CL

Ch. 3: The Kidney

Objectives

  • Identify kidney functions and structure.

  • Diagram nephron functions.

  • Describe renal blood circulation.

  • Explain glomerular filtration and urine formation.

  • Discuss tubular reabsorption and secretion mechanisms.

  • Outline renal acid-base regulation.

  • Explain tubular transport capacity and renal threshold.

  • Compare countercurrent mechanisms in urine concentration.

Kidney Structure

  • Major areas: Capsule, Cortex, Medulla.

  • Nephrons (~1.3 million per kidney) as functional units:

    • Glomeruli for plasma filtration.

    • Loop of Henle for urine concentration.

Renal Circulation

  • Blood flow: Renal artery → Afferent arterioles → Glomeruli → Efferent arterioles → Peritubular capillaries.

  • Hydrostatic pressure drives filtration (55 mmHg).

Glomerular Filtration

  • Selective barrier: capillary endothelium, basement membrane, podocytes.

  • Determines filtration based on size and charge.

Urine Formation

  • Plasma filtration: 180,000 mL/day, final urine: 600-1800 mL.

  • Involves filtration, reabsorption, and secretion.

Tubular Reabsorption and Secretion

  • Reabsorption: Water, salts, glucose back to blood.

  • Secretion: H+ ions, NH3 for waste elimination.

  • Active and passive transport mechanisms.

Acid-Base Regulation

  • Maintained by buffering systems: bicarbonate, pulmonary, and renal excretion mechanisms.

  • Regulation of H+ ions and bicarbonate reabsorption.

Tubular Transport Capacity (Tm)

  • Maximal capacity for solute reabsorption (e.g., glucose: 350 mg/min).

  • Beyond Tm, substances appear in urine.

Antidiuretic Hormone (ADH)

  • Controls water reabsorption in collecting ducts.

  • Related to blood pressure and plasma osmolality changes.

Countercurrent Mechanisms

  • Countercurrent multiplier in Loop of Henle concentrates urine.

  • Urea cycle enhances medulla's osmotic gradient.