Study Notes from Marieb Human Anatomy & Physiology, Chapter 25 on The Urinary System

Marieb, Human Anatomy & Physiology Twelfth Edition, prepared by Justin A. Moore, American River College.
Copyright © 2025 Pearson Education, Inc. All rights reserved.

Purpose of the Urinary System: - Eliminate wastes and toxins via urine. - Assist with understanding renal diseases, aiding in advising patients with renal issues.

Regulate Internal Environment: - Regulates total body water volume and solute concentration. - Regulates ion concentrations in extracellular fluid (ECF). - Ensures long-term acid-base balance. - Excretes metabolic wastes, toxins, and drugs. - Produces erythropoietin (RBC production regulator). - Produces renin (enzyme for synthesizing angiotensin, involved in blood pressure regulation). - Activates vitamin D (forming calcitriol, involved in ECF regulation). - Carries out gluconeogenesis (assists liver during prolonged fasting).

Components: - Kidneys: Main organs for urine formation. - Ureters: Transport urine from kidneys to urinary bladder. - Urinary Bladder: Temporary storage reservoir for urine. - Urethra: Transports urine out of the body.

Position: Retroperitoneal, bean-shaped kidneys located in the superior lumbar region. - Right kidney is slightly lower than the left due to the liver’s position. - Rib cage offers some protection to the upper parts.
Size and Shape: Similar to a large bar of soap, with convex lateral and concave medial surfaces containing the renal hilum. - Renal hilum: Deep cleft for the entry/exit of ureters, blood vessels, lymphatics, and nerves, leading to renal sinus.
Supporting Tissue Layers: - Renal Fascia: Outer layer of dense fibrous connective tissue. - Perirenal Fat Capsule: Fatty cushion. - Fibrous Capsule: Transparent organ capsule to prevent infection spread.

Regions: - Renal Cortex: Light-colored, granular appearance, external region. - Renal Medulla: Darker, reddish-brown area consisting of cone-shaped renal pyramids and renal columns. - Renal Pelvis: Funnel-shaped tube that collects urine, continuous with the ureter. - Major calyces branch into minor calyces which collect urine from pyramids.

Perirenal Fat: Provides cushioning but lower parts of kidneys are more susceptible to trauma. - Renal artery injuries can lead to hematuria; surgical treatments may be required.

Nephrons are the structural and functional units that filter blood and form urine.
Approximately 1 million nephrons per kidney.
Each nephron consists of a renal corpuscle (filtration component) and a renal tubule (where the filtrate is processed).

Consists of: - Glomerulus: Tuft of fenestrated capillaries allowing efficient formation of filtrate. - Glomerular Capsule (Bowman’s Capsule): Hollow structure surrounding the glomerulus with two layers: - Parietal Layer: Simple squamous epithelium. - Visceral Layer: Modified epithelial cells (podocytes) with filtration slits allowing selective passage of substances.

Structure: Approximately 1.2 in. long, consisting of simple epithelia with unique histological characteristics.
Major Parts: - Proximal Convoluted Tubule (PCT): Confined to cortex, made of cuboidal epithelium with microvilli for reabsorption. - Nephron Loop (Loop of Henle): U-shaped structure with descending and ascending limbs; descending limb is permeable to water, ascending limb is impermeable to water but pumps out solutes. - Distal Convoluted Tubule (DCT): Confined to cortex, thinner than PCT with fewer microvilli.
Collecting Ducts: Each collecting duct collects filtrate from multiple nephrons, with principal and intercalated cells facilitating urine concentration and acid-base balance.

Blood Supply: - Kidneys receive about 1200 ml of blood (one-fourth of cardiac output) per minute, crucial for cleansing and adjusting blood composition. - Arterial flow breakdown: - Aorta → Renal Artery → Segmental Artery → Interlobar Artery → Arcuate Artery → Cortical Radiate Artery. - Venous flow: Cortical radiate → Arcuate → Interlobar → Renal Veins (no segmental veins).
Nerve Supply: - Sympathetic vasomotor fibers from renal plexus control blood flow and influence urine formation.

Glomerulus: Specialized for filtration, fed/drained by arterioles, maintaining high glomerular blood pressure (BP). - Peritubular Capillaries: Surround renal tubules, low-pressure, pick up reabsorbed substances. - Vasa Recta: Thin-walled blood vessels associated with juxtamedullary nephrons, involved in maintaining osmotic gradients.

Urine formation involves three key processes: - Glomerular Filtration: Passive process where hydrostatic pressure forces fluids and solutes through the filtration membrane into glomerular capsule. - Tubular Reabsorption: Reclaims most tubular contents from filtrate back into blood (includes both active and passive transport mechanisms). - Tubular Secretion: Moves additional substances from blood into filtrate following the steps of reabsorption but in reverse.

Filtration Membrane Functionality: - Excludes blood proteins and cells, allowing solutes smaller than plasma proteins to pass into the capsule. - Composed of: - Fenestrated Endothelium: Excludes blood cells. - Basement Membrane: Repels large anions through negatively charged glycoproteins. - Podocyte Foot Processes: Form filtration slits restricting protein flow.
Net Filtration Pressure (NFP): Determined by hydrostatic pressures and colloid osmotic pressures affecting filtration rates.
Regulation of Glomerular Filtration Rate (GFR): Must be stable for homeostasis and involves both intrinsic and extrinsic control mechanisms.

Selective Transepithelial Process: substances can follow: - Transcellular Route: Across tubule cells. - Paracellular Route: Between tubule cells (through “leaky” junctions particularly in the PCT).
Regulation of Water Reabsorption: Multiple hormones (like ADH and aldosterone) regulate both passive and active transport of solutes into peritubular capillaries.

Importantly manages the disposal of drugs, metabolites, and any excess ions that the body needs to eliminate.

Micturition involves the contraction of the bladder detrusor muscle, opening of the internal urethral sphincter, and voluntary control of the external urethral sphincter.

Triggered by stretch receptors in the bladder wall; involves autonomic nervous system pathways and somatic control for external sphincter regulation.

Embryonic Kidney Development: Three successive sets of embryonic kidneys form, with the metanephros developing into adult kidneys around week 5 of gestation.
Common Congenital Anomalies: Include horseshoe kidney, hypospadias, and polycystic kidney disease.
Urinary Findings by Age: Highlights changes from childhood (increased instances of UTIs) to aged populations (decline in kidney function).

Identified Substances: Glucose (glycosuria), proteins (proteinuria), ketone bodies, hemoglobin, bile pigments, erythrocytes, and leukocytes (pyuria) could indicate various pathologies related to kidney and urinary function.