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The principal disadvantage of the single-circuit system is that gill capillaries offer so much resistance to blood flow that blood pressure to body tissues is greatly reduced. With the evolution of lung breathing and the elimination of gills between the heart and aorta, vertebrates developed a high...

The principal disadvantage of the single-circuit system is that gill capillaries offer so much resistance to blood flow that blood pressure to body tissues is greatly reduced. With the evolution of lung breathing and the elimination of gills between the heart and aorta, vertebrates developed a high pressure double circulation: a systemic circuit that provides oxygenated blood to the capillary beds of the body organs; and a pulmonary circuit that serves the lungs. The beginning of this major evolutionary change probably resembled the condition seen in lungfishes and amphibians. In modern amphibians, the atrium is completely separated by a partition into two atria. The ventricle is undivided, but venous and arterial bloods remain mostly separate by the arrangement of vessels leaving the heart and differential blood pressures in these vessels. Separation of the ventricles is nearly complete in some reptiles (crocodilians), and is completely separate in birds and mammals. Mammalian heart The four-chambered mammalian heart is a muscular organ located in the thorax and covered by a tough, fibrous sac the pericardium. Blood returning from the lungs collects in the left atrium, passes into the left ventricle, and is pumped into the body's (systemic) circulation. Blood returning from the body flows into the right atrium, and passes into the right ventricle, which pumps it into the lungs. Backflow of blood is prevented by two sets of valves that open and close passively in response to pressure differences between the heart chambers. The left atrioventricular (bicuspid) and the right atrioventricular (tricuspid) valves separate the cavities of the atrium and ventricle in each half of the heart. Where the great arteries, the pulmonary from the right ventricle, and the aorta from the left ventricle, leave the heart, semi lunar valves prevent back flow into the ventricles. Contraction is called systole, and relaxation diastole. When the atria contract (systole), the ventricles relax (diastole), that is, ventricular systole is accompanied by atrial diastole. The rate of the heartbeat depends on age, sex, and especially exercise. Exercise may increase cardiac output (volume of blood forced from either ventricle each minute) more than fivefold. Both heart rate and stroke volume (volume of blood forced from either ventricle per beat) increase. Heart rates among vertebrates vary with the general level of metabolism and body size. Ectothermic codfish have a heart rate of approximately 30 beats per minute: endothermic rabbits of about the same weight, 200 beats per minute. Small animals have higher heart rates than larger animals (elephant, 25 beats per minute, human, 70 per minute, cat, 125 per minute, mouse, 100 per minute, and 4g shrew, the smallest mammal, the heart rate approaches 800 beats per minute). Excitation and control of the heart the vertebrate heart is a muscular pump composed of cardiac muscle. Cardiac muscle resembles skeletal muscle (both are types of striated muscle), but cardiac cells are branched. Unlike skeletal muscle, vertebrate cardiac muscle does not depend on nerve activity to initiate a contraction. Instead, regular contractions are established by specialised cardiac muscle cells, called pacemaker cells. In a tetrapod, the pacemaker is in the sinoatrial (SA). Electrical activity initiated in the pacemaker spreads over the muscle of the two atria and then, after a slight delay, to the secondary pacemaker, the atrioventricular (AV) node, at the top of the ventricles. The control (cardiac) centre in the brain is located in the medulla and connects to the heart by two sets of nerves. Impulses sent along the parasympathetic vagus nerves, apply a braking action to the heart rate, and impulses sent along the sympathetic nerves, speed it up. Both sets of nerves terminate in the SA node. Arteries All vessels leaving the heart are called arteries, whether carrying oxygenated blood (aorta) or deoxygenated blood (pulmonary artery). To withstand high, pounding pressures, the largest arteries closest to the heart have thick layers of elastic fibres, very little smooth muscle (elastic arteries), and tough inelastic connective fibres. The elasticity of these arteries allows them to yield to the surge of blood leaving the heart during ventricular systole and then to compress the fluid column during ventricular diastole. This elasticity maintains the high blood pressure created by the heartbeat. Arteries further away from the heart possess smoother muscle and less elastic fibres, and can increase or decrease their diameter, regulating pressure and flow oscillations that occur with each heartbeat, before the blood reaches body organs. As arteries branch and narrow into arterioles, the walls are composed primarily of only one or two layers of smooth muscle. Contraction of this muscle narrows the arterioles and reduces the flow of blood to body organs, diverting it to where it is most needed. Capillaries Capillaries are present in enormous numbers, forming extensive networks in nearly all tissues, and they are extremely narrow, averaging in mammals at about 8um in diameter, only slightly wider than the red blood cells passing through them. A single layer of thin endothelial cells, held together by a delicate basement membrane, forms their walls. Because larger molecules such as plasma proteins cannot pass through the capillary wall, an almost protein-free filtrate is forced outwards. This fluid movement irrigates the interstitial space, providing tissue cells with oxygen, glucose, amino acids, and other nutrients, and carrying away metabolic waste. Excess fluid in the interstitial space is collected and removed by lymph capillaries of the lymphatic system and eventually, this fluid, called lymph, is returned to the circulatory system via larger lymph vessels. Veins Venules and veins, into which capillary blood drains for its return journey to the heart, are thinner walled, less elastic, and of considerably larger diameter than corresponding arteries and arterioles. Blood pressure in the venous system is low, so venous return is accomplished by valves in the veins, the body muscles surrounding the veins, and the rhythmical action of the lungs. Veins that lift blood from the extremities to the heart contain valves, which divide the long column of blood into segments. When skeletal muscles contract, as in even slight activity, veins are squeezed, and the blood within them moves towards the heart, because valves within the veins keep blood from slipping back. Lymphatic system The lymphatic system of vertebrates is an extensive network of thin-walled vessels that arise as blind-ended lymph capillaries in most tissues of the body. These unite to form a treelike structure of increasingly larger lymph vessels, which finally drain into veins in the lower neck. A principal function of the lymphatic system is to return to the blood the excess fluid (lymph), filtered across capillary walls into interstitial spaces. This fluid (lymph) is similar to plasma but has a lower concentration of protein. Large molecules, especially fats absorbed from the gut, also reach the circulatory system by way of the lymphatic system. The lymphatic system also plays a central role in the body\'s defences. Located at intervals along the lymph vessels are lymph nodes that have several defence-related functions, e.g. removing bacteria and producing white blood cells. Respiration Energy from food is released by oxidative processes; oxygen for this purpose is taken into the body across a respiratory surface. Physiologists distinguish two separate but interrelated respiratory processes: cellular respiration, the oxidative processes that occurs within cells, and external respiration, the exchange of oxygen and carbon dioxide between the organism and its environment. In this section, we describe external respiration and the transport of gases from respiratory surfaces to body tissues. In single-celled organisms, oxygen is acquired and carbon dioxide is liberated by direct diffusion across surface membranes. As animals became larger and evolved, a waterproof covering, specialised devices such as lungs and gills, evolved to increase the effective surface for gas exchange. In addition, because gases diffuse so slowly through living tissue, a circulatory system was necessary to distribute gases to and from deep tissues. With evolution of special oxygen-transporting blood proteins such as haemoglobin, which seems to have evolved in conjunction with the circulatory system, the oxygen-carrying capacity of blood increased greatly. Another strategy employed by living animals is to greatly increase the surface of the body relative to its mass. Many multicellular animals can supply part or all of oxygen requirements by direct diffusion. Flatworms are an example of this strategy. Epithelial respiration frequently supplements gill or lung breathing in larger animals such as amphibians and fishes (e.g. an eel exchanges 60% of its oxygen and carbon dioxide through its highly vascular skin and during hibernation, and frogs and turtles exchange all their respiratory gases through their skin while submerged in ponds or springs). Problems of aquatic and aerial breathing How an animal respires is determined largely by the nature of its environment. The two great arenas of animal evolution, water and land, are vastly different in their physical characteristics. The most obvious difference is that air contains far more oxygen, at least 20 times more than water. The density of water is about 800 times greater and the viscosity, 50 times greater, than that of air. Furthermore, gas molecules diffuse 10,000 times more rapidly in air than in water. These differences mean that aquatic animals must have evolved very efficient ways of removing oxygen from water. Yet, even the most advanced fishes with highly efficient gills and pumping mechanisms, may use as much as 20% of their energy just extracting oxygen from water. By comparison, the cost for mammals to breathe is only 1% to 2% of their resting metabolism. Respiratory surfaces must be thin and always kept wet with a fine film of fluid to allow the diffusion of gases between the environment and the underlying circulation. To keep res piratory membranes moist and protected from injury, air breathers have, in general, developed invaginations of the body surface and then added pumping mechanisms to move air in and out of the body. In general, evaginations of the body surface, such as gills, are most suitable for aquatic respiration; invaginations, such as lungs and tracheae, are best for air breathing. Tracheal systems Insects, centipedes, millipedes, and some spiders have a highly specialised respiratory system, which is simple, direct, and very efficient. It is a branching system of tubes (tracheae) that extends to all parts of the body. The smallest end channels are fluid-filled tracheoles that end at the plasma membranes of body cells. Air enters and leaves the tracheal system through valve like openings (spiracles). Respiratory pigments are pre sent in insect blood, but because the cells have a direct pipeline to the outside, an insect\'s respiration is independent of its circulatory system. Consequently, insect blood plays no direct role in oxygen transport. Gills Gills of various types are effective respiratory devices for life in water. Gills may be simple external extensions of the body surface, but most efficient are the internal gills of fishes and arthro pods. Fish gills are thin filamentous structures, richly supplied with blood vessels arranged so that blood flow is opposite to the flow of water across the gills (counter current flow), providing the greatest possible extraction of oxygen. Water flows over the gills in a steady stream, pulled and pushed by an efficient, two-valved, branchial pump. Lungs of a sort are found in invertebrates (pulmonate snails, scorpions, some spiders, and some small crustaceans), but these structures cannot be very efficiently ventilated. Ventilated lungs (by muscle movements), which produce a rhythmic exchange of air, are characteristic of terrestrial vertebrates. Most rudimentary of vertebrate lungs are those of lungfishes, which supplement and replace gill respiration during droughts. Amphibian lungs vary from simple, smooth-walled, bag like lungs of salamanders to the subdivided lungs of frogs and toads. Total surface area for gas exchange is increased in reptile lungs, as they are subdivided into many interconnecting air sacs. Most elaborate of all are mammalian lungs containing millions of small sacs (alveoli), each with a rich vascular network. A disadvantage of lungs is that gas is exchanged between blood and air only in the alveoli, located at the ends of a branching tree of air tubes (trachea, bronchi, and bronchioles). Air must enter and exit a lung through the same channel. After exhalation, the air tubes are filled with 'used' air from the alveoli, which, during the following inhalation, is pulled back into the lungs. In fact, lung ventilation in humans is so inefficient that in normal breathing only approximately one-sixth of the air in the lungs is replenished with each inspiration. In its passage to the air sacs, air in mammalian lungs undergoes three important changes: (1) It is filtered free from most dust and other foreign substances. (2) It is warmed to body temperature. (3) It is saturated with moisture. The lungs consist of a great deal of elastic connective tissue. They are covered by a thin layer of tough epithelium known as the visceral pleura. A similar layer, the parietal pleura, lines the inner surface of the walls of the chest. The two layers of the pleura are in contact and slide over one another as the lungs expand and contract. The 'space' between the pleura, called the pleural cavity, maintains a partial vacuum, which helps keep the lungs expanded. The chest cavity is bounded by the spine, ribs, and breastbone, and floored by the diaphragm, a dome-shaped, muscular partition between the chest cavity and abdomen. A muscular diaphragm is found only in mammals. During inspiration, the ribs are pulled upwards and the diaphragm flattens. The resultant increase in volume of the chest cavity causes air pressure in the lungs to fall below atmospheric pressure: air rushes in through passageways to equalise the pressure. Normal expiration is a less active process than inspiration, muscles relax, the ribs and diaphragm return to their original position, the chest cavity decreases in size, the elastic lungs deflate, and air exits. Breathing is normally involuntary and automatic, but can come under voluntary control. Neurons in the medulla of the brain regulate normal, quiet breathing. Carbon dioxide rather than oxygen has the greatest effect on respiratory rate. Respiratory gas transportation In some invertebrates, respiratory gases are simply carried, dissolved in body fluids. However, solubility of oxygen is so low in water that it is adequate only for animals with low rates of metabolism. Consequently, in many invertebrates and all vertebrates, special coloured proteins (respiratory pigments) transport nearly all oxygen and a significant amount of carbon dioxide. In vertebrates, these respiratory pigments are contained in blood cells. The most widespread respiratory pigment in the animal kingdom is haemoglobin, a red, iron containing protein present in all vertebrates and many invertebrates. Haemoglobin holds oxygen in a loose, reversible chemical combination so that it can be released to tissues. When the oxygen concentration is high, (in the capillaries of lung alveoli), haemoglobin binds oxygen. In tissues where the prevailing oxygen partial pressure is low, haemoglobin releases its stored oxygen reserves, and binds to carbon dioxide. Invertebrate respiratory pigments include Hemocyanin, a blue, copper-containing protein in crustaceans and most molluscs, and chlorocruorin, a green, iron-containing pigment found in polychaete tube worms. Digestion All organisms require energy to maintain their highly ordered and complex structure. This energy is chemical bond energy, released by transforming complex compounds, acquired from the environment, into simpler ones. The ultimate source of energy for life on Earth is sunlight, which is captured by chlorophyll molecules in green plants, and transformed into chemical bond energy (food energy). Almost all animals are heterotrophic organisms that depend on organic compounds of plants and other animals to obtain the materials they need. Since the food of animals, normally the complex tissues of other organisms, is usually too bulky to be absorbed directly by cells, it must be transformed (digested), into small soluble molecules. Ingestion of foods and their simplification by digestion are only initial steps in nutrition. Foods reduced by digestion to soluble, molecular form are absorbed into the circulatory system and transported to the body\'s tissues. There, they are assimilated into the structures of cells or are oxidised to yield energy and heat. Food not immediately used is stored for future use. Waste produced by oxidation must be excreted. Food products unsuitable for digestion are egested in faeces. Few animals can absorb nutrients directly from their external environments (exceptions are internal parasites). Most animals are active feeders that have evolved numerous specialisations for obtaining food. With food procurement as one of the most potent driving forces in animal evolution, natural selection has placed a high priority on adaptations for exploiting new sources of food and the means of food capture and intake. Methods of gathering particulate food include suspension feeding, e.g. polycheate worms, bivalve molluscs, hemichordates, fairy shrimps, water fleas, and barnacles. One form of suspension feeding, often called filter feeding, has evolved frequently and includes animals such as herring, basking sharks, flamingos, and baleen whales. Another type of particulate feeding exploits deposits of disintegrated organic material (detritus) that accumulates on and in the substratum; this type is called deposit feeding, e.g. many annelids and some hemichordates, scaphopod molluscs, certain bivalve molluscs, and some sedentary polychaete worms. Methods of feeding on food masses are varied and are greatly dependant on the food source utilised. Predators must be able to locate, capture, hold, and swallow prey. Most carnivorous animals simply seize food and swallow it intact, although some employ toxins that paralyse or kill their prey at time of capture. Although no true teeth appear among invertebrates, many have beaks or tooth-like structures for biting and holding e.g. Polychaete Nereis, which possesses a muscular pharynx armed with a chitinous jaw that can be everted with great speed to seize prey. Fish, amphibians, and reptiles use their teeth principally to grip prey and © prevent its escape until they can swallow it whole. Birds lack teeth, but their bills often have serrated edges or the upper bill is hooked for seizing and tearing prey. Many invertebrates are able to reduce food size by shredding, (e.g. shredding mouthparts of many crustaceans) or tearing (e.g. beak-like jaws of cephalopod molluscs). Insects have three pairs of appendages on their heads that can serve as jaws, teeth, chisels, tongues, or sucking tubes. True mastication, chewing of food as opposed to tearing/crushing, is found only among mammals with the development of true teeth. Herbivorous animals have evolved special devices for crushing, cutting, or scraping plant material, e.g. snail radula. Insects such as locusts have grinding and cutting mandibles; herbivorous mammals such as horses/cattle use wide, corrugated molars for grinding. All these mechanisms disrupt the tough cellulose cell wall to accelerate its digestion by intestinal microorganisms, as well as releasing cell contents for direct enzymatic breakdown. Thus, herbivores can digest food that carnivores cannot, and in doing so, convert plant material into protein for consumption by carnivores/omnivores. Fluid feeding is characteristic of parasites, both endoparasites and ectoparasites, e.g. leeches, lampreys, parasitic crustaceans, and insects such as fleas and mosquitoes. In the process of digestion, organic foods are mechanically and chemically broken into small units for absorption. Although food solids consist principally of carbohydrates, proteins, and fats, the very components that form the body of the consumer, these components must first be reduced to their simplest molecular units and dissolved before they can be assimilated. Each animal reassembles some of these digested and absorbed units into organic compounds of the animal\'s own unique pattern. In protozoa and sponges, digestion is entirely intracellular. A food particle is enclosed within a food vacuole by phagocytosis. Digestive enzymes are added, and the products of digestion are absorbed into the cell cytoplasm. Food waste is simply extruded from the cell by exocytosis. There are important limitations to intracellular digestion. Only particles small enough to be phagocytized can be used, and every cell must be able to secrete all necessary enzymes. These limitations were resolved with evolution of an alimentary system in which extracellular digestion of large food masses could take place. In extracellular digestion, certain cells line the lumen (cavity) of alimentary canals, and specialise in forming various digestive secretions, whereas other cells function largely, or entirely, in absorption. Ingested food is exposed to various mechanical, chemical, and bacterial treatments, to different acidic and alkaline phases, and to digestive juices that are added at appropriate stages as food passes through. Metazoan alimentary canals can be divided into five major regions: Reception Conduction and storage Grinding and early digestion Terminal digestion and absorption Water absorption and concentration of solids. Food progresses from one region to the next, allowing digestion to proceed in sequential stages. Receiving region/reception The first region of an alimentary canal consists of devices for feeding and swallowing. These include mouthparts (for example, mandibles, jaws, teeth, radula, bills), buccal cavity, and muscular pharynx. Most Metazoans have salivary glands (buccal glands) that produce lubricating secretions containing mucus to assist swallowing. Salivary glands often have other specialised functions such as secretion of toxic enzymes for quieting struggling prey and secretion of salivary enzymes to begin digestion. The tongue is a vertebrate innovation, usually attached to the floor of the mouth that assists in food, manipulation and swallowing. Tongues are also used as chemo-sensors and possess taste buds that are used to determine the palatability of foods, although it can also assist in the capture of prey, e.g. chameleons, woodpeckers, anteaters or as an olfactory sensor (lizards, snakes). 2. 3. Conduction and storage region The oesophagus of vertebrates and many invertebrates serves to transfer food to the digestive region. In many invertebrates (annelids, insects, octopus) the oesophagus is expanded into a crop, used for food storage before digestion among vertebrates. Only birds have a crop. Region of grinding and early digestion In most vertebrates, and in some invertebrates, the stomach provides initial digestion as well as storage and mixing of food with digestive juices. The mechanical breakdown of food, especially plant food with its tough cellulose cell walls, often continues in herbivorous animals by grinding and crushing devices in the stomach (birds are assisted by swallowed stones and grit, arthropods, by hardened linings). Herbivorous vertebrates have evolved several strategies for exploiting cellulose-splitting microorganisms to derive nutrition from plant food, which can only be broken down by the enzyme cellulase, which no metazoan animals can produce. The guts of these animals have fermentation chambers filled with bacteria producing the enzyme, e.g. the four chambered stomach of cattle and the ceacum of rodents. Stomachs of carnivorous and omnivorous vertebrates are typically U-shaped muscular tubes provided with glands that produce enzymes and strong acids, the latter an adaptation that probably arose for killing prey and halting bacterial activity. In humans, gentle peristaltic waves pass over the filled stomach at a rate of approx imately three a minute. Churning is most vigorous at the intestinal end where food is steadily released into the duodenum, the first region of the small intestine. A pyloric sphincter regulates the flow of food into the intestine. Deep tubular glands in the stomach wall secrete gastric juice containing enzymes. 4. Region of terminal digestion and absorption: intestine The importance of an intestine varies widely among animal groups. In invertebrates that have extensive digestive diverticula, in which food is digested and phagocytised, an intestine may serve only as a pathway for conducting waste out of the body. In other invertebrates with simple stomachs, and in all vertebrates, intestines are sites for digestion and absorption. Devices for increasing the internal surface area of an intestine are highly developed in vertebrates, but are generally absent among invertebrates. Coiling of their intestine is common amongst all vertebrate groups and reaches its highest development in mammals, in which the length of the intestine may exceed eight times the length of their body. Lampreys and sharks have longitudinal or spiral folds in their intestine. Other vertebrates have developed elaborate folds (amphibians, reptiles, birds, and mammals) and minute finger-like projections called villi (birds and mammals). Electron microscopy reveals that each cell lining the intestinal cavity additionally is bordered by hundreds of short, delicate processes called microvilli. These processes, together with larger villi and intestinal folds, may increase the internal surface area of an intestine more than a million times, as compared to a smooth cylinder of the same diameter. This elaborate surface greatly facilitates the absorption of food molecules. 5. © OXL/CN/AN 2022 Region of water absorption and concentration of solids The large intestine consolidates the indigestible remnants of digestion by reabsorption of water to form solid or semisolid faeces for removal from the body by defecation. Reabsorption of water is of special significance in insects, especially those living in dry envi ronments, which must conserve nearly all water entering the rectum. In reptiles and birds, which also produce nearly dry faeces, most water is reabsorbed in the cloaca; white paste-like faeces are formed containing both indigestible food waste and uric acid. The colon of humans contains enormous numbers of bacteria, which first enter the sterile colon of a newborn infant with its food. In adults, approximately one-third of the dry weight of faeces is bacteria; these include harmless bacteria as well as bacteria that can cause serious illness should they escape into the abdomen or bloodstream. Bacteria degrade organic waste in faeces and provide some nutritional benefit by synthesising certain vitamins (vitamin K and small quantities of some B vitamins). Immunity Immunity is a very complex subject, and, unfortunately in a course of this size, only a brief outline is possible. The immune system is located throughout the body of an animal, and it is as crucial to survival as the respiratory, circulatory, nervous, skeletal, or any other system. Every animal\'s environment is filled with an incredible number of parasites and potential parasites such as flatworms, nematodes, arthropods, bacteria, and viruses. Whether any parasites can survive in that animal (the host), and the severity of disease the parasite may cause, depends largely on the response of the host's defence system. An animal is susceptible to a parasite if it cannot eliminate it before it becomes established. The host is resistant if its physiological status prevents establishment and survival of the parasite. It is important to remember that these terms are relative, not absolute; for example, one individual organism may be more or less resistant than another, and a single individual may be more or less resistant at different times of its life, depending on age, health, and environmental exposure. Most animals show some degree of innate (non specific) immunity, a mechanism of defence that does not depend on prior exposure to the invader. In addition, vertebrates (and invertebrates to a lesser extent) develop acquired (specific) immunity, which is specific to a particular non-self material, requires time for its development, and occurs more quickly and vigorously on secondary exposure. Frequent resistance conferred by immune mechanisms is not complete. In some instances, a host may recover clinically and be resistant to a specific challenge, but some parasites may remain and reproduce slowly (toxoplasmosis, and malaria). Physical and chemical barriers Intact integument of most animals provides a barrier to invading organisms. In addition, a variety of antimicrobial substances are present in body secretions of vertebrates. Chemical defences present in many vertebrates include a low PH in the stomach and vagina and enzymes in secretions of the alimentary tract. Mucus is produced by mucus membranes lining the digestive and respiratory tract of vertebrates, and contains parasiticidal substances such as IgA and Lysozyme. IgA is a class of antibody that can cross cellular barriers easily and is an important protective agent in mucus of the intestinal epithelium. IgA is also present in saliva and sweat. Lysozyme is an enzyme that attacks the cell wall of many bacteria. Various cells, including those involved in the acquired immune response, liberate protective compounds. A family of low-molecular-weight glycoprotein, the interferons, are released by a variety of eukaryotic cells in response to invasion by intracellular parasites (including viruses) and other stimuli. Tumour necrosis factor (TNF) is produced mainly by macrophages, and is a major mediator of inflammation and, in sufficient concentration, causes fever. Fever in mammals is one of the most common symptoms of infection. The protective role of fever, if any, remains unclear, but high body temperatures may destabilise certain viruses and bacteria. The intestine of most animals harbours a population of bacteria, which tends to inhibit establishment of pathogenic microbes. Host defences do not harm these beneficial bacteria. Microorganisms that successfully enter an organism will encounter the cells and mechanisms of the innate immune system. The innate response is usually triggered when microbes are identified by pattern recognition receptors, which recognise components that are conserved among broad groups of microorganisms. Innate immune defences are non-specific, meaning these systems respond to pathogens in a generic way. This system does not confer long lasting immunity against a pathogen. The innate immune system is the dominant system of host defence in most organisms. The adaptive immune system evolved in early vertebrates, and allows for a stronger immune response as well as immunological memory, where each pathogen is 'remembered' by a signature antigen. The adaptive immune response is antigen-specific and requires the recognition of specific 'non-self' antigens during a process called antigen presentation. Antigen specificity allows for the generation of responses that are tailored to specific pathogens or pathogen-infected cells. The ability to mount these tailored responses is maintained in the body by 'memory cells'. Should a pathogen infect the body more than once, these specific memory cells are used to quickly eliminate it. The cells of the adaptive immune system are special types of leukocytes, called lymphocytes. B cells and T cells are the major types of lymphocytes and are derived from stem cells in the bone marrow. B cells are involved in the humoral immune response, whereas T cells are involved in response. When B cells and T cells are activated and begin to replicate, some of their offspring will become long-lived memory cells. Throughout the lifetime of an animal, these memory cells will remember each specific pathogen encountered and can mount a strong response if the pathogen is detected again. This is 'adaptive', because it occurs during the lifetime of an individual as an adaptation to infection with that pathogen, and prepares the immune system for future challenges.