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LSS Urinary System

Alexandra Burke-Smith

Water intake  Water load  a decrease in plasma osmolarity  sensed by hypothalamic osmoreceptors which decrease ADH release  decrease in water permeability of collecting duct  increase in urine flow rate + volume (increased fluid loss will tend to raise plasma osmolarity – negative feedback) o Basically sodium reabsorption occurs without water reabsorption  low urine osmolarity (50mosmol/l) + diuresis o Urea is also not recycled as much, therefore the concentration gradient generated in the loop of Henle is smaller which reduces water reabsorption  Dehydration  decreased plasma osmolarity  sensed by hypothalamic osmoreceptors  thirst + ADH release  increased collecting duct water permeability  decreased urine flow rate o Increased urea recycling  larger gradient in loop of Henle  more water reabsorption NB: Feedback Control via ADH keeps plasma osmolarity in a normal range – also determines urine output + water balance Diabetes insipidus 

Disorder of water balance, caused by: o No/insufficient ADH production o Mutant ADH receptor  no detection o Mutant aquaporin  no response to ADH binding Results in polyuria (>30l/day) + polydipsia (unremitting thirst)

Control of Sodium + Potassium excretion Urinary System 6 – Dr Paul Kemp (

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As discussed in the previous lecture, water balance is used to regulate plasma osmolarity. It can then be said that the level of salt determines the ECF volume (plasma makes up part of the ECF volume). If we know the number of mosmoles of salt in the ECF, as well its concentration; we can determine the volume of ECF. As sodium is the most prevalent solute in the ECF, the knock on effect is that it is most important in regulating ECF volume. Humans are closed systems, therefore changes in the volume of the ECF in turn affects the blood volume + pressure Increased dietary sodium  increased osmolarity  increased ECF volume (water moves into an area of higher osmolarity)  increased blood volume + pressure o water retention also leads to increase in body weight Decreased dietary sodium  decreased osmolarity  decreased ECF volume (Water moves into an area of high osmolarity)  decreased blood volume + pressure o water loss also leads to decrease in body weight In reality, the body can’t let the ECF osmolarity change outside of the regulated range of 285-296mosmol/L, therefore sodium levels must be regulated. This is done by balancing the sodium excretion according to out dietary intake

Control of sodium excretion   

The bulk of sodium is reabsorbed in the PCT (65%) through co-transport mechanisms which also drive the reabsorption of bicarbonate, glucose and amino acids. 25% more is absorbed in the loop of Henle, 8% in the DCT Reabsorption in the collecting duct is variable, but up to 2% 19

Alex's Urinary  

Alex's Urinary