**4. Events at Different Segments of Nephron

How do kidneys maintain water balance during drastic changes in water intake? (when dehydrated or overhydrated)

• If dehydrated = Produce urine that is more concentrated than plasma (hypertonic)

• If overhydrated = Produces urine that is diluted than plasma (hypotonic)

Excretion of hypertonic urine (less water, more solutes)

• Requires

• How

• Occur

Requires: Antidiuretic hormone (ADH or arginine vasopressin) to alter transport (permeability) characteristics of collecting duct

How:

• ADH = hormone that reduces water loss by increasing water reabsorption back into the blood

• So ADH makes the CD more permeable to water by inserting aquaporins in principal cells

• Allowing water to leave the duct and be reabsorbed into the blood

• Producing concentrated urine as this leaves only solutes in the urine

Occurs: Dehydrated to retain water volume in the body

1.Proximal Tubule

• What occurs here

• Permeable to

• Tubular interstitial fluid are

What occurs here: Reabsorption and secretion

Permeable to: Water

Tubular interstitial fluid are: Isotonic

Descending Loop of Henle #ff00b3

• Water permeability

• Not permeable to

• What type of transport doesn’t occur here

• So what type of transport does

• What occurs

  • What diffuses out

  • What secreted in

• Therefore what occurs to the tubular and interstitial fluid

  • What else helps produce it

Water permeability: Permeable

Not permeable to: Solutes

What type of transport doesn’t occur here: Active transport

So what type of transport does: Passive transport

What occurs:

• Water diffuses out of tubules due to osmotic gradient

  • Between tubular lumen and interstitial fluids of renal medulla

• Sodium secreted into tubules

Therefore what occurs to the tubular and interstitial fluid: Becomes hypertonic

• What else produces it: Transport of Na+ and Cl- into interstitium without water

Ascending Loop of Henle #9800ff

• Water permeability

• So permeable to

• Therefore tubular fluid leaving ascending LoH is

• What is actively reabsorbed

  • Aided by which hormone

  • How

    • How is it sodium linked

Water permeability: NOT permeable

Permeable to: Solutes

Therefore tubular fluid leaving ascending LoH is: Hypotonic

What is actively reabsorbed: Sodium and chloride

• Hormone: Aldosterone from adrenal gland

• How: By co-transporters of Na+, Cl- and K+ —> cell

  • How is it sodium linked: Na-L-ATPase pump maintains the low intracellular sodium concentration

How is an osmotic gradient produced in the interstitial fluids of renal medulla and what is this mechanism called?

• Because the asce- and desce- limb of LoH are close together in medulla

• And tubular fluid flows in opposite directions

Called: Countercurrent mechanism

Where does interstitial fluid osmolality increase?

From outer zones —> inner zones of renal medulla

Counter-Current Mechanism: LoH

• Involves

• Interaction between

Involves: 2 fluids flowing in parallel but opposite directions in 2 U shaped tubes

Interaction between:

• Flow of filtrate through LoH (countercurent multiplier) AND

• Flow of blood through vasa recta blood vessels (countercurrent exchanger)

Collecting ducts in deep medulla permeable to?

Urea

Distal Tubule #ff8e00

• In the presence of ADH, what occurs

• In the absence of ADH, what occurs

In the presence of ADH: Water is reabsorbed and tubular fluid becomes isotonic

In the absence of ADH: Tubular fluid remains hypotonic

Collecting Duct

• What occurs here

• What are the target cells for ADH

• What occurs in the presence of ADH

  • Inhibits

• What occurs in the absence of ADH

  • Promotes

What occurs here: Regulation of electrolytes (Cl-, K+, H+ and HCO3-)

What are the target cells for ADH: Principal cells

What occurs in the presence of ADH: Stimulates the insertions of aquaporins allow for reabsorption of water

• Inhibits: Diuresis (increase amount of urine production)

What occurs in the absence of ADH: Luminal cell membrane of principal cells are water impermeable so water enters urine

• Promotes: Diuresis

So in what order does the fluid flow through?

• PCT

• Descending limb of LoH

• Ascending limb of LoH

• DCT

Micturition #ffc200

• What

• What stimulates micturition reflex

  • Activated by

• How does urine reach bladder continuously

• What does the bladder do during urine accumulation

• When does micturition reflex activate

  • What happens

• What nerves are responsible for bladder contraction

What: Expulsion of urine

What stimulates micturition reflex: Stretch receptors in bladder wall

• Activated by: Distension from accumulating urine

How does urine reach bladder continuously: Through ureters

What does the bladder do during urine accumulation: Relax

When does micturition reflex activate: When bladder stretch becomes great enough to stimulate reflex centers in spinal cord

• What happens:

  • Smooth muscle sphincter in bladder neck relaxes

  • Detrusor muscle contracts

What nerves are responsible for bladder contraction: Sacral parasympathetic nerves