PGY300 renal lec 5 - REGULATION OF WATER BALANCE...

REGULATION OF WATER
BALANCE
PHYSIOLOGY 300
DR. JOHN A. KRASNEY

READING
•
Chapter 20, pages 642-651
•
Animations:
Urinary System, Early Filtrate Processing
pages 16-23, Late Filtrate Processing.
Fluid and Electrolytes, Water Homeostasis
pages 1-28
http://www.cellphys.ubc.ca/undergrad_files/30
Univ British Columbia

Reabsorption of Water
Water reabsorption is passive and follows
osmotic gradients.
80% of reabsorption of water and electrolytes
occurs in proximal tubule (Obligatory).
Remaining ~ 20% is reabsorbed in distal parts
of the nephron according to the needs of the
body (Facultative).

Facultative Reabsorption of
Water by the Kidney
•
Loop of Henle
•
Distal Tubule
•
Collecting Duct

Obligatory Excretion of Osmoles
•
In order to maintain ECF osmolarity the kidneys
must excrete 600 mOsm/day depending on solute
intake.
•
If urine were always isoosmotic to plasma (300
mOsm/L) then we would have to excrete 2 L of
urine per day.
•
The kidneys concentrate the urine so we only
excrete 1.5 L/day (450 mosm/L).
•
Uosm is higher than P osm if the kidneys are
concentrating the urine (Uosm/Posm).

How can the Kidney produce a urine with such a wide range
of osmolarities?

relative
medulla percentage maximal
species thickness long loops urine mOsm
beaver 1.3 0 495-770
pig 1.6 3 1100
human 3.0 14 1400-1480
cat 4.8 100 2100-3250
porpoise --- --- 1833
jerboa 9.3 33 >6000
desert mouse 10.7 100 5700-6400
Sea water = 1100-1150 mOsm vs Posm=300 mosm/l
Renal Concentrating Ability depends upon the Thickness
of the Medulla

•
Interaction between the
flow of filtrate through the
loop of Henle
(countercurrent multiplier)