1. SBI 4U: Metablic Processes Homeostasis: The Kidneys
Water and pH Balance
Homeostasis: The Kidneys
Section 1.3

2. SBI 4U: Metablic Processes
WATER BALANCE
Human water reserves are depleted faster than food reserves; 2.0 L of water is lost a day through 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 body mass will cause thirst, 5% will bring extreme pain and collapse, and 10% will cause death.
Section 1.3

3. SBI 4U: Metablic Processes
Osmotic Pressure
A pressure exerted on fluids caused by differences in solute concentration or the “pressure to do osmosis”
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 its osmotic pressure.
Section 1.3

4. SBI 4U: Metablic Processes
Osmotic Pressure
In the body if osmotic pressure is high it means the blood has lost water and is hypertonic with respect to the tissues around it. Water levels are unbalanced. There is pressure for the water to move from the body cells to the blood.
If osmotic pressure is low the concentrations of body fluids are near to equilibrium and there is less pressure for osmosis.
i.e.) Water levels are balanced.
Section 1.3

5. Regulation of Water Balance
SBI 4U: Metablic Processes
Regulation of Water Balance
ADH (anti-diuretic hormone) from the pituitary regulates the osmotic pressure of body fluids by causing the kidneys to increase water reabsorption.
Osmotic pressure decreases as water levels balance.
Urine is more concentrated due to increased reabsorption of water back into the body.
**Refer to Figure on pg 457
Concentrated Urine
ADH  Water Reabsorption
Osmotic Pressure
Section 1.3

6. Water Balance Feedback Loop
SBI 4U: Metablic Processes
Water Balance Feedback Loop
Osmotic Pressure (-)
Osmoreceptors (Hypothalamus)
nerve pathway
Posterior Pituitary
ADH
Collecting Ducts (Kidneys)
Water Reabsorption
Osmoreceptors in the hypothalamus detect changes in osmotic pressure
(sweating / dehydration cause the blood to be more concentrated which increases osmotic pressure and pulls water into the blood from the cells, the cells of the hypothalamus shrink).
Section 1.3

7. Water Balance Feedback Loop
SBI 4U: Metablic Processes
Water Balance Feedback Loop
Osmotic Pressure (-)
Osmoreceptors (Hypothalamus)
nerve pathway
Posterior Pituitary
ADH
Collecting Ducts (Kidneys)
Water Reabsorption
Message sent to the pituitary gland to release ADH into the blood.
The body also has the sensation of thirst which is a behavioural response.
Drinking fluids will also help to decrease osmotic pressure.
Section 1.3

8. Water Balance Feedback Loop
SBI 4U: Metablic Processes
Water Balance Feedback Loop
Osmotic Pressure (-)
Osmoreceptors (Hypothalamus)
nerve pathway
Posterior Pituitary
ADH
Collecting Ducts (Kidneys)
Water Reabsorption
At kidneys, ADH makes the collecting ducts and distal tubules more permeable to water, increasing reabsorption into the blood and making urine more concentrated.
Section 1.3

9. Water Balance Feedback Loop
SBI 4U: Metablic Processes
Water Balance Feedback Loop
Osmotic pressure lowers.
Once the osmotic pressure decreases, the cells of the hypothalamus swell and the pituitary gland stops the release of ADH. (negative feedback)
Osmotic Pressure (-)
Osmoreceptors (Hypothalamus)
nerve pathway
Posterior Pituitary
ADH
Collecting Ducts (Kidneys)
Water Reabsorption
Section 1.3

10. SBI 4U: Metablic Processes
pH Balance
Carbon dioxide produced during cellular respiration forms carbonic acid in the blood.
Carbonic acid and other excess acids ionize to produce H+ ions.
A build-up of H+ ions lowers the pH.
Despite this and the variety of foods and fluids consumed with varying pH levels, the body pH remains relatively constant, between 7.3 and
Section 1.3

11. SBI 4U: Metablic Processes
Buffer systems
Buffer systems maintain acid-base balance by absorbing excess H+ ions or ions that act as bases.
Ex. The bicarbonate-carbon dioxide buffer system.
H2O + CO2 H2CO3 HCO3- + H+
Bicarbonate ion buffers excess H+ (reverse reaction)
Carbonic acid is produced which breaks down to form carbon dioxide and water.
Carbon dioxide is exhaled at the lungs
Section 1.3

12. The bicarbonate-carbon dioxide buffer system:
SBI 4U: Metablic Processes
The bicarbonate-carbon dioxide buffer system:
Section 1.3

13. SBI 4U: Metablic Processes
Buffer systems
H2O + CO2 H2CO3 HCO3- + H+
At the kidneys, carbon dioxide is actively transported from the blood into the cells of the tubules to drive the reaction forward.
Carbonic acid is produced and breaks down to form bicarbonate ions and H+
The bicarbonate ion is reabsorbed back into the blood, restoring the buffer.
The H+ combines with ammonia or phosphate ions to be excreted in the urine.
Section 1.3

14. The bicarbonate-carbon dioxide buffer system:
SBI 4U: Metablic Processes
The bicarbonate-carbon dioxide buffer system:
Section 1.3

15. SBI 4U: Metablic Processes
Hey LOOK at this!
Section 1.3

16. SBI 4U: Metablic Processes
Kidney Disorders
Bright’s Disease
Aka nephritis
Nephritis includes many diseases that lead to inflammation of the nephron.
For example the glomerular capillaries can be destroyed by microbes resulting in increased permeability and the filtering of proteins.
This reduces the osmotic pressure on water to be reabsorbed and increases urine output.
Section 1.3

17. SBI 4U: Metablic Processes
Kidney Disorders
Kidney Stones
Precipitation of mineral solutes
Sharp-sided stones can lodge in pelvis or ureter
In some cases high energy shock waves can be used to break up the stone to help dislodge and be voided.
Section 1.3

18. Homework
Pg 465 #1-9
For #9, refer to pg