Renal Corpuscle and Glomerulus Overview

Renal Corpuscle

• The renal corpuscle is comprised of two structures: the glomerulus and Bowman's capsule.
• The glomerulus is a tuft of capillaries that filters blood.
• Bowman's capsule encases the glomerulus and collects the filtered fluid.

Glomerulus

• The glomerulus is fed by the afferent arteriole and drained by the efferent arteriole.
• The capillaries within the glomerulus are fenestrated capillaries, meaning they have pores in the endothelial cells.
• These pores are approximately 50 to 100 nanometers in diameter.
• Formed elements (red blood cells, white blood cells, and platelets) are too large to pass through the fenestration pores.
• The glomerular filtration membrane consists of the endothelial lining of the fenestrated capillaries and the glomerular basement membrane.
• The fenestrated capillaries allow small proteins, water, electrolytes, nutrients, and waste products to pass through.

Glomerular Basement Membrane

• The glomerular basement membrane (GBM) is a specialized structure located between the endothelial cells and the podocytes.
• The GBM consists of three layers:
  • Lamina densa: Contains Type IV collagen and laminins, providing structural support.
  • Lamina rara interna: Consists of negatively charged heparan sulfate, located closer to the endothelial cells.
  • Lamina rara externa: Also consists of negatively charged heparan sulfate, located closer to the podocytes.
• The negative charge of the GBM repels negatively charged molecules like plasma proteins, helping prevent them from entering Bowman's capsule.
• Positively charged molecules, like electrolytes, pass through more readily.

Podocytes

• Podocytes are specialized epithelial cells that wrap around the capillaries of the glomerulus.
• They have foot-like processes called pedicels, which interdigitate with adjacent podocytes, forming filtration slits.
• The filtration slits are approximately 25 to 30 nanometers in diameter.
• Nephrin is a key protein located in the filtration slits, forming a structure called the slit diaphragm.
• The slit diaphragm acts as a final barrier, allowing only molecules smaller than 7 to 9 nanometers to pass through.

Components Filtered Through the Renal Corpuscle

• The following substances are readily filtered through the renal corpuscle:
  • Water
  • Electrolytes (sodium, potassium, calcium, magnesium, chloride, bicarbonate)
  • Nutrients (glucose, amino acids, lipids)
  • Waste products (urea, creatinine, lactic acid)
• The following substances are repelled by the glomerular basement membrane:
  • Plasma proteins (albumin, immunoglobulins, fibrinogen)

Filtration Process Recap

• Substances must pass through three barriers to be filtered into Bowman's capsule:
  • Fenestrated capillaries: Allows substances less than 50 to 100 nanometers in diameter to pass.
  • Glomerular basement membrane: Repels negatively charged molecules; positively charged particles pass readily.
  • Filtration slits: Allows substances less than 25 to 30 nanometers to pass.
  • Slit diaphragm (nephrin): Allows substances less than 7 to 9 nanometers to pass.
• The filtered substances then collect in Bowman's space and enter the proximal convoluted tubule.

Mesangial Cells

• Mesangial cells are important for glomerular structure.
• Mesangial cells phagocytose macromolecules that get stuck in the slit diaphragm.
• Mesangial cells have contractile activity that controls blood flow through the glomerular capillaries.
• Mesangial cells are connected to JG cells via gap junctions.

Juxtaglomerular (JG) Cells

• JG cells are located in the juxtaglomerular apparatus, next to the glomerulus.
• JG cells produce renin, which raises blood pressure.
• JG cells are pressure receptors (baroreceptors) that sense changes in blood pressure.
• JG cells receive signals from mesangial cells via gap junctions.

Glomerular Filtration Rate (GFR)

• GFR is the amount of plasma filtered from the glomerulus into Bowman's capsule per minute.
• The average GFR is 125 milliliters per minute.
• About 1200 milliliters of plasma flow through the glomerulus per minute.
• Only 625 milliliters per minute are used in filtration, and the rest passes through.
• Only 20% (~125 milliliters per minute) of the 625 milliliters of plasma used in filtration is actually filtered.

Factors Affecting GFR

• GFR is determined by net filtration pressure (NFP) and the filtration coefficient (KF).
• NFP is the difference between pressures pushing things out and pressures pulling things in.
• KF represents the permeability and surface area of the glomerulus.

Pressures Affecting GFR

• Pressures forcing out:
  • Glomerular hydrostatic pressure (GHP) – the pressure pushing plasma out of the glomerular capillaries into Bowman's space; ~55 mmHg.
  • Capsular osmotic pressure (COP) – the pressure pulling things out of the capillaries; assumed to be 0 mmHg under normal conditions.
• Pressures pulling in:
  • Colloid osmotic pressure (COP) – the pressure exerted by plasma proteins (like albumin) to keep water in the bloodstream; ~30 mmHg.
  • Capsular hydrostatic pressure (CHP) – the pressure built up in Bowman's capsule as fluid filters in, pushing fluid back into the glomerular capillaries; ~15 mmHg.
• Net Filtration Pressure:
  • NFP = GHP + COP - COP - CHP
  • NFP ~ 10 mmHg

Clinical Correlations

• Glomerular Hydrostatic Pressure:
  • Increased systemic blood pressure leads to increased GHP.
  • Decreased systemic blood pressure leads to decreased GHP.
• Colloid Osmotic Pressure:
  • Increased plasma proteins increase COP (e.g., multiple myeloma).
  • Decreased plasma proteins decrease COP (e.g., hypoproteinemia).
• Capsular Hydrostatic Pressure:
  • Kidney stones (renal calculi) larger than 5 millimeters can obstruct flow and increase CHP.
  • Hydro nephrosis, caused by renal ptosis, can also increase CHP.

Filtration Coefficient (KF)

• KF = surface area of the glomerulus x permeability of the glomerulus
• KF impacts GFR: increases to KF increase GFR. decreases to KF decrease GFR
• Conditions that affect KF:
  • Diabetic nephropathy: Thickening of the glomerulus caused by protein and other deposits, leading to decreased surface area and a lower GFR.
  • Glomerulonephritis: Damage to the glomerulus, making the basement membrane more porous, which can increase GFR and cause protein loss in urine.

This quiz explores the structure and function of the renal corpuscle, focusing on the glomerulus and Bowman's capsule. Discover how these components work together to filter blood and manage essential substances. Test your knowledge on the intricacies of kidney filtration.