Renal & Urological Systems Notes

Renal & Urological Systems

The Urinary System

The urinary system consists of the following components:

  • Kidney (A)
  • Ureter (B)
  • Urinary Bladder (C)
  • Urethra (D)
  • Renal Arteries (E)
  • Renal Veins (E)

Functions of the Urinary System

The urinary system performs several vital functions:

  1. Excretion of Metabolic Wastes
  2. Maintain Water-Salt Balance of Blood
  3. Maintain Acid-Base Balance of Blood (Buffer System): The buffer system is represented by the following equilibrium: H2CO3
  4. Secretion of Hormones: Important for various regulatory processes.

Examples of metabolic wastes include:

  • Amino Acids $\rightarrow$ NH3 + CO2 (Liver) = Urea
  • Creatinine (From breakdown of Creatine Phosphate)
  • Uric Acid (From breakdown of nucleic acids)

The urinary system also regulates:

  • Blood pressure and blood volume
  • Na^+, K^+ and Ca^{2+} levels

An example of a hormone secreted by the kidneys is Erythropoietin, which stimulates blood production.

Structure of the Kidney

The kidney's structure includes:

  • Renal Artery (A)
  • Renal Vein (B)
  • Ureter (C)
  • Renal Cortex (D)
  • Renal Medulla

Detailed Kidney Structure

  • Ureter (A)
  • Renal Pelvis (B)
  • Renal Pyramid (C): Part in Medulla, part in Cortex
  • Renal Cortex (D)
  • Renal Medulla (E)

The kidney also contains:

  • Nephrons (C)
  • Collecting Duct (D)

Structure of a Nephron

The nephron, the functional unit of the kidney, consists of:

  • Renal Artery (A)
  • Renal Vein (B)
  • Afferent Arteriole (C)
  • Efferent Arteriole (D)
  • Glomerulus (E)
  • Glomerular Capsule (F)
  • Proximal Convoluted Tubule (G)
  • Loop of Henle - Descending (H)
  • Loop of Henle - Ascending (I)
  • Peritubular Capillaries (J)
  • Distal Convoluted Tubule (K)
  • Collecting Duct (L)

Filtration Membrane

The filtration membrane includes:

  • Podocytes
  • Foot Processes
  • Filtration Slits

The structure also contains:

  • Efferent Arteriole
  • Afferent Arteriole
  • Glomerular capsular space
  • Glomerular capillary covered by podocyte-containing visceral layer of glomerular capsule
  • Parietal layer of glomerular capsule
  • Proximal convoluted tubule
  • Fenestrations
  • Glomerular capillary endothelium

Nephron Function: Reabsorption of Water

  • Proximal CT: Permeable to water
  • Distal CT: NOT permeable to water
  • Active transport of Na^+
  • Aldosterone: Increases Na^+ Reabsorption (comes from adrenal glands)
  • Collecting Duct: Sometimes permeable to water, depending on ADH (Antidiuretic Hormone)
    • ADH $\rightarrow$ Permeable
    • No ADH $\rightarrow$ Not permeable
    • Some Urea diffuses back out

Mechanisms of Urine Formation

  1. Glomerular filtration
  2. Tubular Reabsorption
  3. Tubular Secretion

Urine Formation Details

  1. Glomerular Filtration:
    • Salts (Na^+, K^+, Ca^{2+}, Mg^{2+} etc.)
    • Water
    • Organic Nutrients
    • Nitrogen Wastes (Urea, etc.)
  2. Tubular Reabsorption:
    • Na^+, K^+, Ca^{2+}, Mg^{2+}
    • Water
    • All Organic Nutrients:
      • Glucose, Amino Acids, etc.
  3. Tubular Secretion:
    • Toxins made soluble by Liver
    • Urea
    • Creatinine
    • H^+ and excess K^+

Osmotic Gradient in the Renal Medulla

The osmolarity increases from the cortex (300 mOsm) to the inner medulla (1200 mOsm).

Loop of Henle: Countercurrent Mechanism

The countercurrent mechanism involves:

  • Collecting duct
  • Osmolality of interstitial fluid (mOsm)
  • Blood from efferent arteriole
  • To vein
  • Active and passive transport of NaCl and H_2O
  • Urea Recycling

Juxtaglomerular Apparatus

  • Cells Between: Afferent Arteriole Distal CT
  • When blood pressure is low: secretes Renin (enzyme)
    • Angiotensinogen $\rightarrow$ Angiotensin I $\rightarrow$ Angiotensin II
      • Vasoconstrictor
      • Aldosterone: Increases Na^+ reabsorption $\rightarrow$ More water reabsorbed $\rightarrow$ Blood pressure increases
      • (ACE-I’s and ARB’s)

Nephron Function & Biohumoral-Regulation (cont.)

  • Atrial Natriuretic Peptide (ANP), aka “hormone”
    • When BP is high
    • Released from Atria
    • Inhibits Renin secretion
    • Na^+ not reabsorbed = excreted
    • Water excreted (because water follows the salt)
    • BP decreases
  • Diuretics
    • Chemicals that increases water content of urine
    • Cause “dehydration”; can also cause electrolyte imbalances
    • RX: Lasix (furosemide), Hydrochlorothiazide, Triamterene
  • Alcohol inhibits ADH secretion
  • Caffeine increases BP and inhibits Na^+ reabsorption

Nephron Function & Biohumoral- Regulation (cont.)

  • Why “Kidney Failure”?
    • ARF (Acute Renal Failure)
      • dehydration, hemorrhage, medications, CVA, MI
    • CRF (Chronic Renal Failure)
      • CKD stages I to V
      • Chronic uncontrolled HTN, DM, autoimmune d/o, stones, congenital anomalies