Cell tissue, nutrient needs

The circulatory system carries out many important functions that contribute to homeostasis. It obtains oxygen from the lungs nutrients from the gastrointestinal tract and hormones from the endocrine glands and it delivers these substances to the tissues that need them. Furthermore, it removes metabolic waste products, such as carbon dioxide, lactic acid, and urea, from the tissues. Finally, it contributes to the actions of the immune system by transporting antibodies and leukocytes to areas of infection. Overall, the circulatory system plays a vital role in maintenance of optimal conditions for cell and tissue function. 

Atrophy is the cellular response of size reduction. This response lessens the cell s oxygen, organelle (specialized subunit within a cell with a specific function), and nutrient needs. This may be an effective reaction to diminished resources. Hypertrophy is an increase in cell size. This can be beneficial when increased capacity is demanded of a cell that does not normally divide, such as cardiac or skeletal muscle cells. Hyperplasia refers to an increase in cellular number by division. This is only possible in cells capable of mitosis. During metaplasia one mature ceU type transforms to another mature cell type. This process is used when scar tissue, for example, replaces normal functioning tissue in response to chronic irritation or inflammation. 

Iron is indispensable in the human body (see Mineral nutrients). The average adult body contains 3 grams of iron. About 65% is found in hemoglobin, which carries oxygen from the lungs to the various parts of the body. Iron is also needed for the proper functioning of cells, muscles, and other tissues (4). 

With few exceptions, small particles of vegetable foods are generally stripped of their more accessible nutrients during digestion in the GI tract. In this way starch, protein, fat and water-soluble small components (sugars, minerals) are usually well absorbed. This is not always the case, however, for larger food particles or for molecules that cannot diffuse out of the celF tissue. Neither is it the case for the lipid-soluble components. These need to be dissolved in lipid before they can be physically removed from the cell to the absorptive surface, since the cell wall is unlikely to be permeable to lipid emulsions or micelles, and the presence of lipases will strip away the solvating lipid. 

Hypoxia is a deficiency of oxygen needed to maintain cellular homeostasis. It may be caused by a reduction in blood supply, namely ischemia, decreased cardiopulmonary function, and diminished oxygen-carrying capacity of the blood. Subsequent oxygen deficiency in tissues leads to depressed aerobic metabolism and, thus, insufficient ATP synthesis. Reductions in blood flow also exacerbate oxygen deprivation by impairing delivery of nutrients, such as glucose, and the removal of metabolic wastes, such as C02, from affected cells. 

The pathways of metabolism must be coordinated so that the produc tion of energy or the synthesis of end products meets the needs of the. cell. Further, individual cells do not function in isolation but, rather, are I part of a community of interacting tissues. Thus, a sophisticated com munication system has evolved to coordinate the functions of the body. Regulatory signals that inform an individual cell of the metabolic state of the body as a whole include hormones, neurotransmitters, and the availability of nutrients. These, in turn, influence signals generated within the cell (Figure 8.5). 

The system that circulates blood is the cardiovascular system, shown in schematic form in Figure 9.1. It consists of two major components the muscular part of the heart, called the myocardium, and the network of blood vessels composed of arteries, veins, and capillaries. Blood circulation is the body s transportation system that supplies tissues with the oxygen, nutrients and their metabolites, and hormones that they need for their function. Blood carries carbon dioxide, encapsulated dead cell matter, and other wastes away from tissues. Circulating blood is crucial to maintaining body homeostasis, with temperature, pH, and other crucial parameters kept within the narrow ranges required for good health. A number of toxicants have adverse effects on the cardiovascular system. 

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