Glomerular Filtration Flashcards
What can pass through the glomerular filter?
Water Salts Small proteins Glucose Urea
What percentage of the blood which passes through the renal artery is filtered?
20%
What cannot pass through the filter?
Large proteins
Cells
What causes the pressure to be increased inside the glomerulus?
Afferent arteriole has a slightly larger diameter than the efferent
What repels proteins from being filtered?
The podocyte glycocalyx and the basement membrane have negatively charged glycoproteins
What is the ultrafiltrate?
The water and solutes forced through the filter
What are the physical forces involved in filtering of plasma and where are they? Do they favour or oppose filtration?
Hydrostatic pressure in the capillary - favours filtration
Hydrostatic pressure in Bowman’s capsule - opposes filtration
Osmotic pressure difference between capillary and tubular lumen - opposes filtration
Structure of the basement membrane of the filter?
Acellular
Gelatinous
Layer of collagen and glycoproteins
What are the three layers of the filtration barrier?
Capillary endothelium
Basement membrane
Podocytes
If you get loss of the negative charge on the glycoproteins, what can this lead to?
Proteinuria
What is clearance?
The volume of plasma from which a substance can be completely cleared to the urine per unit of time
What is renal blood flow? Value?
Volume of blood delivered to kidneys per unit of time
1.1L/min
Define renal plasma flow. Value?
Volume of blood plasma delivered to kidneys per unit time
605ml
What is measured to calculate renal plasma flow?
Haematocrit (volume percentage of red blood cells in blood)
This = 0.55
1.1 x 0.55 = 605ml/min
Value of the glomerular filtration rate?
125ml
20% of renal plasma flow
What is the filtration fraction?
Proportion of a substance that is actually filtered
How is the filtration fraction calculated?
GFR / RPF
What do you use to calculate a patient’s GFR?
Clearance of something eg urea
How is clearance rate worked out? (Equation)
Clearance rate =
{urine conc of P x urine flow (ml/min)} /
plasma conc of p
(so if a compound is completely cleared from the plasma that is filtered, then the rate it appears in the urine is the GFR)
Define glomerular filtration rate
Volume of plasma from which any substance is completely removed by the kidney in a given amount of time
Normal GFR for males and females?
Males = 115-125 ml/min Females = 90-100 ml/min
What conditions must there be when choosing a substance to use to measure the GFR?
Must be freely filtered across the glomerulus
Must not be reabsorbed, secreted or metabolised by cells of the nephron
Must pass directly into the urine
What are substances that can be used to measure GFR?
Creatinine and inulin
What is autoregulation used for?
To keep the GFR within normal limits when arterial BP is within the physiological limit
What is the myogenic response in autoregulation?
If arterial blood pressure rises, get afferent arteriole constriction.
If it falls, get afferent arteriole dilation
How does tubular glomerular feedback occur
Changes in GFR causes changes in tubular flow rate
This causes a change in the amount of NaCl reaching the distal tubule
If there is an increase in arterial pressure, what happens in tubular glomerular feedback?
Increase in arterial pressure - increase in GFR - increase in [Na] and [Cl] in distal tubule
Macula densa cells respond - cause release in adenosine which causes vasoconstriction of afferent arteriole
What happens if NaCl concentration decreases in distal tubule (for tubular glomerular feedback)
Macula densa cells cause release of prostaglandins which cause vasodilation of afferent arteriole
What is general overflow aminoacidura?
All amino acids are present in the urine.
What can general overflow aminoacidura be caused by?
- Inadequate deamination in the liver
- Increased GFR
- Often seen in early pregnancy
What is specific overflow aminoacidura?
Only a specific amino acid is in the urine
What can cause specific overflow aminoacidura?
Genetic inability to break down an amino acid
Eg phenylalanine in PKU due to lack of phenylalanine hydroxylase
Which amino acid is usually associated with renal stone formation? Why?
Cysteine (cystinuria)
It is an insoluble amino acid, especially in acidic urine
