Urinary System Overview

Introduction to the Urinary System

• Definition: The urinary system removes metabolic wastes produced by the body’s cells.
• Main Function: Kidneys remove metabolic wastes from circulation and produce urine.

Organs and Functions of the Urinary System

Organs

• Kidneys: Paired organs that produce urine.
• Urinary Tract: Eliminates urine, including:
  • Ureters: Paired tubes transporting urine from kidneys to bladder.
  • Urinary Bladder: Muscular sac that stores urine.
  • Urethra: Exit tube for urine from bladder to outside body.

Process of Urination

• Urination (Micturition): Process of eliminating urine involves the contraction of the muscular urinary bladder, forcing urine through the urethra.

Functions of the Urinary System

1. Excretion: Removal of metabolic wastes from body fluids.
2. Elimination: Discharge of wastes from the body.
3. Homeostatic Regulation: Regulates volume and solute concentration of blood.

Homeostatic Functions of the Urinary System

Regulating Blood Volume and Pressure

• Adjusts volume of water lost in urine.
• Releases erythropoietin for RBC formation and renin for water/Na/Cl balance.
• Controls quantities of sodium, potassium, and chloride lost in urine.
• Maintains calcium ion levels via calcitriol synthesis.

Stabilizing Blood pH

• Controls loss of hydrogen and bicarbonate ions in urine.

Nutrient Conservation

• Preserves valuable nutrients while removing metabolic wastes (e.g., urea, uric acid).
• Assists liver in detoxifying poisons and deaminating amino acids during starvation.

The Kidneys and Nephrons

Kidney Location and Structure

• Location: On either side of the vertebral column; left kidney is slightly superior to right.
• Protection & Stabilization:
  1. Fibrous Capsule: Collagen fibers covering the kidney.
  2. Perinephric Fat: Adipose tissue surrounding the capsule for insulation/protection.
  3. Renal Fascia: Dense fibrous outer layer anchoring kidney.

Kidney Dimensions

• Typical adult kidney dimensions: about 10 cm long, 5.5 cm wide, and 3 cm thick; weighs around 150 g.
• Hilum: Medial indentation where renal artery, renal nerves enter, and renal vein, ureter exit.

Renal Structures

• Renal Cortex: Superficial region in contact with fibrous capsule.
• Renal Pyramids: 6 to 18 triangular structures with bases abutting cortex; tips project into renal sinus.
• Renal Columns: Tissue bands separating adjacent renal pyramids.
• Kidney Lobe: Contains a renal pyramid, area of cortex, and tissues of renal columns; produces urine.
• Renal Pelvis: Collection area for urine formed in kidney, connecting to ureter.

Blood Supply to the Kidneys

• Kidneys receive 20-25% of total cardiac output (~1200 mL/min).
• Blood flow through each kidney via renal arteries.
• Renal Nerves: Adjust urine formation by influencing blood flow at nephron and stimulating renin release.

Nephrons: Functional Units of Kidney

• Structure: Each nephron consists of renal corpuscle and renal tubule, with ~1 million per kidney.
• Renal Corpuscle: Contains Glomerular (Bowman’s) capsule and Glomerulus (capillary network).
  • Podocytes: Cells with foot processes that wrap around glomerular capillaries, involved in filtration.

Filtration Process

• Filtration Membrane: Consists of fenestrated endothelium, basement membrane, and foot processes of podocytes.
• Blood pressure forces water and small solutes into capsular space, producing a protein-free filtrate.

Renal Tubule Functions

• Proximal Convoluted Tubule (PCT): Reabsorbs 99% of filtrate including organic nutrients and ions.
• Nephron Loop (Loop of Henle): Reabsorbs water and selected ions.
• Distal Convoluted Tubule (DCT): Reabsorption of water and ions; actively secretes unwanted substances.
• Collecting System: Transports tubular fluid from nephrons to renal pelvis, adjusting composition and determining final urine volume.

Renal Physiology: Urine Formation

1. Filtration: Water and solutes exit blood into filtrate.
2. Reabsorption: Moves water and solutes from filtrate back into blood.
3. Secretion: Transports solutes from blood into filtrate.

Metabolic Wastes

• Main wastes: Urea, Creatinine, and Uric acid produced by cellular metabolic processes.
• Removal is crucial as it helps regulate blood volume and composition.

Glomerular Filtration Rate (GFR)

• GFR: The volume of filtrate produced each minute: averages 125 mL/min.
• Regulated by:
  1. Autoregulation (local level).
  2. Hormonal regulation (renin-angiotensin-aldosterone system and natriuretic peptides).
  3. Autonomic regulation (sympathetic division of ANS).

Hormonal Regulation

• RAAS: Release of renin in response to decreased blood flow, regulates blood pressure and GFR by vasoconstriction and sodium retention.
• Atrial Natriuretic Peptide (ANP): Released in response to increased blood volume, promotes GFR and inhibits renin.

Reabsorption and Secretion

• Involves diffusion, osmosis, and carrier-mediated transport.
• Types of transport include facilitated diffusion, active transport, co- and counter-transport.
• Transport Maximum (Tm): Rate of reabsorption increases until carrier proteins are saturated; excess remains in tubular fluid.

Key Points on Transport Mechanisms

• Sodium reabsorption critical for overall renal function and fluid balance.
• Hormones (e.g., aldosterone, ADH) modulate activity of ion pumps for effective reabsorption.

Urine Composition and Transport

• Urine is a clear, sterile liquid reflecting nephron activities and metabolic waste elimination.
• Analyzing Urine: Urinalysis is used to detect various metabolites and can indicate kidney function or other medical conditions.

Anatomy of Urinary System Components

• Ureters: Muscular tubes carrying urine from kidneys to bladder, prevent backflow of urine.
• Urinary Bladder: Stores urine, regulated by internal sphincter and detrusor muscle contractions.
• Urethra: Transports urine from bladder to outside of body; varies significantly between genders.

Reflexes for Urination

• Storage Reflex: Maintains urination control; inhibits bladder contraction while stimulating internal sphincter closure.
• Voiding Reflex: Occurs when bladder is full, promotes urination by contracting detrusor muscle and relaxing sphincters.

Age Considerations

• Infants have no voluntary control over urination due to undeveloped spinal connections.
• Incontinence: Loss of voluntary control can be due to various medical or physiological issues.