Diff: 6 Page Ref: 40428
5) The regulation of mean arterial pressure involves the interaction between extrinsic and intrinsic mechanisms. While the intrinsic mechanisms can affect mean arterial pressure, they alsoplay an important role in the distribution of cardiac output to individual organs. Discuss the role of the intrinsic mechanisms in the control of organ resistance.Answer: Intrinsic control refers to the concept that arterioles within an organ respond to the prevailing local environment in a manner that determines organ resistance (autoregulation). Smooth muscle cells of arterioles dilate and constrict in response to specific changes within an organ. The local factors that modulate arteriolar tone include metabolic activity of the organ, blood flow, perfusion pressure, and locally secreted chemical messengers. The metabolic activityof an organ can induce changes in a number of chemicals including oxygen, carbon dioxide, potassium, and hydrogen ions. Of these, oxygen is the only ion that decreases with increases in tissue metabolic activity. Ischemia (reduced tissue oxygen) results from a mismatch between the delivery and consumption of oxygen. Since oxygen causes arterioles to constrict, ischemia dilates arterioles. This dilation in response to increased metabolic activity is termed active hyperemia.In addition, increases in metabolic byproducts like carbon dioxide, hydrogen ions, and potassium that occur in active tissue cause arterioles to dilate. The concentration of these metabolites depends upon blood flow: greater flow removes more of the byproducts from tissues.Increases in blood flow following a reduction in blood flow is termed reactive hyperemia.Arterioles also respond to changes in perfusion pressure. An increase in perfusion pressure stretches the smooth muscle cells of the arteriole, causing that vessel to constrict. Once the vesselconstricts, blood flow is reduced, which eventually stimulates an increase in blood flow as a consequence of an increased generation of metabolic byproduct. This interaction between flow and perfusion pressure that tends to keep blood flow within an organ constant is referred to as flow autoregulation. The endothelial cells play an important role in modifying arteriolar tone through the release of chemical messengers. Examples of these chemical messengers that induce a dilation are nitric oxide and prostacyclin, whereas endothelin induces a constriction. In addition, other chemicals that modify tone can be released.Diff: 8 Page Ref: 40529
6) The processes of filtration and absorption across the endothelial cells of capillaries is driven by Starling's forces. List the components of Starling's forces and describe how those forces determine the absorption of water across capillaries, including a discussion of how those forces
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