Structure of Kidney — Revision Notes

⚡ 30-Second Revision

Kidney: — Bean-shaped, retroperitoneal organs.
Gross Anatomy: — Outer cortex, inner medulla (renal pyramids), renal columns, calyces, renal pelvis, hilum.
Nephron: — Functional unit. ~1 million per kidney.
Renal Corpuscle: — Glomerulus + Bowman's capsule (site of ultrafiltration).
Glomerulus: — Capillary tuft, high pressure, supplied by afferent, drained by efferent arteriole.
Bowman's Capsule: — Visceral layer has podocytes with filtration slits.
Renal Tubule: — PCT, Loop of Henle, DCT, Collecting Duct.
PCT: — Bulk reabsorption (water, glucose, amino acids, Na+).
Loop of Henle: — Descending limb (water permeable), Ascending limb (solute permeable, creates medullary gradient).
DCT: — Regulated reabsorption/secretion (Na+, K+, H+).
Collecting Duct: — Final water reabsorption (ADH), urea recycling.
Nephron Types: — Cortical (short loop, 85%), Juxtamedullary (long loop, 15%, crucial for concentration).
Blood Supply: — Afferent arteriole $\rightarrow$ Glomerulus $\rightarrow$ Efferent arteriole $\rightarrow$ Peritubular capillaries/Vasa recta.
JGA: — Macula densa (DCT) + JG cells (afferent arteriole) $\rightarrow$ Renin secretion, GFR regulation.

2-Minute Revision

The kidney, a pair of bean-shaped organs, is crucial for blood filtration and maintaining body fluid balance. Its gross structure includes an outer cortex and an inner medulla containing renal pyramids.

Blood enters via the renal artery at the hilum, and urine exits via the ureter from the renal pelvis. The microscopic functional unit is the nephron, with each kidney housing about a million.

A nephron begins with the renal corpuscle (glomerulus + Bowman's capsule) where blood is ultrafiltered. The filtered fluid then enters the renal tubule, comprising the Proximal Convoluted Tubule (PCT) for bulk reabsorption, the Loop of Henle (descending and ascending limbs) which establishes the medullary osmotic gradient, the Distal Convoluted Tubule (DCT) for fine-tuning, and finally the Collecting Duct for final water reabsorption and urine concentration.

Nephrons are either cortical (short loops) or juxtamedullary (long loops, vital for concentrated urine). The Juxtaglomerular Apparatus (JGA), formed by the DCT and afferent arteriole, regulates filtration and blood pressure.

5-Minute Revision

Let's quickly review the essential structure of the kidney, a vital organ for excretion and osmoregulation.

Gross Anatomy: The kidney is a reddish-brown, bean-shaped organ located retroperitoneally. It's covered by a fibrous capsule. Internally, it's divided into an outer renal cortex and an inner renal medulla.

The medulla contains 8-18 cone-shaped structures called renal pyramids, separated by extensions of the cortex known as renal columns of Bertin. The apex of each pyramid, the renal papilla, drains into minor calyces, which merge into major calyces, finally forming the large, funnel-shaped renal pelvis.

The renal pelvis narrows to become the ureter, transporting urine to the bladder. Blood vessels, nerves, and the ureter enter/exit at the medial indentation called the hilum.

Microscopic Anatomy: The Nephron: The functional unit is the nephron, with over a million per kidney. Each nephron has two main parts:

1
Renal Corpuscle: — Located in the cortex, it consists of the glomerulus (a capillary tuft) and Bowman's capsule (a double-walled cup). The glomerulus is supplied by the afferent arteriole and drained by the efferent arteriole. The inner layer of Bowman's capsule has specialized cells called podocytes with foot processes (pedicels) that form filtration slits, allowing ultrafiltration of blood.
2
Renal Tubule: — A long, convoluted tube that modifies the filtrate.

* Proximal Convoluted Tubule (PCT): Highly coiled, in the cortex. Extensive brush border for bulk reabsorption of water, glucose, amino acids, and ions. * Loop of Henle: U-shaped, dips into the medulla.

The descending limb is permeable to water. The ascending limb is impermeable to water but actively transports solutes (Na+, Cl-) out, establishing the medullary osmotic gradient. * Distal Convoluted Tubule (DCT): Coiled, in the cortex.

Involved in regulated reabsorption and secretion, influenced by hormones like aldosterone. * Collecting Duct: Receives filtrate from several DCTs, extends through cortex and medulla. Final water reabsorption (under ADH control) and urea recycling, crucial for urine concentration.

Nephron Types:

Cortical Nephrons (85%): — Short loops of Henle, mainly in cortex.
Juxtamedullary Nephrons (15%): — Long loops of Henle extending deep into medulla, essential for concentrating urine, associated with vasa recta (capillaries parallel to loop).

Juxtaglomerular Apparatus (JGA): A specialized structure where the DCT contacts the afferent arteriole. Composed of macula densa (in DCT, senses NaCl) and juxtaglomerular (JG) cells (in afferent arteriole, secrete renin). It regulates GFR and blood pressure.

Understanding these structures and their precise locations is key to comprehending the entire process of urine formation and renal regulation.

Prelims Revision Notes

Kidney Structure: NEET Quick Recall

I. Gross Anatomy:

Location: — Retroperitoneal, T12-L3, right kidney slightly lower.
Shape/Size: — Bean-shaped, ~10-12 cm long.
Coverings: — Renal capsule (fibrous), adipose capsule (fat), renal fascia (anchoring).
Hilum: — Medial indentation; entry/exit for renal artery, renal vein, ureter, nerves, lymphatics.
Renal Sinus: — Cavity within hilum, contains renal pelvis, fat, vessels.
Regions:

* Cortex: Outer, granular, reddish-brown. Contains renal corpuscles, PCT, DCT. * Medulla: Inner, darker. Contains renal pyramids (8-18), loops of Henle, collecting ducts. * Renal Columns of Bertin: Cortical extensions between pyramids.

Collecting System: — Renal papilla $\rightarrow$ Minor calyx $\rightarrow$ Major calyx $\rightarrow$ Renal pelvis $\rightarrow$ Ureter.

II. Microscopic Anatomy: The Nephron (Functional Unit)

~1 million per kidney.
A. Renal Corpuscle (in Cortex):

* Glomerulus: Capillary tuft. Afferent arteriole (wider) $\rightarrow$ Glomerulus $\rightarrow$ Efferent arteriole (narrower). High pressure for ultrafiltration. * Bowman's Capsule: Double-walled cup.

* Parietal layer: Simple squamous epithelium. * Visceral layer: Podocytes (specialized cells with pedicels forming filtration slits). * Filtration Membrane: Glomerular endothelium (fenestrated) + Basement membrane + Podocyte filtration slits.

Filters water, small solutes; blocks cells, large proteins.

B. Renal Tubule:

* Proximal Convoluted Tubule (PCT): Highly coiled, in cortex. Brush border (microvilli), abundant mitochondria. Site of bulk reabsorption (65% water, Na+, Cl-, K+; 100% glucose, amino acids).

Secretes H+, ammonia, drugs. * Loop of Henle: U-shaped, dips into medulla. * Descending Limb: Thin, highly permeable to water, impermeable to solutes. * Ascending Limb: Thin & thick segments.

Impermeable to water. Actively transports solutes (Na+, Cl-, K+) out into medulla, establishing medullary osmotic gradient. * Distal Convoluted Tubule (DCT): Coiled, in cortex. Fewer microvilli.

Regulated reabsorption (Na+, Cl-, water) and secretion (K+, H+) under hormonal control (aldosterone, ADH). * Collecting Duct: Receives filtrate from several DCTs. Extends through cortex/medulla.

Final water reabsorption (ADH), urea recycling. Merges into papillary ducts.

III. Nephron Types:

Cortical Nephrons (85%): — Renal corpuscle in outer cortex. Short loop of Henle, barely enters medulla. Primarily peritubular capillaries.
Juxtamedullary Nephrons (15%): — Renal corpuscle deep in cortex. Long loop of Henle, extends deep into inner medulla. Crucial for urine concentration. Associated with vasa recta.

IV. Juxtaglomerular Apparatus (JGA):

Location: Where DCT contacts afferent arteriole.
Components:

* Macula Densa: Specialized DCT cells, sense NaCl concentration in filtrate. * Juxtaglomerular (JG) Cells: Modified smooth muscle cells in afferent arteriole, secrete renin.

Function: Regulates GFR and blood pressure (Renin-Angiotensin-Aldosterone System - RAAS).

Vyyuha Quick Recall

To remember the parts of the nephron in order: Great People Love Drinking Coffee.

Glomerulus
PCT (Proximal Convoluted Tubule)
Loop of Henle
DCT (Distal Convoluted Tubule)
Collecting Duct