Circulatory system regulation

All of the organ systems in the body, except the reproductive system, contribute to the maintenance of homeostasis (see Table 1.1). For example, the gastrointestinal tract digests foods to provide nutrients to the body. The respiratory system obtains oxygen and eliminates carbon dioxide. The circulatory system transports all of these materials and others from one part of the body to another. The renal system eliminates wastes and plays a role in regulating blood volume and blood pressure. 

The adenohypophysis does not have a direct anatomical connection with the hypothalamus therefore, regulation of hormone secretion by way of neuronal signals is not possible. Instead, these two structures are associated by a specialized circulatory system and the secretion of hormones from the adenohypophysis is regulated by hormonal signals from the hypothalamus (see Figure 10.2). Systemic arterial blood is directed first to the hypothalamus. The exchange of materials between the blood and the interstitial fluid of the hypothalamus takes place at the primary capillary plexus. The blood then flows to the adenohypophysis through the hypothalamic-hypophyseal portal veins. Portal veins are blood vessels that connect two capillary beds. The second capillary bed in this system is the secondary capillary plexus located in the adenohypophysis. 

Therefore, depending upon the degree of constriction of the vascular smooth muscle, these vessels may alter their diameter, and consequently their blood flow, across a very wide range. For this reason, the arterioles are the major resistance vessels in the circulatory system. In fact, the primary function of arterioles is to regulate the distribution of the cardiac output and to determine which tissues receive more blood and which receive less, depending upon the tissue s and the body s needs. 

As mentioned previously, the arterioles are the major resistance vessels in the circulatory system. Because the walls of these vessels contain primarily smooth muscle, they are capable of significant changes in their radius. Therefore, regulation of blood flow to the tissues is carried out by the arterioles. 

The capability of LDL receptors to remove LDL cholesterol from the circulation can rationalize these clinical observations. If little cholesterol is available in the diet, the cells of the peripheral tissues respond by up-regulating the number of LDL receptors on the cell surface. The higher concentration of receptors means that more of the cholesterol will be removed from the circulatory system. Because the inappropriate deposition of cholesterol is a major contributor to blocked arteries, if the cholesterol is removed from the circulation, less risk of blockage exists. On the other hand, if a large amount of cholesterol exists in the diet, and the cells have enough for their needs, they will synthesize fewer LDL receptors, less cholesterol will be removed from the circulatory system, and the risk of artery disease increases further. 

The blood is the body s main transport system. Although the transport and delivery of oxygen to the cells of the tissues is carried out by specialized cells, other vital components such as nutrients, metabolites, electrolytes, and hormones, are all carried in the noncellular fraction of the blood, the plasma Some components, such as glucose, are dissolved in the plasma others, for example, lipids and steroid hormones, are bound to carrier proteins for transport. The osmotic pressure of the plasma proteins regulates the distribution of water between the blood and the tissues. Plasma proteins in conjunction with platelets maintain the integrity of the circulatory system through the process of clotting. 

The microcirculation is comprised of blood vessels (arterioles, capillaries, and venules) with diameters of less than approximately 150 /xm. The importance of the microcirculation is underscored by the fact that most of the hydrodynamic resistance of the circulatory system Hes in the microvessels (especially in arterioles) and most of the exchange of nutrients and waste products occurs at the level of the smallest microvessels. The subjects of microcirculatory research are blood flow and molecular transport in microvessels, mechanical interactions and molecular exchange between these vessels and the surrounding tissue, and regulation of blood flow and pressure and molecular transport. Quantitative knowledge of microcirculatory mechanics and mass transport has been accumulated primarily in the past 30 years owing to significant innovations in methods and techniques to measure microcirculatory parameters and methods to analyze microcirculatory data. The development of these methods has required joint efforts... 

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