Renal Physiology (Glomerular Filtration)

What is glomerular filtration?

"Non-selective" filtering of everything except plasma proteins and blood cells.

What are the (2) important differences between capillaries in the glomerulus and capillaries elsewhere in the body?

1. Glomerular capillaries are much more permeable than capillaries elsewhere. 2. Filtration occurs along the entire length of the capillary

What are the (3) forces involved in Glomerular filtration?

1. Glomerular capillary blood pressure. 2. Plasma-colloid osmotic pressure. 3. Bowman's capsule hydrostatic pressure.

What are the (2) mechanisms that contribute to autoregulation?

1. Myogenic mechanism. 2. Tubuloglomerular feedback mechanism.

What (3) things does glomerular filtration rate depend on?

1. Net filtration rate. 2. Amount of glomerular capillaries involved. 3. Capillary permeability.

To be filtered, a substance must pass through what (3) of a glomerular membrane?

1. Pores between endothelial cells of glomerular capillary. 2. Acellular basement membrane. 3. Filtration sites between foot processes of podocytes.

What are the (3) layers of the glomerular membrane?

1. Wall of glomerular capillary. 2. Acellular layer (basement membrane). 3. Inner layer of Bowman's capsule.

How much more is the glomerular membrane more permeable to substances than any other capillaries in the systemic system of the body.

100 times.

What is the normal pressure that 20% of the plasma entering the glomerulus is filtered at?

10mm Hg.

What value is the force opposed from hydrostatic pressure in Bowman's capsule?

15 mmHg.

How much of the cardiac output do kidneys receive?

20-25%.

What value is the force opposed from Plasma-colloid Osmotic Pressure?

30 mmHg.

What is the average value of glomerular capillary blood pressure?

55 mmHg.

What occurs to the following factors if the arterial blood pressure was increased: - Glomerular capillary blood pressure? - Net filtration pressure? - GFR?

Glomerular capillary blood pressure - increased. Net filtration pressure - increased. GFR - increased.

What is the resistance (low/high) in the efferent arteriole of the glomerulus?

High resistance.

In regards to the myogenic mechanism, when does the afferent arteriole automatically constrict?

In response to being stretched by an increase in blood pressure.

If arterial pressure rises, how can the net filtration pressure and GFR be reduced to normal?

Vasoconstriction of afferent arteriole.

If arterial pressure falls, how can the net filtration pressure and GFR be reduced to normal?

Vasodilation of afferent arteriole.

During auto regulation what does the kidneys (to a degree) maintain (2)?

- Constant blood flow into glomerular capillaries. - Constant capillary blood pressure. = Contant GFR.

What are the (2) mechanisms that control glomerular filtration rate?

1. Autoregulation (prevents spontaneous change). 2. Extrinsic sympathetic control (long term regulation of blood pressure).

What is the term that describes the layers of the glomerular membrane acting as a whole?

Acts as a 'molecular filter'.

What is the anatomy of the glomerular membrane?

Are highly adapted to filtering as they have additional holes in membrane in which substances can pass between capillary and tubule section.

How is the plasma volume restored?

Baroreceptor reflex triggers vasoconstriction in afferent arteriole leading into glomerulus decreasing GFR = decreased urine volume.

Why is the glomerular blood pressure higher than capillary pressure else where?

Because the diameter of the afferent arteriole (leading into the glomerulus) is larger than the diameter of the efferent arteriole (leading out of the glomerulus).

What is the purpose of high resistance in the efferent arteriole of the glomerulus?

Blood pressure does not decline along the length of the glomerular capillaries. This force tends to push fluid out of the capillaries (glomerulus) into the Bowman's capsule.

What is the driving force for glomerular filtration?

Blood pressure in the capillaries of the glomerulus.

What is the outline of the extrinsic sympathetic control of GFR?

Decrease in plasma volume results in a decrease in blood pressure detected by baroreceptors initiating reflexes co-ordinated by cardiovascular control centre in the brain.

What is the function of the macula densa cells?

Detect changes in the rate fluid that flows over them in the tubule.

What forces favour (1) and oppose (2) filtration of the glomerulus?

Favor = Glomerular capillary blood pressure. Oppose = Plasma-colloid Osmotic Pressure (Oncotic) and Hydrostatic pressure in Bowman's capsule.

What is glomerular capillary blood pressure?

Fluid pressure exerted by the blood within the glomerular capillaries.

What is the result of extrinsic sympathetic control of GFR?

Increase in total peripheral resistance and cardiac output BUT plasma volume still reduced.

If blood pressure was the only variable that changed then: - Increased blood pressure = __ GFR. - Decreased blood pressure = __ GFR.

Increased blood pressure = Increased GFR. Decreased blood pressure = Decreased GFR.

Where are the podocytes of a glomerular membrane found?

Inner layer of Bowman's capsule.

What is the result of a decreased GFR on macula densa cells?

Less fluid than normal is filtered - release vasodilator chemicals (e.g. bradykinin).

What is the major determinant of altering the glomerular filtration rate?

Major determinant is the regulation of glomerular capillary blood pressure.

In males and females what is the amount of plasma that is filtered across the glomerulus into Bowman's capsule?

Males = 125 ml/min. Females = 115 ml/min.

What is the function of podocytes?

Manage how much filtrate is produced by closing or separating pores. Formation of filtration slits that forms a sieve.

What is the filtration coefficient?

Measure of a membrane's permeability to fluid filtered. (Podocyte contraction or relaxation).

What is the result of an increased GFR on macula densa cells?

More fluid than normal is filtered - release vasoconstrictor chemicals (e.g. endothelin).

What is the equation for calculating the net filtration pressure?

Net Filtration Pressure = Glomerular capillary blood pressure - (Plasma colloid osmotic pressure + Bowman's capsule hydrostatic pressure). Favour filtration - ( opposing filtration + opposing filtration).

What is Plasma-colloid Osmotic Pressure?

Plasma proteins cannot be filtered therefore there is an unequal distribution of plasma proteins across the membrane.

Why is there a higher concentration of water in the Bowman's capsule?

Plasma proteins in the glomerular capillaries cannot be exchanged across membrane - plasma proteins absent in the Bowman's capsule.

What is the result of GFR when podocytes contract?

Podocytes contract; flattens the foot processes = number of slits decreased = glomerular filtration decreased.

What is the result of hydrostatic pressure in Bowman's capsule?

Pressure tends to push fluid out of the Bowmans' capsule into the capillaries.

Both the mechanisms controlling GFR are directed at adjusting glomerular blood flow by?

Regulating diameter of vessel and thus the resistance.

What is involved in the myogenic mechanism?

Responds to changes in pressure within the vascular components of the nephron.

What is involved in the tubuloglomerular feedback mechanism?

Senses changes in flow through the tubular component of the nephron.

In regards to the myogenic mechanism, when does the afferent arteriole automatically relax?

When pressure is removed.