Glomerular Filtration Membrane

Glomerular Filtration Membrane and Net Filtration Pressure

Glomerular filtration is the first step in urine formation.

Glomerular filtration is the process by which ultrafiltrate is formed from small, positively charged molecules; large cells and proteins remain in the blood.

It occurs within the renal corpuscle, which is the first part of the nephron.

It is driven by dynamically opposing pressures (aka, Starling pressures) that are exerted by the contents of the blood and ultrafiltrate.

Glomerular Filtration Membrane

Three layers:

Glomerular capillary wall; fenestrations allow passage of small and medium-sized molecules.

Basement membrane allows positively charged molecules only.

Visceral layer of Glomerular/Bowman's capsule; podocytes allow passage of only smallest molecules through filtration slit and slit diaphragm.

The membrane is selectively permeable: only small and positively charged molecules pass freely.

Key components of blood within the capillary:

Large molecules (such as blood cells); blocked by capillary wall.

Negatively charged molecules; blocked by basement membrane.

Mid-sized molecules (such as proteins), blocked by visceral layer of Bowman's capsule.

Small molecules (water, glucose, amino acids, nitrogenous wastes); pass freely through filtration membrane to become part of ultrafiltrate.

Clinical Correlation:

Glomerulonephritis involves inflammation of the filtration membrane, which alters its permeability and inhibits proper filtration, which can be fatal. Signs include the presence of blood or proteins in the urine.

Net Filtration Pressures

Hydrostatic pressures (P):

The forces that blood and ultrafiltrate fluids exert on the filtration membrane; "push" things against the filtration membrane.

Oncotic pressures (aka, colloid osmotic pressures):

The forces that proteins within the blood and ultrafiltrate exert to draw water towards them; they "suck" them through the filtration membrane.

Typical example values:

Pressures exerted by blood in glomerular capillary: – Hydrostatic pressure = 60 mmHg. – Oncotic pressure = 29 mmHg; remember that this is the force blood proteins exert to "suck" water into the capillaries.

Pressures exerted by the ultrafiltrate in Bowman's capsule: – Hydrostatic pressure = 15 mmHg. – Oncotic pressure = approximately 0 mmHg because the filtration membrane prohibited the passage of large proteins into Bowman's capsule.

Thus, high capillary hydrostatic pressure is a major driving force of net filtration pressure.

Net Filtration Pressure Equation (NFP):

Describes the outcome of opposing pressures across the filtration membrane.

A positive net filtration pressure value means that filtration will occur.

Equation: (Glomerular hydrostatic pressure + Bowman's capsule oncotic pressure) - (Bowman's capsule hydrostatic pressure + Glomerular oncotic pressure)

The first half of the equation measures forces that favor filtration.

The second half measures forces that oppose it.

When we plug in the values from our diagram, we get: (60 + 0) - (15 + 29) = 16 mmHg. The positive value means that filtration will occur.

Clinical Correlations

Glomerular Filtration Membrane

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