Lecture 3 - Filtration Flashcards
What does filtration in the kidneys essentially produce?
A protein free filtrate of plasma
What is the GFR?
Glomerular filtration rate
What is the average GFR a day?
180L/day
What are the 3 main processes that go on in the kidneys?
Filtration
Reabsorption
Secretion
How much blood flows through the kidneys every min?
~1200mls (20-25% of CO)
In less than how many minutes does the blood volume equal to the total BV pass through the kidneys?
<5 mins
How many red bloods are filtered through Bowman’s capsule?
NONE!!
Most of the plasma and all the RCCs pass into efferent arterioles and into the peritubular capillaries, then into the renal vein
What is the normal GFR?
125mls/min
How do you work out renal plasma flow?
Plasma is ~55% of total BV
55% of 1200mls/min = 660mls/min
What is the filtration fraction?
Is the amount of renal plasma that becomes glomerular filtrate
What is glomerular filtration dependent on?
Balance between hydrostatic forces favouring filtration and oncotic pressure favouring reabsorption
What things determine the filterability of solutes across Bowmen’s capsule?
Molecular size
Electrical charge
Shape
What are the three layers of the filtration membrane in the kidneys?
- Fenestrated epitheliium
- Basal lamina of glomerulus
- Slit membrane between pedicles
What does fenestrated epithelium prevent?
Filtration of RCCs
What does the basal lamina prevent?
Filtration of larger proteins
What does the slit membrane between pedicles prevent?
Filtration of medium sized proteins
Why is glomerular capillary pressure greater than most of the capillaries in the body?
Afferent arteriole is short and wide and offers little resistance to flow (so blood arriving at glomerulus has high hydrostatic force)
Efferent arteriole long and narrow + so offers high post capillary resistance
What is the golden rule of circulation when it comes to hydrostatic pressure?
If you high resistance, hydrostatic pressure upstream is increased while pressure downstream is decreased
Why is it essential that hydrostatic pressure is greater than oncotic pressure at the glomerulus?
So that only filtration occurs at the glomerular capillaries
What is the primary factor affecting the GFR in physiology?
The glomerular capillary pressure which is determined by the diameter of the afferent and efferent arterioles
How does sympathetic stimulation affect the afferent and efferent arterioles?
Constricts both but greater sensitivity of afferent arteriole
How do circulating catecholamines affect the afferent and efferent arterioles?
Constrict primarily the afferent arteriole
How does angiotensin II affect the afferent and efferent arterioles?
Low levels - efferent only
High levels - efferent and afferent
Describe the effect on GFR in the following scenario:
Afferent arteriolar constriction
Decreased GFR
Describe the effect on GFR in the following scenario:
Efferent arteriolar constriction
Increased GFR
Describe the effect on GFR in the following scenario:
Afferent arteriolar dilatation
Increased GFR
What is the process of the renal vasculature instrinsically adjusting its resistance in response to BP?
Autoregulation
Over what MBP is autoregulation effective?
60-130mmHg
below this filtration falls and ceases altogether below 50mmHg
What happens in the kidneys if mean arterial pressure increases?
Automatic increase in arterial arteriolar constriction preventing a rise in PGC
(also dilates if BP falls)
Does autoregulation of the kidneys rely on nerves or hormones?
Neither
In which situations may autoregulation of the kidneys be overriden? Why is this? How is it done?
Where BP faces serious compromise, e.g. haemorrhage
Activates sympathetic nerves which liberates more blood for immediately more improtant organs
What can prolonged reduction in renal blood flow lead to?
Irreparable damage which may lead to death due to disruption in kidneys role in haemostasis
What is the hydrostatic pressure in the peritubular capillaries like?
Hydrostatic pressure is very low as the efferent arteriole offers resistance along its entire length
So this favours reabsorption in the peritubular capillaries
Where is >99% of the filtrate reabsorbed?
Peritubular capillaries
Why is oncotic pressure higher in the efferent tubule and peritubular capillaries higher?
20% of plasma has been filtered out so this concentrates plasma proteins
What % of plasma is filtered out in the Bowmen’s capsule?
20%
Why is reabsorption favoured in the peritubular capillaries?
As oncotic pressure is greater than hydrostatic pressure
What percentage of: Water Glucose Na Urea Filtered at the glomerulus are reabsorbed within the tubule (mainly PCT)?
Water - 99.5%
Glucose - 100%
Na - 99.5%
Urea - 50%
