8.2 Renal_Glomerular filtration

THREE BASIC RENAL PROCESSES (URINE FORMATION)

Complex: Urine formation depends on glomerular filtration, tubular reabsorption, and tubular secretion, which together move fluid from blood to tubule, back to blood, and then from blood to tubule, like a three-step quality-control system.
Simple: Filter it, take back what you need, add extra waste.

GLOMERULAR FILTRATION RATE (GFR)

Complex: The kidneys filter ~180 L/day (125 mL/min) but excrete only 1–2 L/day because ~99% of filtrate is reabsorbed, like filling a pool and draining only a bucket.
Simple: A lot is filtered, very little becomes urine.

FILTRATION BARRIER (STRUCTURAL INTEGRITY)

Complex: The glomerular membrane has three layers—fenestrated capillary endothelium, negatively charged basement membrane, and podocyte filtration slits—that prevent proteins from entering filtrate (<0.03%), like a triple-layer coffee filter.
Simple: Three layers block proteins from leaking out.

GLOMERULAR CAPILLARY BLOOD PRESSURE (MAIN DRIVING FORCE)

Complex: Glomerular capillary blood pressure (~55 mmHg) is the dominant force favoring filtration and is maintained by arterial blood pressure and higher resistance in the efferent versus afferent arteriole, like narrowing a hose outlet to increase pressure.
Simple: Strong blood pressure pushes fluid into the capsule.

PRESSURES OPPOSING FILTRATION

Complex: Bowman’s capsule hydrostatic pressure (~15 mmHg) and plasma colloid osmotic pressure (~30 mmHg) oppose filtration by pushing fluid back into capillaries, like water pushing against a door.
Simple: Two forces try to stop filtration.

NET FILTRATION PRESSURE & GFR RELATIONSHIP

Complex: Net filtration pressure equals GCP − CP − COP (≈10 mmHg), and GFR is directly proportional to this pressure and the filtration coefficient (Kf), like flow depending on pressure and filter size.
Simple: More pressure means more filtering.