Lecture 33: Body Water - Distribution and Regulation Flashcards
What three things is renal filtration defined by?
renal blood flow
filtration barrier
driving forces
What is the equation for renal clearance?
Cs = (Us x V)/Ps
What substance is used to estimate GFR?
creatinine
What is GFR?
a specific case of clearance
What is the clearance detecting and how did we get there?
We have a specific concentration of a substance in the plasma and a certain volume of urine per time, and we are detecting the concentration of the substance in urine - this makes up the volume of plasma that has been cleared
What is the filtration fraction and how can it be calculated?
it is how much blood in the kidney is actually filtered and it defines the ratio between perfusion and filtration in the kidney (GFR/RPF)
What is the filtered load and how can it be caculated?
the amount of substance filtered per time (GFR x Ps)
Why is water homeostasis so important?
Because it is needed to maintain blood pressure
What are the two main reasons for maintaining blood pressure?
we need to get nutrients to the tissues and to get CO2 out
What percentage of out total body weight does water take up?
55-60%
What percentage of our total body weight does ECF make up?
33%
What percentage of our total body weight does ICF make up?
66%
What does the ECF consist of and what percentage of the ECF do these two things make up?
- plasma (20% of the ECF)
- interstitial fluid (80% of the ECF)
What is meant by the ICF?
The fluid inside the cells
What is meant by the interstitial fluid?
the fluid in between the cells
Define osmolarity
the concentration of a solution expressed as the total number of solute particles per litre
What is osmolarity based on?
the number of osmotically active ions or solutes
How osmotically active is 145 mM NCl?
145 mM Na+ + 145 mM Cl- = 290 m osmol/L
How osmotically active is 145 mM glucose?
145 m osmol/L
How does osmolarity and spez gravity relate to each other? How do they differ?
Osmolarity measures the osmotic activity of ions or solutes whereas spez gravity measures the density of the solution. Spez gravity is just an estimate of osmolarity
Using NaCl and glucose as an example, define isosmotic
isosmotic means they have the same osmolarity
for example, 145 mM of NaCl has the same osmolarity as 290 mM glucose (290 m osmol/L)
Using NaCl and glucose as an example, define hyposmotic
hyposmotic means one has a lower osmolarity
for example, 145 mM of glucose has a lower osmolarity as 145 mM NaCl (145 m osmol/L vs 290 m osmol/L)
Using NaCl and glucose as an example, define hyperosmotic
hyperosmotic means one has a higher osmolarity
for example, 145 mM of NaCl has a higher osmolarity as 145 mM glucose (290 m osmol/L vs 145 m osmol/L)
Define tonicity
The measure of the osmotic pressure gradient between two solutions
What is tonicity based on?
the effect of a solution of cells
What does an isotonic solution not change?
is does not change the water homeostasis between the ECF and ICF
Despite the ECF and ICF having different concentrations of ions and different pH values, what is always kept in balance?
the osmolarity
What are the three main ways we intake water?
food
ingested fluid
water formed by catabolism
What is our main output of water?
urine from the kidneys
In order to reabsorb water, what first needs to be reabsorbed?
Na+
What are the four important places within the nephron where sodium is reabsorbed, in order?
PCT (67%)
TAL (25%)
DCT (5%)
CD (3%)
What percentage of Na+ is reabsorbed?
99%
What are the three important places within the nephron where water is reabsorbed, in order?
PCT (67%)
tDL (25%)
CD (2-8%)
Describe the reabsorption of water (and hence, Na+) in the proximal tubule
Na+ flows through the leaky cuboidal epithelium down its concentration gradient into the cells which decreases the local osmolarity in the tubular lumen and so h2o enters the cell. It does this via osmosis through aquaporins (transcellular) and via very leaky tight junctions (paracellular)
How is glucose related to the reabsorption of water?
transporters such as the sodium glucose transporter use the sodium gradient to reabsorb glucose
How does the absorption of water differ between different parts of the nephron loop?
The descending limb is permeable to water but impermeable to solutes. Solutes remain as water leaves, so the tubular fluid reaching the turn of the nephron loop has a higher osmotic concentration that it did at the start.
The ascending limb is impermeable to water and other solutes and selectively to Na+ and Cl-.
Water reabsorption in the nephron loop is the same as/higher than/lower than sodium reabsorption?
the same as
Changing water contents changes
osmolarity
How does the fluid shift due to changes in osmolarity?
fluid shifts between the ECF and ICF to equalise
Changing water content changes
cell size
Describe what happens to the flow of water in RBC when the cells are in a hypertonic environment
there is a higher concentration of solutes outside the cell than inside the cell so water flows out of the cell which means they gets smaller
Describe what happens to the flow of water in RBC when the cells are in a hypotonic environment
there is a higher concentration of solutes inside the cell than outside the cell so water flows into the cell which means they gets larger and can burst
Describe what happens to the flow of water in RBC when the cells are in an isotonic environment
there is an equal flow of water into and out of the cells
Describe the changes in body osmolarity when we are dehydrated?
There is a water loss from the ECF only so the ECF osmolarity increases (gets more concentrated with solute). Water moves out of the cell until the ICF and ECF are osmotically balanced but the cells will get smaller
Describe the changes in body osmolarity when we are hyperhydrated?
There is a water gained from the ECF only so the ECF osmolarity decreases (gets less concentrated with solute). Water moves into the cell until the ICF and ECF are osmotically balanced but the cells will get larger
What hormone regulates body osmolarity?
ADH
Describe the activation of ADH if ECF osmolarity changes
- changes in plasma (ECF) osmolarity are detected by osmoreceptors in the hypothalamus
- this stimulates the pituitary gland to secrete more/less ADH
- ADH alters permeability of the renal collecting duct
Does ADH increase or decrease permeability of the renal collecting duct? How would ADH secretion be different when we are hyperhydrated or dehydrated?
ADH increases the permeability of the collecting duct so ADH secretion would decrease if we were hyperhydrated by increase if we were dehydrated
Where is ADH made and where is it released?
ADH is made in the cell body of central neurons in the hypothalamus and is transported via axons to the posterior pituitary gland and it is released from here into the bloodstream
What are the two main stimuli for the release of ADH?
- increased ECF osmolarity
2. decreased blood volume
What is the action of ADH?
it inserts water channels (aquaporins) in the luminal membrane of CD to increase H2O reabsorption in the collecting duct
What are the two types of water absorption in the nephron?
obligatory and facultative
Where does obligatory water regulation take place? Is it regulated? It accounts for what percentage of the total water reabsorption?
- in the PCT and nephron loop
- no it is not regulated
- 92%
Where does facultative water regulation take place? Is it regulated? It accounts for what percentage of the total water reabsorption?
- in the DCT and collecting duct
- regulated by ADH
- accounts for up to 8%
Facultative water reabsorption occurs via ______ epithelia and only occurs ____cellularly
tight
trans
Describe the water reabsorption in the CD without ADH
Water is not reabsorbed in the DCT or the CD so all the fluid reaching the DCT is lost in the urine. No facultative water reabsorption occurs, and the person then produces large amount of very dilute urine
What is diuresis? Does this happen with or without ADH?
- excessive production of urine
- without ADH
What is anti-diuresis? Does this happen with or without ADH?
- reduced production of urine
- with ADH
Describe the water reabsorption in the CD with ADH
ADH finds its receptor o the basolateral side of the CD cells. This, via intracellular signalling cascades, causes aquaporins to appear in the apical surfaces of the plasma membranes lining the DCT and CD. This increases the absorption of water and the result is very concentrated urine
What are aquaporins?
water channels
