0-1 Chapter 24 - water, electrolytes, acid-base balance Flashcards
Balance
cellular function requires a fluid medium with a carefully controlled composition
•balances maintained by the collective action of the urinary, respiratory, digestive, integumentary, endocrine, nervous, cardiovascular, and lymphatic systems
three types of homeostatic balance
water balance
electrolyte balance
acid-base balance
water balance
•average daily water intake and loss are equal
electrolyte balance
the amount of electrolytes absorbed by the small intestine balance with the amount lost from the body, usually in urine
acid-base balance
•the body rids itself of acid (hydrogen ion –H+) at a rate that balances metabolic production
Body Water
- newborn baby‟s body weight is about 75% water
- young men average 55% -60%
- women average slightly less
- obese and elderly people as little as 45% by weight
total body water (TBW)
of a 70kg (150 lb) young male make is about 40 liters
major fluid compartments of the body
–65% intracellular fluid (ICF)
–35% extracellular fluid (ECF)
–35% extracellular fluid (ECF)
- 25% tissue (interstitial) fluid
- 8% blood plasma and lymphatic fluid
- 2% transcellular fluid „catch-all‟ category
transcellular fluid „catch-all‟ category
–cerebrospinal, synovial, peritoneal, pleural, and pericardial fluids
–vitreous and aqueous humors of the eye
–bile, and fluids of the digestive, urinary, and reproductive tracts
Water Movement Between Fluid Compartments
•fluid continually exchanged between compartments
•water moves by osmosis
•because water moves so easily through plasma membranes, osmotic gradientsnever last for very long
•if imbalance arises, osmosis restores balance within seconds so the intracellular and extracellular osmolarity are equal
–if osmolarity of the tissue fluid rises, water moves out of the cell
–if it falls, water moves in
osmosis from one fluid compartment to another is determined by
the relative concentrations of solutes in each compartment
–electrolytes–the most abundant solute particles, by far
–sodium salts in ECF
–potassium salts inICF
electrolytes
electrolytesplay the principal role in governing the body‟s water distribution and total water content
fluid balance
when daily gains and losses are equal (about 2,500 mL/day)
Water gains come from two sources:
–preformed water (2,300 mL/day)
•ingested in food (700 mL/day) and drink (1600 mL/day)
–metabolic water (200 mL/day)
•by-product of aerobic metabolism and dehydration synthesis
sensible water loss
is observable
–1,500 mL/ day is in urine
–200 mL/day is in feces
–100 mL/day is sweat in resting adult
insensible water loss
is unnoticed
–300 mL/day in expired breath
–400 mL/day is cutaneous transpiration
•diffuses through epidermis and evaporates
–does not come from sweat glands
–loss varies greatly with environment and activity
obligatory water loss
output that is relatively unavoidable
•expired air, cutaneous transpiration, sweat, fecal moisture, and urine output
thirst
mainly governs fluid intake
dehydration
–reduces blood volume and blood pressure
–increases blood osmolarity
osmoreceptors in hypothalamus
–respond to angiotensin II produced when BP drops and to rise in osmolarity of ECF with drop in blood volume
–osmoreceptors communicate with the hypothalamus and cerebral cortex
hypothalamus produces
antidiuretic hormone
•promotes water conservation
cerebral cortex produces
conscious sense of thirst
intense sense of thirst
with 2-3% increase in plasma osmolarity or 10-15% blood loss
salivation
is inhibited with thirst
•sympathetic signals from thirst center to salivary glands
long term inhibition of thirst
–absorption of water from small intestine reduces osmolarity of blood
•stops the osmoreceptor response, promotes capillary filtration, and makes the saliva more abundant and watery
•changes require 30 minutes or longer to take effect
short term inhibition of thirst
–cooling and moistening of mouth quenches thirst
–distension of stomach and small intestine
–30 to 45 min of satisfaction
•must be followed by water being absorbed into the bloodstream or thirst returns
–short term response designed to prevent overdrinking
Regulation of Water Output
only way to control water output significantly, is through variation in urine volume
–kidneys can‟t replace water or electrolytes
–only slow rate of water and electrolyte loss until water and electrolytes can be ingested
mechanisms:
–changes in urine volume linked to adjustments in Na+ reabsorption
•as Na+is reabsorbed or excreted, water follows
–concentrate the urine through action of ADH
ADH secretion stimulated by
hypothalamic osmoreceptors in response to dehydration
aquaporins synthesized in response to
ADH
–membrane proteins in renal collecting ducts whose job is to channel water back into renal medulla, Na+is still excreted
–slows decrease in water volume and increased osmolarity –concentrates urine
ADH release inhibited when
blood volume and pressure is too high or blood osmolarity too low
•effective way to compensate for hypertension
Disorders of Water Balance
the body is in a state of fluid imbalance if there is an abnormality of total volume, concentration, or distribution of fluid among the compartments
fluid deficiency
fluid output exceeds intake over long period of time
volume depletion
(hypovolemia)
–occurs when proportionate amounts of water and sodium are lost without replacement
–total body water declines, but osmolarity remains normal
–hemorrhage, severe burns, chronic vomiting, or diarrhea
hypovolemia
volume depletion
dehydration
dehydration(negative water balance)
–body eliminates significantly more water than sodium
–total body water declines, osmolarity rises
–lack of drinking water, diabetes, ADH hyposecretion (diabetes insipidus), profuse sweating, overuse of diuretics
infants more vulnerable to dehydration than adults due to
high metabolic rate that demands high urine excretion, immature kidneys cannot concentrate urine effectively, greater ratio of body surface to mass
most serious effects
circulatory shock due to loss of blood volume, neurological dysfunction due to dehydration of brain cells, infant mortality from diarrhea
Fluid Balance in Cold Weather
the body conserves heat by constricting blood vessels of the skin forcing blood to deeper circulation
–raises blood pressure which inhibits secretion of ADH
–increases secretion of atrial natriuretic peptide
–urine output is increased and blood volume reduced
cold air is drier and
increases respiratory water loss also reducing blood volume
cold weather respiratory and urinary loses cause
a state of reduced blood volume (hypovolemia)
–exercise will dilate vessels in skeletal muscles
–insufficient blood for rest of the body can bring on weakness, fatigue, or fainting (hypovolemic shock)
Dehydration from Excessive Sweating
1) water loss from sweating
2) sweat produced by capillary filtration
3) blood volume and pressure drop, osmolarity rises
4) blood absorbs tissue fluid to replace loss
5) tissue fluid pulled from ICF
6) all three compartments lose water
7) 300 mL from tissue fluid and 700 mL from ICF
fluid excess
less common than fluid deficiency because the kidneys are highly effective in compensating for excessive intake by excreting more urine
–renal failure can lead to fluid retention
two types of fluid excesses
volume excess
hypotonic hydration
volume excess
- both Na+ and water retained
- ECF remains isotonic
- caused by aldosterone hypersecretion or renal failure
hypotonic hydration
(water intoxication) (positive water balance)
•more water than Na+ retained or ingested
•ECF becomes hypotonic
–can cause cellular swelling
–pulmonary and cerebral edema
fluid sequestration
a condition in which excess fluid accumulates in a particular location
•total body water may be normal, but volume of circulating blood may drop to a point causing circulatory shock
most common form
edema -abnormal accumulation of fluid in the interstitial spaces, causing swelling of the tissues
hemorrhage
another cause of fluid sequestration
•blood that pools in the tissues is lost to circulation
pleural effusion
several liters of fluid can accumulate in the pleural cavity
•caused by some lung infections
physiological functions of electrolytes
–chemically reactive and participate in metabolism
–determine electrical potential (charge difference) across cell membranes
–strongly affect osmolarity of body fluids
–affect body‟s water content and distribution
major cations
–Na+, K+, Ca2+, and H+
major anions
–Cl-, HCO3-(bicarbonate), and PO43-
great differences between electrolyte concentrations of
blood plasma and intracellular fluid (ICF)
–have the same osmolarity (300 mOsm/L)
concentrations in tissue fluid (ECF) differ only
slightly from those in the plasma
sodium
principal ions responsible for the resting membrane potentials
–inflow of sodium through membrane gates is an essential event in the depolarization that underlies nerve and muscle function
principal cation in ECF
–sodium salts accounts for 90 -95% of osmolarity of ECF
–most significant solute in determining total body water and distribution of water among the fluid compartments
Na+-K+pump
–exchanges intracellular Na+ for extracellular K+
–generates body heat