Body Fluid Compartments and Water Balance

Question 1

for us to be healthy, what does water balance need to be like?

Answer 1

needs to be equal / at 0.

Question 2

how is water distributed in the body?

Answer 2

60% of BW (of men) is fluid -> for a 70kg male = 42L

• intracellular fluid: 40% - 28L
• extracellular fluid: 20% - 14L
  a) plasma: 3L
  b) interstitial fluid: 10.5L (fluid around the cells - extension of plasma fluid)
  c) other ECF - e.g. lymph, eye humor, synovial fluid etc

Question 3

(how can you think of relationship between plasma and interstitial fluid?) / what are similar or differences between them?

difference between ECF and ICF?

Answer 3

(extension of plasma, just located around cells)

*important to remember*

-Plasma and interstitial fluid: same electrolyte components BUT plasma has proteins - e.g. albumin and anions c.f. interstitial fluid

Intracellular fluid c.f. extracellular fluid: ICF has very little Na+. Lots of K+ and PO4^3- - (buffers acid / base situation), protein anions (think - sea with Na+ is ECF, banana tree: ICF). ECF has high Na+ and Cl-

Question 4

what are the main solutes need to think about in human physiology?

Answer 4

Na+, Cl-, glucose and urea

Question 5

how can we classify solutes?

which one of these ^ has greater osmotic power?

what is osmotic pressure?

Answer 5

electrolytes:

• inorganic salts (Na+, K+, Cl-), all acids and bases, some proteins

non electrolytes:

• glucose, urea, lipids etc

electrolytes have greater osmotic power than non-electrolytes

osmotic pressure: process that controls movement of solvents across a membrane. movement occurs when there are differences in osmotic pressure across a membrane.

Question 7

what is osmolarity and osmolality?

which is preffered?

Answer 7

osmolarity: the measure of solute conc expressed as no. of osmoles (Osm) of solute per litre of solution - mOsm / L

osmolality: the measure of osmoles (Osm) of soluter per kg of solvent - Osm / kg.

OSMOLALITY IS PREFFERED - (because is per unit mass, so not affected by changes in temp / pressure)

Question 8

what is the osmolality of plasma?

Answer 8

normal plasma osmolality: 290-300 mOsm/kg

(dont need to memorise this - will probs be given)

Question 9

what happens to cells in an hypertonic medium / hypotonic medium?

Answer 9

hypertonic medium: more solutes outside cell -> water moves out of cell. cell shrink

Hypotonic medium: more solutes in cell than outside -> water moves inside to the cell. swell

Question 10

what are sources of intracellular osmolality?

why does this cause a problem? whats the solution?

Answer 10

sources of intracellular osmolality:

1. charged molecules and their counter ions
2. smaller metabolites and their counter ions
3. small inorganic ions -> cause Donnan effect

Problem: all of the above attract counter ions, which could create a constant flow of water to counteract ion imbalance into the cell. cause cell repture?

solution: animal and bacteria cells actively pump out inorganic ions (esp K), so that cytoplasm contains a lower total conc of inorganic ions than the ECF.

Question 11

what are different types of membrane transport can use?

Answer 11

1. simple diffusion
2. facilitated diffusion

( both 1&2 make use of gradient)

3. primary active transport - needs energy

Question 12

what facilitates the movement of water from one side of cell membrane to another?

Answer 12

Aquaporins

Question 13

explain how sodium potassium exchanger works

Answer 13

Primary active transport:

• Na+ / K- pump must break ATP -> ADP to pump 3 Na ions outside cell
• 2 K ions are then transported into cell

Both transported agaisnt a gradient

Question 15

where is Na+ mostly found?

why is Na+ important?

Answer 15

- in extracellular fluid / outside the cell (Na+ is the predominant extracellular cation)

- plays a key role in fluid and electrolyte balance: Na+ has strong osmotic power, where Na+ goes, water follows.

-

Question 16

• how do you describe the relationship between plasma and ECF sodium? what does this mean?
• what happens if plasma volume and osmolality are regulated?

Answer 16

- Plasma and ECF sodium are in equilibrium

• If plasma volume is regulated - so will ECF volume
• If plasma volume and osmolality are regulated -> then both total water and total sodium will be regulated

Question 17

explain how total body water is regulated?

Answer 17

- water input into body from diet and metabolic water = BODY WATER

• Body water + solutes = BODY FLUID OSMOLALITY
• Body fluid osmolality is monitored by osmoreceptors in CNS: informs body to drink
• Hormones (ADH) inform rest of body how to modulate body fluid osmolality
• Kidneys regulate water content through urination.

-

Question 18

which regions in the brain control thirst?

how are these structures characterised? - what does this allow?

Answer 18

- Subfornical organ: in the hypothalumus

• (Organum Vasculosum of the lamina terminalis (OVLT) - dont need to know
• characterised by: a) extensive vasculature b) lack of normal blood brain barrier

- allow for linkage between CNS and peripheral blood flow

Question 19

explain thirst mechanism & CNS

Answer 19

1. increase in plasma osmolality: (less water in plasma. solutes are concetrated)

a) decrease in saliva = dry mouth -> feedback into hypothalamus thirst center
b) osmoreceptors in hypothalamus -> feedback into hypothalamus thirst center

@ the hypothalamus thirst centre:

a) sensation of thirst -> decide to drink -> water moistens mouth / throat -> water absorbed in GI tract = decrease in plasma osmolality

2. Plasma volume decreases (less efficient feedback)

a) causes blood pressure to decrease -> feedback into hypothalamus thirst center. also triggers the renin-angiotensin mechanism.

Question 20

1. where are the osmoreceptors found in brain?
2. what is another stimuli that triggers sensation to drink?

Answer 20

1. osmoreceptors: in hypothalamus
2. baroreceptors: detect when there is decreased blood volume (in _great veins, right atrium of hear_t -> relied to vasomoto center) -> relayed to hypothalamus

Question 21

what is the main stimulus for the thirst sensation?

Answer 21

increase in plasma osmolality

Question 22

what secretes ADH? when is it released?

what does ADH do (basic)

Answer 22

released from: posterior pituitary

released when: increased plasma osmolality (osmoreceptors in hypo), decreased plasma volume (baroreceptors in great veins and right atrium)

function: ADH makes cells of collecting duct and distal tubule permeable to water - high ADH = concentrated water.

Question 23

what are factors what can trigger ADH?

Answer 23

all processes that lose water: prolonged fever, excessive sweating, vom, diarrhea, blood loss

Question 24

what is cellular mechanism of ADH making less concentrated?

Answer 24

• ADH binds to receptor on cell
• changes transcriptional activity of nucleus
• more aquaporin made & transported to membrane
• more water goes into cell via aquaporin from filtrate of urine
• water goes from cell into blood

Question 25

describe mechanism of aldosterone working

(where made? released from? what does it cause to do? result?)

Answer 25

aldosterone:

released from: adrenal cortex

released when: reduced Na+ or increased K+

function: increases reabsorbtion of Na or increased K secretion
result: homeostatic plasma levels of Na+ and K+.

(also if u reabsorb Na+ (water follows Na) = helps reabsorb water)

Question 26

how does aldosterone make kidney cells recapture sodium?

Answer 26

• aldosterone binds to kidney cells
• causes to produce more

a) ENAC (Na channel) - can reabsorb more Na

b) Na/K pump ( pumps Na in / K out)

-

Question 27

how does atrial natruiretic peptide work? released from? releaed when?

Answer 27

Atrial natriuretic peptide (ANP)

released from: cardiac Atria

released when: increased blood volume (stretches atria)

function: promotes excretion of Na+ and Cl-, in turn decreases water reabsorbtion (brings blood volume back towards normal)(acts on hypothalamus and adrenal cortex up the mechanism.)

Question 28

what horomone does same role as aldoesterone (i think)

Answer 28

angiotension II - promotes urinary reabsorbtion of Na and Cl (and water) when dehy.

(It causes increases in blood pressure, influences renal tubuli to retain sodium and water, and stimulates aldosterone release from adrenal gland.)

Question 29

ADH aka?

Answer 29

vasopressin

Question 30

what happens at glomerulus?

where is water reabsorbed in kidney nephron - which bit is dependent on individual water balance?

Answer 30

at glomerulus: filtrates everthing in blood apart from proteins (bold move! - needs to be corrected)

at proximal tubule: 80% water reabsorbed. active reabsortion of solutes

at distal part of tubule: variable reabsorbtion of the remaining 20% water. How much reabsorbtion is determined by aldosterone and ADH

Question 31

how does variable reabsoprtion occur in the nephron?

Answer 31

depends on relative presence of:

-aquaporins

• ENAC
• Na/K pumps

in kidneys

(activated by ADH and aldoesterone)

Question 32

what are typical values for the osmolality of urine and dailiy urine production?

Answer 32

healthy individuals:

random urine osmolality: 50 to 1400 mOsm/Kg

24 hour urine osmolality: 500 to 800 mOsm / kg

water deprivation test (restrict 12-14 hrs of fluid): should be greater than 800mOsm/kg