40 Questions
What percentage of body weight is water in men?
Sixty percent
What percentage of total body water is in the plasma?
Eight percent
What is the primary determinant of water distribution in the body?
Osmotic content of the ICF and ECF
What is the concentration of potassium in the intracellular fluid?
110 mmol/l
What is the concentration of sodium in the extracellular fluid?
135 mmol/l
What is the term for the equal osmotic concentrations of the ICF and ECF?
Isotonic
What percentage of body water is in the extracellular fluid?
Thirty three percent
What is the primary mechanism of water transport in the body?
Free permeability
What is the normal range of osmolality of the ECF?
282-295 mmol/kg of water
What happens to the ECF osmolality when there is water deprivation?
It increases
What is the function of vasopressin in the body?
To make the renal collecting ducts permeable to water
What is the maximum urine concentration that can be achieved in humans?
1200 mmol/l
What happens to water in the ICF when ECF osmolality increases?
It moves to the ECF
What is the result of stimulating the hypothalamic thirst center?
Increased water intake
What is the effect of vasopressin on the renal collecting ducts?
It makes them permeable to water
What is the outcome of restoring ECF osmolality?
Renal water retention
What is the minimum daily water intake necessary for the maintenance of water balance?
1100 ml
What is the normal ECF sodium concentration?
135-145 mmol/l
What is the normal daily intake of sodium in the western world?
100-200 mmol/day
What is the obligatory sodium loss via the kidneys, skin and gut?
Less than 10 mmol/day
What is the rate of sodium secretion into the gut?
1000 mmol/day
What is the rate of sodium filtration by the kidneys?
25000 mmol/day
What percentage of the body's sodium is freely exchangeable?
70%
What is the consequences of a partial failure of sodium reabsorption?
Sodium homeostasis will be compromised
What is the minimum change in osmolality that osmoreceptors can detect?
1%
What is the plasma osmolality level at which vasopressin becomes detectable in the plasma?
280 mmol/kg
What happens to vasopressin secretion when the ECF osmolality falls?
It is inhibited
What type of solute can diffuse readily across cell membranes and increase ICF osmolality?
Urea
What happens to vasopressin secretion when there is a decrease in plasma volume of more than 10%?
It increases
What is the primary stimulus for vasopressin release when there is a decrease in plasma volume?
Hypovolemia
What is the result of vasopressin secretion when ECF osmolality increases?
Concentrated urine production
What type of receptors besides osmoreceptors affect vasopressin secretion?
Baroreceptors and volume receptors
Which of the following stimulates vasopressin secretion?
Increased ECF osmolality
What is the result of severe water depletion on the brain?
Cerebral dehydration
How should water be given to a patient with water depletion?
Orally or via nasogastric tube
What is the goal of fluid replacement in water depletion?
Correct approximately two-thirds of the deficit in the first 24 hours
What is the effect of water depletion on ECF osmolality?
Increased ECF osmolality
Which of the following inhibits vasopressin secretion?
Alcohol
What is the result of rapid fluid replacement in severe water depletion?
Cerebral oedema
Which of the following stimulates the thirst center?
Increased ECF osmolality
Study Notes
Water Distribution
- Water accounts for approximately 60% of body weight in men and 55% in women, with 66% of this water being in the intracellular fluid (ICF) and 33% in the extracellular fluids (ECF)
- Only 8% of total body water is in the plasma
Sodium Distribution
- The body of an adult man contains approximately 3000 mmol of sodium, with 70% of it being freely exchangeable and the remainder complexed in bone
- Most cell membranes are permeable to sodium, which is maintained by active pumping of sodium from ICF to ECF by Na+-K+ ATPase
Water and Sodium Homeostasis
- The minimum daily water intake necessary for the maintenance of water balance is approximately 1100 ml, which increases if losses are abnormally large
- Sodium input and output are normally balanced, with the kidneys filtering 25,000 mmol/day and the vast majority being regained by reabsorption in the gut and renal tubule
- Excessive sodium intake can be harmful, such as in the pathogenesis of hypertension
Osmolality and Vasopressin
- Changes in body water content independent of the amount of solute will alter the osmolality, which is normally maintained in the range of 282-295 mmol/kg of water
- A slight increase in ECF osmolality will stimulate the hypothalamic thirst center and osmoreceptors, which causes the release of vasopressin (antidiuretic hormone or ADH)
- Vasopressin renders the renal collecting ducts permeable to water, permitting water reabsorption and concentration of the urine
Physiological Responses to Water Loss
- Water loss will increase ECF osmolality, stimulating vasopressin release and promoting water reabsorption
- Redistribution of water from ICF to ECF will occur, increasing ECF volume and decreasing ICF volume
- Renal water retention will occur, and increased water intake will help restore ECF osmolality to normal
Vasopressin Secretion
- Vasopressin concentration rises sharply if plasma osmolality increases above 282 mmol/kg
- A small decrease in blood volume has little effect on vasopressin secretion, but a decrease in plasma volume of more than 10% will stimulate vasopressin release
- Other stimuli affecting vasopressin secretion include angiotensin II, arterial and venous baroreceptors, and volume receptors
Water Depletion
- Water depletion will occur if water intake is inadequate or losses are excessive
- Loss of water from ECF causes an increase in osmolality, stimulating thirst and vasopressin secretion
- Severe water depletion can cause cerebral dehydration, which may lead to cerebral hemorrhage or oedema
Learn about the distribution of water in the human body, including its composition in men and women, and the percentage of water in intracellular and extracellular fluids.
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