Digestive System Anatomy
Capillaries connect to arterioles on one end and venules on the other. The plasma functions as a transportation medium for these substances as they move throughout the body. Unlike iron from a bottle of iron supplements, the iron in our formulas is like that of any plant such as celery or spinach and is a medicine for the body. Arteries flow into smaller arteries called arterioles, and then into capillaries. Details of oxygen transport:
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The composition of the fluid varies markedly depending on its source and is regulated more or less precisely by homeostasis. Blood and coelomic fluid are often physically separated by the blood-vessel walls; where a hemocoel a blood-containing body cavity exists, however, blood rather than coelomic fluid occupies the cavity.
The composition of blood may vary from what is little more than the environmental water containing small amounts of dissolved nutrients and gases to the highly complex tissue containing many cells of different types found in mammals. Lymph essentially consists of blood plasma that has left the blood vessels and has passed through the tissues. It is generally considered to have a separate identity when it is returned to the bloodstream through a series of vessels independent of the blood vessels and the coelomic space.
Coelomic fluid itself may circulate in the body cavity. In most cases this circulation has an apparently random nature, mainly because of movements of the body and organs.
In some phyla, however, the coelomic fluid has a more important role in internal distribution and is circulated by ciliary tracts. Blood is circulated through vessels of the blood vascular system.
Blood is moved through this system by some form of pump. The simplest pump, or heart, may be no more than a vessel along which a wave of contraction passes to propel the blood. In the latter animals, the heart is usually a specialized, chambered, muscular pump that receives blood under low pressure and returns it under higher pressure to the circulation.
Where the flow of blood is in one direction, as is normally the case, valves in the form of flaps of tissue prevent backflow. A characteristic feature of hearts is that they pulsate throughout life and any prolonged cessation of heartbeat is fatal. Contractions of the heart muscle may be initiated in one of two ways. In the first, the heart muscle may have an intrinsic contractile property that is independent of the nervous system.
This myogenic contraction is found in all vertebrates and some invertebrates. In the second, the heart is stimulated by nerve impulses from outside the heart muscle. The hearts of other invertebrates exhibit this neurogenic contraction. Chambered hearts, as found in vertebrates and some larger invertebrates, consist of a series of interconnected muscular compartments separated by valves. The first chamber, the auricle, acts as a reservoir to receive the blood that then passes to the second and main pumping chamber, the ventricle.
Expansion of a chamber is known as diastole and contraction as systole. As one chamber undergoes systole the other undergoes diastole, thus forcing the blood forward.
The series of events during which blood is passed through the heart is known as the cardiac cycle. Contraction of the ventricle forces the blood into the vessels under pressure, known as the blood pressure. As contraction continues in the ventricle, the rising pressure is sufficient to open the valves that had been closed because of attempted reverse blood flow during the previous cycle.
At this point the ventricular pressure transmits a high-speed wave, the pulse, through the blood of the arterial system. After leaving the heart, the blood passes through a series of branching vessels of steadily decreasing diameter.
The smallest branches, only a few micrometres there are about 25, micrometres in one inch in diameter, are the capillaries , which have thin walls through which the fluid part of the blood may pass to bathe the tissue cells.
The capillaries also pick up metabolic end products and carry them into larger collecting vessels that eventually return the blood to the heart. In vertebrates there are structural differences between the muscularly walled arteries, which carry the blood under high pressure from the heart, and the thinner walled veins , which return it at much reduced pressure.
Although such structural differences are less apparent in invertebrates, the terms artery and vein are used for vessels that carry blood from and to the heart, respectively. In the latter animals, the blood leaving the heart passes into a series of open spaces, called sinuses , where it bathes internal organs directly.
Such a body cavity is called a hemocoel, a term that reflects the amalgamation of the blood system and the coelom. To maintain optimum metabolism, all living cells require a suitable environment, which must be maintained within relatively narrow limits. An appropriate gas phase i.
Direct diffusion through the body surface supplies the necessary gases and nutrients for small organisms, but even some single-celled protozoa have a rudimentary circulatory system. Cyclosis in many ciliates carries food vacuoles—which form at the forward end of the gullet cytopharynx —on a more or less fixed route around the cell, while digestion occurs to a fixed point of discharge. For most animal cells, the supply of oxygen is largely independent of the animal and therefore is a limiting factor in its metabolism and ultimately in its structure and distribution.
The nutrient supply to the tissues, however, is controlled by the animal itself, and, because both major catabolic end products of metabolism—ammonia NH 3 and carbon dioxide CO 2 —are more soluble than oxygen O 2 in water and the aqueous phase of the body fluids, they tend not to limit metabolic rates.
The diffusion rate of CO 2 is less than that of O 2 , but its solubility is 30 times that of oxygen. This means that the amount of CO 2 diffusing is 26 times as high as for oxygen at the same temperature and pressure.
The oxygen available to a cell depends on the concentration of oxygen in the external environment and the efficiency with which it is transported to the tissues. Dry air at atmospheric pressure contains about 21 percent oxygen, the percentage of which decreases with increasing altitude. Well-aerated water has the same percentage of oxygen as the surrounding air; however, the amount of dissolved oxygen is governed by temperature and the presence of other solutes.
For example, seawater contains 20 percent less oxygen than fresh water under the same conditions. The rate of diffusion depends on the shape and size of the diffusing molecule, the medium through which it diffuses, the concentration gradient, and the temperature.
These physicochemical constraints imposed by gaseous diffusion have a relationship with animal respiration. Investigations have suggested that a spherical organism larger than 0. Many invertebrates are small, with direct diffusion distances of less than 0. Considerably larger species, however, still survive without an internal circulatory system.
A sphere represents the smallest possible ratio of surface area to volume; modifications in architecture, reduction of metabolic rate, or both may be exploited to allow size increase. Sponges overcome the problem of oxygen supply and increase the chance of food capture by passing water through their many pores using ciliary action. The level of organization of sponges is that of a coordinated aggregation of largely independent cells with poorly defined tissues and no organ systems.
The whole animal has a relatively massive surface area for gaseous exchange, and all cells are in direct contact with the passing water current. Among the eumetazoan multicellular animals the cnidarians sea anemones , corals , and jellyfish are diploblastic, the inner endoderm and outer ectoderm being separated by an acellular mesoglea.
Sea anemones and corals may also grow to considerable size and exhibit complex external structure that, again, has the effect of increasing surface area. Their fundamentally simple structure—with a gastrovascular cavity continuous with the external environmental water—allows both the endodermal and ectodermal cells of the body wall access to aerated water, permitting direct diffusion.
This arrangement is found in a number of other invertebrates, such as Ctenophora comb jellies , and is exploited further by jellyfish , which also show a rudimentary internal circulatory system. The thick, largely acellular, gelatinous bell of a large jellyfish may attain a diameter of 40 centimetres 16 inches or more.
The gastrovascular cavity is modified to form a series of water-filled canals that ramify through the bell and extend from the central gastric pouches to a circular canal that follows the periphery of the umbrella. Ciliary activity within the canals slowly passes food particles and water, taken in through the mouth, from the gastric pouches where digestion is initiated to other parts of the body.
Ciliary activity is a relatively inefficient means of translocating fluids, and it may take up to half an hour to complete a circulatory cycle through even a small species.
To compensate for the inefficiency of the circulation, the metabolic rate of the jellyfish is low, and organic matter makes up only a small proportion of the total body constituents. The central mass of the umbrella may be a considerable distance from either the exumbrella surface or the canal system, and, while it contains some wandering amoeboid cells, its largely acellular nature means that its metabolic requirements are small. While ciliary respiratory currents are sufficient to supply the requirements of animals with simple epithelial tissues and low metabolic rates, most species whose bodies contain a number of organ systems require a more efficient circulatory system.
Many invertebrates and all vertebrates have a closed vascular system in which the circulatory fluid is totally confined within a series of vessels consisting of arteries, veins, and fine linking capillaries. Insects, most crustaceans, and many mollusks, however, have an open system in which the circulating fluid passes somewhat freely among the tissues before being collected and recirculated. The distinction between open and closed circulatory systems may not be as great as was once thought; some crustaceans have vessels with dimensions similar to those of vertebrate capillaries before opening into tissue sinuses.
Compared with closed systems, open circulatory systems generally work at lower pressures, and the rate of fluid return to the heart is slower. Blood distribution to individual organs is not regulated easily, and the open system is not as well-adapted for rapid response to change. The primary body cavity coelom of triploblastic multicellular organisms arises from the central mesoderm, which emerges from between the endoderm and ectoderm during embryonic development.
The fluid of the coelom containing free mesodermal cells constitutes the blood and lymph. The composition of blood varies between different organisms and within one organism at different stages during its circulation. Essentially, however, the blood consists of an aqueous plasma containing sodium, potassium, calcium, magnesium, chloride, and sulfate ions; some trace elements; a number of amino acids; and possibly a protein known as a respiratory pigment.
If present in invertebrates, the respiratory pigments are normally dissolved in the plasma and are not enclosed in blood cells. The constancy of the ionic constituents of blood and their similarity to seawater have been used by some scientists as evidence of a common origin for life in the sea.
In many marine invertebrates, such as echinoderms and some mollusks, the osmotic and ionic characteristics of the blood closely resemble those of seawater.
Other aquatic, and all terrestrial, organisms, however, maintain blood concentrations that differ to some extent from their environments and thus have a greater potential range of habitats. In addition to maintaining the overall stability of the internal environment, blood has a range of other functions. It is the major means of transport of nutrients, metabolites, excretory products, hormones, and gases, and it may provide the mechanical force for such diverse processes as hatching and molting in arthropods and burrowing in bivalve mollusks.
Invertebrate blood may contain a number of cells hemocytes arising from the embryonic mesoderm. Many different types of hemocytes have been described in different species, but they have been studied most extensively in insects, in which four major types and functions have been suggested: While the solubility of oxygen in blood plasma is adequate to supply the tissues of some relatively sedentary invertebrates, more active animals with increased oxygen demands require an additional oxygen carrier.
The oxygen carriers in blood take the form of metal -containing protein molecules that frequently are coloured and thus commonly known as respiratory pigments. The most widely distributed respiratory pigments are the red hemoglobins , which have been reported in all classes of vertebrates, in most invertebrate phyla, and even in some plants. Hemoglobins consist of a variable number of subunits, each containing an iron—porphyrin group attached to a protein.
The distribution of hemoglobins in just a few members of a phylum and in many different phyla argues that the hemoglobin type of molecule must have evolved many times with similar iron—porphyrin groups and different proteins. The green chlorocruorins are also iron—porphyrin pigments and are found in the blood of a number of families of marine polychaete worms.
There is a close resemblance between chlorocruorin and hemoglobin molecules, and a number of species of a genus, such as those of Serpula , contain both, while some closely related species exhibit an almost arbitrary distribution.
For example, Spirorbis borealis has chlorocruorin, S. The third iron-containing pigments, the hemerythrins , are violet.
They differ structurally from both hemoglobin and chlorocruorin in having no porphyrin groups and containing three times as much iron, which is attached directly to the protein.
Hemerythrins are restricted to a small number of animals, including some polychaete and sipunculid worms, the brachiopod Lingula , and some priapulids. Hemocyanins are copper-containing respiratory pigments found in many mollusks some bivalves, many gastropods, and cephalopods and arthropods many crustaceans, some arachnids, and the horseshoe crab , Limulus.
They are colourless when deoxygenated but turn blue on oxygenation. The copper is bound directly to the protein, and oxygen combines reversibly in the proportion of one oxygen molecule to two copper atoms.
The presence of a respiratory pigment greatly increases the oxygen-carrying capacity of blood; invertebrate blood may contain up to 10 percent oxygen with the pigment, compared with about 0.
All respiratory pigments become almost completely saturated with oxygen even at oxygen levels, or pressures, below those normally found in air or water. The oxygen pressures at which the various pigments become saturated depend on their individual chemical characteristics and on such conditions as temperature, pH, and the presence of carbon dioxide.
In addition to their direct transport role, respiratory pigments may temporarily store oxygen for use during periods of respiratory suspension or decreased oxygen availability hypoxia. They may also act as buffers to prevent large blood pH fluctuations, and they may have an osmotic function that helps to reduce fluid loss from aquatic organisms whose internal hydrostatic pressure tends to force water out of the body. All systems involving the consistent movement of circulating fluid require at least one repeating pump and, if flow is to be in one direction, usually some arrangement of valves to prevent backflow.
The simplest form of animal circulatory pump consists of a blood vessel down which passes a wave of muscular contraction, called peristalsis , that forces the enclosed blood in the direction of contraction.
Valves may or may not be present. This type of heart is widely found among invertebrates, and there may be many pulsating vessels in a single individual. In the earthworm, the main dorsal aligned along the back vessel contracts from posterior to anterior 15 to 20 times per minute, pumping blood toward the head.
At the same time, the five paired segmental lateral side vessels, which branch from the dorsal vessel and link it to the ventral aligned along the bottom vessel, pulsate with their own independent rhythms. Although unusual, it is possible for a peristaltic heart to reverse direction.
After a series of contractions in one direction, the hearts of tunicates sea squirts gradually slow down and eventually stop.
After a pause the heart starts again, with reverse contractions pumping the blood in the opposite direction. An elaboration of the simple peristaltic heart is found in the tubular heart of most arthropods, in which part of the dorsal vessel is expanded to form one or more linearly arranged chambers with muscular walls. The walls are perforated by pairs of lateral openings ostia that allow blood to flow into the heart from a large surrounding sinus, the pericardium.
The heart may be suspended by alary muscles, contraction of which expands the heart and increases blood flow into it. The direction of flow is controlled by valves arranged in front of the in-current ostia. Chambered hearts with valves and relatively thick muscular walls are less commonly found in invertebrates but do occur in some mollusks, especially cephalopods octopus and squid.
Blood from the gills enters one to four auricles depending on the species and is passed back to the tissues by contraction of the ventricle. The direction of flow is controlled by valves between the chambers. The filling and emptying of the heart are controlled by regular rhythmical contractions of the muscular wall. In addition to the main systemic heart, many species have accessory booster hearts at critical points in the circulatory system.
Cephalopods have special muscular dilations, the branchial hearts, that pump blood through the capillaries, and insects may have additional ampullar hearts at the points of attachment of many of their appendages. The control of heart rhythm may be either myogenic originating within the heart muscle itself or neurogenic originating in nerve ganglia.
The hearts of the invertebrate mollusks, like those of vertebrates, are myogenic. They are sensitive to pressure and fail to give maximum beats unless distended; the beats become stronger and more frequent with increasing blood pressure. Although under experimental conditions acetylcholine a substance that transmits nerve impulses across a synapse inhibits molluscan heartbeat, indicating some stimulation of the heart muscle by the nervous system, cardiac muscle contraction will continue in excised hearts with no connection to the central nervous system.
Tunicate hearts have two noninnervated, myogenic pacemakers , one at each end of the peristaltic pulsating vessel. Separately, each pacemaker causes a series of normal beats followed by a sequence of abnormal ones; together, they provide periodic reversals of blood flow. The control of heartbeat in most other invertebrates is neurogenic, and one or more nerve ganglia with attendant nerve fibres control contraction. Removal of the ganglia stops the heart, and the administration of acetylcholine increases its rate.
Adult heart control may be neurogenic but not necessarily in all stages in the life cycle. The embryonic heart may show myogenic peristaltic contractions prior to innervation. Heart rate differs markedly among species and under different physiological states of a given individual. In general it is lower in sedentary or sluggish animals and faster in small ones.
The rate increases with internal pressure but often reaches a plateau at optimal pressures. Oxygen availability and the presence of carbon dioxide affect the heart rate, and during periods of hypoxia the heart rate may decrease to almost a standstill to conserve oxygen stores. The time it takes for blood to complete a single circulatory cycle is also highly variable but tends to be much longer in invertebrates than in vertebrates.
For example, in isolation, the circulation rate in mammals is about 10 to 30 seconds, for crustaceans about one minute, for cockroaches five to six minutes, and for other insects almost 30 minutes. At the simplest levels of metazoan organization, where there are at most two cell layers, the tissues are arranged in sheets.
The necessity for a formal circulatory system does not exist, nor are the mesodermal tissues, normally forming one, present. The addition of the mesodermal layer allows greater complexity of organ development and introduces further problems in supplying all cells with their essential requirements. Invertebrate phyla have developed a number of solutions to these problems; most but not all involve the development of a circulatory system: Among the acoelomate phyla, the members of Platyhelminthes flatworms have no body cavity, and the space between the gut and the body wall, when present, is filled with a spongy organ tissue of mesodermal cells through which tissue fluids may percolate.
Dorsoventral back to front flattening, ramifying gut ceca cavities open at one end , and, in the endoparasitic flatworm forms, glycolytic metabolic pathways which release metabolic energy in the absence of oxygen reduce diffusion distances and the need for oxygen and allow the trematodes and turbellarians of this phylum to maintain their normal metabolic rates in the absence of an independent circulatory system. The greatly increased and specialized body surface of the cestodes tapeworms of this phylum has allowed them to dispense with the gut as well.
Most of the other acoelomate invertebrate animals are small enough that direct diffusion constitutes the major means of internal transport. One acoelomate phylum, Nemertea proboscis worms , contains the simplest animals possessing a true vascular system.
In its basic form there may be only two vessels situated one on each side of the straight gut. The vessels unite anteriorly by a cephalic space and posteriorly by an anal space lined by a thin membrane. The system is thus closed, and the blood does not directly bathe the tissues. The main vessels are contractile, but blood flow is irregular and it may move backward or forward within an undefined circuit. The blood is usually colourless, although some species contain pigmented blood cells whose function remains obscure; phagocytic amoebocytes are usually also present.
Although remaining fundamentally simple, the system can grow more elaborate with the addition of extra vessels.
Pseudocoelomate metazoans have a fluid-filled body cavity, the pseudocoelom , which, unlike a true coelom, does not have a cellular peritoneal lining. Most of the pseudocoelomates e. Muscular body and locomotor movements may help to circulate nutrients within the pseudocoelom between the gut and the body wall. The lacunar system of channels within the body wall of the gutless acanthocephalans spiny-headed worms may represent a means of circulation of nutrients absorbed through the body wall.
Hemoglobin has been found in the pseudocoelomic fluid of a number of nematodes, but its precise role in oxygen transport is not known. Despite their greater potential complexity, many of the minor coelomate phyla e. All of the major and some of the minor phyla have well-developed blood vascular systems, often of open design. While some small segmented worms of the phylum Annelida have no separate circulatory system, most have a well-developed closed system.
The typical arrangement is for the main contractile dorsal vessel to carry blood anteriorly while a number of vertical segmental vessels, often called hearts, carry it to the ventral vessel, in which it passes posteriorly.
Segmental branches supply and collect blood from the respiratory surfaces, the gut, and the excretory organs.
There is, however, great scope for variation on the basic circulatory pattern. Many species have a large intestinal sinus rather than a series of vessels supplying the gut, and there may be differences along the length of a single individual. The posterior blood may flow through an intestinal sinus, the medial flow through a dense capillary plexus, and the anterior flow through typical segmental capillaries.
Much modification and complication may occur in species in which the body is divided into more or less distinct regions with specific functions. Many polychaete worms class Polychaeta , especially the fanworms but also representatives of other families, have many blind-ending contractile vessels. Continual reversals of flow within these vessels virtually replace the normal continuous-flow capillary system.
In most leeches class Hirudinea , much of the coelomic space is filled with mesodermal connective tissue, leaving a series of interconnecting coelomic channels. A vascular system comparable to other annelids is present in a few species, but in most the coelomic channels containing blood strictly coelomic fluid have taken over the function of internal transport, with movement induced by contraction of longitudinal lateral channels.
The blood of many annelids contains a respiratory pigment dissolved in the plasma, and the coelomic fluid of others may contain coelomic blood cells containing hemoglobin. The most common blood pigments are hemoglobin and chlorocruorin, but their occurrence does not fit any simple evolutionary pattern.
Closely related species may have dissimilar pigments, while distant relatives may have similar ones. In many species the pigments function in oxygen transport, but in others they are probably more important as oxygen stores for use during periods of hypoxia.
In addition to internal circulation, many polychaete worms also set up circulatory currents for feeding and respiration. Tube-dwelling worms may use muscular activity to pass a current of oxygenated water containing food through their burrows, while filter-feeding fanworms use ciliary activity to establish complicated patterns of water flow through their filtering fans. The phylum Echiura spoonworms contains a small number of marine worms with a circulatory system of similar general pattern to that of the annelids.
Main dorsal and ventral vessels are united by contractile circumintestinal vessels that pump the colourless blood. Coelomic fluid probably aids in oxygen transport and may contain some cells with hemoglobin. With the exception of the cephalopods, members of the phylum Mollusca have an open circulatory system. The chambered, myogenic heart normally has a pair of posterior auricles draining the gills and an anterior ventricle that pumps the blood through the anterior aorta to the tissue sinuses, excretory organs, and gills.
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In grocery stores, many types of buckwheat can be found. Buckwheat grains, groats and flour are now becoming available in most markets across the U. If possible, look for whole hulled grains, toasted, parboiled and dried groats, which are ready to cook with. If you buy buckwheat flour, it should be kept in the refrigerator or freezer and used within a short notice of time since it naturally contains oils that can go bad quickly. To cook dried buckwheat groats, rinse them well and then combine with water on the stovetop in a 2: Sprouting also reduces enzymes that can make buckwheat hard to digest for some people.
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