Glomerular Filtration

When are renal sympathetic nerves activated?

During stress (e.g. cold, fear, haemorrhage, pain, exercise)

What is the purpose of activating renal sympathetic nerves?

Increases systemic vascular resistance to maintain blood pressure

What can persistent sympathetic activation cause?

Reduced renal blood flow → ↓GFR → possible renal failure

What does GFR assess?

The efficiency of substance clearance from the blood by glomerular filtration

Can GFR diagnose kidney disease?

No – it indicates function but not specific diseases

What are the ideal characteristics of a substance used to measure GFR?

• Stable plasma concentration

• Physiologically inert

• Freely filtered at glomerulus

• Not secreted, reabsorbed, or metabolised

What equation is used for GFR measurement?

GFR × [S]plasma = [S]urine × urine flow rate

What is creatinine?

A muscle metabolism by-product, cleared by kidneys

Why is creatinine used to estimate GFR (eGFR)?

• Produced at a constant rate

• Freely filtered

• Not reabsorbed

• Only slightly secreted

How much plasma volume is filtered in glomerulus?

~20% of plasma entering afferent arteriole

What percentage of filtered fluid is reabsorbed?

>99%

How much urine is typically produced per minute?

~1 ml/min

What two main factors control GFR?

• Net filtration pressure

• Filtration coefficient (dependent on capillary surface area and permeability)

What alters net filtration pressure?

• Hydrostatic pressure (glomerular & Bowman’s)

• Colloid osmotic pressure

• Hydrostatic fluid pressure in capsule

What components make up the filtration barrier?

• Fenestrated endothelium

• Basement membrane

• Podocytes with filtration slits

What causes a decrease in the filtration coefficient?

Reduced capillary surface area (e.g., podocyte damage)