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If the fluid builds up slowly generic 140 mg malegra fxt amex, as in hypothyroidism order malegra fxt 140mg overnight delivery, the pericardial cavity may be able to expand gradually to accommodate this extra volume buy malegra fxt 140mg without a prescription. Premature removal of these drainage tubes, for example, following cardiac surgery, or clot formation within these tubes are causes of this condition. Surface Features of the Heart Inside the pericardium, the surface features of the heart are visible, including the four chambers. There is a superficial leaf- 830 Chapter 19 | The Cardiovascular System: The Heart like extension of the atria near the superior surface of the heart, one on each side, called an auricle—a name that means “ear like”—because its shape resembles the external ear of a human (Figure 19. Auricles are relatively thin-walled structures that can fill with blood and empty into the atria or upper chambers of the heart. Also prominent is a series of fat-filled grooves, each of which is known as a sulcus (plural = sulci), along the superior surfaces of the heart. Located between the left and right ventricles are two additional sulci that are not as deep as the coronary sulcus. The anterior interventricular sulcus is visible on the anterior surface of the heart, whereas the posterior interventricular sulcus is visible on the posterior surface of the heart. From superficial to deep, these are the epicardium, the myocardium, and the endocardium (see Figure 19. The outermost layer of the wall of the heart is also the innermost layer of the pericardium, the epicardium, or the visceral pericardium discussed earlier. It is built upon a framework of collagenous fibers, plus the blood vessels that supply the myocardium and the nerve fibers that help regulate the heart. It is the contraction of the myocardium that pumps blood through the heart and into the major arteries. The muscle pattern is elegant and complex, as the muscle cells swirl and spiral around the chambers of the heart. Deeper ventricular muscles also form a figure 8 around the two ventricles and proceed toward the apex. This complex swirling pattern allows the heart to pump blood more effectively than a simple linear pattern would. Although the ventricles on the right and left sides pump the same amount of blood per contraction, the muscle of the left ventricle is much thicker and better developed than that of the right ventricle. In order to overcome the high resistance required to pump blood into the long systemic circuit, the left ventricle must generate a great amount of pressure. The right ventricle does not need to generate as much pressure, since the pulmonary circuit is shorter and provides less resistance. Both ventricles pump the same amount of blood, but the left ventricle must generate a much greater pressure to overcome greater resistance in the systemic circuit. Note the differences in the relative size of the lumens, the region inside each ventricle where the blood is contained. The innermost layer of the heart wall, the endocardium, is joined to the myocardium with a thin layer of connective tissue. It is made of simple squamous epithelium called endothelium, which is continuous with the endothelial lining of the blood vessels (see Figure 19. Once regarded as a simple lining layer, recent evidence indicates that the endothelium of the endocardium and the coronary 832 Chapter 19 | The Cardiovascular System: The Heart capillaries may play active roles in regulating the contraction of the muscle within the myocardium. The endothelium may also regulate the growth patterns of the cardiac muscle cells throughout life, and the endothelins it secretes create an environment in the surrounding tissue fluids that regulates ionic concentrations and states of contractility. Endothelins are potent vasoconstrictors and, in a normal individual, establish a homeostatic balance with other vasoconstrictors and vasodilators. Internal Structure of the Heart Recall that the heart’s contraction cycle follows a dual pattern of circulation—the pulmonary and systemic circuits—because of the pairs of chambers that pump blood into the circulation. In order to develop a more precise understanding of cardiac function, it is first necessary to explore the internal anatomical structures in more detail. Septa of the Heart The word septum is derived from the Latin for “something that encloses;” in this case, a septum (plural = septa) refers to a wall or partition that divides the heart into chambers. Normally in an adult heart, the interatrial septum bears an oval-shaped depression known as the fossa ovalis, a remnant of an opening in the fetal heart known as the foramen ovale. The foramen ovale allowed blood in the fetal heart to pass directly from the right atrium to the left atrium, allowing some blood to bypass the pulmonary circuit. Within seconds after birth, a flap of tissue known as the septum primum that previously acted as a valve closes the foramen ovale and establishes the typical cardiac circulation pattern. Unlike the interatrial septum, the interventricular septum is normally intact after its formation during fetal development. It is substantially thicker than the interatrial septum, since the ventricles generate far greater pressure when they contract. It is marked by the presence of four openings that allow blood to move from the atria into the ventricles and from the ventricles into the pulmonary trunk and aorta. Located in each of these openings between the atria and ventricles is a valve, a specialized structure that ensures one-way flow of blood. The valves at the openings that lead to the pulmonary trunk and aorta are known generically as semilunar valves. In this figure, the atrioventricular septum has been removed to better show the bicupid and tricuspid valves; the interatrial septum is not visible, since its location is covered by the aorta and pulmonary trunk. Since these openings and valves structurally weaken the atrioventricular septum, the remaining tissue is heavily reinforced with dense connective tissue called the cardiac skeleton, or skeleton of the heart. It includes four rings that surround the openings between the atria and ventricles, and the openings to the pulmonary trunk and aorta, and serve as the point of attachment for the heart valves. The presence of the pulmonary trunk and aorta covers the interatrial septum, and the atrioventricular septum is cut away to show the atrioventricular valves. As much as 20–25 percent of the general population may have a patent foramen ovale, but fortunately, most have the benign, asymptomatic version. Patent foramen ovale is normally detected by auscultation of a heart murmur (an abnormal heart sound) and confirmed by imaging with an echocardiogram. Despite its prevalence in the general population, the causes of patent ovale are unknown, and there are no known risk factors. In nonlife-threatening cases, it is better to monitor the condition than to risk heart surgery to repair and seal the opening. Coarctation of the aorta is a congenital abnormal narrowing of the aorta that is normally located at the insertion of the ligamentum arteriosum, the remnant of the fetal shunt called the ductus arteriosus. If severe, this condition drastically restricts blood flow through the primary systemic artery, which is life threatening. Detectable symptoms in an infant include difficulty breathing, poor appetite, trouble feeding, or failure to thrive. In older individuals, symptoms include dizziness, fainting, shortness of breath, chest pain, fatigue, headache, and nosebleeds. Treatment involves surgery to resect (remove) the affected region or angioplasty to open the abnormally narrow passageway. Failure of the ductus arteriosus to close results in blood flowing from the higher pressure aorta into the lower pressure pulmonary trunk. This additional fluid moving toward the lungs increases pulmonary pressure and makes respiration difficult. Symptoms include shortness of breath (dyspnea), tachycardia, enlarged heart, a widened pulse pressure, and poor weight gain in infants.

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A mushroom-shaped herniation protrudes through the craniotomy defect and its edges become lacerated purchase 140mg malegra fxt mastercard. Gunshot Wounds Gunshot wounds to the brain produce a bullet tract of fairly uniform diameter purchase malegra fxt 140 mg. The tract may become hemorrhagic if the victim survives for more than a few minutes generic malegra fxt 140mg with visa. Even if the bullet does not penetrate a vital center in the brain, death is usually very rapid. The moving bullet transmits a great deal of energy to the brain and produces widespread damage, sometimes evidenced by contusions at some distance from the wound tract. The heat produced when a bullet is fired is not sufficient to sterilize it, nor is the scalp sterile. Brain abscess is the most common infectious complication of penetrating wounds, but meningitis and epidural empyema can also occur. Post-Traumatic Epilepsy Post-traumatic epilepsy is another complication of penetrating wounds (including neurosurgical wounds), probably because the mixed glial-mesenchymal scar that follows these wounds acts as a seizure focus. Cerebral Swelling Hematomas, contusions, and penetrating injuries all carry a significant risk of producing cerebral swelling due to congestion and edema. Contusions may also lead to swelling of an entire cerebral hemisphere, but this is more commonly the result of an ipsilateral acute subdural hematoma. Swelling of the entire brain may occur in children, sometimes following apparently minor trauma. Cerebral Hypoxia Head trauma is frequently accompanied by episodes of hypotension or hypoxia, due either to the head injury itself or to concurrent injuries to the rest of the body. Alone or in combination with raised intracranial pressure, such episodes often result in hypoxic damage to the brain. It is most common in young infants, with the majority of cases occurring before 6 months. Since the 1970’s, this syndrome has been attributed to violent shaking of the infant, whose large head and weak neck muscles allow a whiplash-like effect. These findings may be accompanied by rib fractures (from grabbing the thorax) and by metaphyseal fractures of the long bones, from flailing of the limbs. At autopsy, the subdural hemorrhage is rarely of sufficient volume to cause a significant mass effect, yet the brain is commonly swollen. Axonal spheroids are often seen, especially if immunohistochemical staining for amyloid precursor protein is performed to demonstrate them. The pathophysiology of this disorder is extraordinarily controversial and has given rise to some of the most passionate letters to editors imaginable about a neuropathological topic. One issue is whether the forces generated by shaking are sufficient to cause axonal shearing. Some authors have claimed that this is impossible, that most cases are accompanied by some evidence of impact, and that when this is lacking, there still must have been impact, albeit 163 against an object, such as a cushion, that prevented injury to the scalp or skull. Others have claimed that the only axonal injury directly caused by the shaking is at the junction of the medulla and cervical spinal cord, which leads to apnea, and that any further axonal injury is due to hypoxia and increased intracranial pressure; which they claim produce patterns of axonal injury that can be distinguished from those produced by trauma. They have also proposed that the subdural and retinal hemorrhages are the result of increased intracranial pressure, rather than the direct effect of trauma. Thus, they conclude that the entire syndrome can result from hypoxia without trauma. Related controversies, also with important implications in the prosecution of alleged baby-shaking, involve the reversibility of axonal damage and the question of whether infants can experience a lucent interval between trauma and loss of consciousness. These issues are difficult to resolve because of the absence of disinterested witnesses to the handling of the infants. However, from cases without scalp injury and with a confessed shaking, it seems clear that whatever the mechanism, shaking alone can give rise to subdural and retinal hemorrhages with loss of consciousness and axonal injury. On the other hand, if evidence of direct impact to the head is present, it is probably impossible to tell whether there was shaking or not. Anatomic Considerations The spinal canal becomes narrower when flexed or extended. This is particularly true in the presence of traumatic instability, when the vertebrae or the pieces of fractured vertebrae may be properly aligned when the spine is straight but displaced into the canal with motion. Therefore, it must always be remembered that in the presence of injury to the bony spine, movement of the spine can cause serious compression injury to the spinal cord, even if no such injury occurred initially. The spinal canal is narrowest in its cervical portion, the spine is weakest at this level, and violent motion of the head can place the cervical spine under tremendous stress. Traumatic spinal injuries are thus most commonly cervical, and cervical spine injuries must be ruled out in the presence of violent injuries to the head or face. Cranial or facial trauma can result in tearing of the ligaments that hold the odontoid process in place. If the spinal cord is injured, the level of cord injury will often differ from that of spinal injury. Posterior stab wounds are therefore likely to affect one side more than the other. Direct Injuries to the Spine These are caused by stab wounds or by bullets or other high velocity projectiles. As mentioned above, stab wounds are likely to involve one side more than the other, causing a complete or partial Brown-Sequard syndrome (ipsilateral paralysis and loss of vibratory and positional sensation with contralateral loss of pain and temperature sensation). Stab wounds tend to cause localized damage to the spinal cord with little intraparenchymal hemorrhage. Ascending and descending degeneration occur in a pattern predictable from the anatomy of the severed tracts. As in the brain, the damage from gunshot wounds extends beyond the region penetrated, for similar reasons The injuries tend to extend for several segments in either direction and to be fairly hemorrhagic. Gunshot wounds to the vertebrae can injure the spinal cord without penetrating it, by virtue of transferred energy alone. Bullets frequently become trapped in the spinal canal, and in the early stages of injury they can move. Eventually, whether epidurally or intraparenchymally located, they become encased in fibrous tissue. The adjacent spinal cord loses its normal structure and becomes a mixed glial-mesenchymal scar, poorly demarcated from the meninges to which it had become adherent. Indirect Injuries to the Spinal Cord These are blunt force or compressive injuries secondary to spinal trauma. While they occasionally result from temporary spinal deformities, they are more commonly the result of spinal fracture or subluxation. It should be borne in mind that spinal instability may result in temporary deformity that is not appreciated at the time of examination but that has already caused injury to the cord. Injury to the cervical spine is generally the result of cranial or facial injuries, commonly the result of motor vehicle accidents or falls. Motorcycle and diving accidents are particularly likely to result in such injuries. Thoracic spine fractures are more commonly the result of industrial accidents, such as mining cave-ins or collapsed roofs, in which weight falls on the victim. The immediate effects of spinal cord compression are necrosis, hemorrhage, edema, and inflammation.

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Instead buy 140 mg malegra fxt overnight delivery, there is a series of neurotransmitter-filled bulges called varicosities as an axon courses through smooth muscle cheap 140 mg malegra fxt fast delivery, loosely forming motor units (Figure 10 buy 140mg malegra fxt. Smooth muscle is organized in two ways: as single-unit smooth muscle, which is much more common; and as multiunit smooth muscle. Single-unit muscle has its muscle fibers joined by gap junctions so that the muscle contracts as a single unit. This type of smooth muscle is found in the walls of all visceral organs except the heart (which has cardiac muscle in its walls), and so it is commonly called visceral muscle. Because the muscle fibers are not constrained by the organization and stretchability limits of sarcomeres, visceral smooth muscle has a stress-relaxation response. This means that as the muscle of a hollow organ is stretched when it fills, the mechanical stress of the stretching will trigger contraction, but this is immediately followed by relaxation so that the organ does not empty its contents prematurely. This is important for hollow organs, such as the stomach or urinary bladder, which continuously expand as they fill. The smooth muscle around these organs also can maintain a muscle tone when the organ empties and shrinks, a feature that prevents “flabbiness” in the empty organ. In general, visceral smooth muscle produces slow, steady contractions that allow substances, such as food in the digestive tract, to move through the body. As a result, contraction does not spread from one cell to the next, but is instead confined to the cell that was originally stimulated. Hyperplasia in Smooth Muscle Similar to skeletal and cardiac muscle cells, smooth muscle can undergo hypertrophy to increase in size. Unlike other muscle, smooth muscle can also divide to produce more cells, a process called hyperplasia. This can most evidently be observed in the uterus at puberty, which responds to increased estrogen levels by producing more uterine smooth muscle fibers, and greatly increases the size of the myometrium. Skeletal muscles, excluding those of the head and limbs, develop from mesodermal somites, whereas skeletal muscle in the head and limbs develop from general mesoderm. A myoblast is a muscle-forming stem cell that migrates to different regions in the body and then fuse(s) to form a syncytium, 436 Chapter 10 | Muscle Tissue or myotube. As a myotube is formed from many different myoblast cells, it contains many nuclei, but has a continuous cytoplasm. This is why skeletal muscle cells are multinucleate, as the nucleus of each contributing myoblast remains intact in the mature skeletal muscle cell. However, cardiac and smooth muscle cells are not multinucleate because the myoblasts that form their cells do not fuse. Gap junctions develop in the cardiac and single-unit smooth muscle in the early stages of development. As neurons become active, electrical signals that are sent through the muscle influence the distribution of slow and fast fibers in the muscle. Although the number of muscle cells is set during development, satellite cells help to repair skeletal muscle cells. A satellite cell is similar to a myoblast because it is a type of stem cell; however, satellite cells are incorporated into muscle cells and facilitate the protein synthesis required for repair and growth. These cells are located outside the sarcolemma and are stimulated to grow and fuse with muscle cells by growth factors that are released by muscle fibers under certain forms of stress. Satellite cells can regenerate muscle fibers to a very limited extent, but they primarily help to repair damage in living cells. If a cell is damaged to a greater extent than can be repaired by satellite cells, the muscle fibers are replaced by scar tissue in a process called fibrosis. Because scar tissue cannot contract, muscle that has sustained significant damage loses strength and cannot produce the same amount of power or endurance as it could before being damaged. Smooth muscle tissue can regenerate from a type of stem cell called a pericyte, which is found in some small blood vessels. Pericytes allow smooth muscle cells to regenerate and repair much more readily than skeletal and cardiac muscle tissue. As scar tissue accumulates, the heart loses its ability to pump because of the loss of contractile power. However, some minor regeneration may occur due to stem cells found in the blood that occasionally enter cardiac tissue. Physical Therapist As muscle cells die, they are not regenerated but instead are replaced by connective tissue and adipose tissue, which do not possess the contractile abilities of muscle tissue. It is therefore important that those who are susceptible to muscle atrophy exercise to maintain muscle function and prevent the complete loss of muscle tissue. In extreme cases, when movement is not possible, electrical stimulation can be introduced to a muscle from an external source. This acts as a substitute for endogenous neural stimulation, stimulating the muscle to contract and preventing the loss of proteins that occurs with a lack of use. They are trained to target muscles susceptible to atrophy, and to prescribe and monitor exercises designed to stimulate those muscles. Age-related muscle loss is also a target of physical therapy, as exercise can reduce the effects of age-related atrophy and improve muscle function. The goal of a physiotherapist is to improve physical functioning and reduce functional impairments; this is achieved by understanding the cause of muscle impairment and assessing the capabilities of a patient, after which a program to enhance these capabilities is designed. Some factors that are assessed include strength, balance, and endurance, which are continually monitored as exercises are introduced to track improvements in muscle function. Physiotherapists can also instruct patients on the proper use of equipment, such as crutches, and assess whether someone has sufficient strength to use the equipment and when they can function without it. Smooth muscle is found in the skin, where it is associated with hair follicles; it also is found in the walls of internal organs, blood vessels, and internal passageways, where it assists in moving materials. Skeletal muscles maintain posture, stabilize bones and joints, control internal movement, and generate heat. The striations are created by the organization of actin and myosin resulting in the banding pattern of myofibrils. The cross-bridging of myposin heads docking into actin-binding sites is followed by the “power stroke”—the sliding of the thin filaments by thick filaments. The length of a sarcomere is optimal when the zone of overlap between thin and thick filaments is greatest. A motor unit is formed by a motor neuron and all of the muscle fibers that are innervated by that same motor neuron. Increasing the number of motor neurons involved increases the amount of motor units activated in a muscle, which is called recruitment. The opposite of hypertrophy is atrophy, the loss of muscle mass due to the breakdown of structural proteins. Muscle atrophy due to age is called sarcopenia and occurs as muscle fibers die and are replaced by connective and adipose tissue.

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