Renal 2 - Jan 30 (end of midterm 1 content)

Filtration and Glomerular Filtration Rate (GFR)

Introduction to Renal Filtration

The process of filtration in the kidneys is the initial step in urine formation, occurring as plasma moves from the glomerular capillaries into the kidney tubules. This process is essential for removing waste products and regulating fluid and electrolyte balance in the body.

• Filtration: Movement of plasma and dissolved solutes into the Bowman's capsule; red blood cells and plasma proteins remain in the blood.

• Filtration Fraction: Only about 20% of plasma entering the glomerulus is filtered; the remaining 80% continues to the peritubular capillaries.

• Example: If 1 L/min of plasma enters the kidneys, only 0.2 L/min is filtered into the tubules.

Quantitative Aspects of Filtration

• Cardiac Output: 5 L/min

• Renal Blood Flow: Kidneys receive ~20% of cardiac output (1 L/min)

• Plasma Fraction: 60% of blood is plasma (0.6 L/min)

• Filtration Rate: ~20% of plasma is filtered (0.125 L/min)

• Daily Filtration: $0.125 ext{ L/min} imes 60 ext{ min/hr} imes 24 ext{ hr/day} = 180 ext{ L/day}$

• Normal GFR: 125 mL/min

The Renal Corpuscle and Filtration Barrier

The renal corpuscle consists of the glomerulus and Bowman's capsule, where filtration occurs across a specialized barrier.

• Triple Filtration Barrier:

  1. Capillary endothelial cells (fenestrated)

  2. Basal lamina (extracellular matrix)

  3. Podocyte end feet (form filtration slits)

• Mesangial Cells: Can influence filtration by altering capillary surface area.

Pressures Governing Filtration

Filtration from glomerular capillaries into renal tubules is governed by three main pressures:

• Hydrostatic Pressure (PGC): Favors filtration (~55 mm Hg)

• Colloid Osmotic Pressure (πGC): Opposes filtration due to plasma proteins (~30 mm Hg)

• Bowman's Capsule Hydrostatic Pressure (PBC): Opposes filtration (~15 mm Hg)

Net Filtration Pressure Equation:

$ext{Net Filtration Pressure} = P_{GC} - ext{(π}_{GC} + P_{BC})$

Example: $55 ext{ mm Hg} - (30 ext{ mm Hg} + 15 ext{ mm Hg}) = 10 ext{ mm Hg}$

Glomerular Filtration Rate (GFR)

GFR is the volume of fluid filtered from the glomerular capillaries into Bowman's capsules per unit time.

• Normal GFR: ~125 mL/min or 180 L/day

• Plasma Volume: ~3 L; kidneys filter entire plasma volume ~60 times/day

• Importance: Without reabsorption, plasma would be depleted in ~24 minutes

Factors Influencing GFR:

• Filtration Pressure (hydrostatic, colloid osmotic, and capsule pressures)

• Filtration Coefficient (surface area and permeability of filtration barrier)

Factor	Effect on GFR
Filtration Pressure	Depends on blood pressure and opposing pressures
Filtration Coefficient	Depends on slit surface area and barrier permeability

Autoregulation and Other Factors Influencing GFR

Autoregulation of GFR

GFR remains relatively constant over a wide range of blood pressures due to autoregulatory mechanisms, primarily at the level of the renal arterioles.

• Renal Blood Flow (RBF): Determined by resistance in afferent and efferent arterioles

• Regulation: Mainly occurs at the afferent arteriole

• Relationship: RBF and GFR are not always directly proportional

Change	Effect on RBF	Effect on GFR
Constrict AA	↓	↓
Constrict EA	↓	↑
Dilate AA	↑	↑
Dilate EA	↑	↓

Myogenic Response (Autoregulation)

The myogenic response is a contraction of vascular smooth muscle in the afferent arteriole in response to stretch, helping to maintain stable GFR despite changes in blood pressure.

• Mechanism: Increased pressure stretches smooth muscle, causing contraction and reduced blood flow

• Ion Channels: Stretch activates ion channels, leading to depolarization and calcium influx, triggering contraction

Tubuloglomerular Feedback

This local control pathway involves the juxtaglomerular apparatus, where fluid flow through the nephron influences GFR.

• Macula Densa: Senses increased NaCl delivery, releases paracrine signals (e.g., ATP, adenosine)

• Effect: Signals cause constriction of afferent arteriole, reducing GFR

• Example: Increased NaCl transport in macula densa cells leads to increased cilia movement and feedback signaling

Sympathetic Nervous System Influence

Sympathetic neurons can override local control mechanisms, especially during rapid drops in blood pressure (e.g., hemorrhage, severe dehydration).

• Mechanism: Norepinephrine acts on α1 adrenergic receptors, causing vasoconstriction of afferent and efferent arterioles

• Effect: Decreases GFR to conserve water

Hormonal Regulation of GFR

Hormones such as angiotensin II (vasoconstrictor) and prostaglandins (vasodilators) modulate arteriole resistance and filtration coefficient.

• Angiotensin II: Potent vasoconstrictor, decreases GFR

• Prostaglandins: Vasodilators, increase GFR

• Podocyte Modulation: Changes size of filtration slits, altering permeability

• Mesangial Cell Contraction: Alters capillary surface area available for filtration

Hormone	Action	Effect on GFR
Angiotensin II	Vasoconstriction	↓
Prostaglandins	Vasodilation	↑

Summary

Renal filtration and GFR are tightly regulated by physical pressures, autoregulatory mechanisms, neural input, and hormonal signals. These processes ensure efficient removal of waste while maintaining fluid and electrolyte balance.

Additional info: The notes cover material relevant to Chapter 19 (The Kidneys) and Chapter 20 (Fluid and Electrolyte Balance) in a standard Anatomy & Physiology curriculum.