Urinary System Overview

Images taken from Principles of Anatomy & Physiology (Tortora)Presenter: Claire Dodd, Ph.D.

Functions of the Urinary System

• Manages the volume and composition of bodily fluid reservoirs.

• Organs include:

  • Kidneys (2): Filter blood to produce urine, regulating electrolytes, metabolites, and toxins.

  • Ureters (2): Transport urine from the kidneys to the bladder via peristalsis, which is a series of wave-like muscle contractions.

  • Urinary Bladder (1): Hollow muscular organ that stores urine until excretion, typically holding about 750 mL of urine.

  • Urethra (1): Exit point for urine from the body, differing in length between males (approximately 20 cm) and females (approximately 4 cm), and serves as a common duct for the reproductive system in males.

Specific Functions

• Blood Ionic Composition: Regulation of sodium (Na+), potassium (K+), and chloride (Cl–) levels through hormonal mechanisms, particularly aldosterone.

• Blood pH Regulation: Involves hydrogen ions (H+) and bicarbonate (HCO3–) buffering systems to maintain acid-base balance.

• Blood Volume Regulation: Primarily regulates the volume of water (H2O); sodium and potassium levels affect water retention through osmotic pressure.

• Hormone Production:

  • Calcitriol: Active form of Vitamin D, crucial for calcium metabolism.

  • Erythropoietin: Stimulates red blood cell production in response to low oxygen levels in the blood.

• Excretion: Removes metabolic wastes (e.g., urea, creatinine) and foreign substances (e.g., penicillin) from the body.

• Blood Glucose Regulation: Nephrons can reabsorb glucose and in prolonged fasting states, gluconeogenesis occurs in kidneys.

• Blood Pressure Maintenance: Through the renin-angiotensin-aldosterone system (RAAS), which increases blood volume and constricts blood vessels.

• Blood Osmolarity Maintenance: The kidneys adjust the concentration of the urine to manage osmotic gradients effectively.

Anatomy of the Kidneys

• Location: Positioned retroperitoneally and partially protected by lower ribs.

• Surrounding Structures: Neighboring structures include the inferior vena cava, peritoneum, renal fascia, and the adipose capsule that surrounds the kidneys.

• Blood Supply: Receives 20-25% of resting cardiac output for fluid homeostasis, with renal arteries branching directly from the abdominal aorta.

Kidney Composition

• Cortical Tissue: Contains nephrons; plays a role in blood perfusion and synthesis of erythropoietin.

• Medullary Tissue: Arranged in pyramids, involved in urine concentration and is home to various nephron segments.

Nephrons

• Approximately 1.5 million nephrons per kidney are responsible for filtering and modifying systemic blood.

• Function: Filtering blood to produce filtrate, which passes through various nephron structures.

• Blood flow is regulated by sympathetic innervation to adapt to the needs for filtration and urine formation.

Filtration Process

Nephrons filter blood to produce filtrate, following the path:

• Collecting duct → Papillary duct → Minor calyx → Major calyx → Renal pelvis → Ureter.

Learning Goals

• Identify and describe organs and tissues of the urinary system.

• Describe kidney microanatomy and nephron anatomy, including:

  • Blood circulation.

  • Differences between cortex and medulla.

  • Pathway of filtrate.

Nephrons Anatomy

• Renal Corpuscle: Site of filtration, comprising the glomerulus (a tuft of capillaries) and Bowman’s capsule (which collects the filtrate).

• Renal Tubule: Modifies filtrate through secretion and reabsorption processes, which include proximal tubule, loop of Henle, distal tubule, and collecting duct.

Glomerular Filtration Components

• Filtration Membrane: Composed of glomerulus and Bowman’s capsule, allowing for selective permeability based on size.

• Three Forces Control Filtration:

  • Hydrostatic Pressure: Blood pressure in capillaries promoting filtrate formation.

  • Colloid Osmotic Pressure: Pull from proteins in blood opposing filtration.

  • Capsular Hydrostatic Pressure: Counteracts filtration from the collected fluid in Bowman’s capsule.

Urine Formation and Composition

• Glomerular Filtration Rate (GFR): Averages 125 mL/min in males and 105 mL/min in females; critical for waste removal and maintaining homeostasis. If GFR is too high, essential nutrients are lost; if too low, waste products accumulate.

• Juxtaglomerular Apparatus: Monitors and regulates blood pressure and GFR via renal autoregulation.

Processes in Nephrons

Three Physiological Processes:

1. Glomerular filtration: Initial process in urine formation.

2. Tubular reabsorption: Returns valuable substances like water, glucose, and ions back into the blood.

3. Tubular secretion: Moves wastes and excess ions from the blood into the filtrate.

Reabsorption & Secretion in the Nephron Tubule

• Reabsorption: Primarily occurs in the proximal convoluted tubule with reabsorption of water, ions, and nutrients; less in the loop of Henle and distal convoluted tubule.

• Secretion: Involves H+, K+, and various toxins to maintain homeostasis.

Role of ADH and Hormones

• ADH (Antidiuretic Hormone): Promotes water reabsorption in the kidneys during states of dehydration, concentrating urine to conserve water.

• Counter-current Multiplier: Mechanism in the loop of Henle that develops an osmotic gradient essential for the formation of concentrated urine.

Anatomy of the Bladder and Urethra

• Bladder: Hollow muscular organ with the capacity to expand and contract.

• Ureters: Carry urine from each kidney to the bladder, using peristalsis for movement.

• Urethra: Varies significantly in length between sexes and serves dual functions in males for urine and reproductive system.

Micturition Reflex

• Involves both voluntary and involuntary muscle contractions, triggered by stretch receptors in the bladder wall.

  • Involuntary: During storage, controlled by sympathetic stimulation that inhibits bladder contraction.

  • Voluntary: During voiding, controlled by the somatic nervous system (external urethral sphincter).

Learning Goals Overview

• Define tubular reabsorption/secretion and explain the roles of hormones in these processes.

• Detail the mechanisms of urine concentration, highlighting the kidneys' role in producing calcitriol and erythropoietin, as well as summarize the micturition process.