14.06 Ultrafiltration and Urine Production

Overview of Ultrafiltration in the Nephron

• Definition: Ultrafiltration is the process of separating small molecules from larger ones by passing blood plasma through a semi-permeable membrane in the glomerulus and Bowman’s capsule of each nephron.
• Key Structures:
  • Glomerulus: A capillary network under high pressure that forces small molecules into Bowman’s capsule.
  • Bowman’s Capsule: Collects filtrate produced in the glomerulus.

Filtration Barrier in the Glomerulus

The blood in the glomerular capillaries is separated from Bowman’s capsule by three layers:

1. Endothelium of Capillaries:
   • Contains numerous pores (60–80 nm) that allow small molecules to pass through while blocking larger elements like blood cells and platelets.
2. Basement Membrane:
   • A molecular filter made of collagen and glycoproteins, allowing only small molecules to pass through while blocking large plasma proteins.
3. Podocytes:
   • Specialized epithelial cells in Bowman’s capsule with finger-like projections (slits) that facilitate the movement of small molecules into the capsule.

Composition of Glomerular Filtrate

• Glomerular Filtrate: Very similar to blood plasma but lacks large plasma proteins due to the selective permeability of the filtration barrier.
• Components:
  • Contains small molecules like water, glucose, inorganic ions (Na⁺, K⁺, Cl⁻), urea, and uric acid in similar concentrations to blood plasma.
  • Plasma Proteins and Cells: Remain in the blood due to their size, which prevents them from passing through the basement membrane.

Glomerular Filtration Rate (GFR)

• Definition: The volume of fluid filtered from the blood in all glomeruli of both kidneys per minute, approximately 125 cm³/min in humans.
• Factors Affecting GFR:
  • High Blood Pressure in Glomerulus: The narrow efferent arteriole compared to the afferent arteriole creates high pressure, promoting fluid movement into Bowman’s capsule.
  • Water Potential Gradient: Due to plasma proteins, the blood in the glomerulus has a lower water potential than the filtrate in Bowman’s capsule, which further aids filtration. However, the pressure gradient is the dominant factor driving ultrafiltration.

Process of Ultrafiltration

1. High Pressure: Blood enters the glomerulus at a high pressure, forcing small molecules (e.g., water, ions, glucose, and urea) across the filtration barrier into Bowman’s capsule.
2. Selective Filtration: The filtration barrier blocks larger molecules like proteins and blood cells, allowing only small solutes to pass.
3. Formation of Glomerular Filtrate: The filtrate collected in Bowman’s capsule contains essential molecules for reabsorption as well as waste products for excretion.

Factors Leading to Kidney Filtration Issues

Increases filtration pressure, which can damage the filtration barrier, sometimes allowing proteins to pass into the filtrate, resulting in proteinuria (protein in urine).

Blood Loss:

Reduces blood pressure, potentially decreasing the glomerular filtration rate and leading to temporary kidney failure.

High Blood Pressure:

Increases filtration pressure, which can damage the filtration barrier, sometimes allowing proteins to pass into the filtrate, resulting in proteinuria (protein in urine).