Chapter 15: The Urinary
System
Fluid, Electrolyte, and Acid-Base
Balance
Body Fluids
• water accounts for
>50% of body weight
• important to stabilize
the volumes, solute
concentration, and pH
Homeostasis maintained
• fluid balance
• electrolyte balance
– Na+, K+, Ca2+
• acid/base balance
Key ideas regarding fluids and
electrolytes
• all homeostatic mechanisms monitor changes
in the ECF not ICF
• water is not moved by active transport
• gains/losses will occur if intake is different
from excretion
Fluid Balance
• electrolytes affect movement of water
– what are electrolytes?
– how does water move?
– what forces are responsible?
• body must not lose more water than it
takes in
What are some of the ways fluid is
exchanged?
• general water loss
– about 2500ml/day
• water gains
– eating, drinking,
metabolic generation
Kidneys regulate blood volume
• how?
• blood volume drops -> drop in arterial
pressure -> decreased amount of filtrate
– why?
Kidneys & Antidiuretic Hormone
• osmoreceptors are activated by change in
blood composition
• posterior pituitary stimulated to release
antidiuretic hormone (ADH)
• ADH causes collecting ducts of kidneys to
reabsorb more water
• result?
Kidneys regulate blood electrolyte
composition
• sodium ion (Na+)
– most responsible for osmotic H2O flows
• low blood sodium = dilute blood
– H2O flows out of blood into tissue spaces
• edema
Kidneys & Aldosterone
• normally, 80% filtrate Na+ reabsorbed in PCT
• with aldosterone, remaining Na+ reabsorbed
in DCT and collecting ducts
• water follows salt
– increased water absorption
Renin-angiotensin mechanism
• trigger for aldosterone release
• mediated by juxtaglomerular (JG)
apparatus
Renin-angiotensin
•
•
•
•
JG cells stimulated by low blood pressure
release renin enzyme
renin catalyzes production of angiotensin II
angiotensin II
– causes vasoconstriction
– promotes aldosterone release
• result?
Blood Acid-Base Balance
• blood pH: 7.35-7.45
• if pH rises = alkalosis
• if pH falls = acidosis
Different Acids in Body
• what are some acids in the body?
• how does the body keep the blood pH
from changing with all these acids?
Acids
• donate protons (H+)
• acidity represents how many free H+ are in
solution
Acids
• strong acids dissociate completely
– HCl -> H+ + Cl– cause large changes in pH
• weak acids dissociate partially
– H2CO3 -> H+ + HCO3- + H2CO3
– slight effect on pH
– can release more H+ when pH rises
Bases
• accept protons (H+)
• strong bases dissociate completely
– cause large changes in pH
• weak bases dissociate partially
– HCO3- , NH3
– slight effect on pH
– can tie up more H+ when pH drops
Buffers
• consist of a combination of a weak acid or
weak base and the salt of weak acid/weak
base
– weak acids can donate H+
– weak bases can absorb H+
Buffer Systems
• bicarbonate
• phosphate
• protein
• all work to maintain pH in the fluid
compartments of the body by resisting
changes in pH
Bicarbonate Buffer System
• carbonic acid (H2CO3)
• sodium bicarbonate (NaHCO3) salt
• add strong acid…what happens?
– bicarbonate ions tie up H+ released by strong acid
• add strong base…what happens?
– acid dissociates to free H+ ions to bind with OHreleased by strong base
Buffer Systems
• result is that pH of system changes very
little