Human Excretory System and Nephron
மனிதக் கழிவுநீக்க மண்டலமும் நெஃப்ரானும்
Most nephrons are cortical with a short loop of Henle; only some are juxtamedullary with a long loop. Reverse this and an assertion–reason question is lost.
The idea
The human excretory system consists of a pair of kidneys, a pair of ureters, a urinary bladder and a urethra. Kidneys are reddish-brown, bean-shaped organs between the last thoracic and third lumbar vertebrae, each about 10-12 cm long, 5-7 cm wide, 2-3 cm thick and 120-170 g in weight. Inside, the outer cortex and inner medulla are seen; the medulla is divided into medullary pyramids projecting into calyces, and cortex extending between pyramids forms the renal columns of Bertini. Each kidney has nearly one million nephrons, the functional units. A nephron has a glomerulus (a tuft of capillaries formed by the afferent arteriole and drained by the efferent arteriole) and a renal tubule: Bowman's capsule, proximal convoluted tubule (PCT), Henle's loop with descending and ascending limbs, and distal convoluted tubule (DCT). DCTs of many nephrons open into a collecting duct. The glomerulus with Bowman's capsule is the Malpighian body or renal corpuscle. In the majority of nephrons the loop of Henle is too short and extends only a little into the medulla — cortical nephrons; in some the loop is very long and runs deep into the medulla — juxtamedullary nephrons. The efferent arteriole forms the peritubular capillaries and the vasa recta, which is absent or highly reduced in cortical nephrons.
Why it matters
Blood must be filtered continuously and its useful parts reclaimed; a million tiny filtering-and-reclaiming units give the kidney enormous capacity and fine control.
NEET lens
NEET asks: kidney position (last thoracic to third lumbar), size/weight numbers, columns of Bertini, about one million nephrons, the order of tubule parts, Malpighian body = glomerulus + Bowman's capsule, and cortical vs juxtamedullary nephrons (vasa recta absent/reduced in cortical). PCT, DCT and Malpighian corpuscle lie in the cortex; loop of Henle dips into the medulla.
Clinical significance
Diabetes and high blood pressure slowly damage glomeruli, reducing filtration; when enough nephrons are lost, kidney failure follows and dialysis or transplant is needed.
Introduction
Humans excrete through a pair of kidneys, ureters, a urinary bladder and a urethra. The nephron is the structural and functional unit of the kidney.
Core concept
Kidney: between last thoracic and 3rd lumbar vertebrae; 10-12 cm × 5-7 cm × 2-3 cm; 120-170 g; hilum (ureter, blood vessels, nerves enter) → renal pelvis with calyces. Outer cortex, inner medulla (medullary pyramids into calyces); columns of Bertini = cortex between pyramids. ~1 million nephrons per kidney. Nephron: glomerulus (afferent arteriole in, efferent arteriole out) + renal tubule = Bowman's capsule → PCT → Henle's loop (descending, ascending limbs) → DCT → collecting duct (shared) → renal pelvis. Malpighian body/renal corpuscle = glomerulus + Bowman's capsule. Cortical nephrons (majority) — short loop; juxtamedullary — long loop deep into medulla. Efferent arteriole → peritubular capillaries; vasa recta (U-shaped, parallel to Henle's loop) absent or highly reduced in cortical nephrons. JGA: formed by cellular modifications in the DCT and the afferent arteriole at their point of contact.
Go deeper
Collecting ducts extend from cortex to the inner parts of the medulla and converge as they pass through the pyramids to open into calyces. Malpighian corpuscle, PCT and DCT are located in the cortex.
From Class 10
In Class 10 you saw that nephrons filter blood and form urine. NEET wants the exact parts in order, where each lies, and the two kinds of nephrons.