Urine formation Flashcards
What are the 3 steps of urine formation?
- Glomerular filtration
- Tubular reabsorption
- Tubular secretion
What is formed by glomerular filtration?
Primary urine
Is glomerular filtration selective?
No, everything gets filtered
When does the primary urine become urine?
After reabsorption and secretion
What are the 4 ways the kidneys maximize filtration?
- Large surface area
- Permeability
- Receive higher than normal blood flow
- Have higher than normal blood pressure that comes into the kidneys
How much of the plasma that enters the glomerulus is actually filtered?
20%
What are the 2 regions of the nephron?
Renal corpuscle and renal tubule
What does the renal corpuscle do?
Includes the glomerulus and Bowman’s capsule. Does the glomerular filtration
What does the renal tubule do?
Includes the proximal and distal tubules and the loop of Henle. Does reabsorption and secretion
What is the structure of the glomerulus?
A twisted ball of specialized capillaries that are covered by podocytes, with mesangial cells in between the capillaries
What are mesangial cells?
Loosely connected endothelial cells around the podocytes
What are podocytes?
Specialized epithelial cells that cover the outside of the capillaries, with foot processes that interact with the basement membrane and form filtration slits
Why are podocytes important for determining what gets filtered out of the blood during glomerular filtration?
The foot processes keep red blood cells and proteins inside the blood vessel, so they don’t escape
What are the 3 components of primary urine?
- Water
- Inorganic ions
- Small organic molecules (glucose, amino acids, urea)
Why is fluid driven from the glomerulus to the Bowman’s capsule?
Bulk flow. There is high pressure in the glomerulus and low pressure in the Bowman’s capsule
What are the 3 pressures that net glomerular filtration depends on?
Hydrostatic pressure exerted by the glomerular capillaries (60 mmHg)
Hydrostatic pressure exerted by the lumen of the Bowman’s capsule (15 mmHg)
Oncontic pressure
Which pressure force favours the movement of fluid into the Bowman’s capsule lumen?
Glomerular capillary hydrostatic pressure
Which pressure forces resist the movement of fluid into the Bowman’s capsule lumen?
Bowman’s capsule hydrostatic pressure and oncotic pressure
What is oncotic pressure?
The colloid osmotic pressure created by the high concentration of proteins in the plasma
What is the net filtration pressure?
Difference between the pressure forces that favour inwards movement (glomerular capillaries hydrostatic) and pressure forces that favour outward movement (Bowman’s capsule hydrostatic and oncotic). 60 - 30 - 15 = 15 mmHg
Is the primary urine hypo, iso, or hyperosmotic to the blood?
Isosmotic
What is GFR? What is normal?
The glomerular filtration rate. Measures the rate of primary urine formation and is a very important indicator of kidney function. Normal GFR is 120 ml/min
What was the way we used to measure GFR?
Inject the patient with inulin and then measure the amount of inulin in the patient’s urine after 24 hours
Why was inulin a good way to directly measure GFR?
It isn’t reabsorbed or secreted after the Bowman’s capsule. So the amount of inulin in the urine equals GFR
What changes in the kidney would change the GFR?
Blood flow, blood pressure, permeability
How is the GFR of the entire kidney change?
Vasoconstriction or dilation of afferent or efferent blood vessels
How is the GFR of an individual nephron changed?
Altering the mesangial cells
What do we use now to measure GFR in a person?
Creatinine. Produced endogenously by muscle metabolism, so no need to inject the person. It is also not secreted or reabsorbed
What is plasma clearance?
The volume of plasma cleared of a specific substance over time
How does plasma clearance compared to GFR if a substance isn’t secreted or reabsorbed?
C = GFR
How does plasma clearance compared to GFR if a substance is being secreted?
C > GFR
How does plasma clearance compared to GFR if a substance is reabsorbed?
C < GFR
Do the proximal tubules have controlled or uncontrolled reabsorption?
Uncontrolled
Do the distal tubules have controlled or uncontrolled reabsorption?
Controlled
How much glucose should be in the urine of a healthy person?
None, it should all be reabsorbed
What is the specific clearance rate?
The clearance rate of a particular substance from the plasma
What is the relative clearance rate?
The clearance rate of a particular substance from the plasma relative to GFR
