Maintaining
Water-Salt/Acid-Base
Balances
and
The Effects of Hormones
Urinary System
Control of Blood Composition


Kidneys filter 150-180 liters of blood
plasma AND only produce 1-1.8 liters of
urine in 24 hours
Composition depends on 3 factors
1.
2.
3.
Diet
Metabolism
Urine output
Blood Composition

Kidneys play an important role in
keeping blood composition constant
1.
2.
3.
Excretion of Nitrogen-containing wastes like
urea, uric acid, and creatinine
Maintain water and electrolyte balance
Ensure blood pH 7.35-7.45
Maintain water and electrolyte
balance


Water accounts for 60% of body weight in
females and 50% in males
Water locations in body:
1.
2.
25 Liters in Intracellular Fluid – contained in living
cells
15 Liters in Extracellular Fluid – all body fluids
outside of cells
a.
b.
12 Liters in Interstitial Fluid: Tissue Fluid, Cerebrospinal
Fluid, Serous Fluid, humors of eye, lymph
3 Liters in Blood Plasma
Maintain water and electrolyte
balance

Intake and Output of
body water must be
in balance for body to
remain hydrated.
Maintain water and electrolyte
balance

Type and amount of solutes (sodium,
potassium, and calcium) in body fluids are
vital to overall body homeostasis
Reabsorption of water and
electrolytes regulated by hormones

Falling blood pressure
from blood loss,
excess sweat, or low
levels of sodium in
blood stimulate:
KIDNEYS REGULATE
THE VOLUME OF BLOOD
This is done by two HORMONES:
1.
2.
ADH
ALDOSTERONE
ADH
(ANTIDIURETIC HORMONE)



anti-"increased urine output“
released by pituitary gland
promotes reabsorption of water from
collecting duct and distal convoluted tubule
How does ADH work?


Cells in hypothalamus detect low H2O
content of blood
ADH released into blood, by the
hypothalamus and acts on distal
convoluted tubule and collecting duct
How does ADH work?



More H2O reabsorbed, thus decreasing
the volume of urine (and concentrating it)
So, blood volume increases
As blood becomes more dilute, this is
detected by the hypothalamus, ADH
secretion stop


a negative feedback loop
In some cases of high blood pressure,
DIURETIC DRUGS are prescribed which
inhibit ADH secretion




ALCOHOL also inhibits ADH secretion
Drinking alcohol therefore causes
increased urination ---> dehydration --->
HANGOVER
Beer and alcohol cannot quench your
thirst! You will urinate more liquid than
you take in and the body becomes even
more dehydrated than before.
The content of water in your body is
normally about 50%.
ALDOSTERONE



This is a hormone released by the
ADRENAL GLANDS
Aldosterone acts on the kidney to RETAIN
Na+ (& water) and EXCRETE K+.
Remember from last class

[Na+] in the blood is important to the
kidneys’ ability to reabsorb H2O
ALDOSTERONE

Concentration of sodium in blood, in turn,
regulates secretion of aldosterone
 another


negative feedback loop
If Na+ in blood too low, too little H2O is
reabsorbed, results in HYPOTENSION
If Na+ in blood too high, results in
HYPERTENSION
Fall blood pressure stimulates:

Hypothalamus, which stimulates posterior
pituitary to release ADH (antidiuretic
hormone)

ADH causes less water loss in urine SO blood
volume increases and blood pressure rises
Fall blood pressure stimulates:

Adrenal Cortex which releases Aldosterone



Aldosterone stimulates kidney tubules to
reabsorb more sodium ions (salt)
WATER FOLLOWS SALT
So if water follows salt, less water will be lost in
urine, blood volume increases and blood
pressure rises
Fall blood pressure stimulates:

Kidney to release Renin


Renin leads to the formation of angiotensin II
in the blood
Angiotensin II causes smooth muscle of blood
vessels to constrict which increase blood
pressure
KIDNEYS AND BLOOD pH

Kidneys help maintain blood pH. However,
so do the lungs (respiratory system).



You may recall that CO2 is released from the
lungs when you exhale.
H+ + HCO3- ↔ H2CO3 ↔ H2O + CO2
Based on the acidity of the blood,
chemoreceptors either stimulate or depress
breathing.

If, for example, your blood is slightly alkaline, will
your depth of breathing rate increase or
decrease?
ANSWER: IT WILL DECREASE.
KIDNEYS AND BLOOD pH


The kidneys, too, rid the body of acidic
and basic wastes.
Nephrons vary the amount of H+ and NH3
that they excrete and the amount of
HCO3- and Na+ they reabsorb in order to
keep the pH levels normal.
KIDNEYS AND BLOOD pH

If blood acidic, more H+ and ammonia
(NH3) are excreted, and more sodium
bicarbonate is reabsorbed.
Sodium bicarbonate neutralizes acid:
 Na+HCO3- + HOH --> H2CO3 + NaOH (strong base)

KIDNEYS AND BLOOD pH
 If
blood alkaline, less H+
excreted, less Na+ and HCO3reabsorbed
 Reabsorption and excretion of
ions (e.g. K+, Mg++) by kidneys
also maintains proper
ELECTROLYTE BALANCE of blood.
Ensuring proper blood pH




Blood pH must be
7.35-7.45
Alkalosis: blood pH
above 7.45
Acidosis: blood pH
below 7.35
Several pH controlling
systems
pH control system

Blood Buffer


First line of defense in resisting pH changes
Chemical buffers that act to prevent dramatic
changes in hydrogen ion concentration when
a strong acid or strong base is added
pH control system

Respiratory controls


When CO2 enters
blood from tissue cells,
it is converted to
carbonic acid
Modify blood pH by
retaining CO2
(decrease pH) or by
eliminating more CO2
from blood (increase
blood pH)
pH control system

Renal Mechanism



Only system that can remove metabolic acids
and bases from the body
Kidneys act slowly to change pH (hours to
days to regulate)
pH of urine can vary from 4.5 to 8.0
KIDNEY PROBLEMS



Kidney functions are vital to homeostasis;
problems can be life-threatening
Infections can be detected with
URINALYSIS - look for blood cells and
proteins in urine.
NORMAL URINE NEVER HAS BLOOD
PROTEIN OR BLOOD CELLS IN IT. IT
SHOULD ALSO NOT CONTAIN MORE
THAN TRACE AMOUNTS OF GLUCOSE
KIDNEY PROBLEMS

If water and salts retained, causes fluid
accumulation in body tissues, plus ionic
imbalances (leads to problems in including
loss of consciousness and heart failure).
This condition is called EDEMA.