Renal System and Long-Term Blood Pressure Regulation

Objectives

  • Know LEVEL

    • List the organs of the renal system and state their functions.

  • Apply LEVEL

    • Predict the effect of increased glomerular blood pressure on glomerular filtration rate.

  • Synthesize LEVEL

    • Explain how countercurrent multiplication increases the ability of nephron loops to reabsorb water and ions.

  • Comprehend LEVEL

    • Describe the histology of the urinary bladder and explain how that structure relates to function.

  • Analyze LEVEL

    • Connect renal clearance to GFR; explain the ideal characteristics of a substance whose renal clearance is being used to measure GFR.

  • Evaluate LEVEL

    • Explain the long-term mechanisms by which blood pressure is regulated based on its relationships to total blood volume.

Review of Renal System Functions

  • Receives 20% of cardiac output for filtration.
  • Keeps essential substances, discards toxins.
  • Releases ligands like:
    • Renin: increases blood pressure and volume.
    • Erythropoietin (EPO): stimulates red blood cell production.
  • Regulates total water content.
  • Involvement in:
    • Gluconeogenesis
    • Calcium regulation via calcitriol release.

Anatomy of the Urinary System

Gross Anatomy of the Kidneys
  • Located in retroperitoneal space, extending from T12 to L3.
  • Protected by floating ribs.
Three Connective Tissue Layers Surrounding the Kidneys
  1. Renal Fascia:
    • Most superficial layer
    • Dense connective tissue
    • Surrounds kidneys and adrenal glands
  2. Perinephric Fat Capsule:
    • Layer of adipose tissue
  3. Renal Capsule:
    • Direct cover of kidney surface
Internal Kidney Anatomy
  • Renal Cortex: outer region, granular reddish-brown.
  • Renal Medulla: inner region, striped appearance from renal pyramids.
  • Renal Columns: separate renal pyramids.
  • Major and Minor Calyces: collect urine from renal lobes.
Structure and Function of the Ureters
  • Function: Carry urine from kidneys to the bladder.
  • Capable of peristalsis.
  • Connect to bladder at an angle to prevent backflow.
Microscopic Anatomy of the Ureters
  1. Mucosa:
    • Transitional epithelium stretches to accommodate urine.
  2. Muscularis:
    • Senses distension and initiates peristalsis.
  3. Adventitia:
    • Anchors ureter in place.
Common Features of the Bladder
  • Trigone: triangular area at the bottom, marked by openings for ureters and urethra.
  • Mucosa: transitional epithelium folding into rugae.
  • Detrusor Muscle: contracts to promote urination.
  • Internal Urethral Sphincter: near urethra, thick muscle.
  • External Urethral Sphincter: formed of skeletal muscle.
Male vs. Female Urinary Systems
  • Females: Urethra is 3-5 cm, only for urine transport.
  • Males: Urethra is 20 cm, serves for urine and semen transport.

Kidney Vasculature Overview

  • Renal Artery: enters kidney at hilum, branches into segmental arteries, interlobar arteries, arcuate arteries, cortical radiate arteries, and microscopic afferent arterioles.

Nephron Structure

  • Nephron: structural and functional unit of urine formation.
  • Afferent arterioles lead to glomerulus; filtration occurs here into Bowman’s capsule.
  • Blood drained from glomerulus by efferent arterioles.
Fluid Flow Through Nephron
  1. Proximal Convoluted Tubule (PCT):
    • Simple cuboidal epithelium with microvilli, located in the cortex.
  2. Descending and Ascending Tubes:
    • Alternating thick and thin segments, with ascending permeable to solutes.
  3. Distal Convoluted Tubule (DCT):
    • Simple cuboidal epithelium without microvilli.
Collecting Duct Function
  • Principal cells adjust urine for water, Na+, and K+ balance.
  • Intercalated cells manage acid-base balance.

Filtration Process

Layers of the Filtration Barrier
  1. Capillary Endothelium: Fenestrated; allows passage of particles smaller than cells.
  2. Basement Membrane: Fused; blocks most proteins.
  3. Podocytes: Form filtration slits, preventing passage of large molecules.
Filtration Factors
  • NFP = Net Filtration Pressure:
    • NFP = GBHP - (CHP + BCOP)
    • GBHP: glomerular blood hydrostatic pressure.
    • CHP: capsular hydrostatic pressure.
    • BCOP: blood colloid osmotic pressure.
    • GFR maintains near constant rate (80 - 180 mm Hg).

Renal Clearance and GFR

  • Glomerular Filtration Rate (GFR): Total volume of filtrate formed per minute.

  • Clearance Calculation: C = (U * V) / P

    • U: urine concentration
    • V: urine formation rate
    • P: plasma concentration

  • Substance should pass freely through filtration without reabsorption or secretion to approximate GFR.

Long-Term Blood Pressure Regulation

Renin-Angiotensin-Aldosterone System (RAAS):
  1. Renin released from kidneys in response to low BP.
  2. Converts Angiotensinogen to Angiotensin I, which is then converted to Angiotensin II by ACE.
  3. Angiotensin II stimulates:
    • Thirst and sodium reabsorption.
    • Aldosterone release for Na+ reabsorption.
    • Vasoconstriction increasing blood pressure.

Effects of Aging on Kidney Function

  • Decrease in nephrons (40% lost from age 20 to 70).
  • Damage to filtration membrane; declining GFR (~50%).
  • Loss of sensitivity to ADH leading to sodium loss.
  • Decreased bladder size resulting in frequent urination and nocturia.
  • Loss of muscle tone may lead to urinary incontinence.
Polycystic Kidney Disease (PKD)
  • Genetic disorder causing cysts in kidneys/liver/pancreas.
  • Symptoms: High blood pressure, abdominal swelling, blood in urine, infections.
  • No current cure; can lead to kidney failure.