Ch 26, L2

• Proper hydration occurs when water intake equals water output.
• Water intake varies widely among individuals, averaging around 2500 ml per day for adults.
• The majority of water enters the body via ingested liquids and solid foods.
• Body water that is produced by cellular metabolism is called metabolic water, or water of oxidation.

Water output

• Insensible water loss is the water evaporating from the lungs in expired air or diffusing directly through the skin.
• Sensible water loss is any measurable water loss, including urine (about 60%), noticeable sweat, and feces.

Maintaining Osmolality

• Healthy individuals have a remarkable ability to maintain the osmolality (electrolyte-water balance) of their body fluids within a narrow homeostatic balance.
• Increased plasma osmolality triggers thirst, which encourages drinking.
• Thirst is quenched as the mouth's mucosa is moistened, which continues with distension of the stomach and intestines, which in turn switches off the hypothalamic thirst center.
• Antidiuretic hormone (ADH) causes the kidneys to conserve water and excrete concentrated urine.
• Decreased osmolality inhibits thirst and ADH release, prompting the kidneys to excrete large volumes of dilute urine.
• The hypothalamic thirst center governs the thirst mechanism, which is the main driving force for water intake.

Activating the thirst center

• Hypothalamic osmoreceptors detect ECF osmolality via plasma membrane stretch changes caused by water gain or loss; increased osmolality activates these.
• Dry mouth increases the blood osmotic pressure, and salivary glands produce less saliva, reducing water drawn from the blood into salivary ducts.
• A decrease in blood volume or pressure triggers the thirst mechanism, signaled by baroreceptors that directly activate the thirst center, and by angiotensin II.

Regulation of Water Output

• Obligatory water losses are unavoidable and explain why can't survive without drinking for long.
• Kidneys cannot compensate for zero water intake, despite conservation efforts.
• Obligatory water loss = insensible water losses + sensible water loss.
• Kidneys that function normally normally excrete at least 500 ml of water daily, accounting for obligatory water loss in urine.
• Beyond the obligatory water loss, urine volume and solute concentration relies on fluid intake, diet, and water loss.
• Kidneys begin to eliminate excess water around 30 minutes post-ingestion, which is the time required to inhibit ADH release.
• Diuresis peaks around 1 hour after drinking before declining to its lowest level after 3 hours.

Antidiuretic Hormone (ADH)

• The amount of water reabsorbed in the renal collecting ducts is proportional to ADH release.
• Low ADH = most water will not be reabsorbed, so dilute urine and a reduced volume of body fluids occurs.
• High ADH = aquaporins will be inserted in the principal cell apical membranes so most water is reabsorbed and a small volume of concentrated urine is excreted.
• Osmoreceptors trigger or inhibit ADH release from the posterior pituitary.
• Large changes in blood volume or blood pressure influence ADH secretion.
• Decreased blood pressure increases ADH via the baroreceptors and the renin-angiotensin-aldosterone mechanism.
• Factors greatly reducing blood volume, triggering ADH release include severe blood loss, traumatic burns, excessive sweating, vomiting, or diarrhea ,and prolonged fever.

Disorders of Water Balance

• Clinical water imbalances include dehydration, hypotonic hydration, and edema.

Dehydration

• Dehydration is fluid loss, loss of water, or the loss of water and solutes together.
• Hemorrhage, severe burns, prolonged vomiting/diarrhea, excessive sweating, water deprivation, diuretic abuse, diabetes mellitus, or diabetes insipidus results in dehydration.
• Early signs/symptoms of dehydration include sticky oral mucosa, thirst, dry/flushed skin, and decreased urine output (oliguria).
• Prolonged dehydration can result in weight loss, fever, mental confusion, and inadequate blood volume to maintain normal circulation, resulting in hypovolemic shock.

Hypotonic Hydration

• Declining ECF osmolality initiates several compensatory mechanisms.
• A type of cellular overhydration called hypotonic hydration may occur when renal insufficiency exists or when we quickly drink a lot of water.
• In hypotonic hydration, ECF is diluted, so its sodium content is normal, but excess water is present, lowering the sodium concentration (hyponatremia).
• Hyponatremia promotes net osmosis into tissue cells, causing them to swell and that becoming abnormally hydrated.
• Severe metabolic disturbances are evidenced by nausea, vomiting, muscular cramping, and cerebral edema and leads to severe metabolic disturbances
• Uncorrected cerebral edema quickly leads to disorientation, convulsions, coma, and death as it is particularly damaging to neurons.
• Sudden and severe hyponatremia is treated by administering intravenous hypertonic saline to reverse the osmotic gradient and pull water out of the cells.

Edema

• Edema is atypical accumulation of fluid in tissues and is an increase in volume of only the IF (interstitial fluid).
• Extra fluid in the interstitial space can impair tissue function by increasing the distance across which oxygen and nutrients must diffuse between blood and cells.