Lecture 33- body water distribution and regulation Flashcards
How much of the body is water?
- 55%= female
- 60%= male
How much of the body water is ICF, how much is ECF?
- 1/3 ECF
- 2/3 ICF
What is the ECF made of?
- 1/5 plasma
- 4/5 interstitial fluid (between cells)
What is osmolarity?
Based on the number of osmotically active ions or
solutes
What is the osmolarity of NaCl and how does this differ when spilt up into ions?
- 145 mM NaCl
- 145 mM Na+ + 145 mM Cl- = 290 mosmol/L
What are the three classes of osmolarity?
- iso- (same osmolarity)
- hypo- (lower osmolarity)
- hyper-(higher osmolarity) solutions
What is tonicity? What effect does an isotonic solution have?
-Based on the effect of a solution on cells
-An isotonic solution does NOT change water
homeostasis between cells
Under normal conditions how does the osmolarity of the ICF versus the ECF compare?
- The same (275-295 mosmol/L)
- This means there is not usually any movement of water from ICF to ECF or vice versa
Where do we get water from?
- Food
- Fluid
- Water formed by catabolism
Where do we lose water from? Where do we lose the most?
- Lungs
- Skin diffusion
- Skin by sweat
- Kidneys (urine)
- Intestines (in faeces)
Lose the most from kidneys (1400mL)
What is important about the input and output of water?
- Must be equal
- The amount lost from the kidneys is adjusted so that it is
What is a case where we lose more water?
In extreme exercise
What is the driver for water reabsorption?
Sodium
Is there much active secretion of sodium?
No, instead just the stuff that is not reabsorbed exits through urine (not much)
Where are the four important places in the nephron that sodium is reabsorbed?
- Proximal Convoluted Tubule (67%)
- Thick Ascending Limb of nephron loop (25%)
- Distal Convoluted Tubule (5%)
- Collecting Duct (3%)
What are the three important places in the nephron that water is reabsorbed?
- Proximal Convoluted Tubule (67%)
- thin Descending Limb of nephron loop(25%)
- Collecting Duct (2 - 8%)
Why is it important to tightly regulate water and sodium?
Maintain blood volume and therefore blood pressure
How does the reabsorption of water occur in the proximal convoluted tubule (main place)?
-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
How does the reabsorption of water occur in the nephron loop?
- 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.
How does water reabsorption occur in the collecting duct?
Facultative water reabsorption:
- tight epithelia
- only transcellular
- regulated by ADH
- accounts for 2-8% of total water reabsorption
What is obligatory water reabsorption, where does it occur?
- not regulated!!
- accounts for 92% of total water reabsorption
- occurs in proximal convoluted tubule+ start of nephron loop
What effect does changing water content have?
- 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
What happens to a cell in a hypotonic solution?
Water enters cell causing swelling and eventually lysing
What happens to a cell in a hypertonic solution?
Water leaves cell causing shriveling
What is the hormone responsible for regulating body osmolarity?
Antidiuretic hormone (ADH)
How does ADH hormone work to regulate body osmolarity?
-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
In dehydrating what ultimately occurs as a result of increase in ADH?
ECF osmolarity increased therefore water shifts out of the ICF increasing ECF volume and decreasing sodium concentration
In hyperhydration what ultimately occurs as a result of a decrease in ADH?
Water loss decreases the ECF volume the osmolarity increase and sodium ion concentration increases
What does diuresis mean and what is the result?
Without ADH, large volume of dilute urine produced from the collecting duct.
What does anti-diuresis mean and what is the result?
With ADH, small volume of concentrated urine produced from the collecting duct.
What does ADH do in collecting duct?
- 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
How is volume regulated in the juxtaglomerular complex?
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;
What hormone is responsible for regulating sodium reabsorption in the collecting duct?
Aldosterone
