Filtration Flashcards
What do the kidneys filter?
Blood
What is the main function of the kidneys?
To maintain homeostasis of the body
Where does filtration occur within the kidneys?
Glomerular capillaries
What is the purpose of filtration?
To form an essentially protein-free filtrate of plasma
How much is the Glomerular Filtration Rate (GFR)?
180L/day
What % of the cardiac output do the kidneys receive?
20-25%
How long does it take for total blood volume to pass through the kidneys?
Approx. five minutes
Important to note that none of the RBC’s or WBC’s and only a fraction of the plasma are filtered into the Bowman’s capsule.
Where does the rest of the plasma pass?
Passes via efferent arterioles into peritubular capillaries and then into the renal vein
What is glomerular filtration dependant on the balance of?
The balance between hydrostatic forces favouring filtration and oncotic pressure forces favouring reabsorption
RECAP- what % of total blood volume does plasma make up?
55%
The glomerular barrier is selectively permeable. What factors determine if a substance can cross?
Molecular size
Electrical charge
Shape
List some substances which are freely filtered by the glomerular barrier.
Sodium, potassium. chlorine, water, urea, glucose, sucrose, polyethylene-glycol
Give some examples of substances which cannot pass through the glomerular barrier.
Haemoglobin
Serum albumin
There are three layers of filtration in the glomerular membrane.
What are the three layers?
- Fenestration of endothelial cells
- Basal lamina of glomerulus
- Slit membrane between pedicels
What is the function of the fenestration of endothelial cells as a filter layer?
Prevents filtration of blood cells but allows all components of blood plasma to flow in
What is the function of the basal lamina as a filter layer?
Prevents filtration of larger proteins
What is the function of the slit membrane as a filter layer?
Prevents filtration of medium size proteins
Describe the structure of afferent arterioles.
Short and wide
Describe the structure of efferent arterioles.
Long and narrow
-> think a before e in alphabet so longer way through alphabet, longer structure
Why is glomerular capillary pressure higher than most of the other capillaries in the body?
Arrives in afferent arterioles which are short and wide and blood arriving at the glomerulus still has a high hydrostatic pressure
Regarding the golden rule of circulation, what happens to hydrostatic pressure if there is high resistance?
Upstream- increased pressure
Downstream- decreased pressure
Does hydrostatic pressure favour filtration or reabsorption?
Filtration
Does oncotic pressure favour filtration or reabsorption?
Reabsorption
What happens to oncotic and hydrostatic pressure as blood passes through the glomerulus?
Oncotic pressure increases but never exceeds high hydrostatic pressure
->as hydrostatic pressure favours filtration
What is the only process which occurs at the glomerular capillaries?
Filtration
What is the net filtration pressure?
10 mm Hg
-> this is because hydrostatic pressure is usually around the range of 55mmHg and then oncotic pressure (30mmHg) and fluid pressure (15mmHg) counteract so….
50-30-15=10
idk if that makes sense sorry gal
In normal physiology, what affects glomerular filtration rate?
Afferent and efferent arteriolar diameter and the balance of resistance between them
However, GFR can be affected by extrinsic factors like what?
- Sympathetic nerves
- Circulating catecholamines
- Angiotensin II
Which are more sensitive to sympathetic nerves causing constriction; afferent or efferent arterioles?
Afferent
Give some examples of catecholamines and what their function is.
Adrenaline, noradrenaline
Act as vasoconstrictors
What role does angiotensin II have on GFR?
Vasoconstrictor which works at low concs. on efferents and high concs. on both efferent and afferent
Renal vasculature also exhibits a well-developed intrinsic ability to adjust resistance in response to changes in arterial blood pressure.
This essentially keeps blood flow and GFR constant.
What is this known as?
Autoregulation
Okayy imma reverse the question- what is autoregulation regarding filtration?
Renal vasculature has intrinsic ability to adjust resistance in response to changes in arterial blood pressure.
This essentially keeps blood flow and GFR constant.
In humans, when BP is between 60-130, what happens to rate of glomerular filtration?
No changes as normal BP
In humans, when BP is below 60, what happens to rate of glomerular filtration?
Filtration would decrease
->if under 50, filtration stops
What happens if there is an increase in mean arterial pressure?
Automatic increase in afferent arteriolar constriction, preventing a rise of glomerular capillary pressure
->another form of autoregulation
Does autoregulation depend on anything?
No, independent of nerves or hormones
->occurs in denervated and isolated perfused kidneys
In situations where blood volume or pressure face serious compromise, e.g. haemorrhage, what happens?
Activation of sympathetic nerves and autoregulation can be overridden
->this means blood goes to more important organs
However, while reduction in renal blood flow can supply organs with as much as 800ml /min, what happens if renal blood flow is reduced for a prolonged period of time?
Irreparable damage which can lead to death
100% of plasma volume enters the glomerular capillaries via which type of arteriole?
Afferent arteriole
20% of the plasma volume is filtered out, what happens to the rest?
Enters efferent arteriole and passes onto peritubular capillaries
Out of 20% of plasma which is filtered out, how much is reabsorbed?
19%
What happens to the 1% of plasma which is fiiltered?
Excreted
Which type of capillary is responsible for reabsorption?
Peritubular capillaries
