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Alternatively heart attack from weed purchase 6.25 mg carvedilol, degenerative changes may represent an attempt by the cells to repair a matrix where the primary cause of damage has been excessive mechani cal loading (Adams et al 2013) blood pressure chart elderly order carvedilol 12.5 mg on-line. Excessive loading does not necessarily imply trauma; normal loading is excessive if the matrix has become abnormally weak on account of an unfavourable genetic inheritance or age. In poorly vascularized tissues, such as cartilage and tendon, low cell density and inadequate transportation of metabolites could lead to a vicious circle of minor injury, frustrated repair, tissue weakening and further injury. Mechanotransduction Various mechanisms have been proposed to explain how cells in mus culoskeletal tissues detect mechanical loading. An influential review that explains delayed-onset muscle soreness following exercise. A recent account of the mechanics, biology and pathology of spinal tissues, including the origins of back pain. A classic account of the biomechanics of animal movement by one of the founding fathers of biomechanics. Buckingham M, Bajard L, Chang T et al 2003 the formation of skeletal muscle: from somite to limb. Farlay D, Boivin G, Panczer G et al 2005 Longterm strontium ranelate administration in monkeys preserves characteristics of bone mineral crystals and degree of mineralization of bone. A classic experiment concerning cartilage mechanobiology that has not been bettered. Holland A, Ohlendieck K 2013 Proteomic profiling of the contractile appa ratus from skeletal muscle. Junger S, GantenbeinRitter B, Lezuo P et al 2009 Effect of limited nutrition on in situ intervertebral disc cells under simulatedphysiological loading. Peng B, Chen J, Kuang Z et al 2009 Diagnosis and surgical treatment of back pain originating from endplate. Roberts S, Menage J, Duance V et al 1991 Collagen types around the cells of the intervertebral disc and cartilage end plate: an immunolocalization study. A classic animal experiment concerning bone mechanobiology, which is still widely cited. Thambyah A, Broom N 2007 On how degeneration influences loadbearing in the cartilagebone system: a microstructural and micromechanical study. Wang H, Listrat A, Meunier B et al 2014 Apoptosis in capillary endothelial cells in ageing skeletal muscle. The muscle in these tubes is of two types: smooth muscle is characteristic of the walls of blood vessels, whereas cardiac muscle provides the walls of the heart chambers with their powerful contractile pumping action. The general characteristics and classification of muscle tissues are given on page 103. Smooth muscle also forms an important contractile element in the walls of many other organ systems of the body. Smooth muscle is also referred to as involuntary muscle because its activity is neither initiated nor monitored consciously. It is much more variable, in both form and function, than either striated or cardiac muscle, a reflection of its varied roles in different systems of the body. Smooth muscle is typically found in the walls of tubular structures and hollow viscera. The account that follows will therefore be concerned with the generic properties of smooth muscle. The more specialized morphologies of smooth muscle are described in the appropriate regional chapters. Such an arrangement achieves both close packing and a more efficient transfer of force from cell to cell. This appearance contrasts markedly with that of skeletal muscle cells, which show a consistent diameter in cross-section and peripherally placed nuclei throughout their length. Smooth muscle has no attachment structures equivalent to the fasciae, tendons and aponeuroses associated with skeletal muscle. There is a special arrangement for transmitting force from cell to cell and, where necessary, to other soft tissue structures. These elements bridge the gaps between adjacent cells and provide mechanical continuity throughout the fascicle. At the boundaries of fascicles, the connective tissue fibres become interwoven with those of interfascicular septa, so that the contraction of different fascicles is communicated throughout the tissue and to neighbouring structures. The components of the reticular network, the ground substance and collagen and elastic fibres, are synthesized by the smooth muscle cells themselves, not by fibroblasts or other connective tissue cells, which are rarely found within fasciculi. Their length can range from 15 µm in small blood vessels to 200 µm, and even to 500 µm or more in the uterus during pregnancy. The nucleus is single, located at the midpoint, and often twisted into a corkscrew shape by the contraction of the cell. Individual cells are spindle-shaped with a single central nucleus, aligned in parallel with neighbouring cells in a fasciculus. In several places, the plasma membranes of adjacent cells are closely approximated at gap junctions (arrows). For clarity, some structural features have been separated for illustration in different cells. The spindle-shaped cells interdigitate with their long axes parallel; mechanical continuity between the cells is provided by a reticular layer of elastin and collagen fibres. The cytoskeletal framework consists of intermediate filament arrays (mainly longitudinal) and bundles of actin and myosin filaments (shown in separate cells) inserted into cytoplasmic dense bodies and submembraneous dense plaques to form a three-dimensional network. The sarcolemma contains anchoring desmosomes (adherens junctions), gap junctions and caveolae. Vascular smooth muscle in human kidney tissue, showing a cytoplasm packed densely with microfilaments (actin and myosin), cytoplasmic dense bodies (arrows) and submembraneous dense plaques (arrowheads).

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Fetuses with in utero muscular dystrophies hypertension 9 code order carvedilol 25 mg with visa, or other conditions that result in small or atrophied muscles heart attack maroon 5 order carvedilol 12.5 mg without prescription, have webs of skin, pterygia, which pass across the flexor aspect of the joints and severely limit movement. Multiple pterygium syndrome is characterized by webbing across the neck, the axillae and antecubital fossae. Usually, the legs are maintained straight and webbing is not seen at the hip and knee. These conditions may be recog nized on prenatal ultrasound examination by the appearance of fixed, immobile limbs in bizarre positions, or by webbing in limb flexures. Specific syndromes, lethal multiple pterygium syndrome and congenital muscular dystrophy, have been described. The workload undertaken by the musculoskeletal system before birth is relatively light because the fetus is supported by the amniotic fluid and, therefore, under essentially weightless conditions. The load on the muscles and bones is generated by the fetus itself, with little gravitational effect. The reduction of gravitational force afforded by the supporting fluid means that all parts of the fetus are subject to relatively equal forces and that the position assumed by the fetus relative to gravity is of little consequence. This is important to ensure the normal modelling of fetal bones, especially the skull. Skulls of premature babies may become distorted as a result of the weight of the head on the mattress, despite regular changes in position, and the application of oxygen therapy via a mask attached by a band around the head can cause dysostosis of the occipital bone. Later, one main vessel, the axial artery, supplies the limb and the terminal plexus. The development of the vasculature in the limb precedes the mor phological and molecular changes that occur within the limb mesen chyme as tissues begin to form. Cartilage differentiation within the chick limb bud occurs only after local vascular regression begins, and only in areas with few or no capillaries (Hallmann et al 1987). It is not known whether the presence, or lack, of blood vessels affords different local environmental stimuli for mesenchymal cells (by varying the supply of nutrients to the tissue), or whether the local environment is controlled by the endothelial cells. Similarly, it is not clear whether inductive factors from the limb mesenchyme cause the changes that occur in the pattern of blood vessels. Work on chick wing buds suggests that the position of the central artery in the primitive limb bud vascu lature depends on Shh signalling from the polarizing region (Davey et al 2007). Simple movements of an extremity have been observed sporadically as early as the seventh week of gestation in human embryos. Combined movements of limb, trunk and head commence between 12 and 16 weeks of gestation. Movements of the embryo and fetus encourage normal skin growth and flexibility, in addition to the progressive maturation of the muscu loskeletal system. Movements of the fetus often involve slow and asym metric twisting and stretching movements of the trunk and limbs, which resemble athetoid movements. There may also be rapid, repeti tive, wideamplitude limb movements, similar to myoclonus. By term, the quality of the movements has generally matured to smooth, alternating movement of the limbs, with medium speed and intensity. The reduced effect of gravity in utero may cause certain fetal movements to appear, on ultrasonography, more fluent than the equiva lent movements observed postnatally. As the limb enlarges, the marginal vein can be subdivided into pre and postaxial veins, which run along their respective borders and which are the precursors of the superficial veins of the limb. Generally, the preaxial (superficial) veins join the deep veins at the proximal joint, and the postaxial (superficial) veins join the deep veins at the distal joint of the limb. In: Ferretti P, Copp A, Tickle C et al (eds) Embryos, Genes and Birth Defects, 2nd ed. Zeller R, LopezRios J, Zuniga A 2009 Vertebrate limb development; moving towards integrative analysis of organogenesis. Chevallier A, Kieny M, Mauger A 1977 Limbsomite relationship: origin of the limb musculature. Lewis J, Chevallier A, Kieny M et al 1981 Muscle nerve branches do not develop in chick wings devoid of muscle. An account that reviews and contrasts recent work on both chick and mouse models on the importance of movement in the development of the locomotor apparatus. A review of recent work using chicken embryos to explore the importance of movement in development of the locomotor apparatus. Sugimoto Y, Takimoto A, Akiyama H et al 2013 Scx+/Sox9+ progenitors contribute to the establishment of the junction between cartilage and tendon/ligament. Summerbell D, Lewis J, Wolpert L 1973 Positional information in chick limb morphogenesis. Sürmeli G, Akay T, Ippolito G et al 2011 Patterns of spinal sensorymotor connectivity prescribed by a dorsoventral positional template. Weintraub H, Davis R, Tapscott S et al 1991 the MyoD gene family: nodal point during specification of the muscle cell lineage. A review of the molecular basis of limb development particularly focusing on how cell­cell interactions are integrated. Zuniga A, Zeller R, Probst S 2012 the molecular basis of human congenital limb malformations. A presentation of recently identified genes involved in limb development and their relevance to human congenital limb defects. This effort started with the formalizations of human, fruit fly and mouse anatomy in the 1990s and now includes all the major model organisms (Bard 2005, Druzinsky et al 2013). The types of data that are associated with tissues currently include gene expression, diseases and abnormal phenotypes.

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Low-frequency stimulation alleviates postural instability and on-medication gait freezing and falling hypertension online cheapest generic carvedilol uk, symptoms that conventional medication and surgery fail to improve prehypertension causes symptoms buy genuine carvedilol. The relative extent of the pedunculopontine nucleus has been outlined based on choline acetyltransferase immunohistochemistry. An explanation of the abnormalities in basal ganglia function involved in dystonia. A review presenting a persuasive argument for functional similarities and intimate reciprocal connections between pedunculopontine nucleus and other basal ganglia structures. A landmark publication setting out a conceptual framework for the way in which the basal ganglia and cerebral cortex process different types of information through largely distinct parallel circuits based on known anatomical connectivity. A review that highlights some limitations of the anatomical model of basal ganglia function. They contain the primary motor and sensory cortices, the highest levels at which motor activities are controlled and to which general and special sensory systems project, and which provide the neural substrate for the conscious experience of sensory stimuli. Association areas are both modality-specific and multimodal, enabling complex analyses of the internal and external environment and of the relationship of the individual with the external world. The elements of the limbic system are particularly concerned with memory and the emotional aspects of behaviour, and provide an affective overtone to conscious experience as well as an interface with subcortical areas such as the hypothalamus, through which widespread physiological activities are integrated. Other cortical areas, primarily within the frontal region, are concerned with the highest aspects of cognitive function and contribute to personality, judgment, foresight and planning. The configuration of the main cerebral sulci and gyri provides the basis for dividing the hemispheres into frontal, parietal, occipital, temporal, insular and limbic lobes. The internal white matter contains association fibres limited to each hemisphere, commissural fibres linking corresponding areas of both hemispheres, and projection fibres connecting the cerebral cortex of each hemisphere with subcortical, brainstem and spinal cord nuclei. Some of these bundles (tracts, fasciculi) are relatively well defined macroscopically and microscopically, while others are less easy to identify. A detailed knowledge of the threedimensional anatomical interrelationships of white matter tracts is a requisite for the planning, intraoperative monitoring and execution of neurosurgical resective procedures. Current understanding of these relationships owes much to the seminal work of Josef Klingler and his meticulous dissection of white matter tracts using formalinfixed, freeze-thawed brains (Agrawal et al 2011). The superolateral surface is convex and lies beneath the bones of the cranial vault; the frontal, parietal, temporal and occipital lobes correspond approximately in surface extent to the overlying bones from which they take their names. The frontal and parietal lobes are separated from the temporal lobe by the prominent lateral (Sylvian) fissure. The inferior surface is divided by the anterior part of the lateral fissure into a small anterior orbital part and a larger posterior tentorial part. The orbital part is the concave orbital surface of the frontal lobe and rests on the floor of the anterior cranial fossa. The posterior part is formed by the basal aspects of the temporal and occipital lobes, and rests on the floor of the middle cranial fossa and the upper surface of the tentorium cerebelli, which separates it from the superior surface of the cerebellum. The medial surface is flat and vertical, separated from the opposite hemisphere by the longitudinal fissure and the falx cerebri. Anteriorly, the cerebral hemisphere terminates at the frontal and temporal poles, and posteriorly at the occipital pole. The cerebral sulci delineate the brain gyri and are extensions of the subarachnoid space (Butler and Hodos 2005, Sarnat and Netsky 1981, Park et al 2007, Chi et al 1977, Nishikuni and Ribas 2013, Ono et al 1990, Catani and Thiebaut de Schotten 2012, Duvernoy 1991, Naidich et al 2013). The main sulci have depths of 1­3 cm, and their walls harbour small gyri that connect with each other (transverse gyri). Sulci that separate the transverse gyri vary in length and depth, and may become visible as incisures at the surface of the brain. The sulci of the superolateral and inferior surfaces of the hemisphere are usually orientated towards the nearest ventricular cavity. Sulci that are usually continuous include the lateral fissure and the callosal, calcarine, parieto-occipital, collateral and, generally, the central sulcus. On the superolateral surface of the hemisphere, the frontal and temporal regions are each composed of three horizontal gyri (superior, middle and inferior frontal and temporal gyri). The central area is composed of two slightly oblique gyri (pre- and postcentral gyri). The occipital region is composed of two or three less well-defined gyri (superior, middle and inferior occipital gyri). The orbital part of the inferior surface is covered by the orbital gyri and the basal aspect of the rectus gyri, and the tentorial part of the inferior surface is covered by the basal aspects of the inferior temporal, inferior occipital and lingual gyri, and the fusiform gyrus. Non-pyramidal cells, also called stellate or granule cells, are divided into spiny and non-spiny types. The extent to which this organization aids the understanding of cortical functional organization is debatable, but the use of cytoarchitectonic description to identify regions of cortex is common. These are the molecular or plexiform layer; external granular lamina; external pyramidal lamina; internal granular lamina; internal pyramidal (ganglionic) lamina; and multiform (or fusiform/pleiomorphic) layer. Homotypical variants, in which all six laminae are found, are called frontal, parietal and polar, names that link them with specific cortical regions in a somewhat misleading manner. Large pyramidal neurones are found in the greatest densities in agranular cortex, which is typified by the numerous efferent projections of pyramidal cell axons. Although it is often equated with motor cortical areas such as the precentral gyrus (area 4), agranular cortex also occurs elsewhere. In the granular type of cortex the granular layers are maximally developed and contain densely packed stellate cells, among which small pyramidal neurones are dispersed. However, it does receive efferent fibres, derived from the scattered pyramidal cells, although they are less numerous than elsewhere. Granular cortex occurs in the postcentral gyrus (somatosensory area), striate area (visual area) and superior temporal gyrus (acoustic area), and in small areas of the parahippocampal gyrus. Despite its very high density of stellate cells, especially in the striate area, it is almost the thinnest of the five main types.

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It contains both decussating and commissural fibres that connect diencephalic and mesencephalic nuclei: the interstitial and dorsal nuclei of the posterior commissure located within the periventricular grey matter; nucleus of Darkschewitsch of the periaqueductal grey matter; interstitial nucleus of Cajal located at the rostral end of the oculomotor nucleus and closely linked with the medial longitudinal fasciculus; and posterior thalamic blood pressure medication makes me pee buy carvedilol 25 mg on line, pretectal blood pressure chart vaughns cheap carvedilol on line, tectal and habenular nuclei. Habenular commissure the habenular commissure lies between the habenulae, small protuberances of the thalami located at the distal ends of the striae medullaris. B, Cortical gyri of the insula exposed by removal of the frontal, temporal and parietal opercula. C, Removal of the insular cortex, extreme capsule, claustrum and external capsule to expose the lateral aspect of the putamen. E, Removal of part of the temporal lobe to show the internal capsule fibres converging on the crus cerebri of the midbrain. F, Removal of the optic tract and superficial dissection of the pons and upper medulla, emphasizing the continuity of the corona radiata, internal capsule, crus cerebri, longitudinal pontine fibres and the medullary pyramid. Anterior thalamic radiations interconnect the medial and anterior thalamic nuclei and various hypothalamic nuclei and limbic structures with the frontal cortex. The genu of the internal capsule is usually regarded as containing corticobulbar fibres, which are mainly derived from area 4 and terminate mostly in the contralateral motor nuclei of cranial nerves. Anterior fibres of the superior thalamic radiation, between the thalamus and cortex, also extend into the genu. The posterior limb of the internal capsule includes the corticospinal or pyramidal tract. The fibres concerned with the upper limb are anterior, and the more posterior regions contain fibres representing the trunk and lower limbs. To join the internal capsule, as well as converging, the corticospinal tract fibres undergo an internal rotation of approximately 90 degrees, since the main axes of the precentral gyrus and of the internal capsule genu and posterior limb are almost perpendicular. Throughout their convergence and rotation the fibres keep their somatotopical motor arrangement, originated according to the homuncular cortical representation, and end up having an anteriorcranial to a posterior-caudal disposal along the genu and the anterior portion of the internal capsule posterior limb. Radiologically and surgically, this important portion of the internal capsule can have its topography estimated from the position of the interventricular foramen (of Monro) that lies medially and adjacent to the internal capsule genu, which contains the corticonuclear bundle. Other descending axons include frontopontine fibres, particularly from areas 4 and 6, and corticorubral fibres, which connect the frontal lobe to the red nucleus. Most of the posterior limb also contains fibres of the superior thalamic radiation (the somaesthetic radiation) ascending to the postcentral gyrus. The retrolenticular part of the internal capsule contains parietopontine, occipitopontine and occipitotectal fibres. It also includes the posterior thalamic radiation and the optic radiation, and interconnections between the occipital and parietal lobes and caudal parts of the thalamus, especially the pulvinar. The fibres of the optic radiation arise from the lateral geniculate body and the pulvinar, join the retrolenticular and the sublenticular parts of the internal capsule, run within the sagittal stratum over the inferior horn and ventricular atrium, and project posteriorly, passing superiorly and inferiorly to the posterior horn as part of the posterior thalamic peduncle to reach both the superior and inferior lips of the calcarine sulcus. Within the temporal lobe, the fibres of the optic radiation are located along the depths of the superior and middle temporal gyri about 2 cm from the brain surface, inferior to the vertical segment of the superior longitudinal fasciculus, and always superior to the inferior temporal sulcus. In coronal sections, the optic radiation appears predominantly flat anteriorly and comma-shaped posteriorly. The sublenticular part of the internal capsule contains temporopontine and some parietopontine fibres, the auditory radiation from the medial geniculate body to the superior temporal and transverse temporal gyri (areas 41 and 42), and a few fibres that connect the thalamus with the temporal lobe and insula. Fibres of the auditory radiation sweep anterolaterally below and behind the lentiform complex to reach the cortex, and are superior to the inferior horn, the atrium and the optic radiation. The primary somatosensory, visual and auditory areas give rise to ipsilateral corticocortical connections to the association areas of the parietal, occipital and temporal lobes, respectively, which then progressively project towards the medial temporal limbic areas: notably, the parahippocampal gyrus, entorhinal cortex and hippocampus. From here, connections pass to cortex in the walls of the superior temporal sulcus, and so on to the posterior parahippocampal gyrus, and on into limbic cortex. Similarly, for the visual system, the primary visual cortex (area 17) projects to the parastriate cortex (area 18), which in turn projects to the peristriate region (area 19). Information then flows to inferotemporal cortex (area 20), to cortex in the walls of the superior temporal sulcus, then to medial temporal cortex in the posterior parahippocampal gyrus, and so to limbic areas. The auditory system shows a similar progression from primary auditory cortex to temporal association cortex, and so to the medial temporal lobe. In addition to this stepwise outward progression from sensory areas through posterior association cortex, connections also occur at each stage with parts of the frontal cortex. The next step in the outward progression, the superior parietal lobule (area 5), is interconnected with the premotor cortex (area 6), and this in turn is connected with area 7 in the inferior parietal lobule. This has reciprocal connections with prefrontal association cortex on the lateral surface of the hemisphere (areas 9 and 46), and temporal association areas, which connect with more anterior prefrontal association areas and, ultimately in the sequence, with orbitofrontal cortex. Similar stepwise links exist between areas on the visual and auditory association pathways in the occipitotemporal lobe and areas of the frontal association cortex. All neocortical areas are connected with subcortical regions, although their density varies between areas. All areas of the neocortex receive afferents from more than one thalamic nucleus, and all such connections are reciprocal. The vast majority of, if not all, cortical areas project to the striatum, tectum, pons and brainstem reticular formation. Additionally, all cortical areas are reciprocally connected with the claustrum; the frontal cortex connects with the anterior part and the occipital lobe with the posterior part. All cortical areas receive topographically organized cholinergic projections from the basal forebrain, noradrenergic fibres from the locus coeruleus, serotoninergic fibres from the midbrain raphe nuclei, dopaminergic fibres from the ventral midbrain, and histaminergic fibres from the posterior hypothalamus. Widely separated, but functionally interconnected, areas of cortex share common patterns of connections with subcortical nuclei, and within the neocortex. For example, contiguous zones of the striatum, thalamus, claustrum, cholinergic basal forebrain, superior colliculus and pontine nuclei connect with anatomically widely separated areas in the prefrontal and parietal cortex, which are themselves interconnected. Although there is debate as to their exact composition, in broad outline, the temporal stem lies anterior to the inferior horn and connects anteromedial temporal structures to the basolateral frontal portion of the hemisphere. The sagittal stratum corresponds to fibres running along the inferior limiting sulcus of the insula forming the roof and lateral walls of the inferior horn and ventricular atrium.

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From the centre to the periphery blood pressure chart form 6.25 mg carvedilol purchase amex, the vascular tree shows three main modifications pulse pressure pregnancy cheap generic carvedilol uk. The arteries increase in number by repeated bifurcation and by sending out side branches, in both the systemic and the pulmonary circulation. For example, the aorta, which carries blood from the heart to the systemic circulation, gives rise to about 4 × 106 arterioles and four times as many capillaries. The arteries also decrease in diameter, although not to the same extent as their increase in number, so that a hypothetical cross-section of all the vessels will show an increase in total area with increasing distance from the heart. At its emergence from the heart, the aorta of an adult man has an outer diameter of approximately 30 mm (cross-sectional area of nearly 7 cm2). The diameter decreases along the arterial tree until it is as little as 10 µm in arterioles (each with a cross-sectional area of about 80 µm2). However, given the enormous number of arterioles, the total cross-sectional area at this level is approximately 150 cm2, more than 200 times that of the aorta. The walls of arteries decrease in thickness towards the periphery, although this is not as substantial as the reduction in vessel diameter. Consequently, in the smallest arteries (arterioles), the thickness of the wall represents about half the outer radius of the vessel, whereas in a large vessel it represents between one-fifteenth and one-fifth. Venules, which return blood from the capillaries, converge on each other, forming a progressively smaller number of veins of increasingly large size. As with arteries, the hypothetical total cross-sectional area of all veins at a given level reduces nearer to the heart. Eventually, only the two largest veins, the superior and inferior venae cavae, open into the heart from the systemic circulation. A similar pattern is found in the pulmonary circulation but here the vascular loop is shorter and has fewer branch points, and consequently, the number of vessels is smaller than in the systemic circulation. The total end-to-end length of the vascular network in a typical adult is twice the circumference of the earth. Large arteries, such as the thoracic aorta and subclavian, axillary, femoral and popliteal arteries, lie close to a single vein that drains the same territory as that supplied by the artery. Other arteries are usually flanked by two veins, satellite veins (venae comitantes), which lie on either side of the artery and have numerous cross-connections; the whole is enclosed in a single connective tissue sheath. The artery and the two satellite veins are often associated with a nerve, and when they are surrounded by a common connective tissue sheath they form a neurovascular bundle. The close association between the larger arteries and veins in the limbs allows counterflow exchange of heat. This mechanism promotes heat transfer from arterial to venous blood and thus helps to preserve body heat. Counterflow exchange mechanisms are found in the microcirculation, as in the arterial and venous sinusoids of the vasa recta in the renal medulla. Here, countercurrent exchange retains solutes at a high concentration in the medullary interstitium, with efferent venous blood transferring solutes to the afferent arterial supply; this mechanism is essential for concentration of the urine. In functional terms, four main classes of vessel are described: conducting and distributing vessels (large arteries), resistance vessels (small arteries but mainly arterioles), exchange vessels (capillaries, sinusoids and small venules) and capacitance vessels (veins). Although muscle cells and elastic tissue are present in all arteries, the relative amount of elastic material is greatest in the largest vessels, whereas the relative amount of smooth muscle increases progressively towards the smallest arteries. The large conducting arteries that arise from the heart, together with their main branches, are characterized by the predominantly elastic properties of their walls. Distributing vessels are smaller arteries supplying the individual organs, and their walls are characterized by a well-developed muscular component. Resistance vessels include the smallest arteries and arterioles, and are highly muscularized. They provide the major part of peripheral resistance to blood flow and so cause the largest drop in blood pressure before the blood flows into the tissue capillary beds. Capillaries, sinusoids and small (postcapillary) venules are collectively termed exchange vessels. Their thin walls allow exchange between blood and the interstitial fluid that surrounds all cells: this is the essential function of a circulatory system. Arterioles, capillaries and venules constitute the microvasculature, the structural basis of the microcirculation. Larger venules and veins form an extensive but variable, largevolume, low-pressure system of vessels conveying blood back to the heart. Their high capacitance is due to the significant distensibility (compliance) of their walls, so that the content of blood is high even at low pressures. Veins contain the greatest proportion of blood, reflecting their large relative volume. Blood from the gastrointestinal tract (with the exception of the lower part of the anal canal) and from the spleen, pancreas and gallbladder drains to the liver via the portal vein. The portal vein ramifies within the substance of the liver like an artery and ends in the hepatic sinusoids. These drain into the hepatic veins, which in turn drain into the inferior vena cava. Blood supplying the abdominal organs thus passes through two sets of capillaries before it returns to the heart. The first provides the organs with oxygenated blood, and the second carries deoxygenated blood, rich in absorption products from the intestine, through the liver parenchyma. A venous portal circulation also connects the median eminence and infundibulum of the hypothalamus with the adenohypophysis. In essence, a venous portal system is a capillary network that lies between two veins, instead of between an artery and a vein, which is the more usual arrangement in the circulation. This maintains a relatively high-pressure system, which is important for renal filtration.

References

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