Water Balance

We must output how much urine per day

500mL

Why must we output 500mL of urine per day

Waste removal

Urine output can be as high as what

18L per day - if 10% of normal GFR is being absorbed (normally 99% is reabsorbed - don’t need to know 99% - just context)

What is considered sensible & insensible water loss

Sensible water loss can be easily measured (urine)

Insensible water loss can’t be easily measured - through skin & lungs

Insensible water loss can be as high as what during exercise & in hot conditions

Can be as high as 3.5L/hr from skin during exercise and in hot conditions

Sources of input of water into our system

Food

Drink

Metabolism (water is a waste product of metabolism)

Water output routes from the body

Urine

Faeces

Evaporation

Exhaled air

Water moves from a region of high/low osmolarity to a region of high/low osmolarity

Water moves from region of low osmolarity to region of high osmolarity

Under normal circumstances is osmotic pressure inside and outside cell equal

yes

What measures water conc. in the body

Osmoreceptors - they sense Posm

Where are osmoreceptors located in the body

Paraventricular Nucleus (PVN) of the Hypothalamus

Changes in osmoreceptor activity leads to the regulation of what

ADH secretion by the posterior pituitary gland

What are the main cells that are responsive to ADH in the kidney

the CNT (Connecting Tubule) cells and Principal (P) cells are the main ADH-responsive cells in the kidney - they are in the distal nephron

Normal osmolarity of cells & the ECF

280-300 mOsm/L

Osmoreceptors fire above ____mOsm/L - triggering the release of ADH

Osmoreceptors fire above 290mOsm/L - triggering the release of ADH

why is osmolarity so tightly controlled

↑ ECF Osmolarity → Cells shrink

↓ ECF Osmolarity → Cells Swell

What is Arginine Vasopressin (AVP) (or Vasopressin (VP))

ADH

ADH is a peptide made of how many amino acids

9 - it is a nonapeptide

ADH is broken down where

Liver

Is ADH fast/slow acting

Fast

Is ADH cleared by the kidney & degraded by the liver fast/slow

Fast - osmolarity needs to be tightly regulated (minute-to-minute)

Where in the nephron is responsible for creating a water-saving environment in the kidneys

Loop of Henle

Vasa Recta

How does the loop of Henle create a water saving environment

Interstitial osmolarity gets higher as you go deeper from the cortex into the medulla of the kidney.

The descending limb of the Loop of Henle is permeable to water but impermeable to solutes.

The water gets pulled out of the descending loop of Henle into the interstitium to balance osmolarity

What happens in the ascending loop of Henle

The (thick) ascending limb is impermeable to water but actively transports Na⁺ into the interstitium.

There is a unique arrangement of medullary peritubular capillaries (vasa recta) that allow for the recycling of urea between the loop & the collecting duct

We like to make the medulla hypertonic for lots of water reabsorption. How do we make the medulla optimally hypertonic

Transport salt, but not water, out of the loop of Henle and into the interstitium (the “single effect”)

Problem: Salt transport can only achieve a limited degree of hypertonicity. How do we magnify the degree of hypertonicity to achieve 1200 mOsm? - Countercurrent multiplication

How does countercurrent multiplication work

As the descending limb descends deeper into the medulla, it becomes more concentrated with solutes (like sodium and chloride) as water is reabsorbed into the body. This causes the filtrate to become more concentrated the deeper it goes.

The ascending limb pumps out salts but is impermeable to water, so it becomes less concentrated and makes the surrounding interstitial fluid more concentrated.

The water reabsorbed from the descending limb is taken away in vasa recta but the salt pumped out into the interstitial fluid of the medulla is not - this increases the medullary osmolarity without dilution from the water released.

The absence of ADH will result in urine with higher/lower osmolarity

Lower

fill in the line of the graph to show the change in osmolarity of tubular fluid

An increase in blood volume is detected by what

Baroreceptors

Volume receptors

Tonic firing of baroreceptors & volume receptors due to increased blood volume has what effect

Their tonic firing inhibits the supraoptic nucleus (SON) and paraventricular nucleus (PVN) of the hypothalamus, which are key centres for vasopressin (antidiuretic hormone, ADH) release.

ADH is not released. More water is excreted

An increase in plasma osmolarity is detected by what

osmoreceptors

If osmoreceptors are stimulated due to elevated plasma osmolarity what happens

They cause the release of ADH which makes the kidneys retain water

2 factors NB in the regulation of Urine

osmolarity & H2O balance

Where can you find a high osmolarity

Medulla of kidney

What regulates H2O permeability of CD (collecting duct)

ADH

What is the body’s response to hypovolemia

What is the body’s response to hypervolemia

We measure water balance by measuring what

osmolarity

Role of neurons in regulating a hyperosmotic state

Neurons shrink in response to surrounding hyperosmotic solution

They send more action potentials causing more ADH to be released

Change in osmolarity particularly effects neurons - neurons don't cope very well with osmotic challenges - this means almost nothing in the body is as tightly regulated as osmolarity

There is only 1 mechanism for saving water - what is it

create an osmotic gradient

can we make the blue 1200 any higher

Yes - with a longer loop of Henle

What are vasa recta in the kidney

capillaries that run parallel to the loops of Henle in the renal medulla. They play a crucial role in maintaining the countercurrent exchange system

How does ADH cause reabsorption of water

ADH causes the insertion of aquaporins in the collecting duct - water can’t normally escape from the collecting duct but it can now escape because of the aquaporins - water leaves the collecting duct due to the hyperosmotic environment outside the collecting duct

Effect of alcohol on ADH

Alcohol suppresses ADH - you pee loads on the night out. The next morning you are dehydrated & ADH is working on overdrive so you pee out a small dark pee because ADH is allowing the body to retain as much water as possible

The movement of water is always secondary to what

movement of salt