K5 - The Distal Tubule, Collecting Ducts and Urination Flashcards
What is the tubular fluid leaving the loop of henle and entering the distal tubules relationship to plasma?
hypo-osmotic
what is the concentration of the surrounding interstitial fluid of the renal cortex?
300mosmol/l
the distal tubule empties into where?
collecting duct
What is very important for salt balance?
residual load
what hormones regular ion and water balance?
- antidiuretic hormone (ADH)
- aldosterone
- atrial natriuretic hormone
- parathyroid hormone
How does ADH regulate water balance?
Water reabsorption increases
How does aldosterone regulate ion balance?
Na+ reabsorption increases
H+/K+ secretion increases
How does the atrial natriuretic hormone regulate ion balance?
Na+ reabsorption decreases
How does parathyroid hormone (PTH) regulate ion channel?
Ca2+ reabsorption increases and PO4 3- reabsorption decreases
What does the distal tubule have low permeability to?
water and urea
Where is urea concentrated in?
tubular fluid
what occurs in the early distal tubule?
Na+, K+ and 2Cl- transport
what occurs in the late distal tubule?
Ca2+ reabsorption
H+ secretion
Na+ reabsorption
K+ reabsorption
What part of the collecting duct has low ion permeability and permeability to water influenced by ADH?
the late collecting duct
what is synthesised by the supraoptic nucleus in the hypothalamus?
octapeptide
Where does ADH secretion occur?
posterior pituitary
where is ADH stored?
In granules in the posterior pituitary
When is ADH released into the blood?
When action potentials down the nerves lead to Ca2+ dependent exocytosis
What is the plasma half life of ADH secretion?
10-15 mins
Describe the effect of ADH on water permeability of the collecting duct?
- ADH acts on type 2 vasopressin receptor on basolateral membrane
- Once bound ATP is converted into cyclic AMP
- this increases the aquaporins at the luminal membrane therefore more water is reabsorbed to the blood
If there is no ADH in the blood what happens to the aquaporin concentration at the luminal membrane?
they become internalised back into the cytoplasm and therefore reduce the permeability of the cells for water reabsorption
What enables hypertonic urine formation?
In the presence of maximal [ADH]plasma water moves from the collecting duct lumen along the osmotic gradient into the medullary interstitial fluid
What does High ADH cause?
- high water permeability down osmotic gradient,
- concentrated urine
what enables hypotonic urine?
- low ADH
- low water permeability down osmotic gradient,
- dilute urine
what effect does ADH have on salt balance?
no effect
what are the features of the Proximal tubule and descending loop of Henle?
- Na+ reabsorbed by transcellular mechanisms (**PT only)
- Water moves between inter-cellular junctions
- These produce net fluid reabsorption
- Na+ and water coupling
what are the properties of the Ascending Loop of Henlé and Distal Tubule?
- Low water permeability of epithelium (ADH-dependent)
- Na+ & Cl- reabsorption
These produce dilution of urine - Na+ & Cl- coupling
What are the properties if the collecting duct?
- Modulation of water / urea permeability
- This contributes to determination of final urine osmolarity
What are the properties of the Distal Tubule / Collecting Duct?
Site of hormone action (ADH, Aldosterone, Atrial Natriuretic Peptide
Where is urine temporarily stored?
Where is urine temporarily stored?
In the bladder
What is the name for the process of the bladder emptying?
Micturation or urination
What two mechanisms is micturition governed by?
- The micturition reflex
2. Voluntary control
How many ml of urine can the urinary bladder accommodate before stretch receptors within its wall initiate the micturition reflex?
250- 400ml
what is the micturation reflex?
stretch receptors within walls of bladder are stretched leading to involuntary emptying of the bladder by simultaneous bladder contraction and the opening of both internal and external urethral sphincters
how can micturation reflex be voluntary prevented?
By deliberate tightening of the external sphincter and surrounding pelvic diaphragm
