1. Biology 30S Excretory System
Role of the Nephrons
Biology 30S
Excretory System

3. More on The Kidneys… The kidneys filter 120ml of blood per minute!
only 1 ml (1%) of this turns into urine that will eventually leave the body.
That leaves 119ml of fluid called filtrate to be returned back to the blood stream.

4. If that didn’t happen….
A person would have to pee once every 3 minutes
And drink 1 Litre of fluid every 10 minutes in order to maintain Homeostasis!

5. The Kidneys are therefore very efficient at…
Returning good stuff back to the blood
AND
Removing wastes by making urine

6. Urine Production by Kidneys
This happens in 3 main steps:
Filtration
Reabsorption
Secretion

8. Filtration Blood enters glomerulus under high pressure
Pressure causes 20% of blood plasma (120 ml of 600 ml) to be forced into the Bowman’s Capsule (BC)
Water, salt, glucose, amino acids (good stuff) and urea (bad stuff) all pass through into BC
Blood Cells and proteins do NOT enter the BC – they are too large to leave the capillaries
Filtrate is ISOTONIC to Plasma

9. Filtration Renal Veins  Efferent Arterioles
Renal Artery  Afferent Arteriole  Glomerulus  BC
_____________________________________________________ _________
BLOOD Filtrate
FILTRATION

10. Filtration Bowman’s Capsule separates materials in plasma
Filtrate!
Bowman’s Capsule separates materials in plasma
little stuff goes through
big stuff doesn’t

11. Reabsorption
Useful materials such as sugars and salt ions are reabsorbed back into the blood stream.
“Good” materials that can still be used by the body are sent back to the blood.
Reabsorption happens as filtrate passes sequentially through nephron.
Reabsorption takes place in all regions of the nephron but one: the ascending loop of Henle.

12. Reabsorption: PCT
As filtrate enters the PCT ~80% of the salt ions (Na+ & K+), glucose and amino acids are actively transported out & back into the blood stream.
Cl- ions will flow passively out of the PCT following the positively charge K+ & Na+.
As the [ ] of these molecules drops inside the PCT, water will diffuse out of the PCT passively by osmosis.

13. Reabsorption: Descending Loop of Henle
Both sodium (Na+) and potassium (K+) diffuse back into the Loop of Henley.
Water continues to move out of the Loop of Henle into the capillary network.
At this point, filtrate is more concentrated (hypertonic) with respect to salt ions that it was in the PCT due to water moving out and salts being more added.
This causes a concentration gradient to form

14. Reabsorption: Ascending Loop of Henle
As the filtrate proceeds up into the ascending loop of Henle, the Cl- ions are actively pumped back out of nephron.
Na+ ions passively follow Cl- out of tubule and into the medulla.
Since the Loop of Henle is impermeable to water, water cannot leave this part of the nephron.
Filtrate will become more dilute again.

15. Counter Current Mechanism
This is a “current” created by the ascending L of H
Purpose: to get huge amounts of water to be reabsorbed back to the blood
Method: pumping salt to the medulla, which then draws water to the medulla. From the medulla, water goes back to the blood
Causes less water to be lost in the urine and more water to be returned to the blood

16. Reabsorption: DCT
Water continues to diffuse out of the nephron and back into the blood.
The medulla is hypertonic to the filtrate in regards to salt concentration
Due to continued re-absorption of water, the filtrate becomes more and more concentrated with wastes, mainly urea.
The hormone ADH can regulate the amount of water that diffuses.
The amount of water that diffuses from the DCT back into the blood depends on bodily needs.

17. Reabsorption: CT
The same thing that happens in the DCT happens in the CT

18. Good stuff goes back to blood
Reabsorption
Good stuff goes back to blood

19. Secretion
Wastes and toxins are actively transported from the blood into the Nephron.
This “bad stuff” is “fast tracked” into the PCT, DCT, and CT of the Nephron.
Excess things like H+ ions, OH- ions, glucose and salt and Toxins like ammonia, and drugs

20. Secretion
Bad stuff moves from blood into nephron

21. Filtration, Reabsorption & Secretion What you Need to Know!
Filtration: movement of plasma from the glomerulus capillaries to the Bowman’s Capsule
Little things pass through
Big things do not
Reabsorption: the movement of materials from the nephron tubules back into the blood stream.
Occurs in the PCT, L of H, DCT & CT

22. Filtration, Reabsorption & Secretion What you Need to Know!
Counter Current Mechanism: a “current” created by the ascending L of H
Purpose: to get huge amounts of water to be reabsorbed back to the blood
Secretion: Wastes and toxins are fast tracked from the blood for excretion
Happens in the PCT, DCT, and CT of the Nephron.

24. The Nephron!

25. Plasma & Filtrate Alike
Both are clear, slightly yellow liquids
Both can be found in the kidney
Both contain water, salts, nutrients (sugars) & nitrogenous wastes
Plasma that is forced from the glomerulus through Bowman’s Capsule becomes filtrate
Filtrate used to be plasma

26. Plasma & Filtrate Differences

27. Filtrate & Urine Alike Both are clear, yellow liquids
Both can be found in the kidney
Both contain water, salts and nitrogenous wastes
Both pertain to the excretion of Nitrogenous wastes
Once filtrate drips out of the collecting tubules of the nephron and into the renal pelvis, it is now called urine
Urine used to be filtrate

28. Filtrate & Urine Difference

29. Plasma Filtrate Urine Filtrate, filtered plasma, in nephron
Urine, modified filtrate, in collecting tubule
Plasma, in blood vessels