Urinary System

Urinary System Overview

• The urinary system is responsible for eliminating excess water, salts, and physiological wastes by producing urine.
    - Composed of the following structures:
      - Kidneys
      - Ureters
      - Urinary Bladder
      - Urethra
    - Note: The urinary tract includes the ureters, bladder, and urethra; the kidneys are not part of the tract.

Functions of the Urinary System

• The primary functions include:
    - Adjusting blood volume and blood pressure.
    - Regulating plasma concentrations of sodium, potassium, chloride, and other ions.
    - Stabilizing blood pH.
    - Conserving valuable nutrients.
    - Removing drugs, toxins, and metabolic wastes from the bloodstream.

Gross Anatomy of the Urinary System

• Major Structures:
    - Kidneys
    - Ureters
    - Urinary Bladder
    - Urethra
• Additional structures visible in an anterior view include:
    - Inferior vena cava
    - Aorta
    - Adrenal gland

Connective Tissues Supporting the Kidneys

• Fibrous Capsule:
    - A layer of collagen fibers covering the outer surface of the kidney, projecting collagen fibers through the perinephric fat to the renal fascia.
• Perinephric Fat:
    - A thick layer of adipose tissue surrounding the kidney.
• Renal Fascia:
    - Dense, fibrous outer layer that anchors the kidney to surrounding structures.

Internal Structure of the Kidney

• Kidney Anatomy:
    - Fibrous Capsule - Outer covering of the kidney.
    - Renal Cortex - Superficial layer of the kidney.
    - Renal Medulla - Inner layer, contains renal pyramids and columns.
    - Renal Pyramid - Cone-shaped structure extending from the cortex to the renal papilla.
    - Renal Column - Tissue separating the pyramids.
    - Kidney Lobe - Comprises a renal pyramid, overlying cortex, and adjacent renal columns.

Collecting Structures of the Kidney

• Minor Calyx:
    - Collects urine from one lobe of the kidney.
• Major Calyx:
    - Collects urine from 4-5 minor calyces.
• Renal Pelvis:
    - Collects urine from major calyces.
• Hilum:
    - Entry region for vessels, nerves, and ureter.
• Ureter:
    - Transports urine away from the kidney.

Nephrons

• Definition:
    - Microscopic functional units of the kidney responsible for urine production.
• Types of Nephrons:
    - Cortical Nephrons:
      - Comprise approximately 85% of kidney nephrons, primarily located in the cortex, responsible for most regulatory functions.
    - Juxtamedullary Nephrons:
      - About 15% of nephrons, characterized by long nephron loops extending deep into the medulla, essential for producing concentrated urine.

Structure of a Nephron

• Components of a Nephron:
    - Renal Corpuscle:
      - Composed of Bowman's Capsule (Glomerular Capsule) and the Glomerulus.
    - Renal Tubule:
      - Includes:
        - Proximal Convoluted Tubule (PCT)
        - Nephron Loop (Loop of Henle)
        - Distal Convoluted Tubule (DCT).

Processes within Nephrons

• Renal Corpuscle:
    - Site of filtration; blood pressure forces water and solutes from glomerular capillaries into the capsular space, producing filtrate which is protein-free and similar to blood plasma.
• Renal Tubule:
    - Tubular passage approximately 50 mm long, receives and modifies filtrate to form urine.
    - PCT:
      - Reabsorbs nutrients from the filtrate.
    - Nephron Loop:
      - Establishes an osmotic gradient for water reabsorption.
    - DCT:
      - Adjusts tubular fluid composition through reabsorption and secretion.

Collecting System

• Comprises a series of tubes carrying tubular fluid away from nephrons:
    - Collecting Duct:
      - Collects fluid from many nephrons, carries it through the renal medulla.
      - Lined with:
        - Intercalated Cells (secrete and reabsorb hydrogen and bicarbonate ions).
        - Principal Cells (reabsorb water and secrete potassium).
    - Papillary Duct:
      - Collects fluid from multiple collecting ducts and delivers it to the minor calyx.

Blood Supply to the Kidneys

• Arterial Flow:
    - Renal Artery: Branches off the aorta.
      - Progresses through:
        - Segmental Arteries
        - Interlobar Arteries
        - Arcuate Arteries
        - Cortical Radiate Arteries
        - Afferent arterioles supply blood to individual nephrons.
    - Glomerulus: Site of filtration.
    - Efferent Arterioles: Exit the glomerulus.
    - Peritubular Capillaries and Vasa Recta (parallel nephron loop, crucial for urine concentration).
• Venous Flow:
    - Involves:
      - Cortical radiate veins, arcuate veins, interlobar veins, and renal veins, returning blood to the inferior vena cava.
    - Notably, no segmental veins are present.

Nephron Innervation

• Each kidney contains approximately 1.25 million nephrons, innervated by renal nerves with most being sympathetic postganglionic fibers.
• Functions of Nerve Fibers:
    - Adjust blood flow and pressure at glomeruli.
    - Stimulate the release of renin.

Urine Formation Processes

• Three primary processes:
    - Filtration:
      - Blood pressure forces water and solutes across glomerular capillary membranes into the capsular space.
    - Reabsorption:
      - Transport of water and solutes from tubular fluid into the peritubular fluid.
    - Secretion:
      - Transport of solutes from peritubular fluid into tubular fluid.

Glomerular Structure and Function

• Glomerular Capsule:
    - Forms the outer wall of the renal corpuscle and covers the glomerular capillaries.
• Filtration Membrane Composition:
    - Fenestrated Glomerular Capillaries - Large pores allow fluid to pass; prevents most plasma proteins from entering.
    - Dense Layer: Specialized basement membrane.
    - Filtration Slits - Create an additional barrier via podocytes.

Factors Controlling Glomerular Filtration

• Glomerular Hydrostatic Pressure (GHP):
    - Average of 50 mm Hg; pushes water and solutes out of plasma into filtrate.
• Capsular Colloid Osmotic Pressure:
    - Typically 0 mm Hg; few plasma proteins enter capsular space.
• Blood Colloid Osmotic Pressure (BCOP):
    - Averages approximately 25 mm Hg, opposes filtration.
• Capsular Hydrostatic Pressure (CsHP):
    - Averages 15 mm Hg; opposes GHP.
• Net Filtration Pressure (NFP):
    - Approximately 10 mm Hg, forces water and dissolved substances from glomerular capillaries into the capsular space.

Glomerular Filtration Rate (GFR)

• Amount of filtrate produced by the kidneys per minute; averages 125 mL/min (180 L/day), with about 99% reabsorbed.

Control of GFR

• Two levels of control stabilizing GFR:
    - Autoregulation: Local level adjustments.
    - Central Regulation: Involves endocrine response from kidneys and neural response via sympathetic nervous system.

Reabsorption in Renal Tubule

• Mainly occurs in Proximal Convoluted Tubule (PCT):
    - Reabsorbs >99% of glucose, amino acids, some ions, and about 108 liters of water daily.

Secretion in Distal Convoluted Tubule (DCT)

• Modification of tubular fluid composition through reabsorption (Na+ for K+) and secretion of toxins/drugs.

Countercurrent Multiplication

• Occurs in the Nephron Loop:
    - Thin descending limb is permeable to water; thick ascending limb actively transports Na+ and Cl–, creating an osmotic gradient necessary for water reabsorption.

Urine Production Summary

• Filtration happens at the renal corpuscle.
• PCT, nephron loop, and DCT play roles in reabsorbing critical ions and water while concentrating urine.

Concentration of Urine

• Regulation of water reabsorption (facultative via ADH) leads to changes in urine concentration, with possible levels reaching ~450 mOsm/L at the papillary duct.

Pathophysiology of the Urinary System

• Renal Failure:
    - Impairs many body systems; symptoms include reduced urine production, fluid balance disturbances, hypertension, and anemia.
• Chronic Renal Failure:
    - Gradual deterioration; management focuses on reducing strain on the urinary system.
• Acute Renal Failure:
    - Rapid deterioration; possible recovery if initial incident addressed properly.
• Renal Failure Treatment: Options include dialysis and kidney transplant.

Urinary Tract

• Composed of:
    - Ureters (muscular tubes conducting urine to the bladder)
    - Urinary Bladder (muscular organ holding up to a liter of urine)
    - Urethra (extending from the bladder to the body exterior, differing between sexes in length and function).

Key Features of the Urinary Bladder

• Trigone region: triangular area channeling urine into the urethra.
• Muscular layers: Form the detrusor muscle aiding in urine expulsion.

Micturition Reflex

• Coordinates urination through local reflex pathways and central pathways involving the cerebral cortex, regulated by stretch receptors signaling bladder fullness.

Urinary Disorders

• Primary signs include alterations in urine volume or appearance:
    - Polyuria: Excessive urine production.
    - Oliguria: Reduced urine production.
    - Anuria: Severely reduced urine output.
    - Both oliguria and anuria indicate severe kidney distress or potential failure.