Water Handling

Question 1

too much water in the brain can cause __ ___. why is this bad?

Answer 1

can cause cerebral edema. compresses the blood vessels that run near the surface of the brain and the skull, leading to ischemia.

Question 2

too little water in the brain can cause ___ ____.

Answer 2

intracranial hemorrhage. the vessels shink, causing them to be more fryable.

Question 3

explain the cerebral adaptation process to abnormal [Na] concentratiosn

Answer 3

• the brain can reduce the number of particles tomaintain osmolarity and regulate the brain “size”. there is a volume sensor that can sense osmole concentration
• can also create osmotically active neurotransmitters to prevent water from leaving if it senses dehydration.

when the brain has already adapted by losing osmoles. If we rapidly change its balance, even if it is off but compensated, we can cause osmotic demyelination/further brain damage.

Question 4

explain what can happen if you overcorrecct hyponatremia too quickly.

Answer 4

in hyponatremia, there is a decreased amount of serum sodium. water then moves inside the cell (where there is currently more sodium than in serum), while ATPase pumps work to pump Na+ out of cell to increase the reduced resume sodium level. Overall, this process can allow the brain and body cells to function relatively normally despite lower levels of sodium.

IF you treat hyponatremia too quickly, when the brain has already adapted by losing osmoles. If we rapidly change its balance, even if it is off but compensated, we can cause osmotic demyelination/further brain damage.

Question 5

water handling is driven by ___ and ___ activation

Answer 5

by serum sodium concentration, and RAS activation

Question 6

outline the feedback loop governing regulation of plasma osmolality thrgouh control of arginine vasopressin secretion and thirst

Answer 6

• an increase in plasma osmolality activates hypothalamic oassmoreceptors
• these osmoreceptors stimulate vasopressin secretion by the posterior pituitary gland
• the increase in plasma vasopressin leads to an increase in renal water REABSORPTION and a decrease in water excretion.
• osmosesnsing in the hypothalamus also stimulates THIRST AND DRINKING to help restore plasma osmolality

Question 7

feedback loop governing regulation of plasma osmolality thrgouh control of arginine vasopressin secretion and thirst

• an increase in plasma osmolality activates __ oassmoreceptors
• these osmoreceptors stimulate __ gland
• the increase in plasma vasopressin leads to an increase in renal water __ and a decrease in water __.
• osmosesnsing in the hypothalamus also stimulates __ AND __ to help restore plasma osmolality

Answer 7

• an increase in plasma osmolality activates hypothalamic oassmoreceptors
• these osmoreceptors stimulate vasopressin secretion by the posterior pituitary gland
• the increase in plasma vasopressin leads to an increase in renal water REABSORPTION and a decrease in water excretion.

– osmosesnsing in the hypothalamus also stimulates THIRST AND DRINKING to help restore plasma osmolality

Question 8

ineffecitve vs effective osmole

Answer 8

Ineffective osmole; a particle that has the same concentration inside and outside the cells (ex/ urea, glucose, alcohols)

Effect osmoles; have different concentrations in side and outside of the cell, causing fluid shifts (na+ (outside cell higher), K+, higher inside the cell, glucose if insulin is problematic, mannitol (draws fluid out of the brain– effective ormolu)

Question 9

outline effective and ineffective osmoles

Answer 9

Question 10

osmolality vs tonicity

Answer 10

Osmolality = sum of osmotically active molecules

Tonicity = sum of effective osmoles ~ [Na] + [K]. In serum can ignore [K]

Question 11

outline how angiotensin II can override the osmolality signal (recall the osmolality is a reflection of osmotically effective solutes aka SODIUM)

Answer 11

The signal from angiotensin II will always override the signal based on the serum sodium concentration (ex/ if ADH is “turned off” but angiotensin II is being produced, angiotensin will still promote ADH production)– the body considers the need to preserve blood volume more important than needing to have a normal sodium concentration.

An abnormal Na+ concentration is due to abnormal water handling.

Question 12

Why is hyponatremia a water problem?

Answer 12

The hormonal system (RAS) does not respond to serum sodium concentration. The stimuli of RAS is the affect of pressure from blood volume-- therefore its more so water.

Hyponatremia is due to too much water. The water intake is too much compared to water loss (ex/ ingestion too much water, but in chronic hyponatremia involves reduced renal excretion of water.) our clinical approach to hyponatremia is to explain why the kidney is not excreting enough water.

Question 13

explain normal renal handling of water based on nephron components

Answer 13

Glomerulus; freely filters water. The amount of water cleared in the kidney is reflected by GFR. The PCT reabsorbs 2/3 sodium that is filtered, therefore we also reabsorb 2/3 of th water in PCT.

The descending LOH water is also reabsorbed, but Na+ is left behind.

In ascending loop, Na+ is removed from lumen and put in interstitial, creating a hypertonic interstate, water stays in the tubules, causing dilution of filtrate-“desalination vs the Na+K+Cl-.

In the collecting tubule there is ADH dependence. Fine tuning of concentration of urine depending on what’s going on in the body– ex/ if theres dehydration.

Knowing that water follows osmoles, we need to excrete a significant amount of osmoles to regulate water and draw it out.

Question 14

mechanisms that cause abnormal renal handling (causing hyponatremia– too much water) (4)

Answer 14

1. inadequate GFR– water doesn’t leave the blood in the first place– can’t get excreted
2. impaired desalination– hereditary, acquired
3. antidiuresis; inappropriate release of ADH, then reduces ability to excrete water, causing buildup and hypnatremia
4. too few osmoles– can’t drag out water to begin with.

Question 15

2 main causes of abnormal renal handling of water causing hyperntatremia (too little water)

Answer 15

1. diabetes insipidus
2. too many osmoles

Can be due to excessive loss of water because of resistance to ADH– diabetes insidious.

Can be losing too much water if you have an excess amount of osmoles in the urine/

Question 17

note:

• The brain is particularly vulnerable to changes in cellular water content
• Water handling is driven by [Na] and RAS activation
• Abnormal serum [Na] is due to abnormal water handling
• The kidney regulates the amount of water in the body
• The kidney is usually at fault when there is abnormal [Na] (i.e., abnormal amount of water)– but if the sodium is normal and the other solutes are off, its usually a solute problem (due to shift, intake, or loss of the solute), rather than a water problem