Lecture 32- Glomerular function Flashcards
What is the specific form of filtration that the kidney does? Where does this occur?
- Ultrafiltration, essentially a sieve that acts on a very small scale.
- In the glomerulus.
What does filtration in the kidney create?
A plasma-like filtrate of the blood
Is the glomerulus selective when it filters?
- No, not particularly selective
- Only criteria is size and sometimes charge
How much urine is produced per day? What does this mean?
- Produces only 1.5L of urine per day
- Therefore, 99% of what is filtered goes back
How much of the cardiac output does the kidney get and how does this compare to other organs?
- 20-25% of cardiac output (CO) or 1-1.2L/min
- brain= 13%
- heart= 4%
Why is there a high flow to the kidney?
For filtration (reabsorption + secretion processes) rather than metabolism
What are the three things that make up the filtration barrier?
- Fenestrated endothelium
- Basement membrane and
- Podocytes with foot processes and filtration slits
What does the filtration barrier let through? What does the filtration barrier not let through?
- Small substances (low molecular mass) are freely filtered
- Large substances (high molecular mass) are NOT filtered
What are driving forces determined by?
- forces / pressure gradients between glomerular capillary / capsular space
- permeability of glomerular capillary
- surface area of glomerular capillary
What are the two positive forces influencing glomerular filtration?
- Glomerular hydrostatic pressure (GHP) = blood pressure (+ 50 mmHg)
- Capsular colloid osmotic pressure (CsCOP) = no protein in capsular space (+ 0 mmHg)
What are the two negative forces influencing glomerular filtration?
- Blood colloid osmotic pressure (BCOP) = albumin, proteins left behind (- 25 mmHg)
- Capsular hydrostatic pressure (CsHP) = corpuscule embedded in tissue (- 15 mmHg)
Net filtration pressure is…
calculated by positive forces- negative forces
How can we define renal filtration?
By calculating renal clearance: the rate at which substance S is cleared by the kidneys per unit time
How is renal clearance calculated?
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)
What does the renal clearance equation apply to?
For all substances that can be detected in plasma and urine
What is the glomerular filtration rate (GFR)?
- Amount of fluid/plasma filtered per unit time
- 125mL/min (180L/day)
What are three features of glomerular filtration rate?
- Tightly regulated as also effects blood pressure
- Variation from person to person as nephron numbers vary
- Declines slowly from age 30
For be able to estimate GFR from renal clearance a substance must…
- not be reabsorbed from the tubule
- not be secreted into the tubule
- not be metabolised
Just has to be filtered
What are two substances that meet the criteria for being able to predict GFR from renal clearance?
- 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
What is plasma creatine an indicator for? What happens if it is low?
- 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
What is the term given for the amount of blood that once reaches the kidney is filtered? What percentage is this?
- Filtration fraction
- 20% (GFR/ renal blood flow)
What is a filtered load? What is the unit?
- Amount of a particular substance (solute) filtered per minute
- GFR x solute plasma conc.
- Unit = mol/time
