Physiology 5- distal tubule, collecting ducts and urination

Question 1

How can the osmolarity of the tubular fluid entering the distal tubule be described?

Answer 1

Hypo-osmotic to plasma (about 100mosmol/l)

Question 2

What effects do the following hormones have on ion reabsorption in the distal tubule?

a) aldosterone
b) ANP
c) parathyroid hormone

Answer 2

a) increases Na+ reabsorption, and increases H+ and K+ secretion
b) increases Na+ secretion
c) Increases Ca+ reabsorption, decreases phosphate reabsoprtion

Question 3

How does ADH bring about an increase in water permeability in the distal tubule/collecting duct?

Answer 3

Increases number of aquaporins at the luminal membrane

Question 4

Where is ADH synthesised and secreted?

Answer 4

Synthesized in the hypothalamus, secreted in the posterior pituitary

Question 5

Outline how a) hypotonic and b) hypertonic urine is formed.

Answer 5

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

Question 6

Summarise the effect of ADH on urine volume and osmolarity.

Answer 6

Decreases urine volume and increases it’s osmolarity.

Question 7

Where along the nephron is osmolarity at it’s greatest?

Answer 7

At the “turn” in the loop of Henle- water has been reabsorbed, leaving a high salt concentration

Question 8

How much urine can the bladder hold before the micturation reflex is initiated?

Answer 8

250-450mls

Question 9

How does urination remain under voluntary control?

Answer 9

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