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The concept of homeostasis (maintaining a continuously balanced internal environment) permeates all physiological thinking, and was first developed from studies with mammals, although it applies to all life, from single-celled organisms to multicellular complex organisms. Potential changes in the in...

The concept of homeostasis (maintaining a continuously balanced internal environment) permeates all physiological thinking, and was first developed from studies with mammals, although it applies to all life, from single-celled organisms to multicellular complex organisms. Potential changes in the internal environment arise from different sources, metabolic activities requiring a constant supply of materials, (oxygen, nutrients, salts), which are used up in cellular activities. These activities also produce waste products that must be removed. In addition, the internal environment responds to changes in the organism\'s external environment. Changes are stabilised by the physiological mechanisms of homeostasis. In more complex metazoans, homeostasis is maintained by the coordinated activities of the circulatory, nervous, and endocrine systems, and by organs that serve as sites of exchange with the external environment. These include kidneys, lungs (or gills), digestive tract and integument. Through these organs, oxygen, nutrients, minerals and other constituents of body fluids enter, water is exchanged, heat is lost, and metabolic wastes are eliminated. Thus, systems within an organism function in an integrated way to maintain a constant internal environment around a set point. In variables such as PH, temperature, osmotic pressure, glucose levels, and oxygen levels, they activate physiological mechanisms and return the variable to its set point. Water and osmoregulation Aquatic animals Most marine invertebrates are in osmotic equilibrium (salt balance) with their environment. As their body surfaces are permeable to salts and water, body fluid concentration rises or falls with changes in concentrations of seawater. Because such animals are not able to regulate the osmotic pressure of their body fluid (salt concentration), they are called osmotic conformers. Invertebrates in the open sea are seldom exposed to osmotic fluctuations, as the ocean is a highly stable environment, and, therefore, they have evolved very limited abilities to withstand osmotic change. When exposed to dilute seawater, they die quickly because their body's cells cannot tolerate dilution. Coastal and estuarine conditions are for more variable than those of the open ocean. Here, animals must cope with large, often abrupt changes in salinity as tides mix with freshwater, draining from rivers. These animals can survive a wide range of salinity changes, with varying powers of osmotic regulation. For example, a brackish-water shore crab, (Eriocheir) can resist dilution of body fluids by dilute (brackish) seawater. Although the concentration of salts in the body fluids falls, it does so less rapidly than the fall in seawater concentration. This crab is classified as a hyperosmotic regulator, as it maintains its body fluids at a more concentrated level than the surrounding water. By regulating against excessive dilution, it protects cells from extreme changes. However, their limited capacity for osmotic regulation means they will die if exposed to greatly diluted seawater. Coastal invertebrates face considerable water regulation problems; firstly, because body fluids are osmotically more concentrated than the dilute seawater outside, water flows into the body, especially across the thin, permeable membranes of the gills (the more concentrated solution, inside the crab, pulls water in). Were this inflow of water allowed to continue, its body fluids would soon become diluted and unbalanced. This problem is solved by the kidneys (antennal glands located in the crab\'s head), which excrete excess water as dilute urine. The second problem is salt loss. Again, because the animal is saltier than its environment, it cannot avoid loss of salt ions by outward diffusion across its gills. Additionally, salt is lost in urine. To compensate for salt loss, special salt-secreting cells in the gills actively remove salts from the environment and move them into the blood, maintaining the internal osmotic concentration. This is an active transport process (requires energy), because transport is against a concentration gradient from a lower salt concentration (in dilute seawater) to a higher one (in blood). Freshwater animals must keep the salt concentration of their body fluids higher than that of the water in which they live. Water enters their bodies osmotically, and salt is lost by diffusion outwards, thus, they need a highly efficient hyperosmotic regulator. Several adaptations have made the move to freshwater possible; firstly, the scaled, mucus-covered body surface of a fish is remarkably waterproof. Secondly, water that enters the body by osmosis (via gills) is pumped out by the kidney, which forms very dilute urine. Secondly, special salt-absorbing cells located in the gills, transport salts, (present in small quantities in fresh water), into the blood. This process, together with dietary salt, replaces salt loss. Amphibians living in water also compensate for salt loss by actively absorbing salt from the water through their skin. Marine fishes maintain the salt concentration of their body fluids at approximately one-third that of seawater and are hypo-osmotic regulators. Modern bony fishes are descendants of earlier freshwater bony fishes that moved back into the sea during the Triassic period approximately 200 million years ago. Over millions of years, freshwater fishes have established a salt concentration in body fluid equivalent to around 1/3 of that of seawater. Body fluid of terrestrial vertebrates is also similar to dilute seawater, a fact related to their ancient marine heritage. With seawater around them, marine fish lose water, and gain salt, thus, a marine bony fish risks desiccation. To compensate for water loss, a marine fish drinks seawater, which is absorbed via the intestine. The major sea salt, sodium chloride, is carried by blood to the gills, where specialised salt-secreting cells excrete it back into the sea. Ions remaining in the intestinal residue (magnesium, sulphate, calcium) are voided with faeces or excreted by kidneys. Sharks and rays (elasmobranches) have achieved osmotic balance differently. The salt compo sition of a shark\'s blood is similar to that of bony fishes, but their blood also carries large concentrations of organic compounds, especially urea. Urea is a metabolic waste that most animals quickly excrete. The shark's kidney, however, conserves urea, allowing it to accumulate in the blood, raising blood osmolarity to equal or slightly exceeding that of seawater. With the osmotic difference between blood and seawater removed, environmental osmotic equilibrium occurs. Terrestrial animals Animal bodies are mostly water, and all metabolic activities proceed in water, thus, conducting life in a dry environment poses serious problems. Yet many animals, like the plants preceding them, moved onto land. Terrestrial animals lose water by evaporation (from respiratory/body surfaces), excretion (in urine), and elimination in faeces. Losses of water are replaced by food, drinking water, and retaining metabolic water, formed in cells by the oxidation of foods. Certain arthropods, (desert cockroaches) are able to absorb water vapour directly from the atmosphere. Excretion Excretion of waste presents a special problem in water conservation. The primary end product of protein breakdown is ammonia, a highly toxic material. Fishes easily excrete ammonia by diffusion across their gills, as there is an abundance of water to wash it away. Terrestrial insects, reptiles, and birds have no convenient way to rid themselves of toxic ammonia; instead, they convert it into uric acid, a non-toxic, almost insoluble compound. This conversion enables them to excrete semi solid urine with little water loss. Marine birds and turtles have evolved an effective solution for excreting large loads of salt eaten with their food. Located above each eye is a special salt gland capable of excreting a highly concentrated solution of sodium chloride. Marine lizards and turtles shed their salt gland secretion as tears. Salt glands are important accessory organs of salt excretion in these animals, because their kidneys cannot produce concentrated urine, as can mammalian kidneys. Invertebrates Contractile vacuole The tiny, spherical, intracellular vacuole of protozoa and freshwater sponges is not a true excretory organ, since ammonia and other nitrogenous waste from metabolism readily diffuse into the surrounding water. The contractile vacuole is an organ of water balance in freshwater protozoa, in which it expels excess water gained by osmosis. As water enters the protozoan, a vacuole grows and, finally, empties its contents through a pore on the surface. This cycle is repeated rhythmically. Contractile vacuoles are common in freshwater protozoa, sponges, and radiate animals (such as hydra), but rare or absent in marine forms of these groups, which are isomotic with seawater and, consequently, neither lose nor gain too much water. More complex invertebrates have excretory organs that are basically tubular structures, forming urine by first producing a fluid secretion of the blood. This fluid secretion enters the proximal end of the tubule and is modified continuously as it flows down the tubule. The final product is urine. Nephridium The most common type of invertebrate excretory organ is the nephridium, a tubular structure designed to maintain appropriate osmotic balance. One of the simplest arrangements is the flame cell system of acoelomates (flat-worms) and some pseudocoelomates. In planaria and other flatworms, the protonephridial system takes the form of two highly branched duct systems distributed throughout the body. Fluid enters the system through specialised 'flame cells', moves slowly down the tubules, and is excreted through pores that open at intervals on the body surface. In the tubule, water and metabolites valuable to the body are recovered by reabsorption, leaving waste behind to be expelled. As flatworms have no circulatory system to deliver waste, the flame-cell system is extensively branched throughout a flatworm\'s body. Arthropod kidney I n s e c t s h a v e a system of Malpighian tubules, which are located at the junction of the midgut and hindgut (rectum). Salts, especially potassium, are actively secreted into the tubules from the surrounding arthropod hemolymph (blood). Water, uric acid, and other waste follow. This fluid drains into the rectum, where solutes and water are actively reabsorbed, and waste is left to be excreted. Vertebrate kidney From comparative studies of development, biologists believe the kidney of the earliest vertebrates was composed of segmentally arranged tubes, each resembling an invertebrate nephridium, extending the length of the coelomic cavity. Each tubule opened at one end into the coelom by a nephrostome and at the other end into a common archinephric duct. This ancestral kidney is called an archinephros ('ancient kidney'), a segmented kidney very similar to an archinephros is found in the embryos of hagfishes and caecilians. Almost from the beginning, the reproductive system, which develops beside the excretory system from the same segmental blocks of trunk mesoderm, used the nephric duct system for reproductive products. Thus, even though the two systems have nothing functionally in common, they are closely associated by their use of common ducts. Kidneys of living vertebrates developed from this primitive plan. During embryonic development of amniote vertebrates, there is a succession of three developmental stages of kidneys: pronephros, mesonephros, and metanephros. Most, but not all, of these stages are observed in other vertebrate groups. In all vertebrate embryos, the pronephros is the first kidney to appear. It is located anteriorly in the body and becomes part of the permanent kidney only in adult hagfish. In all other vertebrates, the pronephros degenerates during development and is replaced by a more centrally located mesonephros, which is the functional kidney of embryonic amniotes (reptiles, birds, and mammals), and contributes to the adult kidney of fishes and amphibians. The metanephros, characteristic of adult amniotes, is distinguished in several ways from the pronephros and mesonephros. It is more caudally located and it is a much larger, more compact structure containing a very large number of nephric tubules. It is drained by a new duct, the ureter, which developed when the old archinephric duct was relinquished to the reproductive system of the male for sperm transport. Thus, three successive kidney types, pronephros, mesonephros, and metanephros, succeed each other embryologically, and, to some extent, phylogenetically, in amniotes. Vertebrate kidney function The organisation of kidneys differs somewhat in different groups of vertebrates, but in all basic functional units are the nephrons, and three well defined physiological processes, which are filtration, reabsorption, and secretion, form urine. This discussion focuses mainly on the mammalian kidney, which is the most completely understood regulatory organ. The two human kidneys are small organs comprising less than 1% of body weight. Yet, they receive a remarkable 20% to 25% of the total cardiac output, some 2000 litres of blood each day. This vast blood flow is channelled to approximately two million nephrons, which form the bulk of the two kidneys. Each nephron begins with an expanded chamber, the Bowman\'s capsule, containing a tuft of capillaries, the glomerulus, which together are called the renal corpuscle. Blood pressure in the capillaries forces a protein-free filtrate into a Bowman\'s capsule and along a renal tubule, consisting of several segments that perform different functions in the process of urine formation. The filtrate passes first into a proximal convoluted tubule, and then into a long, thin-walled loop of Henle, which drops deep into the inner portion of the kidney, (the medulla) before returning to the outer portion, (the cortex) where it joins a distal convoluted tubule. From the distal tubule, the fluid empties into a collecting duct, which drains into the renal pelvis. Here, the urine is collected before being carried by the ureter to the urinary bladder. Urine that leaves the collecting duct is very different from the filtrate produced in the renal corpuscle. During its travel, through the renal tubule and collecting duct, both the composition and concentration of the original filtrate change. Some solutes such as glucose, amino acids, and sodium are reabsorbed while other materials, such as hydrogen ions and urea, are concentrated in urine. Blood from the aorta enters each kidney through a large renal artery, which divides into a branching system of smaller arteries. The arterial blood reaches the glomerulus through an afferent arteriole and leaves by an efferent arteriole. From the efferent arteriole, the blood travels to an extensive capillary network that surrounds and supplies the proximal and distal convoluted tubule and the loop of Henle. This capillary network provides a means for the pickup and delivery of materials that are reabsorbed or secreted by the kidney tubules. From these capillaries, veins that unite to form the renal vein collect blood. This vein returns blood to the vena cava. Urine formation begins in the glomerulus, which acts as a specialised mechanical filter in which a protein-free filtrate of the plasma is driven by the blood pressure across capillary walls and into the fluid filled space of the Bowman\'s capsule. Solute molecules, small enough to pass through the capillary wall, are carried through with the water in which they are dissolved. Red blood cells and plasma proteins, however, are too large to pass. The filtrate continues through the renal tubular system where it undergoes extensive modification before becoming urine. Human kidneys form approximately 180 litres of filtrate each day, a volume many times more than total blood volume. The final urine volume in humans averages 1.2 litres per day. The conversion of filtrate into urine involves two processes as follows: (1) The modification of the composition of the filtrate through tubular reabsorption and secretion. (2) Changes in the total osmotic concentration of urine through regulation of water excretion. Approximately 60% of the filtrate volume and virtually all of the glucose, amino acids, vitamin, required salts, and other valuable nutrients, are reabsorbed by active transport in the proximal convoluted tubule. Water is withdrawn passively from the tubule, as it osmotically follows the active reabsorption of solutes. Distal convoluted tubules make the final adjustment of filtrate composition. In the distal convoluted tubule, sodium reabsorption is controlled by aldosterone, a steroid hormone from the adrenal gland. Aldostrone secretion is usually coupled with increasing the secretion of antidiuretic hormone, which promotes water conservation by the kidney. Thirst is stimulated by decreased blood volume and increased blood osmolarity. The varying ability of different mammals to form concentrated urine correlates closely with the length of the loops of Henle. Beavers, which have no need to conserve water in their aquatic environment, have short loops and can concentrate their urine only to about twice the osmolarity of blood. Humans, with relatively longer loops, can concentrate urine 4.2 times that of blood. As we would anticipate, desert mammals have much greater urine concentrating powers. A camel can produce urine 8 times the plasma concentration, a gerbil 14 times, and an Australian hopping mouse 22 times. In this creature, the greatest urine concentrator of all, the loops of Henle extend to the tip of a long renal papilla that pushes out into the mouth of the ureter. Temperature regulation We have seen that a fundamental problem facing an animal is keeping its internal environment in a state that permits normal cell function. Biochemical activities are sensitive to the chemical environment, and the discussion thus far has examined how the chemical environment is stabilised. Biochemical reactions are also extremely sensitive to temperature. All enzymes have an optimum functioning temperature; at temperatures above or below this optimum, enzyme function is impaired. When body temperature drops too low, metabolic processes slow down, reducing the amount of energy available. If body temperature rises too high, metabolic reactions become unbalanced, enzymatic activity is hampered or even destroyed. Thus, animals can survive only in a restricted range of temperature, usually between 0° C to 40° C. Animals must either find a habitat where they do not have to contend with temperature extremes, or they must develop a means of stabilising their metabolism independent of external temperature extremes. All animals produce heat from cellular metabolism, but in most, the heat is conducted away as fast as it is produced. In these animals, the ectotherm's (majority of animals) body temperature is determined solely by the environment. Many ectotherms exploit their envi ronment behaviourally to select areas of more favourable temperature, but the source of energy used to increase body temperature comes from the environment, not from within the body. Alternatively, some animals are able to generate and retain enough heat to elevate their own body temperature to a high but stable level. Because the source of their body heat is internal, they are called endotherms (birds, mammals, a few reptiles, and fast swimming fishes). Endothermy allows birds and mammals to stabilise their internal temperature, allowing biochemical processes and nervous system functions to proceed at steady high levels of activity. Endotherms can thus remain active in winter and exploit habitats denied to ectotherms. Ectotherms have a wide variety of behavioural methods for maintaining a more stable temperature; however, most ectotherms can adjust their metabolic rates to the prevailing temperature, such that the intensity of metabolism remains mostly unchanged. This is called temperature compensation and involves complex biochemical and cellular adjustments, enabling a fish or salamander, to have almost the same level of activity in both warm and cold environments. This metabolic regulation is a form of homeostasis. Most mammals (endotherms) have body temperatures between 36° C and 38° C, somewhat lower than those of birds (40° C - 42° C). Constant temperature is maintained by a balance between heat production and loss, a complex activity when these animals alternate between periods of rest and bursts of heat-producing activity. Heat is produced by the animal\'s metabolism (oxidation of foods, basal cellular metabolism, and muscular contraction). Because much of an endotherm's daily caloric intake is required to generate heat, especially in cold weather, an endotherm must eat more food than a similar sized ectotherm. Heat is lost by radiation, conduction, and convection (air movement) to a cooler environment, and by water evaporation. If the animal becomes too cool, it can generate heat by increasing muscular activity (exercise or shivering), and by decreasing heat loss by increasing its insulation. If it becomes too warm, it decreases heat production and increases heat loss. Small desert animals escape the heat by becoming nocturnal and using underground burrows Animals such as camels and desert antelopes (gazelle, oryx, and eland) possess a number of adaptations for coping with heat and dehydration. Mechanisms for controlling water loss and preventing overheating are closely linked. Glossy, pallid fur reflects direct sunlight and resists heat. Heat is lost by convection and conduction from the underside of elands, where the fur is very thin. Fat tissue, an essential food reserve, is concentrated in a single hump on the back, instead of being uniformly distributed under the skin, where it would impair loss of heat by radiation. Elands avoid evaporative water loss, by permitting their body temperature to drop during the cool night and then to rise slowly during the day, as the body stores heat. Only when the body temperature reaches 41° C, must elands prevent further rise through evaporative cooling by sweating and panting. They conserve water by producing concentrated urine and dry faeces. In cold environments, mammals and birds use several mechanisms to maintain homeothermy: In all mammals living in the cold regions of the earth, fur thickness increases in winter, up to 50%. Thick underhair is the principal insulating layer, with longer guard hairs serving as protection against wear, and as coloration. To prevent extremities from becoming areas of heat loss, they are allowed to cool to low temperatures, often approaching freezing point. Heat in warm arterial blood is not lost from the body, as counter current heat exchange system exists between the outgoing warm blood and the returning cold blood. Arterial blood in the leg of an arctic mammal or bird passes in close contact with a network of small veins. When arterial blood reaches the foot, it has transferred nearly all of its heat to the veins, returning blood to the body core. Thus, little heat is lost to the surrounding cold air from poorly insulated distal regions of the leg. Counter current heat exchanges in appendages are also common in aquatic mammals such as seals and whales. The extremities of mammals and birds living in cold environments must function at low temperatures. Fats, in these areas, have very low melting points; perhaps 30° C lower than ordinary body fats. In severe cold conditions, all mammals produce more heat by augmented muscular activity (exercise or shivering). Another source of heat is increased oxidation of foods, especially from stores of brown fat. This mechanism is called 'non-shivering thermogenesis'. Behavioural adaptations to cold include small rodents exploiting the excellent insulating qualities of snow by living under it in runways on the forest floor, where their food is also located. Endothermy is energetically expensive. The problem is especially acute for small birds and mammals, which, because of their intense metabolism, may require daily food intake approaching their own weight. Some very small mammals, such as bats, maintain high body temperatures when active, but allow their body temperature to drop when inactive and asleep. This is called daily torpor. Many small and medium-sized mammals in northern tem perate regions solve the problem of winter food shortage and low temperatures by entering a prolonged and controlled state of dormancy, known as hibernation. True hibernators (e.g. marmots), prepare for hibernation by storing body fat. Entry into hibernation is gradual with a fall in metabolism and heart rate (normally 200 per minute to 4-5 per minute). Mammals such as bears enter a state of prolonged sleep, not true hibernation; with little or no drop in body temperature although heart rate slows from 40 to 10 beats a minute. Circulation and respiration Single-celled organisms live in direct contact with their environments. They obtain nutrients and oxygen and release waste directly across their cell surface. These organisms are so small that no special internal system of transport, beyond normal streaming movements of cytoplasm, is required. Even some simple multicellular forms, such as sponges, cnidarians, and flatworms, lack the internal complexity and metabolic demands that would require a circulatory system. Most other multicellular organisms, because of their increased size, activity, and complexity, need a specialised circulatory system to transport nutrients and respiratory gases to and from all tissues of their body. Circulatory systems have acquired additional functions; water, electrolytes, hormones, and the many other constituents of body fluids, are distributed and exchanged between different organs and tissues. An effective response to injury and disease is accelerated by an efficient circulatory system. Homeothermic birds and mammals depend on blood circulation to conserve or dissipate heat as required for temperature maintenance. Body fluids the body fluid of a single-celled organism is cellular cytoplasm, a liquid-gel substance in which the various membrane systems and organelles are suspended. In multicellular animals, body fluids are divided into two types, intracellular and extracellular. The intracellular fluid is the collective fluid inside all the body\'s cells. The extracellular fluid is that surrounding the cells. Cells are bathed by their own aqueous environment, the extracellular fluid that buffers them from often harsh physical and chemical changes occurring outside the body. Extracellular fluid is further subdivided into blood plasma and interstitial (intercellular) fluid. Blood vessels contain plasma, whereas interstitial fluid, (tissue fluid) occupies spaces surrounding the cells in the body. Nutrients and gases passing between vascular plasma and cells must traverse this narrow fluid separation. Interstitial fluid is constantly formed from plasma by filtration through capillary walls. All these fluid spaces (plasma, interstitial, and intracellular) differ from each other in solute composition, although all are mostly water. Animals are 70% to 90% water, of this, 50% is cell water, 15% is interstitial-fluid water, and the remaining 5% is blood plasma. Plasma serves as the pathway of exchange between body cells and the outside environment. Exchanges of respiratory gases, nutrients, and waste are accomplished by specialised organs (kidney, lung, gill, alimentary canal), as well as by the skin. Body fluids contain many inorganic and organic substances in solution. Principal among these are inorganic electrolytes and proteins. Sodium, chloride, and bicarbonate salts are the main extracellular electrolytes, whereas potassium, magnesium, phosphate salts and proteins are the major intracellular electrolytes. The two types of extracellular fluid have similar compositions except that plasma has more proteins, which are mostly too large to filter through capillary walls into interstitial fluid. Blood composition Among invertebrates that lack a circulatory system, (flatworms and cnidarians) it is not possible to distinguish a true 'blood' These organisms possess a clear, watery tissue fluid containing phagoeytic cells, a little protein, and a mixture of salts similar to seawater. The 'blood' (hemolymph) of invertebrates with open circulatory systems is slightly more complex. Invertebrates with closed circulatory systems, maintain a clear separation between blood contained within blood vessels and tissue (interstitial) fluid surrounding the blood vessels. In vertebrates, blood is a complex liquid tissue composed of plasma and formed elements, mostly red cells (erythrocytes), suspended in plasma. The composition of mammalian blood is: 1. 2. 3. 4. 5. 6. 90% water. Dissolved solids, consisting of plasma proteins (albumin, globulins, fibrinogen- coagulation). Glucose, amino acids, electrolytes, various enzymes, antibodies, hormones, metabolic wastes, traces of many other organic and inorganic materials. Dissolved gases, especially oxygen, carbon dioxide, and nitrogen. Red blood cells (erythrocytes), containing haemoglobin for the transport of oxygen and carbon dioxide, red blood cells of humans and other mammals lack nuclei, but those of all other vertebrates have nuclei. White blood cells (leukocytes), serving as scavengers and as defensive cells. Cell fragments (platelets in mammals) or ceils (thrombocytes in other vertebrates) that function in blood coagulation. Red blood cells, or erythrocytes, are present in enormous numbers in blood (approx 54 million per millilitre of blood in men and 48 million in women). In mammals and birds, red cells form continuously from large nucleated erythroblasts in red bone marrow (in other vertebrates, kidneys and spleen are more important for red blood cell production). In mammals, the nucleus shrinks during development and is eventually lost from the cell by exocytosis, along with ribosomes, mitochondria, and most enzyme systems. What is left is a biconcave disc packed with about 280 million molecules of the blood-transporting pigment, haemoglobin. An erythrocyte enters the circulation for an average life span of four months. During this time, it may journey 11,000km, squeezing repeatedly through capillaries, which are sometimes so narrow that the erythrocyte must bend to pass through. At last, it fragments and is quickly engulfed by large scavenger cells, called macrophages, which are located in the liver, bone marrow, and spleen. Iron from haemoglobin is used again: the rest of the haem is converted to bilirubin, a brown bile-pigment, which is excreted with the faeces, giving them their characteristic colour. White blood cells, or leukocytes, form a wandering system of protection for the body. In humans they number approximately 50,000 to 100,000 per millilitre of blood (1 white cell: 500 to 1000 red cells). There are several kinds of white blood cells: granulocytes (subdivided into ncutrophils, basophils, and eosinophils), and agranulocytcs, the lymphocytes and monocytes. Circulation Most animals have evolved mechanisms for transporting materials among various regions of their body. For sponges and radiates, seawater, propelled by ciliary, or body movements, passes through channels or compartments to facilitate the movement of food, respiratory gases, and waste. True circulatory systems, (containing vessels through which blood moves) are essential to animals so large or so active that diffusion processes alone cannot supply their oxygen needs. A circulatory system having a full complement of component (propulsive organ, arterial distribution system, capillaries, and venous return system) is fully recognisable in annelid worms. In earthworms, there are two main vessels, a dorsal vessel carrying blood towards the head, and a ventral vessel that flows posteriorly, delivering blood throughout the body, by way of segmental vessels and a dense capillary network. The dorsal vessel drives the blood forwards by peristalsis serving as a heart. Many smaller segmental vessels, which deliver blood to tissue capillaries, are actively contractile as well. Many invertebrates have an open circulation (no small blood vessels or capillaries connecting arteries with veins). In insects, other arthropods, and most molluscs, blood sinuses, collectively called a hemocoel, replace capillary beds, which are found in animals with closed systems. In arthropods, the heart and all viscera lie in the hemocoel, bathed by blood. Blood enters the heart through valved openings, the ostia, and the heart\'s contractions, which resemble a forward-moving peristaltic wave, propel blood into a limited arterial system. Blood is distributed to the head and other organs, then, it escapes into the hemocoel. It is routed through the body and appendages by a system of baffles and longitudinal membranes (septa) before returning to the heart. Many arthropods have auxiliary hearts or contractile vessels to boost blood flow. In vertebrates, the principal differences in the blood-vascular system involve the gradual separation of the heart into two separate pumps, as vertebrates evolved from aquatic life with gill breathing to fully terrestrial life with lung breathing. A fish heart contains two main chambers in series, an atrium and a ventricle. Blood makes a single circuit through a fish\'s vascular system; it is pumped from the heart to the gills, where it is oxygenated, flowing into the dorsal aorta and distributed to body organs, it finally returns by veins to the heart.