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A&P: Chp 25

Terms in this set (47)

Fluid Compartments
Water occupies two main fluid compartments:

1. Intracellular fluid (ICF) - about two thirds by volume, in cells

2. Extracellular fluid (ECF) - consists of two major subdivisions
-Plasma - fluid portion of blood
-Interstitial fluid (IF) - fluid in spaces between cells

Other ECF - lymph, cerebrospinal fluid, eye humors, synovial fluid, serous fluid, and gastrointestinal secretions
Composition of Body Fluids
**Water is the universal solvent

Solutes are broadly classified into:
-Electrolytes (charged atoms) - inorganic salts, all acids and bases, and some proteins

-Nonelectrolytes - examples include glucose, lipids, creatinine, and urea

*Electrolytes have greater osmotic
*Water moves according to osmotic gradients (high concentration to low) (isn't much dissolved to dissolved)
Fluid Movement Among Compartments
-Compartmental exchange: regulated by osmotic and hydrostatic pressures

-Net leakage of fluid from the blood is picked up by lymphatic vessels and returned to bloodstream

-Exchanges between interstitial and intracellular fluids are complex bc selective permeability of the cellular membranes

-Two-way water flow is substantial
Extracellular and Intracellular Fluids
-Ion fluxes are restricted and move selectively by active transport

-Nutrients, respiratory gases, and wastes move unidirectionally

-Plasma is the only fluid that circulates throughout the body and links external and internal environments

-Osmolalities of all body fluids are equal; changes in solute concentrations are quickly followed by osmotic changes
Water Balance and ECF Osmolality
To remain properly hydrated, water intake = water output
Water intake sources:
-Ingested fluid (60%) and solid food (30%)
-Metabolic water or water of oxidation (10%)

Water output:
-Urine (60%) and feces (4%)
-Insensible losses (28%), sweat (8%)

Increases in plasma osmolality trigger thirst and release of antidiuretic hormone (ADH)
Regulation of Water Intake
Hypothalamic thirst center is stimulated:
-By a decline in plasma volume of 10%-15%
-By increases in plasma osmolality of 1-2%
-Via baroreceptor input, angiotensin II, and other stimuli

Thirst is quenched as soon as we begin to drink water

Feedback signals that inhibit the thirst centers include:
-Moistening of the mucosa of the mouth and throat
-Activation of stomach and intestinal stretch receptors
Regulation of Water Output
Obligatory water losses include:
-Insensible water losses from lungs and skin

-Water that accompanies undigested food residues in feces

Obligatory water loss reflects the fact that:
-Kidneys excrete 900-1200 mOsm of solutes to maintain blood homeostasis

-Urine solutes must be flushed out of the body in water
Influence and Regulation of ADH
-Water reabsorption in collecting ducts is proportional to ADH release

-Low ADH levels produce dilute urine and reduced volume of body fluids

-High ADH levels produce concentrated urine

-Hypothalamic osmoreceptors trigger or inhibit ADH release

*Factors that specifically trigger ADH release include prolonged fever; excessive sweating, vomiting, or diarrhea; severe blood loss; and traumatic burns
Dehydration
-Water loss exceeds water intake and the body is in negative fluid balance

Causes: hemorrhage, severe burns, prolonged vomiting or diarrhea, profuse sweating, water deprivation, and diuretic abuse

Signs and symptoms: cottonmouth, thirst, dry flushed skin, and oliguria

-Prolonged dehydration may lead to weight loss, fever, and mental confusion

-Other consequences include hypovolemic shock and loss of electrolytes
Hypotonic Hydration
-Renal insufficiency or an extraordinary amount of water ingested quickly can lead to cellular overhydration, or water intoxication

-ECF is diluted - sodium content is normal but excess water is present

-The resulting hyponatremia promotes net osmosis into tissue cells, causing swelling

-These events must be quickly reversed to prevent severe metabolic disturbances, particularly in neurons
Edema
Atypical accumulation of fluid in the interstitial space, *leading to tissue swelling

-Caused by anything that increases flow of fluids out of the bloodstream or hinders their return

Factors that accelerate fluid loss include:
-Increased blood pressure, capillary permeability
-Incompetent venous valves, localized blood vessel blockage
-Congestive heart failure, hypertension, high blood volume
Electrolyte Balance
Electrolytes: salts, acids, and bases, but electrolyte balance usually refers only to salt balance

Important for:
-Neuromuscular excitability
-Secretory activity
-Membrane permeability
-Controlling fluid movements

Salts enter the body by ingestion and are lost via perspiration, feces, and urine
Sodium in Fluid and Electrolyte Balance
Sodium holds a central position in fluid and electrolyte balance

Sodium salts:
-Account for 90-95% of all solutes in the ECF
-Contribute 280 mOsm of the total 300 mOsm ECF solute concentration

Sodium is the single most abundant cation in the ECF

Sodium is the only cation exerting significant osmotic pressure
Regulation of Sodium Balance: Aldosterone
Sodium reabsorption
-65% of sodium in filtrate is reabsorbed in the proximal tubules
-25% is reclaimed in the loops of Henle

Aldosterone high= all remaining Na+ is actively reabsorbed

Water follows sodium if tubule permeability has been increased with ADH

Renin-angiotensin mechanism triggers release of aldosterone

-Mediated by the juxtaglomerular apparatus, which releases renin in response to:
*Sympathetic nervous system stimulation
*Decreased filtrate osmolality
*Decreased stretch (due to decreased blood pressure)

Renin catalyzes the production of angiotensin II, which prompts aldosterone release

Adrenal cortical cells release aldosterone by elevated K+ levels in the ECF

Aldosterone: diminish urine output and increase blood volume slowly
Cardiovascular System Baroreceptors
Baroreceptors alert brain of increases in blood volume (hence increased blood pressure)
-Sympathetic nervous system impulses to the kidneys decline
-Afferent arterioles dilate
-Glomerular filtration rate rises
**Sodium and water output increase

Pressure diuresis = decreases blood pressure

-Drops in systemic blood pressure lead to opposite actions and systemic blood pressure increases

-Since sodium ion concentration determines fluid volume, baroreceptors = "sodium receptors"
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