Page 1059 - Fundamentals of anatomy physiology

1046 Unit 5 Environmental Exchange
this reason, changes in the ICF in one cell have no direct
effect on the composition of the ICF in distant cells and
tissues, unless those changes also affect the ECF.
2. No receptors directly monitor fluid or electrolyte balance. In
other words, receptors cannot detect how many liters of
water or grams of sodium, chloride, or potassium the body
contains. Nor can they count how many liters or grams we
gain or lose throughout the day. But receptors can monitor
plasma volume and osmotic concentration. The plasma vol-
ume and osmotic concentration are good indicators of the
state of fluid balance and electrolyte balance for the body
as a whole, because fluid continuously circulates between
interstitial fluid and plasma, and because exchange occurs
between the ECF and the ICF.
3. Cells cannot move water molecules by active transport. All
movement of water across plasma membranes and epithe-
lia takes place passively, in response to osmotic gradients
established by the active transport of specific ions, such as
sodium and chloride. You may find it useful to remember,
“water follows salt.” As we saw in earlier chapters, when
sodium and chloride ions (or other solutes) are actively
transported across a membrane or epithelium, water fol-
lows by osmosis. p. 1020 This basic principle accounts
for water absorption across the digestive epithelium, and
for water conservation by the kidneys.
27 4. The body’s content of water or electrolytes will increase if dietary of the kidneys. p. 1022 The higher the plasma concentration
gains exceed losses to the environment, and will decrease if losses
exceed gains. This basic rule is important when you consider
the mechanics of fluid balance and electrolyte balance. Ho-
meostatic adjustments affect the balance between urinary
excretion and dietary absorption. As we saw in Chapter
26, circulating hormones regulate renal function. These
hormones can also produce complementary changes in
behavior. For example, the combination of angiotensin II
and aldosterone can give you a sensation of thirst—which
stimulates you to drink fluids—and a taste for heavily salted
foods.
An Overview of the Primary Regulatory
Hormones
Three hormones mediate physiological adjustments to fluid
balance and electrolyte balance. They are (1) antidiuretic hor-
mone (ADH), (2) aldosterone, and (3) the natriuretic peptides
(ANP and BNP). We introduced and discussed these hormones
in earlier chapters. We summarize their effects next. For a more
detailed review, refer to the appropriate sections of Chapters 18,
21, and 26. The interactions among these hormones are il-
lustrated in Figures 18–19b, 21–15, 21–16, and 26–11 (pp. 667,
774, 777, 1017).
Antidiuretic Hormone
The hypothalamus contains special cells known as osmore-
ceptors, which monitor the osmotic concentration of the ECF.
These cells are sensitive to subtle changes: A 2 percent change in
osmotic concentration (approximately 6 mOsm/L) is enough
to alter osmoreceptor activity.
The population of osmoreceptors includes neurons that
secrete ADH. These neurons are located in the anterior hypo-
thalamus. Their axons release ADH near fenestrated capillaries
in the posterior lobe of the pituitary gland. The rate of ADH
release varies directly with osmotic concentration: The higher
the osmotic concentration, the more ADH is released.
Increased release of ADH has two important effects: (1) It
stimulates water conservation by the kidneys, reducing urinary
water losses and concentrating the urine; and (2) it stimulates
the hypothalamic thirst center, promoting the intake of fluids.
As we saw in Chapter 21, the combination of decreased water
loss and increased water gain gradually restores the normal
plasma osmotic concentration. pp. 772–774
Aldosterone
The secretion of aldosterone by the adrenal cortex plays a major
role in determining the rate of Na1 absorption and K1 loss
along the distal convoluted tubule (DCT) and collecting system
of aldosterone, the more efficiently the kidneys conserve Na1.
Because “water follows salt,” the conservation of Na1 also in-
creases water retention. As Na1 is reabsorbed, Cl2 follows (see
Figure 26–14a, p. 1024), and water follows by osmosis as so-
dium and chloride ions move out of the tubular fluid. Aldoste-
rone also increases the sensitivity of salt receptors on the tongue.
This effect may increase your consumption of salty foods.
Aldosterone is secreted in response to increasing K1 or de-
creasing Na1 levels in the blood reaching the adrenal cortex, or
in response to the activation of the renin–angiotensin-aldoste-
rone system. As we saw in earlier chapters, renin release occurs
in response to any of three changes. They are (1) a decrease in
plasma volume or blood pressure at the juxtaglomerular com-
plex of the nephron; (2) a decrease in the osmotic concentration
of tubular fluid at the DCT; or, as we will soon see, (3) decreas-
ing Na1 or increasing K1 concentrations in the renal circulation.
Natriuretic Peptides
The natriuretic (natrium, sodium; -uretic, urine) peptides, atrial
natriuretic peptide (ANP) and brain natriuretic peptide (BNP),
are released by cardiac muscle cells in response to abnormal
stretching of the heart walls. Increased blood pressure or in-
creased blood volume causes this stretching. Among their other
effects, these peptides reduce thirst and block the release of
ADH and aldosterone that might otherwise lead to water and