L4: glomerular filtration Flashcards
Basic kidney anatomy
1: 20
- covered by a transparent fibrous renal capsule
- outer layer = cortex
- inner layer = medulla
- renal pyramids - 5-8
- renal columns - contain the major blood vessels
- renal papilla - all urine filled collecting ducts empty into these
- renal pelvis - urine to ureter
- ureter - urine to bladder
Basic anatomy of nephron
- nephrons are hollow tubes - closed at the proximal end and open at the distal end
- each functions as an independent unit and produces a minuscule quantity of urine
- 1.25 million nephrons per kidney
- 3-4mm long
Regions of the nephron
- renal corpuscle - bowman’s capsule and glomerulus (capillaries)
- tubule - PCT, loop of Henle, DCT
- collecting tubule
Anatomy of the bowman’s capsule/glomerulus
- arriving afferent arteriole becoming a capillary which coils inside the bowman’s capsule to leave as an efferent arteriolar
- coiled capillary = glomerulus
- visceral layer of BC - inner wall of BC whither lines glomerular capillaries
- parietal layer of BC - outer wall of BC
- the next part of the nephron is PCT fro, bowman’s capsule
Glomerular filtration - stage 1 of urine formation
- blood enters the glomerulus which are coiled networks of capillaries with large pores
- plasma filters out of capillaries into the Bowman’s space
- blood cells and proteins do not pass into the Bowman’s space but almost everything else does - plasma, low molecular weight substances, electrolytes etc
What are podocytes?
They are highly specialised epithelial cells of the visceral layer of the BC that wrap themselves around the glomerular capillaries.
Pic at 18:30
What is the 3 part filtration barrier of BC?
Pic at 21:20
- endothelial cells - fenestrated cells lining the glomerular capillaries that have large pores. Lots of plasma forced out through these due to high hydrostatic pressure.
- basement membrane - a fibrous layer that is uncharacteristically thick (100-200nm) compared to most BMs (40-50nm). It excludes proteins on the basis of their size and negative charge as the BM has negative charged proteoglycans on it. Dry small molecules can easily enter the BM regardless of charge, but repulsive or attractive forces reduce or facilitated the rate of movement.
- podocytes - epithelial cells of the bowman’s capsule visceral layer that have interdigital into processes and cover the entire glomerular area.
How is pressure generated in the glomerulus ?
Filtration occurs because of high pressure in the glomeruli.
The wide afferent arteriolar and narrower efferent arteriolar generates a high hydrostatic pressure in the capillaries which favours net filtration of plasma into the nephron.
Starlings forces - pressures on fluid in normal capillaries NOT GLOMERULAR
CHP at arterial end = 35 mmHg
COP at arterial end = 25mmHg
So net flow of fluid out of arterial end of capillary
CHP at venule end = 15 mmHg, because fluid is lost at the arterial end, so pressure drops
COP at venule end = 25mmHg, oncotic pressure is fairly constant.
So net flow of fluid into the venule end of the capillary.
Surrounding tissue outside of capillary also has its own hydrostatic pressure, IHP. Very low since fluid in interstitium is dispersed.
Also has its own oncotic pressure IOP, which can try and drawn fluid out but is not as powerful as the capillary proteins, so again a low value. Negligible but included for completion sake.
Starlings forces prevent the accumulation of fluid outside the capillaries. This is not wanted in the glomerulus though.
(Table summarising at min 2 of part 2)
Starlings forces in the glomerulus
Net filtration pressure favours outward fluid movement.
- glomerular hydrostatic pressure = 50mmHg (bigger than normal capillary)
- glomerular oncotic pressure is 25mmHg due to having the same proteins etc passing through the blood
- BCs hydrostatic pressure = 15mmHg, wants to move fluid back into glomerulus
- BCs oncotic pressure = 0mmHg due to no proteins in the bowman’s capsule
50-25-15 = 10mmHg outwards pressure
Composition of glomerular filtrate
Composition of glomerular filtrate is identical to plasma composition with respect to water, electrolytes, glucose and waste products.
Very little protein in glomerular filtrate however.
So sodium, potassium, chloride, bicarbonate, glucose, urea, uric acid and creatinine should all be the same or very similar.
70,000Da is the cut-off for the filtration barrier. Albumin has a molecular weight of 69,000Da but is not filtered, why?
Not filtered and kept out of the filtrate due to its negative charge and so is repelled by the basement membrane of the BC.
Major causes of proteinuria
Proteinuria = protein in urine due to disruption of the filtration barrier
- diabetes mellitus - excessive plasma glucose damages filter - corrodes endothelium
- hypertension - even higher glomerular hydrostatic pressure damages the filter
- glomerulonephritis - inflammatory damage due to immunological attack
Effects of decreased plasma protein levels on fluid movement across capillaries
(Pt2 min 5)
If there are capillaries in the kidneys which are damaged and allow proteins to leave them, the oncotic pressure (COP) will be decreased. So less fluid will be drawn back into the capillaries, so interstitial fluid volume increases, leading to oedema.
What can excessive proteinuria cause?
Low level of protein in the plasma which can lead to oedema.
Can cause bubbly or frothy urine.
Swollen around eyes and swollen hand and feet.