Week 2 - B - Physiology 3(A) - Starling forces and GFR Flashcards
What is the rate of excretion equal to in the kindeys?
The rate of excretion is equal to = Rate of glomerular filtration (GF) + Rate of tubular secretion (TS) - rate of tubular reabsorption (TR)
What percentage of the cardiac output flows through the kidneys? What percentage of the plasma entering the kidney is filtered into the proximal tubule?
25 % of the cardiac output goes to the kidneys Only 20% of the plasma entering the kidney is filtered, the rest of the 80% is passed into the efferent arteriole
What drains into the glomerulus and what does the glomerular capillaries drain into? Which has a bigger diameter?
Affernet arteriole drains into the glomerular capillaries which drains into the efferent arteriole Afferent arteriole has a greater diameter
The pores found between the endothelial cells of the glomerular capillaries are about 100x bigger than the normal endotherliar pores What are the three filtration barries between the glomerular capillaries and the lumen of the bowman’s capsule? What are the 3 layers collectively known as?
Glomerular capillary endothelium (fenestrated) Basement membrane (basal lamina) Foot process of podocytes with intermittent filtration slits Known as the glomerular membrane
The basement membrane is a plasma protein barrier made up of negative proteins What does it function to repel?
Repels large negative plasma proteins such as albumin
The net filtration pressure is composed of hydrostatic and oncotic pressures What is the balance of the hydrostatic and oncotic pressures known as?
This is known as the Starling Forces
What are the 4 starling forces contributing to the net filtration pressure in the kindeys?
Glomerular capillary blood pressure Bowman’s capsule hydrostatic (fluid) pressure Glomerular capillary oncotic pressure Bowman’s capsule oncotic pressure
What is the most important starling force? What is the pressure of this force?
The glomerular capillary blood pressure (pressure = 55 mmHg)
What does the glomerular capillary blood pressure favour?
It favours the process of filtration in the blood
The efferent arteriole is thinner than the afferent arteriole and therefore there is a backflow of pressure which is why it is important for the glomerular capillary blood pressure to exist There is also a build up of pressure in the bowman’s capsule, what is this known as and what does it cause?
This is the Bowman’s capsule hydrostatic (fluid) pressure which opposes filtration
What is the value of the Bowman’s capsule hydrostatic pressure?
Valued at 15mmHg
What is the other force which opposes filtration known as? What is the value of its pressure?
This is the Glomerular capillary oncotic (colloid osmotic) pressure - 30mmHg
What is the final starling force and what is it value?
This is the Bowman’s capsule oncotic pressure = 0mmHg
What is the oncotic pressure caused by and what does it cause? hence why is the bowman’s capsule oncotic pressure 0?
It is related to the presence of plasma proteins (notibly albumin) and causes the pull of water into the circulatory system (will pull into the the fluid around if there are proteins in the bowman’s capsule) Since there is not meant to be plasma protein in bowmans capsule - the pressure is 0
What can preoteinuria cause due to the decreasing Oncotic pressure to pull fluid (water) into the blood?
The loss of proteins can cause oedema as there is not as strong an oncotic pressure and therefore fluid leaks into adjacent tissues
State the pressures for each of the Starling forces
Glomerular capillary blood pressure = 55mmHg Bowman’s capsule hydrostatic pressure = 15mmHg Glomerular capillary oncotic pressure = 30mmHg Bowman’s capsule oncotic pressure = 0mmHg
What is the overall Net filtration pressure?
Net filtration pressure = forces favouring filtration - opposing filtration forces = (55+0) - (15+30) = 10
A normal GFR (glomerular filtration rate) is around 125mls/min What is the cut off for normal?
Normal is believed to be an eGFR of at least 60ml/min
Define the glomerular filtration rate?
GFR = rate at which protein-free plasma is filtered from the glomeruli into the Bowman’s capsule per unit time.
What is the major determinant of eGFR?
The glomerular capillary blood fressure (favours filtration) (BPgc)
GFR = Kf × net filtration pressure Kf is the filtration coefficient, what does this mean?
Filtration coefficient is a measure of the permeability of the glomerular membrane
Regulation of renal blood flow and glomerular filtration rate is regulated by what?
Extrinsic regulation fo GFR and Autoregulation of GFR (intrinsic)
What is the extrinsic regulator of GFR?
This is the sympathetics from the barorecptor reflex
If there is an increased arterial blood flow into the glomerulus, what normally happens?
This causes an increased glomerular capillary blood pressure causing increased NFP leading to an increased GFR (GFR = Kf x NFP)
Where are the baroreceptors located and what is their nerve innervation?
Aortic arch - vagus nerve (CN X) Carotid sinus - glossopharyngeal nerve (CN IX)
What do the baroreceptors do in response to a low blood volume eg due to haemorrhage?
* Fall in blood volume causes a decrease in arterial pressure * This leads to an increased sympathetic activity by the barorecptors to increase heart rate and blood volume * This causes generalised arteriolar vasoconstriction including the afferent arteriole * This causes a decreased BPgc - decreased NFP and a decreased GFR which decreases the urine production to stop fluid loss
Where do the baroreceptors fire to to cause the increase in sympathetic activty in low blood volume?
Relay the information to the medulla which will cause an increase in sympathetics to the heart in low blood volume
What prevents short term changes in systemic arterial pressure affecting GFR? eg during exercise have a greater MABP
Autoregulation
Autoregulation is intrinsic to the kindeys What are the two forms of autoregulation?
Myogenic and tubuloglomerular feedback
How does the myogenic response in autoregulation work? Myogenic - originating in muscle tissue (rather than from nerve impulses).
It works due to when vascular smooth muscle is stretched (eg by arterial blood pressure increasing), it contracts thus constricting the arteriole
How does tubuloglomerular feedback work?
When GFR rises, this increases the salt (NaCl) output into the tubule, this is sensed at the distal convoluted tubule by the macula densa cells which release vasocative mediators to bring about contraction of the afferent smooth muscle and therefore maintaining the GFR
If there is an increase in hydrostatic pressure of bowman’s capsule, say due to a kidney stone, what effect will this have on GFR?
This will cause increased opposition of filtration decreasing the net filtration pressure and causing a decreased GFR
Say a person suffers from diarrhoea If you become dehydrated, plasma proteins become more concentrated as a result What happens now?
Because the plasma proteins are more concentrated, this increases the Glomerular capillary oncotic pressure leading to a greater pull of fluids into the plasma decreasing the NFP and GFR
If we lose plasma proteins in an eg severely burned patient – this will decrease capillary oncotoic pressure leading to an increase in the GFR If the surface area for filtration decreases, what happens?
This will decrease the permeability of the glomerular membrane as there is less area of it decreasing the filtration coefficient which will lead to a decreased GFR
