Ch 17

Kidney and Urinary System Functions

• Definition and Relationship
    - Kidney functions are synonymous with urinary system functions.
    - The kidney is the primary functional organ of the urinary system.

• Urinary Tract Components
    - Ureters: Tubes that transport urine from the kidneys to the bladder.
    - Urinary Bladder: Stores urine.
    - Urethra: Excretes urine from the body.

• Functions of the Urinary System
    - The urinary system has numerous functions which include:
      - Filtration System for Blood:       - Metabolic waste excretion, notably nitrogenous wastes like urea and creatinine.
          - Urea:
              - Produced from the conversion of ammonia from amino acids in the liver.
              - Excess amino acids from a high protein diet result in increased urea production which is excreted in urine.
          - Creatinine:
              - A byproduct of creatine metabolism, essential for short-term energy in muscles.
              - Excreted through urine.
      - Regulatory Functions:
          - Control of solids and fluids.
          - Maintenance of water balance.
          - Maintenance of electrolyte balance affecting blood pressure.
          - Acid-base balance regulation for blood pH stability.
          - Drug metabolism and waste excretion (90% focus on metabolic waste).

Urinary System Anatomy

• Kidney Gross Anatomy
    - Cortex: The outer region of the kidney.
    - Medulla: The inner region containing renal pyramids.
      - Renal pyramids are grouped in the medulla, critical for osmolarity differences between areas.

• Nephron Structure:
    - The functional unit of the kidney.
      - It’s composed of:
          - Bowman's Capsule: The start of the nephron, capturing ultrafiltrate.
          - Proximal Convoluted Tubule (PCT): First tubule where significant reabsorption occurs.
          - Loop of Henle: Contains descending and ascending limbs, crucial for creating countercurrent osmotic gradients.
          - Distal Convoluted Tubule (DCT): Involved in fine-tuning substance reabsorption and secretion.
          - Collecting Duct: Final structure where urine is concentrated.

• Urinary Tract Illustrated
    - Ureters (transport), bladder (storage), urethra (excretion).
    - Infections: UTIs are more common in females due to shorter urethra anatomy.

Kidney Physiology

• Nephrons and Collecting Ducts
    - Millions of nephrons exist per kidney; function alongside collecting ducts.
    - Filtrate Production:
      - Ultrafiltration: Occurs in the glomerulus.
      - Blood enters through afferent arterioles; substances filtered into Bowman's capsule.

• Filtration Processes
    - Glomerular Filtration Rate (GFR): Measurement of fluid filtered per unit time; regulated intrinsically by:
      - Myogenic Response: Afferent arteriole constricts with increased pressure, reducing GFR.
      - Tubuloglomerular Feedback: Macula densa senses increases in sodium or water in DCT, signaling constriction of afferent arteriole, reducing GFR.

• Hormonal Regulation
    - The Renin-Angiotensin-Aldosterone System (RAAS):
      - Renin: Secreted by JG cells in afferent arterioles due to decreased pressure.
      - Angiotensin I: Converted from angiotensinogen by renin.
      - Angiotensin II: Formed by ACE in lungs, leads to vasoconstriction and stimulates aldosterone secretion which increases sodium reabsorption in the kidneys.

Sodium and Potassium Regulation

• Role of Sodium:
    - Sodium is crucial for regulating blood volume and pressure.
    - Aldosterone increases sodium reabsorption, promoting water retention, and raising blood pressure.

• Potassium Regulation:
    - Increased sodium reabsorption leads to increased potassium secretion in the DCT and collecting duct.
    - Acidosis Effects: Increased hydrogen ions can lead to hyperkalemia as potassium is retained.

• Aldosterone Effects:
    - Produced in the adrenal cortex, regulates sodium reabsorption and potassium excretion.
    - Increased blood volume results from increased sodium and subsequent water retention.

Acid-Base Balance Regulation

• Hydrogen Ion and Bicarbonate Regulation:
    - Kidneys manage blood pH via secretion of hydrogen ions and reabsorption of bicarbonate.
    - Carbonic anhydrase enzymes catalyze conversions to maintain bicarbonate buffer systems in blood.
• Bicarbonate is reabsorbed from tubular fluid into the blood, ensuring proper blood pH.

Clinical Diuretics

• Types of Diuretics:
    - Carbonic Anhydrase Inhibitors, Thiazides, Potassium-Sparing, Loop Diuretics
    - Function: Diuretics increase urine production by various mechanisms impacting the nephron's abilities to reabsorb water or electrolytes.
• Commonly used in clinical settings to manage hypertension and fluid overload issues.