Chapter 24: Fluid and pH Balance
Introduction:
Cellular function requires a fluid medium with a carefully controlled composition
Three types of homeostatic balance
● Fluid balance
● Electrolyte balance
● Acid-base balance
Balances maintained by collective action of urinary respiratory, digestive, integumentary,
endocrine, nervous, cardiovascular, and lymphatic systems
Fluid Balance:
Expected Learning Outcomes:
● Name the major fluid compartments and explain how water moves from one to
another
● List the body’s sources of water and routes of water loss
● Describe the mechanism of regulating water intake and output
● Describe some conditions in which the body has a deficiency or excess of water
or an improper distribution of water among the fluid compartments
● Average healthy adult’s weights is about 50-60% water.
Fluid Compartments
Major fluid compartments of the body
● 65% intracellular fluid (ICF)
● 35% extracellular fluid (ECF)
○ 25% tissue (interstitial) fluid
○ 8% blood plasma and lymphatic fluid
○ 2% 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
Constantly moving between spaces due to osmosis
Osmosis:
Osmosis from one fluid compartment to another is determined by the relative
concentrations of solutes in each compartment
Electrolytes play the principal role in governing the body’s water distribution and total
water content
● Sodium salts in ECF
● Potassium salts in ICF
Water Gain and Loss
Fluid balance: usually about 2,500 mL/day
Gains come from two sources
● Preformed water (2,300 mL/day)
○ Ingested in food (700mL/day) and drink (1,600 mL/day)
, ● Metabolic Water (200 mL/day)
○ By-product of aerobic metabolism and dehydration synthesis
Losses are categorized as sensible or insensible
● 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
Typical Water Intake and Output:
Obligatory water loss: output that is relatively unavoidable
● Expired air, cutaneous transpiration, sweat, fecal moisture, and minimum urine
output (400 mL/day)
Regulation of Fluid Intake:
Thirst mainly governs fluid intake
Dehydration
● Reduces blood volume and pressure
● Increases blood osmolarity
Osmoreceptors in hypothalamus
● Response to angiotensin II produced when BP drops and also respond to rise in
osmolarity to ECF
● Osmoreceptors communicate with other hypothalamic neurons and with cerebral
cortex
Hypothalamus
● Produces antidiuretic hormone
● Sympathetic signals from thirst center to salivary glands reduce salivation
○ Cerebral cortex produces conscious sense of thirst due to dry mouth
○ Intense sense of thirst with 2% t 3% increase in plasma osmolarity or 10%
to 15% blood loss
Regulation if Fluid Output:
Change in urine volume main way to influence water output
● Kidneys can slow rate of water and electrolyte loss until water electrolysis can be
ingested
Renal water output
● Changes in urine volume influenced by:
○ Aldosterone: acts on tubule; Na+ is reabsorbed and water follows
○ Natriuretic peptide: decreasing in Na+ reabsorption in collecting duct
(increase excretion in urine)
○ ADH: causes collecting ducting to increase the number of aquaporin
proteins (more aquaporins mean more water reabsorption…. less urine
Introduction:
Cellular function requires a fluid medium with a carefully controlled composition
Three types of homeostatic balance
● Fluid balance
● Electrolyte balance
● Acid-base balance
Balances maintained by collective action of urinary respiratory, digestive, integumentary,
endocrine, nervous, cardiovascular, and lymphatic systems
Fluid Balance:
Expected Learning Outcomes:
● Name the major fluid compartments and explain how water moves from one to
another
● List the body’s sources of water and routes of water loss
● Describe the mechanism of regulating water intake and output
● Describe some conditions in which the body has a deficiency or excess of water
or an improper distribution of water among the fluid compartments
● Average healthy adult’s weights is about 50-60% water.
Fluid Compartments
Major fluid compartments of the body
● 65% intracellular fluid (ICF)
● 35% extracellular fluid (ECF)
○ 25% tissue (interstitial) fluid
○ 8% blood plasma and lymphatic fluid
○ 2% 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
Constantly moving between spaces due to osmosis
Osmosis:
Osmosis from one fluid compartment to another is determined by the relative
concentrations of solutes in each compartment
Electrolytes play the principal role in governing the body’s water distribution and total
water content
● Sodium salts in ECF
● Potassium salts in ICF
Water Gain and Loss
Fluid balance: usually about 2,500 mL/day
Gains come from two sources
● Preformed water (2,300 mL/day)
○ Ingested in food (700mL/day) and drink (1,600 mL/day)
, ● Metabolic Water (200 mL/day)
○ By-product of aerobic metabolism and dehydration synthesis
Losses are categorized as sensible or insensible
● 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
Typical Water Intake and Output:
Obligatory water loss: output that is relatively unavoidable
● Expired air, cutaneous transpiration, sweat, fecal moisture, and minimum urine
output (400 mL/day)
Regulation of Fluid Intake:
Thirst mainly governs fluid intake
Dehydration
● Reduces blood volume and pressure
● Increases blood osmolarity
Osmoreceptors in hypothalamus
● Response to angiotensin II produced when BP drops and also respond to rise in
osmolarity to ECF
● Osmoreceptors communicate with other hypothalamic neurons and with cerebral
cortex
Hypothalamus
● Produces antidiuretic hormone
● Sympathetic signals from thirst center to salivary glands reduce salivation
○ Cerebral cortex produces conscious sense of thirst due to dry mouth
○ Intense sense of thirst with 2% t 3% increase in plasma osmolarity or 10%
to 15% blood loss
Regulation if Fluid Output:
Change in urine volume main way to influence water output
● Kidneys can slow rate of water and electrolyte loss until water electrolysis can be
ingested
Renal water output
● Changes in urine volume influenced by:
○ Aldosterone: acts on tubule; Na+ is reabsorbed and water follows
○ Natriuretic peptide: decreasing in Na+ reabsorption in collecting duct
(increase excretion in urine)
○ ADH: causes collecting ducting to increase the number of aquaporin
proteins (more aquaporins mean more water reabsorption…. less urine
