Urinary System: Structure, Function, and Filtration Mechanisms

Overview of the Urinary System

Introduction

The urinary system is essential for maintaining homeostasis by removing waste products from the body. The kidneys are the primary functioning organs, responsible for filtering blood and producing urine.

• Organs of the Urinary System: Kidneys (2), Ureters (2), Urinary bladder (1), Urethra (1)

• Main Function: Rids the body of metabolic wastes and regulates fluid balance.

Functions of the Kidneys

• Removal of Metabolic Wastes: Eliminates waste via urine, including nitrogenous wastes.

• Regulation of Fluid and Electrolyte Balance: Maintains blood osmolarity by conserving or eliminating water and ions.

• Regulation of Acid-Base Balance: Adjusts blood pH by managing hydrogen and bicarbonate ions.

• Maintenance of Blood Pressure: Controls blood volume and produces enzymes affecting vascular resistance.

• Regulation of Erythropoiesis: Releases erythropoietin (EPO) to stimulate red blood cell production.

• Other Metabolic Functions: Detoxifies blood, activates vitamin D, and performs gluconeogenesis.

Excretion and Nitrogenous Wastes

Excretion

Excretion is the process of separating and eliminating wastes from body fluids. Four body systems contribute to excretion:

• Respiratory System: Removes CO2 and water.

• Integumentary System: Excretes water, salts, lactic acid, and urea via sweat.

• Digestive System: Eliminates water, salts, bile pigments, cholesterol, and food residue.

• Urinary System: Excretes metabolic wastes, toxins, drugs, hormones, salts, H+, and water.

Nitrogenous Wastes

Nitrogenous wastes are metabolic byproducts that must be eliminated:

• Urea: Formed from protein catabolism; amino acids are deaminated to produce ammonia, which is converted to urea by the liver.

• Uric Acid: Product of nucleic acid catabolism.

• Creatinine: Product of creatine phosphate catabolism in muscles.

• Blood Urea Nitrogen (BUN): Measures nitrogenous waste in blood; normal range is 10–20 mg/dL.

• Azotemia: Elevated BUN, indicating renal insufficiency.

• Uremia: Clinical syndrome due to high nitrogenous waste; symptoms include diarrhea, vomiting, dyspnea, and cardiac arrhythmia. Treatment: hemodialysis or organ transplant.

Waste Type	Source
Urea	Protein catabolism
Uric Acid	Nucleic acid catabolism
Creatinine	Creatine phosphate catabolism

Anatomy of the Kidney

External Structure and Location

The kidneys are retroperitoneal organs located against the posterior abdominal wall. They are protected by the lower rib cage, fat, and fibrous capsule. The right kidney is typically lower due to the position of the liver.

• Renal Capsule: Dense connective tissue covering the kidney.

• Fat Capsule: Cushions and protects the kidney.

• Renal Hilum: Entry/exit point for vessels, nerves, and lymphatics.

Gross Anatomy

• Renal Cortex: Outer region containing nephrons.

• Renal Medulla: Inner region with renal pyramids and columns.

• Renal Pelvis: Funnel-shaped structure collecting urine.

• Major/Minor Calyces: Collect urine from papillary ducts.

Renal Circulation

Blood flow through the kidney is highly organized and essential for filtration.

• Renal Artery → Segmental Artery → Interlobar Artery → Arcuate Artery → Interlobular Artery → Afferent Arteriole → Glomerulus → Efferent Arteriole → Peritubular Capillaries/Vasa Recta → Interlobular Vein → Arcuate Vein → Interlobar Vein → Renal Vein

Structure	Function
Glomerulus	Filtration of blood plasma
Peritubular Capillaries	Reabsorption and secretion
Vasa Recta	Maintains medullary osmotic gradient

The Nephron

Structure and Types

Each kidney contains about 1.2 million nephrons, the functional units responsible for urine formation.

• Renal Corpuscle: Composed of the glomerulus and Bowman's capsule; filters blood plasma.

• Renal Tubule: Converts filtrate into urine through reabsorption and secretion.

Nephron Type	Features
Cortical	85% of nephrons; short loops; peritubular capillaries
Juxtamedullary	15% of nephrons; long loops; vasa recta; maintain medullary salinity gradient

Renal Corpuscle

• Glomerular Capsule: Collects filtrate from glomerulus.

• Afferent Arteriole: Larger diameter, increases pressure for filtration.

• Efferent Arteriole: Smaller diameter, maintains glomerular pressure.

Microscopic Anatomy of the Nephron

• Proximal Convoluted Tubule (PCT): Reabsorbs majority of filtrate.

• Nephron Loop (Loop of Henle): Establishes osmotic gradient.

• Distal Convoluted Tubule (DCT): Further reabsorption and secretion.

• Collecting Duct: Final concentration of urine.

Pathway of Filtrate: Glomerular capsule → PCT → Nephron loop → DCT → Collecting duct → Papillary duct → Minor calyx → Major calyx → Renal pelvis → Ureter → Urinary bladder → Urethra

Renal Physiology I: Glomerular Filtration

Basic Stages of Urine Formation

• Glomerular Filtration: Blood is filtered at the glomerulus.

• Tubular Reabsorption: Useful substances are reabsorbed into the blood.

• Tubular Secretion: Additional wastes are secreted into the filtrate.

• Water Conservation: Water is reabsorbed to concentrate urine.

Filtration Membrane

The filtration membrane in the renal corpuscle allows selective passage of water and small solutes while preventing the passage of blood cells and large proteins.

• Fenestrated Endothelium: Pores (70–90 nm) exclude blood cells; highly permeable.

• Basement Membrane: Proteoglycan gel with negative charge; excludes molecules >8 nm and repels albumin.

• Filtration Slits: Formed by podocyte extensions (pedicels); 30 nm slits with negative charge, further restricting large anions.

Turned Back	Passed Through Filter
Blood cells	Water
Plasma proteins	Electrolytes
Large anions	Glucose
Protein-bound minerals/hormones	Amino acids
Most molecules >8 nm	Fatty acids
	Vitamins
	Urea
	Uric acid
	Creatinine

Filtration Selectivity

• Molecules <3 nm: Freely pass (water, electrolytes, glucose, fatty acids, amino acids, nitrogenous wastes, vitamins).

• Low molecular weight substances bound to plasma proteins: Cannot pass (most calcium, iron, thyroid hormone).

• Unbound fraction: Passes freely into filtrate.

Additional info: These notes cover the structure and function of the urinary system, focusing on the kidney's anatomy, nephron types, and the mechanisms of glomerular filtration. The filtration membrane's selectivity is crucial for maintaining blood composition and producing urine efficiently.