The Urinary System

Chapter 25: The Urinary System

Introduction to the Kidneys

  • Role of Kidneys:

    • Regulation of body’s internal environment:

    • Maintains total body water volume and solute concentration.

    • Regulates ion concentrations in extracellular fluid (ECF).

    • Ensures long-term acid-base balance.

    • Excretion:

    • Excretes metabolic wastes, toxins, and drugs.

    • Production of hormones:

    • Produces erythropoietin (RBC production regulation).

    • Produces renin (enzyme aiding in Angiotensin synthesis, involved in blood pressure regulation).

    • Activates vitamin D, forming calcitriol (hormone involved in ECF regulation).

    • Gluconeogenesis:

    • Assists liver during prolonged fasting.

Overview of the Urinary System

  • Components of the Urinary System:

    • Kidneys: Main organs of urine production.

    • Ureters: Transport urine from kidneys to urinary bladder.

    • Urinary Bladder: Temporary storage reservoir for urine.

    • Urethra: Transport urine out of the body.

Anatomy and Location of the Kidneys

  • External Features:

    • Bean-shaped, retroperitoneal organs located in the superior lumbar region.

    • The right kidney is positioned lower than the left, due to the crowded liver.

    • Rib cage provides some protection to the kidneys.

    • Each kidney is topped by an adrenal (suprarenal) gland.

    • Overall Size: Comparable to a large bar of soap with a convex lateral surface and a concave medial surface containing the renal hilum.

  • Supportive Tissue Layers:

    • Renal Fascia: Outer layer of dense fibrous connective tissue that anchors them.

    • Perirenal Fat Capsule: Provides cushioning.

    • Fibrous Capsule: Transparent layer that prevents the spread of infection.

Internal Anatomy of the Kidneys

  • Regions:

    • Renal Cortex:

    • Light-colored, superficial region with a granular appearance.

    • Renal Medulla:

    • Darker reddish-brown region consisting of:

      • Cone-shaped medullary (renal) pyramids.

      • Renal columns separating the pyramids.

    • Renal Pelvis: Funnel-shaped tube where urine collects before entering the ureter.

  • Urine Flow System:

    • Urine collects in minor calyces from renal papillae, merges into major calyces, and from there flows into the renal pelvis.

Clinical Aspects of Kidney Function

  • Homeostatic Imbalances:

    • Renal Trauma: The lower parts of the kidneys (especially the right) are susceptible to blunt trauma.

    • Hematuria: Presence of blood in urine is often a sign of injury or disease.

    • Infections: Pyelonephritis, characterized by fever and lower back pain, can arise from ascendant infections or hematogenous spread.

Blood and Nerve Supply to the Kidneys

  • Blood Supply: The kidneys receive a significant blood supply; renal arteries deliver about 1,200 ml/min (one-fourth of cardiac output).

    • Arterial Flow Pathway:

    • Abdominal aorta → Renal artery → Segmental artery → Interlobar artery → Arcuate artery → Cortical radiate artery.

    • Venous Return: Cortical radiate → Arcuate → Interlobar → Renal veins.

  • Nerve Supply:

    • Sympathetic fibers from the renal plexus control blood flow and influence urine formation.

Nephron: Functional Unit of the Kidney

  • Structure of Nephron:

    • Approximately 1 million nephrons per kidney, each comprising:

    • Renal corpuscle: Includes glomerulus and glomerular (Bowman’s) capsule.

    • Renal tubule:

      • Proximal convoluted tubule (PCT).

      • Nephron loop.

      • Distal convoluted tubule (DCT).

    • Each empties into one of thousands of collecting ducts.

Renal Corpuscle

  • Components:

    • Glomerulus: A tuft of fenestrated capillaries specialized for filtration.

    • Glomerular Capsule: Cup-shaped structure surrounding glomerulus, composed of two layers:

    • Parietal Layer: Simple squamous epithelium.

    • Visceral Layer: Made of podocytes with filtration slits.

Renal Tubule and Collecting Duct

  • Structure: Each tubule section has unique histology and functions.

    • Proximal Convoluted Tubule (PCT): Involved primarily in reabsorption and secretion.

    • Nephron Loop:

    • Descending limb allows for water reabsorption, ascending limb involves solute reabsorption but no water.

    • Distal Convoluted Tubule (DCT): Last section of nephron; connects to collecting ducts which transport urine to the renal pelvis.

Types of Nephrons

  • Cortical Nephrons: 85%, mostly located in the cortex with short nephron loops.

  • Juxtamedullary Nephrons: Originate near the cortex-medulla junction, with long nephron loops, crucial for urine concentration.

Capillary Beds and Nephron Function

  • Glomerulus: Specialized for filtration, receives blood via afferent arterioles and drains via efferent arterioles.

  • Peritubular Capillaries: Surround renal tubules, facilitating reabsorption from filtrate back into the blood.

  • Vasa Recta: Key in urine concentration, preserve medullary osmotic gradient.

Juxtaglomerular Complex (JGC)

  • Composed of macula densa (NaCl sensors in DCT), granular cells (JG cells that release renin), and extraglomerular mesangial cells (signal transport between JG cells and macula densa).

Urine Formation Processes

  • Overview: Formed through glomerular filtration, tubular reabsorption, and secretion.

  • Glomerular Filtration: Passive process driven by hydrostatic pressure, creating filtrate which is plasma devoid of proteins.

  • Tubular Reabsorption: Selective process reclaiming water and nutrients.

Phases of Urine Formation

  1. Filtration: Occurs in the glomerulus via hydrostatic pressure.

  2. Reabsorption: Most occurs in PCT, nephron loop, and DCT, involving both active and passive transport mechanisms.

  3. Secretion: Moves selected substances from blood into nephron for elimination.

Regulation of Glomerular Filtration Rate (GFR)

  • GFR is crucial for maintaining homeostasis, involving intrinsic (myogenic response and tubuloglomerular feedback) and extrinsic controls (sympathetic and hormonal mechanisms).

  • Key Pressure Influencers:

    • Net Filtration Pressure (NFP): Determined by blood pressures and the properties of the filtration membrane.

Hormonal Regulation

  • Renin-Angiotensin-Aldosterone System (RAAS): In response to low BP stimulates renin release, increasing blood volume and pressure.

Urinalysis and Pathological Indicators

  • Composition of normal urine: 95% water and 5% solutes including nitrogenous wastes.

  • Abnormal Urine Constituents:

    • Glucose (glycosuria), proteins (proteinuria), blood (hematuria), etc., indicating various potential diseases or disorders.

Anatomy of Ureters, Bladder, and Urethra

  • Ureters: Tubes delivering urine from kidneys to bladder.

  • Urinary Bladder: Stores urine, capable of distension and involuntary during resting state.

    • Three layers: Mucosa, muscular layer (detrusor), fibrous adventitia.

  • Urethra: Muscular tube that directs urine out of the body, featuring internal (smooth muscle) and external (skeletal muscle) sphincters.

Clinical Considerations

  • Renal Calculi (kidney stones): Can obstruct urine flow, causing pain.

  • Urinary Tract Infections (UTIs): Common, particularly in females due to anatomical proximity.

  • Micturition: Process of urination involving smooth and skeletal muscle control. \n- Incontinence: Can result from various factors, including childbirth or surgery.

Developmental Aspects

  • Developmental kidney anomalies can lead to significant health issues, including horseshoe kidney, hypoplastic kidneys, and polycystic kidney disease.

  • GFR changes with age: Decreases with age, affecting kidney function and urine regulation.

End of Chapter 25 Notes