Lecture 32- Glomerular function

Question 1

What is the specific form of filtration that the kidney does? Where does this occur?

Answer 1

• Ultrafiltration, essentially a sieve that acts on a very small scale.
• In the glomerulus.

Question 2

What does filtration in the kidney create?

Answer 2

A plasma-like filtrate of the blood

Question 3

Is the glomerulus selective when it filters?

Answer 3

• No, not particularly selective

- Only criteria is size and sometimes charge

Question 4

How much urine is produced per day? What does this mean?

Answer 4

• Produces only 1.5L of urine per day

- Therefore, 99% of what is filtered goes back

Question 5

How much of the cardiac output does the kidney get and how does this compare to other organs?

Answer 5

• 20-25% of cardiac output (CO) or 1-1.2L/min
• brain= 13%
• heart= 4%

Question 6

Why is there a high flow to the kidney?

Answer 6

For filtration (reabsorption + secretion processes) rather than metabolism

Question 7

What are the three things that make up the filtration barrier?

Answer 7

• Fenestrated endothelium
• Basement membrane and
• Podocytes with foot processes and filtration slits

Question 8

What does the filtration barrier let through? What does the filtration barrier not let through?

Answer 8

• Small substances (low molecular mass) are freely filtered

- Large substances (high molecular mass) are NOT filtered

Question 9

What are driving forces determined by?

Answer 9

• forces / pressure gradients between glomerular capillary / capsular space
• permeability of glomerular capillary
• surface area of glomerular capillary

Question 10

What are the two positive forces influencing glomerular filtration?

Answer 10

• Glomerular hydrostatic pressure (GHP) = blood pressure (+ 50 mmHg)
• Capsular colloid osmotic pressure (CsCOP) = no protein in capsular space (+ 0 mmHg)

Question 11

What are the two negative forces influencing glomerular filtration?

Answer 11

• Blood colloid osmotic pressure (BCOP) = albumin, proteins left behind (- 25 mmHg)
• Capsular hydrostatic pressure (CsHP) = corpuscule embedded in tissue (- 15 mmHg)

Question 12

Net filtration pressure is…

Answer 12

calculated by positive forces- negative forces

Question 13

How can we define renal filtration?

Answer 13

By calculating renal clearance: the rate at which substance S is cleared by the kidneys per unit time

Question 14

How is renal clearance calculated?

Answer 14

Clearance (Cs) = Us x V / Ps (in mL/min)

Us = concentration of S in urine (mg/L or mol/L)
V = volume of urine produced per unit time (mL/min or L/hour)
Ps = concentration of S in plasma (mg/L or mol/L)

Question 15

What does the renal clearance equation apply to?

Answer 15

For all substances that can be detected in plasma and urine

Question 16

What is the glomerular filtration rate (GFR)?

Answer 16

• Amount of fluid/plasma filtered per unit time

- 125mL/min (180L/day)

Question 17

What are three features of glomerular filtration rate?

Answer 17

• Tightly regulated as also effects blood pressure
• Variation from person to person as nephron numbers vary
• Declines slowly from age 30

Question 18

For be able to estimate GFR from renal clearance a substance must…

Answer 18

• not be reabsorbed from the tubule
• not be secreted into the tubule
• not be metabolised

Just has to be filtered

Question 19

What are two substances that meet the criteria for being able to predict GFR from renal clearance?

Answer 19

• Inulin: polysaccharide (sugar) not metabolized by body. Also not found in body, must be injected.
• Creatinine: waste product produced by muscles. Already in the body so most commonly used clinically

Question 20

What is plasma creatine an indicator for? What happens if it is low?

Answer 20

• An indicator for kidney function
• If plasma creatinine is low both kidneys are working (125 mL/min)
• If plasma creatine is high then only one kidney is working

Question 21

What is the term given for the amount of blood that once reaches the kidney is filtered? What percentage is this?

Answer 21

• Filtration fraction

- 20% (GFR/ renal blood flow)

Question 22

What is a filtered load? What is the unit?

Answer 22

• Amount of a particular substance (solute) filtered per minute
• GFR x solute plasma conc.
• Unit = mol/time