Excretory Products and Their Elimination
Syllabus Unit: Human Physiology
Why it matters
Chapter overview
The kidneys filter nitrogenous waste from blood and precisely regulate the body's water and electrolyte balance, one nephron at a time. NEET tests the excretory system's gross and microscopic anatomy, the three-step mechanism of urine formation, the hormonal feedback loops that fine-tune it, and the disorders that arise when kidney function fails.
Biology Syllabus Human Physiology 20%–22% weightage
Live Animation1 / 4
1KidneyFilters blood, forms urine
2UreterCarries urine to bladder
3Urinary BladderStores urine
4UrethraExpels urine
1Human Excretory System & the Nephron
A pair of bean-shaped kidneys, each packed with about a million nephrons, is the body's master filtration system.
Learning Objectives
- Name the four organs of the human excretory system in the order urine passes through them.
- Label the two main regions of kidney internal structure: cortex and medulla.
- Describe the nephron's key structural parts from Bowman's capsule to the collecting duct.
- Distinguish cortical nephrons from juxtamedullary nephrons.
Key Terms
NephronThe structural and functional unit of the kidney; each kidney contains about a million, responsible for filtering blood and forming urine.
GlomerulusA tuft of capillaries inside Bowman's capsule where blood filtration begins.
Bowman's CapsuleThe cup-shaped end of the nephron that encloses the glomerulus and collects the filtrate.
Loop of HenleThe hairpin-shaped tubule segment (descending + ascending limbs) that creates the medullary concentration gradient.
The human excretory system consists of a pair of **kidneys**, a pair of **ureters**, a **urinary bladder**, and a **urethra** — together removing nitrogenous waste and regulating body fluid balance.
Kidney gross anatomy
- Cortex — the outer region, containing Bowman's capsules and convoluted tubules of the nephrons.
- Medulla — the inner region, organised into pyramids; contains the loops of Henle and collecting ducts.
- Renal pelvis — funnel-shaped space that collects urine and channels it into the ureter.
The nephron: structural unit, ~1 million per kidney
- Malpighian (renal) corpuscle — the glomerulus (capillary tuft) enclosed by Bowman's capsule; site of filtration.
- Proximal convoluted tubule (PCT) — reabsorbs most filtered glucose, amino acids, salts and water.
- Loop of Henle — descending limb permeable to water; ascending limb permeable to ions but not water; together build the medullary osmotic gradient.
- Distal convoluted tubule (DCT) — fine-tunes reabsorption, secretes H+/K+; sensitive to aldosterone.
- Collecting duct — receives filtrate from many nephrons; final water reabsorption under ADH control before urine exits to the renal pelvis.
Key PointJuxtamedullary nephrons (~15%) have long loops of Henle deep into the medulla and are key to concentrating urine; cortical nephrons (~85%) have short loops confined mostly to the cortex.