Physiology of body fluid volume Flashcards
What is the osmolarity of the fluid leaving the loop of henle? What does this make it?
100 mosmol
hyposmotic
What is the collecting duct bathed in?
progressively increasing concentrations of intersitial fluid as it descends through the medulla
Where does hormone regulation of fluid and NaCl affect?
distal tubule and collecting duct
>95% of the ions in filtrate have been reabsorbed before this though
What is the action of antidiuretic hormone?
increased water absorption
What is the action of aldosterone?
increased Na absorption and increased H/K secretion
What is the action of atrial natiuretic hormone?
decreased Na reabsorption
What is the action of parathyroid hormone?
increased Ca reabsorption
decreased phosphate reabsorption
What ions are transported in the early distal tubule?
Na, K and Cl
What ions are transported in the late distal tubule?
Ca, Na and K REABSORBED
H SECRETED
What are the characterisitics of the collecting duct in terms of water and ions?
early - permeable to ions
late - low ion permeability but high water and urea permeability
What type of hormone is ADH?
neuropeptide - synthesised by nerve cells
What happens when action potentials reach the ADH nerves?
ADH is released through Ca dependant exocytosis
What type of ADH receptors does the basolateral membrane express?
type 2
What happens when ADH binds to receptors?
cAMP increases which initiates the insertion of aquaporins (normally stored in vesicles) into the apical membranes of tubular cells
What does decreased ADH cause?
aquaporins become internalised
What is the purpose of aquaporins?
to create an equilibrium between the collecting duct fluid and tubular fluid
Why would high volumes of urine come about due to the collecting duct?
in the collecting duct the membrane is impermeable to water so there is no water reabsorption
Does ADH have an effect on salt reabsorption?
NO
What does increased osmolarity trigger?
hypothalamic osmoreceptors -> hypothalamic neurons -> increased thirst
What does decreased ECF volume trigger?
decreased arterial BP -> left atrial volume receptors activated -> hypothalamic neurones -> increased ADH and arterial vasodilation
What are the two types of diabetes insipidus?
central - no ADH production
nephrogenic - produce ADH as normal but defect in Type 2 ADH receptors
What stimulates ADH release?
nicotine
What inhibits ADH release?
alcohol
What does stimulation of stretch receptors in the upper GI tract cause?
feed forward initiation of ADH
What is aldosterone secreted in response to?
increased K
decreased Na
What is the role of aldosterone?
secrete K
reabsorb Na
What would happen if there was no aldosterone?
tons of salt would be secreted from the body
Should there be any K in the urine?
NO
90% of it is absorbed in proximal tubule
rest is absorbed in distal
Where is angiotensin converting enzyme expressed?
outside of endothelial cells
What 3 things stimulate renin release?
- reduced pressure in the efferent arteriole (low BP)
- low salt - sensed by macula densa cells
- increased sympathetic activity as a result of low BP - granular cells release renin and are innervated by sympathetic nervous system
How does aldosterone increase the reabsorption of Na?
in the distal and collecting tubules, it increases the expression of sodium apical channels and basolateral sodium/potassium pump expression
this is done by change in gene expression and therefore
What is atrial naturetic peptide?
released by the heart and stored in atrial muscle cells
When is ANP released?
when atrial muscle cells are stretched due to an increase in circulating plasma volume
What does ANP do?
promotes excretion of Na and diuresis - which decreases plasma volume and BP
What are the two reflexes that control micturition?
micturation reflex (can hold 250-400ml before stretch receptors are triggered) and voluntary control (tightening of external spinchter and pelvic diaphragm)
