6: Reabsorption of Water: Collecting Ducts

The main 2 functions of the distal tubule is to?

• Regulate secretion of K+ into the Urine

• The absorption of Na+ out of the filtrate

In the distal tubule, more…?

• NacI is absorbed.

• there are no aquaporins present so no water is absorbed from the filtrate

The collecting ducts first form in the?

• Cortex

What forms the collecting ducts ?

• Convergence of Many Distal Tubules

The collecting ducts must pass through the hyper?

• Hypertonic Renal Medulla

What can the collecting ducts pass after passing through the Hypertonic Renal Medulla?

• Can pass Urine into the Calyces

The fluid of that enters the collecting duct is normally very?

• Hypertonic (≈100mOsm)

Why is the fluid than enters the collecting duct Hypertonic?

• due to the actions of the distal tubule removing more solutes

The collecting duct is permeable to water but completely?

• Completely Impermeable to NacI

the high concentration of NaCl in the renal medulla interstitial fluid cannot move?

• Into the collecting ducts.

The hypertonic environment of the renal medulla is usually?

• Constant

the permeability of the collecting duct varies depending on the?

• Hydration Status of the body

In the collecting duct, hormones maintain whole?

• body salt/water balance by regulating the rate of water reabsorption.

the permeability of the collecting duct is altered by the number of?

• Aquaporins

• present in the plasma membrane of the collecting duct epithelial cell

The more aquaporins there are in the apical plasma membrane, the?

• Permeability of the Membrane

Fluid not reabsorbed that leaves the collecting duct….?

• Becomes Urine

Aquaporins diagram

Aquaporins (water channels) can be stored in?

• Internal Vesicles

The posterior pituitary gland secretes anti?

• Antidiuretic Hormone (ADH)

When is ADH released?

• During a state of Dehydration

As little as ..% increase in plasma osmolality can induce increased ADH release?

• 1%

When ADH binds to its receptors on the epithelial cell surface, cAMP acts as a….?

• Second messengers,

• Triggering Aquaporins.

The process of Exocytosis does ?

• Insert the aquaporins into the plasma membrane

• does not result in the release of any contents into the extracellular fluid

In the absence of ADH, the aquaporins are removed from the?

• Removed from the Plasma Membrane

• Via Endocytosis

ADH Water Balance Diagra,

Bullet point describe Dehydration and irs ADH effect on Water Balance?

• Dehydration = ADH Release

• ADH increases Concentration of Aquaporins.

• More Water Retained in Collecting Ducts

• Urine = More Concentrated?

Bullet point describe Over-Hydration and irs ADH effect on Water Balance?

• Overly Hydrated Inhibits ADH Production

• Lack of ADH promotes the internalisation of aquaporins in the collecting ducts.

• Less water is retained from the collecting ducts

• Urine is more Diluted.

The highest possible urine osmolarity is equivalent to the osmolarity?

• of medullary interstitium (≈1,400mOsm).

Under normal conditions how much Urine is produced daily?

• 1.5 L

• minimum of 400ML

Diabetes insipidus is a?

• Rare Disease

• not linked to T1D or T2D

Diabetes insipidus is characterised by?

• Polyuria

• Chronic Thirst

What are the 2 Types of Diabetes Insipidus?

• central diabetes insipidus

• nephrogenic diabetes insipidus

Describe Central Diabetes Insipidus?

• Central diabetes insipidus results from the insufficient secretion of ADH

Describe Nephrogenic Diabetes Insipidus

• Nephrogenic diabetes insipidus develops when the kidneys do not respond to ADH circulating

• Can occur due to Lithium perscription.