Renal system: L38 - Body water

Question 1

What percentage of our body mass is water for males and females?

Answer 1

Males: 60%
Females: 55%

Question 2

What percentage of our water is ICF and ECF?

Answer 2

2/3 is ICF, 1/3 is ECF

Question 3

What percentage of our ECF is interstitial fluid and plasma? What percentage of blood is plasma?

Answer 3

Interstitial fluid: 4/5
Plasma: 1/5
Blood is 50-60% plasma.

Question 4

What is the ICF composed of?

Answer 4

Mainly K+ ions (150mmol/L) and contributes most to ICF osmolarity.

Question 5

What is the normal overall osmolarity for both ECF and ICF?

Answer 5

Around 275-295 mosmol/L for both ICF and ECF as the membranes are permeable so there will be osmotic balance achieved to equalise solute concentration in each compartment.

Question 6

What is the ECF composed of?

Answer 6

Mainly Na+ ions (140mmol/L) and contributes most to ECF osmolarity. Cl- is also dominant (110mmol/L) and can be remembered by NaCl comes as a package.

Question 7

What stimulates ADH release?

Answer 7

Osmolarity changes.

Question 8

How would cells shrink/swell in terms of osmolarity? What are the consequences of this?

Answer 8

Water moves between compartments through semipermeable membrane, to the compartment of higher osmolarity to equalise osmolarity between compartments. Unfortunately, this causes changes in volumes of cells. For example, a high osmolarity in cells means water will go into the cell causing cell swelling and possible burst, a low osmolarity in cells means water will rush out of the cell into the ECF causing cell shrinkage.
In both cases cell structure and thus function will be impaired. Therefore it is important to regulate water volume to regulate size.

Question 9

What are the typical intakes and outputs that affect water balance in our body?

Answer 9

Inputs: Water in food, drink, from metabolism.
Outputs: Lungs, skin, sweat, faeces, urine.

Question 10

Explain the negative feedback loop of ADH release.

Answer 10

Osmoreceptors in the hypothalamus detect changes in plasma osmolarity. These then send action potentials to the posterior pituitary gland which increase/decrease ADH secretion in response. An increase in ADH causes increased permeability of water in collecting duct, so more water is either retained or excreted in response. This balances changes in total body water, plasma osmolarity and cell volume therefore remain stable.

Question 11

Explain where ADH is made, where it is stored, how it is released.

Answer 11

Anti-diuretic hormone is made in the hypothalamus and stored in the nerve terminal of the posterior pituitary. When stimulus of increased ECF osmolarity or a decrease in blood volume, increase in frequency of action potentials sent along the neurons. This stimulates rapid release of ADH into surround capillaries, because this is modulated by nerves its referred to as neurosecretion.

Question 12

What does ADH do in extreme conditions of requiring water reabsorption?

Answer 12

Normally, ADH inhibits water excretion by kidneys (by increasing collecting duct permeability), but in extreme conditions it can induce thirst and cause vasoconstriction (to maintain blood pressure when body water severely reduced). It is also able to respond to significantly reduced blood volume (e.g following haemorrhage).

Question 13

Discuss how ADH changes permeability of collecting duct.

Answer 13

Increased ADH = Increased aquaporins in membrane of CD = water able to pass through them to be reabsorbed into the medulla (and then capillaries) = decreased volume of urine excreted = increased total body water.
ADH works to modulate genes and increase aquaporin synthesis. ADH also increases exocytosis of those aquaporins already been synthesised. Because of this, only 0.5% of filtrate will be excreted, the result will be a small volume of concentrated urine passed. The medulla is hyperosmotic, which creates a gradient to enable fluid to move out of the collecting duct.

Question 14

What are the physiological limits of kidney water handling? As in max/min amount that you can urinate, and max/min amount of osmolarity of urine your body can deal with.

Answer 14

Limits of urine osmolarity: Lower limit = 50mosmol/L this is very dilute urine with low levels of ADH, Upper limit = 1200 mosmol/L this is very concentrated urine with high levels of ADH.
Limits of urine volume: Lower limit is 0.5L/day (high ADH), Upper limit is 20L/day (low ADH).

Question 15

How is thirst triggered?

Answer 15

With normal regulations of osmolarity, ADH will regulate urine output without the need to trigger sensation of thirst. Triggers to thirst include:
ECF osmolarity must exceed the dipsogenic threshold at 295 mosmol/L.
Hypovolaemia = Low blood volume, with associated low blood pressure.
Angiotensin II which acts to increase blood pressure (in low blood volume) through vasoconstriction.

Question 16

What is the maximum drinking response?

Answer 16

The limit to how much a person can drink to combat thirst which is about 25/L day.

Question 17

What is significant about isosmotic fluid gains/losses?

Answer 17

A loss of isosmotic fluid can occur when a person loses electrolytes as well as water.
A gain of isosmotic fluid can occur when a person gains electrolytes as well as water.
In these cases, osmolarity is not an issue as it is not changed between ECF or ICF, but instead of problems with cell size, there are changes in volume of blood and consequently changes in blood pressure.

Question 18

Explain how isosmotic fluid loss occurs and consequences of isosmotic fluid loss.

Answer 18

Examples include: diarrhoea, burns and vomiting. Fluid is lost from the ECF, but there is no change in osmolarity. Because of this, water will not move into the ECF from the ICF to compensate. Cells therefore remain healthy, but circulating blood volume drops ( and therefore blood pressure also).

Question 19

Explain how isosmotic fluid gain occurs and consequences of this.

Answer 19

Example is when a person is given too much intravenous fluid in hospital. Fluid is added to the ECF, but no change in osmolarity. Because of this, water will not move from ECF into the ICF. Cell therefore remains healthy, but there is an increase in blood volume and therefore increased blood pressure.

Question 20

Briefly, how come isosmotic fluid gains/losses not a problem?

Answer 20

Changes in the blood circulating volume is normally not an issue if kidneys are functioning, kidney is able to compensate for increases and decreases in ECF volume caused by isosmotic gains/losses via sodium homeostasis (explained in L39).