1. SBI 4U: Metablic Processes
9.5 Urine Formation
The formation of urine occurs in three processes:
Section 1.3

2. SBI 4U: Metablic Processes
Step 1: FILTRATION
The movement of fluids and solutes from the blood into the Bowman's capsule of the nephron.
The glomerulus is a high-powered filter, acting at 1.5 times the normal pressure of a capillary.
Body contains 2.75 L of blood plasma, and is filtered 65 times/day
Section 1.3

3. Not all solutes make it through.
DO: H2O, NaCl, Glucose, Amino Acids, H+, urea, uric acid
DON'T (too large): plasma proteins, red blood cells, platelets

4. SBI 4U: Metablic Processes
Step 2: REABSORPTION
The transfer of essential solutes and most water back into the surrounding cells and the blood stream.
For every 120 mL of fluid (amount filtered each minute), 1 mL of urine is formed, 119 mL of fluid and solutes is reabsorbed.
Occurs mainly in the proximal tubule, loop of Henle and distal tubule.
Occurs by active and passive transport.
Section 1.3

5. Specialized ion pumps, transport K+, Na+ and Cl-
Inner walls of the tubule have microscopic extensions called microvilli that increase surface area for reabsorption
Movement of water is facilitated by membrane proteins called aquaporins (water channels)

6. Active Transport of NaCl
major site of NaCl reabsorption is the loop of Henle, (followed by the distal tubule and collecting ducts)
Na+ is pumped from the tubules into the surrounding cells of the kidney
Cl- follows by diffusion
Water follows by osmosis

7. Passive Transport of Water
SBI 4U: Metablic Processes
Passive Transport of Water
Kidneys help maintain water balance
Water is reabsorbed back into the cells and bloodstream by osmosis for 2 reasons:
The blood in the efferent arteriole has a very high [plasma proteins] and low [water], therefore it draws water from the nephron tubules and surrounding cells into the blood.
Section 1.3

8. As solutes (glucose, K+, NaCl, etc) are drawn out of the tubules through active and passive transport they create an osmotic gradient. Their concentration becomes high in the cells surrounding the tubules.
Therefore, water moves out of the nephron into the surrounding cells once more, again by osmosis.

9. As water gets reabsorbed, the solutes left in the nephron (urea, uric acid, excess essential solutes) become quite concentrated.
Some urea and uric acid will diffuse back into the blood, however most of the urea and uric acid will be left to form urine.
The material left in the nephron is the filtrate.

11. SBI 4U: Metablic Processes
STEP 3: SECRETION
The movement of wastes from the blood into the nephron
Purpose:
to release any toxins/drugs that were not filtered
Maintain the electrolyte balance of the body (ions)
Acid-base balance (usually it is an acid being secreted). Secretion of H+ ions
Section 1.3

12. The bicarbonate ion (HCO3-) is never secreted since it is used as a buffer in the maintenance of our blood pH.
Acidic juices like cranberry cause our urine to be quite acidic which helps protect against UTI’s and prevent kidney stones.

13. Copy Table 1 on page 452 into your notes

14. Kidney Disease
Many kidney disorders can be diagnosed through urinalysis.
Diabetes Mellitus – insufficient insulin secretion, causes blood sugar to rise. Excess sugar remains in the nephron. Urinate more frequently
Diabetes Insipidus – inadequate production/secretion of ADH, leads to frequent urination. Urine is often clear, and the patient is always thirsty.

15. Bright’s Disease – inflammation of the kidneys, inadequate filtration at the glomerulus leading to the presence of protein in urine

16. Kidney stones – buildup of mineral solutes, that can combine with calcium to produce crystals that accumulate and forms stones. Can be acidic or alkaline.
- causes pain &
bleeding as they
pass through the
ureter and urethra
Loss of kidney function requires the use of dialysis (filtering machine)

18. Homework
Finish Pg. 454 # 2 – 6, 9, 10
Prelab sheet – due tomorrow