Fluid volume mechanisms regulating

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. 

Isotonicity of the extracellular space is regulated by (i.) thirst mechanism, (2.) ADH, and (S.) dilution and concentration potential of the kidneys. Maintenance of extracellular isovoiaemia is effected by a change in renal sodium excretion. For this reason, disturbances in the sodium supply primarily result in changes in the extracellular fluid volume. Isohydria is also continually regulated within the normal range. 

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. 

Fig. 4.10. Body fluid homeostasis (constant body water balance). Intake is influenced by availability of fluids and food, thirst, hunger, and the ability to swallow. The rates of breathing and evaporation and urinary volume influence water loss. The body adjusts the volume of urinary excretion to compensate for variations in other types of water loss and for variations in intake. The hormones aldosterone and antidiuretic hormone (ADH) help to monitor blood volume and osmolality through mechanisms regulating thirst and sodium and water balance.

Ruid volume also plays a part in regulation of fluid levels in the body. Several mechanisms, in addition to ADH, respond to the sensation of low or high fluid volumes and osmolality. Neural mechanisms, through sensory receptors, sense low blood volume in the blood vessels and stimulate a sympathetic response resulting in constriction of the arterioles, which, in turn, result in a decrease in blood flow to... 

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

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. 

Physiologically, in both normal and hypertensive individuals, blood pressure is maintained by moment-to-moment regulation of cardiac output and peripheral vascular resistance, exerted at three anatomic sites (Figure 11-1) arterioles, postcapillary venules (capacitance vessels), and heart. A fourth anatomic control site, the kidney, contributes to maintenance of blood pressure by regulating the volume of intravascular fluid. Baroreflexes, mediated by autonomic nerves, act in combination with humoral mechanisms, including the renin-angiotensin-aldosterone system, to coordinate function at these four control sites and to maintain normal blood pressure. Finally, local release of vasoactive substances from vascular endothelium may also be involved in the regulation of vascular resistance. For example, endothelin-1 (see Chapter 17) constricts and nitric oxide (see Chapter 19) dilates blood vessels. 

The composition and volume of extracellular fluid are regulated by complex hormonal and nervous mechanisms that interact to control its osmolality, volume, and pH. 

The pH of extracellular fluid is kept within very narrow limits (7.35-7.45) by buffering mechanisms (see also Chapter 1), the lungs, and the kidneys. These three systems do not act independently. For example, in acute blood loss release of ADH and aldosterone restores the blood volume and renal regulation of the pH leads to shifts in K+ and Na+ levels. 

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