Glomerular Filtration Flashcards
How many litres a day of filtrate is filtered in the glomerulus?
180l/day
How many times a day does the total plasma pass the filtration barrier?
65
Describe the structure of the glomerulus
Afferent arteriole moves into the Bowman’s capsule where it forms a capillary bed for filtration to occur, then filtered blood leaves through the efferent arteriole
What is the filtration barrier?
A highly specialised blood filtration interface that is permeable to small solutes but relatively impermeable to macromolecules
What are the components of the filtration barrier?
Endothelial cells, basement membrane and podocytes
What is the main function of the endothelial cells of the filtration barrier?
To filter blood cells and platelets
What is the main function of the basement membrane of the filtration barrier?
It is the main barrier for filtration. It is made up of glycoproteins such as collagen, laminin and fibronectin that create a mesh to act as a filtration barrier (have -ve charge)
What is the main function of podocytes of the filtration barrier?
They provide structural support from trabeculae and pedicles. Also play a role in phagocytosis. Filtrate moves through slit pores
What are the molecular determinants of filtration?
Size, shape and charge
Discuss the molecular determinants of filtration
As molecular weight increases, filtration decreased
-ve charged molecules are less likely to be filtered due to -ve charge of BM except when size of molecule is very small
What is Starling’s hypothesis?
It states that the fluid movement across the wall of the capillary is dependent on the balance between the hydrostatic pressure gradient and the oncotic pressure gradient across the capillary
What is the equations for Starling’s forces?
GFR is proportional to (Pcap + OPbc) - (Pbc + OPcap)
Why does the oncotic pressure in the bowman’s capsule not to be considered in the calculation of the GFR?
Very few proteins are filtered into the glomerular filtrate so the number is so small it doesn’t need to be considered
How does the Pbc change down the capillary?
It’s a constant - stays at about 20mmHg
How does the Pcap change down the capillary?
It starts at just over 60mmHg and ends at just under 60mmHg, therefore averages at 60mmHg
How does the oncotic pressure in the capillary change down the capillary?
It starts at around 25mmHg and ends at about 35mmHg, average of about 30mmHg
How does the net filtration rate of all other capillaries differ from glomerular capillaries?
In glomerular capillaries the net filtration rate starts at approx 15mmHg and ends at approx 5mmHg
In other capillaries, about half way down, filtrate begins to move back in to the capillary
Fluid never moves back in to glomerular capillaries
Discuss the process of autoregulation when blood pressure increases or decreases
Increased arterial blood pressure: increased renal blood flow therefore increased Pcap and increased GFR. This stimulates autoregulatory mechanisms. Resistance of the afferent arteriole is increased, RBF and Pcap decrease, decreasing GFR
Decreased arterial blood pressure: decreased renal blood flow therefore decreased Pcap and decreased GFR. This stimulates autoregulatory mechanisms. Resistance of the afferent arteriole is decreased, RBF and Pcap increase, increasing GFR
What are the theories thought to underpin autoregulation?
Myogenic theory and tubuloglomerular feedback theory
Discuss the myogenic theory
Changes in pressure inhibit or stimulate stretch receptors which signals the muscle to contract/relax
Discuss the tubuloglomerular feedback theory
Macula densa cells have cilia that detect flow of tubular fluid. A change in the rate of fluid flow leads to the release of vasoactive chemicals (vasodilators and vasoconstrictors)
