Lecture 33: Body water - distribution and regulation Flashcards
What % of our body weight is water?
55-60% of our body weight
- Males 60%
- Females 55%
What proportion of our TBW is ECF?
1/3
What proportion of our TBW is ICF?
2/3
What proportion of ECF is plasma?
1/5
What proportion of ECF is interstitial fluid?
4/5
What is osmolarity based on?
The number of osmotically active ions or solutes
What can osmolarity be estimated by?
Specific gravity
What does iso-osmotic mean?
Same osmolarity
What does hypo-osmotic mean?
Lower osmolarity
What does hyper-osmotic mean?
Higher osmolarity
What is tonicity based on?
The effect of a solution on cells
What does an isotonic solution not do?
Change water homeostasis between cells
What is the sodium concentration in the ECF?
145 mmol/L
What is the potassium concentration in the ECF?
4-5 mmol/L
What is the sodium concentration in the ICF?
15 mmol/L
What is the potassium concentration in the ICF?
150 mmol/L
What is particular about the osmolarity between the the ICF/ECF
They are the same/similar
Total body water remains relatively what?
Constant
Intake and loss of water must be in what?
Balance
Urine output is adjusted to do what?
Maintain balance
What is water output most effected by?
Kidneys producing urine
What are the 4 important places within the nephron where sodium is reabsorbed?
- Proximal convoluted tubule(PCT)
- Thick ascending loop (TAL)
- Distal convoluted tubule (DCT)
- Collecting duct (CD)
What % of sodium is reabsorbed in the PCT?
67%
What % of sodium is reabsorbed in the TAL?
25%
What % of sodium is reabsorbed in the DCT?
5%
What % of sodium is reabsorbed in the CD?
3%
What are the 3 important places within the nephron water is reabsorbed?
- PCT
- Thin descending loop (tDL)
- CD
What % of water is reabsorbed in the PCT?
67%
What % of water is reabsorbed in the tDL?
25%
What % of water is reabsorbed in the CD?
2-8%
What % of water is excreted as urinary secretion?
0.5 - 8% of the filtered load
What is needed to make you own Powerade?
4
- Water
- Sugar
- Sodium
- Flavour/vitamins
What is water reabsorption in the proximal tubule driven by?
Sodium reabsorption
What do transporters such as the sodium glucose-cotransporter use to reabsorb glucose?
The sodium gradient
The thick ascending loop reabsorbs sodium into the interstitial generating what?
A high osmotic medullary gradient (HOMG)
`The thin descending loop is leaky epithelium facilitating what?
Water reabsorption
Water reabsorption in the thin descending loop can occur how?
(2)
- Via aquaporins (transcellular)
- Via the paracellular pathway
Obligatory water reabsorption is not what?
Regulated
Obligatory water reabsorption account for what % of total water reabsorption
92% of total water reabsorption
Facultative water reabsorption in the CD has what type of epithelium?
Tight epithelia
What 2 types of water reabsorption can occur in the CD?
- Obligatory water reabsorption
- Facultative water reabsorption
Facultative water reabsorption uses only what type of pathway?
Transcellular
What is regulated reabsorption in the CD regulated by?
ADH
Facultative water reabsorption account for what % of total water reabsorption?
2-8% of total water reabsorption
Changing water contents does what?
Changes osmolarity
What does fluid shift between to equalise osmolarity?
ECF - ICF
What does changing water contents do to a cell?
Changes the size of the cell
What does a change in the size of cell do?
2
- Cell structure altered
- Cell functions become impaired
What does a hypertonic solution do to red blood cells?
Causes water to move out of the cell and it to shrivel
What does an isotonic solution do to red blood cells?
Causes water to move in/out at a constant rate
What does a hypotonic solution do to red blood cells?
Causes water to move into the cell and it to swell
What are red blood cells dependent on?
An isotonic environment
What do hypertonic/hypotonic environments to to red blood cells?
Alter their function
What does TBW changes alter?
Plasma (ECF) osmolarity
What are changes to osmolarity of ECF detected by?
Osmoreceptors in the hypothalamus (Brain)
When changes to osmolarity of ECF are detected what happens?
It stimulates the pituitary gland to secrete more/less ADH
What does ADH alter the permeability of?
The renal collecting duct
When the permeability of the renal collecting duct is altered by ADH what happens?
Water is retained or excreted to balance initial change in TBW
What happens to water re-absorption and urine production without ADH?
Diuresis - Large volume of dilute urine
What happens to water re-absorption and urine production with ADH?
Anti-diuresis - small volume of concentrated urine
ADH in the blood stream finds what?
Its receptor on the basolateral side of CD cells
What does ADH via intracellular signalling cascades increase the number of?
Aquaporins in the apical membrane of CD
What does an increase in aquaporins in the apical membrane of CD do?
Increases water permeability of the apical membrane
What can macula dense cells sense?
Changes in sodium reaching the distal tubule and therefore perfusion of the nephron
If sodium levels reaching the distal tubule is too low what happens?
It leads to a release of renin from juxtaglomerular cells
What is a decrease in ECF volume by fluid loss detected by?
Baroreceptors/kidney
What does a decrease in ECF volume detected by the kidneys causes the kidneys to do?
(2)
- Renin release leading to angiotensin II activation
- Increased aldosterone
What does renin release leading to angiotensin II activation and increased aldosterone lead to?
(2)
- Decrease sodium loss in urine
- Decreased water loss in urine
What is an increase in ECF volume or fluid gain detected by?
Cardiac cells
What is an increase in ECF volume detected by cardiac cells to do?
Cause the release of natriuretic peptides
What do natriuretic peptides have an effect on?
3
- Hypothalamus
- Kidneys
- Blood vessels
What does the hypothalamus, kidneys and blood vessels cause when activated by natriuretic peptides?
(2)
- Increase in sodium loss in urine
- Increased water loss in urine
