GLOMERULAR FILTRATION

A. Role: formation of the initial fluid entering the nephron

B. Composition of Glomerular Filtrate: ultrafiltrate of plasma, due to size-selective filtration: nonselectively permeable to small particles but not very permeable to larger particles, such as blood cells and plasma proteins, so contains all the dissolved substances at the same concentration as in blood plasma up to the size of plasma proteins

Note: Certain conditions, such as kidney inflammation (nephritis) can increase glomerular permeability, permitting larger particles, such as plasma proteins to appear in the urine (e.g. albuminuria); major damage can lead to blood cells appearing in urine

C. Mechanism: passive filtration

1. Forces: hydrostatic pressure determined by renal arterial and venous pressures, and by vasomotor state of afferent and efferent arterioles; osmotic pressure determined by plasma protein concentration

2. Process: Normally, the hydrostatic pressure difference (forcing fluid out of the glomerular capillaries) is larger than the osmotic force (tending to retain fluid in the capillaries) so the result is net filtration

D. Glomerular Filtration Rate (GFR)

1. Normal value: 120-125 ml/min (for 70 kg human)

2. Control: the separate afferent and efferent arterioles permit the kidney to reduce blood flow while still maintaining normal glomerular pressure and filtration

3. If arterial pressure drops too low, glomerular capillary pressure eventually drops also, so filtration slows or ceases and the kidney can no longer function