Lecture 4 - Why the kidney works backwards Flashcards
What is total body water in an average male?
60% of body weight (70kg man) => ~42L
What is total body water in an average female?
50% of body weight (60kg woman) => ~30L
Why do females have less total body water?
Females have more adipose tissue than males, which have a low affinity for water
How much water is lost per day by just breathing?
0.5L/day
How much salt is lost in sweat?
Not much
30 mmol/L
Approx how much water is gained by food and drink per day?
2.2L/day
Approx how much water is produced by metabolism per day?
0.3L/day
Approx how much water is lost insensibly via skin and lungs per day?
0.9L/day
approx how much water is lost in urine per day?
1.5L/day
approx how much water is lost in the faeces per day?
0.1L/day
how much water do we turn over in a gentle environment?
2.5/30L which is ~5-8% of water ever day
we turn over more in hot/dry environments
How much is the ECF volume regulated to?
~15% or more
How much water and salt do we typically consume more than we need per day?
20%
Why doesnt the bladder -> tube concept of the kidney work?
There are no water or urea pumps, so those substances can’t enter the kidney
How do we get past without water and urea pumps?
- separate out large volume of ECF from plasma
- reabsorb ~99% of this back into the plasma.
Water, salts, urea which we dont want to keep is left behind - Achieving the ideal kidney but backwards
What is the aim of filtration?
To make a large volume of protein-free plasma (aka standard, ordinary extracellular fluid)
What is the normal GFR rate?
125ml per min per 1.73m^2
What is the average GFR rate for a male? for a female?
male = 7.5L/h => 180L/day female = 6L/H = 144L/day
What is the osmolarity of ECF?
~300mosmol/L
more like 290mosmol/L though!
What happens to the fluid after filtration?
- it passes through a tubule that is not very permeable to urea
- salt and water are reabsorbed which leaves behind most of the urea and a small amount of remaining water
What happens in the reabsorption step?
- salt is actively pumped out of the tubule using ATP as fuel
- Water follows salts, amino acids and glucose
- Fluid which is removed from the tube is approx = to ECF concentration (~300mosmol/L )
How much of the fluid has not been reabsorbed so far?
~20%
what happens if the concentration of glucose is over the transport maximum?
the renal losses of glucose are linear
What is the transport maximum of glucose?
10-11 and 16.6mmol/L plasma glucose.
some glucose is lost at 10+ mmol/L
When does glucose loss become linear?
at 16+mmol/L
What are the two mechanisms of sodium reabsorption
- sodium reabsorption primarily by cotransport: with bicarbonate, glucose, amino acids
this occurs in the early segment - Sodium reabsorbed mainly with chloride once other substances have been reabsorbed
this occurs in the second segment
What are the concentrations along the proximal tubule?
the amount of Na decreases, but there is no change in the concentration of Na
HCO3, glucose, amino acids also decrease
Cl increases along the tube
Water follows salt by paracellular osmosis so the osmolarity is stable and equivalent to ECF concentration
Describe the composition of the end of the proximal tubule
- Tubular fluid has decreased from 6L/h to 1.2L/h
- approximately 80% of GFR is reabsorbed
- there is no change in Sodium concentration
- There are no glucose or amino acids here
- The fluid is iso osmotic as the missing osmoles are made up by urea
- Urea concentration is ~2.4x higher than it was in the original glomerular filtrate and ECF
