Kidneys: Basic Renal Processes- Glomerular Filtration

Dr. Huising, Fall 2024, Lecture 19

Glomerular Filtration

• Nondiscriminant filtration of a protein-free plasma from the glomerulus into Bowman’s capsule

The glomerular membrane is…

• Considerably more permeable than capillaries elsewhere

• The glomerular capillary wall consists of a single layer of flattened endothelial cells (with added holes to make it leaky)

What is the major force that causes glomerular filtration?

• Glomerular capillary blood pressure

• Blood pressure pushes filtrate, forces involved in glomerular filtration, glomerular filtration rate

• Increase in blood pressure = increase in filtration

To be filtered, a substance must pass through..

1. The pores between and the fenestrations (capillary pore) within the endothelial cells of the glomerular capillary (100 times more permeable to H₂O and solutes than regular capillaries)

2. An acellular basement membrane (collagen for structural strength, negative charged glycoproteins to repel proteins, particularly smaller proteins like albumin)

3. The filtration slits between the foot processes of the podocytes in the inner layer of Bowman’s capsule

What is filtration?

• Act of the kidneys pushing plasma of the blood out of the capillaries of the glomerulus and into Bowman’s capsule

What are three forces that contribute to glomerular filtration rate (GFR)?

1. Glomerular capillary blood pressure

2. Plasma-colloid osmotic pressure

3. Bowman’s capsule hydrostatic pressure

Glomerular Capillary Blood Pressure

• The fluid pressure within the glomerular capillaries

• Favors filtration

• Variable

• About 55 mmHg

Plasma-Colloid Osmotic Pressure

• Caused by unequal distribution of protein between plasma (contains protein) and glomerular filtrate (no protein)

• Water wants to move down the osmotic gradient into glomerulus/ back into plasma

• Opposes filtration

• Constant

• About 30 mmHg

Bowman’s Capsule Hydrostatic Pressure

• The fluid pressure by the filtrate in Bowman’s capsule

• For more fluid/ filtrate to enter, the existing needs to be pushed out, which requires pressure

• Opposes filtation

• Constant

• About 15 mmHg

What is net filtration pressure?

• The sum of glomerular capillary blood pressure, plasma-colloid osmotic pressure, and Bowman’s capsule hydrostatic pressure

• Favors filtration

• Controlled by glomerular pressure

Glomerular Filtration Rate

• Depends on

  • net filtration pressure (major)

  • glomerular surface area available for penetration (minor)

  • permeability of the glomerular membrane (minor)

What are the controlled adjustments of the GFR?

• Autoregulation

  • Myogenic

  • Tubularglomerular Feedback

• Extrinsic Sympathetic Control

  • Baroreceptor Reflex

• Vasoconstriction: decrease in glomerular capillary blood pressure → decrease in net filtration → decrease in GFR

• Vasodilation: increase in glomerular capillary blood pressure → increase in net filtration → increase in GFR

Myogenic (autoregulation)

• Local response within arteriolar smooth muscle wall to stretch

• Increase in blood pressure → vasodilation → decrease in blood pressure

Tubularglomerular Feedback (autoregulation)

• Adjsutments to afferent arteriolar pressure in response to salt concentration in the loop of Henle at the juxtaglomerular apparatus

• Maintains constant GFR across a large blood pressure range, allows glomerular capillary blood pressure to stay constant even when upstream efferent arteriole blood pressure increases

1. Increased blood pressure leads to increased GFR and increases the salt delivery to the distal tubules

2. Macula densa cells (specialized tubule cells in the juxtaglomerular apparatus) detect this increase in salt delivery and release paracrine factors that constrict the adjacent afferent arteriole

3. Glomerular capillary pressure and GFR decrease, and salt is pushed out

Baroreceptor Reflex in Extrinsic Control (GFR)

• A sudden drop in blood pressure causes the body to respond with general vasoconstriction to help increase the blood pressure

• At the same time, constriction of afferent arterioles reduces GFR and volume of filtrate and urine produced