The Urinary System: Structure, Function, and Physiology

The Urinary System

Overview and Functions

The urinary system is essential for maintaining homeostasis by regulating the composition and volume of blood, removing metabolic wastes, and controlling fluid and electrolyte balance.

• Excretion of metabolic wastes: Removes nitrogenous wastes such as urea, creatinine, and uric acid from the body.

• Regulation of blood volume and pressure: Adjusts water and salt balance, influencing blood pressure.

• Electrolyte balance: Maintains proper levels of ions such as Na+, K+, Ca2+, and Cl-.

• Acid-base balance: Regulates blood pH by excreting hydrogen ions and reabsorbing bicarbonate.

• Hormone production: Produces erythropoietin (stimulates red blood cell production) and renin (regulates blood pressure).

Organs of the Urinary System

• Kidneys: Filter blood, form urine, and regulate various homeostatic functions.

• Ureters: Muscular tubes that transport urine from the kidneys to the urinary bladder.

• Urinary bladder: Hollow organ that stores urine until micturition (urination).

• Urethra: Tube that conveys urine from the bladder to the exterior of the body.

Kidney Location and Protective Coverings

• Location: Retroperitoneal, on either side of the vertebral column, between T12 and L3 vertebrae.

• Protective coverings:

  • Renal capsule: Tough fibrous layer directly covering the kidney.

  • Adipose capsule: Fatty layer providing cushioning and insulation.

  • Renal fascia: Dense connective tissue anchoring the kidney to surrounding structures.

Internal Anatomy of the Kidneys

• Cortex: Outer region containing renal corpuscles and convoluted tubules.

• Medulla: Inner region composed of renal pyramids, which contain loops of Henle and collecting ducts.

• Renal pelvis: Funnel-shaped cavity collecting urine from major calyces and leading to the ureter.

• Renal columns: Extensions of cortical tissue between pyramids.

Nephrons: Structure and Function

Nephrons are the functional units of the kidney, responsible for urine formation.

• Renal corpuscle: Consists of the glomerulus (capillary tuft) and Bowman's capsule; site of filtration.

• Renal tubule: Includes proximal convoluted tubule (PCT), loop of Henle (nephron loop), distal convoluted tubule (DCT), and collecting duct.

• Function: Filters blood, reabsorbs needed substances, and secretes wastes into urine.

Types of Nephrons

Type	Location	Structure	Function
Cortical	Mostly in cortex	Short loops of Henle	Majority of nephrons; perform most filtration and reabsorption
Juxtamedullary	At cortex-medulla junction	Long loops of Henle extending deep into medulla	Concentrate urine via countercurrent mechanisms

Blood and Nerve Supply to the Kidneys

• Renal arteries: Branch from the abdominal aorta and supply blood to the kidneys.

• Arterioles: Afferent arterioles supply glomeruli; efferent arterioles drain them.

• Venous drainage: Blood exits via renal veins to the inferior vena cava.

• Nerve supply: Sympathetic fibers regulate blood flow and renin release.

Ureters, Urinary Bladder, and Urethra

• Ureters: 25-30 cm long tubes with smooth muscle; propel urine by peristalsis.

• Urinary bladder: Muscular sac (detrusor muscle) with transitional epithelium; stores urine (capacity ~500 mL).

• Urethra: Thin-walled tube; length varies by sex (shorter in females).

Urine Formation Processes

1. Glomerular filtration: Passive process in which hydrostatic pressure forces water and solutes from blood into Bowman's capsule.

2. Tubular reabsorption: Selective movement of substances from filtrate back into blood (e.g., glucose, amino acids, water).

3. Tubular secretion: Active transport of substances (e.g., H+, K+, drugs) from blood into filtrate.

Filtration Membrane and Glomerular Filtration Rate (GFR)

• Filtration membrane: Composed of fenestrated endothelium, basement membrane, and podocyte filtration slits; allows passage of water and small solutes, blocks proteins and cells.

• Effective filtration pressure (EFP): Net pressure driving filtration, calculated as:

$\text{EFP} = (\text{Glomerular hydrostatic pressure}) - (\text{Capsular hydrostatic pressure} + \text{Blood colloid osmotic pressure})$

• Glomerular filtration rate (GFR): Volume of filtrate formed per minute by both kidneys (normal: 120-125 mL/min).

• Regulation: Intrinsic (renal autoregulation) and extrinsic (neural and hormonal) controls adjust GFR.

Tubular Reabsorption

• Proximal convoluted tubule (PCT): Reabsorbs most water, ions, glucose, and amino acids.

• Loop of Henle: Descending limb reabsorbs water; ascending limb reabsorbs Na+ and Cl-.

• Distal convoluted tubule (DCT) and collecting duct: Reabsorption regulated by hormones (aldosterone, ADH).

Tubular Secretion and Acid-Base Balance

• Secretion: Removes substances such as H+, K+, NH4+, creatinine, and drugs from blood into filtrate.

• Acidification of urine: Secretion of H+ helps regulate blood pH.

• Buffering mechanisms: Phosphate and ammonia buffer excess H+ in urine.

Countercurrent Multiplier Mechanism

• Purpose: Creates an osmotic gradient in the medulla to concentrate urine.

• Mechanism: Long loops of Henle (especially in juxtamedullary nephrons) and vasa recta maintain the gradient by countercurrent flow.

• Result: Allows kidneys to produce concentrated urine and conserve water.

Composition of Normal and Abnormal Urine

• Normal urine: 95% water, 5% solutes (urea, creatinine, uric acid, ions).

• Abnormal constituents: Glucose (glycosuria), proteins (proteinuria), blood (hematuria), ketones (ketonuria), bile pigments.

Micturition (Urination) Mechanism

• Process: Bladder fills, stretch receptors trigger reflex, detrusor muscle contracts, internal and external urethral sphincters relax.

• Control: In infants, micturition is reflexive; in adults, voluntary control develops with maturation of the nervous system.

Example: In diabetes mellitus, glucose appears in urine due to exceeding the renal threshold for glucose reabsorption (glycosuria).