Urinary System Physiology

Question 1

How is renal blood flow regulated?

Answer 1

Intrinsic mechanisms -> Autoregulation- this fails at 80mmHg

1) Myogenic -> increased blood flow leads to distension of blood vessles -> contraction of smooth muscles blood vessels -> increased vascular resistance -> reduced renal flow
2) Active renal metabolites induce vasodilatation

Question 2

Which microscopic structures are found in the glomerulus to aid its function?

Answer 2

1) Fenestrated capillary endothelium - free passage of water and electrolytes for filtration
2) Negatively charged Basement membrane - attracts postitive/neutral molecules
3) Epithelium of bowman’s capsule has podocytes - not continuous and has gaps for filtratoin

Question 3

Summarise Glomerular filtration

Answer 3

Glomerulus - first site of urine formation
High filtration rate 120ml/min
Hydrostatic pressure 40mmHg
Osmotic pressure generated by plasma proteins (opposite pressure to hydrostatic pressure) ~25mmHg
Net filtration pressure of ~15mmHg
As filtration progresses, osmotic pressure increases as fluid is removed. No further fluid filtered
Filter fluid = plasma - proteins/blood cells
= Ultrafiltrate (300mosmol/L)

Question 4

What happens to the ultrafiltrate at the PCT?

Answer 4

Proximal convoluted tubule absorbs 70% of the
filtered sodium; chloride follows by electrostatic
attraction; water follows the absorption of NaCl
as a result of the osmotic gradient.
Transport of NaCl- is an energy-dependent process;
it relies on an ATP-dependent pump.

Question 5

What happens to the ultrafiltrate at Loop of Henle?

Answer 5

Descending loop of Henle is permeable to NaCl
and water.
Due to the high osmolality of the surrounding
medulla, water is removed from the ultrafiltrate
and NaCl is added as ions move down the
concentration gradient between the medulla and
the ultrafiltrate.
Ultrafiltrate in the descending loop of Henle is
concentrated by the passive addition of NaCl from
the medulla and the reduction in volume due to
the absorption of water
The ascending loop of Henle is responsible for the
active reabsorption of Na
(which again leads to
passive Cl- absorption). However, this region is impermeable to water; the transport of NaCl leads to a decrease in the osmolality of the ultrafiltrate but
an increase in the osmolality of the surrounding
medullary parenchyma.
As the fluid enters the ascending limb
ions are removed but water remains, as this area is
impermeable to water. The osmolality is decreased
as ions are removed;

Question 6

How is phosphate reabsorption regulated?

Answer 6

Reabsorption of phosphate is controlled by PTH. High levels of PTH inihibit phosphate reabsorption from proximal tubule.
Reabsorpion occurs with co-carrier of phosphate and sodium.