Module 4 Section 3 (Glomerular Filtration) Flashcards
Describe the forces that regulate glomerular filtration.
Glomerular Capillary Blood Pressure:
- This is the pressure exerted by the blood in the glomerular capillaries.
- Regular capillaries have a BP of about 18 mmHg
- Glomerular capillary pressure is on average 55 mmHg
• This is b/c of the afferent arteriole diameter being larger than the diameter of the efferent arterioles, which incr resistance to blood leaving the glomerular capillaries.
• This also prevents glomerular capillary pressure from decreasing along their length, further favouring filtration.
Plasma-Colloid Oncotic Pressure:
- The presence of large proteins in the plasma that can’t be filtered produces a oncotic force that resists the movement of water into Bowman’s capsule.
- The pressure is about 30 mmHg.
Bowman’s Capsule Hydrostatic Pressure:
- This is the pressure of the fluid in Bowman’s capsule and it also resists the movement of water out of the glomerular capillaries.
- The pressure is around 15 mmHg.
- Thus, net filtration pressure = the glomerular capillary BP - the sum of the plasma-colloid oncotic and Bowman’s capsule hydrostatic pressures.
- Net Filtration Pressure = 55 mmHg -(30 mmHg + 15 mmHg) = 10 mmHg
Describe how glomerular filtration is regulated by the body, both intrinsically and extrinsically.
B/c changes in GFR are directly proportional to glomerular capillary BP, autoregulatory (or intrinsic) mechanisms are in place to prevent sudden swings in GFR
- This is primarily done by regulating the diameter of the afferent arterioles (constricting the afferent arterioles will decr glomerular capillary BP and dilating the afferent arterioles will incr it
Intrinsic:
1) Myogenic activity
- When an incr pressure stretches the afferent arteriole walls, they automatically constrict to reduce blood flow to the glomerular capillaries, thus preventing an incr in GFR
- The opposite is also true: if BP decr, the afferent arterioles will dilate to incr blood flow and prevent a decr in GFR
2) Tubuloglomerular feedback (TGF)
- The juxtaglomerular apparatus is the area of the distal tubule that passes through the region where the afferent and efferent arterioles join the glomerulus.
• It’s a combo of vascular and tubular cells.
- Specialized tubular cells in this area are collectively called = macula densa (can sense changes in the salt level of the tubular fluid)
• If there is an incr arterial pressure that increases the GFR, more fluid than normal will flow through the distal tubule
• This also means there is an incr salt delivery.
• In response, the macula densa releases ATP, which is degraded to adenosine
• This adenosine acts on the afferent arterioles to cause constriction and reduce GFR
• The opposite is also true
Extrinsic:
1) Sympathetic control of GFR
- It innervates the afferent arterioles
- Ex: a haemorrhage (sudden loss of blood volume followed by a drop in arterial pressure) would initiate responses to normalize BP
- At the level of the kidney, this incr sympathetic activity would constrict the afferent arterioles, which would decr glomerular capillary pressure, decr GFR, reducing urine production
- This is the mechanism by which depleted plasma volumes can be corrected
** Check chart on slide 12 **
Explain why the kidneys receive a greater proportion of cardiac output, relative to its weight.
The total blood flow to the kidneys is 625 mL/min
- Considering, only 55% of whole blood is filterable plasma, we can adjust renal blood flow to 1140mL/min
- Since total CO = 5000mL/min at rest, we can now calculate that the kidneys receive around 22% of the total CO
• This proportion far exceeds what would be expected based on tissue size (kidney weight is only about 1% of total body weight)
** Check chart on slide 13 **
What is the glomerulus?
The glomerulus is a network of capillaries located at the beginning of a nephron.
Discuss the process of blood flow in relation to the glomerulus.
1) Blood is filtered across the walls of the glomerulus (capillary network) through the glomerular membrane, which then yields its filtrate into the Bowman’s capsule.
2) The filtrate then enters the renal tubule of the nephron
3) Blood is received into the glomerulus via afferent arterioles
4) Blood exits through the efferent arterioles
Where does the glomerulus receive its blood supply from? Where does it exit into?
It receives its blood supply from an afferent arteriole
The glomerular capillaries exit into the efferent arterioles
What is the glomerular filtration rate (GFR)?
It’s the rate at which blood is filtered through all of the glomeruli
It’s also the measure of the overall renal function
In order for blood to be filtered, the fluid must pass through 3 layers that make up the glomerular membrane. What are these layers?
1) The glomerular capillary wall:
- It consists of a single layer of endothelial cells
- It also contains many large pores that make it 100x more permeable to fluids and solutes than regular capillaries.
• The pores are of such size that large plasma proteins can’t pass through, but smaller ones, such as albumin, can
2) The basement membrane:
- This layer contains no cells and is composed of collagen to provide structural strength, and glycoproteins to discourage the filtration of small plasma proteins
- B/c the glycoproteins are -, they help to repel any proteins that do get through the capillary walls
- Only about 1% of filtered albumin will pass into Bowman’s capsule.
3) The inner layer of Bowman’s capsule:
- This layer is composed of podocytes that form narrow filtration slits b/w them that allow fluid to pass into Bowman’s capsule
What are podocytes?
These are cells that wrap around the capillaries of the glomerulus
How is the glomerular filtration rate regulated?
GFR is dependent upon filtration pressure and the glomerular surface area available and how permeable the membrane is
- Collectively, these properties are called the filtration coefficient (Kf)
- Filtration Coefficient (Kf) x Filtration Pressure = Glomerular Filtration Rate
• Average male = 125mL/min
• Average female = 115 mL/min
Normally plasma-colloid osmotic pressure and Bowman’s capsule hydrostatic pressure do not vary much and are essentially considered constants. However, both of these values can change due to pathological conditions (someone w/ a kidney stone that obstructs the ureter will have an increased Bowman’s capsule hydrostatic pressure, again decreasing GFR).
Using what you know now about how GFR can be decreased, and the example about kidney stones, think of another way by which GFR could be decreased.
Someone with severe diarrhea will be dehydrated, as they are losing more fluid than they are taking in.
This results in decr BP due to the decr plasma volume. There is also an incr in plasma-colloid osmotic pressure that results in a decr GFR.
Discuss controlled changes in the GFR.
Net filtration pressure equation:
- Glomerular Capillary Blood Pressure -(Plasma-colloid Oncotic Pressure + Bowman’s Capsule Hydrostatic Pressure) = Net Filtration Pressure
- Even though plasma-colloid osmotic pressure and Bowman’s capsule hydrostatic pressure generally don’t change, glomerular capillary blood pressure can be regulated.
Glomerular filtration rate equation:
- Filtration Coefficient (Kf) x Net Filtration Pressure = Glomerular Filtration Rate (GFR)
As you can see from these two equations, changes in GFR are proportional to changes in glomerular capillary blood pressure.
Using what you have learned thus far about the autoregulation of GFR, discuss what you believe would occur to the glomerular capillary BP, net filtration pressure, and the glomerular filtration rate if there was vasoconstriction in the afferent arteriole? What would occur if there was vasodilation of the afferent arteriole? Explain your reasoning.
Recall that net filtration pressure is proportional to the glomerular capillary pressure, as both the plasma-colloid oncotic and Bowman’s capsule hydrostatic pressures are nearly constant. Also recall that the glomerular filtration rate (GFR) is = to the filtration pressure x the filtration coefficient.
- Therefore, any change in glomerular capillary pressure is proportionally reflected in both the filtration pressure and glomerular filtration rate.
In the case of vasoconstriction of the afferent arteriole, there would be a decr in glomerular capillary BP, a decr in net filtration pressure, and a decr in glomerular filtration rate.
In the case of vasodilation of the afferent arteriole, there would be an incr in glomerular capillary BP, an incr in net filtration pressure, and an incr in glomerular filtration rate.
In a healthy person, about 20% of the plasma enters the kidneys and becomes the glomerular filtrate. What does this mean in terms of GFR and total blood volume to the kidneys??
This means that if GFR = 125 mL/min, the total blood flow to the kidneys must be 5 x 125 (625 mL/min)
What do you think is the functional significance of the kidneys receiving such a high proportion of total CO?
This large proportion of CO signifies the importance of the kidneys! Its primary purpose is not to deliver O2 and nutrients, but rather to deliver blood for “cleaning”. Delivery of this large volume of blood also allows the kidneys to maintain tight control of volume and electrolyte conc of the body’s water pools and to eliminate wastes efficiently.
