Urinary System: Kidneys, Ureters, Bladder, and Urethra

Kidneys

Feature	Description
Location	Retroperitoneal (T12-L3)
Weight	120-160 g each
Size	11 × 6 × 3 cm
Hilum	Medial border (renal artery, vein, ureter, lymphatics, nerves)
Capsule	Fibrous (perirenal fat, Gerota fascia)

Gross Structure

Region	Description
Cortex	Outer (glomeruli, proximal/distal tubules)
Medulla	Inner (loops of Henle, collecting ducts)
Renal pyramids	8-18 per kidney (apex = papilla)
Renal columns	Cortex between pyramids (Bertin)
Minor calyces	Cup papillae (8-18 per kidney)
Major calyces	2-3 per kidney (converge to pelvis)
Renal pelvis	Funnel-shaped → ureter

Blood Supply

Artery	Branch	Territory
Renal artery	From aorta (L1-L2)	Kidney
Segmental arteries	5 branches	Renal segments
Interlobar arteries	Between pyramids	Medulla
Arcuate arteries	At corticomedullary junction	Cortex
Interlobular arteries	Into cortex	Afferent arterioles
Afferent arteriole	→ Glomerulus	Filtration
Efferent arteriole	→ Peritubular capillaries	Reabsorption

Renal blood flow: ~1200 mL/min (20-25% of cardiac output)

Nephron

Types:

Nephron	Percentage	Loop of Henle
Cortical	85%	Short, does not enter medulla
Juxtamedullary	15%	Long, deep into medulla (concentrating ability)

Nephron segments:

Segment	Function
Glomerulus	Filtration (ultrafiltrate of plasma)
Proximal convoluted tubule (PCT)	Reabsorption: Na⁺, Cl⁻, HCO₃⁻, glucose, amino acids, water (65%)
Loop of Henle (descending)	Water reabsorption (concentrating)
Loop of Henle (ascending)	Na⁺/K⁺/Cl⁻ reabsorption (diluting segment)
Distal convoluted tubule (DCT)	Na⁺/Cl⁻ reabsorption (thiazide-sensitive), Ca²⁺ reabsorption
Connecting tubule	Fine regulation
Collecting duct	Water (ADH), Na⁺ (aldosterone), acid-base regulation

Glomerulus

Structure	Description
Capillary tuft	Fenestrated endothelium (70-100 nm pores)
Basement membrane	Type IV collagen, laminin (negatively charged)
Podocytes	Visceral layer of Bowman capsule (foot processes, slit diaphragms)
Mesangium	Mesangial cells + matrix (support, phagocytosis, contraction)

Filtration barrier:

1. Fenestrated endothelium
2. Basement membrane (charge and size selective)
3. Slit diaphragm (podocytes)

Filtration rate (GFR): ~125 mL/min (180 L/day)

Net filtration pressure: (Pgc - Pbs) - (πgc - πbs) = ~10 mmHg

Juxtaglomerular Apparatus

Cell	Location	Function
Juxtaglomerular (granular) cells	Afferent arteriole	Renin secretion
Macula densa	DCT (adjacent to afferent arteriole)	Na⁺ sensing (tubuloglomerular feedback)
Extraglomerular mesangial (Lacis) cells	Between afferent and efferent	Communication

Ureters

Feature	Description
Length	25-30 cm
Diameter	3-5 mm
Wall	Mucosa (urothelium), muscularis (inner longitudinal, outer circular), adventitia
Constrictions	Ureteropelvic junction, pelvic brim, ureterovesical junction
Peristalsis	1-5 contractions/min

Urinary Bladder

Feature	Description
Capacity	400-600 mL (voiding), up to 1000 mL (distended)
Location	Pelvis (behind pubic symphysis)
Wall	Mucosa (urothelium), submucosa, detrusor muscle (3 layers), serosa (superior)
Trigone	Triangular area between ureteric orifices and internal urethral orifice

Urethra

Feature	Male	Female
Length	18-20 cm	3-5 cm
Parts	Prostatic, membranous, spongy (penile)	Single
Sphincters	Internal (smooth, sympathetic), external (skeletal, S2-S4)	External (skeletal)

Micturition Reflex

Phase	Mechanism
Filling	Sympathetic (relaxation of detrusor, contraction of internal sphincter)
Voiding	Parasympathetic (contraction of detrusor, relaxation of sphincters)
Voluntary control	Somatic (pudendal nerve) controls external sphincter

Pontine micturition center (Barrington nucleus): Coordinates the reflex

Overview of Filtration and Reabsorption

Substance	Filtered (per day)	Excreted (per day)	% Reabsorbed
Water	180 L	1-2 L	99%
Na⁺	25,000 mmol	100-200 mmol	99%
Glucose	180 g	0	100%
Urea	50 g	25 g	50%
HCO₃⁻	4,500 mmol	0-2 mmol	~100%
Creatinine	1.8 g	1.8 g	0%