Ch. 26 – Fluid, Electrolyte, and Acid-Base Balance – short answer essay

question

Name the body fluid compartments, noting their locations and the approximate fluid volume in each.
answer

The body fluid compartments include the intracellular fluid compartment, located inside the cells with fluid volume of approximately 25 liters, and the extracellular fluid compartment (plasma and interstitial fluid), located in the external environment of each cell with fluid volume of approximately 15 liters. (p. 996)
question

Explain why and how ECF osmolality is maintained.
answer

It is important to control the extracellular fluid (ECF) osmolality because the ECF determines the ICF volume and underlies the control of the fluid balance in the body. The ECF is maintained by both thirst and the antidiuretic hormone (ADH). A rise in plasma osmolality triggers thirst and the release of ADH; a drop in plasma osmolality inhibits thirst and ADH. (pp. 999-1000)
question

Explain why and how sodium balance, ECF volume, and blood pressure are jointly regulated.
answer

Sodium is pivotal to fluid and electrolyte balance and to the homeostasis of all body systems because it is the principal extracellular ion. While the sodium content of the body may be altered, its concentration in the ECF remains stable because of immediate adjustments in water volume. The regulation of the sodium-water balance is inseparably linked to blood pressure and entails a variety of neural and hormonal controls: (1) aldosterone—increases the reabsorption of sodium from the filtrate; water follows passively by osmosis, increasing blood volume (and pressure). The renin-angiotensin mechanism is an important control of aldosterone release; the juxtaglomerular apparatus responds to: (a) decreased stretch (due to decreased blood pressure), (b) decreased filtrate osmolality, or (c) sympathetic nervous system stimulation, resulting ultimately in aldosterone release from the adrenal cortex. (2) ADH—osmoreceptors in the hypothalamus sense solute concentration in the ECF: increases in sodium content stimulate ADH release, resulting in increased water retention by the kidney (and increasing blood pressure). (3) Atrial natriuretic peptide—released by cells in the atria during high-pressure situations, it has potent diuretic and natriuretic (sodium-excreting) effects; the kidneys do not reabsorb as much sodium (therefore water) and blood pressure drops. (pp. 1002-1007)
question

Explain how the chemical buffer systems resist changes in pH.
answer

Chemical acid-base buffers prevent pronounced changes in H+ concentration by binding to hydrogen ions whenever the pH of body fluids drops and releasing them when pH rises. (p. 1009)

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