Chapter 25: The Urinary System

General Functions of the Kidney and Urinary System

The urinary system is essential for maintaining homeostasis by regulating the composition and volume of blood, removing waste products, and controlling electrolyte and acid-base balance.

• Excretion: Removal of metabolic wastes, toxins, and drugs from the blood.

• Regulation: Controls blood volume, blood pressure, and osmolarity.

• Homeostasis: Maintains electrolyte, acid-base, and fluid balance.

• Hormone Production: Secretes erythropoietin (stimulates RBC production) and renin (regulates blood pressure).

• Metabolic Functions: Activates vitamin D and performs gluconeogenesis during fasting.

Organs of the Urinary System

The urinary system consists of four main organs:

• Kidneys: Filter blood and produce urine.

• Ureters: Transport urine from kidneys to bladder.

• Urinary Bladder: Stores urine until excretion.

• Urethra: Conducts urine from bladder to outside the body.

Kidney Location and External Anatomy

The kidneys are retroperitoneal, located on either side of the vertebral column between T12 and L3.

• External Anatomy: Each kidney is bean-shaped, with a hilum (entry/exit for vessels and ureter), surrounded by a fibrous capsule, adipose tissue, and renal fascia.

Internal Gross Anatomy of the Kidney

The kidney is organized into distinct regions, each with specialized functions.

• Renal Cortex: Outer region; contains most nephrons.

• Renal Medulla: Inner region; contains renal pyramids.

• Renal Pyramids: Cone-shaped structures; contain collecting ducts.

• Renal Columns: Extensions of cortex between pyramids.

• Minor and Major Calyces: Collect urine from pyramids; minor calyces merge to form major calyces.

• Renal Pelvis: Funnel-shaped cavity; collects urine and leads to ureter.

Blood Vessels of the Kidney

The kidneys receive blood via the renal arteries, which branch into smaller vessels to supply nephrons.

• Renal artery → segmental arteries → interlobar arteries → arcuate arteries → cortical radiate arteries → afferent arterioles → glomerulus → efferent arterioles → peritubular capillaries/vasa recta → cortical radiate veins → arcuate veins → interlobar veins → renal vein

The Nephron: Functional and Structural Unit

Each kidney contains about 1 million nephrons, which filter blood and form urine.

• Renal Corpuscle: Consists of the glomerulus and Bowman's (glomerular) capsule.

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

Anatomical Parts of the Nephron

• Glomerulus: Capillary network for filtration.

• Bowman's Capsule: Surrounds glomerulus; collects filtrate.

• Proximal Convoluted Tubule (PCT): Reabsorbs water, ions, nutrients.

• Nephron Loop (Loop of Henle): Descending limb (permeable to water), ascending limb (permeable to ions).

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

• Collecting Duct: Final adjustments to urine; regulated by hormones.

Classes of Nephrons

• Cortical Nephrons: Located mostly in cortex; short loops of Henle.

• Juxtamedullary Nephrons: Located near medulla; long loops of Henle; important for concentrating urine.

Capillary Beds of the Nephron

• Glomerulus: Filtration of blood.

• Peritubular Capillaries: Reabsorption and secretion around PCT and DCT.

• Vasa Recta: Surrounds loop of Henle; maintains medullary concentration gradient.

Juxtaglomerular Complex (JGC/JGA)

The JGC regulates blood pressure and filtration rate.

• Location: Where DCT contacts afferent arteriole.

• Components: Macula densa (senses NaCl), granular cells (secrete renin), extraglomerular mesangial cells.

• Function: Regulates GFR and blood pressure via renin release.

Renal Processes

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

• Reabsorption: Return of useful substances from filtrate to blood.

• Secretion: Addition of waste and excess substances from blood to filtrate.

These processes occur in different parts of the nephron:

• Filtration: Glomerulus

• Reabsorption: PCT, loop of Henle, DCT, collecting duct

• Secretion: PCT, DCT, collecting duct

Filtration Process and Filtration Membrane

Filtration occurs at the glomerulus, where blood pressure forces plasma through a specialized membrane.

• Filtration Membrane: Consists of three layers:

  • Fenestrated endothelium of glomerular capillaries

  • Basement membrane

  • Podocyte foot processes (visceral layer of Bowman's capsule)

• Function: Produces a cell- and protein-free filtrate.

Filtrate vs. Blood vs. Plasma

• Filtrate: Similar to plasma but lacks proteins and cells.

• Blood: Contains cells, proteins, and plasma.

• Plasma: Liquid component of blood; contains proteins.

Pressures Affecting Filtration

Three pressures determine net filtration pressure (NFP):

• Glomerular hydrostatic pressure (outward): Promotes filtration.

• Capsular hydrostatic pressure (inward): Opposes filtration.

• Blood colloid osmotic pressure (inward): Opposes filtration.

Net Filtration Pressure (NFP):

• Equation:

• Where HPg = glomerular hydrostatic pressure, OPg = glomerular osmotic pressure, HPc = capsular hydrostatic pressure.

Regulation of Glomerular Filtration Rate (GFR)

GFR is the volume of filtrate formed per minute by both kidneys.

• GFR depends on: NFP, surface area, and permeability of filtration membrane.

• Relationship: If NFP increases, GFR increases; if NFP decreases, GFR decreases.

Intrinsic Controls (Renal Autoregulation)

• Myogenic Mechanism: Smooth muscle in afferent arteriole responds to pressure changes.

• Tubuloglomerular Feedback: Macula densa senses NaCl; adjusts afferent arteriole diameter.

Extrinsic Controls

• Sympathetic Nervous System: Constricts afferent arterioles, reduces GFR during stress.

• Hormonal Mechanism (RAAS): Renin released, activates angiotensin II, stimulates aldosterone, increases Na+ reabsorption and blood pressure.

Definitions of Key Terms

• Reabsorption: Movement of substances from filtrate back into blood.

• Secretion: Movement of substances from blood into filtrate.

• Renal Plasma Clearance: Volume of plasma cleared of a substance per unit time.

• Micturition: Process of urination.

Reabsorption Process

Reabsorption occurs throughout the renal tubule and collecting duct.

• Transcellular Transport: Active and passive movement through cells (e.g., sodium).

• Paracellular Transport: Movement between cells (e.g., some ions).

• Osmosis: Water reabsorption.

• Active Transport: Requires energy (e.g., sodium).

• Passive Transport: Does not require energy (e.g., water, some ions).

• Water cannot be actively transported.

Glucose and Amino Acid Reabsorption

• Normally, all glucose and amino acids are reabsorbed in the PCT.

• Transport Maximum: If exceeded (e.g., in diabetes mellitus), glucose appears in urine.

Nephron Loop (Loop of Henle) Structure and Function

• Juxtamedullary Nephrons: Have long loops; crucial for urine concentration.

• Descending Limb: Permeable to water, impermeable to solutes.

• Ascending Limb: Permeable to solutes (Na+, Cl-), impermeable to water.

Countercurrent Multiplication

Countercurrent multiplication creates a concentration gradient in the renal medulla, allowing for water reabsorption and urine concentration.

• Arrangement: Opposing flow in descending and ascending limbs.

• Purpose: Enables formation of concentrated urine.

Collecting Duct Permeability and ADH

• Location: Runs through medulla to renal pelvis.

• ADH (Antidiuretic Hormone): Increases water permeability by inserting aquaporins.

• Without ADH: Collecting duct is impermeable to water; urine is dilute.

• With ADH: Collecting duct is permeable; urine is concentrated.

• ADH is secreted by: Posterior pituitary in response to increased plasma osmolarity.

Controls Over Sodium Reabsorption and Potassium Secretion

• Aldosterone: Increases Na+ reabsorption and K+ secretion in DCT and collecting duct.

• RAAS: Renin-angiotensin-aldosterone system regulates aldosterone release.

Secretion Process

• Opposite of reabsorption: Substances move from blood to filtrate.

• Active and passive transport: Both mechanisms involved.

• Potassium and hydrogen ion secretion: Linked; helps regulate acid-base and electrolyte balance.

• Acidosis/Alkalosis: Affects H+ secretion; hyper/hypokalemia affects K+ secretion.

Renal Plasma Clearance

Clearance depends on filtration, reabsorption, and secretion.

Kidney's Role in Acid/Base and Electrolyte Balance

• Acid/Base Balance: Secretes H+, reabsorbs HCO3-.

• Electrolyte Balance: Regulates Na+, K+, Cl-, Ca2+.

Normal Composition of Urine and Urinalysis

• Normal Urine: Water, urea, creatinine, ions.

• Abnormal Results: Erythrocytes (bleeding), glucose (diabetes), proteins (kidney damage).

Bladder Anatomy

• Detrusor Muscle: Smooth muscle for contraction.

• Transitional Epithelium: Allows stretching.

• Rugae: Folds for expansion.

• Adventitia: Outer connective tissue.

Ureters and Bladder Connection

• Ureters: Enter bladder obliquely; prevents reflux by closing during bladder contraction.

Urethral Sphincters: Location and Function

Male vs. Female Urethra

• Male: Longer, passes through prostate and penis; also carries semen.

• Female: Shorter, only carries urine.

Micturition Mechanism

Micturition is the process of emptying the bladder, controlled by reflexes and voluntary actions.

• Guarding Reflex: Prevents involuntary urination.

• Voiding Reflex: Initiates urination.

• Detrusor Muscle: Innervated by parasympathetic nervous system.

• Internal Urethral Sphincter: Innervated by sympathetic nervous system.

• External Urethral Sphincter: Innervated by somatic nervous system.

Additional info: The urinary system is critical for maintaining overall health by regulating waste removal, fluid balance, and blood pressure.