Physiology of the Kidneys

I. Structure and Function of the Kidneys

• Kidney Functions:
  1. Regulation of extracellular fluid environment including:
  • a. Volume of blood plasma (affects blood pressure)
  • b. Wastes
  • c. Electrolytes
  • d. pH
  • e. Secrete erythropoietin

II. Gross Structure of the Urinary System

• Urinary Pathway:

  1. Urine is produced in kidney nephrons → drains into renal pelvis → down ureter to urinary bladder → exits through urethra.
  2. Urine transport occurs via peristalsis.

• Kidney Structure:

  1. Two distinct regions:
  • a. Renal cortex
  • b. Renal medulla (consists of renal pyramids and columns)

• Each Pyramid:

  • Drains into minor calyx → major calyx → renal pelvis

III. Kidney Stones (Nephrolithiasis)

• Formation:
  1. Hard objects made of crystallized minerals or waste.
  2. 80% are calcium stones; others may be magnesium ammonium phosphate or uric acid.
• Tendency to Form Stones:
  • Increased with dehydration.
• Obstruction and Pain:
  • Large stones can obstruct urine flow; smaller stones (usually < 5 mm) may cause intense pain if passed to ureters.
• Treatment Options:
  1. Medications
  2. Lithotripsy (shock waves)
  3. Surgery

IV. Control of Micturition

• Detrusor Muscles:

  1. Smooth muscle lining the bladder wall; innervated by parasympathetic neurons (acetylcholine binds to muscarinic receptors).
  2. Internal (smooth muscle) and external (skeletal muscle) urethral sphincters surround urethra.

• Nerve Control:

  1. Stretch receptors in the bladder send signals to S2-S4 spinal cord regions.
  2. Reflexes allow bladder guarding (inhibits emptying) & voiding reflex (urination initiation).

V. Urinary Incontinence

• Definition:

  • Uncontrolled urination due to loss of bladder control; multiple causes.

• Stress Urinary Incontinence:

  • Leakage during activities increasing abdominal pressure (sneezing, coughing).
  • More common in women post-childbirth or aging; treatable by surgical sling.

• Overactive Bladder:

  • Uncontrolled detrusor contractions; causes frequent urination & urgency.
  • Diagnosed by urodynamic testing.

VI. Microscopic Kidney Structure

• Nephron:

  1. Functional unit; >1 million per kidney.
  2. Consists of tubules & associated blood vessels; filtration, reabsorption, urine formation occurs here.

• Nephron Tubules:

  1. Glomerular capsule surrounds glomerulus (renal corpuscle).
  2. Filtrate passes through:
  • Proximal convoluted tubule
  • Loop of Henle (descending & ascending limbs)
  • Distal convoluted tubule
  • Collecting duct

VII. Glomerular Filtration

• Filtration Process:
  1. Glomerular capillaries are fenestrated; allow water & solutes out but not blood cells/proteins.
  2. Filtrate Formation:
  • Takes place in glomerular capsule, driven by hydrostatic pressure.
  1. Glomerular Filtration Rate (GFR):
  • 115-125 ml/min; significant daily filtration (~180 L).

VIII. Reabsorption of Salt and Water

• Overview:
  1. Filtrate is processed via reabsorption; 180 L filtered but only 1-2 L excreted.
  2. Most reabsorption (~85%) occurs in proximal tubules & descending loop of Henle (unregulated).
• Proximal Tubule Reabsorption consists of both active (Na+ pumping) and passive (follows water/Cl−) processes.
• Countercurrent Multiplication:
  1. Mechanism important for urine concentration, involves ascending and descending limbs of Loop of Henle and vasa recta.
  2. Establishes concentration gradients through Sodium & Urea transport.

IX. Hormonal Controls

• Antidiuretic Hormone (ADH):
  1. Influences water reabsorption in collecting ducts.
  2. Regulates by insertion of aquaporins in response to osmolarity changes.

X. Kidney Diseases

• Acute Renal Failure:
  • Sudden inability to regulate blood volume, caused by reduced blood flow.
• Chronic Conditions:
  • Glomerulonephritis: Autoimmune damage resulting in proteinuria & edema.
  • Renal Insufficiency: Deterioration of kidney function leading to uremia; dialysis may be necessary.

XI. Diuretics**

• Usage:
  1. Medications used to manage blood pressure and fluid retention.
  2. Types include loop diuretics, thiazides, and potassium-sparing diuretics.