Filtration

Filtration takes place in the glomerulus

• ~20% of plasma volume randomly filters out of the blood into the nephron tubule

→ formed elements and plasma proteins DO NOT leave blood

→ smaller solutes in the plasma enter the tubule

Glomerular capillary

• Fenestrated — very leaky, big pores

Podocytes

• create foot processes, which are part of the glomerular filtering mechanism

Mesangial cells

• surround capillaries

• contract/relax to alter blood flow through capillaries

• secrete cytokines and are associated with inflammatory response

• help to maintain the glomerular basement membrane (also part of the filtering mechanism)

The three layers of the glomerular filter (from innermost to outermost)

• Capillary fenestrations

→ size of pores in endothelial cells’ membrane filters cellular elements

• Glomerular basement membrane

→ electrical charge repels plasma proteins

• Filtration slits formed by podocytes

→ size of openings limits what solutes can move through

Glomerular filtration rate (GFR)

• The amount of filtration that happens on the renal corpuscle depends on 3 pressures…

• Capillary hydrostatic pressure (P sub H)

→ pushes outward against the walls of the capillary

• Colloid osmotic pressure in capillary

→ osmotic pressure pulling fluid into the capillary

• Bowman’s capsule fluid (hydrostatic pressure) pressure (P sub fluid)

→ pressure that resists movement of fluid into the capsule

Glomerular filtration rate = sum of all 3 pressures

= 10 mmHg pushing fluid into tubule

Changes in resistance in the afferent and efferent arterioles alters…

• renal hydrostatic pressure (P sub H) and therefore overall GFR

Resistance at both sites is modulated by the sympathetic nervous system AND…

locally

• constricts: GFR decreases

• dilates: GFR increases

→ sympathetic control (tonic control)

→ norepinephrine onto alpha 1 receptors