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The precapillary sphincters purchase 160mg malegra fxt plus with visa impotence pills for men, circular smooth muscle cells that surround the capillary at its origin with the metarteriole discount 160 mg malegra fxt plus otc erectile dysfunction natural remedies diabetes, tightly regulate the flow of blood from a metarteriole to the capillaries it supplies. Their function is critical: If all of the capillary beds in the body were to open simultaneously, they would collectively hold every drop of blood in the body and there would be none in the arteries, arterioles, venules, veins, or the heart itself. When the surrounding tissues need oxygen and have excess waste products, the precapillary sphincters open, allowing blood to flow through and exchange to occur before closing once more (Figure 20. If all of the precapillary sphincters in a capillary bed are closed, blood will flow from the metarteriole directly into a thoroughfare channel and then into the venous circulation, bypassing the capillary bed entirely. In addition, an arteriovenous anastomosis may bypass the capillary bed and lead directly to the venous system. Although you might expect blood flow through a capillary bed to be smooth, in reality, it moves with an irregular, pulsating flow. This pattern is called vasomotion and is regulated by chemical signals that are triggered in response to changes in This OpenStax book is available for free at http://cnx. For example, during strenuous exercise when oxygen levels decrease and carbon dioxide, hydrogen ion, and lactic acid levels all increase, the capillary beds in skeletal muscle are open, as they would be in the digestive system when nutrients are present in the digestive tract. During sleep or rest periods, vessels in both areas are largely closed; they open only occasionally to allow oxygen and nutrient supplies to travel to the tissues to maintain basic life processes. Precapillary sphincters located at the junction of a metarteriole with a capillary regulate blood flow. An arteriovenous anastomosis, which directly connects the arteriole with the venule, is shown at the bottom. The walls of venules consist of endothelium, a thin middle layer with a few muscle cells and elastic fibers, plus an outer layer of connective tissue fibers that constitute a very thin tunica externa (Figure 20. Venules as well as capillaries are the primary sites of emigration or diapedesis, in which the white blood cells adhere to the endothelial lining of the vessels and then squeeze through adjacent cells to enter the tissue fluid. Because they are low-pressure vessels, larger veins are commonly equipped with valves that promote the unidirectional flow of blood toward the heart and prevent backflow toward the capillaries caused by the inherent low blood pressure in veins as well as the pull of gravity. In terms of scale, the diameter of a venule is measured in micrometers compared to millimeters for veins. Comparison of Arteries and Veins Arteries Veins Conducts blood away from Direction of blood flow Conducts blood toward the heart the heart General appearance Rounded Irregular, often collapsed Pressure High Low Wall thickness Thick Thin Relative oxygen Higher in systemic arteries Lower in systemic veins concentration Lower in pulmonary arteries Higher in pulmonary veins Present most commonly in limbs and in veins Valves Not present inferior to the heart Table 20. Any blood that accumulates in a vein will increase the pressure within it, which can then be reflected back into the smaller veins, venules, and eventually even the capillaries. Increased pressure will promote the flow of fluids out of the capillaries and into the interstitial fluid. Most people experience a daily accumulation of tissue fluid, especially if they spend much of their work life on their feet (like most health professionals). Edema has many potential causes, including hypertension and heart failure, severe protein deficiency, renal failure, and many others. In order to treat edema, which is a sign rather than a discrete disorder, the underlying cause must be diagnosed and alleviated. This disorder arises when defective valves allow blood to accumulate within the veins, causing them to distend, twist, and become visible on the surface of the integument. Varicose veins may occur in both sexes, but are more common in women and are often related to pregnancy. More than simple cosmetic blemishes, varicose veins are often painful and sometimes itchy or throbbing. The use of support hose, as well as elevating the feet and legs whenever possible, may be helpful in alleviating this condition. Laser surgery and interventional radiologic procedures can reduce the size and severity of varicose veins. As there are typically redundant circulation patterns, that is, 898 Chapter 20 | The Cardiovascular System: Blood Vessels and Circulation anastomoses, for the smaller and more superficial veins, removal does not typically impair the circulation. There is evidence that patients with varicose veins suffer a greater risk of developing a thrombus or clot. Veins as Blood Reservoirs In addition to their primary function of returning blood to the heart, veins may be considered blood reservoirs, since systemic veins contain approximately 64 percent of the blood volume at any given time (Figure 20.

An absolute monocytosis (>1 X 109/L) is present and immature erythrocytes and granulocytes may also be present buy malegra fxt plus 160mg without a prescription erectile dysfunction treatment options articles. The bone marrow is hypercellular with proliferation of abnormal myelocytes cheap 160 mg malegra fxt plus visa erectile dysfunction causes weed, promonocytes, and monoblasts, and there are <20% blasts. Chylous A body effusion that has a milky, opaque appearance due to the presence of lymph fluid and chylomicrons. Circulating leukocyte The population of neutrophils actively circulating pool within the peripheral blood stream. Can be detected by the identification of only one of the immunoglobulin light chains (kappa or lambda) on B cells or the presence of a population of cells with a common phenotype. Clot Extravascular coagulation, whether occurring in vitro or in blood shed into the tissues or body cavities. Retraction of the clot occurs over a period of time and results in the expression of serum and a firm mass of cells and fibrin. Cold agglutinin disease Condition associated with the presence of cold- reacting autoantibodies (IgM) directed against erythrocyte surface antigens. Colony forming unit A visible aggregation (seen in vitro) of cells that developed from a single stem cell. Colony stimulating factorCytokine that stimulates the growth of immature leukocytes in the bone marrow. The common pathway includes three rate-limiting steps: (1) activation of factor X by the intrinsic and extrinsic pathways, (2) conversion of prothrombin to thrombin by activated factor X, and (3) cleavage of fibrinogen to fibrin. Compensated hemolytic A disorder in which the erythrocyte life span is disease decreased but the bone marrow is able to increase erythropoiesis enough to compensate for the decreased erythrocyte life span; anemia does not develop. Complement Any of the eleven serum proteins that when sequentially activated causes lysis of the cell membrane. Congenital Heinz body Inherited disorder characterized by anemia due hemolytic anemia to decreased erythrocyte lifespan. Erythrocyte hemolysis results from the precipitation of hemoglobin in the form of heinz bodies, which damages the cell membrane and causes cell rigidity. Contact group A group of coagulation factors in the intrinsic pathway that is involved with the initial activation of the coagulation system and requires contact with a negatively charged surface for activity. Continuous flow analysisAn automated method of analyzing blood cells that allows measurement of cellular characteristics as the individual cells flow singly through a laser beam. Contour gating Subclassification of cell populations based on two characteristics such as size (x-axis) and nuclear density (y-axis) and the frequency (z- axis) of that characterized cell type. Coverglass smear Blood smear prepared by placing a drop of blood in the center of one coverglass, then placing a second coverglass on top of the blood at a 45° angle to the first coverglass. Cyanosis Develops as a result of excess deoxygenated hemoglobin in the blood, resulting in a bluish color of the skin and mucous membranes. Cytochemistry Chemical staining procedures used to identify various constituents (enzymes and proteins) within white blood cells. Useful in differentiating blasts in acute leukemia, especially when morphologic differentiation on romanowsky stained smears is impossible. Cytokine Protein produced by many cell types that modulates the function of other cell types; cytokines include interleukins, colony stimulating factors, and interferons. This occurs because the primary hemostatic plug is not adequately stabilized by the formation of fibrin. Döhle bodies An oval aggregate of rough endoplasmic reticulum that stains light gray blue (with Romanowsky stain) found within the cytoplasm of neutophils and eosinophils. It is associated with severe bacterial infection, pregnancy, burns, cancer, aplastic anemia, and toxic states. The antibody reacts with erythrocytes in capillaries at temperatures below 15°C and fixes complement to the cell membrane. Upon warming, the terminal complement components on erythrocytes are activated, causing cell hemolysis. Dysfibrinogenemia A hereditary condition in which there is a structural alteration in the fibrinogen molecule. Dyspoiesis Abnormal development of blood cells frequently characterized by asynchrony in nuclear to cytoplasmic maturation and/or abnormal granule development. Echinocyte A spiculated erythrocyte with short, equally spaced projections over the entire outer surface of the cell.

However malegra fxt plus 160mg discount erectile dysfunction nitric oxide, there are also ovarian ectopic pregnancies (in which the egg never left the ovary) and abdominal ectopic pregnancies (in which an egg was “lost” to the abdominal cavity during the transfer from ovary to uterine tube 160 mg malegra fxt plus with amex erectile dysfunction lisinopril, or in which an embryo from a tubal pregnancy re-implanted in the abdomen). Once in the abdominal cavity, an embryo can implant into any well-vascularized structure—the rectouterine cavity (Douglas’ pouch), the mesentery of the intestines, and the greater omentum are some common sites. Tubal pregnancies can be caused by scar tissue within the tube following a sexually transmitted bacterial infection. The scar tissue impedes the progress of the embryo into the uterus—in some cases “snagging” the embryo and, in other cases, blocking the tube completely. Implantation in a uterine tube causes bleeding, which appears to stimulate smooth muscle contractions and expulsion of the embryo. If an ectopic pregnancy is detected early, the embryo’s development can be arrested by the administration of the cytotoxic drug methotrexate, which inhibits the metabolism of folic acid. Even if the embryo has successfully found its way to the uterus, it does not always implant in an optimal location (the fundus or the posterior wall of the uterus). Placenta previa can result if an embryo implants close to the internal os of the uterus (the internal opening of the cervix). As the fetus grows, the placenta can partially or completely cover the opening of the cervix (Figure 28. Embryonic Membranes During the second week of development, with the embryo implanted in the uterus, cells within the blastocyst start to organize into layers. Some grow to form the extra-embryonic membranes needed to support and protect the growing embryo: the amnion, the yolk sac, the allantois, and the chorion. At the beginning of the second week, the cells of the inner cell mass form into a two-layered disc of embryonic cells, and a space—the amniotic cavity—opens up between it and the trophoblast (Figure 28. Cells from the upper layer of the disc (the epiblast) extend around the amniotic cavity, creating a membranous sac that forms into the amnion by the end of the second week. Early in development, amniotic fluid consists almost entirely of a filtrate of maternal plasma, but as the kidneys of the fetus begin to function at approximately the eighth week, they add urine to the volume of amniotic fluid. Floating within the amniotic fluid, the 1330 Chapter 28 | Development and Inheritance embryo—and later, the fetus—is protected from trauma and rapid temperature changes. On the ventral side of the embryonic disc, opposite the amnion, cells in the lower layer of the embryonic disk (the hypoblast) extend into the blastocyst cavity and form a yolk sac. The yolk sac supplies some nutrients absorbed from the trophoblast and also provides primitive blood circulation to the developing embryo for the second and third week of development. When the placenta takes over nourishing the embryo at approximately week 4, the yolk sac has been greatly reduced in size and its main function is to serve as the source of blood cells and germ cells (cells that will give rise to gametes). During week 3, a finger-like outpocketing of the yolk sac develops into the allantois, a primitive excretory duct of the embryo that will become part of the urinary bladder. Together, the stalks of the yolk sac and allantois establish the outer structure of the umbilical cord. The last of the extra-embryonic membranes is the chorion, which is the one membrane that surrounds all others. The development of the chorion will be discussed in more detail shortly, as it relates to the growth and development of the placenta. Embryogenesis As the third week of development begins, the two-layered disc of cells becomes a three-layered disc through the process of gastrulation, during which the cells transition from totipotency to multipotency. The embryo, which takes the shape of an oval-shaped disc, forms an indentation called the primitive streak along the dorsal surface of the epiblast. A node at the caudal or “tail” end of the primitive streak emits growth factors that direct cells to multiply and migrate. Cells migrate toward and through the primitive streak and then move laterally to create two new layers of cells. The first layer is the endoderm, a sheet of cells that displaces the hypoblast and lies adjacent to the yolk sac. The cells of the epiblast that remain (not having migrated through the primitive streak) become the ectoderm (Figure 28. Whereas the ectoderm and endoderm form tightly connected epithelial sheets, the mesodermal cells are less organized and exist as a loosely connected cell community. The ectoderm gives rise to cell lineages that differentiate to become the central and peripheral nervous systems, sensory organs, epidermis, hair, and nails. Mesodermal cells ultimately become the skeleton, muscles, connective tissue, heart, blood vessels, and kidneys.