1. SBI 4U: Metablic Processes
WATER BALANCE
Kidneys maintain water balance in the body.
Humans' water reserves are depleted faster than their food reserves. Under normal activity levels we lose 2.0 L of water a day in perspiration, urine and exhaled air. Increased exercise leads to increased water loss.
To maintain fluid levels we must consume 2L of fluids daily. A drop in fluid intake by 1% of your body mass will cause thirst, a 5% decrease will bring extreme pain and collapse, and a decrease of 10% will cause death.
More water in means more urine out and vice versa.
Water balance is regulated by our nervous and endocrine systems.
Section 1.3

2. ENDOCRINE REGULATION OF WATER BALANCE
SBI 4U: Metablic Processes
ENDOCRINE REGULATION OF WATER BALANCE
ADH (anti-diuretic hormone) regulates the osmotic pressure* of body fluids by causing the kidneys to increase water reabsorption (at the collecting ducts).
*Osmotic Pressure: A hydrostatic pressure caused by a difference in the amounts of solutes between solutions that are separated by a semi-permeable membrane.
If osmotic pressure is increased in a solution it means it is hypertonic and water will move in (pressure to do osmosis). If osmotic pressure is decreased in a solution there is less pressure for osmosis – it it near to equilibrium. In our body when we become dehydrated, either by decreasing water intake or increasing water loss (ie. Sweating) – water is lost from our bloodstream, this increases it's osmotic pressure.
When ADH is released, water reabsorption is increased to decrease osmotic pressure and urine is more concentrated due to increased reabsorption of water back into the body.
⇧ADH --> ⇧ Water Reabsorption --> Concentrated Urine
Section 1.3

3. NERVOUS CONTROL of WATER BALANCE
SBI 4U: Metablic Processes
NERVOUS CONTROL of WATER BALANCE
Osmoreceptors in the hypothalamus detect changes in osmotic pressure (sweating / dehydration cause the blood to be more concentrated which increases osmotic pressure, pulls water into the bloodstream from the cells, the cells of the hypothalamus shrink).
Message sent to the pituitary gland to release ADH into the blood.
At kidneys, ADH makes the collecting ducts and distal tubules more permeable to water, increasing reabsorption into the blood and urine more concentrated.
Osmotic pressure lowers.
⇧ Osmotic Pressure
(-) Lowers
OSMORECEPTORS
(HYPOTHALAMUS)
NERVES
PITUITARY GLAND
ADH
COLLECTING DUCTS
(KIDNEYS)
⇧ Water Reabsorption
Section 1.3

4. SBI 4U: Metablic Processes
The body also has the sensation of thirst which is a behavioural response. Drinking fluids will also help to decrease osmotic pressure.
Once the osmotic pressure decreases, the cells of the hypothalamus swell and the pituitary gland stops the release of ADH. (negative feedback!)
Page 356 #1-5, 9, 10 (6)
Normal Osmotic Pressure
OSMORECEPTORS
(HYPOTHALAMUS)
NERVES
PITUITARY GLAND
ADH
COLLECTING DUCTS
(KIDNEYS)
⇧ Water Reabsorption
Section 1.3

5. SBI 4U: Metablic Processes
Section 1.3

6. REGULATING BLOOD PRESSURE
SBI 4U: Metablic Processes
REGULATING BLOOD PRESSURE
Kidneys are also a part in the blood pressure feedback loop:
Blood pressure is too low (dehydration / blood loss)
Blood pressure receptors in the juxtaglomerular apparatus detect low blood pressure.
Special cells in the apparatus release renin (enzyme used to convert the blood plasma protein angiotensinogen to angiotensin).
Angiotensin causes blood vessels to constrict (decrease volume increases pressure) and causes the adrenal glands in the cortex of the kidneys to release aldosterone
Aldosterone causes the distal tubule and the collecting ducts to increase Na+ and water reabsorption.
LOW Pressure
⇧ Pressure
Blood Vessels Constrict
JGA
Collecting Ducts
Increase Water Reabsorption
Renin
Angiotensinogen
Angiotensin
Adrenal Gland
Aldosterone
Section 1.3

7. SBI 4U: Metablic Processes
REGULATING BLOOD pH
Kidneys are important in regulating blood pH:
Despite the variety of foods and fluids consumed with varying pH levels, the pH of the body remains relatively constant, between 7.3 and 7.5.
During cellular respiration our cells produce carbon dioxide which diffuses into the blood and combines with water to form carbonic acid. Carbonic acid then ionizes to release protons which lowers the pH.
Excess protons are buffered by the bicarbonate ion in the blood.
H2O + CO H2CO HCO H+
Carbonic acid
Bicarbonate ion
Section 1.3

8. SBI 4U: Metablic Processes
H2O + CO H2CO HCO H+
Due to this buffering system, the process is reversed, the bicarbonate ion combines with protons to form carbonic acid, which breaks down into water and carbon dioxide.
CO2 will be eliminated by the lungs. Excess protons will be eliminated by the kidneys. The buffering system helps maintain the pH of the blood in the meantime.
Thus the kidneys help maintain the buffer by:
ensuring that the bicarbonate ions are returned to the bloodstream (always reabsorbed) and actively reabsorbing CO2, which reverses the buffering system so more HCO3- is produced.
Using any excess protons in the creation of waste products that are secreted and subsequently urinated out.
Homework: p 356 #7,8
Section 1.3