Red blood cells are cells present in blood; the only known vertebrates without red blood cells are the crocodile icefish. While they no longer use hemoglobin, remnants of hemoglobin genes can be found in their genome. Vertebrate red blood cells consist of hemoglobin, a complex metalloprotein containing heme groups whose iron atoms temporarily bind to oxygen molecules in the lungs or gills and release them throughout the body. Oxygen can diffuse through the red blood cell's cell membrane. Hemoglobin in the red blood cells carries some of the waste product carbon dioxide back from the tissues. Myoglobin, a compound related to hemoglobin, acts to store oxygen in muscle cells.
The color of red blood cells is due to the heme group of hemoglobin. The blood plasma alone is straw-colored, but the red blood cells change color depending on the state of the hemoglobin: when combined with oxygen the resulting oxyhemoglobin is scarlet, when oxygen has been released the resulting deoxyhemoglobin is of a dark red burgundy color. However, blood can appear bluish when seen through skin. Pulse oximetry takes advantage of the hemoglobin color change to directly measure the arterial blood oxygen saturation using colorimetric techniques. Hemoglobin has a high affinity for carbon monoxide, forming carboxyhemoglobin, a bright red in color. Flushed, confused patients with a saturation reading of 100% on pulse oximetry are sometimes found to be suffering from carbon monoxide poisoning. Having oxygen-carrying proteins inside specialized cells was an important step in the evolution of vertebrates as it allows for less viscous blood, higher concentrations of oxygen, better diffusion of oxygen from the blood to the tissues.
The size of red blood cells varies among vertebrate species. The red blood cells of mammals are shaped as biconcave disks: flattened and depressed in the center, with a dumbbell-shaped cross section, a torus-shaped rim on the edge of the disk; this shape allows for a high surface-area-to-volume ratio to facilitate diffusion of gases. However, there are some exceptions concerning shape in the artiodactyl order, which displays a wide variety of bizarre red blood cell morphologies: small and ovaloid cells in llamas and camels, tiny spherical cells in mouse deer, cells which assume fusiform, lanceolate and irregularly polygonal and other angular forms in red deer and wapiti. Members of this order have evolved a mode of red blood cell development different from the mammalian norm. Overall, mammalian red blood cells are remarkably flexible and deformable so as to squeeze through tiny capillaries, as well as to maximize their apposing surface by assuming a cigar shape, where they efficiently release their oxygen load.
Red blood cells in mammals are unique amongst vertebrates. Red blood cells of mammals cells have nuclei during early phases of erythropoiesis, but extrude them during development as they mature; the red blood cells without nuclei, called reticulocytes, subsequently lose all other cellular organelles such as their mitochondria, Golgi apparatus and endoplasmic reticulum. The spleen acts as a reservoir of red blood cells. In some other mammals such as dogs and horses, the spl
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the outer surfaces of organs and blood vessels throughout the body, as well as the inner surfaces of cavities in many internal organs. An example is the outermost layer of the skin. There are three principal shapes of epithelial cell: squamous and cuboidal; these can be arranged in a single layer of cells as simple epithelium, either squamous, columnar, or cuboidal, or in layers of two or more cells deep as stratified, either squamous, columnar or cuboidal. In some tissues, a layer of columnar cells may appear to be stratified due to the placement of the nuclei; this sort of tissue is called pseudostratified. All glands are made up of epithelial cells. Functions of epithelial cells include secretion, selective absorption, transcellular transport, sensing. Epithelial layers contain no blood vessels, so they must receive nourishment via diffusion of substances from the underlying connective tissue, through the basement membrane.
Cell junctions are well employed in epithelial tissues. In general, epithelial tissues are classified by the number of their layers and by the shape and function of the cells; the three principal shapes associated with epithelial cells are—squamous and columnar. Squamous epithelium has cells; this is found as the lining of the mouth, the blood vessels and in the alveoli of the lungs. Cuboidal epithelium has cells whose height and width are the same. Columnar epithelium has cells taller. By layer, epithelium is classed as either simple epithelium, only one cell thick or stratified epithelium having two or more cells in thickness or multi-layered – as stratified squamous epithelium, stratified cuboidal epithelium, stratified columnar epithelium, both types of layering can be made up of any of the cell shapes. However, when taller simple columnar epithelial cells are viewed in cross section showing several nuclei appearing at different heights, they can be confused with stratified epithelia; this kind of epithelium is therefore described as pseudostratified columnar epithelium.
Transitional epithelium has cells that can change from squamous to cuboidal, depending on the amount of tension on the epithelium. Simple epithelium is a single layer of cells with every cell in direct contact with the basement membrane that separates it from the underlying connective tissue. In general, it is found where filtration occur; the thinness of the epithelial barrier facilitates these processes. In general, simple epithelial tissues are classified by the shape of their cells; the four major classes of simple epithelium are: simple squamous. Simple squamous. Simple cuboidal: these cells may have secretory, absorptive, or excretory functions. Examples include small collecting ducts of kidney and salivary gland. Simple columnar. Non-ciliated epithelium can possess microvilli; some tissues are referred to as simple glandular columnar epithelium. These secrete mucus and are found in stomach and rectum. Pseudostratified columnar epithelium; the ciliated type is called respiratory epithelium as it is exclusively confined to the larger respiratory airways of the nasal cavity and bronchi.
Stratified epithelium differs from simple epithelium. It is therefore found where body linings have to withstand mechanical or chemical insult such that layers can be abraded and lost without exposing subepithelial layers. Cells flatten as the layers become more apical, though in their most basal layers the cells can be squamous, cuboidal or columnar. Stratified epithelia can have the following specializations: The basic cell types are squamous and columnar classed by their shape. Cells of epithelial tissue are scutoid shaped packed and form a continuous sheet, they have no intercellular spaces. All epithelia is separated from underlying tissues by an extracellular fibrous basement membrane; the lining of the mouth, lung alveoli and kidney tubules are all made of epithelial tissue. The lining of the blood and lymphatic vessels are of a specialised form of epithelium called endothelium. Epithelium lines both the outside and the inside cavities and lumina of bodies; the outermost layer of human skin is composed of dead stratified squamous, keratinized epithelial cells.
Tissues that line the inside of the mouth, the esophagus, the vagina, part of the rectum are composed of nonkeratinized stratified squamous epithelium. Other surfaces that separate body cavities from the outside environment are lined by simple squamous, columnar, or pseudostratified epithelial cells. Other epithelial cells line the insides of the lungs, the gastrointestinal tract, the reproductive and urinary tracts, make up the exocrine and endocrine glands; the outer surface of the cornea is covered with fast-growing regenerated epithelial cells. A specialised form of epithelium – endothelium forms the inner lining of blood vessels and the heart, is known as vascular endotheliu
A tooth is a hard, calcified structure found in the jaws of many vertebrates and used to break down food. Some animals carnivores use teeth for hunting or for defensive purposes; the roots of teeth are covered by gums. Teeth hardness; the cellular tissues that become teeth originate from the embryonic germ layer, the ectoderm. The general structure of teeth is similar across the vertebrates, although there is considerable variation in their form and position; the teeth of mammals have deep roots, this pattern is found in some fish, in crocodilians. In most teleost fish, the teeth are attached to the outer surface of the bone, while in lizards they are attached to the inner surface of the jaw by one side. In cartilaginous fish, such as sharks, the teeth are attached by tough ligaments to the hoops of cartilage that form the jaw; some animals develop only one set of teeth. Sharks, for example, grow a new set of teeth. Rodent incisors grow and wear away continually through gnawing, which helps maintain constant length.
The industry of the beaver is due in part to this qualification. Many rodents such as voles and guinea pigs, but not mice, as well as leporidae like rabbits, have continuously growing molars in addition to incisors. Teeth are not always attached to the jaw. In many reptiles and fish, teeth are attached to the palate or to the floor of the mouth, forming additional rows inside those on the jaws proper; some teleosts have teeth in the pharynx. While not true teeth in the usual sense, the dermal denticles of sharks are identical in structure and are to have the same evolutionary origin. Indeed, teeth appear to have first evolved in sharks, are not found in the more primitive jawless fish – while lampreys do have tooth-like structures on the tongue, these are in fact, composed of keratin, not of dentine or enamel, bear no relationship to true teeth. Though "modern" teeth-like structures with dentine and enamel have been found in late conodonts, they are now supposed to have evolved independently of vertebrates' teeth.
Living amphibians have small teeth, or none at all, since they feed only on soft foods. In reptiles, teeth are simple and conical in shape, although there is some variation between species, most notably the venom-injecting fangs of snakes; the pattern of incisors, canines and molars is found only in mammals, to varying extents, in their evolutionary ancestors. The numbers of these types of teeth vary between species; the genes governing tooth development in mammals are homologous to those involved in the development of fish scales. Study of a tooth plate of a fossil of the extinct fish Romundina stellina showed that the teeth and scales were made of the same tissues found in mammal teeth, lending support to the theory that teeth evolved as a modification of scales. Teeth are among the most distinctive features of mammal species. Paleontologists use teeth to determine their relationships; the shape of the animal's teeth are related to its diet. For example, plant matter is hard to digest, so herbivores have many molars for chewing and grinding.
Carnivores, on the other hand, have canine teeth to tear meat. Mammals, in general, are diphyodont. In humans, the first set starts to appear at about six months of age, although some babies are born with one or more visible teeth, known as neonatal teeth. Normal tooth eruption at about six months can be painful. Kangaroos and manatees are unusual among mammals because they are polyphyodonts. In Aardvarks, teeth lack enamel and have many pulp tubules, hence the name of the order Tubulidentata. In dogs, the teeth are less than humans to form dental cavities because of the high pH of dog saliva, which prevents enamel from demineralizing. Sometimes called cuspids, these teeth are shaped like points and are used for tearing and grasping food Like human teeth, whale teeth have polyp-like protrusions located on the root surface of the tooth; these polyps are made of cementum in both species, but in human teeth, the protrusions are located on the outside of the root, while in whales the nodule is located on the inside of the pulp chamber.
While the roots of human teeth are made of cementum on the outer surface, whales have cementum on the entire surface of the tooth with a small layer of enamel at the tip. This small enamel layer is only seen in older whales where the cementum has been worn away to show the underlying enamel; the toothed whale is a suborder of the cetaceans characterized by having teeth. The teeth differ among the species, they may be numerous, with some dolphins bearing over 100 teeth in their jaws. On the other hand, the narwhals have a giant unicorn-like tusk, a tooth containing millions of sensory pathways and used for sensing during feeding and mating, it is the most neurologically complex tooth known. Beaked whales are toothless, with only bizarre teeth found in males; these teeth may be used for feeding but for demonstrating aggression and showmanship. In humans there are 20 primary teeth, 28 to 32 of what's known as permanent teeth, in addition to other four being third molars or wisdom teeth, each of which may or may not g
A nose is a protuberance in vertebrates that houses the nostrils, or nares, which receive and expel air for respiration alongside the mouth. Behind the nose are the olfactory mucosa and the sinuses. Behind the nasal cavity, air next passes through the pharynx, shared with the digestive system, into the rest of the respiratory system. In humans, the nose is located centrally on the face and serves as an alternative respiratory passage during suckling for infants. On most other mammals, it is located on the upper tip of the snout. Acting as the first interface between the external environment and an animal's delicate internal lungs, a nose conditions incoming air, both as a function of thermal regulation and filtration during respiration, as well as enabling the sensory perception of smell. Hair inside nostrils filter incoming air, as a first line of defense against dust particles and other potential obstructions that would otherwise inhibit respiration, as a kind of filter against airborne illness.
In addition to acting as a filter, mucus produced within the nose supplements the body's effort to maintain temperature, as well as contributes moisture to integral components of the respiratory system. Capillary structures of the nose warm and humidify air entering the body. During exhalation, the capillaries aid recovery of some moisture as a function of thermal regulation, again; the wet nose of dogs is useful for the perception of direction. The sensitive cold receptors in the skin detect the place where the nose is cooled the most and this is the direction a particular smell that the animal just picked up comes from. In amphibians and lungfish, the nostrils open into small sacs that, in turn, open into the forward roof of the mouth through the choanae; these sacs contain a small amount of olfactory epithelium, which, in the case of caecilians lines a number of neighbouring tentacles. Despite the general similarity in structure to those of amphibians, the nostrils of lungfish are not used in respiration, since these animals breathe through their mouths.
Amphibians have a vomeronasal organ, lined by olfactory epithelium, unlike those of amniotes, this is a simple sac that, except in salamanders, has little connection with the rest of the nasal system. In reptiles, the nasal chamber is larger, with the choanae located much further back in the roof of the mouth. In crocodilians, the chamber is exceptionally long, helping the animal to breathe while submerged; the reptilian nasal chamber is divided into three parts: an anterior vestibule, the main olfactory chamber, a posterior nasopharynx. The olfactory chamber is lined by olfactory epithelium on its upper surface and possesses a number of turbinates to increase the sensory area; the vomeronasal organ is well-developed in lizards and snakes, in which it no longer connects with the nasal cavity, opening directly into the roof of the mouth. It is smaller in turtles, in which it retains its original nasal connection, is absent in adult crocodilians. Birds have a similar nose with the nostrils located at the upper rear part of the beak.
Since they have a poor sense of smell, the olfactory chamber is small, although it does contain three turbinates, which sometimes have a complex structure similar to that of mammals. In many birds, including doves and fowls, the nostrils are covered by a horny protective shield; the vomeronasal organ of birds is either under-developed or altogether absent, depending on the species. The nasal cavities in mammals are both fused into one. Among most species they are exceptionally large occupying up to half the length of the skull. In some groups, including primates and cetaceans, the nose has been secondarily reduced, these animals have a poor sense of smell; the nasal cavity of mammals has been enlarged, in part, by the development of a palate cutting off the entire upper surface of the original oral cavity, which becomes part of the nose, leaving the palate as the new roof of the mouth. The enlarged nasal cavity contains complex turbinates forming coiled scroll-like shapes that help to warm the air before it reaches the lungs.
The cavity extends into neighbouring skull bones, forming additional air cavities known as paranasal sinuses. In cetaceans, the nose has been reduced to the nostrils, which have migrated to the top of the head, producing a more streamlined body shape and the ability to breathe while submerged. Conversely, the elephant's nose has elaborated into a long, manipulative organ called the trunk; the vomeronasal organ of mammals is similar to that of reptiles. In most species, it is located in the floor of the nasal cavity, opens into the mouth via two nasopalatine ducts running through the palate, but it opens directly into the nose in many rodents, it is, lost in bats, in many primates, including humans. Fish have a good sense of smell. Unlike that of tetrapods, the nose has any role in respiration. Instead, it consists of a pair of small pouches located behind the nostrils at the front or sides of the head. In many cases, each of the nostrils is divided into two by a fold of skin, allowing water to flow into the nose through one side and out through the other.
The pouches are lined by olfactory epithelium, include a series of internal folds to increase the surface area. In some teleosts, the pouches branch off into additional sinus-like cavities, while in coelacanths, they form a series of tubes. Unlike tetrapods, the nasal epithelium of fishes does not include