Physiology 5- distal tubule, collecting ducts and urination Flashcards
How can the osmolarity of the tubular fluid entering the distal tubule be described?
Hypo-osmotic to plasma (about 100mosmol/l)
What effects do the following hormones have on ion reabsorption in the distal tubule?
a) aldosterone
b) ANP
c) parathyroid hormone
a) increases Na+ reabsorption, and increases H+ and K+ secretion
b) increases Na+ secretion
c) Increases Ca+ reabsorption, decreases phosphate reabsoprtion
How does ADH bring about an increase in water permeability in the distal tubule/collecting duct?
Increases number of aquaporins at the luminal membrane
Where is ADH synthesised and secreted?
Synthesized in the hypothalamus, secreted in the posterior pituitary
Outline how a) hypotonic and b) hypertonic urine is formed.
a) Minimal ADH secretion means less water is reabsorbed
b) increased ADH secretion causes water to move along the medullary osmotic gradient into the tubular interstitial fluid
Summarise the effect of ADH on urine volume and osmolarity.
Decreases urine volume and increases it’s osmolarity.
Where along the nephron is osmolarity at it’s greatest?
At the “turn” in the loop of Henle- water has been reabsorbed, leaving a high salt concentration
How much urine can the bladder hold before the micturation reflex is initiated?
250-450mls
How does urination remain under voluntary control?
External urinary sphincter is contracted (voluntarily). Release of this contraction allows the external sphincter to relax which causes the internal sphincter to relax, allowing urination
