Anatomy & Physiology Study Notes: The Urinary System

Urinary System Overview

Introduction

The urinary system is essential for maintaining homeostasis by filtering blood, removing waste products, and regulating fluid and electrolyte balance. It consists of several organs that work together to produce, store, and eliminate urine.

Main Organs of the Urinary System

• Kidneys: Paired. Posterior organs. Filter waste products from the blood causing the filtrate/waste to turn into fluid to produce urine.

• Ureters: Transport urine from kidneys to bladder.

• Urinary Bladder: Stores urine until excretion.

• Urethra: How urine is expelled from the body.

Functions of the Urinary System

Key Functions

• Filter waste products from the blood and convert the filtrate into urine.

• Storage of Urine: Urine is temporarily stored in the bladder.

• Expulsion of Urine: Urine is expelled from the body via the urethra.

• Regulation of Blood Volume: Adjusts the amount of water excreted to control blood volume.

• Regulation of Erythrocyte Production: Kidneys release erythropoietin to stimulate red blood cell production.

• Regulation of Ion Levels: Maintains balance of ions such as sodium, potassium, and calcium.

• Regulation of Acid-Base Balance: Controls excretion of hydrogen and bicarbonate ions to maintain pH.

Kidney Anatomy

Structure of the Kidney

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

• Renal Medulla: Inner region with renal pyramids and collecting ducts.

• Renal Pyramids: Funnel-shaped (triangle) cavity that collects urine and channels it to the ureter.

• Corticomedullary junction: Where the base of the renal pyramids that sit directly next to the renal cortex.

• Major (the large part) and Minor (gets urine funneled into it) Calyces: Collect urine from renal papillae. location: The apex of the renal pyramids.

• Renal pelvis: Extensions of cortex between pyramids. the biggest part where the urine gets funneled into. Brings the urine directly into the ureter.

Nephron: Functional Unit of the Kidney

Nephron Structure

The nephron is the microscopic unit responsible for filtration and urine formation. your kidneys contains about 2.5 million nephrons total.

• Renal Corpuscle: Includes the glomerulus (capillary network) and Bowman's capsule (surrounds glomerulus).

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

Two Types of Nephrons

Type	Location	Function
Cortical Nephrons	85% of nephrons; mostly in cortex	Responsible for most filtration and reabsorption
Juxtamedullary Nephrons	Near medulla; long nephron loops	Important for concentrating urine

Blood Supply to the Kidneys

Renal Circulation

• Renal Artery: Supplies blood to the kidney.

• Segmental, Interlobar, Arcuate, and Interlobular Arteries: Branches that distribute blood within the kidney.

• Afferent Arteriole: Brings blood to the glomerulus.

• Efferent Arteriole: Carries blood away from the glomerulus.

• Peritubular Capillaries & Vasa Recta: Surround tubules for exchange of substances.

Urine Formation

Processes of Urine Formation

• Glomerular Filtration: Movement of water and solutes from blood into Bowman's capsule.

• Tubular Reabsorption: Selective movement of substances from filtrate back into blood.

• Tubular Secretion: Active transport of substances from blood into filtrate.

Key Equations

• Glomerular Filtration Rate (GFR): $GFR = K_f imes (P_{GC} - P_{BS} - ext{Oncotic Pressure})$ where $K_f$ is filtration coefficient, $P_{GC}$ is glomerular capillary pressure, $P_{BS}$ is Bowman's space pressure.

Characteristics of Urine

Urine Composition and Properties

• 95% water, 5% solutes

• Nitrogenous wastes: urea, uric acid, creatinine

• Hormones and drugs may be present

• 1-2 L produced daily (minimum 0.5 L required)

• Normal pH: 4.5-8.0

• Color: ranges from almost clear to dark yellow

Adrenal Glands

Location and Structure

• Located superior to each kidney

• Composed of cortex (outer) and medulla (inner)

• Cortex produces corticosteroids; medulla produces catecholamines (e.g., adrenaline)

Posterior Pituitary Gland

Role in Urinary System

• Stores and releases antidiuretic hormone (ADH) and oxytocin

• ADH increases water reabsorption in kidneys, reducing urine output

Ureters

Structure and Function

• Muscular tubes that transport urine from kidneys to bladder

• Wall layers: mucosa (transitional epithelium), muscularis, adventitia

Urinary Bladder

Structure and Function

• Hollow, muscular organ for urine storage

• Wall layers: mucosa (transitional epithelium), muscularis (detrusor muscle), adventitia

Female vs. Male Urethra

Comparative Anatomy

Feature	Female Urethra	Male Urethra
Length	~4 cm	~20 cm
Function	Urine excretion only	Urine and semen excretion
Location	Anterior to vaginal opening	Runs through penis

Micturition Reflex

Mechanism of Urine Expulsion

• Micturition: Process of expelling urine from the bladder

• Involves relaxation of detrusor muscle and contraction of internal urethral sphincter

• Controlled by spinal reflex and higher brain centers

Example:

When the bladder fills, stretch receptors signal the spinal cord, triggering the micturition reflex for urination.

Additional info: The notes include brief references to hormone regulation (ADH from posterior pituitary, corticosteroids from adrenal cortex) and their roles in kidney function and urine concentration.