Control of plasma osmolality Flashcards
What is normal body fluid osmolality? What is the exception to this?
285-295 mOsm/kg
Exception is urine
How is water taken into the body?
Drinking
How is water lost from the body?
Urine by the kidneys
Faeces from the GI tract
Expired air by the lungs
Sweat from the skin
How are water intake and water loss related to each other ideally?
Water intake = water loss
What happens to plasma osmolality when water intake = water loss? Why?
Remains the same
because have same amount of water per solute
What happens to plasma osmolality when water intake is more than water loss? Why?
Decreases
because have more water per solute
What happens to plasma osmolality when water intake is less than water loss? Why?
Increases
because have less water per solute
What is the relative size of the range of urine osmolality?
Large range
How is the osmolality of urine changed?
By changing the volume of urine
How does changing the volume of urine change its osmolality?
Different amounts of water to same amount of solute
What are the mechanisms that regulate plasma osmolality?
ADH
Thirst
What detects plasma osmolality?
Osmoreceptors
Where are osmoreceptors located?
OVLT in the hypothalamus
How do osmoreceptors in the OVLT in the hypothalamus get into contact with plasma?
The capillary endothelium supplying them is fenestrated
plasma leaks out of capillaries
What produces ADH?
Neurosecretory cells in the supraoptic and paraventricular nuclei of the hypothalamus
What happens to ADH after it is produced?
It is transported down the axon of the neurosecretory cell
stored in vesicles in its tip
Where is the tip of the neurosecretory cell located?
Posterior pituitary gland
What causes ADH to be released?
Osmoreceptors detect an increase in plasma osmolality
action potentials are fired down neurosecretory cells that produce ADH
How much does the plasma osmolality have to increase by for ADH to be released?
1%
How does the amount of ADH released change with plasma osmolality?
Below 285mOsm/kg
amount of ADH released is constant at above 0
As plasma osmolality increases above 285mOsm/kg
amount of ADH released increases proportionately
How is ADH released?
Vesicles containing ADH in the tip of neurosecretory cells exocytose
release ADH into the blood
Where does ADH travel to after it has been released?
Kidneys
What does ADH do in the kidneys?
Increases the expression of aquaporin2 channels on the apical domain of epithelial cells in the later distal convoluted tubule and the collecting ducts
What is the effect of increased expression of aquaporin 2 channels on the apical domain of epithelial cells in the later distal convoluted tubule and the collecting ducts?
More water molecules move through it from the tubular lumen into the epithelial cell
so more water is reabsorbed
Aquaporin3 and aquaporin4 channels on basolateral domain of epithelial cell
water molecules move through them from the epithelial cell into the interstitial fluid
What is the effect of increased water reabsorption on the urine?
Smaller volume of more concentrated urine produced
What is the effect of increased water reabsorption on plasma osmolality?
Plasma osmolality decreases
What stimulates thirst?
Osmoreceptors detect an increase in plasma osmolality
action potentials are fired down neurones
How much does the plasma osmolality have to increase by for thirst to be stimulated?
10%
When does the stimulation for thirst stop?
After a sufficient volume of water has been ingested
so before it has been absorbed into the blood
What does thirst result in?
Increased water intake
What is the importance of the stimulation for thirst stopping after water has been consumed, before it has been absorbed into the blood?
It takes a while for water to be absorbed into the blood
so prevent ingestion of water in the meantime
as this would lead to overhydration
How do the ADH and thirst mechanisms regulating plasma osmolality compare to each other?
ADH mechanism occurs before thirst
so ADH mechanism is faster
ADH mechanism gives short-term solution
whereas thirst gives long-term solution
What else can stimulate ADH release and increased thirst apart from increased plasma osmolality?
Decreased blood volume and blood pressure
How does a decreased blood volume and pressure affect ADH release?
ADH released at lower plasma osmolalities than normal
More ADH released per unit increase in plasma osmolality
What are ADH and the thirst mechanism more sensitive to: changes in plasma osmolarity or changes in blood volume and pressure? How is this shown?
More sensitive to changes in plasma osmolarity
Smaller changes in plasma osmolarity than blood volume and pressure can stimulate ADH and the thirst mechanism
What is more important to control: plasma osmolality or blood volume and blood pressure? How is this shown?
Blood volume and blood pressure
Plasma osmolality will be compromised to maintain normal blood volume and blood pressure
How does an increased blood volume and pressure affect ADH release?
ADH released at higher plasma osmolalities than normal
Less ADH released per unit increase in plasma osmolality
How can ADH cause disease?
Deficiency of ADH
Excess of ADH
What is the disease of ADH deficiency called?
Diabetes insipidus
What are the types of diabetes insipidus?
Central
Nephrogenic
What is central diabetes insipidus?
Low levels of ADH in the plasma
because the hypothalamus and posterior pituitary gland don’t produce and release enough
What are the causes of central diabetes insipidus?
Damage to hypothalamus and posterior pituitary gland
- tumours
- trauma
- infection
What is nephrogenic diabetes insipidus?
Normal levels of ADH in the plasma
but the kidneys are insensitive to it
What are the signs symptoms of diabetes insipidus?
Polyuria - large volume of dilute urine produced
Polydipsia
How is diabetes insipidus treated?
Giving ADH
How can ADH be given?
Injections
Nasal spray
What is the disease of excess ADH called?
Syndrome of inappropriate ADH secretion (SIADH)
What causes SIADH?
High levels of ADH in the blood
excessively produced by the hypothalamus, or other sources
What are the signs and symptoms of SIADH?
Small volume of concentrated urine produced always
High total body fluid
Dilutional hyponatraemia