Fluid regulation mechanisms

Problems arise when there is a failure or a decrease in function in any of the fluid regulation mechanisms. For example, in aging, the thirst mechanism is suppressed. The elderly patient may not drink adequate fluids owing to a lack of the thirst drive, and hypovolemia could result. Problems also arise if the regulating mechanisms fail to excrete excess water from the body, such as might occur with renal failure, and fluid overload results. 

This chapter will discuss cell water, the regulation of cell volume, and the modifications produced by cell pH. These topics are housekeeping functions and do not have the glamour of genetic engineering. Nevertheless, it is quite probable that one of the first requirements for life to evolve was a guarantee of a stable fluid environment. Mechanisms had to be established that would regulate the cell content of water. 

Which mechanisms of fluid regulation respond to high fluid volume in the... 

In the consideration of seals or O rings for the aircraft industry, it is worth knowing the basic fluid flow mechanism in the hydraulic system, since the purpose of seals in aircraft is to preserve the pressure in such a system. Every aircraft hydraulic system has two major parts, the power section and the actuating section. The power section provides fluid flow, regulates and limits pressure. 

When the temporal sequence of adjustments of blood pressure is analysed it seems, that CNS mechanisms (e.g., baroreflexes) will provide regulation of the circulation within seconds to minutes. Other mechanisms, such as the renin-angiotensin-aldosterone system and fluid shifts, occur over minutes to hours. Only the... 

Vasopressin (Rtressin Synthetic) and its derivatives, namely lypressin (Diapid) and desmopressin (DDAVP), regulate the reabsorption of water by the kidneys. Vasopressin is secreted by the pituitary when body fluids must be conserved. An example of this mechanism may be seen when an individual has severe vomiting and diarrhea with little or no fluid intake. When this and similar conditions are present, die posterior pituitary releases the hormone vasopressin, water in die kidneys is reabsorbed into die blood (ie, conserved), and die urine becomes concentrated. Vasopressin exhibits its greatest activity on die renal tubular epithelium, where it promotes water resoqition and smooth muscle contraction throughout die vascular bed. Vasopressin has some vasopressor activity. 

Aldosterone acts on the distal tubule of the nephron to increase sodium reabsorption. The mechanism of action involves an increase in the number of sodium-permeable channels on the luminal surface of the distal tubule and an increase in the activity of the Na+-K+ ATPase pump on the basilar surface of the tubule. Sodium diffuses down its concentration gradient out of the lumen and into the tubular cells. The pump then actively removes the sodium from cells of the distal tubule and into the extracellular fluid so that it may diffuse into the surrounding capillaries and return to the circulation. Due to its osmotic effects, the retention of sodium is accompanied by the retention of water. In other words, wherever sodium goes, water follows. As a result, aldosterone is very important in regulation of blood volume and blood pressure. The retention of sodium and water expands the blood volume and, consequently, increases mean arterial pressure. 

Sodium is the major extracellular cation. Because of its osmotic effects, changes in sodium content in the body have an important influence on extracellular fluid volume, including plasma volume. For example, excess sodium leads to the retention of water and an increase in plasma volume. Increased plasma volume then causes an increase in blood pressure. Conversely, sodium deficit leads to water loss and decreased plasma volume. A decrease in plasma volume then causes a decrease in blood pressure. Therefore, homeostatic mechanisms involved in the regulation of plasma volume and blood pressure involve regulation of sodium content, or sodium balance, in the body. 

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