L2 - Glomerular Filtration Flashcards
What is the role of the glomerulus?
Glomerulus filters plasma
180l/day filtrate
Total plasma passes filtration barrier 65 times a day
What does the glomerulus allow the passage of?
Water and small molecules
What does the glomerulus restrict the passage of?
Blood cells and proteins
What is the structure of the glomerulus?
Capillary bed
Filtration occurs within the capillaries
Plasma enters via the afferent arteriole and leaves via the efferent arteriole
What are the 3 layers of the filtration barrier?
Endothelial cells
Basement membrane
Epithelial cells - podocytes
What does the endothelial cell filtration barrier consist of?
Flat large nuclei
Circular fenestrations
In contact with each other
Filters blood cells and platelets
What does the basement membrane filtration barrier consist of?
Continuous - main barrier for filtration
Composed of glycoproteins - collagen, laminin, fibronectin
Negatively charge
Filtration based on molecular shape, size and charge
What does the epithelial cell filtration barrier consist of?
Trabecula – projections from cell body
Pedicles – projections from trabecula
- They interdigitate creating slit pores
- Slit pores allow filtrate to move through
Maintenance of structure
Phagocytosis of foreign bodies
If the filtration: plasma ratio is 0 what does this show?
None of the plasma has been filtered
If the filtration: plasma ratio is 0.5 what does this show?
Half of the plasma has been filtered
What 3 things determine what is filtered?
Molecular size
Charge
Shape
How does molecular size affect filtration?
As the mass of the molecule increases the likelihood of filtration decreases
At 70,000 Daltons it will not be filtrated
How does charge affect filtration
Negatively charged molecules are less likely to be filtered
- Tested this by removing the negative charge of dextran molecule
This becomes insignificant if the molecule is very small
What is the filtration co-efficient?
Kf
A measure of the permeability of the filtration barrier
What are starling forces?
GFR alpha = forces favouring – forces opposing = (Pcap + OPbc) – (Pbc + OPcap) - P = hydrostatic pressure - OP = oncotic pressure - cap = capillary - bc = Bowman’s capsule
Which two pressures favour filtration?
Hydrostatic pressure capillary
Oncotic pressure in Bowmans capsule
Summary of forces in the Bowmans capsule?
Volume of fluid is constant
Pressure is constant
Summary of forces in the glomerular capillary?
Hydrostatic pressure averages at 60 mmHg
- Along the capillary as plasma volume decreases as does the pressure
Oncotic pressure starts at 20 mmHg and rises to 30 mmHg
- Determined by the concentration of proteins
- Losing fluid from the capillary increases protein concentration
GFR equation
GFR
= Kf (Pcap + OPbc)–(Pbc + OPcap)
= 125 ml/min
What is the single nephron GFR?
50 nl/min
What helps in autoregulation?
Afferent arteriole resistance?
What series of events happens after an increase in arterial BP?
Increase RBF
Increase GFR
Autoregulation increases resistance afferent arteriole Decrease RBF and Pcap
Decrease GFR
What series of events happens after a decrease in arterial BP?
Decrease RBF
Decrease GFR
Autoregulation decreases resistance afferent arteriole Increase RBF and Pcap
Increase GFR
Autoregulation of GFR - what is the myogenic theory?
Response is a property of afferent arteriole smooth muscle
- Increase arterial BP
- Stretches afferent arteriole
- Afferent arteriole constriction
- Increase afferent arteriole resistance
Autoregulation of GFR - what is the tubuloglomerular feedback theory?
Response is a property of juxtaglomerular apparatus
Macula densa cells in the distal tubule
- Have cilia that detect the rate of flow of tubular fluid
- Release vasoactive chemicals
What do macula densa cells do in response to and increase in GFR and flow?
Release vasoconstrictors
Leads to afferent arteriole constriction
