Kidneys

Kidney (Excretory System)

Learning Objectives

  • Describe the structure and function of the nephron.

  • Explain the processes of filtration, reabsorption, secretion, and excretion.

  • Relate renal physiology to body fluid regulation and homeostasis.

Anatomy of the Kidney

  • Cortex   - Contains glomeruli and convoluted tubules.

  • Medulla   - Contains loops of Henle and collecting ducts.

  • Renal Pelvis   - Drains into the ureter.

  • Vascular Pathway   - Renal artery → afferent arteriole → glomerulus → efferent arteriole → peritubular capillaries → renal vein.

The Nephron – Functional Unit

  • Types of Nephrons
      - Cortical Nephrons
        - Comprise 85% of nephrons.     - Characterized by short loops of Henle.     - Primarily function in bulk reabsorption.   - Juxtamedullary Nephrons
        - Have longer loops of Henle.     - Create osmotic gradient for urine concentration.

The Nephron – Main Segments

  1. Glomerular Capsule

  2. Proximal Convoluted Tubule

  3. Loop of Henle

  4. Distal Convoluted Tubule

  5. Collecting Duct

Four Major Processes of the Nephron

  1. Filtration    - Movement from blood to tubule (glomerulus → Bowman’s capsule).

  2. Reabsorption    - Movement from tubule to blood (nutrients, ions, water).

  3. Secretion    - Movement from blood to tubule (toxins, drugs, ions).

  4. Excretion    - Removal of urine from the body.

Glomerular Filtration

  • Components of Glomerular Filtration:
      - Fenestrated Capillaries
      - Basement Membrane
      - Podocyte Slit Diaphragms

  • Super-filtrate Components
      - Contains water, ions, and small molecules (notably excluding larger proteins).

  • Key Cells Involved:
      - Macula Densa
      - Juxtaglomerular Cells
      - Renal Nerve

Glomerular Filtration - Determinants

  • Hydrostatic Pressure:
      - Glomerular blood pressure that drives filtration.

  • Osmotic Pressure:
      - Pressure from proteins in the blood that oppose filtration.

  • Capsular Pressure:
      - Pressure exerted by fluid in Bowman’s capsule that opposes filtration.

  • Glomerular Filtration Rate (GFR):
      - Normal value: ext 125extmL/min=180extL/dayext{~}125 ext{ mL/min} = 180 ext{ L/day}.

  • Intrinsic Control:
      - Myogenic control and tubuloglomerular feedback regulating blood flow based on kidney demand.

  • Extrinsic Control:
      - Influenced by overall heart rate and blood pressure.

Tubular Reabsorption

  • Proximal Convoluted Tubule (PCT):
      - Reabsorbs approximately 65% of Na⁺ and water, as well as glucose and amino acids.

  • Loop of Henle:
      - Descending Limb:
        - Permeable to water only, facilitating reabsorption.   - Ascending Limb:
        - Actively reabsorbs Na⁺, K⁺, Cl⁻, but is impermeable to water.

  • Distal Convoluted Tubule (DCT):
      - Hormonal control in reabsorption:
        - Aldosterone: Secreted to increase Na⁺ reabsorption and K⁺ secretion.     - Antidiuretic Hormone (ADH): Promotes water reabsorption by inserting more aquaporins (type 2) in the collecting ducts.

Key Molecules and Ions in Reabsorption

  • Reabsorbed Substances:
      - Glucose, Na⁺, Amino Acids, Ca²⁺, Vitamins, Mg²⁺.

  • Waste Substances:
      - Urea, Creatinine, Uric Acid, H⁺, NH₄⁺, K⁺

Tubular Secretion

  • Proximal Convoluted Tubule (PTC):
      - Responsible for the elimination of H⁺, K⁺, drugs, and organic anions.

  • Loop of Henle:
      - No significant secretion occurs.

  • Distal Convoluted Tubule (DCT):
      - Primarily secretes K⁺.

Extrinsic Regulation of Kidney Function

  1. Aldosterone
      - Increases Na⁺ reabsorption and K⁺ secretion within the distal nephron.

  2. Antidiuretic Hormone (ADH)
      - Increases water reabsorption by signalling for aquaporin channels.

  3. Atrial Natriuretic Peptide (ANP)
      - Counteracts Na⁺ reabsorption → results in increased urine output.

  4. Renin-Angiotensin System
      - Critical for regulating blood pressure and GFR.

  5. Sympathetic Autonomic Nervous System (SANs)
      - Increases heart rate and blood pressure.

Intrinsic Control Mechanisms

  • Myogenic Mechanism:
      - Regulates afferent arteriole diameter in response to blood pressure changes:
        - ↑ BP: Afferent arteriole stretches, contracts, reduces blood flow, stabilizes GFR.
        - ↓ BP: Afferent arteriole relaxes, dilates, increases blood flow, stabilizes GFR.

  • Tubuloglomerular Feedback:
      - Mechanism involving the macula densa, which adjusts GFR based on NaCl levels in fluid:     - High NaCl Levels: Macula densa detects this, releases adenosine → constricts the afferent arteriole → decreases GFR.
        - Low NaCl Levels: Macula densa responds by releasing prostaglandins, signals renin release, stimulates efferent constriction (via angiotensin II) → restores GFR.

Clinical Correlation – Renal Pathophysiology

  • Condition
      - Acute Kidney Injury:
        - Mechanism: Rapid loss of GFR.
        - Key Finding: Elevated Blood Urea Nitrogen (BUN) and Creatinine levels.   - Chronic Kidney Disease:
        - Mechanism: Progressive nephron loss.
        - Key Findings: Anemia, electrolyte imbalance.   - Glomerulonephritis:
        - Mechanism: Immune-mediated inflammation.
        - Key Findings: Proteinuria, hematuria.   - Diabetes Mellitus:
        - Mechanism: Causes damage to glomeruli.
        - Key Findings: Glycosuria, microalbuminuria.   - Key Clinical Measures:
        - GFR: Measure of filtration efficiency.     - Creatinine Clearance: Estimates renal function.
        - BUN: Connects to protein metabolism and kidney clearance.

Key Takeaways

  • The nephron serves as the core functional unit responsible for filtration, reabsorption, secretion, and excretion.

  • Regulation of GFR ensures that filtration remains consistent despite fluctuations in blood pressure.

  • Hormonal mechanisms finely adjust water and ion balance in the body.

  • Understanding renal pathophysiology highlights the critical links between kidney function and overall systemic health.