urinary system anatomy

Overview

  • Functions of the urinary system include:

    • Adjusting blood volume and blood pressure.

    • Regulating blood plasma concentrations of sodium, potassium, chloride, and other ions.

    • Stabilizing blood pH.

    • Conserving valuable nutrients by preventing their loss in urine.

    • Eliminating metabolic wastes.

    • Removing drugs and toxins from the bloodstream.

Components of the Urinary System

  • Kidneys

    • Function: Filters blood, creating urine.

  • Ureters

    • Function: Carries urine to the bladder.

  • Bladder

    • Function: Temporary storage for urine.

  • Urethra

    • Function: Carries urine to the body's exterior.

Kidneys

  • Location:

    • Retroperitoneal (located behind the peritoneum).

    • Positioned between the T12 and L3 vertebrae.

    • The right kidney is slightly lower than the left due to liver positioning.

  • Features:

    • Connective tissue coverings:

    • Renal (fibrous) capsule: Collagen fibers covering the kidney's outer surface.

    • Renal fat pad (adipose capsule): Adipose tissue that surrounds and protects the kidney.

    • Renal fascia: Anchors the kidney to surrounding structures.

    • Regions:

    • Renal cortex: Outer area.

    • Renal medulla: Middle region.

    • Renal hilum: Curved entry/exit area for blood vessels, nerves, and ureter.

    • Renal sinus: Empty space within the kidney not involved in filtering blood.

  • Structural Features:

    • Renal pyramid: Striped appearance due to nephron's collecting system.

    • Renal papilla: Pointed tip of each pyramid.

    • Renal column: Tissue extending from cortex between pyramids.

  • Urinary Collection System:

    • Minor calyx: Collects urine from each pyramid.

    • Major calyx: Formed from merging minor calyces.

    • Renal pelvis: Formed from merging major calyces, urine is carried to the ureter.

Nephron

  • Basic unit of the kidney:

    • Responsible for filtering blood and creating urine.

    • Produces filtrate similar to blood plasma but without proteins, which is then modified and concentrated.

  • Components:

    • Renal corpuscle:

    • Glomerulus: Knot of capillaries filtering blood.

    • Glomerular capsule: Collects the filtrate from the capillaries.

    • Renal tubule:

    • Proximal convoluted tubule (PCT):

    • Nephron loop:

      • Descending limb (thin)

      • Ascending limb (thick)

    • Distal convoluted tubule (DCT)

    • Collecting system:

    • Collecting duct: Carries tubular fluid through the medulla.

    • Papillary duct: Collects urine from several collecting ducts and delivers to a minor calyx.

Blood Flow Through the Kidney

  • Arteries:

    • Renal artery

    • Segmental artery

    • Interlobar artery

    • Arcuate artery

    • Cortical radiate artery

    • Glomerulus

  • Veins:

    • Peritubular capillaries

    • Cortical radiate veins

    • Arcuate veins

    • Interlobar veins

    • Renal veins

    • Note: There are no Segmental veins, but arteries share identical nomenclature.

  • Capillary Function:

    • Afferent arteriole: Brings blood into the glomerulus.

    • Efferent arteriole: Carries blood away from the glomerulus.

    • Peritubular capillaries: Wraps around PCT and DCT for reclaiming water and solutes post-filtration.

    • Vasa recta: Surrounds nephron loop of juxtamedullary nephrons.

Ureters

  • Function:

    • Transports urine from the kidneys to the bladder.

  • Structure:

    • Retroperitoneal: Located posterior to the peritoneum.

    • Mucosa: Composed of transitional epithelium.

    • Muscular Layer: Consists of circular and longitudinal muscle layers.

    • Connective Tissue: Continuous with the peritoneum.

  • Movement of Urine: Peristalsis (wave-like contractions) forces urine to the bladder against gravity.

Bladder

  • Function: Temporary storage for urine.

  • Structure:

    • Mucosa: Transitional epithelium that changes shape as the bladder fills.

    • Detrusor muscle: Muscular layer composed of 2 longitudinal and 1 circular muscle.

    • Supporting ligaments:

    • Middle umbilical ligament

    • Right and left lateral umbilical ligaments.

  • Internal Structures:

    • Rugae: Temporary folds in an empty bladder.

    • Ureteral openings: Right and left openings.

    • Urethral opening: Exit point for urine.

    • Trigone: Region bounded by openings; always contains urine, even after voiding.

Urethra

  • Function: Carries urine from bladder to the outside of the body.

  • Structure:

    • Varies along the tract with:

    • Transitional epithelium

    • Stratified columnar

    • Stratified squamous

  • Sphincters:

    • Internal urethral sphincter: Smooth muscle, involuntary, located at the bladder neck.

    • External urethral sphincter: Skeletal muscle, voluntary, located in the urogenital diaphragm.

Micturition Reflex

  • Involves:

    • Local pathway: Muscle and nerve signals local to the bladder and urethra.

    • Central pathway: Involves the CNS.

  • Local pathway process:

    • Starts with bladder wall stretching.

    • Sends signals to the sacral spinal cord.

    • Parasympathetic fibers relay instructions for the detrusor muscle to contract.

  • Central pathway process:

    • Parasympathetic fibers relay information to the thalamus about bladder fullness.

    • Communication to the cerebral cortex allows voluntary relaxation or contraction of the external urethral sphincter leading to urination.

Renal Function

  • Homeostasis Maintenance:

    • Regulates blood volume and composition.

    • Removes metabolic wastes including:

    • Urea: Byproduct of amino acid catabolism.

    • Creatinine: Produced in skeletal muscle.

    • Uric acid: Byproduct of RNA recycling.

Filtration, Reabsorption, and Secretion

  1. Filtration: Blood pressure in the glomerulus pushes water and solutes into the glomerular capsule.

  2. Reabsorption: Most water and solutes are reclaimed from the proximal convoluted tubule (PCT).

  3. Secretion: Extra solutes are removed from tubular fluid in the distal convoluted tubule (DCT) or nephron loop.

Renal Corpuscle Structure

  • Components:

    • Afferent arteriole: Blood enters glomerulus.

    • Efferent arteriole: Blood exits glomerulus.

    • Juxtaglomerular complex: Cells leading to renin secretion in response to drops in glomerular pressure.

    • Podocytes: Cells creating filtration slits surrounding the glomerulus.

  • Mesangial Cells: Control capillary diameter and blood flow between loops of the glomerular capillaries.

  • Filtration Membrane: Comprises the endothelium (capillary), connective tissue layer, and filtration slits formed by podocytes.

Glomerular Filtration Rate (GFR)

  • Definition: Amount of filtrate produced by kidneys per minute (approx. 90-130 mL/min/1.73 m²).

  • Influencing Factors Include:

    • Glomerular hydrostatic pressure (GHP)

    • Blood colloid osmotic pressure (BCOP)

    • Capsular hydrostatic pressure (CsHP)

    • Capsular colloid osmotic pressure (CsCOP).

    • Net filtration pressure calculation:
      extNetFiltrationPressure=GHP−BCOP−CsHP−CsCOPext{Net Filtration Pressure} = GHP - BCOP - CsHP - CsCOP

    • Example Calculation Based on Values:
      50−25−15−0=10extmmHg50 - 25 - 15 - 0 = 10 ext{ mm Hg}