Term 2 Lecture 11: Loop Of Henle

Question 1

Filtrate formation

Answer 1

The kidneys can excrete varying concentrations depending on the bodys state of hydration

uncontrolled osmotic reabsorption occurs in the proximal tubule

fluid entering the LoH is isotonic with the ECF

descending LoH has obligatory reabsorption of water causing establishment and maintenance of the vertical osmotic gradient

Question 2

Descending limb

Answer 2

highly permeable to water reabsorption by osmosis
AQP-1 channels always open
No Na+ reabsorption

Question 3

Ascending limb

Answer 3

Actively transports NACL from lumen into interstitial fluid via channels
impermeable to water (no aquaporins)

Question 4

steps from proximal tubule to collecting duct

Answer 4

1) isometric fluid leaves the proximal tubule and becomes progressively more concentrated in the descending LoH
2) removal of solute in the thick ascending LoH creates hypoosmotic fluid
3) hormones control distal nephron permeability to water and solutes
4) urine osmolarity depends on reabsorption in the collection duct

Question 5

Mechanism of countercurrent multiplication

Answer 5

Imagine you have a new nephron and nothing has happened to it yet - these are the processes to set up an osmotic gradient:

The filtrate in the proximal tubule is isoosmotic with the interstitial fluid surrounding the tubule so no water movement occurs. The filtrate moves through to the distal tubule and into the collecting duct

Potassium/Sodium pumps in the ascending limb are activated to pump out NaCl and K+ moving NaCl into the interstitial fluids around the LoH increasing osmolarity of the interstitial fluid and decreasing the osmolarity of the filtrate in the ascending limb.

Filtrate in descending limb increases its osmolarity so water flows osmotically out of the descending limb into the more concentrated interstitial fluid. Passive movement of water occurs until osmolarity equilibriates.

So when transport of NaCl is established at the ascending limb of the LoH, water passively moves osmotically out of the descending limb of LoH. Establishing a 200mOsm/L gradient.

A gradient of osmolarity that is low at the top matching extracellular (interstitial) fluid osmolarity (isotonic.) Down to a very concentrated osmolarity at the hairpin bend of the LoH.

The fluid in the distal part of the LoH is hypotonic to excrete a urine more dilute than normal body fluids.
This is important to maintain water balance

Question 6

Vasopressin controlled variable water reabsorption

Answer 6

(vasopressin causes insertion of aquaporin channels into membrane)

Occurs in the final tubular segments
obligatory water reaabsorption
proximal tubule (65% water filtered out here)
LoH (15% water filtered out here)
20% of the water remains in the lumen to enter the distal and collecting tubules for variable reabsorption under hormonal control

Question 7

Water reabsorption in the final segments of tubule

Answer 7

36 L/day i.e. 13x the amount of plasma water in the entire circulatory system is reabsorbed.

100mOsm/L leaves the LoH
-hypotonic to surrounding interstitial fluid of the renal cortex (which is 300mOsm/L)
Distal tubule passes through the cortex and then empties into the collecting duct which descends into the medulla - increasing concentration from 300 to 1200mOsm/L

Distal and collecting duct are impermeable to water, vasopressin acts to insert aquaporins and allow osmosis to occur

Question 8

Role of vasopressin

Answer 8

Anti-diuretic hormone (ADH)
presence of vasopressin in the tubular lumen wall makes it permeable to water
vasopressin plays a role in neurotransmission in other parts of the body where it causes blood vessel constriction.
Vasopressin is produced in the hypothalamus and released from the posterior pituitary gland.

Question 9

Trigger for vasopressin release is plasma osmolarity

Answer 9

high plasma osmolarity (high salt):
is detected by the hypothalamus and it releases vasopressin to reabsorb water and reduce blood osmolarity to normal levels

low plasma osmolarity: vasopressin is not stimulated so channels are not inserted and water is not reabsorbed into the plasma from the distal/collecting tubule as plasma is dilute enough already

Question 10

Vasopressin insertion step by step

Answer 10

1) blood-borne vasopressin diffuses from the peritubular capillaries and binds with its receptor V2 on the basolateral membrane of distal or collecting tubule principal cells
2) This binding activates cyclic AMP (cAMP) which is the second messenger in this pathway
3) cyclic AMP increases the opposite luminal membranes permeability to water by promoting the release of vasopressin from storage vesicles in the cell and insertion of vasopressin (regulated AQP-2 water channels) into the membrane. This membrane is impermeable to water in the absence of vasopressin
4)water enters the principal cells through the always open AQP-3 or -4 channels which are permanently positioned at the basolateral border and then enters the blood - in this way it is reabsorbed

• so we reabsorb as little or as much water as we need depending on our hydration state

(see diagram notebook 3)

Question 11

Blood volume and osmolarity activate osmoreceptors

Answer 11

stimulus>receptor>afferent pathway>route to hypothalamus

-Osmolarity greater than 280mOsm/L > hypothalamic osmoreceptors > interneurons to hypothalamus

-Decreased atrial stretch due to lower blood volume > atrial stretch receptors > sensory neurons to hypothalamus

-Decreased blood pressure > carotid and aortic baroreceptors > sensory neuron to hypothalamus

The hypothalamus is the integrating centre once stimulated the hypothalamus neurons synthesise vasopressin.

Efferent pathway:
Vasopressin is released from the posterior pituitary

Effector: distal/ collecting duct principal cells

Tissue response: Insertion of water pores (AQP-2) into apical membrane resulting in increased water reabsorption to conserve water in the plasma.

Question 12

Osmoreceptors are stretch sensitive neurons that increase firing rate as osmolarity increases

Answer 12

They are located near the hypothalamus in the third ventricle of the brain by the cerebrospinal fluid - from which they can sense the fluids osmolarity.

They are connected to interneurons in the hypothalamus that are connected to hypothalamic neurons that make vasopressin.

Cells shrink when dehydrated, non-specific cationic channels linked to actin filaments open depolarising cells

When plasma osmolarity is below 280mOsm/L osmoreceptors are inactive and vasopressin release from pituitary ceases

Question 13

Circadian rhythm

Answer 13

At night a lot of vasopressin is released so most water is reabsorbed into the plasma allowing humans to sleep through the night and release concentrated urine in the morning.

Alcohol inhibits vasopressin action so less water is reabsorbed so after consuming a certain amount of alcohol you need to urinate more frequently

Nocturnal enuresis
bedwetting - usually in children
caused by reduced vasopressin production
this can be treated by artificial vasopressin Desmopressin - administered as a nasal spray

Question 14

Head injury effect on vasopressin production

Answer 14

Head injuries if severe enough can sever the pituitary stalk causing many physiological problems including the lack of ability to produce vasopressin to regulate water balance causing trouble reabsorbing water and excess production of urine.
This is treated with regular Desmopressin

Question 15

Vasopressin water regulation summary

Answer 15

> Water moves into interstitial fluid by osmosis
- intrinsic cortex - hyperosmotic medulla

> small amount of urine produced normal output 1ml/min
lowest possible volume 0.3ml/min highest possible 25ml/min

> Urine must always be produced, minimum volume essential to excrete waste products is 0.5L per day

Question 16

The renal countercurrent multiplier: why is the osmotic gradient dynamic in the LoH?

Answer 16

Because in the vasorector (peritubular capillaries around LoH) blood flows in the opposite direction to the filtrate in the LoH.

This countercurrent continually moves ions and water away from the gradient so it never equilibriates.

Water moves by osmosis from the descending LoH into progressively more concentrated interstitial fluid.

25% Na+ and K+ is reabsorbed from the ascending limb by active ion transporters in the thick portion of the ascending LoH.

NHCC symporter uses energy stored in Na+ concentration gradient to transport Na+ K+ and 2Cl- from lumen to epithelial cells of the ascending limb

Na+ K+ ATPase removes Na+ from cells on the basolateral side epithelium.
K+ and Cl- moved by cotransporter proteins or open channels

NHCC mediated transport can be inhibited by drugs known as loop diuretics e.g. Furosemide (Lasix)

Question 17

countercurrent blood flow for warmth in animals feet

Answer 17

You can see countercurrent blood flow in ducks in the blood flow in their feet and the pads of the paws of dogs and cats. This prevents their feet from freezing.
Warm blood flows in and cold blood out. Heat is transferred across a temperature gradient keeping the venous blood warm

Question 18

High bp can be reduced by a diuretic

Answer 18

Diuretics cause increased volume of urine excretion which in turn decreases blood plasma volume.
The diuretic Furosemide works by blocking Na+K+ channels preventing Na+ reabsorption reducing the osmotic gradient in the LoH so that less water is reabsorbed and therefore volume of urine produced is increased within 20 minutes.
The problem with Furosemide is that it causes you to excrete a lot of K+ which is essential for normal cardiac excitability and deficiency can cause upset cardiac rhythm - detrimental to health.
For this reason potassium sparing loop diuretics are far more commonly used, they still act on the Na+K+ channels but allow partial reabsorption of K+ without NaCl reabsorption - thus allowing continued normal cardiac rhythm

Question 19

Summary so far

Answer 19

Proximal tubule
-67% Na+ reabsorbed here
Active, uncontrolled and plays a pivotal role in reabsorption of glucose, aas, Cl-, water and urea
-65% water reabsorbed here
Passive, obligatory osmosis following active Na+ reabsorption

Loop of Henle
-25% Na+ reabsorbed here
Active, uncontrolled, NaCl reabsorption from ascending limb helps establish the medullary interstitial vertical osmotic gradient which is important in the kidneys ability to produce urine of varying concentrations and volumes depending on the bodys needs

Distal and collecting tubule
-8% Na+ absorbed here
Active, variable and subject to aldosterone control, important in the regulation of ECF volume and long-term control of blood pressure. Linked to K+ and H+ secretion.
-20% water absorbed here
Passive, not linked to solute reabsorption, variable quantities of “free” water reabsorption subject to vasopressin control, driving force is the vertical osmotic gradient in the medullary interstitial fluid established by the LoH, important in regulating ECF osmolarity.