Lecture 33- body water distribution and regulation

Question 1

How much of the body is water?

Answer 1

• 55%= female

- 60%= male

Question 2

How much of the body water is ICF, how much is ECF?

Answer 2

• 1/3 ECF

- 2/3 ICF

Question 3

What is the ECF made of?

Answer 3

• 1/5 plasma

- 4/5 interstitial fluid (between cells)

Question 4

What is osmolarity?

Answer 4

Based on the number of osmotically active ions or

solutes

Question 5

What is the osmolarity of NaCl and how does this differ when spilt up into ions?

Answer 5

• 145 mM NaCl

- 145 mM Na+ + 145 mM Cl- = 290 mosmol/L

Question 6

What are the three classes of osmolarity?

Answer 6

• iso- (same osmolarity)
• hypo- (lower osmolarity)
• hyper-(higher osmolarity) solutions

Question 7

What is tonicity? What effect does an isotonic solution have?

Answer 7

-Based on the effect of a solution on cells
-An isotonic solution does NOT change water
homeostasis between cells

Question 8

Under normal conditions how does the osmolarity of the ICF versus the ECF compare?

Answer 8

• The same (275-295 mosmol/L)

- This means there is not usually any movement of water from ICF to ECF or vice versa

Question 9

Where do we get water from?

Answer 9

• Food
• Fluid
• Water formed by catabolism

Question 10

Where do we lose water from? Where do we lose the most?

Answer 10

• Lungs
• Skin diffusion
• Skin by sweat
• Kidneys (urine)
• Intestines (in faeces)

Lose the most from kidneys (1400mL)

Question 11

What is important about the input and output of water?

Answer 11

• Must be equal

- The amount lost from the kidneys is adjusted so that it is

Question 12

What is a case where we lose more water?

Answer 12

In extreme exercise

Question 13

What is the driver for water reabsorption?

Answer 13

Sodium

Question 14

Is there much active secretion of sodium?

Answer 14

No, instead just the stuff that is not reabsorbed exits through urine (not much)

Question 15

Where are the four important places in the nephron that sodium is reabsorbed?

Answer 15

• Proximal Convoluted Tubule (67%)
• Thick Ascending Limb of nephron loop (25%)
• Distal Convoluted Tubule (5%)
• Collecting Duct (3%)

Question 16

What are the three important places in the nephron that water is reabsorbed?

Answer 16

• Proximal Convoluted Tubule (67%)
• thin Descending Limb of nephron loop(25%)
• Collecting Duct (2 - 8%)

Question 17

Why is it important to tightly regulate water and sodium?

Answer 17

Maintain blood volume and therefore blood pressure

Question 18

How does the reabsorption of water occur in the proximal convoluted tubule (main place)?

Answer 18

-water reabsorption in the proximal tubule (67% of the
filtered load) is driven by Na+ reabsorption (isosmotic!!)
-transporters such as the sodium glucose-cotransporter
use the sodium gradient to reabsorb glucose

Question 19

How does the reabsorption of water occur in the nephron loop?

Answer 19

• The thin descending limb has leaky epithelium facilitating water reabsorption via aquaporins (transcellular) and the paracellular pathway. As impermeable to solutes, solute concentration increases.
• The Thick Ascending Limb then reabsorbs Na+ (and Cl-) into the interstitium generating a High Osmotic Medullary Gradient (HOMG). Impermeable to water therefore, solute concentration decreases.

Question 20

How does water reabsorption occur in the collecting duct?

Answer 20

Facultative water reabsorption:

• tight epithelia
• only transcellular
• regulated by ADH
• accounts for 2-8% of total water reabsorption

Question 21

What is obligatory water reabsorption, where does it occur?

Answer 21

• not regulated!!
• accounts for 92% of total water reabsorption
• occurs in proximal convoluted tubule+ start of nephron loop

Question 22

What effect does changing water content have?

Answer 22

• Changes osmolarity
• Fluid shifts between ECF - ICF to equalise and the volume of the compartments change
• In other words changing water content changes cell size and thus cell structures are altered and cell functions may become impaired

Question 23

What happens to a cell in a hypotonic solution?

Answer 23

Water enters cell causing swelling and eventually lysing

Question 24

What happens to a cell in a hypertonic solution?

Answer 24

Water leaves cell causing shriveling

Question 25

What is the hormone responsible for regulating body osmolarity?

Answer 25

Antidiuretic hormone (ADH)

Question 26

How does ADH hormone work to regulate body osmolarity?

Answer 26

-TBW changes alter plasma (ECF) osmolarity
-This is detected by osmoreceptors in hypothalamus (brain)
-This stimulates the pituitary gland to secrete more/less ADH
-ADH alters permeability of renal collecting duct (CD)
so water retained/excreted to balance initial change in TBW
-Plasma osmolarity stabilizes again and thus cell volume stabilizes

Question 27

In dehydrating what ultimately occurs as a result of increase in ADH?

Answer 27

ECF osmolarity increased therefore water shifts out of the ICF increasing ECF volume and decreasing sodium concentration

Question 28

In hyperhydration what ultimately occurs as a result of a decrease in ADH?

Answer 28

Water loss decreases the ECF volume the osmolarity increase and sodium ion concentration increases

Question 29

What does diuresis mean and what is the result?

Answer 29

Without ADH, large volume of dilute urine produced from the collecting duct.

Question 30

What does anti-diuresis mean and what is the result?

Answer 30

With ADH, small volume of concentrated urine produced from the collecting duct.

Question 31

What does ADH do in collecting duct?

Answer 31

• ADH in the bloodstream finds its receptor on basolateral side of CD cells
• ADH via intracellular signaling cascades increases the number of aquaporins in the apical membrane increasing water permeability of the
  apical membrane

Question 32

How is volume regulated in the juxtaglomerular complex?

Answer 32

The macula densa cells can sense changes in sodium reaching the distal tubule and therefore perfusion of the nephron; if it is too low this leads to a release of renin from juxtaglomerular cells;

Question 33

What hormone is responsible for regulating sodium reabsorption in the collecting duct?

Answer 33

Aldosterone