8.1 Renal Nephrons

NEPHRONS:

FUNCTIONAL UNITS OF THE KIDNEY produce urine by filtering plasma and modifying it along the tubules

TYPES OF NEPHRONS

Cortical nephrons (80–85%) have short loops for routine filtration, while juxtamedullary nephrons (15–20%) have long loops and vasa recta to create urine concentration gradients, like shallow vs deep wells.

NEPHRON COMPONENTS:

VASCULAR + TUBULAR

Each nephron has intertwined vascular and tubular components that allow blood processing and filtrate modification, like pipes wrapped around sorting channels.

THREE KEY PROCESSES

Filtration moves protein-free plasma into Bowman’s capsule, reabsorption returns needed substances to blood, and secretion adds wastes to filtrate, like screening, reclaiming, and dumping.

RENAL CORPUSCLE

(FILTRATION SITE)

contains the glomerulus and Bowman’s capsule where high pressure (wider afferent, narrower efferent arteriole) drives filtration through fenestrae and podocyte slits, like squeezing liquid through a fine sieve.

SECOND CAPILLARY NETWORK

(PERITUBULAR CAPILLARIES & VASA RECTA)
Blood leaving the glomerulus forms peritubular capillaries or vasa recta for reabsorption, secretion, and medullary gradient maintenance, like delivery trucks picking up reclaimed goods.

PROXIMAL TUBULE (BULK REABSORPTION)

The proximal tubule reabsorbs 65–70% of filtrate in an iso-osmotic manner (~300 mOsm), like grabbing most valuables immediately after sorting.

LOOP OF HENLE: DESCENDING LIMB

The descending limb is permeable to water, causing water to leave and filtrate to concentrate as it descends, like a sponge losing water in salty air.
Simple: Water leaves, filtrate gets concentrated.

LOOP OF HENLE: ASCENDING LIMB

Complex: The ascending limb is impermeable to water but reabsorbs solutes, creating dilute filtrate without changing volume, like removing salt but keeping the same cup size.
Simple: Salt leaves, water stays.

DISTAL TUBULE (FINE-TUNING)

Complex: The distal tubule fine-tunes Na⁺, K⁺, and Cl⁻ under hormonal control, adjusting balance like a thermostat.
Simple: Small adjustments happen here.

COLLECTING DUCT (FINAL CONTROL)

Complex: The collecting duct, though not part of the nephron, determines final urine composition via ADH and aldosterone, after which no changes can occur, like sealing a bottle.
Simple: This is the final decision point.

BOTTOM LINE

Complex: Less than 1% of filtrate becomes urine after precise, hormone-controlled processing across nephron segments, like refining crude oil into fuel.
Simple: Almost everything is reused, very little becomes urine.