Water Homeostasis

Question 1

What is the total body water (in a 70kg male)?

What % of this is the body weight?

Answer 1

42L

60% of body weight

Question 2

What is the total body water (42L) split into?

Answer 2

Intracellular fluid - 28L (40% of body weight)

Extracellular fluid - 14L (20% of body weight)

Question 3

What is the predominant electrolyte in intracellular fluid?

Answer 3

K+

Question 4

What are the predominant electrolytes in extracellular fluid?

Answer 4

Na+
Chloride
Bicarbonate

Ca2+ especially in heart & muscle

Question 5

What is the extracellular fluid split into and measurements?

Answer 5

Interstitial fluid (surrounds cells) - 11L
Plasma (in blood vessels) - 3L

Question 6

What is sensible & insensible loss & examples?

Answer 6

Sensible = measurable - urine, vomit
Insensible = immeasurable - sweat, breath

Question 7

What is the main regulator of water loss?

Answer 7

Kidneys

Question 8

Where does water intake come from?

Answer 8

Drink, diet and IV fluid

Question 9

What’s the regulating hormones for water homeostasis?

Answer 9

ADH, aldosterone and Atrial natriuretic peptide

Question 10

What is osmolality?

Answer 10

Measure of the number of dissolved particles by kg of fluid

Conc/KG solution

Question 11

What is osmolarity?

Answer 11

Measure of the number of dissolved particles per L of fluid

Conc/Litre solution

Question 12

What is osmotic pressure?

Answer 12

Pressure applied to a solution, by a pure solvent, required to prevent inward osmosis through a semipermeable membrane

Question 13

*What is oncotic pressure?

Answer 13

Albumin pressure on Capillery wall keeping fluid in

Question 14

• what is hydrostatic pressure?

Answer 14

Pressure difference between Capillary blood (plasma) and interstitial fluid –water & solutes move from plasma into interstitial fluid

Question 15

What happens when water is lost from extracellular fluid?

Answer 15

Increase in solutes / decrease in water
Increase in osmolality in ECF
Change detected by osmoreceptors in the hypothalamus
Results in ADH release from posterior pituitary gland
ADH acts to increase water reabsorbtion in the collecting ducts of the kidney in order to dilute the solute and return water in ECF to normal

Question 16

What happens when there is decreased renal blood flow?

Answer 16

Decreased water in ECF=decrease in effective circulating volume=decrease in renal blood flow
Renin release from juxtaglomerular cells in kidneys
Raas system- renin converts angiotensinogen to angiotensin I
Angiotensin converting enzyme (ACE) then converts angiotensin I into angiotensin II
Triggers the release of aldosterone from the adrenal cortex above the kidneys
Aldosterone increases Na+ reabsorption in the kidneys (water also reabsorbed) in exchange for potassium or hydrogen excretion
stimulates ADH release
Sodium reabsorption brings water with it to return water in ECF to normal

Question 17

What is water controlled by?

Answer 17

Changing osmolality (sodium)

Question 18

What does RAAS stand for and what does it do?

Answer 18

Renin angiotensin aldosterone system

Maintain BP

Question 19

When is the RAAS system activated and by what cells?

Answer 19

Decreased BP (juxtaglomerular cells in afferent arteriole-kidney)
Decreased NaCl (macula densa of DCT)

Question 20

What happens in the RAAS system?

Answer 20

Renin (from the juxtaglomerular cells in kidneys) turns angiotensinogen (from liver) to angiotensin I

ACE (angiotensin converting enzyme) from the lungs turns angiotensin I to angiotensin II

This stimulates the release of ADH & Aldosterone

Stimulates sympathetic NS too

Question 21

What does ADH do?

Answer 21

Secreted by posterior pituitary gland
Acts on APII proteins on collecting ducts
Increase collecting duct permeability - reabsorbs more water
Increase extracellular fluid= increased BP

Question 22

What does aldosterone do?

Answer 22

Secreted by adrenal cortex (suprarenal cortex)
Increased Na+ reabsorbtion in ascending limb of Loop of Henle - excretes K+ in exchange

H2O follows Na+ (obligatory H2O movements)

Increase BP as ECF increases

Question 23

What can excess water cause?

Answer 23

 oedema

Question 24

What causes dehydration?

Answer 24

Water deprivation, vomiting, diarrhoea, burns, heavy sweating, diabetes insipidus (urinate a lot since too little ADH) diabetes mellitus, drugs

Question 25

What are the consequences of dehydration?

Answer 25

Thirst, dry mouth, in elastic skin, sunken eyes, raised haematocrit (viscosity of blood), weight loss, confusion, hypotension

Question 26

What are the causes and consequences of water excess?

Answer 26

High intake/decreased loss of water, excess ADH

Hyponatraemia (low Na levels), cerebral over perfusion (due to high blood vol and pressure)- causes headaches, confusion & convulsions

Question 27

What is serous effusion?

Answer 27

Excess water in a body cavity

Question 28

What is oedema?

Answer 28

Excess water in the intercellular tissue space

Question 29

What is high & low sodium called?

Answer 29

Hypernatraemia

Hyponatraemia

Question 30

What is high & low potassium called?

Answer 30

Hyperkalaemia

Hypokalaemia

Question 31

What is high & low calcium called?

Answer 31

Hypercalcaemia

Hypocalcaemia