Maintenance of the Body
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Body Fluids
(pp. 991–993)
Body Water Content
(p. 991)
Water accounts for 45–75% of body weight, depending on age,
sex, and amount of body fat.
Fluid Compartments
(p. 991)
About two-thirds (25 L) of body water is found within cells in the
intracellular fluid (ICF) compartment; the balance (15 L) is in the
extracellular fluid (ECF) compartment. Te ECF includes plasma
and interstitial fluid.
Composition of Body Fluids
(pp. 991–993)
Solutes dissolved in body fluids include electrolytes and
nonelectrolytes. Electrolyte concentration is expressed in mEq/L.
Plasma contains more proteins than does interstitial fluid;
otherwise, extracellular fluids are similar. Te most abundant
ECF electrolytes are sodium, chloride, and bicarbonate ions.
Intracellular fluids contain large amounts of protein anions and
potassium, phosphate, and magnesium ions.
Fluid, Electrolyte, and Acid/Base Balance; Topic:
Introduction to Body Fluids, pp. 1–8.
Fluid Movement Among Compartments
(p. 993)
Substances usually pass through the plasma and interstitial fluid
to reach the intracellular fluid.
Osmotic and hydrostatic pressure regulate fluid exchanges
between compartments: (a) Hydrostatic pressure forces filtrate
out of the capillaries and colloid osmotic pressure pulls filtrate
back in. (b) Water moves freely between the ECF and the ICF by
osmosis, but solute movements are restricted by size, charge, and
dependence on transport proteins. (c) Water flows always follow
changes in ECF osmolality.
Fluid, Electrolyte, and Acid/Base Balance; Topic:
Introduction to Body Fluids, pp. 19–23.
Water Balance and ECF Osmolality
(pp. 993–997)
Sources of body water are ingested foods and fluids and
metabolic water.
Water leaves the body primarily via the lungs, skin,
gastrointestinal tract, and kidneys.
Regulation of Water Intake
(pp. 994–995)
Increased plasma osmolality triggers the thirst mechanism,
mediated by hypothalamic osmoreceptors. Tirst, inhibited by
distension of the gastrointestinal tract by ingested water and then
by reduced osmolality, may be damped before body needs for
water are met.
Regulation of Water Output
(p. 995)
Obligatory water loss is unavoidable and includes insensible
water losses from the lungs, the skin, and in feces, and about
500 ml of urine output daily.
Beyond obligatory water loss, the volume of urinary output
depends on water intake and loss via other routes and reflects the
influence of antidiuretic hormone on the renal collecting ducts.
Influence of Antidiuretic Hormone (ADH)
(p. 995)
Antidiuretic hormone causes aquaporins (water channels) to be
inserted in the cell membranes of the collecting ducts, so that
most filtered water is reabsorbed. ADH release is triggered if ECF
osmolality is high, or if a large drop in blood volume or pressure
Disorders of Water Balance
(pp. 995-997)
Dehydration occurs when water loss exceeds water intake over
time. It is evidenced by thirst, dry skin, and decreased urine
output. A serious consequence is hypovolemic shock.
Hypotonic hydration occurs when body fluids are excessively
diluted and entering water makes cells swell. Te most serious
consequence is cerebral edema.
Edema is an abnormal accumulation of fluid in the interstitial
space (increased IF), which may impair blood circulation.
Fluid, Electrolyte, and Acid/Base Balance; Topic:
Water Homeostasis, pp. 1–27.
Electrolyte Balance
(pp. 997–1004)
Most electrolytes are obtained from ingested foods and fluids.
Salts, particularly NaCl, are o±en ingested in excess of need.
Electrolytes are lost in perspiration, feces, and urine. Te kidneys
are most important in regulating electrolyte balance.
The Central Role of Sodium in Fluid and Electrolyte Balance
(pp. 997–999)
Sodium salts are the most abundant solutes in ECF. Tey exert
the bulk of ECF osmotic pressure and control water volume and
distribution in the body.
Regulation of Sodium Balance
(pp. 999–1000)
Sodium ion balance is linked to ECF volume and blood pressure
regulation and involves both neural and hormonal controls.
Aldosterone promotes Na
reabsorption to maintain blood
volume and blood pressure.
Declining blood pressure and falling filtrate NaCl concentration
stimulate the granular cells to release renin. Renin, via
angiotensin II, enhances systemic blood pressure, Na
reabsorption, and aldosterone release.
Atrial natriuretic peptide, released by certain atrial cells in
response to rising blood pressure (or blood volume), causes
systemic vasodilation and inhibits renin, aldosterone, and ADH
release. Hence, it enhances Na
and water excretion, reducing
blood volume and blood pressure.
Estrogens and glucocorticoids increase renal retention of sodium.
Progesterone promotes enhanced sodium and water excretion in
Chapter Summary
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