GLOMERULAR GLOMERULAR FUNCTION FUNCTION

Professor Harbindar Jeet Professor Harbindar Jeet SinghSingh

Faculty of MedicineFaculty of MedicineUniversiti Teknologi MARAUniversiti Teknologi MARA

Objectives

1. Describe the structures of the glomerulus

2. Explain the barriers to filtration in the glomerulus

3. Describe the forces that determine glomerular filtration

4. Describe the role of afferent and efferent arteriole in the control of filtration

5. Explain the concept of auto-regulation of renal blood flow and GFR

Under resting conditions, about 20% of the cardiac output in humans perfuses the kidneys (the kidneys constitute only about 0.5% of the human body mass).

The rate of blood flow is about 1000 ml per minute or 400 mL/100 g of tissue per minute.

About 120 ml/min is filtered.

About 1 mL/min of urine is produced normally.

The metabolic energy requirement of urine production is about 10% of basal oxygen consumption.

However, renal arterio-venous O2 difference studies show a rate of blood flow that far exceeds metabolic demand.

Formation of urine involves three main processes

1. Glomerular Ultrafiltration

2. Tubular Reabsorption

3. Tubular Secretion

It is a process that separates the plasma water and its non-protein constituents from blood cells and protein macromolecules.

It occurs at the glomeruli under pressure through the perm-selective glomerular capillary wall.

It is qualitatively similar to that occurring in the systemic capillaries but the two are quantitatively different.

Glomerular filtration

Principle Determinants of Glomerular Ultrafiltration

Starling equation states that the rate of fluid movement (J) across a given point of a capillary wall is determined by the net balance between

1. the transcapillary hydrostatic (hydraulic) pressure gradient (ΔP),

2. colloid osmotic pressure (Δπ)

3. hydraulic permeability of the filtration barrier (k).

J = k ( ΔP – Δπ)

In addition to this glomerular filtration is also determined by the surface area (S) available for filtration.

The total single-nephron glomerular filtration rate can therefore be expressed by the following equation

SNGFR = kS(ΔP – Δπ)

We can determine the Ultrafiltration coefficient from SNGFR and net ultrafiltration pressures.

Direct measurement of glomerular capillary pressures in numerous mammalian species reveal pressures of between 45-50 mmHg or approximately 40% of the mean aortic pressure.

Measurement of pressures in the Bowman capsule reveal pressures of between 10 - 12 mmHg.

Measurement of plasma protein concentration in femoral blood (CA) and blood plasma collected from surface efferent arterioles (CE) reveal concentrations of 50-60 g/L and 80-90 g/L protein respectively.

From these the calculated intra-capillary oncotic pressure at the afferent end is about 20 mmHg and at the efferent end it is about 35 mmHg.

Therefore the net ultrafiltration pressure (PUF) can be calculated from these values

PUF = PGC – ( PBC + π A)

Using stop flow techniques it has been possible to estimate the net filtration pressure and the calculated pressure has been confirmed by many researchers.

The four pressures that determine of ultrafiltration

I. Glomerular capillary hydrostatic pressure (PGC)2. Glomerular capillary oncotic pressure (GOP)3. Bowman’s capsule oncotic pressure (BOP)4. Bowmen’s capsule hydrostatic pressure (PBC)

Net filtration pressure = (PGC+BOP) – (GOP + PBC)

Barrier to filtration

The glomerular capillary membrane is similar to other capillariesexcept that it has three, instead of two, major layers:

the endothelium, basement membrane, and an epithelial layer (podocytes).

Together they make up the filtration barrier.

In addition, the basement membrane contains negatively charged proteoglycans (chondroitin sulfate and heparin sulphate proteoglycans, which impart selectivity to the basement membrane.

Ultrafiltration occurs through the (i) fenestrations in the glomerular capillary endothelium (pore), (ii) filtration slits between the glomerularepithelial foot processes.

The endothelium contains holes with a diameter of 500 to 1000 Å.

The basement membrane consists of three filamentous layers (lamina rara interna, lamina densa and lamina rara externa).

The first and third of these layers are fused with the endothelium and epithelium, respectively.

The foot processes or pedicels are separated by filtration slits measuring about 250 to 600 Å in width and each gap is bridged by a thin diaphragm. The diaphragms contain pores of 40 by 140 Å (4-14 nm).

It has been estimated that capillary endothelium accounts for only approximately 2% of the total hydraulic resistance, the basement membrane accounts for nearly 50% of the resistance, and the epitheliallayer (podocytes) for the remaining 48%.

In addition to the pore size other factors like shape of a molecule, its flexibility and deformability and also its charge influence its passage through the glomerular barrier.

All the three layers of the glomerular basement membrane possess fixed negatively charged sites that influence the filtration of macromolecules.

The anionic sites in the glomerular basement membrane consist of glycosaminoglycans rich in heparin (glycosialoproteins).

In minimal change nephropathy the proteinuria is a result of the loss of these negative charges on the basement membranes.

The glomerular capillary wall therefore, in addition to discriminating on the basis of size, also acts as an electrostatic barrier.

Substance Molecular weight Filterability

Water 18 1.0Sodium 23 1.0Glucose 180 1.0Inulin 5,500 1.0Myoglobin 17,000 0.75Albumin 69,000 0.005

1. Glomerular Transcapillary hydraulic pressure

Ultrafiltration of fluid across the walls of the glomerular capillary occurs only when the local transcapillary hydraulic pressure exceeds the opposing oncotic pressure. SNGFR increases as the transcapillary hydraulic pressure increases. But after a while this increase becomes non-linear. This is because, the formation of ultrafiltrate increases the glomerular plasma oncotic pressure which partially offsets, the increment in ΔP.

It has been suggested there may also be a concomitant decrease in Kf

Preglomerular and postglomerular resistances

Effects of selective variations in the four determinants of GFR

2. Ultrafiltration/glomerular coefficient

Changes in filtration coefficient will influence SNGFR if it is not large enough to yield filtration pressure equilibrium.

If the filtration coefficient is large enough to yield filtration pressure equilibrium, then further increases will not increase SNGFR anymore.

Reductions in Kf have been demonstrated in a variety of experimental conditions, including, experimental glomerulonephritis, obstructive uropathy, gentamicin nephrotoxicity and certain forms of hypertension

3. Colloid Osmotic Pressure/Afferent protein Concentration

Theoretically, both SNGFR and SNFF vary reciprocally with colloid osmotic pressure.Numerous studies in vitro using isolated perfused rat kidney have shown that reductions in colloid osmotic pressure leads to increases in average GFR.

However studies in vivo report that, reductions in colloid osmoticpressure either reveal no changes in GFR or the increases are less then predicted.

It has been noted that increases in colloid osmotic pressure are associated with parallel changes in the filtration coefficient. The possible mechanism for this is that in vivo changes in colloid osmotic pressure may be associated with alterations in the activity of any number of vasoactive systems and that this might account for the reductions in the filtration coefficient through mesangial cell contraction.

This failure of SNGFR to increase may be a result of marked decreases in filtration coefficient

4. Glomerular Plasma Flow rate

Increase in glomerular plasma flow rate increases single nephron GFR.

This happens because increased glomerular plasma flow rate slows the rise in glomerular capillary oncotic pressure and filtration ensues through out the glomerular capillary. This effect is only evident within the normal plasma flow rate.

Although filtration increases the filtration fraction decreases.

Auto-regulation of GFR

The relative constancy of GFR and renal blood flow despite marked variation in mean arterial pressure (80-140 mm Hg) is referred to as auto-regulation.

The major function of auto-regulation in the kidney is to maintain a relatively constant GFR and to allow precise control of renal excretion of water and solutes.

This phenomenon can be demonstrated in denervated and isolated kidneys suggesting that it is intrinsic to the kidneys.

Auto-regulation of GFR is a consequence of auto-regulation of glomerular plasma flow and glomerular capillary pressure.

Two main mechanisms have been proposed to explain auto-regulation.

1. Myogenic reflex2. Tubulo-glomerular feedback

Myogenic reflex

Increased transmural pressure

Pressure-induced activation of the smooth muscle

Increased calcium release/influx (Cai)

Contraction of the smooth muscle

Increased resistance

Decreased Blood flow.

Both spontaneous action potential and smooth muscle contraction are blocked by verapramil and the mechanosensitive cation channel blocker, gadolinium.

Myogenic contraction is also prevented in calcium-free medium and by L-type calcium channel blocker, diltiazem.

Roles for EDRF and ET in this mechanisms remain unclear.

Auto-regulation of the afferent arteriole is greatly attenuated in diabetickidneys.

Insulin treatment and inhibition of endogenous prostaglandin production partially restores this myogenic vasoconstriction

Tubulo-glomerular feedback

There is an inverse relationship between SNGFR and delivery of fluid to the distal tubule (maculadensa).

The decline in SNGFR in response to increased distaldelivery is mainly due to increases in pre-and post-glomerular resistance and decreases in renal plasma flow and not due to changes in hydrostatic pressure in the glomerular capillaries per se.

The precise mechanism is not fully understood but animal experiments suggest that:

a) Distal chloride delivery may be the signal responsible for initiating this feedback as

i) The response was only observed with chloride-containing iso-osmotic perfusates

ii) Blockade of ion transport with diuretics or an unfavourable voltage inhibits the response.

b) Distal tubular fluid osmolality

i) It has been observed that the chloride concentration could be reduced without inhibiting the response

ii) It has been observed that both increased osmolality and decreased sodium chloride concentration of the perfusate mobilize calcium from intracellular stores, leading to an increase in intracellular calcium concentration in the macula densa cells.

iii) Increasing intracellular calcium ionophore enhances the TGF response

c). Potassium concentration may influence TGF

i) The response was significantly attenuated when tubules were perfused with K+ free medium containing a K+ channel blockerthat prevented efflux of endogenous K+ into the perfused segment.

ii) TGF responsiveness was restored by adding KCL to the perfusate.

Neural regulation of glomerular filtration rate.

Microscopic studies have revealed that the renal vasculature - including both the smooth muscle and myo-epithelioid cells of the afferent arteriole and the efferent arteriole, the macula densa cells of the distal tubule, and the glomerular mesangium – is richly innervated.

These nerves include

1. Renal efferent sympathetic adrenergic nerves

2. Renal afferent sensory fibres containing peptides such as substance P

There is no efferent parasympathetic cholinergic innervations.

Acetylcholinestrase is however found in adrenergic nerves.

Renal albumin handling

Analysis of urine of healthy individuals shows that < 100 mg/dl of albumin is excreted in the urine per day

With a daily GFR of 150-180 litres in an adult, daily filtered load of albumin is in the range of 3300 – 5760 mg, which is about 4.3% of the total plasma albumin

Therefore less than 1% of the filtered albumin is lost in the urine

Albumin is reabsorbed mainly in the proximal tubule by receptor-mediated endocytosis involving the binding proteins megalin (517-kDa)and cubulin (460-kDa).

EEV

SE

LY

LE

RE

Fusion

Endocytic pathway for albumin

EEV - Early endocytic vesicleSE - Sorting endosomeRE - Recycling endosomeLE - Late endosomeLY - Lysosome

Cubilin

Megalin

Proximal tubular cell

pH

THANK YOU

e.g.

1. Hydrostatic pressure remains relatively constant in glomerular capillaries, whereas it declines in the extra-renal capillaries.

2. Glomerular capillary oncotic pressure rises along the capillary, whereas it remains constant in systemic capillaries.

3. The hydrostatic pressure in Bowman’s space is considerably higher than the systemic tissue hydrostatic pressure.

4. The decline in net ultrafiltration pressure in the glomerular capillaries is due to a rise in oncotic pressure whereas in systemic capillaries it is due to a fall in hydrostatic pressure along the capillary.

5. Movement of fluid in glomerular capillaries is in one direction.

6. Glomerular capillaries are less permeable to proteins than systemic capillaries

7. The filtration coefficient (Kf) is much larger for glomerular capillaries than systemic capillaries

8. The rate of filtration depends upon the rate of plasma flow

through the glomerular capillaries. The higher the plasma flow the greater the filtration. (This is because the increased flow decreases the rate of rise of oncotic pressure)

Afferent Efferent